LACK OF EFFECT OF OUABAIN ON CALCIUM HOMEOSTASIS IN RAT PLATELETS - COMPARATIVE-STUDY WITH HUMAN PLATELETS

The precise mechanisms of Ca2+ handling in rat platelets are not fully understood. We sought to determine whether rat platelets possess a Na +-Ca2+ exchanger. First, we investigated the time course of the effect of ouabain (10(-4) M) on cytosolic sodium concentration ([Nai](i)) an d Ca2+ homeostasis in platelets from Wistar rats in comparison with th ose from humans. Ouabain increased platelet [Na+](i) in both rat and h uman platelets. Whereas ouabain induced a time-dependent increase in b asal and thrombin-stimulated (0.3 units/ml) cytosolic calcium concentr ation ([Ca2+](i)) in human platelets, no change was found in rat plate lets. Furthermore, 90-min pretreatment of rat platelets with ouabain d id not affect the [Ca2+](i) response to thrombin (0.1-1.0 units/ml) or a maximal dose of ionomycin (5 mu M). Also the decline in [Ca2+](i) a fter the peak response evoked by these agonists in the absence of extr acellular Ca2+ was not changed by ouabain pretreatment. Similarly, rep lacement of extracellular Na+ had no influence on any of these determi nations. Thus decreasing the plasma membrane Na+ gradient did not affe ct basal [Ca2+](i), thrombin-induced mobilization of Ca2+ from intrace llular stores, internal Ca2+ discharge capacity, or Ca2+ extrusion fro m cytosol of rat platelets. In contrast to human platelets, a Na+-Ca2 exchange mechanism dose not appear to play a significant role in Ca2 homeostasis of rat platelets.