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Abstract

In stimulated human platelets dense-granule secretion in response to the 'weak agonists' ADP, adrenaline, platelet activating factor and low concentrations of thrombin as well as Ca²⁺ mobilisation in response to thrombin are enhanced by a Na⁺/H⁺ exchanger. In the present study the role of this antiport in collagen stimulated human platelets was examined. While stimulation of platelets loaded with the fluorescent intracellular pH-sensitive dye, bis-carboxyethyl-5-(6)-carboxyfluorescein (BCECF) with thrombin resulted in the activation of the Na⁺/H⁺ exchanger, activation of this antiport did not occur in collagen-stimulated platelets. The lack of antiport activity in response to collagen using BCECF-loaded platelets correlated with the lack of any functional role of the antiport in collagen stimulated platelets. In the presence of a Na⁺/H⁺ exchange inhibitor, ethylisopropylamiloride, neither collagen-induced platelet aggregation or dense-granule secretion was affected. Furthermore, while the removal of extracellular Na⁺ (Na⁺ext), a condition that also prevents activation of the antiport, inhibited dense-granule secretion in response to a low concentration of thrombin, collagen-induced secretion was potentiated. This potentiatory effect could not be attributed to changes in either the membrane potential or in collagen-induced phospholipase C or protein kinase C activity. The present results indicate that in contrast to the 'weak agonists' (1) collagen-induced platelet activation does not require activation of the Na⁺/H⁺ exchanger and (2) Na⁺ext per se is an inhibitor of collagen-induced secretion.