Down regulation of α₂ 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 μunits/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 as 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 α₂ adrenergic receptors numbers of these cells from 413 ± 92/platelet to 206 ± 84/platelet as determined by Scatchard analysis of [³H]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 ± 0.06 nM).