DIURNAL-VARIATION IN MELATONIN EFFECT ON ADENOSINE-TRIPHOSPHATE AND SEROTONIN RELEASE BY HUMAN PLATELETS

The effect of the pineal hormone melatonin on adenosine diphosphate-induced human platelet aggregation and adenosine triphosphate release was assessed in platelet-rich plasma obtained from normal volunteers at 08.30 and 20.30 h. In 10-7-10-5 mol/l concentrations melatonin inhibited ADP-induced platelet aggregation only in the evening (p >0.05). ADP-induced ATP release, an index of platelet secretory processes, showed a generally greater, dose-dependent inhibition after adding melatonin (10-9-10-5 mol/l) at 20.30 h as compared with 08.30 h. The inhibitory activity of melatonin (10-9-10-5 mol/l) on [3H]serotonin release elicited by thrombin in washed human platelets obtained from normal volunteers was dose-dependent; the effect was generally greater at 20.30 h. The activity of the potent platelet anti-aggregating agent prostacyclin did not exhibit diurnal differences with respect to impairing ADP-induced platelet-rich plasma aggregation. These results indicate the existence of a diurnal variation of sensitivity to melatonin in human platelets.