Biomedical subjects

Deborah A Berkich

Publications and source records attributed to Deborah A Berkich.

5 recordsLinked to original sources

PubMed2005

Analysis of glucose metabolism in diabetic rat retinas.

This study was conceived in an effort to understand cause and effect relationships between hyperglycemia and diabetic retinopathy. Numerous studies show that hyperglycemia leads to oxidative stress in the diabetic retinas, but the mechanisms that generate oxidative stress have not been resolved. Increased electron pressure on the mitochondrial electron transfer chain, increased generation of cytosolic NADH, and decreases in cellular NADPH have all been cited as possible sources of reactive oxygen species and nitrous oxide. In the present study, excised retinas from control and diabetic rats were exposed to euglycemic and hyperglycemic conditions. Using a microwave irradiation quenching technique to study retinas of diabetic rats in vivo, glucose, glucose-derived metabolites, and NADH oxidation/reduction status were measured. Studying excised retinas in vitro, glycolytic flux, lactate production, and tricarboxylic acid cycle flux were evaluated. Enzymatically assayed glucose 6-phosphate and fructose 6-phosphate were only slightly elevated by hyperglycemia and/or diabetes, but polyols were increased dramatically. Cytosolic NADH-to-NAD ratios were not elevated by hyperglycemia nor by diabetes in vivo or in vitro. Tricarboxylic acid cycle flux was not increased by the diabetic state nor by hyperglycemia. On the other hand, small increases in glycolytic flux were observed with hyperglycemia, but glycolytic flux was always lower in diabetic compared with control animals. An observed decrease in activity of glyceraldehyde-3-phosphate dehydrogenase may be partially responsible for slow glycolytic flux for retinas of diabetic rats. Therefore, it is concluded that glucose metabolism, downstream of hexokinase, is not elevated by hyperglycemia or diabetes. Metabolites upstream of glucose such as the sorbitol pathway (which decreases NADPH) and polyol synthesis are increased.

Animals↗

PubMed2004

Neuroglial metabolism in the awake rat brain: CO2 fixation increases with brain activity.

Glial cells are thought to supply energy for neurotransmission by increasing nonoxidative glycolysis; however, oxidative metabolism in glia may also contribute to increased brain activity. To study glial contribution to cerebral energy metabolism in the unanesthetized state, we measured neuronal and glial metabolic fluxes in the awake rat brain by using a double isotopic-labeling technique and a two-compartment mathematical model of neurotransmitter metabolism. Rats (n = 23) were infused simultaneously with 14C-bicarbonate and [1-13C]glucose for up to 1 hr. The 14C and 13C labeling of glutamate, glutamine, and aspartate was measured at five time points in tissue extracts using scintillation counting and 13C nuclear magnetic resonance of the chromatographically separated amino acids. The isotopic 13C enrichment of glutamate and glutamine was different, suggesting significant rates of glial metabolism compared with the glutamate-glutamine cycle. Modeling the 13C-labeling time courses alone and with 14C confirmed significant glial TCA cycle activity (V(PDH)((g)), approximately 0.5 micromol x gm(-1) x min(-1)) relative to the glutamate-glutamine cycle (V(NT)) (approximately 0.5-0.6 micromol x gm(-1) x min(-1)). The glial TCA cycle rate was approximately 30% of total TCA cycle activity. A high pyruvate carboxylase rate (V(PC), approximately 0.14-0.18 micromol x gm(-1) x min(-1)) contributed to the glial TCA cycle flux. This anaplerotic rate in the awake rat brain was severalfold higher than under deep pentobarbital anesthesia, measured previously in our laboratory using the same 13C-labeling technique. We postulate that the high rate of anaplerosis in awake brain is linked to brain activity by maintaining glial glutamine concentrations during increased neurotransmission.

Adenosine Triphosphate↗

PubMed2003

Mitochondrial membrane potentials in ischemic hearts.

