Stimulation of coronary guanylate cyclase by nicorandil (SG-75) as a mechanism of its vasodilating action.

Since there is evidence suggesting that nicorandil (SG-75) relaxes coronary arterial smooth muscle by increasing cGMP levels, the effects of this vasodilator on soluble guanylate cyclase from bovine coronary arteries were studied more closely. It was found that nicorandil stimulated guanylate cyclase dose-dependently (3-30 mM) up to 100-fold the control value. Similar to nitroglycerin but in contrast to sodium nitroprusside, cysteine (0.5-20 mM) was required to obtain this stimulation. All other investigated thiols, except thiosalicylic acid which was partially able to mimic the cysteine effect, were ineffective. As evident from time course studies, nicorandil induced stimulation of guanylate cyclase was characterized by a lag-phase which could be avoided by preincubating the enzyme with nicorandil. The stimulatory effect of nicorandil was diminished in the presence of methylene blue, ferricyanide or hydroquinone. These results give further evidence that a) nicorandil exerts its vasodilating effect via stimulation of guanylate cyclase and b) nitrate esters, such as nitroglycerin or nicorandil, stimulate the enzyme, at least in vitro, only in the presence of cysteine or, to a lesser extent, thiosalicylic acid.