DOWN REGULATION OF ALPHA-2 ADRENERGIC-RECEPTOR NUMBERS IN PLATELETS BY INSULIN

Incubation of platelets from normal volunteers, who had not taken any medication at least for 2 weeks, with insulin (200-mu-units/ml), resulted in the inhibition of the potentiation of ADP-induced platelet aggregation in the presence of (-)-epinephrine by 50-60% when compared with the control. The inhibitory effect of insulin was not related to the increased cyclic AMP level or decreased thromboxane A2 synthesis in these cells. It was found that the treatment of platelets with insulin decreased alpha-2 adrenergic receptors numbers of these cells from 413 +/- 92/platelet to 206 +/- 84/platelet as determined by Scatchard analysis of [H-3]yohimbine binding. The affinity of the receptors (1.05 +/- 0.02 nM) remained essentially unchanged due to the treatment of platelets with the hormone (1.40 +/- 0.60 nM).