Kinetic differences between thrombin-induced and ADP-induced calcium influx and release from internal stores in fura-2-loaded human platelets.

The rapid (less than 1 sec) time course of [Ca2+]i rises was measured in fura-2-loaded, aspirin treated human platelets by stopped flow fluorimetry. With thrombin, which is known to cause substantial, rapid hydrolysis of phosphatidylinositol-4,5-bisphosphate, the mean delay before a detectible rise in [Ca2+]i in medium containing 1 mM Ca2+ o was 250 +/- 10 msec (S.E.M., n = 11). With ADP, which is reported not to stimulate phosphatidylinositol-4,5-bisphosphate hydrolysis, the delay under the same conditions was only 20 +/- 10 msec (S.E.M., n = 26). In the absence of external Ca2+, with 1 mM EGTA, the measured delays were 300 +/- 20 msec for thrombin and 210 +/- 10 msec for ADP. Times to peak were also faster for ADP than thrombin. These results suggest that thrombin and ADP promote Ca2+ influx in different ways. It is also possible that the process generating Ca2+ influx differs from that which triggers release from internal stores.