Effect of enalapril and quinapril on forearm vascular ACE in man

Different ACE inhibitors can be distinguished in vitro by their affinity for converting enzyme in vascular and other tissues. Quinapril appears to be amongst the more effective inhibitors of vascular tissue ACE in vitro. This study assesses the in vivo effect of single oral doses of quinapril and enalapril, in attenuating the vasoconstrictive action of angiotensin I (AI) (which we have previously shown depends on its conversion to angiotensin II (AII) by vascular ACE) in the forearm resistance vessels of man.

The design was of randomized, open, placebo controlled, two way crossover type, Forearm blood flow (FABF) was measured simultaneously in both forearms by mercury in silastic strain gauge plethysmography. AI infusion were via a fine bore cannula in the left brachial artery with the right arm serving as a control.

Mean plasma ACE on placebo was 34.3 U.l-1. Both quinapril and enalapril produced a similar degree of plasma ACE inhibition reducing concentrations to 2.8 U.l-1 and 2.6 U.l-1 respectively. Quinapril caused a significantly greater inhibition of AI induced vasoconstriction with a 30.0% reduction compared with 67.0% and 85.0% for enalapril and placebo respectively. Enalapril attenuated AI induced vasoconstriction to a greater degree than placebo but the difference was not significantly different.

These results indicate that when quinapril and enalapril are administered as single 20 mg doses, each of which produces the same degree of plasma ACE inhibition and blood pressure reduction- quinapril inhibits vascular ACE to a greater degree than both enalapril and placebo.