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 less than 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.