Activation of the alpha(2A)-adrenoceptor mediates deceleration of the deaggregation component of the response to ADP or 5-HT in human platelets in vitro

Platelet aggregation requires the concomitant activation of at least one G( i)- and one G(q)-coupled receptor. Epinephrine (EPI) amplifies the response elicited by a number of agonists for platelet aggregation. This study test ed the hypothesis that platelet alpha (2A)-adrenoceptor activation causes d eceleration of the deaggregation component of the platelet aggregation resp onse when activated concomitantly with the G(q)-coupled adenosine diphospha te (ADP) P2Y(1) or 5-hydroxytryptamine(2A) receptor. The time course of the aggregation response of human platelet-rich plasma following activation of combinations of two or three receptors was assessed by turbidometry using lepirudin anticoagulation. Simultaneous activation of specific two- and thr ee-receptor combinations was achieved using selective antagonists for the P 2Y(12) and P2Y(1) receptor subtypes. Steady-state and kinetic parameters, o btained using a four-compartment kinetic model, were used to assess the eff ects on the net aggregation response. Graded alpha (2A)-adrenoceptor activa tion was associated with a concentration-dependent decrease of the rate con stant of deaggregation. Activation of both ADP receptor subtypes and the al pha (2A)-adrenoceptor produced a concentration-dependent, mutual amplificat ion of the aggregation response. In addition, when three receptors were sim ultaneously activated, mutual amplification of the aggregation response was observed at physiologically relevant concentrations of epinephrine or nore pinephrine (NE) and ADP. We propose that similar to the P2Y12 receptor, act ivation of the alpha (2A)-adrenoceptor decelerates the deaggregation compon ent shifting the balance toward increased net aggregation. The effects of E PI and NE on the aggregation response may contribute to the mechanism of in creased thrombotic risk present in certain pathophysiological and disease s tates.