REVERSAL OF THE CA2+ PUMP OF BLOOD-PLATELETS

In this study, the endoplasmic Ca2+ transport ATPase of blood platelet s was compared with Ca2+ ATPase of sarcoplasmic reticulum skeletal mus cle. Similar to the muscle enzyme, the Ca2+ ATPase from platelets was found to catalyse ATP reversible arrow P-i exchange both in the presen ce and in the absence of a transmembrane Ca2+ gradient. When platelet vesicles are loaded with Ca2+ and diluted in medium containing ADP, P- i and EGTA, the ATPase catalyses Ca2+ efflux coupled to synthesis of A TP. The stoichiometry between Ca2+ ion released and ATP synthesized by platelet Ca2+ ATPase is 1, while that of skeletal muscle is 2. Thapsi gargin, a specific inhibitor of sarcoplasmic/endoplasmic reticulum Ca2 + ATPases, inhibited both the Ca2+-dependent ATPase activity and the r eversal of the platelet Ca2+ pump. The possibility is discussed that t he differences observed between the two transport systems is related t o the distinct amino acid sequences of the enzymes.