Effect of systemic alpha(-2) adrenergic blockade on the morning increase in platelet aggregation in normal subjects

To assess whether activation of platelets by catecholamines contributes to the increase in platelet aggregation associated with either the assumption of upright posture or exercise, we studied the effect of oral yohimbine on these phenomena. Whole blood platelet aggregation and plasma catecholamine levels were measured before and after standing and at peak exercise in untr eated normal subjects and after oral yohimbine. Neurochemical indexes indic ated systemic alpha(2)-receptor blockade by yohimbine. Yohimbine reduced th e ortho-static increase in platelet aggregation response by 63 +/- 11%, but exercise-induced increase in aggregation was unaffected, Thus, alpha(2)-ad renergic blockade attenuates the orthostatic increase in platelet aggregati on. Agents designed to inhibit the morning surge in catecholamine levels or block platelet alpha(2) adrenoceptors may reduce the risk of thrombotic va scular events in atherosclerosis. (C) 1999 by Excerpta Medico, Inc.