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Glucocorticoid and its effect on cardiac glucose utilization Puthanveetil, Prasanth Nair

Abstract

Glycogen is an immediate source of glucose for cardiac tissue to maintain its metabolic homeostasis. However, its excess brings about cardiac structural and physiological impairments. Previously, we have demonstrated that in hearts from dexamethasone (DEX) treated animals, glycogen accumulation was enhanced. We examined the influence of DEX on glucose entry and glycogen synthase as a means of regulating the accumulation of this stored polysaccharide. Following DEX, cardiac tissue had limited contribution towards the development of whole body insulin resistance. Measurement of GLUT4 at the plasma membrane revealed an excess presence of this transporter protein at this location. Interestingly, this was accompanied by an increase in GLUT4 in the intracellular membrane fraction, an effect that was well correlated to an increased GLUT4 mR.NA. Both total and phosphorylated AMPK increased following DEX. Immunoprecipitation of AS 160 followed by Western blotting demonstrated no change in Akt phosphorylation at Ser473 and Thr308 in DEX treated hearts. However, there was a significant increase in AMPK phosphorylation at Thr172, which correlated well with AS 160 phosphorylation. In DEX hearts, there was a considerable reduction in the phosphorylation of glycogen synthase, whereas GSK-3-β phosphorylation was augmented. Our data suggest that AMPK mediated glucose entry, combined with activation of glycogen synthase and reduction in glucose oxidation (Qi, D., et al. Diabetes 53:1790, 2004), act together to promote glycogen storage. Our data suggest that in the presence of intact insulin signaling, AMPK mediated glucose entry, combined with activation of glycogen synthase and the previously reported reduction in glucose oxidation, act together to promote glycogen storage. Should these effects persist chronically, they may explain the increased morbidity and mortality observed with long term excesses in endogenous or exogenous glucocorticoids.

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