Furosemide-induced renal medullary hypoperfusion in the rat: role of tissue tonicity, prostaglandins and angiotensin II
1 Laboratory of Renal and Body Fluid Physiology, M. Mossakowski Medical Research Centre, Polish Academy of Sciences, Warsaw, Poland
Abstract
Furosemide (frusemide)-induced renal medullary hypoperfusion provides a model for studies of the dependence of local circulation on tissue tonicity. We examined the role of medullary prostaglandins (PG) and adenosine (Ado) as possible mediators of the response to furosemide. Furosemide was infused I.V. at 0.25 mg kg–1 h–1 in anaesthetized rats, untreated or treated with intramedullary indomethacin (Indo) or Ado. An integrated set-up was used to measure renal medullary laser-Doppler flux (MBF) and medullary ionic tonicity (electrical admittance, Y), and to infuse Indo and Ado directly into the medulla. The cortical flux was measured on kidney surface. The excretion of water, sodium and total solute was also determined. Intramedullary Indo (1 mg kg–1 h–1) decreased MBF 18 ± 5% and increased tissue Y 14 ± 3% (both significant); the treatment abolished the post-furosemide decrease in MBF (–22% in untreated group) and enhanced slightly the increase in renal excretion. Intramedullary Ado (5 mg kg–1 h–1) did not change baseline MBF or Y; the post-furosemide decreases in MBF (–22%) and Y, and the increase in renal excretion were preserved. We conclude that a decrease in intramedullary PG activity secondary to decreased medullary hypertonicity mediates the fall in medullary perfusion in response to furosemide; the hypoperfusion may help restore the initial tonicity. Together with the earlier evidence on the dependence of post-furosemide medullary hypoperfusion on angiotensin II, the study exposes its interaction with PG in the control of medullary circulation. Adenosine is not involved in medullary vascular responses to decreased tissue hypertonicity.
, http://www.100md.com
Introduction
One of the most prominent features of the renal medulla is its osmotic hypertonicity. The processes of accumulation of solutes in the medullary interstitium and the mechanisms responsible for the maintenance or dissipation of the cortico-papillary osmotic gradient in the kidney have been the subject of vast research; many studies indicated that decreases or increases in the medullary blood flow (MBF) may result in inverse changes (retention or wash-out) of medullary solute concentration (Sadowski & Dobrowolski, 2003). On the other hand, we showed recently that, vice versa, primary changes in medullary interstitial tonicity, induced by an inhibition of NaCl transport from the ascending limbs of medullary loops of Henle to the interstitium with furosemide, may modulate perfusion of the medulla (Dobrowolski et al. 2000, 2001). This modulation may be physiologically important, considering the postulated crucial role of the renal medulla in the long term control of arterial pressure (Cowley, 1997; Bergstroem & Evans, 2004).
, 百拇医药
Our earlier studies documented that a decrease in medullary hypertonicity induced by furosemide was paralleled by a decrease in MBF (Dobrowolski et al. 2000, 2001). The decrease could be prevented by a blockade of angiotensin II (Ang II) receptors of the AT1 type with losartan, which suggested that the renin–angiotensin system was involved in the response (Dobrowolski et al. 2001). This accorded well with the long standing evidence on the stimulation by furosemide of renin synthesis (reviewed in Puschett & Winaver, 1992), mediated by an inhibition of NaCl transport in macula densa cells (Martinez-Maldonado et al. 1990). However, we thought it unlikely that Ang II should directly mediate the post-furosemide decrease in MBF. In the anaesthetized rat exogenous Ang II did not change or even increased MBF and after losartan no increase in MBF was seen (Nobes et al. 1991; Ortiz et al. 1998; Bdzyska et al. 2002).
, http://www.100md.com
In the present study we tested the hypothesis that a decrease in the intramedullary activity of vasodilator prostaglandins (PGs) was responsible for the fall in MBF observed after furosemide administration. PG activity is higher in the renal medulla compared to the cortex (Pflueger et al. 1999; Kompanowska-Jezierska et al. 2001), PG receptors mediating vasodilatation are present in medullary vessels (Purdy & Arendshorst, 2000), and there is evidence that PGs maintain medullary circulation in anaesthetized, operated rats (Conrad & Dunn, 1992; Kompanowska-Jezierska et al. 1999; Pallone et al. 2003). Furthermore, medullary PG activity was reported to change directly with experimentally induced alterations in the local tonicity (Dannon et al. 1978; Yang et al. 1999), and furosemide treatment decreased expression of PG cyclooxygenases and PGE2 content in the rat inner medulla (Castrop et al. 2002). Thus, a suppression of the tonic influence of vasodilator PG could be a key factor in a decrease in MBF in response to the furosemide-induced decrease in medullary hypertonicity.
, 百拇医药
In the present study we examined the proposed mediatory role of intrarenal PG using an experimental set-up enabling simultaneous measurement of medullary perfusion and local tissue ionic hypertonicity as well as a selective suppression of interstitial PG activity in the medulla (Dobrowolski & Sadowski, 2004).
The same experimental approach was also used to study the role of adenosine (Ado), another potential mediator of furosemide action on the renal medullary vasculature. Since Ado's major source is the breakdown of ATP by tubular Na+,K+-ATPase, its synthesis and interstitial concentration would change directly with the rate of tubular NaCl transport. Moreover, there is evidence to the involvement of adenosine in the control of medullary circulation (Zou et al. 1999). In this study the response to furosemide was examined during renal medullary infusion of adenosine, which should prevent a local deficiency of this agent.
, 百拇医药
Methods
Male Wistar rats weighing 280–330 g, maintained on dry pellet diet and given free access to water, were anaesthetized with intraperitoneal thiopentabarbital (Thiopental, 100 mg (kg body wt)–1; Biochemie GmbH, Vienna, Austria). The procedures were approved by the Ethical Committee of the Medical Research Centre, Polish Academy of Sciences, Warsaw, Poland.
Surgical preparations
In order to compensate for fluid losses, during surgical preparation 3% bovine albumin in Ringer solution was infused at 6.6 ml kg–1 h–1. Body temperature was maintained at about 37°C by means of a heating pad. The surgical and other preparatory techniques were as previously described (Dobrowolski et al. 2000). Briefly, the left kidney was exposed from a subcostal flank incision and placed in a plastic holder similar to that used for micropuncture experiments; the inside of the holder was padded in a way that the dorsal curvature of the kidney (and not the side surface) was facing upwards. This was done to enable placement in the kidney of an integrated probe (see below). The left ureter was cannulated for timed urine collection. To ensure constant renal perfusion pressure (RPP) throughout experiments, a screw-controlled snare was placed on the aorta above the origin of the left renal artery. Before the start of the experiment, the aorta was slightly pre-constricted to bring RPP (measured below the constriction site) to 105–110 mmHg; later during experiment the snare was released or tightened to maintain a pressure constant.
, 百拇医药
Experimental procedures and measurements
At the end of surgical preparations an infusion of bovine serum albumin was replaced by isotonic saline solution at 6.6 ml kg–1 h–1. Thereafter an integrated probe (a laser-Doppler probe plus admittance electrode encompassed within a stainless steel cannula), as described in detail in a recent paper (Dobrowolski & Sadowski, 2004) was inserted into the kidney from its dorsal surface, along the cortico-papillary axis, to the depth of about 5.5 mm. The probe enables simultaneous measurement of (1) medullary blood flow (laser-Doppler flux, MBF), using the Perimed measuring system (Jarfalla, Sweden) and a PF 402 needle probe, and (2) electrical admittance (Y) of medullary tissue (an index of interstitial ion concentration), i.e. conductive properties of the tissue between the stainless steel cannula of the laser-Doppler probe and a platinum–iridium admittance electrode. This sort of an ‘admittance cell’ was connected to a laboratory conductance meter (Mera Elwro, Wrocaw, Poland). In addition to MBF and Y measurement, this set-up enables infusion of fluids directly into the medulla, via the space between the admittance electrode and the guide cannula encapsulating it. For measurement of cortical blood flow (CBF) another Perimed laser-Doppler probe (type PF 407/415) was placed on the kidney surface.
