Long-Term Efficacy of Captopril (SQ 14,225) In 2-Kidney Renal Hypertensive Rats

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.

Captopril: pharmacology, metabolism and disposition

By inhibiting ACE, captopril blocks the conversion of AI or AII and augments the effects of bradykinin both in vitro and in vivo. In rats, dogs, and monkeys with 2-kidney renal hypertension, orally administered captopril rapidly and markedly reduces blood pressure; this antihypertensive effect apparently occurs via a renin-dependent mechanism; that is, the inhibition of ACE. In 1-kidney renal hypertension studies in rats and dogs, it was determined that oral doses of captopril markedly lowered blood pressure, but only after several days of dosing; the mechanism is thought to be non-renin dependent. In SHR, daily oral doses of captopril progressively lowered blood pressure; normal levels were attained by the sixth month. In all species studied, the reduction in blood pressure resulted from a reduction in total peripheral resistance; cardiac output remained unchanged or increased. In humans, captopril reduces blood pressure in patients with essential hypertension with low, normal, and high renin levels, and in patients with renovascular hypertension and hypertension associated with chronic renal failure. In hypertensive patients with high plasma renin activity, captopril apparently exerts most of its pharmacologic effects through inhibition of ACE. The means by which captopril reduces high blood pressure associated with low or normal PRA is not known, but it is clear that captopril does not act on an overactive plasma renin-angiotensin system in these cases. The antihypertensive effect of captopril is enhanced when it is given in combination with a diuretic or after salt depletion. Captopril was rapidly and well absorbed in all species tested, including man. Studies in rodents indicated that ingestion of food caused a reduction in the extent of absorption and bioavailability of captopril. Captopril and/or its metabolites were distributed extensively and rapidly throughout most tissues of normal rats; no radioactivity was detected in the brain. In vitro and in vivo, captopril formed disulfide bonds with albumin and other proteins. This binding was reversible in nature. In vitro studies in blood indicates that the disulfide dimer of captopril and mixed disulfides of captopril with L-cysteine and glutathione were formed. In intact blood cells, captopril remained in the reduced form (sulfhydryl), whereas in whole blood or plasma, captopril was converted to its disulfide dimer and other oxidative products. Biotransformation of captopril may involve both enzymatic and nonenzymatic processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Vascular wall renin in spontaneously hypertensive rats. Potential relevance to hypertension maintenance and antihypertensive effect of captopril

Relationships among systolic blood pressure (SBP), plasma renin activity (PRA), arterial renin concentrations (ARC), and venous renin concentrations (VRC) were examined in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats before and after treatment with captopril. The ARC was elevated in SHR relative to WKY whereas VRC was not. Similarly, ARC was related to SBP (r = 0.69, p less than 0.01) whereas PRA was not (r = 0.04). Captopril (100 mg/kg daily by mouth for 8 days) decreased blood pressure significantly in both SHR and WKY. PRA as well as ARC and VRC were all increased by captopril. Bilateral nephrectomy virtually eliminated PRA but ARC was not significantly reduced over a 24-hour period. Bilateral nephrectomy also markedly attenuated the acute antihypertensive effects of captopril in SHR; however, a modest effect was still apparent. It is suggested that ARC in SHR, being higher than in WKY, may play a role in the genesis or maintenance of hypertension in this model. Furthermore, the effects of captopril in both intact and nephrectomized SHR may be related to the ability of captopril to inhibit the vascular formation of angiotensin II. Finally, vascular renin is probably not renal in origin and responds to typical feedback inhibition as unmasked by captopril administration.

Captopril: clinical pharmacology and benefit-to-risk ratio in hypertension and congestive heart failure

Captopril, a competitive antagonist of angiotensin converting enzyme, has been marketed in the United States for the treatment of resistant hypertension. Despite extensive study, its exact mechanism of action remains unclear; decreased renin-angiotensin-aldosterone and sympathoadrenal system activity as well as increased bradykinin and prostaglandin E and F activity have been postulated. The drug decreases peripheral vascular resistance. Controlled trials in resistant hypertension of various etiologies and chronic congestive heart failure have demonstrated sustained effectiveness and therapeutic benefits. Side effects include skin rash, loss of taste, proteinuria, and leukopenia; higher doses and concomitant renal dysfunction appear to be predisposing factors. The benefit-to-risk ratio for captopril clearly justifies its use in resistant cases of hypertension and congestive heart failure, but further experience is needed to evaluate its use in milder forms of these diseases.

Studies on the mechanism of the enhancement of the antihypertensive activity of captopril by a diuretic in spontaneously hypertensive rats

Captopril (30 mg/kg/day orally for two days) in spontaneously hypertensive rats (SHR) inhibited serum angiotensin converting enzyme (ACE) activity 92.3%; increased plasma renin activity (PRA) 18-fold and reduced mean arterial blood pressure (MABP) 19 mm Hg. Hydrochlorothiazide (HCTZ) (100 mg/kg-day 1; 10 mg/kg-day 2, orally) increased PRA 3-fold but did not affect serum ACE or MABP. HCTZ plus captopril inhibited serum ACE 95.2%; increased PRA 38-fold and reduced MABP 47.5 mm Hg. Captopril or HCTZ plus captopril did not alter the responses of isolated aortic strips to norepinephrine (NE), serotonin, angiotensin II (AII) or isoproterenol. Pressor responses of conscious SHR to AII and NE were unaltered by captopril or HCTZ plus captopril although the bradykinin-induced depressor responses were significantly but equally potentiated. These results suggest that the potentiating effect of HCTZ is due to some mechanism that shifts the animal's blood pressure maintenance system to a renin-dependent state and is not due to changes in vascular reactivity.

