Effects of anoxia and depolarization on the movement of carbon atoms derived from glucose into macromolecular fractions in rat brain slices

Formation of glycerol from glucose in rat brain and cultured brain cells. Augmentation with kainate or ischemia

An increase in the concentration of glycerol in the ischemic brain is assumed to reflect degradation of phospholipids of plasma membranes. However, glycerol could, theoretically, be formed from glucose, which after glycolytic conversion to dihydroxyacetone phosphate, could be converted to glycerol-3-phosphate and hence to glycerol. We show here that (13)C-labeled glycerol accumulate in incubation media of cultured cerebellar granule cells and astrocytes incubated with [(13)C]glucose, 3 mmol/L, demonstrating the formation of glycerol from glucose. Co-incubation of cerebellar granule cells with kainate, 50 micromol/L, led to increased glucose metabolism and increased accumulation of [(13)C]glycerol. Accumulation of [(13)C]glycerol and its precursor, [(13)C]glycerol-3-phosphate, was evident in brain, but not in serum, of kainate-treated rats that received [U-(13)C]glucose, 5 micromol/g bodyweight, intravenously and survived for 5 min. Global ischemia induced by decapitation also caused accumulation of [(13)C]glycerol and [(13)C]glycerol-3-phosphate. These results show that glycerol can be formed from glucose in brain; they also demonstrate the existence of a cerebral glycerol-3-phosphatase activity. Ischemia-induced increases in brain glycerol may, in part, reflect an altered metabolism of glucose, in which glycerol formation, like lactate formation, acts as a redox sink.

Protein kinase C as an early and sensitive marker of ischemia-induced progressive neuronal damage in gerbil hippocampus

In the model of transient brain ischemia of 6-min duration in gerbils we have estimated: 1. The concentration of brain gangliosides: A significant decrease to about 70% of control was observed selectively in the hippocampus at 3 and 7 d after ischemia. 2. The activity of Na+,K(+)-ATPase: The enzyme activity was not affected in either hippocampus nor in cerebral cortex. 3. The malonaldehyde (MDA) concentration: The levels of MDA had increased at 30 min after ischemia up to 123 and 129% of control in hippocampus and cerebral cortex, respectively. 4. Immunoreactivity of protein kinase C detected by Western blotting: In hippocampus the early translocation toward membranes was followed by a decrease in total enzyme content at 6, 24, 72, and 96 h of postischemic recovery. Also, a sharp increase of 50 kDa isoform (PKM) was noticed immediately and at the early recovery times. The behavior of these biochemical markers of ischemic brain injury in the hippocampus after the short (6 min) insult was contrasted with their reaction in the cerebral cortex as well as after prolongation of the ischemia to 15 min. These results taken together indicate that an early increase in PKC translocation followed by a decrease is the most symptomatic for selective, delayed, postischemic hippocampal injury, resulting from short duration (6 min) ischemia of the gerbil brain.

Hexose monophosphate pathway activity in normal and hypoxic rat brain

Experiments were undertaken to define the role of the alternative route of glucose metabolism in the hexose monophosphate pathway (HMP) during energy balanced, mild brain hypoxia. In similar hypoxic model in spite of the lack of the deficit of high energy compounds, the significant acceleration of glycolysis and inhibition of macromolecular syntheses (lipid, proteins, and nucleic acids) were previously observed. The HMP activity, although directly coupled to intracellular synthetic processes, has not been defined and little is known about the mechanisms of its regulation under brain hypoxia. HMP activity was examined in the rat brain in vivo by estimation of the increment of 6-phosphogluconate concentration after inhibition of its oxidation as achieved by injection of 6-aminonicotinamide. The activity of this alternative route of glucose metabolism was estimated to be 0.4 mmol/h/kg w.w. in the brain cortex and 0.7 mmol/h/kg w.w. in the brain stem. During 2 h of mild hypoxia (7% O2 in N2) the HMP activity dropped to 30% of control level, whereas during first hour of reoxygenation increased to 200% of control. Increased activity of HMP in posthypoxic brain during reoxygenation also was observed in vitro by measuring the rate of [1-14C]- and [6-14C]glucose conversion to 14CO2 in cerebral cortical slices. The possible mechanism of the rapid changes in the activity of HMP induced by hypoxia is discussed. The results suggest that the brain glucose metabolism under mild hypoxia is reoriented toward energy producing pathway (glycolysis) partially at the expense of HMP. The mechanism of this regulation seems not to be directly triggered by energy deficit. Activity of HMP in the brain is in accord with the intracellular synthetic processes and their demands on the metabolites produced by this pathway. Relying upon that, the posthypoxic stimulation of HMP would indicate the metabolic recovery during reoxygenation.

