The Effects of Nucleotides on the Expression of @-Adrenergic Adenylate Cyclase Activity in Membranes from Turkey Erythrocytes

Purine nucleotides augment catecholamine-induced activation of adenylate cyclase in the turkey erythrocyte plasma membrane. This effect occurs with greatest sensitivity for GTP (K, 1.4 X low8 M). The nucleotides do not change the K, for isoproterenol but increase the adenylate cyclase activity for any given stimulatory concentration of isoproterenol. The basal adenylate cyclase measured in the absence of hormone is unaffected by the nucleotides. The increment in enzyme activity is specifically a P-adrenergic function because it is completely blocked by propranolol, a P-adrenergic blocker, and unaffected by phentolamine, an cY-adrenergic blocker. It is proposed that GTP enhances adenylate cyclase by altering the interaction between hormone and receptor. The interaction between tritiated isoproterenol and the catechol-specific site of the /3-adrenergic receptor complex is not affected. It is therefore proposed that the receptor site specific for the ethanolamine function of the catecholamine molecule is influenced by the nucleotides.