Interaction of nitric oxide and cGMP with signal transduction in activated platelets

Although nitric oxide (NO), one of the endothelium-derived relaxing factors, prevents formation of platelet aggregates, the mechanism by which this occurs is not fully understood. Accordingly, the effect of NO on signal transduction of gel-filtered human platelets was measured and compared with that of a cell-permeant guanosine 3',5'-cyclic monophosphate (cGMP) analogue, 8-bromo-cGMP (8-BrcGMP). NO inhibited the rise in intracellular Ca2+ concentration ([Ca2+]i), phosphorylation of the 47-kDa substrate (p47) of protein kinase C (PKC), serotonin secretion, and phosphatidic acid production induced by thrombin or the endoperoxide analogue U-46619. Similar effects were seen with 8-BrcGMP, and NO induced a concentration-related rise in cGMP. Neither NO nor 8-BrcGMP inhibited platelet aggregation, [Ca2+]i mobilization, or serotonin secretion induced by the Ca2+ ionophores A23187 or ionomycin or directly activated PKC purified from platelets. However, both NO and 8-BrcGMP enhanced p47 phosphorylation induced by the Ca2+ ionophores without augmenting phosphatidic acid production. Thus, if [Ca2+]i is elevated, a rise in cGMP enhances PKC activation. Both NO and 8-BrcGMP, however, prevent Ca2+ mobilization and platelet aggregation induced by receptor-mediated agonists by interfering with signal transduction at a point proximal to phospholipase C activation.