Abstract
Objectives: Hypoxia promotes vasodilation of coronary arteries as a protective response to improve blood flow to the myocardium. Although diminished formation of ATP and enhanced phosphorylation/activation of AMP-activated protein kinase (AMPK) in vascular smooth muscle are known to mediate hypoxia-induced vasorelaxation, the contribution of glycogen toward AMPK phosphorylation and contractility remains unclear. Methods: Using surgically-removed endothelium-denuded human saphenous vein segments ex vivo, the present study has examined serotonin (5-HT)-induced smooth muscle contractility by isometric tension measurements and AMPK phosphorylation by immunoblot analysis under normoxic (95% O2/5% CO2) and hypoxic conditions (95% N2/5% CO2, 30 min). Results: Under normoxic glycogen-enriched conditions, the maximal contractile response (Emax) and sensitivity (pEC50) to 5-HT-induced contractility were 145 ± 3% and 6.7 ± 0.2, respectively (n = 4). Induction of hypoxia diminished the glycogen content by ~95% (3.7 ± 1 under hypoxia versus 73.6 ± 5 μg/mg protein under normoxia; n = 2). Importantly, glycogen depletion led to diminution in 5-HT-induced maximal contractility to 37 ± 11% with an accompanying exaggerated increase in AMPK phosphorylation, compared with normoxic conditions (n = 4). Inclusion of exogenous D-glucose (5.5 mM) prevented the exaggerated increase in AMPK phosphorylation thereby restoring 5-HT-induced maximal contractility to 125 ± 6% (n = 4). Parallel studies that included either L-glucose or 2-deoxy-D-glucose (non-metabolizable forms of glucose) did not show any changes in contractility or AMPK phosphorylation. Conclusion: The present findings suggest that depletion of smooth muscle glycogen during hypoxia may limit the availability of intracellular glucose in vein grafts, thereby enhancing AMPK phosphorylation to promote vasorelaxation.
Published Version
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