Metabolic activation of AMP kinase in vascular smooth muscle

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Abstract
AMP-activated kinase (AMPK) is a highly conserved heterotrimeric kinase that functions as a metabolic master switch to coordinate cellular enzymes involved in carbohydrate and fat metabolism that regulate ATP conservation and synthesis. AMPK is activated by conditions that increase AMP-to-ATP ratio, such as exercise and metabolic stress. In the present study, we probed whether AMPK was expressed in vascular smooth muscle and would be activated by metabolic stress. Endothelium-denuded porcine carotid artery segments were metabolically challenged with 2-deoxyglucose (10 mM) plus N2 (N2-2DG). These vessels exhibited a rapid increase in AMPK activity by 1 min that was near maximal by 20 min. AMPK inactivation on return to normal physiological saline was ∼50% in 1 min and fully recovered by 5 min. Immunoprecipitation of the α1- and α2-catalytic subunit followed by immunoblot analysis for [P]Thr172-AMPK indicates that α1-AMPK accounts for all activity. Little if any α2-AMPK was detected in carotid smooth muscle. AMPK activity was not increased by contractile agonist (endothelin-1) or by the reported AMPK activators 5-aminoimidazole-4-carboxamide ribofuranoside (2 mM), metformin (2 mM), or phenformin (0.2 mM). AMPK activation by N2-2DG was associated with a rapid and pronounced reduction in endothelin-induced force and reduced phosphorylation of Akt and Erk 1/2. These data demonstrate that AMPK expression differs in vascular smooth muscle compared with striated muscles and that activation and inactivation after metabolic stress occur rapidly and are associated with signaling pathways that may regulate smooth-muscle contraction