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([Ca^(2+)]_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[Ca^(2+)]_i of platelets four to five times in the presence of 1 mmol/L external calcium.but there was a much smaller increase in[Ca^(2+)]_i in the absence of external calcium.It seems that thrombin increased[Ca^(2+)]_i partly by release of internal Ca^(2+),but mainly by stimulating influx.J-894 markedly inhibited the rise of[Ca^(2+)]_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.
