Blockade of Receptor-operated Calcium Channels by Mibefradil (Ro 40-5967): Effects on Intracellular Calcium and Platelet Aggregation

The goal of the present study was to evaluate if mibefradil, a novel nondihydropyridine Ca2+ antagonist, could block receptor-operated calcium channels (ROCC) present in human platelets and to determine the functional consequences of this blockade. Therefore, the effect of mibefradil on increases in intracellular Ca2+ concentrations and aggregation of human platelets induced by platelet activating factor (PAF) was examined. In order to differentiate effects on Ca2+ mobilization from intracellular stores from those on Ca2+ influx through ROCC, intracellular Ca2+ concentrations were measured either in fura-2-loaded platelets or in cells loaded with both BAPTA and fura-2. Mibefradil totally and dose dependently inhibited PAF-induced Ca2+ influx with a maximal effective concentration of 10 microM, but at this concentration only reduced Ca2+ mobilization from intracellular stores. A similar effect was observed when platelets were stimulated with ADP, suggesting that mibefradil was indeed interfering with ROCC and not specifically with PAF receptors. In the same range of concentrations, mibefradil inhibited Ca(2+)-dependent platelet aggregation induced by PAF. This effect was most likely due to the inhibition of ROCC, as Ca(2+)-independent aggregation induced by phorbol-myristyl-acetate (PMA) was insensitive to mibefradil. We conclude that mibefradil, which has previously been described to be an antagonist for L- and T-Type Ca2+ channels, also blocks receptor-operated Ca2+ channels. This blockade seems to be functionally relevant for platelet aggregation.