Studies on the mechanism of the synergistic antihypertensive activity of captopril and hydrochlorothiazide following acute administration in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHR), captopril at 30 and hydrochlorothiazide (HCTZ) at 100 mg/kg/day p.o. for 2 days lowered mean arterial blood pressure (MABP) 16 and 10 mm Hg, respectively. Treatment with the combination of captopril plus HCTZ for one day lowered MABP to the same extent as captopril alone, but produced a synergistic 44 mm Hg MABP lowering after the second day combination treatment. Bilateral ureteral ligation did not prevent the synergistic antihypertensive effect demonstrating that removal of electrolytes was not the cause of this effect. Cardiovascular responses to angiotensin-I and -II, norepinephrine or bradykinin did not differ in rats given the combination or captopril alone. After the combination treatment for one day, captopril but not HCTZ alone on the second day lowered MABP in rats to the same degree as in those receiving the combination treatment for 2 days, suggesting that the diuretic action per se is unimportant. Captopril and HCTZ increased plasma renin activity (PRA) but only the combination of captopril and HCTZ produced a greater and longer lasting increase of PRA. It is concluded that the mechanism for the synergism is the renin dependency, created by the combination of HCTZ and captopril, resulting in a greater role of the renin system in blood pressure control and increased responsiveness to angiotensin converting enzyme inhibition by captopril.

Influence of angiotensin converting enzyme inhibition with captopril on blood pressure and adrenergic function in normal and sodium restricted rats

Observations with both captopril and teprotide suggest interplay of the renin angiotensin and sympathetic nervous systems during sodium depletion. We therefore examined adrenergic responses in normal or sodium restricted (8 days low sodium chow; trichlormethiazide 3 mg/kg, p.o., days 5-7) normotensive rats orally pretreated with placebo and captopril (3 or 10 mg/kg; twice on day 7, once on day 8) and pithed 2 hrs after the last dose. Consistent hypotension in conscious intact animals was observed only in sodium-depleted groups receiving captopril. Pressor responses to low frequency (2.5 Hz--5 sec) sympathetic stimulation and phenylephrine were reduced in normal sodium, pithed normotensive Sprague-Dawley rats receiving 10, but not 3 mg/kg captopril and low sodium animals receiving both doses. Suppression of phenylephrine by captopril was accentuated in sodium-depleted groups. Pressor responses to angiotensin II were less in all salt-depleted animals receiving either placebo or captopril. Captopril failed to reduce tachycardia to either sympathetic nerve stimulation or isoproterenol. These effects of captopril suggest that angiotensin plays a role in maintenance of vascular, but not cardiac adrenergic function. This role is manifested at a post-junctional site and becomes critical in the sodium deficient state.

Sympathoadrenal and renin-angiotensin systems in the development of two-kidney, one clip renal hypertension in rats

The relative roles of the sympathetic nervous system and renin-angiotensin system in the development of two-kidney renal hypertension were studied using four groups of rats: Group I = vehicle control; Group II = 6-OH-dopamine (2 weeks prior to renal clipping then weekly throughout the study); Group III = adrenal medullectomy plus vehicle; Group IV = 6-OH-dopamine plus adrenal medullectomy. Six weeks after clipping of a single renal artery, plasma renin activity (PRA) was comparably elevated in all groups. However, mean blood pressure (MBP) of Group II was lower than that of Group I controls (154.7 +/- 6.8 vs 197.3 +/- 6.6 mm Hg respectively). The MBP of Group III (207.0 +/- 5.2 mm Hg) was not different from that of Group I whereas in Group IV (134.2 +/- 18.0 mm Hg) it was markedly lower. All groups of rats were given a single dose of captopril (30 mg/kg p.o.) to inhibit the renin-angiotensin system. Despite differences in starting MBP, captopril caused similar reductions (38-50%) of MBP and increases in PRA in all groups. Similar results were obtained in two-kidney renal hypertensive rats with hypertension of 12 weeks' duration. It is concluded that the sympathetic nervous system does not contribute to the elevated PRA in two-kidney renal hypertensive rats but does contribute significantly to the development of hypertension in this model.

Prevention of the development of spontaneous hypertension in rats by captopril (SQ 14,225)

Indirect systolic blood pressure (SBP) was monitored in male, weanling, spontaneously hypertensive rats (SHR) maintained on water for 16 weeks (group A) or on captopril therapy (100 mg/kg/day in drinking water) for 4 (group B), 8 (group C), 12 (group D), 16 (group E) weeks. Weanling SHR of group A developed typical, time-related hypertension over a 16-week observation period. In marked contrast, SHR receiving captopril did not develop hypertension. Discontinuation of captopril after 4, 8, 12 or 16 weeks resulted in the usual development of hypertension. There was a transient increase in water consumption of drug treated rats which returned to normal by 8 weeks of dosing. Conversely, when captopril therapy was discontinued after 4, 8 or 12 weeks, there was a transient decrease in water intake, the magnitude of which was inversely related to the duration of drug therapy. These results demonstrate that daily captopril therapy was able to completely prevent the development of spontaneous hypertension. Explanations for the mechanism(s) underlying the influence of captopril on blood pressure and water intake remain speculative.