PHOSPHATASE INHIBITORS SUPPRESS CA2-MEDIATED INTRACELLULAR CA2+ STOREDEPLETION IN HUMAN PLATELETS( INFLUX INDUCED BY RECEPTOR)

The effects of three phosphatase inhibitors including okadaic acid, ca lyculin A and tautomycin were evaluated on platelet Ca2+ mobilization. Calyculin A and tautomycin at appropriate concentrations appeared to have a selective inhibitory effect on thrombin-induced Ca2+ influx, bu t not on [Ca2+](i) release from intracellular Ca2+ storage sites. In c ontrast, pretreatment with okadaic acid at concentrations that effecti vely lowered Ca2+ influx also suppressed Ca2+ release from intracellul ar Ca2+ stores. In a system that specifically evaluates the effects of agents on Ca2+ influx induced by the Ca2+-depleted state of intracell ular Ca2+ storage sites, the three phosphatase inhibitors attenuated C a2+ influx in a dose dependent manner and showed complete inhibition a t appropriate concentrations. These findings suggest that protein phos phorylation/dephosphorylation plays an important role in mediating sig nals to open Ca2+ channels when Ca2+ depletion in intracellular Ca2+ s tores is caused by thrombin. In contrast, Ca2+ influx induced by thaps igargin, a Ca2+-ATPase inhibitor, was only partially suppressed by pre treatment with each of the three phosphatase inhibitors. Based on thes e findings, we suggest that the Ca2+-depleted state of intracellular C a2+ stores by thapsigargin induces the opening of Ca2+ channels via ph osphatase inhibitor-insensitive pathways. All the phosphatase inhibito rs, at the highest concentrations tested in the present study, only pa rtially inhibited Mn2+ entry induced by thrombin. These findings sugge st that there are at least two types of divalent ion channels on plate let plasma membranes and that one of them, that preferentially allows Mn2+ entry, is resistant to the inhibitory effects of phosphatase inhi bitors.