Inhibition of 5-hydroxytryptamine-potentiated aggregation of human blood platelets by 5-hydroxytryptamine receptor-blocking agents.

Blood platelets possess specific receptors for 5-hydroxytryptamine (5-HT) which differ from the carrier for the amine uptake. Activation of 5-HT receptors causes shape change and aggregation of platelets. We studied the influence of drugs with 5-HT receptor blocking properties on 5-HT-potentiated, ADP-induced aggregation of human blood platelets. The synergistic effect of 5-HT on aggregation was inhibited by the compounds tested in the order of potency: cyproheptadine, pizotifen, spiperone, methysergide, methiothepine , mianserin, ketanserin, dihydroergotamine. The IC50 values lay in the range from 14 to 40 nmol/l. Chlorpromazine and haloperidol were less effective by more than one order of magnitude. Our results indicate a certain similarity between the 5-HT receptors on platelets and the 5-HT receptors on serotonergic neurons and vascular smooth muscle which belong to the 5-HT2 receptor type.