Platelet aggregation and endogenous 5-ht secretion in presence of ca, sr and ba. effects of calcium antagonists

Washed rat platelets aggregation and endogenous serotonin release were studied after thrombin stimulation in the presence of different concentrations of Ca²⁺, Sr²⁺ or Ba²⁺. The extent of platelet aggregation and release was found to depend upon the external concentration of these cations. For all of them, an optimum concentration could be defined. Higher concentrations were shown to inhibit both aggregation and release. Efficiency to support thrombin-induced aggregation was in the order Ca²⁺>Sr²⁺>Ba²⁺. Complete inhibition of aggregation and release induced by thrombin was obtained after a 30 second preincubation with 38 uM nitrendipine, 1 nM Cd^2- or 1 mM Mn²⁺. Inhibition was obtained in the presence of Ca²⁺, Sr²⁺ or Ba²⁺. These results are consistent with the hypothesis that Sr²⁺ and Ba²⁺ are able to support platelet activation acting as Ca²⁺ substitutes. Following thrombin stimulation, they could penetrate the platelets and mimick a rise in cytoplasmic Ca²⁺.