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

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 Kd = 0.7 microM. Half-maximal stimulation of ATP-dependent transport occurs at 0.4 microM 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.