Effects of chronic AICAR administration on the metabolic and contractile phenotypes of rat slow- and fast-twitch skeletal muscles

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Abstract
The present study examined the effects of chronic activation of 5'-AMP-activated protein kinase (AMPK) on the oxidative capacity and myosin heavy chain (MHC) based fibre phenotype of rodent fast- and slow-twitch muscles. Sprague–Dawley rats received daily injections for 4 weeks of the known AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) or vehicle (control). The AICAR group displayed increases in hexokinase-II (HXK-II) activity, expression, and phosphorylation in fast-twitch muscles (P < 0.001) but not in the slow-twitch soleus (SOL). In the AICAR group, citrate synthase (EC 4.1.3.7) and 3-hydroxyacyl-CoA-dehydrogenase (EC 1.1.1.35) were elevated 1.6- and 2.1-fold (P < 0.05), respectively, in fast-twitch medial gastrocnemius (MG), and by 1.2- and 1.4-fold (P < 0.05) in the slower-twitch plantaris (PLANT). No changes were observed in the slow-twitch SOL. In contrast, the activity of glyceraldehyde phosphate dehydrogenase (EC 1.2.1.12) remained unchanged in all muscles. AICAR treatment did not alter the MHC-based fibre type composition in fast- or slow-twitch muscles, as determined by immunohistochemical and electrophoretic analytical methods or by RT–PCR. We conclude that chronic activation of AMPK mimics the metabolic changes associated with chronic exercise training (increased oxidative capacity) in the fast-twitch MG and PLANT, but does not coordinately alter MHC isoform content or mRNA expression.Key words: AMP-activated protein kinase, myosin heavy chain, metabolism, RT–PCR, SDS–PAGE.

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