Kinetics of platelet P-selectin mobilization: Concurrent surface expression and release induced by thrombin or PMA, and inhibition by the NO donor SNAP

Activated platelets and endothelium surface express the cell adhesion molec ule P-selectin (CD62P), which plays an important role in mediating interact ions with leukocytes, Increased levels of a functional soluble form of P-se lectin (sP-selectin) have been reported in several pathological states but it is not clear whether this circulating sP-selectin originates from platel ets and/or endothelial cells. Here we describe the concurrent kinetics of i ntracellular storage, surface expression and release of platelet P-selectin induced by thrombin or the protein kinase C activator PMA. Platelet activa tion with submaximal concentrations of thrombin (0.1 U/ml) resulted in a ra pid decrease of intracellular P-selectin. This decrease of intracellular P- selectin concurred with a gradual increase of surface expression and an ini tial increase of sP-selectin. Our results indicate that intracellular store s of P-selectin were only partly mobilized upon activation with submaximal concentrations of thrombin. A high concentration of thrombin (1.0 U/ml) ind uced a rapid and nearly total decrease of intracellular stores and a more p ronounced, but transient, increase of surface expression. The release of P- selectin was fast and occurred during the initial activation phase. The NO donor SNAP inhibited both surface expression and release of platelet P-sele ctin in a similar manner. PMA (0.1-1.0 mu M) mediated a more slow, gradual and sustained surface expression and release of P-selectin than thrombin. T hus, surface expression and release of platelet P-selectin show different k inetics depending on the mode of activation.