Circadian variation in platelet alpha2-adrenoceptor affinity in normal subjects

Circadian rhythms—prominent and reproducible temporal patterns that occur regularly over a 24-hour period—have been described in a variety of physiologic and morbid events including death. In patients with coronary artery disease, episodes of angina1 and acute myocardial infarction2 occur with a peak frequency between 6 A.M. and 12 noon, with a second but small peak frequency in the late afternoon. Other studies have demonstrated existence of circadian rhythms in plasma catecholamine and cortisol concentrations,3 heart rate and blood pressure,4 fibrinolytic activity5 and platelet aggregability6 in man. We designed this study to prospectively reassess occurrence of circadian variation in platelet aggregation patterns in normal subjects. Because platelet aggregation, particularly in response to epinephrine, is related to β2-adrenergic state,7,8 status of β2-adrenoceptors was also evaluated.