, 百拇医药
After placement of the probes, about 1 h was allowed for stabilization. During this time, solvents of drugs used later during experiments were infused into the medulla at a rate of 1 ml h–1. Subsequently, two 15 min control measurement and urine collection periods were made before furosemide (Hoechst, Germany) was administered I.V., first as a priming dose of 0.25 mg (kg body wt)–1 in 5 ml kg–1 over 5 min, followed by an infusion delivering 0.25 mg kg–1 h–1. This dosage was shown previously to induce strictly defined changes in renal excretion as well as in CBF, MBF and Y (Dobrowolski et al. 2000, 2001). Starting from the beginning of the priming injection, four 5 min and a further 20 min urine collections were made during the infusion of furosemide.
, 百拇医药
After experiments the rats were killed with an overdose of the anaesthetic. The position of the integrated probe in the inner medulla (close to the border with the outer medulla) was verified at the kidney's cross-section.
Protocols
Effects of furosemide during intramedullary drug solvent infusion (n= 9). This group served as a control for studies of effects of furosemide administration in rats given adenosine or indomethacin directly into the renal medulla (see below). Effects of furosemide were quite similar in rats given intramedullary infusion of adenosine or indomethacin solvents (isotonic saline or slightly alkalinized saline, respectively); therefore a common control group was created.
, 百拇医药
Effects of furosemide during intramedullary indomethacin infusion (n= 6). These experiments were designed to examine whether inhibition of prostaglandin synthesis in the renal medulla would modify the responses of medullary blood flow to furosemide. In order to minimize the risk of indomethacin (Indo) leakage out of the medulla, especially to the adjacent cortex, a low dosage was used (1 mg kg–1 h–1 into the medulla, infused throughout the experiment), selected on the basis of the evoked functional effects. This dose increased tissue Y by 14% and decreased MBF by 18%, compared with +23% and –36% changes, respectively, observed earlier in our laboratory after a dose of 5 mg kg–1I.V. infused within 5 min (Kompanowska et al. 1999). At least 40 min were allowed for stabilization of the measured variables before pre-furosemide control periods were made.
, 百拇医药
Effects of furosemide during intramedullary adenosine infusion (n= 6). We examined if high medullary interstitial concentration of adenosine would modify the responses of medullary blood flow to furosemide. Adenosine (Ado) was administered into the medulla at 5 μg kg–1 h–1. This dosage was shown previously (Dobrowolski & Sadowski, 2004) to impair urine concentration without affecting MBF or Y. It was also shown in preliminary experiments that the intramedullary infusion did not alter renal cortical blood flow, in contrast to a significant decrease observed after systemic infusion. At least 40 min were allowed for stabilization of the measured variables before pre-furosemide control periods were made.
, 百拇医药
Statistics
The significance of changes within one group over time was first evaluated by repeat measurement analysis of variance (ANOVA), followed by Student's t test for dependent variables. Differences in mean values between groups were evaluated using the classical one-way ANOVA followed by Student's t test for independent samples with Bonferroni's correction. Absolute values were used for statistical calculations whereas per cent changes are shown in graphs.
, 百拇医药
Results
Of the three pre-treatment regimens (solvent, adenosine and indomethacin), only the last one significantly changed the basal (pre-furosemide) values of some of the parameters measured. Indomethacin moderately increased tissue admittance (Y) 14 ± 3% and decreased MBF 18 ± 5% (both P < 0.05); simultaneously, the renal excretion rate of sodium (UNaV) increased from 0.6 ± 0.1 to 1.5 ± 0.3 μmol min–1 (difference of borderline significance).
, 百拇医药
Table 1 shows the responses of Y, MBF and CBF to furosemide, as observed under intramedullary solvent, indomethacin or adenosine infusion. It is seen that the renal perfusion pressure (RPP) was effectively maintained constant in all the experimental groups. The between-group differences in the basal (pre-furosemide) values did not exceed the usual variability range of the measured parameters. Furosemide significantly decreased Y, MBF and CBF in the solvent and adenosine group but did not alter MBF in rats receiving intramedullary indomethacin.
, http://www.100md.com
Figure 1 shows the time course of Y, MBF and CBF responses to furosemide. It decreased medullary tissue Y, quite similarly in the solvent, indomethacin and adenosine groups. Associated in time was a rapid phase of a decrease in MBF of about 20%, clearly seen in the solvent and adenosine groups. By contrast, no rapid or delayed post-furosemide decrease in MBF was seen in indomethacin-treated rats. Furosemide induced also initial fluctuations in CBF that did not resemble the systematic decrease in Y and MBF. There was also a delayed prolonged decrease in CBF that did not exceed 10%. These responses showed no significant differences between individual pre-treatment groups.
, http://www.100md.com
The curves for rats treated with intramedullary indomethacin (), adenosine () and animals receiving intramedullary solvent infusion (). Values are means ±S.E.M. Begining with the point indicated by the arrow (upper panel) the decreases in the three curves were significant. The crosshatched bar on the time axis indicates the period of maximal renal excretion. *Significantly different from pre-furosemide (baseline) values at P < 0.05 or less.
A summary of the maximal post-furosemide changes in CBF, MBF and Y and of the associated changes in renal excretion is given in Fig. 2. In contrast to unaltered responses to furosemide in rats pre-treated with adenosine, indomethacin prevented a post-furosemide decrease in MBF without significantly affecting the decreases in CBF or Y. In each of the three groups, furosemide induced a substantial and similar increase in the three variables of renal excretion (for water, sodium and total solutes (V, UNaV, UosmV), respectively). However, in the indomethacin group the increase in UosmV was significantly greater than in untreated animals.
, 百拇医药
Upper panel: effects of furosemide on renal cortical (CBF), medullary blood flow (MBF) and medullary tissue admittance (Y) in rats untreated (open columns) or treated with indomethacin (filled columns) or adenosine (hatched columns). Lower panel: effects of furosemide on urine flow (V), sodium excretion (UNaV) and total solute excretion (UosmV)) in rats untreated () or treated with indomethacin () or adenosine (). Data refer to excretion rates recorded 5–15 min after furosemide administration (see also Fig. 1). Values are means ±S.E.M.; number of experiments is given in parentheses. *Significantly different from pre-furosemide (baseline) values at P < 0.05 or less; significantly different from change in CBF at P < 0.03; significantly different from the effect of furosemide without pre-treatment, P < 0.04.
, 百拇医药
Adenosine pre-treatment modified the responses to furosemide only slightly; the maximal MBF, CBF and Y responses as well as the increases in V, UNaV and UosmV did not significantly differ from those seen in solvent-treated rats.
Discussion
In this study we confirmed our earlier reports indicating that furosemide induced an acute substantial decrease in the tissue ionic hypertonicity of the renal medulla (electrical admittance, Y), a direct consequence of an inhibition of NaCl transport in the medullary segments of the loops of Henle. Associated with this was a substantial decrease in medullary perfusion (MBF); the time profiles of MBF and Y changes were remarkably similar (Dobrowolski et al. 2000, 2001).
, 百拇医药
The main finding of the present experiments was that an inhibition of medullary prostaglandin (PG) activity prevented the decrease in MBF. This finding helps explain the mechanism of the decrease in medullary perfusion; however, any attempt at such an explanation must take into consideration, in addition, our earlier observation that the MBF decrease after furosemide was also prevented by an inhibition of Ang II receptors of the AT1 type with losartan.
