Renal prostaglandin E in the hypotensive mechanism of MK-421 in conscious rats

Angiotensin converting enzyme inhibitors improve contractile function of stunned myocardium by different mechanisms of action

Angiotensin converting enzyme (ACE) inhibitors enhance contractile function of myocardium "stunned" by a brief episode of coronary artery occlusion, yet their mechanism(s) of action remain unresolved. In addition to possible hemodynamic effects, ACE inhibitors may stimulate the synthesis of cardioprotective prostaglandins. Furthermore, the beneficial effects of ACE inhibitors that contain a sulfhydryl group may be due in part to the ability of thiol compounds to act as nonspecific antioxidants or direct scavengers of cytotoxic oxygen-derived free radicals. To investigate this question we compared the effects of (1) the sulfhydryl-containing ACE inhibitor zofenopril, (2) the sulfhydryl-containing stereoisomer of captopril (SQ 14,534) with essentially no ACE inhibitor properties, (3) the nonsulfhydryl-containing ACE inhibitor enalaprilat, and (4) solvent alone, given at the time of reperfusion, on recovery of contractile function after 15 minutes of coronary occlusion in the anesthetized open-chest dog. Segment shortening in control animals remained depressed or "stunned" after reperfusion, recovering to only -5 +/- 12% of baseline preocclusion values at 3 hours after reperfusion. In contrast, all three treatment agents attenuated postischemic dysfunction: segment shortening was restored to 33 +/- 12%, 54 +/- 6%, and 83 +/- 5% of baseline values at 3 hours after reflow in dogs treated with SQ 14,534 (p less than 0.05), zofenopril (p less than 0.01), and enalaprilat (p less than 0.01), respectively (all vs control value). These improvements in segment shortening did not appear to be the result of altered oxygen supply or demand after reperfusion, inasmuch as no significant differences in systemic hemodynamic parameters or myocardial blood flow were observed among the groups. In the second phase of the study, we found that the improved contractile function associated with enalaprilat treatment could largely be reversed by infusion of the potent cyclooxygenase inhibitor indomethacin: segment shortening was reduced from 69 +/- 12% at 2 hours after treatment/reperfusion to 38 +/- 12% at 2 hours after indomethacin infusion (p less than 0.01 vs 2 hours after reperfusion). Infusion of indomethacin had no effect, however, on the improved contractile function associated with zofenopril treatment. We therefore conclude that sulfhydryl- versus nonsulfhydryl-containing agents enhance contractile function of stunned myocardium by different mechanisms of action: enalaprilat attenuates postischemic dysfunction at least in part by a prostaglandin-mediated mechanism, whereas the salutary effects of zofenopril and SQ 14,534 may be due in part to the antioxidant properties of the sulfhydryl moiety.

The interaction between indomethacin and captopril or enalapril in healthy volunteers

In this study eight healthy volunteers were involved in a randomized, cross-over trial in which they were treated with either 25 mg of captopril b.i.d. or 20 mg of enalapril o.i.d. alone or in combination with 50 mg of indomethacin b.i.d. in order to detect a difference between both converting enzyme inhibitors when interacting with indomethacin. Before and after each 4-d treatment period, blood pressure (determined by random zero sphygmomanometer), body weight, plasma renin activity, angiotensin converting enzyme, plasma potassium, serum creatinine and the 24-h urinary excretion of 6-keto-prostaglandin F1 alpha were measured. Indomethacin attenuated the decrease of supine diastolic blood pressure during treatment with captopril, but not with enalapril. However, the initial decrease of blood pressure on captopril was greater than on enalapril. Both converting enzyme inhibitors had no effect on the urinary excretion of 6-keto-prostaglandin F1 alpha, while indomethacin reduced it. The results suggest a difference between captopril and enalapril in interaction with indomethacin.

Role of the endogenous angiotensin II in the antihypertensive effect of MK 421 in rats made hypertensive by norepinephrine or vasopressin

To assess the mechanism by which inhibitors of angiotensin converting enzyme (ACE) lower blood pressure, we evaluated the role of endogenous angiotensin II in the antihypertensive effect of MK 421, a long-lasting ACE inhibitor, in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The hypertensive effect of norepinephrine (1.8 mg/kg/day, ip) or vasopressin (7.2 U/kg/day, ip) was inhibited by the simultaneous administration of MK 421 (6 mg/kg/day, ip). Additional administration of angiotensin II at a subpressor dose (36 micrograms/kg/day, ip) did not revert the antihypertensive effect of MK 421 in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The present results suggest that the hypotensive effect of ACE inhibitors may depend on a reduced sensitivity of the vasculature to vasoconstrictor substances. In addition, it is also suggested that the suppressed angiotensin II may not be essential for the antihypertensive effect of ACE inhibitors in rats made hypertensive by chronic infusion of norepinephrine or vasopressin.

