Indirect evidence against a role of the kinin system in the renal hemodynamic effect of captopril in the rat

Contrasting renal effects of chronic administrations of enalapril and losartan on one-kidney, one clip hypertensive rats

OBJECTIVE

To compare the effects of chronic administrations of the angiotensin II antagonist losartan and of the angiotensin I converting enzyme inhibitor enalapril on renal function in sodium-depleted rats with one-kidney, one clip hypertension, and to examine the contribution of endogenous kinins to the effect of enalapril.

METHODS

We administered enalapril and losartan (10 and 30 mg/kg per day, respectively) for 6 days to hypertensive rats that had been subjected to dietary sodium-intake restriction for 6 days prior to treatment and continued to be subjected to this restriction during treatment. In an additional group, administration of enalapril was combined with infusion of the bradykinin B2-receptor antagonist Hoe 140 (300 microg/kg per day subcutaneously via an osmotic pump). Renal function of anesthetized rats was assessed by using a clearance technique.

RESULTS

Despite there being similar falls in arterial pressure, glomerular filtration rate (867 +/- 40 microl/min per g kidney weight in untreated rats) was decreased to a larger extent in enalapril-treated than it was in losartan-treated rats (284 +/- 29 versus 438 +/- 36 microl/min per g kidney weight, P < 0.01). Although infusion of Hoe 140 had no influence on the effect of enalapril on arterial pressure, the level of glomerular filtration achieved in rats of this group (545 +/- 55 microl/min per g kidney weight) was similar to that found in losartan-treated rats. No effect of either treatment on renal plasma flow was detected; as a consequence, the excessive decrease in filtration fraction observed for rats in the enalapril-treated group was corrected by concomitant administration of Hoe 140. Interestingly, administration of enalapril resulted in a greater loss of sodium than did administration of losartan (723 +/- 147 versus 308 +/- 57 micromol during 6 days), and this effect was abolished by infusion of Hoe 140 (353 +/- 42 micromol during 6 days).

CONCLUSION

Administration of enalapril to sodium-depleted rats with one-kidney, one clip hypertension reduces their glomerular filtration rate to a greater extent than does administration of losartan despite these agents having similar effects on systemic blood pressure. Combined administration of enalapril and Hoe 140 has a less marked effect on glomerular filtration rate than does that of enalapril alone. This suggests that kinins play a role in the regulation of efferent arteriolar tone in this rat model.

Prevention of the cardiovascular and renal effects of angiotensin II by endothelin blockade

Angiotensin II (Ang II) stimulates the release and gene expression of endothelin-1 in isolated vascular smooth muscle cells. In 47 Sprague-Dawley rats, we assessed the influence of concomitant treatment by the mixed ET(A)/ET(B) endothelin receptor antagonist bosentan (30 mg/kg per day, gavage) on the effect of a 10-day infusion of Ang II (200 ng/kg per minute, SC, osmotic pump) on arterial pressure, renal hemodynamics (microsphere method), albuminuria, cardiac weight, and carotid structure. Ang II increased systolic arterial pressure (SAP) by 49+/-7 mm Hg. Although bosentan alone did not affect SAP, the development of Ang II-induced hypertension was entirely prevented by the endothelin antagonist. In addition, the reduction in renal blood flow induced by Ang II (4.9+/-0.3 versus 7.4+/-0.2 mL x min-1 x g-1 in control rats) was prevented by concomitant administration of bosentan (8.8+/-0.8 mL x min-1 x g-1). The marked increase in albuminuria observed in rats infused with Ang II (2524+/-961 versus 91+/-6 microg/24 h in control rats) was prevented by bosentan. Similarly, bosentan abolished the increase in heart weight index (from 2.96+/-0.03 to 3.41+/-0.08 mg/g body weight) and carotid media thickness (from 73+/-14 to 108+/-6 microm) induced by Ang II infusion. Of interest, the dipsogenic action of Ang II was not influenced by bosentan. In conclusion, endogenous endothelin contributes to the cardiovascular and renal effects of Ang II.