(Dobrowolski et al. 2001). Apparently, both the presence of PGs in the medulla and the intact renin–angiotensin system are needed for the decrease in MBF that follows the furosemide-induced decrease in the medullary tonicity. In theory, the decrease could depend on a reduction of medullary activity of vasodilator PG or on an increase in the plasma level of vasoconstrictor Ang II. There is good evidence that anaesthesia and surgery are powerful stimulators of both systems (review by Conrad & Dunn, 1992); therefore under baseline conditions of the present experiments (before administration of furosemide) the activity of intrarenal prostaglandins and the Ang II level were presumably high. One may reason that the medullary vasculature would be more likely to constrict in response to elimination of the abundant medullary PGs than to a further elevation of plasma Ang II. Nevertheless, the role of either factor in the MBF decrease after furosemide must be considered.
, 百拇医药
The hypothetical mechanisms of the post-furosemide decrease in medullary perfusion are presented in the diagram in (Fig. 3). An inhibition by furosemide of the Na+–K+–2 Cl– transporter (NKCC2) in the thick ascending limb of the loop of Henle (TALH) reduces the delivery of NaCl to the medullary interstitium and results in a decrease in medullary tissue ionic hypertonicity. This is an inhibitory stimulus for intramedullary PG synthesis. Under baseline conditions PG content is higher in the medulla compared with the cortex (Pflueger et al. 1999; Kompanowska-Jezierska et al. 2001) and PG receptors mediating vasodilatation are present in the medullary vessels (Purdy & Arendshorst, 2000). Furthermore, there is evidence from early (Dannon et al. 1978; Craven et al. 1980) and more recent studies (Yang et al. 1999; Hao et al. 2000) that PG synthesis changes directly with the osmotic tonicity or NaCl concentration in the renal medullary interstitium, and Castrop et al. (2002) recently provided data documenting decreased cyclooxygenase mRNA expression, cyclooxygenase protein content and tissue PGE2 in the inner medulla of rats treated with furosemide.
, 百拇医药
Inhibition by furosemide of the Na+–K+–2 Cl– transporter (NKCC2) in the thick ascending limb of the loop of Henle (TALH) and in the macula densa cells results in a decrease in medullary tissue hypertonicity and in stimulation of the renin–angiotensin system, respectively. The effect of these changes on intramedullary prostaglandins (PGs) is dual: their synthesis is inhibited by decreasing hypertonicity (a major effect) and stimulated by increased circulating Ang II (via AT1 receptors). The post-furosemide decrease in medullary perfusion (MBF) is mainly the result of a decrease in local PG activity; a potential (dashed line) direct effect of Ang II on the medullary vasculature is probably offset by vasodilator action of PGs stimulated by Ang II and by nitric oxide (not shown). We propose that the necessary condition for furosemide to cause a decrease in MBF secondary to a decrease in medullary hypertonicity is a relatively high baseline medullary PG activity. After exclusion of PG synthesis with intramedullary indomethacin (Indo, the PG link area shaded) but also after abolishment of PG stimulation by Ang II via AT1 receptors (losartan pre-treatment), the baseline PG activity is low, no substantial further decrease can be induced with furosemide, and MBF remains at the baseline level. In case Indo diffused out of the medulla, into the cortex, to inhibit PG-stimulated renin release (via macula densa and/or through a direct effect on juxtaglomerular cells, dashed lines), the circulating Ang II would increase little after furosemide application and stimulation of intramedullary PG synthesis would be negligible, an effect that would add to direct inhibition of the synthesis by intramedullary Indo.
, 百拇医药
We suggest that the reduction of intramedullary PGs and of their tonic vasodilator influence on the medullary vessels is a major factor determining the furosemide-induced decrease in MBF. In anaesthetized rats subjected to surgical trauma, PGs were shown to have a major role in maintaining adequate perfusion of the renal medulla, and MBF fell invariably after blockade of PG cyclooxygenase (Sadowski et al. 1997; Kompanowska-Jezierska et al. 1999). In accordance with the above suggestion, in the present experiments an intramedullary Indo infusion prevented the decrease in MBF in response to furosemide.
, 百拇医药
The other mechanism potentially responsible for the decrease in MBF involves a well-documented furosemide-induced stimulation (via a change in NaCl transport in macula densa cells) of renin synthesis (Martinez-Maldonado et al. 1990; Puschett & Winaver, 1992), increased generation of angiotensin II, and the enhanced vasoconstrictor action of the circulating peptide on the medullary vasculature (Fig. 3). However, Ang II can hardly be conceived as an important agent controlling medullary perfusion and a direct mediator of post-furosemide medullary vasoconstriction. We showed recently that in anaesthetized rats exogenous Ang II decreased, whereas losartan increased, perfusion of the cortex but no similar effects were seen in the medulla (Bdzyska et al. 2002, 2003). Most probably, the unresponsiveness of the medullary vasculature to exogenous (and endogenous) Ang II was due to an effective buffering of the constrictor effects by vasodilator agents, such as PG and nitric oxide. It is noteworthy that even a low, subpressor dose of Ang II was found to increase NO content in the medulla of anaesthetized rats (Zou et al. 1998). The diagram in Fig. 3 offers an explanation of why, in spite of the above evidence on ineffectiveness of Ang II as a medullary vasoconstrictor, an inhibition of AT1 receptors with losartan prevented the MBF decrease after furosemide treatment, as described by us earlier (Dobrowolski et al. 2001). We propose that the critical effect of losartan was the blockade of stimulation by Ang II of the medullary PG synthesis, as described by Siragy & Carey, 1996).
, http://www.100md.com
In summary, we conclude that the furosemide-induced decrease in medullary hypertonicity triggers a decrease in the activity of medullary vasodilator PG, followed by medullary vasoconstriction. However, this occurs only when the baseline medullary PG activity is high. In the absence of PG stimulation by Ang II (e.g. after losartan treatment) the initial PG activity is negligible, the medullary hypotonicity does not lead to a further substantial decrease of this activity, and no vasoconstrictor response to furosemide is seen. Therefore, both Indo and losartan prevent the furosemide-induced decrease in MBF.
, http://www.100md.com
The above analysis of the inhibitory effect of Indo on the post-furosemide decrease in MBF assumed that the Indo delivered to the medulla did not diffuse into the adjacent cortical tissue (juxtamedullary glomeruli) to inhibit PG-stimulated renin release (via macula densa and/or through a direct effect on juxtaglomerular cells) (Ito et al. 1989). If the leakage of Indo outside the medulla did occur (dotted lines in the diagram), the stimulation of the renin–angiotensin system by furosemide would be attenuated (Yoshida et al. 1987). Then, in addition to a hypotonicity-dependent inhibition of PG synthesis within the medulla, a decrease in the level of circulating Ang II and reduced stimulation of PG synthesis mediated by AT1 receptors would contribute to a further decrease in intramedullary PG activity. Such a contributory effect cannot be excluded; however, a substantial leakage of Indo into the cortex was unlikely. A low dose of the drug was used for intramedullary infusion, selected after analysis of well-established functional effects. This dose caused a substantially smaller increase in medullary tissue admittance (interstitial hypertonicity) and only a half of the decrease in MBF observed in this laboratory after systemic administration of Indo (Kompanowska-Jezierska et al. 1999); also, the observed increase in sodium excretion was typical for intramedullary administration, unlike a modest decrease or no change seen after systemic administration (Dobrowolski & Sadowski, 2004). It is also clear that the possibility that some part of the effect of furosemide depended on PG inhibition in the cortex would in no way undermine the overall interpretation presented above.
, http://www.100md.com
Our results draw attention to the role of medullary tissue tonicity as a factor participating in the control of medullary circulation. Furthermore, they provide functional evidence for the contribution of the medullary PG and the renin–angiotensin system as mediators of the decrease in MBF induced by medullary interstitial hypotonicity. The medullary hypoperfusion may be physiologically important as it helps restore the cortico-papillary solute gradient by increasing the effectiveness of the countercurrent exchanger of the vasa recta and limiting the wash-out of solutes from the medullary interstitium.