Plasma angiotensins and blood pressure during converting enzyme inhibition

The relationship between plasma angiotensin and the reduction of blood pressure with the angiotensin converting enzyme inhibitor enalapril was studied in rats. Blood pressure was measured in conscious rats with indwelling arterial catheters. To measure angiotensin II, plasma was analyzed by physical separation of angiotensins using high performance liquid chromatography followed by radioimmunoassay. The effects of both single (acute) and long-term (chronic) dosages of enalapril were measured. After a single oral dose of enalapril (10 mg/kg), mean arterial pressure fell from 111 +/- 3 to 86 +/- 3 mm Hg (p less than 0.005). Despite the blood pressure reduction, plasma angiotensin II was unaffected (control, 9.9 +/- 1.8 vs 9.7 +/- 1.1 pg/ml). After a higher single oral dose of enalapril (30 mg/kg), there was a reduction in both mean arterial pressure (81 +/- 5 mm Hg, p less than 0.005) and plasma angiotensin II concentration (2.3 +/- 0.6 pg/ml, p less than 0.01). The chronic effects of converting enzyme inhibition were evaluated in rats given enalapril in their drinking water (30 mg/kg 24 hr) for 1 week or 2 months. Mean arterial pressure remained equally low after chronic administration (for 1 week or 2 months), but plasma angiotensin II increased above normal (after 1 week, 28.9 +/- 8.7, p less than 0.01 vs control; after 2 months, 43.1 +/- 16.2 pg/ml, p less than 0.05 vs control). Although plasma angiotensin converting enzyme activity was undetectable at any time after enalapril administration, there was a partial return of the angiotensin I pressor response with chronic administration. The data are most compatible with actions of converting enzyme inhibitors independent of the blockade of plasma angiotensin II formation.

No evidence on significant roles of the prostaglandin-thromboxane and kallikrein-kinin system in the antihypertensive effect of MK 421 in rats made hypertensive by norepinephrine or vasopressin

Inhibition of angiotensin converting enzyme by MK 421 (6 mg/kg/day ip) induced a significant increase in urinary kinin excretion in norepinephrine-infused rats (1.8 mg/kg/day ip), whereas it had no effect on urinary prostaglandin E2 excretion. In contrast, MK 421 did not induced any significant changes in urinary kinin and prostaglandin E2 excretion in vasopressin-infused rats (7.2 U/kg/day ip). The simultaneous administration of indomethacin (10 mg/kg/day sc), OKY 046 (12 mg/kg/day sc) or aprotinin (100,000 units/kg/day sc) did not affect the antihypertensive effect of MK 421 in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The present results suggest that the hypotensive effect of MK 421 may depend on a reduced sensitivity of the vasculature to vasoconstrictor substances. In addition, it is also suggested that neither the prostaglandin-thromboxane or kallikrein-kinin systems are essential for the antihypertensive effect of MK 421 in these models of hypertension.

Enalapril. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension and congestive heart failure

Enalapril maleate is an orally active angiotensin-converting enzyme inhibitor. It lowers peripheral vascular resistance without causing an increase in heart rate. Enalapril 10 to 40 mg/day administered either once or twice daily is effective in lowering blood pressure in all grades of essential and renovascular hypertension, and shows similar efficacy to usual therapeutic dosages of hydrochlorothiazide, beta-blockers (propranolol, atenolol and metoprolol) and captopril. Most patients achieve adequate blood pressure control on enalapril alone or with hydrochlorothiazide. In patients with severe congestive heart failure resistant to conventional therapy, enalapril improves cardiac performance by a reduction in both preload and afterload, and improves clinical status long term. Enalapril appears to be well tolerated, with few serious adverse effects being reported. It does not induce the bradycardia associated with beta-blockers or the adverse effects of diuretics on some laboratory values. In fact, the hypokalaemic effect of hydrochlorothiazide is attenuated by the addition of enalapril. The incidence of the main (but rare) side effects of hypotension in hypovolaemic patients and reduced renal function in certain patients with renovascular hypertension, which are also seen with captopril, might be reduced by careful dosage titration, discontinuation of diuretics, and monitoring of at-risk patients. Thus, enalapril is a particularly worthwhile addition to the antihypertensive armamentarium, as an alternative for treatment of all grades of essential and renovascular hypertension. It also shows promise in the treatment of congestive heart failure.