Insufficient Supply of Reducing Equivalents to the Respiratory Chain in Cerebral Cortex during Severe Insulin-Induced Hypoglycemia in Cats

The ability of endogenous substrates in brain to substitute for glucose as sources for energy metabolism during insulin-induced hypoglycemia was studied. The ratio of the arteriovenous difference of glucose to the arteriovenous difference of oxygen in the cerebral cortex was measured during progressive hypoglycemia in paralyzed, artificially ventilated cats that were anesthetized with pentobarbital sodium and nitrous oxide. The ratio did not change when blood glucose fell from a mean of 7.68 to approximately 2 mumol/ml. Below 2 mumol/ml the ratio decreased, indicating that substrates other than the glucose supplied by the blood were being utilized. In another series of experiments, changes in the redox state of respiratory chain NAD were monitored from the cerebral cortex using microfluorometry during the onset of hypoglycemia and the recovery. Hypoglycemia severe enough to produce isoelectric EEG was accompanied by an oxidation of NADH, demonstrating that the supply of reducing equivalents to the respiratory chain was decreased. Recovery from hypoglycemia, produced by intravenous glucose injections, was accompanied by an increase in blood glucose concentrations, the return of EEG activity, and a decrease in the NAD/NADH ratio. When blood glucose concentration reached 2.23 during the recovery, further increases in blood glucose had no effect on the redox state of NAD. Although alternative substrates appear to be utilized for energy metabolism during severe hypoglycemia, they cannot fully replace glucose as the source of reducing equivalent to the respiratory chain.