, 百拇医药
The post-furosemide decrease in tubular NaCl transport could reduce the breakdown of ATP by Na+,K+-ATPase and limit the generation of adenosine (Ado). Assuming an initial tonic vasodilator influence of Ado on the medullary vasculature via A2 receptors (Zou et al. 1999), furosemide administration could lead to medullary vasoconstriction, and the delivery of exogenous Ado to the medulla would prevent the effect. However, Ado pre-treatment did not significantly modify the decrease of MBF after furosemide treatment, which speaks against an involvement of this agent in the response but, obviously, not against its role in the control of medullary circulation in general.
, http://www.100md.com
The post-furosemide increase in renal excretion tended to be greater in indomethacin-treated than in control or adenosine-treated rats (Fig. 2) whereas some of the early renal pharmacological studies provided evidence for antagonistic rather than synergistic effects of PG blockade and furosemide on renal excretion, even though the results were not entirely consistent (reviewed in Yoshida et al. 1987). This discrepancy may depend on different routes of administration of inhibitors of PG cyclooxygenase. In our experiments intramedullary Indo alone induced natriuresis, in agreement with our recent report (Dobrowolski & Sadowski, 2004) but in contrast to modest antinatriuresis or no change in sodium excretion observed after systemic administration of the drug (reviewed in Conrad & Dunn, 1992). Further specific studies of tubular transport would be required to explain the apparent facilitation of the action of furosemide observed after elimination of medullary PG.
, 百拇医药
References
Bdzyska B, Grzelec-Mojzesowicz M, Dobrowolski L & Sadowski J (2002). Differential effect of angiotensin II on blood circulation in the renal medulla and cortex of anaesthetised rats. J Physiol 538, 159–166.
Bdzyska B, Grzelec-Mojzesowicz M & Sadowski J (2003). Prostaglandins but not nitric oxide protect renal medullary perfusion in anaesthetized rats receiving angiotensin II. J Physiol 548, 875–880.
, 百拇医药
Bergstroem G & Evans RG (2004). Mechanisms underlying the antihypertensive functions of the renal medulla. Acta Physiol Scand 181, 475–486.
Castrop H, Vitzthum H, Schumacher K, Schweda F & Kurtz A (2002). Low tonicity mediates a downregulation of cyclooxygenase-1 expression by furosemide in the rat renal papilla. J Am Soc Nephrol 13, 1136–1144.
Conrad KP & Dunn MJ (1992). Renal prostaglandins and other eicosanoids. In Renal Physiology, ed. Windhager EE, Vol. II, pp. 1707–1757. Oxford University Press, New York, Oxford.
, 百拇医药
Cowley AW Jr (1997). Role of the renal medulla in volume and arterial presure regulation. Am J Physiol 273, R1–R15.
Craven PA, Briggs R & DeRubertis FR (1980). Calcium dependent action of osmolality and adenosine 3'5'monophosphate accumulations in rat inner medulla: Evidence for a relationship to calcium responsive arachidonate release and prostaglandin synthesis. J Clin Invest 65, 529–542.
Dannon A, Knapp HR, Oelez O & Oates JA (1978). Stimulation of prostaglandin biosynthesis in the renal papilla by hypertonic mediums. Am J Physiol 234, F64–F67.
, 百拇医药
Dobrowolski LB, Bdzyska B, Grzelec-Mojzesowicz M & Sadowski J (2001). Renal vascular effects of frusemide in the rat: influence of salt loading and the role of angiotensin II. Exp Physiol 86, 611–616.
Dobrowolski LB, Bdzyska B & Sadowski J (2000). Differential effect of frusemide on renal medullary and cortical blood flow in the anaesthetised rat. Exp Physiol 85, 785–789.
Dobrowolski L & Sadowski J (2004). Renal medullary infusion of indomethacin and adenosine: effects on local blood flow, tissue ion content and renal excretion. Kidney Blood Press Res 27, 29–34.
, 百拇医药
Hao CM, Yull F, Blackwell T, Kmhoff M, Davis LS & Breyer MD (2000). Dehydration activates an NF-kB-driven, COX2-dependent survival mechanism in renal medullary interstitial cells. J Clin Inves 106, 973–982.
Ito S, Carretero OA, Abe K, Beierwaltes WH & Yoshinaga K (1989). Effect of prostanoids on renin release from rabbit afferent arterioles with and without macula densa. Kidney Int 35, 1138–1144.
Kompanowska-Jezierska E, Berndt TJ & Knox FG (2001). Prostaglandin E2 concentrations in rat renal cortical and medullary interstitium: effect of volume expansion and renal perfusion pressure. Acta Physiol Scand 172, 287–289.
, http://www.100md.com
Kompanowska-Jezierska E, Walkowska A & Sadowski J (1999). Exaggerated volume expansion natriuresis in rats preloaded with hypertonic saline: a paradoxical enhancement by inhibition of prostaglandin synthesis. Acta Physiol Scand 167, 189–194.
Martinez-Maldonado M, Gely R, Tapia E & Benabe JE (1990). Role of macula densa in diuretics-induced renin release. Hypertension 16, 261–268.
Nobes MS, Harris PJ, Yamada H & Mendelsohn FAO (1991). Effects of angiotensin on renal cortical and papillary blood flow measured by laser-Doppler flowmetry. Am J Physiol 261, F998–F1006.
, 百拇医药
Ortiz MC, Fortepiani LA, Ruiz-Marcos FM, Atucha NM & Garcia-Estan J (1998). Role of AT1 receptors in the renal papillary effects of acute and chronic nitric oxide inhibition. Am J Physiol 274, R760–R766.
Pallone TI, Zhang Z & Rhinehart K (2003). Physiology of the renal medullary circulation. Am J Physiol 284, F253–F266.
Pflueger AC, Gross JM & Knox FG (1999). Adenosine-induced renal vasoconstriction in diabetes mellitus rats: role of prostaglandins. Am J Physiol 277, R1410–R1417.
, http://www.100md.com
Purdy KE & Arendshorst WJ (2000). EP1 and EP4 receptors mediate prostaglandin E2 actions in the microcirculation of rat kidney. Am J Physiol 279, F755–F764.
Puschett JB & Winaver J (1992). Effects of diuretics on renal function. In Renal Physiology, ed. Windhager EE, Vol. II, pp. 2335–2406. Oxford University Press, New York, Oxford.
Sadowski J & Dobrowolski L (2003). The renal medullary interstitium: focus on osmotic hypertonicity. Clin Exp Physiol Pharmacol 30, 119–126.
, 百拇医药
Sadowski J, Kompanowska-Jezierska E, Dobrowolski L, Walkowska A & Bdzyska B (1997). Simultaneous recording of tissue ion content and blood flow in rat renal medulla: evidence on interdependence. Am J Physiol 273, F658–F662.
Siragy HM & Carey RM (1996). The subtype-2 (AT2) angiotensin receptor regulates renal cyclic guanosine 3,5-monophosphate and AT1 receptor-mediated prostaglandin E2 production in conscious rats. J Clin Invest 97, 1978–1982.
, 百拇医药
Yang T, Schnermann JB & Briggs JP (1999). Regulation of cyclooxygenase-2 expression in renal medulla by tonicity in vivo and in vitro. Am J Physiol 277, F1–F9.
Yoshida M, Suzuki-Kusaba M & Satoh S (1987). Participation of the prostaglandin system in furosemide-induced changes of renal function in anesthetized rats. Ren Physiol 10, 25–32.
Zou A-P, Nithipatikom K, Li PL & Cowley AW Jr (1999). Role of renal medullary adenosine in the control of blood flow and sodium excretion. Am J Physiol 276, R790–R798.
