Inhibition of thrombin-induced Ca2+ influx in platelets by R59949, an inhibitor of diacylglycerol kinase

Objectives The aim of this study was to determine whether diacylglycerol kinase (DGK) is involved in transplasmalemmal Ca2+ influx of platelets. Methods Effects of R59949, an inhibitor of diacylglycerol kinase, on intracellular Ca2+ concentration ([Ca2+]i) and mRNA expression of DGK isozymes were investigated using washed human platelet suspensions. Key findings Thrombin-induced increase in [Ca2+]i was significantly inhibited by pretreatment of platelets with R59949, while thapsigargin-induced increase in [Ca2+]i was comparable in platelets with and without R59949 pretreatment. Thapsigargin-induced increase in [Ca2+]i was markedly attenuated in the presence of SKF-96365. In the presence of SKF-96365, thrombin-induced increase in [Ca2+]i was significantly attenuated, and additional treatment with R59949 caused a further decrease in [Ca2+]i. Pretreatment of platelets with 1-butanol significantly attenuated thrombin-induced increase in [Ca2+]i, while thrombin-induced increase in [Ca2+]i was augmented in the presence of propranolol. mRNA expression of DGK-a and DGK-?, which are known to be inhibited by R59949, in platelets was confirmed by RT-PCR analysis. Conclusions R59949 inhibited a store-depletion-insensitive component of transplasmalemmal Ca2+ entry induced by thrombin, while store-operated Ca2+ entry was not affected by R59949. The results of this study suggest that phosphatidic acid is involved in thrombin-induced Ca2+ influx of platelets.