Muscle contraction causes an increase in activity of 5'-AMP-activated protein kinase (AMPK). This study was designed to determine whether chronic chemical activation of AMPK will increase mitochondrial enzymes, GLUT-4, and hexokinase in different types of skeletal muscle of resting rats. In acute studies, rats were subcutaneously injected with either 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR; 1 mg/g body wt) in 0.9% NaCl or with 0.9% NaCl alone and were then anesthetized for collection and freezing of tissues. AMPK activity increased in the superficial, white region of the quadriceps and in soleus muscles but not in the deep, red region of the quadriceps muscle. Acetyl-CoA carboxylase (ACC) activity, a target for AMPK, decreased in all three muscle types in response to AICAR injection but was lowest in the white quadriceps. In rats given daily, 1 mg/g body wt, subcutaneous injections of AICAR for 4 wk, activities of citrate synthase, succinate dehydrogenase, and malate dehydrogenase were increased in white quadriceps and soleus but not in red quadriceps. Cytochrome c and delta-aminolevulinic acid synthase levels were increased in white, but not red, quadriceps. Carnitine palmitoyl-transferase and hydroxy-acyl-CoA dehydrogenase were not significantly increased. Hexokinase was markedly increased in all three muscles, and GLUT-4 was increased in red and white quadriceps. These results suggest that chronic AMPK activation may mediate the effects of muscle contraction on some, but not all, biochemical adaptations of muscle to endurance exercise training.

AMP-activated protein kinase (AMPK) is activated during muscle contraction in response to the increase in AMP and decrease in phosphocreatine (PCr). Once activated, AMPK has been proposed to phosphorylate a number of targets, resulting in increases in glucose transport, fatty acid oxidation, and gene transcription. Although it has been possible to directly observe phosphorylation of one of these targets, acetyl-CoA carboxylase (ACC) in vitro, it has been more difficult to obtain direct evidence of ACC phosphorylation in contracting skeletal muscle. In these experiments using a phosphoserine antibody to ACC and a phosphothreonine antibody to AMPK, evidence was obtained for phosphorylation and activation of ACC in vitro, in gastrocnemius muscle electrically stimulated at different frequencies, and in muscle from rats running on the treadmill. Significant negative linear correlations between phospho-ACC and ACC activity were observed in all models (P < 0.01). The decline in ACC activity was related to the decrease in PCr and the rise in AMP. A relationship between phospho-AMPK (threonine 172) and activity of AMPK immunoprecipitated with anti-alpha(2) subunit antibody preparation was also observed. These data provide the first evidence of a direct link between extent of phosphorylation of these proteins at sites recognized by the antibodies and activity of the enzymes in electrically stimulated muscle and in muscle of rats running on the treadmill.

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Evidence is accumulating for roles of AMP-activated protein kinase (AMPK) in controlling glucose uptake, fatty acid oxidation and gene expression in skeletal muscle. Relatively little is known, however, about the control of expression of the AMPK subunit isoforms. Marked differences are noted in subunit expression as a function of muscle fibre type. Expression of the γ 3 subunit isoform increases in fast-twitch red fibres of the rat in response to training. All subunit isoforms are expressed to a lesser extent in rats treated with propylthiouracil (PTU; an inhibitor of thyroid hormone synthesis) for 3 weeks compared with rats given excess thyroid hormones for 3 weeks. An approx. 2-fold increase in acetyl-CoA carboxylase was observed in gastrocnemius of hyperthyroid rats compared with experimentally hypothyroid rats. Thyroid state therefore appears to be one important factor controlling expression of these proteins in skeletal muscle.

This study was designed to determine whether the reductions in GLUT-4 seen in 3-day-denervated muscles can be prevented through chemical activation of 5'-AMP-activated protein kinase (AMPK). Muscle AMPK can be chemically activated in rats using subcutaneous injections with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). In this study, the tibial nerve was sectioned on one side; the other was sham operated but without nerve section. Acute injections of AICAR resulted in significantly increased AMPK activity in denervated gastrocnemius but not soleus muscles. Acetyl-CoA carboxylase activity, a reporter of AMPK activation, declined in both gastrocnemius and soleus in both denervated and contralateral muscles. Three days after denervation, GLUT-4 levels were significantly decreased by approximately 40% in gastrocnemius muscles and by approximately 30% in soleus muscles. When rats were injected with AICAR (1 mg/g body wt) for 3 days, the decline in GLUT-4 levels was prevented in denervated gastrocnemius muscles but not in denervated soleus muscles. The extent of denervation-induced muscle atrophy was similar in AICAR-treated vs. saline-treated rats. These studies provide evidence that some effects of denervation may be prevented by chemical activation of the appropriate signaling pathways.