Zou AP, Wu F & Cowley AW Jr (1998). Protective effect of angiotensin II-induced increase in nitric oxide in the renal medullary circulation. Hypertension 31, 271–276., 百拇医药(Leszek Dobrowolski and Ja)
Abstract
Furosemide (frusemide)-induced renal medullary hypoperfusion provides a model for studies of the dependence of local circulation on tissue tonicity. We examined the role of medullary prostaglandins (PG) and adenosine (Ado) as possible mediators of the response to furosemide. Furosemide was infused I.V. at 0.25 mg kg–1 h–1 in anaesthetized rats, untreated or treated with intramedullary indomethacin (Indo) or Ado. An integrated set-up was used to measure renal medullary laser-Doppler flux (MBF) and medullary ionic tonicity (electrical admittance, Y), and to infuse Indo and Ado directly into the medulla. The cortical flux was measured on kidney surface. The excretion of water, sodium and total solute was also determined. Intramedullary Indo (1 mg kg–1 h–1) decreased MBF 18 ± 5% and increased tissue Y 14 ± 3% (both significant); the treatment abolished the post-furosemide decrease in MBF (–22% in untreated group) and enhanced slightly the increase in renal excretion. Intramedullary Ado (5 mg kg–1 h–1) did not change baseline MBF or Y; the post-furosemide decreases in MBF (–22%) and Y, and the increase in renal excretion were preserved. We conclude that a decrease in intramedullary PG activity secondary to decreased medullary hypertonicity mediates the fall in medullary perfusion in response to furosemide; the hypoperfusion may help restore the initial tonicity. Together with the earlier evidence on the dependence of post-furosemide medullary hypoperfusion on angiotensin II, the study exposes its interaction with PG in the control of medullary circulation. Adenosine is not involved in medullary vascular responses to decreased tissue hypertonicity.
, http://www.100md.com
Introduction
One of the most prominent features of the renal medulla is its osmotic hypertonicity. The processes of accumulation of solutes in the medullary interstitium and the mechanisms responsible for the maintenance or dissipation of the cortico-papillary osmotic gradient in the kidney have been the subject of vast research; many studies indicated that decreases or increases in the medullary blood flow (MBF) may result in inverse changes (retention or wash-out) of medullary solute concentration (Sadowski & Dobrowolski, 2003). On the other hand, we showed recently that, vice versa, primary changes in medullary interstitial tonicity, induced by an inhibition of NaCl transport from the ascending limbs of medullary loops of Henle to the interstitium with furosemide, may modulate perfusion of the medulla (Dobrowolski et al. 2000, 2001). This modulation may be physiologically important, considering the postulated crucial role of the renal medulla in the long term control of arterial pressure (Cowley, 1997; Bergstroem & Evans, 2004).
, 百拇医药
Our earlier studies documented that a decrease in medullary hypertonicity induced by furosemide was paralleled by a decrease in MBF (Dobrowolski et al. 2000, 2001). The decrease could be prevented by a blockade of angiotensin II (Ang II) receptors of the AT1 type with losartan, which suggested that the renin–angiotensin system was involved in the response (Dobrowolski et al. 2001). This accorded well with the long standing evidence on the stimulation by furosemide of renin synthesis (reviewed in Puschett & Winaver, 1992), mediated by an inhibition of NaCl transport in macula densa cells (Martinez-Maldonado et al. 1990). However, we thought it unlikely that Ang II should directly mediate the post-furosemide decrease in MBF. In the anaesthetized rat exogenous Ang II did not change or even increased MBF and after losartan no increase in MBF was seen (Nobes et al. 1991; Ortiz et al. 1998; Bdzyska et al. 2002).
, http://www.100md.com
In the present study we tested the hypothesis that a decrease in the intramedullary activity of vasodilator prostaglandins (PGs) was responsible for the fall in MBF observed after furosemide administration. PG activity is higher in the renal medulla compared to the cortex (Pflueger et al. 1999; Kompanowska-Jezierska et al. 2001), PG receptors mediating vasodilatation are present in medullary vessels (Purdy & Arendshorst, 2000), and there is evidence that PGs maintain medullary circulation in anaesthetized, operated rats (Conrad & Dunn, 1992; Kompanowska-Jezierska et al. 1999; Pallone et al. 2003). Furthermore, medullary PG activity was reported to change directly with experimentally induced alterations in the local tonicity (Dannon et al. 1978; Yang et al. 1999), and furosemide treatment decreased expression of PG cyclooxygenases and PGE2 content in the rat inner medulla (Castrop et al. 2002). Thus, a suppression of the tonic influence of vasodilator PG could be a key factor in a decrease in MBF in response to the furosemide-induced decrease in medullary hypertonicity.
, 百拇医药
In the present study we examined the proposed mediatory role of intrarenal PG using an experimental set-up enabling simultaneous measurement of medullary perfusion and local tissue ionic hypertonicity as well as a selective suppression of interstitial PG activity in the medulla (Dobrowolski & Sadowski, 2004).
The same experimental approach was also used to study the role of adenosine (Ado), another potential mediator of furosemide action on the renal medullary vasculature. Since Ado's major source is the breakdown of ATP by tubular Na+,K+-ATPase, its synthesis and interstitial concentration would change directly with the rate of tubular NaCl transport. Moreover, there is evidence to the involvement of adenosine in the control of medullary circulation (Zou et al. 1999). In this study the response to furosemide was examined during renal medullary infusion of adenosine, which should prevent a local deficiency of this agent.
, 百拇医药
Methods
Male Wistar rats weighing 280–330 g, maintained on dry pellet diet and given free access to water, were anaesthetized with intraperitoneal thiopentabarbital (Thiopental, 100 mg (kg body wt)–1; Biochemie GmbH, Vienna, Austria). The procedures were approved by the Ethical Committee of the Medical Research Centre, Polish Academy of Sciences, Warsaw, Poland.
Surgical preparations
In order to compensate for fluid losses, during surgical preparation 3% bovine albumin in Ringer solution was infused at 6.6 ml kg–1 h–1. Body temperature was maintained at about 37°C by means of a heating pad. The surgical and other preparatory techniques were as previously described (Dobrowolski et al. 2000). Briefly, the left kidney was exposed from a subcostal flank incision and placed in a plastic holder similar to that used for micropuncture experiments; the inside of the holder was padded in a way that the dorsal curvature of the kidney (and not the side surface) was facing upwards. This was done to enable placement in the kidney of an integrated probe (see below). The left ureter was cannulated for timed urine collection. To ensure constant renal perfusion pressure (RPP) throughout experiments, a screw-controlled snare was placed on the aorta above the origin of the left renal artery. Before the start of the experiment, the aorta was slightly pre-constricted to bring RPP (measured below the constriction site) to 105–110 mmHg; later during experiment the snare was released or tightened to maintain a pressure constant.
, 百拇医药
Experimental procedures and measurements
At the end of surgical preparations an infusion of bovine serum albumin was replaced by isotonic saline solution at 6.6 ml kg–1 h–1. Thereafter an integrated probe (a laser-Doppler probe plus admittance electrode encompassed within a stainless steel cannula), as described in detail in a recent paper (Dobrowolski & Sadowski, 2004) was inserted into the kidney from its dorsal surface, along the cortico-papillary axis, to the depth of about 5.5 mm. The probe enables simultaneous measurement of (1) medullary blood flow (laser-Doppler flux, MBF), using the Perimed measuring system (Jarfalla, Sweden) and a PF 402 needle probe, and (2) electrical admittance (Y) of medullary tissue (an index of interstitial ion concentration), i.e. conductive properties of the tissue between the stainless steel cannula of the laser-Doppler probe and a platinum–iridium admittance electrode. This sort of an ‘admittance cell’ was connected to a laboratory conductance meter (Mera Elwro, Wrocaw, Poland). In addition to MBF and Y measurement, this set-up enables infusion of fluids directly into the medulla, via the space between the admittance electrode and the guide cannula encapsulating it. For measurement of cortical blood flow (CBF) another Perimed laser-Doppler probe (type PF 407/415) was placed on the kidney surface.
