Regulation of gluconeogenesis during exposure of young rats to hypoxic conditions

Abstract

1. Two-day-old rats were exposed at constant temperature to atmospheres containing air and nitrogen with the air content varied in steps from 100 to 0%. By using this system of graded hypoxia a comparison was made between rates of gluconeogenesis from lactate, serine and aspartate in the whole animal and the concentrations of several liver metabolites. 2. Gluconeogenesis, expressed as the percentage incorporation of labelled isotope into glucose plus glycogen, proceeds linearly for 30min when the animals are incubated in a normal air atmosphere, but is completely suppressed if the atmosphere is 100% nitrogen. 3. Preincubation of animals for between 5 and 30min under an atmosphere containing 19% air results in the attainment of a new steady state with respect to gluconeogenesis and hepatic concentrations of ATP, ADP, AMP, lactate, pyruvate, β-hydroxybutyrate and acetoacetate. 4. When lactate (100μmol), aspartate (20μmol) or serine (20μmol) was injected, it was shown that the more severe the hypoxia the greater the depression of gluconeogenesis. Under conditions when gluconeogenesis was markedly inhibited there were no changes in the degree of phosphorylation of hepatic adenine nucleotides, but free [NAD+]/[NADH] ratios fell in both cytosol and mitochondrial compartments of the liver cell. 5. Measurements of total liver NAD+ and NADH showed that the concentrations of these nucleotide coenzymes changed less with anoxia, in comparison with the concentration ratio of free coenzymes. 6. Calculations showed that the difference in NAD+–NADH redox potentials between mitochondrial and cytosol compartments increased with the severity of hypoxia. 7. From the constancy of the concentrations of adenine nucleotides it is concluded that liver of hypoxic rats can conserve ATP by lowering the rate of ATP utilization for gluconeogenesis. Gluconeogenesis may be regulated in turn by the changes in mitochondrial and cytosol redox state.