Sodium-Calcium Exchange in Platelet Plasma Membrane Vesicles

Abstract

Unstimulated platelets maintain submicromolar concentration of cytosolic Ca²⁺ (~100 μM) and a steep plasma membrane Ca²⁺ gradient. Since passive inward diffusion of Ca²⁺ will cause platelet activation, mechanisms in addition to Ca²⁺ sequestration by endoplasmic reticulum must exist to remove Ca²⁺ from cytosol. In other tissues low cytosolic Ca²⁺ is achieved, in part, by plasma membrane ATP-dependent Ca²⁺ efflux^1,2 and Na⁺-Ca²⁺ exchange^3,4. Although platelets possess Ca²⁺-Mg²⁺-ATPase activity, the available evidence suggests that this enzyme is present only in the inner membranes^5,6. Whether Na⁺-Ca²⁺ exchange activity occurs in platelets is not known. We investigated the involvement of Na⁺-Ca²⁺ exchange in this process. This study establishes that platelet membranes enriched in plasma membrane markers exhibited Na⁺-Ca²⁺ exchange activity.