The membrane potential modulates thrombin-stimulated Ca2+ mobilization and platelet aggregation

G protein-coupled receptors can be directly modulated by changes in transmembrane voltage in a variety of cell types. Here we show that, while changes in the membrane voltage itself do not induce detectable modifications in the cytosolic Ca2+ concentration, platelet stimulation with thrombin or the PAR-1 and PAR-4 agonist peptides SFLLRN and AYPGKF, respectively, results in Ca2+ release from intracellular stores that is sensitive to the membrane depolarisation. Direct activation of G proteins or phospholipase C by AlF4− and m-3M3FBS, respectively, leads to Ca2+ release that is insensitive to changes in the membrane potential. Thapsigargin-, as well as OAG-induced Ca2+ entry are affected by the membrane voltage, probably as a result of the modification in the driving force for Ca2+ influx; however, hyperpolarisation does not enhance thrombin- or OAG-evoked Ca2+ entry probably revealing the presence of a voltage-sensitive regulatory mechanism. Transmembrane voltage also modulates the activity of the plasma membrane Ca2+-ATPase (PMCA) most likely due to a decrease in the phosphotyrosine content of the pump. Thrombin-stimulated platelet aggregation is modulated by membrane depolarisation by a mechanism that is, at least partially, independent of Ca2+. These observations indicate that PAR-1 and PAR-4 receptors are modulated by the membrane voltage in human platelets.