Pharmacological characterization of nicorandil by 86Rb efflux and isometric vasorelaxation studies in vascular smooth muscle.

Nicorandil and cromakalim were found to stimulate 86Rb efflux (a marker of K+ ions) from resting preparations of rabbit aorta. This action was suppressed by 10(-5) M glibenclamide, an antagonist of K(+)-channel openers in vascular smooth muscle. Through intracellular production of cyclic GMP, and subsequent suppression of cellular Ca2+ activation, nitrovasodilators interfere indirectly with the activation of Ca(2+)-dependent ion channels. 8-Bromo-cyclic GMP and sodium nitroprusside antagonized the Ca(2+)-dependent 86Rb efflux induced by 3 x 10(-7) M norepinephrine. When nicorandil and cromakalim were investigated in the same experimental setup in the presence of glibenclamide to suppress stimulation of K+ channels, only nicorandil also suppressed the norepinephrine-induced increase of the 86Rb efflux. These results confirm that nicorandil acts as both an opener of K+ channels and a nitrovasodilator. Nicorandil relaxed helical strips from rabbit aorta contracted by 10(-7) M norepinephrine, but in contrast to the relaxant action of cromakalim, this response was not antagonized by the use of glibenclamide, indicating a greater importance of the nitrovasodilator mechanism than of the K(+)-channel-opening activity for relaxation in this tissue. However, when the nitrate-like action of nicorandil was suppressed by 10(-5) M methylene blue, the K(+)-channel-opening activity was unmasked on addition of 10(-4) M glibenclamide at high concentrations of nicorandil.