Protein kinase C-dependent and -independent mechanisms of dense granule exocytosis by human platelets.

We examined the mechanisms of ATP release by human platelets using Ro-31-7549, a specific inhibitor of protein kinase C. Ro-31-7549 almost completely inhibited TPA-induced platelet aggregation and ATP release at 5-10 microM in washed platelets and in platelet-rich plasma. However, it suppressed thrombin- and U46619-induced ATP release by only 48% and 21%, respectively, and had little effect on aggregation in washed platelet suspensions containing serum or in platelet-rich plasma. The addition of GRGDS to prevent aggregation inhibited this residual thrombin-induced release by 53% and the residual U46619 release by 100% in the presence of Ro-31-7549. In washed platelet suspensions free of serum or plasma, Ro-31-7549 almost completely inhibited the ATP release and partially suppressed the aggregation induced by these agonists. These results suggested that there are protein kinase C-dependent and -independent mechanisms for ATP release by human platelets and that activation of the latter mechanism may depend on aggregation and plasma factors.