Chronic kinin blockade and effect of ramipril in renal adaptation to sodium restriction

The contribution of endogenous kinins to impairment in renal adaptation to a 6-day period of dietary sodium withdrawal associated with treatment with ramipril (5 mg/kg per day) and losartan (30 mg/kg per day) was evaluated by use of concomitant chronic administration of the bradykinin B2-receptor antagonist Hoe 140 (150 or 300 micrograms/kg per day via subcutaneous osmotic pump). A similar level of higher cumulative sodium excretion was observed in ramipril- and losartan-treated rats compared with untreated animals, and the effect of ramipril was not affected by Hoe 140. Similarly, the fall in arterial pressure and the renal vasodilatation associated with ramipril and losartan were not modified by Hoe 140. Glomerular filtration rate (785 +/- 73 microL/min per g KW in untreated sodium-depleted rats) decreased to a larger extent in ramipril-treated rats compared with losartan-treated rats (371 +/- 78 and 550 +/- 55 microL/min per g KW, respectively). Hoe 140 markedly prevented the alteration in glomerular filtration rate associated with ramipril, thus resulting in a final glomerular filtration rate (543 +/- 41 microL/min per g KW) similar to that observed with losartan. These findings demonstrate that despite a lack of influence on arterial pressure and sodium balance, accumulation of kinins markedly contributes to deterioration of the glomerular filtration rate induced by ramipril in sodium-depleted rats.

Renal circulation and blockade of the renin-angiotensin system. Is angiotensin-converting enzyme inhibition the last word?

The mechanism by which angiotensin-converting enzyme (ACE) inhibition influences renal perfusion and function has assumed growing importance as alternatives for blocking the system have emerged. Neither renin inhibitors nor angiotensin II (Ang II) antagonists are likely to trigger responses similar to ACE inhibitor-induced involvement of kinins, prostaglandins, or nitric oxide. Several observations suggest species variation in the contribution of these pathways to the renal response to ACE inhibition. In humans, recent investigation suggests that virtually all of the renal response is due to a fall in Ang II formation. Perhaps most persuasive is the surprising observation that the renal hemodynamic response to renin inhibitors exceeds by more than 50% the response to ACE inhibition in healthy humans. To the extent that kinins or prostaglandins contribute to the renal response to ACE inhibition, one would anticipate a smaller response to renin inhibition. Possible explanations include an unanticipated additional action of renin inhibitors, better tissue penetration of these highly lipophilic agents, or more effective blockade of Ang II formation through an action at the rate-limiting step or non-ACE-dependent Ang II generation. Substantial evidence favors the latter two possibilities. Whatever the explanation, these observations raise the intriguing possibility that the undoubted therapeutic efficacy of ACE inhibition in renal injury, documented most rigorously for type I diabetes mellitus, might be exceeded with the newer classes of agent.

Sodium and angiotensin in hypertension induced by long-term nitric oxide blockade

The influence of dietary sodium restriction and angiotensin II blockade on hypertension induced by a 25-day period of administration of the inhibitor of nitric oxide synthesis NG-nitro-L-arginine-methyl ester (10 mg/kg twice daily by gavage) was assessed in Wistar rats fed a normal or low sodium diet. In addition, the angiotensin II receptor blocker losartan (30 mg/kg once daily by gavage) was administered before and during NG-nitro-L-arginine-methyl ester in rats fed the normal sodium diet. At the end of the studies, conscious systolic arterial pressure increased similarly in NG-nitro-L-arginine-methyl ester-treated groups maintained on normal or low sodium intake. Moreover, a 25% reduction in cardiac output due to a decrease in stroke volume was observed in both groups. A slight but significant cardiac hypertrophic response was observed in hypertensive rats irrespective of sodium intake. At the completion of studies, plasma renin activity was similar to corresponding controls in the hypertensive groups on normal or low sodium intake. Losartan totally prevented the development of hypertension as well as the decrease in stroke volume and cardiac hypertrophy associated with NG-nitro-L-arginine-methyl ester treatment in rats on normal sodium intake. In conclusion, hypertension resulting from long-term blockade of nitric oxide synthesis was not affected by dietary sodium restriction. A crucial role for the renin-angiotensin system was demonstrated in this new model of hypertension.

