Mechanism of relaxant effects of nicorandil on the dog coronary artery.

In order to search into the mechanism of the relaxant effects of nicorandil, a new coronary vasodilator, on the dog coronary artery, the effects of this substance on the potassium contracture were studied, and compared with those of nitroglycerin and nifedipine. All the three substances induced a dose-related relaxation (nifedipine greater than nitroglycerin greater than nicorandil). However, the increase in La3+-resistant 45Ca uptake was inhibited only by nifedipine. In a calcium-free EGTA containing medium caffeine induced a phasic contraction in the fully-depolarized preparation. Nicorandil produced an inhibition of this contraction, while nitroglycerin and nifedipine did not. It was concluded that nicorandil differs from the so-called calcium antagonists in the mechanism of relaxant action on the coronary artery. It also differs from nitroglycerin in that it produced an inhibition of caffeine-induced contraction.