A Difference in Mode of Antagonism between Optical Isomers of a Potent Selective Alpha₁-Adrenoceptor Blocker (YM-12617) and Norepinephrine in Isolated Rabbit Iris Dilator and Aorta

Abstract

Optical isomers of YM-12617, a potent and selective alpha₁ -adrenoceptor blocker, were tested on the rabbit iris dilator and aorta. The order of potency was R(-)- isomer>racemate>S(+)-isomer. The R(-)-isomer and racemate behaved as an essentially irreversible antagonist to norepinephrine in the iris dilator where the efficacy of norepinephrine was small, although the S(+)-isomer was a competitive antagonist. These drugs behaved as a competitive antagonist of norepinephrine in the aorta where the efficacy of norepinephrine was large.