Platelet Activation Profiles on TiO2: Effect of Ca2+ Binding to the Surface

Surface ion equilibrium is hypothesized to play an important role in defining the interactions between foreign materials and biological systems. In this study, we compare two surfaces with respect to their ability to activate adhering platelets. One is a commonly used implant material TiO2, which binds Ca2+, and the other one is glass, which does not. We show, that in the presence of Ca2+, TiO2 acts as an agonist, activating adhering platelets and causing the expression on their surface of two well-known activation markers, CD62P ( P-selectin) and CD63. On the contrary, in the absence of Ca2+, platelets adhering on TiO2 express only one of the two markers, CD63. Platelets adhering on glass, as well as platelets challenged with soluble agonists in solution, express both markers independently of whether Ca2+ is present or not. The expression of CD62P and CD63 is indicative of the exocytosis of the so-called alpha- and dense granules, respectively. It is a normal response of platelets to activation. Differences in the expression profiles of these two markers point to differential regulation of the exocytosis of the two kinds of granules, confirming the recent notion that platelets can tune their microenvironment in a trigger-specific fashion.