CLOCK regulates circadian platelet activity

In this study, to elucidate the circadian regulation of platelet functions and its involvement in hemostatic activity, we examined platelet aggregation in the Clock mutant mice. Circadian fluctuation in tail bleeding observed in WT was remarkably weaker in Clock mutant mice, and the bleeding was continuously low at almost the trough levels of WT mice. Day/night fluctuations in the maximum aggregation rates of platelet induced by ADP and collagen observed in WT mice were not rhythmic and remained continuously high in Clock mutant mice. These results suggest that these time-of-day differences in hemostatic activity were governed by the endogenous circadian clock and the Clock mutation increased platelet activity in vivo. © 2008 Elsevier Ltd. All rights reserved.