Verapamil, an antagonist at 5-hydroxytryptamine receptors of human blood platelets

In human blood platelets verapamil and D600 (2-methoxyverapamil) in therapeutic concentrations inhibited the shape change reaction induced by 5-hydroxytryptamine (5HT) but not that induced by ADP. The N-methylated derivatives (D575 and D890) had much less effect. The inhibitory action of verapamil was independent of external Ca2+. Nitrendipine and diltiazem (20 microM) had no effect on the 5HT- and the ADP-induced shape change reactions. Since both these shape change reactions are mediated by a rise in cytoplasmic free Ca2+, it is concluded that the inhibition of the 5HT effect by verapamil and D600 was not due to their interference with calcium channels but rather to an antagonistic action on 5HT2-receptors. This view is supported by the finding that verapamil but not D575 competed with [3H]ketanserin and [3H]spiroperidol for their specific binding sites on membranes of rat cerebral cortex.