Divalent cation-dependent and independent surface expression of thrombospondin on thrombin-stimulated human platelets

Abstract

Thrombospondin (TSP), a platelet a-granule protein, becomes expressed on the surface of thrombin-stimulated platelets. The surface expression of this protein occurs through two distinct mechanisms. At low platelet concentrations (1 x 108/mL), a divalent ion-independent, low-capacity mechanism predominates. At higher cell concentrations, a divalent ion-dependent, higher capacity mechanism prevails that can account for >90% of all the TSP surface expression measured. This mechanism requires the presence of both calcium and magnesium (Ca + Mg). The dependence of the divalent ion-dependent surface expression on platelet concentration suggests that release of the molecule from the cell followed by its binding to the cell surface mediates this component of the endogenous TSP-platelet interaction. These data are consistent with a two-receptor model for the platelet surface expression of the endogenous TSP pool.