Reduction of adenylate cyclase activity in lysates of human platelets by the alpha-adrenergic component of epinephrine.

The effects of the alpha- and beta-adrenergic components of epinephrine on adenylate cyclase activity were studied in lyastes of human platelets. The beta-adrenergic component increased enzyme activity between 20 and 50%, whereas the alpha-adrenergic component caused a concentration-dependent reduction of adenylate cyclase activity. The maximal inhibition of adenylate cyclase was 50 to 60% with about 100 muM epinephrine; about 1 muM epinephrine caused half-maximal inhibition. The alpha-adrenergic reduction of adenylate cyclase activity appeared to be immediate and was specific for platelets from species in which alpha-adrenergic stimulation caused aggregation. Adenylate cyclase activity measured in the presence of prostaglandins of fluoride was also reduced by the alpha-adrenergic component of epinephrine, but the inhibitory effect was less than on basal activity. In contrast, the 5'-guanylylimidodiphosphate-stimulated enzyme was not affected by stimulation of alpha-adrenergic receptors.