Phosphatidylinositol 3,4,5-trisphosphate triggers platelet aggregation by activating Ca²⁺ influx

Exogenous phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P₃] stimulates the aggregation of washed rabbit platelets in a Ca²⁺- and dose- dependent manner. This aggregation is reversible at low PtdIns(3,4,5)P₃ levels, but becomes irreversible when the concentration exceeds a threshold of about 20 μM. Other D-3 and D-4 phosphoinositides examined, including phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P₂], phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂], and phosphatidylinositol 3-monophosphate [PtdIns(3)P], fail to exert appreciable platelet activation at comparable concentrations. In addition, PtdIns(3,4,5)P₃ can reverse the inhibitory effect of wortmannin on thrombin-induced platelet aggregation. Taken together with the observation that PtdIns(3,4,5)P₃ is readily incorporated into cell membranes, these findings reaffirm the second messenger role of PtdIns(3,4,5)P₃ in thrombin receptor activation. The existence of a PtdIns(3,4,5)P₃-dependent Ca²⁺ entry system on platelet membranes is supported by the partial inhibition of thrombin-induced Ca²⁺ influx by wortmannin. Evidence suggests that this system differs from receptor-operated nonselective Ca²⁺ channels. However, the mechanism by which PtdIns(3,4,5)P₃ facilitates Ca²⁺ entry remains unclear. Although PtdIns(3,4,5)P₃ has been known to stimulate phospholipase C-γ (PLC-γ), internal Ca²⁺ mobilization does not play a significant role in the cytosolic Ca²⁺ increase in response to PtdIns(3,4,5)P₃ stimulation. Collectively, these data provide a putative link between PtdIns(3,4,5)P₃ and Ca²⁺ signaling, which may, in part, account for the regulatory function of PtdIns(3,4,5)P₃ during platelet aggregation. Moreover, this study bears out the notion that individual PI 3-kinase lipid products play distinct roles in the regulation of cellular functions.