The group at C2 of N-ethylnicotinamide determines the vasodilator potencies and mechanisms of action of nicorandil and its congeners in canine coronary arteries.

The relaxant mechanisms of action of nicorandil and its congeners (SG-86, SG-103, SG-209 and SG-212) on large coronary arteries were investigated in isolated canine circumflex arteries contracted with 25 mmol/l KCl or 10(-7) mol/l U46619, a thromboxane A2 analogue. SG-212, SG-86, SG-209 and SG-103 were obtained by replacement of the nitroxy group of nicorandil by bromine, the hydroxy, acetoxy and nicotinoyloxy groups, respectively. Nicorandil (10(-6)-10(-3) mol/l), SG-212 (3 x 10(-4)-10(-2) mol/l), SG-209 (10(-4)-10(-2) mol/l), SG-103 (3 x 10(-4)-10(-2) mol/l), and SG-86 (10(-3)-10(-2) mol/l) all produced a concentration-dependent relaxation in KCl- or U46619-contracted arteries. The order of relaxant potency was as follows: Nicorandil much greater than SG-209 greater than SG-212 = SG-103 greater than SG-86. The relaxant effect of nicorandil was not affected by glibenclamide but antagonized by methylene blue. In the presence of glibenclamide, the concentration-relaxation curves for SG-209 underwent rightward parallel shifts. The relaxant effect of SG-209, however, was not affected by methylene blue. The concentration-relaxation curves for SG-212 underwent rightward parallel shifts only to a limited extent in the presence of glibenclamide, but they were not affected by methylene blue. The relaxant effect of SG-103 was affected by neither glibenclamide or methylene blue. The relaxant effect of SG-86 was not affected by glibenclamide. The relaxant effect of nicorandil accompanied an increase in cyclic-GMP levels but that of SG-209 did not.(ABSTRACT TRUNCATED AT 250 WORDS)