Inhibitory effect of captopril on renal responses to frusemide in sodium-restricted rats

Interactive effects of indomethacin, angiotensin II and frusemide on renal haemodynamics and natriuresis in man

The responses of renal haemodynamic and natriuretic indices to the oral prostaglandin synthetase inhibitor indomethacin (200 mg), to infused angiotensin II (1 ng min-1 kg-1) and to the combination of the two were studies in placebo-controlled fashion in eight normal male subjects both prior to and following administration of intravenous frusemide (20 mg). As compared with placebo, angiotensin II infusion alone caused significant reductions in absolute rate of sodium excretion, fractional sodium excretion, urine flow rate and effective renal plasma flow (all P < 0.001 vs placebo) but had no effect on glomerular filtration rate. The only change observed in these parameters with indomethacin alone was a small but significant reduction in urine flow rate (P < 0.005 vs placebo). As compared with the effects of angiotensin II alone, indomethacin pre-treatment followed by angiotensin II infusion led to much greater falls in absolute rate of sodium excretion, fractional sodium excretion, urine flow rate and effective renal plasma flow (all P < 0.0001 vs placebo) associated with a significant reduction in glomerular filtration rate (P < 0.0001) not observed with angiotensin II alone. Frusemide administration at the midpoint of each study limb resulted in each case in a prompt 15 to 20 fold increase in natriuresis. The renal haemodynamic and natriuretic effects of angiotensin II, indomethacin and their combination were not qualitatively different from those observed in the pre-frusemide phase. Our findings provide a clear demonstration in man of the important homeostatic role of renal prostaglandins in preserving renal function, particularly glomerular filtration, under conditions of elevated circulating angiotensin II.

Furosemide-induced natriuresis is augmented by ultra-low-dose captopril but not by standard doses of captopril in chronic heart failure

BACKGROUND

Ten chronic heart failure patients were studied on three occasions in randomized double-blind fashion to compare the acute hemodynamic, neurohormonal, and renal sodium-handling responses to 1 mg captopril versus 25 mg captopril, both in the absence of loop diuretic therapy and during furosemide-stimulated natriuresis.

METHODS AND RESULTS

Compared with placebo, 1 mg captopril caused nonsignificant decreases in mean arterial pressure and circulating angiotensin II level and had no effect on glomerular filtration rate as determined by 51Cr-EDTA elimination. Captopril (25 mg) produced marked suppression of serum angiotensin II with or without oral furosemide (both p less than 0.002), a marked decrease in mean arterial pressure (p less than 0.001) that was accentuated by furosemide (p less than 0.00001), and a decrease in glomerular filtration rate (p = 0.0007). No difference from placebo in renal sodium excretion was noted with either 1 or 25 mg captopril in the absence of furosemide. In contrast, while 25 mg captopril caused slight attenuation of the natriuretic response to furosemide, 1 mg captopril significantly enhanced furosemide-induced natriuresis (p less than 0.05). No correlation was found in our patients between the natriuretic effect of furosemide and either absolute mean arterial pressure or change in mean arterial pressure during the furosemide phase of each study session. This suggests that blood pressure is not the important factor mediating the divergent renal responses to furosemide of the two captopril dosage regimens.

CONCLUSIONS

We propose that in the face of furosemide-induced postglomerular vasodilatation in chronic heart failure, captopril at a starting dose of 1 mg (but not 25 mg) preserves enough circulating angiotensin II to maintain efferent arteriolar tone and thus glomerular filtration, while offsetting the antinatriuretic renal tubular effects of angiotensin II.

Captopril augments both basal and frusemide-induced natriuresis in normal man by suppression of circulating angiotensin II

1. We studied the renal effects of reinfusing low dose angiotensin II (1 ng kg-1 min-1) into seven salt-replete healthy volunteers after pretreatment with the angiotensin converting enzyme (ACE) inhibitor captopril (25 mg) to establish whether the natriuretic and renal haemodynamic responses to ACE inhibition in normal man result from suppression of circulating angiotensin II. In the same subjects we also studied the effect of captopril (25 mg) with and without exogenous angiotensin II (1 ng kg-1 min-1) on the natriuretic response to intravenous frusemide (20 mg). 2. In the pre-frusemide study captopril increased absolute and fractional excretion of sodium and paraaminohippurate clearance but had no effect on inulin clearance. 3. Reinfusion of angiotensin II after captopril pretreatment completely suppressed the renal effects of ACE inhibition, yielding renal vasoconstrictor and antinatriuretic effects equivalent to those produced by infused angiotensin II in the absence of captopril. 4. Frusemide increased renal sodium excretion without affecting paraaminohippurate or inulin clearance. Captopril augmented frusemide-induced natriuresis and again this effect was reversed by angiotensin II reinfusion. 5. We conclude that captopril augments both basal and frusemide-induced renal sodium excretion in normal man. Our findings suggest that these renal responses to ACE inhibition may be mediated by inhibition of circulating angiotensin II, specifically its renal tubular salt-retaining actions, rather than via effects on other neurohumoral systems.

