Kinetic characterization and substrate requirement for the Ca2+ uptake system in platelet membrane

An ATP-dependent mechanism for Ca2+ uptake in human platelet membrane fractions has been identified and characterized. Ca2+ uptake into a membrane fraction is shown to be stimulated at low concentrations of ATP and Ca2+ and to require magnesium ions. Initial rate kinetics, using Eadie-Scatchard analysis, indicated a single class of calcium uptake sites in the presence of ATP, with a Kd for free [Ca2+] of 0.145 microM. Ca2+ uptake in the presence of several ATP concentrations demonstrates that ATP binds to at least two sites, representing high and low affinities of 3.21 and 80.1 microM, respectively. The neuroleptic drug fluphenazine inhibited ATP-stimulated calcium uptake (IC50 = 55 microM), suggesting this ATP-dependent Ca2+ uptake system may provide a useful ion-transport model with which to study neuroleptic therapy in humans.