EVIDENCE FOR REGULATED COUPLING OF A(1) ADENOSINE RECEPTORS BY PHOSPHORYLATION IN ZUCKER RATS

Studies were designed to find the molecular basis for previous observations that lipolysis is less active and A(1) adenosine receptor signaling is more active in adipocytes from obese than from lean Zucker rats. With quantitative immunoblot procedures for detection, G(i) alpha(1) and G(s) alpha(45) levels were found anomalously low in obese compared with lean membranes (50 and 30%, respectively), but other Ga: subunit levels were normal. However, the sensitivity of the receptor-Gi protein to GTP was about 5- to 10-fold higher in obese compared with lean membranes when assessed from 1) the ability of GTP to inhibit forskolin-stimulated adenylyl cyclase in the presence of an adenosine receptor agonist and 2) the ability of a nonhydrolyzable guanine nucleotide analogue to alter Al adenosine receptor agonist binding. Alkaline phosphatase treatment of isolated adipocyte membranes from obese but not lean animals decreased guanine nucleotide sensitivity of agonist binding. Surprisingly, solubilized adipocyte A(1) adenosine receptors from all animals exhibited the same high sensitivity to guanine nucleotides as that of intact obese membranes, and this high sensitivity could be decreased 20-fold by treatment with alkaline phosphatase. These data suggest that protein phosphorylation may regulate coupling of the Al adenosine receptor in rat adipocyte membranes.