Chronic ethanol feeding increases the quantity of G alpha s-protein in rat liver plasma membranes

The liver is a primary target for both acute and chronic effects of ethanol. Because ethanol is known to alter the function of guanine nucleotide regulatory proteins (G-proteins), changes in hepatic G-proteins could contribute to the adverse effects of ethanol on liver function. Male Wistar rats were fed a liquid diet containing 36% of calories as ethanol for 4 weeks. Control rats were pair-fed or allowed free access to a diet that isocalorically substituted maltose dextrins for ethanol. Liver plasma membranes were isolated and separated into basolateral and canalicular fractions by sucrose-density gradients. Enrichment of marker enzymes (5'-nucleotidase for canalicular membranes and forskolin-stimulated adenylyl cyclase activity for basolateral membranes) was not affected by ethanol feeding. Quantity of G alpha s and G alpha i proteins in membrane fractions was determined by immunoblot. After ethanol feeding, immunoreactive G alpha s protein was increased in liver plasma membranes compared with pair-fed controls. G alpha i and G alpha s were present in both the basolateral and canalicular fractions of the plasma membrane in control and ethanol-fed rats. G alpha s quantity in the basolateral membrane was greater in ethanol-fed rats compared with controls, with no differences in G alpha s observed in canalicular membranes. The quantity of G alpha i did not change in response to ethanol feeding in any of the membrane fractions. Treatment of isolated plasma and basolateral membranes with 10 mumol/L 5'-guanylimidophosphate, a nonhydrolyzable guanosine triphosphate analogue that activates G-proteins, increased cAMP production to a greater extent in ethanol-fed rats compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)