The influence of activating hormones on human platelet membrane Ca2+-ATPase activity

Intact platelets were pretreated with hormones and thereafter membranes were prepared and Ca2+-ATPase activity determined. Thrombin decreased the Vmax of Ca2+-ATPase after pretreatment of intact platelets. Platelet activating factor, vasopressin and ADP also decreased Ca2+-ATPase activity. 12-O-tetradecanoylphorbol-13-acetate (TPA) or A23187 or ionomycin alone had no effect, whilst the simultaneous pretreatment with TPA and Ca2+-ionophore decreased Ca2+-ATPase activity. cAMP elevating agents prostaglandin E1 (PGE1) and forskolin had no influence per se on Ca2+-ATPase, but antagonized the inhibitory effect of thrombin. The data suggest a close connection between phosphoinositide metabolism and the Ca2+-ATPase system.