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

Abstract

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 Gα s and Gα i proteins in membrane fractions was determined by immunoblot. After ethanol feeding, immunoreactive Gα s protein was increased in liver plasma membranes compared with pair-fed controls. Gα i and Gα s were present in both the basolateral and canalicular fractions of the plasma membrane in control and ethanol-fed rats. Gα s quantity in the basolateral membrane was greater in ethanol-fed rats compared with controls, with no differences in Gα s observed in canalicular membranes. The quantity of Gα i did not change in response to ethanol feeding in any of the membrane fractions. Treatment of isolated plasma and basolateral membranes with 10 μmol/L 5′-guanylimidophosphate, a non-hydrolyzable guanosine triphosphate analogue that activates G-proteins, increased cAMP production to a greater extent in ethanol-fed rats compared with controls. These data indicate that ethanol increases the quantity and function of Gα s protein in rat liver plasma membranes.