Store-operated calcium entry and non-capacitative calcium entry have distinct roles in thrombin-induced calcium signalling in human platelets

Phosphatidylserine (PS)-exposing platelets accelerate coagulation at sites of vascular injury. PS exposure requires sustained Ca²⁺ signalling. Two distinct Ca²⁺ entry pathways amplify and sustain platelet Ca²⁺ signalling, but their relative importance in human platelets is not known. Here we examined the relative roles of store-operated Ca²⁺ entry (SOCE) and non-capacitative Ca²⁺ entry (NCCE) in thrombin-induced Ca²⁺ signalling and PS exposure by using two Ca²⁺ channel blockers. BTP-2 showed marked selectivity for SOCE over NCCE. LOE-908 specifically blocked NCCE under our conditions. Using these agents we found that SOCE is important at low thrombin concentrations whereas NCCE became increasingly important as thrombin concentration was increased. PS exposure was reduced by LOE-908, and only activated at thrombin concentrations that also activate NCCE. In contrast, BTP-2 had no effect on PS exposure. We suggest that SOCE amplifies and sustains Ca²⁺ signalling in response to low concentrations of thrombin whereas both NCCE and SOCE are important contributors to Ca²⁺ signalling at higher thrombin concentrations. However, despite being involved in Ca²⁺ signalling at high thrombin concentrations, SOCE is not important for thrombin-induced PS exposure in human platelets. This suggests that the route of Ca²⁺ entry is an important regulator of thrombin-induced PS exposure in platelets.