enalapril and ramiprilat

We reported earlier that the vasodilator response to acetylcholine (ACh) in lungs exposed to indomethacin and preconstricted with an analog of thromboxane (U46619) is converted to vasoconstriction by brief electrolysis of inflowing perfusion medium and suggested that this effect reflected endothelial injury. The purpose of our present study was 2-fold. First, because captopril, a sulfhydryl-containing inhibitor of angiotensin-converting enzyme inhibitor, prevented this effect (we assumed by scavenging electrolysis generated free radicals of oxygen), we determined whether two angiotensin-converting enzyme inhibitors lacking this moiety, namely lisinopril and ramiprilat, provided similar protection. Second, we studied whether electrolysis, like other forms of experimental lung injury, impaired uptake of serotonin (5-HT) by the endothelium. Our study confirmed that within 5 min of electrolytic injury, the ACh response is converted to vasoconstriction. This effect was completely prevented by lisinopril (18 microM) or ramiprilat (30 microM), neither of which affected ACh vasodilatation in control lungs. Lower concentrations of either drug exerted lesser degrees of protection. Five or 20 min after electrolysis, single-pass uptake of [14C]5-HT was significantly (P less than .01; N = 11) lower than control (82.4 +/- 3.4% vs. 71 +/- 3.2 and 46.5 +/- 6%, respectively). In contrast, 5-HT uptake was unaltered by electrolysis in the presence of 18 microM lisinopril. We conclude that loss of ACh vasodilation is an early reflection of lung endothelial injury that is accompanied by reduced [14C]5-HT uptake. Also, the protective property of nonsulfhydryl-containing angiotensin-converting enzyme inhibitors may be related to unexpected antioxidant actions.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Angiotensin-Converting Enzyme Inhibitors; Animals; Enalapril; Endothelium, Vascular; Free Radicals; Lisinopril; Lung; Male; Prostaglandin Endoperoxides, Synthetic; Pyrroles; Rabbits; Ramipril; Serotonin; Vasoconstrictor Agents

The chemiluminescence reaction elicited from luminol in the presence of hydroxyl radicals was concentration-dependently suppressed by captopril, indicating efficacious radical scavenging. As to be expected, ACE inhibitors lacking free sulfhydryl groups (ramipril, enalapril) were inactive. However, the endogenous scavenger and anti-oxidant uric acid proved to be far superior to captopril, when concentrations of both were compared that are realized in vivo. A substantial augmentation of endogenous scavenging ability during therapy with captopril thus seems unlikely. In a model of standardized myocardial hypoxia (isolated working heart of the guinea pig with 30 min low flow perfusion) captopril, ramiprilat and uric acid equally improved post-hypoxic heart function. There was no cardioprotective action of captopril in excess of that accountable for by inhibition of ACE. It seems possible that ACE (kininase II) inhibitors exert cardioprotection via elevated tissue levels of kinins: bradykinin also improved heart performance after low flow perfusion and bradykinin-induced coronary dilatation was markedly enhanced in the presence of ramiprilate, reflecting attenuated degradation by endothelial kininase II.

Topics: Angiotensin-Converting Enzyme Inhibitors; Animals; Bradykinin; Bridged Bicyclo Compounds; Captopril; Coronary Disease; Dose-Response Relationship, Drug; Enalapril; Guinea Pigs; Hemodynamics; Hydroxides; Hydroxyl Radical; Luminescent Measurements; Myocardial Reperfusion Injury; Pyrroles; Ramipril; Sulfhydryl Reagents; Uric Acid

The rate of conversion of ramipril (Hoe 498), a new angiotensin converting enzyme (ACE) inhibitor, to its active metabolite was compared with that of enalapril. After intravenous administration to rats, ramipril was very rapidly deesterified to its active moiety, ramiprilat. The ratio of the active metabolite level to the prodrug level in plasma at 5 min after administration was 10.7 for ramipril, which was about 5 times the ratio for enalapril. The in vitro conversion rates of ramipril were higher than those of enalapril in all rat tissue homogenates examined, including the liver, a main site of metabolism. The apparent Km values of ramipril and enalapril in the liver were 190 and 710 mumol/l, respectively, suggesting that ramipril has a higher affinity for esterase than enalapril. In conclusion, ramipril was superior to enalapril in efficiency of conversion to the active metabolite.

Topics: Angiotensin-Converting Enzyme Inhibitors; Animals; Aorta, Thoracic; Biotransformation; Bridged Bicyclo Compounds; Chromatography, Gas; Enalapril; Enalaprilat; In Vitro Techniques; Kidney; Liver; Lung; Male; Muscle, Smooth, Vascular; Prodrugs; Pyrroles; Ramipril; Rats; Rats, Inbred Strains

Evidence for effects of angiotensin converting enzyme (ACE) on isolated human glomeruli was provided using specific binding of tritium-labeled ramiprilat, a potent inhibitor of ACE. [3H]ramiprilat bound to isolated glomeruli, depending on time and temperature, displaying a KD of 3.8 nmol/L and a Bmax of 853 fmol/mg protein. Specific binding represented more than 90% of total binding. Dissociation occurred rapidly after dilution of the sample with incubation buffer or after addition of an excess of unlabeled inhibitor. Binding of [3H]ramiprilat was also inhibited by increasing concentrations of enalaprilat, another ACE inhibitor. ACE is a zinc-containing enzyme. Addition of EGTA to the assay, which chelates zinc ions, completely prevented binding. This was reversed by divalent Zn2+ and Ca2+ ions, but not by magnesium. Binding of [3H]ramiprilat to isolated glomeruli was maximal at pH 8, which also is optimal for ACE activity. The binding of [3H]ramiprilat to isolated human glomeruli is specific, and resembles the characteristics which have been found earlier for enzyme activity of ACE. Thus, binding of [3H]ramiprilat to isolated glomeruli can be assumed to be directed to ACE.

Topics: Adult; Angiotensin-Converting Enzyme Inhibitors; Enalapril; Enalaprilat; Humans; Hydrogen-Ion Concentration; In Vitro Techniques; Kidney Glomerulus; Protein Binding; Pyrroles; Ramipril