Desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocytes results in a 40-65% decrease in agonist-stimulated adenylate cyclase activity and correlates with increased phosphorylation of beta-adrenergic receptors. To assess the role of phosphorylation in desensitization, membranes from isoprenaline- and dibutyryl cyclic AMP-desensitized turkey erythrocytes were incubated with alkaline phosphatase for 30 min at 37 degrees C, pH 8.0. In both preparations alkaline phosphatase treatment significantly decreased desensitization of agonist-stimulated adenylate cyclase activity by 40-75% (P less than 0.05). Similar results were obtained after alkaline phosphatase treatment of membranes from isoprenaline- and dibutyryl cyclic AMP-desensitized duck erythrocytes. Moreover, alkaline phosphatase treatment of membranes from duck erythrocytes desensitized with 12-O-tetradecanoylphorbol 13-acetate returned agonist-stimulated adenylate cyclase activity to near control values. In all experiments, inclusion of 20 mM-sodium phosphate to inhibit alkaline phosphatase during treatment of membranes attenuated the enzyme9s effect on agonist-stimulated adenylate cyclase activity. In addition, alkaline phosphatase treatment of membranes from control and isoprenaline-desensitized turkey erythrocytes increased the mobility of beta-adrenergic-receptor proteins, specifically photoaffinity-labelled with [125I]iodocyanopindolol-diazirine, on SDS/polyacrylamide-gel electrophoresis. The increased mobility of the beta-adrenergic-receptor proteins after alkaline phosphatase treatment of membranes was again inhibited by 20 mM-phosphate. These results provide additional evidence for a direct role for phosphorylation in desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocytes.
