Differential regulation and relocalization of the platelet P2Y receptors after activation:: A way to avoid loss of hemostatic properties?

In the present study, we investigated the desensitization and trafficking of the P2Y(1) and P2Y(12) receptors after agonist-induced stimulation of platelets or astrocytoma cells transfected with the P2Y(1) or P2Y(12) receptors fused to green fluorescent protein. In platelets and in transfected cells, exposure to 10 muM ADP caused desensitization of the P2Y(1) receptor-driven calcium signal, whereas the P2Y(12) receptor-mediated inhibition of cAMP formation was not affected. Plasma membranes from ADP-stimulated platelets also retained P2Y(12) activity. Agonist-induced P2Y(1) receptor desensitization was accompanied by its internalization in platelets and transfected cells. In contrast, although a substantial fraction of P2Y(12) receptors was rapidly and transiently internalized, most of the P2Y(12) receptors remained at the plasma membrane. Activated P2Y(1) receptors were internalized through a clathrin-dependent pathway in cells and platelets, whereas the P2Y(12) receptors seemed to use a distinct, clathrin-independent pathway. Together, these data indicate that the P2Y(1) and P2Y(12) receptors are differentially regulated upon activation. The absence of desensitization of the Gi protein-coupled P2Y(12) receptor-dependent responses could represent a mechanism to preserve the hemostatic properties of otherwise unresponsive platelets.