Endurance training and human alpha 2-adrenergic receptors on platelets

Trained endurance athletes have a smaller rise in blood pressure, heart rate, and catecholamine response to stress when compared to an untrained, control population. Since pre-synaptic alpha 2-adrenergic receptors modulate the control of epinephrine release from nerve terminals, and since platelets are used as models of monoaminergic neurons, we investigated changes in the number and affinity binding of alpha 2-adrenergic receptors on platelets from endurance athletes and a sedentary population using the radiolabeled ligand yohimbine. We found, compared to a control, sedentary population, trained athletes had a 45% increase in the number of platelet alpha 2-adrenergic receptors (338 +/- 39 receptors/platelet vs 233 +/- 25 receptors/platelet, P less than 0.05) with no change in the dissociation constant (2.53 nM +/- 0.2 vs 2.24 nM +/- 0.2, NS) or the ED50 concentration for the competitive displacement of 2.5 nM yohimbine by epinephrine (2.8 X 10(-6) M vs 3.34 X 10(-6) M, NS). The increase in alpha 2-adrenergic receptors found in athletes may explain their decreased catecholamine response and concomitant physiologic responses to exertion.