ADENOSINE A(1) RECEPTORS MEDIATE CHRONIC ETHANOL-INDUCED INCREASES INRECEPTOR-STIMULATED CYCLIC-AMP IN CULTURED-HEPATOCYTES

Cellular responses to adenosine depend on the distribution of the two adenosine receptor subclasses. In primary cultures of rat hepatocytes, adenosine receptors were coupled to adenylate cyclase via A(1) and A( 2) receptors which inhibit and stimulate cyclic AMP production respect ively. R-(-)-N-6-(2-phenylisopropyl)adenosine (R-PIA), the adenosine A (1) receptor-selective agonist, inhibited glucagon-stimulated cyclic A MP production with an IC50 of 19 nM. This inhibition was blocked by th e A(1)-specific antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPDX). 5'-N-Ethylcarboxamidoadenosine (NECA), an agonist which stimulates A(2 ) receptors, increased cyclic AMP production with an EC(50) of 0.6 mu M. Treatment of primary cultures of rat hepatocytes with 100 mM ethano l for 48 h decreases the quantity and function of the inhibitory guani ne-nucleotide regulatory protein (G(i)), resulting in a sensitization of receptor-stimulated cyclic AMP production [Nagy and deSilva (1992) Biochem. J. 286, 681-686]. When cells were cultured with 2 units/ml ad enosine deaminase, to degrade extracellular adenosine, ethanol-induced increases in cyclic AMP production were completely prevented. Moreove r, the specific A(1)-receptor antagonist, CPDX, also blocked the chron ic effects of ethanol on receptor-stimulated cyclic AMP production. Tr eatment with adenosine deaminase or CPDX also prevented the decrease i n quantity of the alpha subunit protein of G(i) observed in hepatocyte s after chronic treatment with ethanol. Taken together, these results suggest that activation of adenosine A(1) receptors on primary culture s of hepatocytes is involved in the development of chronic ethanol-ind uced sensitization of receptor-stimulated cyclic AMP production.