Ca2+ binding and charge movements in membranes of platelets and sarcoplasmic reticulum

Abstract

The properties of the Ca²⁺-pump system of platelet microsomes isolated without Ca²⁺-precipitating anions are studied. Passive Ca²⁺ binding to the microsomes takes place in a noncooperative manner with K _d = 0.7 μM. Half-maximal stimulation of ATP-dependent transport occurs at 0.4 μM Ca²⁺. The velocity of Ca²⁺ uptake, Ca²⁺ capacity and the level of phosphoprotein in platelet microsomes are significantly lower than in cardiac microsomes. Energization of platelet and muscle microsomes and activation of intact platelets result in opposite charge redistribution in hydrophobic regions of the membranes. It is concluded that these charge movements are caused by Ca²⁺ binding to and dissociation from nonpolar binding sites in the membranes.