Effects of Ouabain on86Rb-Uptake,3H-5-HT-Uptake and Aggregation by 5-HT and ADP in Human Platelets

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Abstract
In the search of sensitive models for actions of digitalis-like substances on intact cells or tissues, the effects of ouabain on human platelets were investigated. In a concentration-dependent manner ouabain 10-8-10-4 M inhibited Na+-K-ATPase activity measured as uptake of 86Rubidium (86Rb), with about 90% inhibition of the total uptake at ouabain .gtoreq. 10-6 M. An almost identical concentration-effect curve was found for platelet uptake of 3H-serotonin (3H-5-HT). The platelet shape change reaction to exogenous 5-HT (1 .times. 10-6 M) was suppressed by ouabain (10-8-10-4M) in a concentration-dependent manner, but with no clear maximum effect within the range testes. Aggregation induced by adenosine-di-phosphate (ADP 2 .times. 10-6 M) was enhanced by ouabain 10-8-10-6 M. At the highest concentration tested the rate of aggregation was increased by 31% and the change in light transmission by 54%. At low concentration (< 10-9M) of ouabain, there was a tendency towards increased aggregation as well as increased uptake of 86Rb, which may be a parallel to observations of positive inotropic effects of low concentration of glycosides, which do not inhibit Na-K+-ATPase. The results show that human platelets can be used as a model tissue for studying effects of cardiac glycosides. This suggests that it may be useful for further investigations of the biological effects of agents with a similar effect profile, e.g. endogenous digitalis-like substances.