Effects of the calmodulin antagonists fendiline and calmidazolium on aggregation, secretion of ATP, and internal calcium in washed human platelets.

Ca-calmodulin dependent phosphorylation of myosin is essential for the induction of platelet shape change and subsequent reactions. Therefore, we studied the effects of the calmodulin antagonists fendiline and calmidazolium on the thrombin-induced aggregation, secretion of ATP, and increases in the intracellular free calcium concentration ([Ca]) in washed human platelets in the absence and presence of extracellular Ca. In Ca free medium, fendiline (10-100 μM) and calmidazolium (3-30 μM) concentration-dependently inhibited aggregation. The effect of fendiline could be partly reversed by extracellular Ca and higher thrombin concentrations. Furthermore, aggregations induced by the calcium ionophore ionomycin and by the protein kinase C-activator 4-β-phorbol 12-myristate 13-acetate were inhibited by fendiline, although to a smaller degree than the thrombin-induced aggregation. Thrombin-induced secretion of ATP was attenuated by low concentrations of fendiline (1-30 μM) and calmidazolium (1 μM) but enhanced by higher concentrations (10-30 and 3-10 μM, respectively), independently of extracellular Ca. Fendiline (1-10 μM) did not affect [Ca] in resting and thrombin-stimulated platelets. At higher concentrations (30-100 μM), it induced increases in [Ca] in unstimulated platelets and attenuated the response to thrombin in Ca free medium, whereas thrombin-induced Ca influx was markedly enhanced. Similar results were obtained with calmidazolium (1-3 μM). These stimulating effects on ATP secretion and on [Ca] of fendiline and calmidazolium may be attributed to interactions with platelet membranes by which the permeability of small cations is increased. In contrast to these actions that would be normally considered platelet-activating, our study demonstrates that the two calmodulin antagonists effectively antagonize [Ca] mediated induction of platelet aggregation. [ABSTRACT FROM AUTHOR]