Excised rat hearts were perfused isovolumically and then made globally ischemic for times varying from 0 to 70 min followed by 50 min of reperfusion. In situ mitochondrial electrical potential gradients (Deltapsi(m)) were measured during reperfusion using the lipophilic cation, 3H-tetraphenylphosphonium. Therefore, it was possible to measure the relationships between mechanical performance, Deltapsi(m), and high energy phosphates as a function of time of ischemia. The absolute value of Deltapsi(m) remained constant and then dropped sharply in parallel with mechanical performance after 35 min of ischemia. Eliminating Ca2+ from the reperfusate medium did not preserve Deltapsi(m) nor increase high energy phosphates during the recovery period. An inhibitor of the mitochondrial permeability transition, cyclosporin A, delayed the fall in Deltapsi(m) but did not eliminate it. The data suggest that the mitochondrial permeability transition plays a role in ischemic cell death but is not triggered by influx of Ca2+ through the plasma membrane.

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PubMed2002

Thyroid hormone regulation of cardiac bioenergetics: role of intracellular creatine.

The effect of thyroid hormone (T(3)) on the content of myocardial creatine (Cr), Cr phosphate (CrP), and high-energy adenine nucleotides and on cardiac function was examined. In the hearts of control and T(3)-treated rats perfused in vitro, while "low" and "high" contractile work was performed, T(3) treatment resulted in a approximately 50% reduction in CrP, Cr, total Cr content (Cr + CrP), and in the CrP-to-Cr ratio. In addition, there was a slight fall in myocardial content of ATP and a large rise in calculated free ADP (fADP), resulting in a significant decrease in the ATP-to-fADP ratio in the hearts of hyperthyroid compared with euthyroid rats. Moreover, there was a substantial decrease in the level of ATP in hearts of T(3)-treated rats under high work conditions. Importantly, the ratio of cardiac work to oxygen consumption was not altered by thyroid status. Treatment with T(3) also resulted in an almost threefold reduction in the content of Na(+)/Cr transporter mRNA in the ventricular myocardium and skeletal muscle but not in the brain. We conclude with the following: 1) changes in the expression of the Na(+)/Cr transporter mRNA correlate with Cr + CrP in the myocardium; 2) hearts of hyperthyroid rats contain lower levels of ATP and higher levels of fADP under both low and high work conditions but no reduction in efficiency of work output; and 3) the reduction in Cr and ATP in hearts of hyperthyroid rats may be the basis for the reduced maximal work capacity of the hyperthyroid heart.

Adenosine Diphosphate↗

PubMedDate not supplied

Evaluation of brain mitochondrial glutamate and alpha-ketoglutarate transport under physiologic conditions.

Some models of brain energy metabolism used to interpret in vivo (13)C nuclear magnetic resonance spectroscopic data assume that intramitochondrial alpha-ketoglutarate is in rapid isotopic equilibrium with total brain glutamate, most of which is cytosolic. If so, the kinetics of changes in (13)C-glutamate can be used to predict citric acid cycle flux. For this to be a valid assumption, the brain mitochondrial transporters of glutamate and alpha-ketoglutarate must operate under physiologic conditions at rates much faster than that of the citric acid cycle. To test the assumption, we incubated brain mitochondria under physiologic conditions, metabolizing both pyruvate and glutamate and measured rates of glutamate, aspartate, and alpha-ketoglutarate transport. Under the conditions employed (66% of maximal O(2) consumption), the rate of synthesis of intramitochondrial alpha-ketoglutarate was 142 nmol/min.mg and the combined initial rate of alpha-ketoglutarate plus glutamate efflux from the mitochondria was 95 nmol/min.mg. It thus seems that much of the alpha-ketoglutarate synthesized within the mitochondria proceeds around the citric acid cycle without equilibrating with cytosolic glutamate. Unless the two pools are in such rapid exchange that they maintain the same percent (13)C enrichment at all points, (13)C enrichment of glutamate alone cannot be used to determine tricarboxylic acid cycle flux. The alpha-ketoglutarate pool is far smaller than the glutamate pool and will therefore approach steady state faster than will glutamate at the metabolite transport rates measured.

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