, 百拇医药
After placement of the probes, about 1 h was allowed for stabilization. During this time, solvents of drugs used later during experiments were infused into the medulla at a rate of 1 ml h–1. Subsequently, two 15 min control measurement and urine collection periods were made before furosemide (Hoechst, Germany) was administered I.V., first as a priming dose of 0.25 mg (kg body wt)–1 in 5 ml kg–1 over 5 min, followed by an infusion delivering 0.25 mg kg–1 h–1. This dosage was shown previously to induce strictly defined changes in renal excretion as well as in CBF, MBF and Y (Dobrowolski et al. 2000, 2001). Starting from the beginning of the priming injection, four 5 min and a further 20 min urine collections were made during the infusion of furosemide.
, 百拇医药
After experiments the rats were killed with an overdose of the anaesthetic. The position of the integrated probe in the inner medulla (close to the border with the outer medulla) was verified at the kidney's cross-section.
Protocols
Effects of furosemide during intramedullary drug solvent infusion (n= 9). This group served as a control for studies of effects of furosemide administration in rats given adenosine or indomethacin directly into the renal medulla (see below). Effects of furosemide were quite similar in rats given intramedullary infusion of adenosine or indomethacin solvents (isotonic saline or slightly alkalinized saline, respectively); therefore a common control group was created.
, 百拇医药
Effects of furosemide during intramedullary indomethacin infusion (n= 6). These experiments were designed to examine whether inhibition of prostaglandin synthesis in the renal medulla would modify the responses of medullary blood flow to furosemide. In order to minimize the risk of indomethacin (Indo) leakage out of the medulla, especially to the adjacent cortex, a low dosage was used (1 mg kg–1 h–1 into the medulla, infused throughout the experiment), selected on the basis of the evoked functional effects. This dose increased tissue Y by 14% and decreased MBF by 18%, compared with +23% and –36% changes, respectively, observed earlier in our laboratory after a dose of 5 mg kg–1I.V. infused within 5 min (Kompanowska et al. 1999). At least 40 min were allowed for stabilization of the measured variables before pre-furosemide control periods were made.
, 百拇医药
Effects of furosemide during intramedullary adenosine infusion (n= 6). We examined if high medullary interstitial concentration of adenosine would modify the responses of medullary blood flow to furosemide. Adenosine (Ado) was administered into the medulla at 5 μg kg–1 h–1. This dosage was shown previously (Dobrowolski & Sadowski, 2004) to impair urine concentration without affecting MBF or Y. It was also shown in preliminary experiments that the intramedullary infusion did not alter renal cortical blood flow, in contrast to a significant decrease observed after systemic infusion. At least 40 min were allowed for stabilization of the measured variables before pre-furosemide control periods were made.
, 百拇医药
Statistics
The significance of changes within one group over time was first evaluated by repeat measurement analysis of variance (ANOVA), followed by Student's t test for dependent variables. Differences in mean values between groups were evaluated using the classical one-way ANOVA followed by Student's t test for independent samples with Bonferroni's correction. Absolute values were used for statistical calculations whereas per cent changes are shown in graphs.
, 百拇医药
Results
Of the three pre-treatment regimens (solvent, adenosine and indomethacin), only the last one significantly changed the basal (pre-furosemide) values of some of the parameters measured. Indomethacin moderately increased tissue admittance (Y) 14 ± 3% and decreased MBF 18 ± 5% (both P < 0.05); simultaneously, the renal excretion rate of sodium (UNaV) increased from 0.6 ± 0.1 to 1.5 ± 0.3 μmol min–1 (difference of borderline significance).
, 百拇医药
Table 1 shows the responses of Y, MBF and CBF to furosemide, as observed under intramedullary solvent, indomethacin or adenosine infusion. It is seen that the renal perfusion pressure (RPP) was effectively maintained constant in all the experimental groups. The between-group differences in the basal (pre-furosemide) values did not exceed the usual variability range of the measured parameters. Furosemide significantly decreased Y, MBF and CBF in the solvent and adenosine group but did not alter MBF in rats receiving intramedullary indomethacin.
, http://www.100md.com
Figure 1 shows the time course of Y, MBF and CBF responses to furosemide. It decreased medullary tissue Y, quite similarly in the solvent, indomethacin and adenosine groups. Associated in time was a rapid phase of a decrease in MBF of about 20%, clearly seen in the solvent and adenosine groups. By contrast, no rapid or delayed post-furosemide decrease in MBF was seen in indomethacin-treated rats. Furosemide induced also initial fluctuations in CBF that did not resemble the systematic decrease in Y and MBF. There was also a delayed prolonged decrease in CBF that did not exceed 10%. These responses showed no significant differences between individual pre-treatment groups.
, http://www.100md.com
The curves for rats treated with intramedullary indomethacin (), adenosine () and animals receiving intramedullary solvent infusion (). Values are means ±S.E.M. Begining with the point indicated by the arrow (upper panel) the decreases in the three curves were significant. The crosshatched bar on the time axis indicates the period of maximal renal excretion. *Significantly different from pre-furosemide (baseline) values at P < 0.05 or less.
A summary of the maximal post-furosemide changes in CBF, MBF and Y and of the associated changes in renal excretion is given in Fig. 2. In contrast to unaltered responses to furosemide in rats pre-treated with adenosine, indomethacin prevented a post-furosemide decrease in MBF without significantly affecting the decreases in CBF or Y. In each of the three groups, furosemide induced a substantial and similar increase in the three variables of renal excretion (for water, sodium and total solutes (V, UNaV, UosmV), respectively). However, in the indomethacin group the increase in UosmV was significantly greater than in untreated animals.
, 百拇医药
Upper panel: effects of furosemide on renal cortical (CBF), medullary blood flow (MBF) and medullary tissue admittance (Y) in rats untreated (open columns) or treated with indomethacin (filled columns) or adenosine (hatched columns). Lower panel: effects of furosemide on urine flow (V), sodium excretion (UNaV) and total solute excretion (UosmV)) in rats untreated () or treated with indomethacin () or adenosine (). Data refer to excretion rates recorded 5–15 min after furosemide administration (see also Fig. 1). Values are means ±S.E.M.; number of experiments is given in parentheses. *Significantly different from pre-furosemide (baseline) values at P < 0.05 or less; significantly different from change in CBF at P < 0.03; significantly different from the effect of furosemide without pre-treatment, P < 0.04.
, 百拇医药
Adenosine pre-treatment modified the responses to furosemide only slightly; the maximal MBF, CBF and Y responses as well as the increases in V, UNaV and UosmV did not significantly differ from those seen in solvent-treated rats.
Discussion
In this study we confirmed our earlier reports indicating that furosemide induced an acute substantial decrease in the tissue ionic hypertonicity of the renal medulla (electrical admittance, Y), a direct consequence of an inhibition of NaCl transport in the medullary segments of the loops of Henle. Associated with this was a substantial decrease in medullary perfusion (MBF); the time profiles of MBF and Y changes were remarkably similar (Dobrowolski et al. 2000, 2001).
, 百拇医药
The main finding of the present experiments was that an inhibition of medullary prostaglandin (PG) activity prevented the decrease in MBF. This finding helps explain the mechanism of the decrease in medullary perfusion; however, any attempt at such an explanation must take into consideration, in addition, our earlier observation that the MBF decrease after furosemide was also prevented by an inhibition of Ang II receptors of the AT1 type with losartan.
