| Popis: |
Nicorandil increases cyclic 3'5'-guanosine monophosphate (cGMP) in vascular smooth muscle. However, high concentrations are required to activate guanylate cyclase (GC). We examined the relationship between activation of GC, increases in cGMP and relaxation in canine mesenteric artery and vein, renal and coronary artery and thoracic aorta. Nicorandil (10-100 microM) relaxed in each of the blood vessels. Relaxation was associated with elevations of cGMP but independent of release of endothelium-derived relaxing factor, and inhibited by methylene blue and hemoglobin. The organic nitrate esters nitroglycerin, pentaerythritol tetranitrate, isosorbide dinitrate, 2-isosorbide mononitrate, and 5-isosorbide mononitrate each behaved in a similar manner. In each blood vessel pentaerythritol tetranitrate was the most potent and 5-isosorbide mononitrate the least potent relaxant and stimulant of cGMP. Each of the organic nitrate esters (1 microM to 1 mM) except nicorandil stimulated soluble GC activity in the presence of 10 mM cysteine. Nicorandil (EC50 38 mM) increased GC activity. Moreover, nicorandil (0.1 microM to 30 microM) did not inhibit cGMP phosphodiesterase. The EC50 for vascular relaxation was directly correlated with the EC50 for elevation of cGMP for each of the agonists in each blood vessel type. The EC50 for activation of GC was directly related to the reciprocal of the rate constant for nitric oxide formation for each of the organic nitrate esters. However, a direct correlation existed between the EC50 for activation of GC and the EC50 for 1) elevation of cGMP and 2) relaxation, for each of the organic nitrate esters except nicorandil. Thus, the high concentrations of nicorandil required to activate GC cannot account for the low concentrations required to elevate cGMP or relax smooth muscle. We postulate that nicorandil may interact with a membrane receptor or release a second messenger, distinct from nitric oxide or endothelium-derived relaxing factor, which then activates GC. This may represent a physiologic mechanism for regulation of GC activity in smooth muscle. |