Influence of captopril on red cell velocity in the vasa recta of the renal medulla

Previous studies from our laboratory have indicated an important role for angiotensin II (AII) in the regulation of renal medullary haemodynamics during normal physiological conditions. In order to investigate further the influence of endogenous AII on the juxtamedullary vascular resistance in anaesthetized rats the velocity of fluorescently-labeled red cells (vRBC) was measured with a cross-correlation technique in the vasa recta before and after infusion of the angiotensin I- converting enzyme inhibitor captopril or vehicle. Irrespective of treatment, vRBC was higher in the descending vasa recta (DVR) than in the ascending vasa recta (AVR). In time control animals VRBC in DVR and AVR and mean arterial blood pressure (MAP) remained stable over the 45 min study period. In animals receiving captopril (3 mg.h-1.kg-1 bw) vRBC increased almost proportionally in DVR and AVR; by 26% in DVR (from 1.02 +/- 0.12 to 1.28 +/- 0.10 mm.s-1, p < 0.05) and by 19% in AVR (from 0.46 +/- 0.05 to 0.55 +/- 0.07 mm.s-1, p < 0.05). MAP decreased by 9% (from 107 +/- 3 to 97 +/- 2 mm Hg, p < 0.05). These results give further support to the suggestion of an involvement of AII in the regulation of juxtamedullary vascular resistance during normal physiological conditions.

Effect of ACE-inhibition on tissue blood flow during acute left ventricular failure in the dog

Anesthetized dogs in acute left ventricular failure were treated with the ACE-inhibitor enalaprilat (MK-422). ACE-inhibition produced a fall in the mean blood pressure and a redistribution of cardiac output to the brain, right ventricle, upper gastrointestinal tract, and the inner and middle part of the renal cortex. The flow to the spleen, adrenals, skin, muscle, fat, lower gastrointestinal tract, and outer renal cortex did not change significantly. The differential sensitivity of the tissues to ACE-inhibition is most likely due to differences in sensitivity to circulating angiotensin II.

Modulation of proximal tubular reabsorption by angiotensin II

In shrinking-drop micropuncture studies in anaesthetized rats proximal tubular fluid reabsorption (JVa) decreased by 36% following intravenous infusion of enalapril. In a separate group of rats enalapril reduced fractional lithium clearance indicating decreased proximal fluid reabsorption. Following enalapril, GFR rose by 46% but absolute proximal reabsorption rose by 22% indicating 48% effectiveness of proximal glomerulo-tubular balance (GTB). Since renal blood flow and glomerular filtration rate (GFR) increased in parallel and arterial pressure fell, fluid uptake and proximal GTB were unlikely to have been decreased by peritubular physical forces. In anaesthetized rats proximal fluid reabsorption and proximal GTB are modulated by endogenous AII through direct stimulation of proximal tubular transport.

The effect of a converting enzyme inhibitor on autoregulation and intrarenal distribution of glomerular filtration in the rat

The role of the renin-angiotensin system in the autoregulation and distribution of the single nephron glomerular filtration rate (SNGFR) in anaesthetized, normotensive rats was investigated. SNGFR in outer cortical (OC) and inner cortical (IC) nephrons of the left kidney were measured with a modified Hanssen technique at three levels of renal arterial pressure (RAP): at a spontaneous arterial pressure; at a value within the autoregulatory limit, 100 mmHg; and at the lower limit of the autoregulatory range, 70 mmHg. This was done in control rats and in rats given a continuous i.v. infusion of the converting enzyme inhibitor (CEI) captopril (3 mg . h-1 X kg-1 BW). In control rats there was complete autoregulation of SNGFR in both OC and IC nephrons when RAP was reduced to 100 mmHg. Further reduction to 70 mmHg elicited different responses among the cortical layers, associated with a decrease in SNGFR. A fractional redistribution of glomerular filtration rate towards IC nephrons was evident. Administration of CEI at spontaneous RAP increased SNGFR in IC nephrons compared with values in control rats, but did not notably alter SNGFR in OC nephrons. Reduction of RAP to 100 mmHg during CEI infusion caused SNGFR to decrease below control values in both OC and IC nephrons, and the autoregulation as found in control rats was impaired. When RAP was lowered to 70 mmHg during CEI administration there was a progressive decrease in SNGFR in all cortical layers, although absolute changes were much greater in IC nephrons than in OC nephrons.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of renin and converting enzyme inhibition in sodium-deficient dogs

