PLATELET ALPHA2 ADRENOCEPTOR CHARACTERISTICS DURING THE MORNING INCREASE IN PLATELET AGGREGABILITY

The incidence of myocardial infarction and sudden cardiac death increases in the morning, as do platelet aggregability and sympathetic activity. We considered the possibility that increased platelet aggregability in the morning was due to an increase in platelet alpha2 adrenergic receptor number or agonist binding affinity, or to a temporal disparity between the increase in sympathetic activity that accompanies arising and the anticipated decrease in platelet high affinity alpha2 adrenoceptors. To evaluate these possibilities, we studied eight healthy male volunteers (20-35 years) before and 1-1.5 and 3-4 h after they arose at 08.00 h and performed routine morning activities. Platelet aggregability was determined in platelet-rich plasma; alpha2 adrenoceptor density and agonist binding affinity were determined in intact platelets and in membrane preparations using the alpha2 selective ligand [3H]-yohimbine. The threshold concentration of epinephrine required to produce biphasic aggregation decreased (i.e. platelet aggregability increased) from 3.6 ± 1.2 to 0.9 ± 0.3 μM (P < 0.05) after arising. However, alpha2 adrenoceptor density (380 ± 71 to 365 ± 51 sites per platelet) and agonist binding affinity assessed simultaneously did not change after arising, suggesting that the increase in platelet aggregability is due to factors extrinsic to the platelets or to an intra-platelet mechanism distal to the receptor level. © 1992 The European Society of Cardiology.