Differential effects of the antianginal drug nicorandil on canine arteries and veins

Nicorandil, a potent coronary vasorelaxant used in the treatment of angina, has differential effects on arteries and veins in vivo. To explain this phenomenon, experiments were designed to characterize the relaxant and inhibitory actions of this compound on canine isolated arteries and veins. Paired rings of canine coronary, femoral, and saphenous arteries and saphenous veins were suspended at optimal length for isometric tension recording in organ chambers containing physiologic salt solution at 37°C and gassed with 95% 0₂-5% CO₂. In certain experiments, one ring of each pair was denuded of the endothelium. Removal of the endothelium did not affect nicorandil-induced relaxations of contracted blood vessels. Nicorandil exerted a differential relaxant effect on arteries and veins contracted with KC1 (order of potency: Saphenous vein > coronary artery > femoral artery). No difference in sensitivity to nicorandil was observed in arteries and veins contracted with prostaglandin F_2α. Contractions of saphenous arteries and veins to nor-epinephrine (NE) were equally sensitive to the inhibitory action of nicorandil. However, contractions of saphenous veins induced by sympathetic nerve stimulation were more sensitive to nicorandil than were contractions of saphenous arteries. Furthermore, nicorandil did not affect contractions to phenylephrine in saphenous veins, although contractions to B-HT 920 were virtually abolished by the compound. Saphenous veins contracted with St 587 were more sensitive to the relaxant action of nicorandil than when contracted with phenylephrine. These results suggest that nicorandil inhibits preferentially con-tractions of canine arteries and veins mediated by α₂rather than α-adrenoceptors. This selectivity of action may be conferred by a differential receptor reserve and/or receptor coupling efficiency between the a-adrenoceptor subtypes for contractile agonists.