(Dobrowolski et al. 2001). Apparently, both the presence of PGs in the medulla and the intact renin–angiotensin system are needed for the decrease in MBF that follows the furosemide-induced decrease in the medullary tonicity. In theory, the decrease could depend on a reduction of medullary activity of vasodilator PG or on an increase in the plasma level of vasoconstrictor Ang II. There is good evidence that anaesthesia and surgery are powerful stimulators of both systems (review by Conrad & Dunn, 1992); therefore under baseline conditions of the present experiments (before administration of furosemide) the activity of intrarenal prostaglandins and the Ang II level were presumably high. One may reason that the medullary vasculature would be more likely to constrict in response to elimination of the abundant medullary PGs than to a further elevation of plasma Ang II. Nevertheless, the role of either factor in the MBF decrease after furosemide must be considered.
, 百拇医药
The hypothetical mechanisms of the post-furosemide decrease in medullary perfusion are presented in the diagram in (Fig. 3). An inhibition by furosemide of the Na+–K+–2 Cl– transporter (NKCC2) in the thick ascending limb of the loop of Henle (TALH) reduces the delivery of NaCl to the medullary interstitium and results in a decrease in medullary tissue ionic hypertonicity. This is an inhibitory stimulus for intramedullary PG synthesis. Under baseline conditions PG content is higher in the medulla compared with the cortex (Pflueger et al. 1999; Kompanowska-Jezierska et al. 2001) and PG receptors mediating vasodilatation are present in the medullary vessels (Purdy & Arendshorst, 2000). Furthermore, there is evidence from early (Dannon et al. 1978; Craven et al. 1980) and more recent studies (Yang et al. 1999; Hao et al. 2000) that PG synthesis changes directly with the osmotic tonicity or NaCl concentration in the renal medullary interstitium, and Castrop et al. (2002) recently provided data documenting decreased cyclooxygenase mRNA expression, cyclooxygenase protein content and tissue PGE2 in the inner medulla of rats treated with furosemide.
, 百拇医药
Inhibition by furosemide of the Na+–K+–2 Cl– transporter (NKCC2) in the thick ascending limb of the loop of Henle (TALH) and in the macula densa cells results in a decrease in medullary tissue hypertonicity and in stimulation of the renin–angiotensin system, respectively. The effect of these changes on intramedullary prostaglandins (PGs) is dual: their synthesis is inhibited by decreasing hypertonicity (a major effect) and stimulated by increased circulating Ang II (via AT1 receptors). The post-furosemide decrease in medullary perfusion (MBF) is mainly the result of a decrease in local PG activity; a potential (dashed line) direct effect of Ang II on the medullary vasculature is probably offset by vasodilator action of PGs stimulated by Ang II and by nitric oxide (not shown). We propose that the necessary condition for furosemide to cause a decrease in MBF secondary to a decrease in medullary hypertonicity is a relatively high baseline medullary PG activity. After exclusion of PG synthesis with intramedullary indomethacin (Indo, the PG link area shaded) but also after abolishment of PG stimulation by Ang II via AT1 receptors (losartan pre-treatment), the baseline PG activity is low, no substantial further decrease can be induced with furosemide, and MBF remains at the baseline level. In case Indo diffused out of the medulla, into the cortex, to inhibit PG-stimulated renin release (via macula densa and/or through a direct effect on juxtaglomerular cells, dashed lines), the circulating Ang II would increase little after furosemide application and stimulation of intramedullary PG synthesis would be negligible, an effect that would add to direct inhibition of the synthesis by intramedullary Indo.
, 百拇医药
We suggest that the reduction of intramedullary PGs and of their tonic vasodilator influence on the medullary vessels is a major factor determining the furosemide-induced decrease in MBF. In anaesthetized rats subjected to surgical trauma, PGs were shown to have a major role in maintaining adequate perfusion of the renal medulla, and MBF fell invariably after blockade of PG cyclooxygenase (Sadowski et al. 1997; Kompanowska-Jezierska et al. 1999). In accordance with the above suggestion, in the present experiments an intramedullary Indo infusion prevented the decrease in MBF in response to furosemide.
, 百拇医药
The other mechanism potentially responsible for the decrease in MBF involves a well-documented furosemide-induced stimulation (via a change in NaCl transport in macula densa cells) of renin synthesis (Martinez-Maldonado et al. 1990; Puschett & Winaver, 1992), increased generation of angiotensin II, and the enhanced vasoconstrictor action of the circulating peptide on the medullary vasculature (Fig. 3). However, Ang II can hardly be conceived as an important agent controlling medullary perfusion and a direct mediator of post-furosemide medullary vasoconstriction. We showed recently that in anaesthetized rats exogenous Ang II decreased, whereas losartan increased, perfusion of the cortex but no similar effects were seen in the medulla (Bdzyska et al. 2002, 2003). Most probably, the unresponsiveness of the medullary vasculature to exogenous (and endogenous) Ang II was due to an effective buffering of the constrictor effects by vasodilator agents, such as PG and nitric oxide. It is noteworthy that even a low, subpressor dose of Ang II was found to increase NO content in the medulla of anaesthetized rats (Zou et al. 1998). The diagram in Fig. 3 offers an explanation of why, in spite of the above evidence on ineffectiveness of Ang II as a medullary vasoconstrictor, an inhibition of AT1 receptors with losartan prevented the MBF decrease after furosemide treatment, as described by us earlier (Dobrowolski et al. 2001). We propose that the critical effect of losartan was the blockade of stimulation by Ang II of the medullary PG synthesis, as described by Siragy & Carey, 1996).
, http://www.100md.com
In summary, we conclude that the furosemide-induced decrease in medullary hypertonicity triggers a decrease in the activity of medullary vasodilator PG, followed by medullary vasoconstriction. However, this occurs only when the baseline medullary PG activity is high. In the absence of PG stimulation by Ang II (e.g. after losartan treatment) the initial PG activity is negligible, the medullary hypotonicity does not lead to a further substantial decrease of this activity, and no vasoconstrictor response to furosemide is seen. Therefore, both Indo and losartan prevent the furosemide-induced decrease in MBF.
, http://www.100md.com
The above analysis of the inhibitory effect of Indo on the post-furosemide decrease in MBF assumed that the Indo delivered to the medulla did not diffuse into the adjacent cortical tissue (juxtamedullary glomeruli) to inhibit PG-stimulated renin release (via macula densa and/or through a direct effect on juxtaglomerular cells) (Ito et al. 1989). If the leakage of Indo outside the medulla did occur (dotted lines in the diagram), the stimulation of the renin–angiotensin system by furosemide would be attenuated (Yoshida et al. 1987). Then, in addition to a hypotonicity-dependent inhibition of PG synthesis within the medulla, a decrease in the level of circulating Ang II and reduced stimulation of PG synthesis mediated by AT1 receptors would contribute to a further decrease in intramedullary PG activity. Such a contributory effect cannot be excluded; however, a substantial leakage of Indo into the cortex was unlikely. A low dose of the drug was used for intramedullary infusion, selected after analysis of well-established functional effects. This dose caused a substantially smaller increase in medullary tissue admittance (interstitial hypertonicity) and only a half of the decrease in MBF observed in this laboratory after systemic administration of Indo (Kompanowska-Jezierska et al. 1999); also, the observed increase in sodium excretion was typical for intramedullary administration, unlike a modest decrease or no change seen after systemic administration (Dobrowolski & Sadowski, 2004). It is also clear that the possibility that some part of the effect of furosemide depended on PG inhibition in the cortex would in no way undermine the overall interpretation presented above.
, http://www.100md.com
Our results draw attention to the role of medullary tissue tonicity as a factor participating in the control of medullary circulation. Furthermore, they provide functional evidence for the contribution of the medullary PG and the renin–angiotensin system as mediators of the decrease in MBF induced by medullary interstitial hypotonicity. The medullary hypoperfusion may be physiologically important as it helps restore the cortico-papillary solute gradient by increasing the effectiveness of the countercurrent exchanger of the vasa recta and limiting the wash-out of solutes from the medullary interstitium.
