Original Article: PDF OnlyBelz Gustav G.; Essig, Jürgen; Wellstein, AntonJournal of Cardiovascular Pharmacology: February 1987 - p 219-224 Free Abstract According to classic pharmacologic theory, agonist/antagonist competition can be used to quantify an antagonist's potency by measurement of agonist dose- response curves in the presence of varying doses of the antagonist. We used this principle to characterize the interaction between angiotensin I (AI) and the angiotensinconverting enzyme (ACE) inhibitor cilazapril in humans. In addition, by comparing the effects of AI and angiotensin II before and after administration of a 30-mg dose of cilazapril, we could show the specific AI antagonism of the ACE inhibitor in humans. To obtain the antagonist's dose-response curves, six healthy male volunteers received five single oral doses of cilazapril, 0.5–8.0 mg. Enalapril, 10 mg, and captopril, 12.5 mg, served as positive controls and placebo as the negative control. Dose-response curves following intravenous infusions of AI were established 4 h after oral ingestion of the ACE inhibitors. Noninvasively measured systolic and diastolic blood pressures and total peripheral resistance assessed AI effects. Cilazapril dose dependently shifted the AI dose-response curve rightward, with 1.0 mg inducing a twofold shift. Enalapril and captopril appear less potent, on a milligram basis, in antagonizing AI effects 4 h after drug intake. The methodology could be a useful tool for a rational testing and comparison of ACE inhibitors in clinical pharmacology. © Lippincott-Raven Publishers.