While renin is a highly specific protease, converting enzyme has at least two principal substrates, angiotensin I and bradykinin. Changes in the rate of formation of angiotensin II or degradation of bradykinin can influence the hypotensive action of angiotensin converting enzyme inhibitors. The present study was designed to determine if there were differences in the maximal blood pressure reduction in Na-deficient dogs after angiotensin converting enzyme or renin inhibitor treatment. Five conscious dogs received 0.1, 0.5, and 1.0 mg/kg of i.v. enalaprilat, a potent angiotensin converting enzyme inhibitor, which reduced blood pressure to 75 +/- 4, 71 +/- 5, and 71 +/- 5 mm Hg. Plasma immunoreactive angiotensin II levels were reduced in a dose-related fashion to 35% of control level at the highest dose. Infusion of a maximally effective dose of a statine-containing renin inhibitor (SCRIP) with the high dose of enalaprilat produced no further fall in blood pressure (68 +/- 7 mm Hg), but immunoreactive angiotensin II levels fell to essentially zero in four of five dogs. The order of drug administration was reversed in another experiment in a group of nine dogs in which SCRIP reduced plasma immunoreactive angiotensin II to 25% of control at 0.04 mg/kg/minute (n = 5), with reduction to near zero levels at higher doses. Maximal blood pressure reduction was achieved at 0.32 to 0.64 mg/kg/minute (76 +/- 4 mm Hg); 1 mg/kg of enalaprilat lowered blood pressure an additional 11 +/- 2 mm Hg (p less than 0.01) while not further decreasing immunoreactive angiotensin II levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal by angiotensins II and III of the effects of converting enzyme inhibition on renal electrolyte excretion in rats

Experiments were carried out in anaesthetized rats to compare the abilities of angiotensin II (A II) and angiotensin III (A III) to reverse the effects of angiotensin converting enzyme inhibition (Teprotide) on salt and water excretion. In rats infused with Teprotide, significant increases in urine flow and sodium excretion were observed and arterial blood pressure decreased. Addition of A II (10 pmol min-1) to the Teprotide infusion reduced renal excretion of sodium and water to control values and excretion of potassium to below control. Blood pressure increased to a value significantly higher than that during the control period. In a separate group of rats the natriuretic and diuretic effects of Teprotide were reversed by a similar dose of A III (10 pmol min-1). A primarily angiotensin-mediated action is indicated for the renal effects of Teprotide. Although angiotensins II and III appeared to be equipotent in their abilities to reverse the renal responses to Teprotide, A II caused an increase in arterial blood pressure that was not seen with A III. In a third group of rats A II (10 pmol min-1) was added to the Teprotide infusion and an aortic snare located between the renal arteries was tightened to prevent any increase in left renal perfusion pressure. During this period the rate of sodium excretion from the left kidney was significantly lower than from the right kidney. It is concluded that in the absence of any accompanying 'pressure natriuresis' A II is more potent than A III in its ability to reverse the natriuretic action of Teprotide. The elevation of blood pressure to above control values by a dose of A II only just sufficient to reverse the Teprotide-induced natriuresis suggests that the concentration of angiotensin in the kidney is higher than in plasma and supports the concept of an intrarenal renin-angiotensin system. In the rat both A II and A III may affect renal salt and water excretion by a combination of mechanisms involving glomerular, vascular and tubular receptors. The possibility is raised that differential effects of the active angiotensins on these mechanisms may participate in the regulation of sodium excretion.

Renal adaptation to sodium deprivation. Effect of captopril in the rat

Dietary sodium restriction is associated with a rapid decrease in urinary sodium excretion and achievement of a new sodium balance within three to five days. In addition, renal vasoconstriction and progressive activation of intrarenal systems with vasoconstrictor (renin-angiotensin) or vasodilating (kallikrein-kinin and prostaglandins) properties are observed. The relationship between sodium homeostasis and the renin-angiotensin system was assessed through the use of captopril in the rat. Treatment with captopril, before and during a six-day period after suppression of dietary sodium, was associated with sodium wasting (urinary sodium always exceeded sodium intake during the observation period); in addition, the normal increase in urinary aldosterone was blunted by about 80 percent. When captopril treatment was given for six days to rats maintained on long-term sodium restriction (at least four weeks) urinary sodium increased, although transiently; at the end of the study, renal vasodilatation together with a redistribution of glomerular blood flow to nonsuperficial glomeruli was observed. These studies indicate that captopril administration markedly blunts the renal and systemic adaptations to a reduced sodium intake in the rat. They suggest that the renin-angiotensin system is probably indispensable in preventing sodium loss when dietary sodium is suppressed.

