Alteration of Na+ homeostasis as a critical step in the development of irreversible hepatocyte injury after adenosine triphosphate depletion

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Abstract

The exposure of isolated hepatocytes to the redox-cycling quinone menadione caused an early loss of mitochondrial membrane potential, adenosine triphosphate (ATP) depletion, and decreased intracellular pH. These alterations were followed by an increase in intracellular Na+ and, ultimately, cell death. If HCO3 was omitted from the incubation buffer, or the hepatocytes were incubated in an acidic medium (pH 6.5) the accumulation of Na+ was markedly reduced. Inhibition of the Na+/H+ exchanger and of the Na+/HCO3 cotransporter by, respectively, amiloride and 4,4′-di-isothiocyano-2,2′-disulfonic acid stilbene (DIDS) suppressed the initial Na+ influx but did not prevent subsequent Na+ accumulation, because amiloride and DIDS inhibited the Na+/K+ pump. The omission of HCO3 from the extracellular medium or the incubation in acidic conditions also prevented menadione toxicity, without interfering with the loss of mitochondrial membrane potential and with ATP depletion. A similar protection was evident when hepatocytes were incubated with menadione in a medium without Na+. The preservation of adequate levels of ATP by supplementing hepatocytes with fructose allowed the initial Na+ load to be recovered and provided partial protection against menadione toxicity. These effects were suppressed if Na+/K+-ATPase was inhibited with ouabain. Taken together, these results indicated that the activation of the Na+/HCO3 cotransporter and of the Na+/H+ exchanger in response to the decrease of intracellular pH stimulated an enhanced influx of Na+. When the activity of the Na+/K+ pump was not able to control Na+ levels because of ATP depletion, such an uncontrolled Na+ influx precipitated irreversible injury and caused hepatocyte death. (HEPATOLOGY 1995; 21:1089–1098.)

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