Heterologous Desensitization of the Inhibitory AI Adenosine Receptor- Adenylate Cyclase System in Rat Adipocytes

Adenosine, acting via AI adenosine receptors, can inhibit adenylate cyclase activity in adipocytes. To assess the effects of chronic adenosine agonist exposure on the AI adenosine receptor system of adipocytes, rats were infused with (-)-phenylisopropyladenosine or vehicle for 6 days and membranes were prepared. Basal as wel1,as isoproterenol-, sodium fluoride-, and forskolin-stimulated adenylate cyclase activities were significantly increased (-2-fold) in membranes from treated animals. (-)-Phenylisopropyladenosine-mediated inhibition of forskolin-stimulated adenylate cyclase activity was significantly ( p = 0.0001) attenuated in membranes from treated rats (20.1 f 2.1% inhibition) versus controls (31.6 f 2.3% inhibition). Prostaglandin El-induced inhibition of forskolin-stimulated adenylate cyclase activity was also attenuated: 11.7 h 3.6 uersus 23.2 2 4.6% ( p = 0.001). Using the AI adenosine receptor agonist radioligand (-)-Ns-(3-[1251]iodo-4hydroxyphenylisopropyl)adenosine, 32% fewer high affinity binding sites were detected in membranes from treated animals ( p < 0.04). Photoaffinity labeling with N6-2-(3-[1251]iodo-4-azidophenyl)ethyladenosine revealed no gross difference in receptor structure. The number of &adrenergic receptors as well as the percentage of receptors in the high affinity state as assessed by (-)-3-[12SI]iodocyanopindolol binding were the same in both groups. In membranes from treated rats, the amount of [cv-~’P]NAD incorporated by pertussis toxin into the a subunit of the inhibitory guanine nucleotide regulatory protein (Ni) was decreased by 37 f 11%. Concurrently, the quantity of label incorporated by cholera toxin into the a subunit of the stimulatory guanine nucleotide regulatory protein (N.) was increased by 44 f 14% in treated membranes. Finally, the capacity of N, solubilized from treated membranes to stimulate adenylate cyclase activity when reconstituted into cyc549 lymphoma cell membranes was enhanced by -50% compared to control. Thus, heterologous desensitization, manifested by a diminished capacity to inhibit adenylate cyclase and an enhanced