Some processes of energy saving and expenditure occurring during ethanol perfusion in the isolated liver of fed rats; a Nuclear Magnetic Resonance study.

BACKGROUND: In the isolated liver of fed rats, a 10 mM ethanol perfusion rapidly induced a rapid 25% decrease in the total ATP content, the new steady state resulting from both synthesis and consumption. The in situ rate of mitochondrial ATP synthesis without activation of the respiration was increased by 27%, implying an increased energy demand. An attempt to identify the ethanol-induced ATP-consuming pathways was performed using 31P and 13C Nuclear Magnetic Resonance. RESULTS: Ethanol (i) transiently increased sn-glycerol-3-phosphate formation whereas glycogenolysis was continuously maintained; (ii) decreased the glycolytic ATP supply and (iii) diminished the intracellular pH in a dose-dependent manner in a slight extend. Although the cytosolic oxidation of ethanol largely generated H+ (and NADH), intracellular pHi was maintained by (i) the large and passive excretion of cellular acetic acid arising from ethanol oxidation (evidenced by exogenous acetate administration), without energetic cost or (ii) proton extrusion via the Na+-HCO3- symport (implying the indirect activation of the Na+-K+-ATPase pump and thus an energy use), demonstrated during the addition of their specific inhibitors SITS and ouabaïn, respectively. CONCLUSION: Various cellular mechanisms diminish the cytosolic concentration of H+ and NADH produced by ethanol oxidation, such as (i) the large but transient contribution of the dihydroxyacetone phosphate/sn-glycerol-3-phosphate shuttle between cytosol and mitochondria, mainly implicated in the redox state and (ii) the major participation of acetic acid in passive proton extrusion out of the cell. These processes are not ATP-consuming and the latter is a cellular way to save some energy. Their starting in conjunction with the increase in mitochondrial ATP synthesis in ethanol-perfused whole liver was however insufficient to alleviate either the inhibition of glycolytic ATP synthesis and/or the implication of Na+-HCO3- symport and Na+-K+-ATPase in the pHi homeostasis, energy-consuming carriers.