The possible involvement of protein phosphatase 1 in thrombin‐induced Ca 2+ influx of human platelets

Protein phosphatase 1 is considered to be involved in thrombin‐induced platelet activation (Murata et al., Biochem Int 26:327–334, 1992). To clarify the mechanism, we examined the effects of protein phosphatase 1 and 2A inhibitors (calyculin A, tautomycin, okadaic acid) on Ca 2+ influx. In the presence of 1 mM Ca 2+ , thrombin‐ (0.1 U/ml) induced platelet aggregation and ATP release were inhibited by calyculin A, while this inhibitory effect was abolished in the absence of Ca 2+ (EGTA 1 mM). Furthermore, thrombin‐induced Mn 2+ influx but not intracellular Ca 2+ mobilization was inhibited by calyculin A in a dose‐related manner. Calyculin A also blocked the ongoing Ca 2+ influx when added 3 min after thrombin stimulation. Similar inhibitory effects were observed with okadaic acid and tautomycin in the same potency sequence as the reported one for protein phosphatase 1 (calyculin A > tautomycin > okadaic acid). These results suggest that the anti‐platelet effects of phosphatase inhibitors are due to the inhibition of Ca 2+ influx and that protein phosphatase 1 plays a key role in the regulation of receptor operated Ca 2+ channel of human platelets.