PLATELET ACTIVATION

This review article describes the different receptors, second-messenge rs and mechanisms involved in platelet activation. Several platelet ag onists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domai ns. These receptors are connected, via GTP regulatory proteins, with c ytoplasmic second-messenger-generating enzymes. Phospholipase C and ad enylate cyclase are the two best-known enzymes, generating inositol tr iphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosp hate and cyclic AMP from ATP respectively. The intraplatelet free calc ium level, which is critical for the activation status of the platelet , is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations, Shape-change occurs with small increases in intrap latelet calcium, while aggregation and secretion of granules take plac e at higher calcium levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.