, 百拇医药
The post-furosemide decrease in tubular NaCl transport could reduce the breakdown of ATP by Na+,K+-ATPase and limit the generation of adenosine (Ado). Assuming an initial tonic vasodilator influence of Ado on the medullary vasculature via A2 receptors (Zou et al. 1999), furosemide administration could lead to medullary vasoconstriction, and the delivery of exogenous Ado to the medulla would prevent the effect. However, Ado pre-treatment did not significantly modify the decrease of MBF after furosemide treatment, which speaks against an involvement of this agent in the response but, obviously, not against its role in the control of medullary circulation in general.
, http://www.100md.com
The post-furosemide increase in renal excretion tended to be greater in indomethacin-treated than in control or adenosine-treated rats (Fig. 2) whereas some of the early renal pharmacological studies provided evidence for antagonistic rather than synergistic effects of PG blockade and furosemide on renal excretion, even though the results were not entirely consistent (reviewed in Yoshida et al. 1987). This discrepancy may depend on different routes of administration of inhibitors of PG cyclooxygenase. In our experiments intramedullary Indo alone induced natriuresis, in agreement with our recent report (Dobrowolski & Sadowski, 2004) but in contrast to modest antinatriuresis or no change in sodium excretion observed after systemic administration of the drug (reviewed in Conrad & Dunn, 1992). Further specific studies of tubular transport would be required to explain the apparent facilitation of the action of furosemide observed after elimination of medullary PG.
, 百拇医药
References
Bdzyska B, Grzelec-Mojzesowicz M, Dobrowolski L & Sadowski J (2002). Differential effect of angiotensin II on blood circulation in the renal medulla and cortex of anaesthetised rats. J Physiol 538, 159–166.
Bdzyska B, Grzelec-Mojzesowicz M & Sadowski J (2003). Prostaglandins but not nitric oxide protect renal medullary perfusion in anaesthetized rats receiving angiotensin II. J Physiol 548, 875–880.
, 百拇医药
Bergstroem G & Evans RG (2004). Mechanisms underlying the antihypertensive functions of the renal medulla. Acta Physiol Scand 181, 475–486.
Castrop H, Vitzthum H, Schumacher K, Schweda F & Kurtz A (2002). Low tonicity mediates a downregulation of cyclooxygenase-1 expression by furosemide in the rat renal papilla. J Am Soc Nephrol 13, 1136–1144.
Conrad KP & Dunn MJ (1992). Renal prostaglandins and other eicosanoids. In Renal Physiology, ed. Windhager EE, Vol. II, pp. 1707–1757. Oxford University Press, New York, Oxford.
, 百拇医药
Cowley AW Jr (1997). Role of the renal medulla in volume and arterial presure regulation. Am J Physiol 273, R1–R15.
Craven PA, Briggs R & DeRubertis FR (1980). Calcium dependent action of osmolality and adenosine 3'5'monophosphate accumulations in rat inner medulla: Evidence for a relationship to calcium responsive arachidonate release and prostaglandin synthesis. J Clin Invest 65, 529–542.
Dannon A, Knapp HR, Oelez O & Oates JA (1978). Stimulation of prostaglandin biosynthesis in the renal papilla by hypertonic mediums. Am J Physiol 234, F64–F67.
, 百拇医药
Dobrowolski LB, Bdzyska B, Grzelec-Mojzesowicz M & Sadowski J (2001). Renal vascular effects of frusemide in the rat: influence of salt loading and the role of angiotensin II. Exp Physiol 86, 611–616.
Dobrowolski LB, Bdzyska B & Sadowski J (2000). Differential effect of frusemide on renal medullary and cortical blood flow in the anaesthetised rat. Exp Physiol 85, 785–789.
Dobrowolski L & Sadowski J (2004). Renal medullary infusion of indomethacin and adenosine: effects on local blood flow, tissue ion content and renal excretion. Kidney Blood Press Res 27, 29–34.
, 百拇医药
Hao CM, Yull F, Blackwell T, Kmhoff M, Davis LS & Breyer MD (2000). Dehydration activates an NF-kB-driven, COX2-dependent survival mechanism in renal medullary interstitial cells. J Clin Inves 106, 973–982.
Ito S, Carretero OA, Abe K, Beierwaltes WH & Yoshinaga K (1989). Effect of prostanoids on renin release from rabbit afferent arterioles with and without macula densa. Kidney Int 35, 1138–1144.
Kompanowska-Jezierska E, Berndt TJ & Knox FG (2001). Prostaglandin E2 concentrations in rat renal cortical and medullary interstitium: effect of volume expansion and renal perfusion pressure. Acta Physiol Scand 172, 287–289.
, http://www.100md.com
Kompanowska-Jezierska E, Walkowska A & Sadowski J (1999). Exaggerated volume expansion natriuresis in rats preloaded with hypertonic saline: a paradoxical enhancement by inhibition of prostaglandin synthesis. Acta Physiol Scand 167, 189–194.
Martinez-Maldonado M, Gely R, Tapia E & Benabe JE (1990). Role of macula densa in diuretics-induced renin release. Hypertension 16, 261–268.
Nobes MS, Harris PJ, Yamada H & Mendelsohn FAO (1991). Effects of angiotensin on renal cortical and papillary blood flow measured by laser-Doppler flowmetry. Am J Physiol 261, F998–F1006.
, 百拇医药
Ortiz MC, Fortepiani LA, Ruiz-Marcos FM, Atucha NM & Garcia-Estan J (1998). Role of AT1 receptors in the renal papillary effects of acute and chronic nitric oxide inhibition. Am J Physiol 274, R760–R766.
Pallone TI, Zhang Z & Rhinehart K (2003). Physiology of the renal medullary circulation. Am J Physiol 284, F253–F266.
Pflueger AC, Gross JM & Knox FG (1999). Adenosine-induced renal vasoconstriction in diabetes mellitus rats: role of prostaglandins. Am J Physiol 277, R1410–R1417.
, http://www.100md.com
Purdy KE & Arendshorst WJ (2000). EP1 and EP4 receptors mediate prostaglandin E2 actions in the microcirculation of rat kidney. Am J Physiol 279, F755–F764.
Puschett JB & Winaver J (1992). Effects of diuretics on renal function. In Renal Physiology, ed. Windhager EE, Vol. II, pp. 2335–2406. Oxford University Press, New York, Oxford.
Sadowski J & Dobrowolski L (2003). The renal medullary interstitium: focus on osmotic hypertonicity. Clin Exp Physiol Pharmacol 30, 119–126.
, 百拇医药
Sadowski J, Kompanowska-Jezierska E, Dobrowolski L, Walkowska A & Bdzyska B (1997). Simultaneous recording of tissue ion content and blood flow in rat renal medulla: evidence on interdependence. Am J Physiol 273, F658–F662.
Siragy HM & Carey RM (1996). The subtype-2 (AT2) angiotensin receptor regulates renal cyclic guanosine 3,5-monophosphate and AT1 receptor-mediated prostaglandin E2 production in conscious rats. J Clin Invest 97, 1978–1982.
, 百拇医药
Yang T, Schnermann JB & Briggs JP (1999). Regulation of cyclooxygenase-2 expression in renal medulla by tonicity in vivo and in vitro. Am J Physiol 277, F1–F9.
Yoshida M, Suzuki-Kusaba M & Satoh S (1987). Participation of the prostaglandin system in furosemide-induced changes of renal function in anesthetized rats. Ren Physiol 10, 25–32.
Zou A-P, Nithipatikom K, Li PL & Cowley AW Jr (1999). Role of renal medullary adenosine in the control of blood flow and sodium excretion. Am J Physiol 276, R790–R798.
Zou AP, Wu F & Cowley AW Jr (1998). Protective effect of angiotensin II-induced increase in nitric oxide in the renal medullary circulation. Hypertension 31, 271–276., 百拇医药(Leszek Dobrowolski and Ja)