Verapamil is a potent inhibitor of 5-HT-induced platelet aggregation

We have studied the effects of verapamil, diltiazem and amlodipine on 5-HT-induced platelet aggregation and compared the results with those obtained for other platelet aggregating agents. Experiments were carried out using both human whole blood and platelet-rich plasma (PRP). Verapamil (but not diltiazem or amlodipine) inhibited 5-HT-induced platelet aggregation at much lower concentrations (IC50 = about 1 microM) than were required for inhibition of aggregation induced by other aggregating agents. Like some other selective inhibitors of 5-HT-induced platelet aggregation, it was not possible to completely overcome the inhibition by increasing the concentration of 5-HT. The antiaggregatory effects of verapamil were similar, but not identical, in whole blood and PRP. These results show that the Ca2+ channel blocker verapamil has some selectivity as an inhibitor of 5-HT-induced platelet aggregation and that this behaviour as a 5-HT antagonist should be taken into account when interpreting any therapeutic benefit ascribed to this drug.