Publications (2)59.65 Total impact
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ABSTRACT: High extracellular glutamate concentrations have been identified as a likely trigger of epileptic seizures in mesial temporal lobe epilepsy (MTLE), but the underlying mechanism remains unclear. We investigated whether a deficiency in glutamine synthetase, a key enzyme in catabolism of extracellular glutamate in the brain, could explain the perturbed glutamate homoeostasis in MTLE. The anteromedial temporal lobe is the focus of the seizures in MTLE, and surgical resection of this structure, including the hippocampus, leads to resolution of seizures in many cases. By means of immunohistochemistry, western blotting, and functional enzyme assays, we assessed the distribution, quantity, and activity of glutamine synthetase in the MTLE hippocampus. In western blots, the expression of glutamine synthetase in the hippocampus was 40% lower in MTLE than in non-MTLE samples (median 44 [IQR 30-58] vs 69 [56-87]% of maximum concentration in standard curve; p=0.043; n=8 and n=6, respectively). The enzyme activity was lower by 38% in MTLE vs non-MTLE (mean 0.0060 [SD 0.0031] vs 0.0097 [0.0042] U/mg protein; p=0.045; n=6 and n=9, respectively). Loss of glutamine synthetase was particularly pronounced in areas of the MTLE hippocampus with astroglial proliferation, even though astrocytes normally have high content of the enzyme. Quantitative immunoblotting showed no significant change in the amount of EAAT2, the predominant glial glutamate transporter in the hippocampus. A deficiency in glutamine synthetase in astrocytes is a possible molecular basis for extracellular glutamate accumulation and seizure generation in MTLE. Further studies are needed to define the cause, but the loss of glutamine synthetase may provide a new focus for therapeutic interventions in MTLE.
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ABSTRACT: Cardiac metabolism becomes more dependent on carbohydrates in congestive heart failure (CHF), and lactate may be used as an important respiratory substrate. Monocarboxylate transporter 1 (MCT1) promotes cotransport of lactate and protons into and out of heart cells and conceivably flux of lactate between cells, because it is abundantly present in the intercalated disk. Six weeks after induction of myocardial infarction (MI) in Wistar rats, left ventricular end-diastolic pressures were >15 mm Hg, signifying CHF. MCT1 and connexin43 protein levels in CHF were 260% and 20%, respectively, of those in sham-operated animals (Sham), and the corresponding mRNA signals were 181% and not significantly changed, respectively. Confocal laserscan immunohistochemistry and quantitative immunogold cytochemistry showed that MCT1 density was much higher in CHF than in Sham both at the surface membrane and in the intercalated disk. In CHF, a novel intracellular pool of MCT1 appeared to be associated with cisternae, some close to the T tubules. In contrast, connexin43 particles, seen exclusively at gap junctions, were substantially fewer. Maximum lactate uptake was 107+/-15 mmol. L(-1). min(-1) in CHF and 42+/-6 mmol. L(-1). min(-1) in Sham cells (P<0.05). The K(m) values were between 7 and 9 mmol/L (P=NS). In cardiomyocytes from CHF rats, (1) the amount of functional MCT1 in the sarcolemma, including in the intercalated disk, is increased several-fold; (2) a new intracellular pool of MCT1 appears; (3) another disk protein, connexin43, is much reduced; and (4) increased reliance on lactate and other monocarboxylates (eg, pyruvate) could provide tight metabolic control of high-energy phosphates.