Metabolic studies on dysmyelinating mutant "pt" rabbit brain in vitro

"Paralytic tremor" (pt) rabbit mutant is characterized by a severe hypomyelination of the CNS, however, it is not defined if the defect in myelinogenesis is an "assembly" or "synthesis" type. In this study, we have compared the general metabolic and biosynthetic properties of the myelinating mutant brain with unaffected controls of the same age. In the brain slices of 4 wk old "pt" rabbits the incorporation of U-[14C]glucose, 6-[3H] galactose, and U-[14C] leucine into macromolecules (total lipids and proteins, galactolipids, and myelin basic protein) was substantially elevated. In isolated myelin fraction, the total reduction of the radioactivity was followed by the increased specific activity of all examined macromolecules. The myelin to homogenate specific activity ratio was similar in control and "pt" rabbits. Distribution of the label and myelin marker, cyclic nucleotide 3'-phosphodiesterase (CNP-ase) among the membranous fractions suggests the partial inhibition of myelin formation in "pt" rabbits on the step of premyelin, unilamellar membranes. 14CO2 yields derived from differently labeled glucose were used for the evaluation of the basal oxidative metabolism in "pt" brain slices. 14CO2 production from U-[14C] glucose was normal. The depolarization of the slices by 50 mM K+ stimulated glucose oxidation to a higher extent in "pt" than in control. Hexose monophosphate pathway (HMP), the route providing much of NADPH required for lipid biosynthesis, did not change significantly by mutation. The activity of glucose 6-phosphate dehydrogenase (Glc-6-P DH), an oligodendroglia enriched, HMP connected enzyme, was slightly lower in "pt" homogenates by 13-17%, whereas CNP-ase was lowered more than 30% in the same samples. All this data suggest that the capacity for the synthesis of myelin constituents is well preserved in the mutant brain and the impairment of myelogenesis is probably caused by increased elimination of already synthesized, myelin-related components.

Patterns of deoxyglucose and glucose labeling in the optic tectum of monocularly stimulated bass

Uniformly labeled deoxyglucose and glucose were used to examine patterns of altered metabolic activity in the optic tectum of largemouth bass. Autoradiographs from fish which viewed moving vertical stripes with one eye show that the metabolites of the two sugars procedure similar patterns of activity-related labeling in the tectum: tangentially arranged bands of increased optical density through the SFGS and the SGC. In addition, aldehyde fixation was found to improve the histological quality of the sections without altering the patterns of labeling.

The effect of hypoxia on the metabolism of labeled glucose and acetate in the rat brain

Glucose consumption and utilization of amino acids, lipids and proteins was measured in the rat brain under normoxia and hypoxia (7%O2:93%N2). After 2 h of hypoxia the brain glucose consumption increased by over 60% of control value. In spite of this increase, the radioactivity of amino acid fraction did not increase or parallel changes of glucose radioactivity in the blood. This strongly suggested that glucose flux into amino acids remained unchanged in hypoxia. Incorporation of 14C from glucose into macromolecules was found to decrease. The above changes demonstrated that the metabolic steps which precede synthesis of amino and tricarboxylic acids were inhibited. In some experiments, the incorporation of 14C from [2-14C]-acetate into a macromolecular fraction was also measured. The amounts of radioactivity found in these fractions were unchanged under hypoxic conditions. Possible differences in the influence of hypoxia on macromolecular synthesis in different metabolic compartments of the brain are discussed.