Effect of converting enzyme inhibition by enalapril on sodium homeostasis in the rat

The effect of oral treatment with the converting enzyme inhibitor enalapril on sodium homeostasis was investigated in the rat. Treatment by enalapril prior to and during a 6 day period following abrupt suppression of dietary Na+ was associated with a sodium wasting state (urinary Na+ always exceeded intake during the observation period) and blunting by 90% of the aldosterone response to Na+ restriction In rats on chronic low Na+ intake, enalapril produced a slight, transient natriuresis together with a marked increase in drinking volume. In Na+ replete rats, enalapril had no influence on sodium balance. Converting enzyme inhibition markedly impaired the systemic and renal response to Na restriction and enalapril had no natriuretic effect in the Na+ replete state.

Attenuation of nephrotoxic acute renal failure in the dog with angiotensin-converting enzyme inhibitor (SQ-20,881)

Angiotensin-converting enzyme inhibitor was used in dogs with uranyl nitrate-induced acute renal failure to evaluate (1) a possible protective effect of angiotensin blockade and (2)the role of angiotensin II in the generation of renal failure in this model. Angiotensin-converting enzyme inhibitor treatment attenuated the fall in glomerular filtration rate and renal blood flow during the first 6 hours after injection of the nephrotoxic agent. A protective effect of similar magnitude was observed whether angiotensin-converting enzyme inhibitor treatment preceded, or shortly followed, the administration of uranyl nitrate. This indicates that angiotensin-converting enzyme inhibitor delivery to its intrarenal site of action remains effective after administration of the nephrotoxin. In addition, protection of glomerular filtration rate correlated with sodium and renal solute excretion. However, combined treatment with angiotensin-converting enzyme inhibitor and furosemide enhanced solute excretion but did not further improve the protection of renal function. Finally, the protective effects of angiotensin-converting enzyme inhibitor on renal function and hemodynamics were abolished by intravenous indomethacin. In conclusion, early, continuous blockade of angiotensin II protects partially against th initiation of acute renal failure. These findings support a major pathogenic role for angiotensin II in the generation phase of acute renal failure in this model. Furthermore, they suggest that an imbalance between vasoconstrictive (angiotensin II) and vasodilating factors (prostaglandins) may be operative in the early phase of uranyl nitrate-induced acute renal failure in the dog.

Effect of converting enzyme inhibition on the renal haemodynamic responses to noradrenaline infusion in the rat

1 The renal haemodynamic responses to a close arterial infusion of noradrenaline (29.7-177.9 nmol kg-1 h-1) were measured in rats anaesthetized with pentobarbitone. Systemic blood pressure was unaffected by noradrenaline infusion at this dose level. Renal blood flow was significantly reduced by 16% while glomerular filtration rate remained unchanged. These responses resulted in a rise in filtration fraction of some 10%. 2 In a separate group of animals, noradrenaline infusion in this manner and at similar dose rate increased plasma renin activity approximately 3 fold. 3 Continuous infusion of the angiotensin converting enzyme inhibitor, teprotide (3.36 mumol kg-1 h-1), had no measurable effect on systemic blood pressure, renal blood flow, glomerular filtration rate or filtration fraction. 4 Infusion of noradrenaline into these animals receiving teprotide caused a significant fall in renal blood flow of 16%. There was a fall in glomerular filtration rate of some 17% which was significantly different from the response observed in the animals not receiving teprotide. There was a consequent small but insignificant fall in filtration fraction. 5 These data show that the regulation of glomerular filtration rate in response to the vasoconstrictor drug, noradrenaline, is partly mediated via the renin-angiotensin system. They provide evidence for a role of intrarenal angiotensin II in regulating glomerular filtration by causing efferent arteriolar vasoconstriction.

Mechanism of the response to captopril in glucocorticoid hypertension

The effect of captopril was explored in salt-depleted methylprednisolone (MP)-hypertensive rats. MP treatment raised BP by 41+/-2 mmHg over 2 weeks. Controls (C) had no change in BP. Sodium balance and weight data indicated a greater salt depletion in MP than in C. On day 15, captopril reduced BP in both MP (20+/-4 mmHg) and C (31+/-4 mmHg). The effect was significantly smaller in MP than in C (p less than 0.05). Plasma renin activity (PRA) was similarly elevated in both groups, consistent with salt depletion. Serum (SCE) and lung-converting enzyme (LCE) activity were similar in MP and C. The diminished antihypertensive effect of captopril in MP is therefore not attributable to differences in PRA, SCE, or LCE. Our data suggest that depressor actions of captopril unrelated to the renin-angiotensin system are impaired in MP. Glucocorticoid-induced changes in vasodilator systems may explain these findings.