Properties and regulation of a trans-plasma membrane redox system in rat liver

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Abstract
Regulation of the reduction of ferricyanide by the isolated perfused rat liver was studied. The rate of reduction was dependent on the rate of supply of ferricyanide and independent of perfusate O2 concentration. The effect of pH was also examined; the rate of reduction was optimal at pH 7.4 and was inhibited to a greater extent by pH values below 7.4 than those above 7.4. The effects of substrates on the rate of ferricyanide reduction was assessed. Reductants of the cytosolic and mitochondrial NADH/NAD+ couple were tested. 2-Hydroxybutyrate (10 mM), lactate (10 mM), glycerol (10 mM) and ethanol (10 mM) each had no effect. Dihydroxyacetone (10 mM) stimulated the rate. Dehydroascorbate (1 mM), stimulated the rate of ferricyanide reduction; the stimulation did not appear to be attributable to the production of reduced substances that were excreted to reduce extracellular ferricyanide. The effects of glucagon and cAMP on the rate of ferricyanide reduction were examined. Glucagon inhibited the rate by .apprx. 30% and half-maximal inhibition occurred at 0.1 nM, corresponding to the concentration at which half-maximal stimulation of glucose release occurred. cAMP stimulated glucose release, but had no significant effect on the rate of ferricyanide reduction. The trans-plasma membrane redox system of liver that reduces extracellular ferricyanide is regulated by glucagon. The rate is also altered by the substrate dihydroxyacetone. The effect of glucagon may be direct as it cannot be mimicked by cAMP and it occurs directly following exposure to the hormone.