Abstract: The intracellular free calcium concentration plays a key role as the second messenger in the regulation of cellular reaction.The changes of cytoplasmic free calcium concentration([Ca²⁺]i) in human platelets loaded with the fluorescent calcium indicator quin-2 were measured in this study. The result showed that thrombin (0.3 U/ml)raised the [Ca²⁺]i of platelets four to five times in the presence of 1 mmol/L external calcium, but there was a much smaller increase in [Ca²⁺]i in the absence of external calcium. It seems that thrombin increased[Ca²⁺]i partly by release of internal Ca²⁺, but mainly by stimulating influx.J-894 markedly inhibited the rise of [Ca²⁺]i in platelets induced by thrombin in a dose dependent manner through decreasing both calcium influx and calcium release. This suggests that J-894 may be a calcium channel blocker.

Key words: Thrombin, Platelet, Calcium, J-894, Quin-2