Circadian Variation In Platelet Aggregation In Healthy Subjects

Platelet plays a crucial role in the physiologic hemostatic course and in the process of artery thrombosis formation. Then, anti-platelet therapy has become one of the most regular therapies in modern cardiovascular disease. In the natural blood circulation, platelet doesn't adhere to natural vessel wall. However, when the vessel is injured, collagen under the vascular endothelia is exposed, platelet will adhere to it quickly and be activated. The platelet activation means that platelet adheres to collagen, aggregates and releases many substances which can promote ability of platelet coagulation and result in thrombosis. So far, the impedance method for measuring aggregation in whole blood is the most advanced method. Many factors have influence on platelet aggregation, such as the heterogeneity of platelet,inducers, and many receptors on platelet membrane. Recently, some studies on diurnal variation in platelet aggregation of healthy people were reported, but these studies were usually small in sample size and contraversial. What is more, there were few studies on Chinese population. Nubile and Malyszko showed a morning increase in vitro platelet aggregation. Jafri et al. showed that there was a trend toward circadian variation in vitro platelet aggregation to epinephrine, but these changes didn't achieve a statistical significance. Meanwhile, Platelet aggregation in response to 4 different aggregating agents were shown a bimodal daily variation with peaks in the morning and in the afternoon by Fujimura's research. However, the sample sizes of these studies are too small, and the conclusions are different. We study whether platelet aggregation has circadian variation change by measuring platelet aggregation in fixed time of a day in healthy Chinese volunteers in order to offer basis to use anti-platelet drugs rationally.Methods: A total of 30 healthy young volunteers (mean age 25.20±1.10 years, men 16, women 14) were selected. The inclusion criteria include as the followings: liver and kidney function are normal, blood routine is normal, female are in non-menstrual period (from ovulatory period to menstrual period). The criteria for exclusion include as the followings: sorts of hematic desease, hemorrhagic disease and bleeding tendency; platelet count >300×10~9/L or <100×10~9/L, receive the therapy of ticlopidine, clopidogrel, dipyridamole, warfarin, nonsteriodal, anti-inflammatory drugs, unfractionated heparin and low molecular weight heparin in four weeks; smokers and drunkers. Blood samples were obtained at 2:00, 6:00, 10:00, 16:00, 20:00h in one day. Aggregation was assessed with a mobile four-channel impedance aggregometer (Chronolog-Log Corporation), which allowed measurement of aggregation the whole blood induced by collagen (2μg/mL), ADP (10μM), ADP (20μM), arachidonic acid (0.5mM) and epinephrine (10μg/mL). Continuous variables were expressed as the mean value plus or minus standard deviation (SD). The repeated measures engineered variance analysis were performed to compare differences for individual time points when overall significant variation or trends were detected during the 24-hour observation period. A p value of <0.05 was considered statistically significant.Results: In healthy subjects epinephrine-induced platelet aggregation in the whole blood was significantly highest at 6:00 when compared to values at 2:00 (P=0.007). Collagen-induced platelet aggregation in the whole blood was highest at 6:00 when compared to values at 2:00, but these changes did not achieve statistical significance (P=0.087). Arachidonic acid- and ADP-induced platelet aggregation in the whole blood was highest at 10:00 when compared to values at 2:00. But these changes also did not achieve statistical significance.Conclusions: We use the impedance method for measuring aggregation in whole blood , demonstrate that platelet aggregation has a circadian rhythm in healthy volunteers in response to epinephrine, with peak level achieved at 6:00 after arising, with valley level at 2:00 when resting. Platelet aggregation in response to other inducers doesn't have a circadian rhythm, but there is a trend, all with valley level achieved at 2:00.