Ouabain increases the calcium concentration in intracellular stores involved in stimulus-response coupling in human platelets.

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Abstract
The effect of ouabain on Ca2+ homeostasis in human platelets was studied using both quin 2 and chlorotetracycline to monitor changes in cytosolic Ca2+ as well as changes in the amount of Ca2+ accumulated in intracellular storage sites. In resting platelets, ouabain induces a concentration- and time-dependent increase in cytosolic Ca2+ concentration and a marked elevation of Ca2+ in the intracellular stores. The amount of Ca2+ mobilized from these stores upon stimulation with thrombin, as well as thrombin-induced secretion of platelet 5-hydroxytryptamine, was increased after preincubation with the glycoside (3 .times. 10-6 M). These data show that ouabain induces an elevation of intracellular Ca2+ levels, most likely mediated via Na+-Ca2= exchange, and that this incremental amount of Ca2+ is accumulated in an intracellular store involved in stimulus-response coupling. This may explain the enhanced functional responses of platelets to agonists in the presence of ouabain and suggests a role for Na+-Ca2+ exchange in Ca2+ homeostasis of the human platelet.