How Maternal Insulin-Induced Hypoglycemia Affects Fetal Brain Neuronal-Glia Function. A Metabolic Study Using 13C MRS Isotopmer Analysis

A Metabolic Study Using l3C MRS Isotopmer Analysis S. Haber and A. Lapidot Deoartment of Organic Chemistrv, The Weizmann Institute of Science, Rehovot 76100, Israel L -u-~ , Introduction metabolites, including lactate, and the amino acids, were not altered during insulin-induced hypoglycemia, the labeling patterns of glutamate, glutamine, lactate and alanine were, in some cases, quite different from the controls. The higher enrichments of lactate and alanine found in the fetal brain relative to the maternal brain, in comparison to control rabbits, may indicate a slight depletion of the pool sizes. This depletion occurs by utilization of these compounds as energy fuels, due to the scarcity of glucose in fetal circulation. Smaller pool sizes lead to lower endogenous dilution of the labeled compounds and, ultimately, to the slightly higher 13C enrichments, relative to controls. The relative enrichments of glutamate C-3 and C-4 in the maternal brain were also quite different under control and hypoglycemic conditions. In control maternal brain, the enrichment of the glutamate C-4 carbon was much higher than that of the. glutamate C-3 carbon, whereas during hypoglycemia, the enrichments of the two carbons were very similar. This may imply a slight drop in energy production. Gltrcose flux zh PC n/qd PDH We suggest that the lower PC/PDH ratio in the fetal brain is a result of higher glucose “channeling” into neurons than into glia. This so-called channeling occurs in order to maintain energy productrion in the neurons, thus preventing damage to vital neuronal function. Lactate oxidation, suggested to occur in the hyhpoglycemic fetal brain, may take place in glia, rather than in neurons, thus sparing glucose for neuronal metabolism. The lack of effect on both the PC and PDH ratio in glia may indicate an equal effect of glucose lack on both the PC and PDH pathways in f&al glia and the fact that lactate, which might be utilized as an alternative substrate is also labeled, thus “masking” a decreased glucose flux in glia. In the maternal brain, an opposite effect on the glutamate FC/PDH ratio was found: it was higher in hypoglycemic maternal brain than in control. This may be due to direct effect of insulin in the brain Since insulin does not cross the placenta, the fetus is only hypoglycemic but not hyperinsulinemic. These suggestions, however, require further verification. Improved maternal glycemic control in diabetic pregnancies, appears to reduce the risk of congenital malformations in the fetus. However, intensive insulin therapy often leads to maternal hypoglycemia, which may be harmful to the developing embryo and fetus. The frequency and severity of hypoglycemia in pregnant IDDM subjects, is even higher than that observed in non pregnant IDDM patients. Since insulin does not cross the placenta, the fetus of the insulininduced hypoglycemic mother is subjected to periods of hypoglycemia. The damage to the fetus caused by hypoglycemia has been suggested to stem from the total dependence of the fetus on glycolysis for energy, during early neurulation; thus the harmful effect of hypoglycemia is greatly diminished once the capacity for aerobic glucose metabolism is developed, during late neurulation; however, contradictory results of persisting harmful effects of maternal hypoglycemia during late neurulation have also been reported. The mechanisms involved in the fetal cerebral response to insulin-induced maternal hypoglycemia are presented here, using l3 C MRS isotopomer analysis of brain extracts (1,2). Methods l~zsl~lin treatment. Healthy, pregnant rabbits were rendered hype Tlycemic by an i.m. injection (5 KU/kg), 217 prior to [U19 Clglucose infusion. Following lh of the infusion, the fetuses were delivered and their brains removed. Results and Discussion Inszrlin-in&c& hypoglyerrric rabbits. Two hours after insulin administration, maternal plasma levels dropped by 60%. Fetal plasma glucose also decreased si mificantly (20 mg/dL). Subsequent infusion of [U1+ Clglucose did not cause any increase in maternal or fetal plasma glucose levels. The drop in circulating glucose did not lead to a drop in cerebral glucose levels, in either the maternal or the fetal brain. The lack of effect of hypoglycemia on cerebral glucose levels may stem from the adaptive protective mechanisms, known to occur in acute hypoglycemia. The issue of increased cerebral blood flow in the hypoglycemic fetus has not, to the best of our knowledge, been studied to date. Thus, we cannot assume that the fetal brain would adapt itself by increasing blood flow. Our data led us to suggest an alternative protective mechanism in the fetal brain. The drop in fetal cerebral lactate levels suggests that in order to maintain normal brain function during hypoglycemia, lactate oxidation is increased, since the fetal brain has the abillity to oxidize other substrates, besides glucose. The lack of effect of hypoglycemia on maternal lactate levels is expected since, in contrast to the fetal brain, under normal and acute glucose deficiency, the adult brain lacks the ability to oxidize alternative energy fuels. In the maternal brain, glucose, /3-HBA and lactate levels were not significantly altered. This may be explained by an increase in cerebral blood flow which provides glucose to the brain, sufficient to maintain normal glucose metabolism during acute hypoglycemia. l 3C labding of. crr&nl inetnbolitrs dwirlg acute hypoglycemia. Although cerebral levels of several Glucose flux via PC and PDH, reflected by its contribution to glutamate and glutamine synthesis in insulin-induced hypoglycemic pregnant and fetal rabbit brain.