A quantitative method of evaluating the diuretic response to furosemide in rats

Furosemide effects are usually evaluated by measuring the urinary excretion rate of Na+ (UVNa) in humans. In the present study, however, UVNa showed a nonlinear relationship with urine flow rate after intravenous injection of furosemide in rats. In contrast, when the urinary excretion rate of (Na+ + K+) (UVNa + K) was plotted against the urine flow rate, a linear regression line was observed, with small interindividual variations in normal rats and in rats with uranyl nitrate-induced acute renal failure (ARF). Piretanide, a loop diuretic, also showed a similar relationship, while other types of diuretics revealed different slope values for the relationship. Although the urinary excretion rate of Cl- (UVCl) vs UVNa + K is expected to show a linear relationship in normal rats, the correlation coefficient of the linear regression line was smaller than that of the urine flow rate vs UVNa + K. Further, the slope of UVCl vs UVNa + K was slightly different in ARF rats. Therefore, UVNa + K provides a better quantitative measure of diuretic response to loop diuretics than UVNa or UVCl.

Cytochrome P-450 and FAD-monooxygenase mediated S- and N-oxygenations

S- and N-Oxygenations are mostly mediated by either the cytochrome P-450 group of enzymes (haemoproteins), or the FAD-containing monooxygenases (flavoproteins). The nucleophilicity of the heteroatom may be an important determinant of which of these microsomal enzymes is utilised for specific oxygenations. This article describes the approaches that could be used for assessing the relative contribution of these microsomal oxidases to a particular reaction, with reference to studies with N,N-dimethylaniline, diethylsulphide, tetrahydrothiophen and dibenzothiophen.

The effects of frusemide, saralasin and hypotension on fetal plasma renin activity and on fetal renal function

1. In eleven chronically catheterized fetal sheep aged 124-142 days, hypotension caused by infusion of sodium nitroprusside (1.6-3.3 mg/h) and competitive antagonism of angiotensin II by saralasin (3.3 mg/h) both caused a fall in fetal urine flow (P less than 0.02 and P less than 0.05, respectively), and in sodium excretion (P less than 0.05 and P less than 0.01) because they both caused a fall in glomerular filtration rate (G.F.R., P less than 0.02 and P less than 0.01). Neither hypotension nor saralasin had any significant effect on fractional sodium reabsorption. Saralasin only caused a significant fall in systolic pressure (P = 0.05) while infusion of sodium nitroprusside caused a fall in both systolic and diastolic pressure (P less than 0.005 and P less than 0.02). 2. Frusemide (6 mg I.V) caused a marked natriuresis and diuresis (F = 24.9, P less than 0.005 and F = 30.5, P less than 0.005). This effect was maximal within 30 min. There was no change in fetal G.F.R. and there was a significant decrease in the fraction of the filtered sodium load that was reabsorbed (F = 10.44, P less than 0.0025). Fetal mean plasma renin activity (p.r.a.) rose progressively throughout (F = 9.3, P less than 0.005). When frusemide was given to fetal sheep which were hypotensive because they were infused with sodium nitroprusside, it still caused a diuresis (F = 5.73, P less than 0.025) and the fraction of the filtered sodium load that was reabsorbed decreased (F = 4.06, P less than 0.05) to a similar extent to that seen in animals given frusemide alone. On the other hand, frusemide was ineffective as a diuretic i.e. it had no effect on fractional sodium reabsorption, when given to fetal sheep which were infused with saralasin. 3. Injection of frusemide was associated with a significant rise in the diastolic pressures of hypotensive fetuses (P less than 0.05). Furthermore, when the infusion of saralasin was terminated 1.5 h after frusemide injection, blood pressure rose significantly (F = 11.19, P less than 0.0005 for systolic pressure and F = 7.15, P less than 0.005 for diastolic pressure) and p.r.a. fell (F = 4.78, P less than 0.025). 4. It is concluded that the fetal renin-angiotensin system can play a significant role in regulation of fetal blood pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute blood pressure and urinary responses to single dose combinations of captopril and diuretics in conscious spontaneously hypertensive rats

The present studies involved oral administration of captopril alone or in combination with individual diuretics in conscious, freely-moving spontaneously hypertensive rats (SHR). Both blood pressure (BP) and urinary fluid and electrolyte excretion were concomitantly measured. Captopril, 10 mg kg-1, caused a decrease in BP (-18 +/- 3 mm Hg) which was not significantly different from control (-10 +/- 2 mm Hg). Diuretics alone also failed to alter BP. However the BP-lowering effect of captopril was readily potentiated by concomitant administration of a single oral dose of frusemide (10 and 30 mg kg-1) in proportion to the magnitude of urinary loss. Hydrochlorothiazide at 1 and 3 mg kg-1 oral and metolazone at 0.3 and 1 mg kg-1 oral induced fluid loss similar to frusemide at 10 mg kg-1 and all produced comparable BP reduction in combination with captopril (25-32 mm Hg). Triamterene did not increase fluid excretion and was ineffective. In SHR with bilateral ureteral ligation, the synergistic effect of frusemide was abolished while that of hydrochlorothiazide still occurred with a delayed onset. It is concluded that concomitant measurement of BP and urinary excretion in conscious, freely-moving SHR is a useful technique for studying new antihypertensive drugs. Using this preparation it was found that enhancement of the acute hypotensive response to captopril with concomitant diuretic therapy occurs within 10 min and is primarily related to the fluid and Na loss, regardless of the diuretic agent used.