YC-1 potentiates nitric oxide- and carbon monoxide-induced cyclic GMP effects in human platelets

Nitric oxide (NO), the physiological activator of soluble guanylyl cyclase (sGC), induces inhibitory effects on platelet activation via elevation of c GMP levels and stimulation of the cGMP-dependent protein kinase. YC-1, a be nzylindazole derivative, was shown to activate sGC in intact platelets, res ulting in inhibition of platelet aggregation. In a previous study, we demon strated that YC-1 not only stimulates purified sGC but also potentiates the stimulatory action of submaximally effective NO and carbon monoxide (CO) c oncentrations. Here, we investigated the potentiating effect of YC-1 in int act platelets. YC-1 together with NO or CO led to complete inhibition of pl atelet aggregation at concentrations that were ineffective by themselves. M aximally effective 2,2-diethyl-1-nitroso-oxyhydrazine (3 mu M) and YC-1 (10 0 mu M) concentrations each elevated the cGMP levels in intact platelets ap proximately 13-fold, and administration of the two drugs together resulted in enormous potentiation of cGMP formation, which greatly exceeded the effe ct on the purified enzyme and yielded a >1300-fold increase in cGMP levels. Similar results were obtained using CO instead of NO. Furthermore, YC-1 no t only stimulated sGC but also inhibited cGMP-hydrolyzing phosphodiesterase s in platelets. The enormous elevation of cGMP levels led to enhanced phosp horylation of the cGMP-dependent protein kinase substrate vasodilator-stimu lated phosphoprotein. Thus, by the combination of two effects (i.e., potent iation of NO-induced sGC stimulation and phosphodiesterase inhibition), YC- 1-like substances are potent activators of the sGC/cGMP pathways and are th erefore interesting candidates to act as modulators of cGMP-mediated effect s, especially within the cardiovascular system.