-AMP-activated protein kinase (AMPK) is a meta-bolic stress sensor present in all eukaryotes. A dominant missense mutation (R225Q) in pig PRKAG3, encoding the muscle-specific ␥3 isoform, causes a marked increase in glycogen content. To determine the functional role of the AMPK ␥3 isoform, we generated transgenic mice with skeletal muscle-specific expression of wild type or mutant (225Q) mouse ␥3 as well as Prkag3 knockout mice. Glycogen resynthesis after exercise was impaired in AMPK ␥3 knock-out mice and markedly enhanced in transgenic mutant mice. An AMPK activator failed to increase skeletal muscle glucose uptake in AMPK ␥3 knock-out mice, whereas contraction effects were preserved. When placed on a high fat diet, transgenic mutant mice but not knock-out mice were protected against excessive triglyceride accumulation and insulin resistance in skeletal muscle. Transfection experiments reveal the R225Q mutation is associated with higher basal AMPK activity and diminished AMP dependence. Our results validate the muscle-specific AMPK ␥3 isoform as a therapeutic target for prevention and treatment of insulin resistance.AMPK 1 is a heterotrimeric serine/threonine protein kinase composed of a catalytic ␣ subunit and non-catalytic ␤ and ␥ subunits (1, 2). The mammalian genome contains seven AMPK genes encoding two ␣, two ␤, and three ␥ isoforms. AMPK signaling is elicited by cellular stresses that deplete ATP (and consequently elevate AMP) by either inhibiting ATP production (e.g. hypoxia) or accelerating ATP consumption (e.g. muscle contraction). AMPK is activated allosterically by AMP and through phosphorylation of Thr 172 in the ␣ subunit by an upstream AMPK kinase, the tumor-suppressor protein kinase LKB1 (3, 4). AMPK is likely to be important for diverse functions in many cell types, but particular interest has been focused on elucidating the role of AMPK in the regulation of lipid and carbohydrate metabolism in skeletal muscle (5-10). AMPK activity has been correlated with an increase in glucose uptake and fatty acid oxidation and an inhibition of glycogen synthase activity and fatty acid synthesis. Exercise, as well as skeletal muscle contractions in vitro, leads to AMPK activation. Pharmacological activation of AMPK also can be achieved using 5-aminoimidazole-4-carboxamide-1-␤-D-ribonucleoside (AICAR). Once taken up by the cell, AICAR is phosphorylated to 5-aminoimidazole-4-carboxamide riboside monophosphate (ZMP) and mimics effects of AMP on AMPK (1, 2). AMPK function is closely related to glycogen storage. AMPK phosphorylates glycogen synthase in vitro (11) and co-immunoprecipitates with glycogen synthase and glycogen phosphorylase from skeletal muscle (12). Mutations of the ␥3 or ␥2 subunit, respectively, affect glycogen storage in pigs (13, 14) or glycogen storage associated with cardiac abnormalities in humans (15). The recent identification of a glycogen-binding domain in the AMPK ␤1 subunit provides a molecular relationship between AMPK and glycogen (16,17). The formation of heterotrimers appears to be...

Lower-body power output, estimated from vertical jump height and body mass, is a sensitive and field expedient measure that can be used to assess the influence of caloric deficit on physical performance after 8 wk of U.S. Army Ranger training. With severe weight loss (>or=13% of body mass), IGF-I and cortisol correlate more closely with soft-tissue tissue adaptations than does testosterone.

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Expression patterns of the three isoforms of the regulatory gamma-subunit of AMP-activated protein kinase (AMPK) were determined in various tissues from adult humans, mice, and rats, as well as in human primary muscle cells. Real-time PCR-based quantification of mRNA showed similar expression patterns in the three species and a good correlation with protein expression in mice and rats. The gamma3-isoform appeared highly specific to skeletal muscle, whereas gamma1 and gamma2 showed broad tissue distributions. Moreover, the proportion of white, type IIb fibers in the mouse and rat muscle samples, as indicated by real-time PCR quantification of Atp1b2 mRNA, showed a strong positive correlation with the expression of gamma3. In samples of white skeletal muscle, gamma3 clearly appeared to be the most abundant gamma-isoform. Differentiation of human primary muscle cells from myoblasts into multinucleated myotubes was accompanied by upregulation of gamma3 mRNA expression, whereas levels of gamma1 and gamma2 remained largely unchanged. However, even in these cultured myotubes, gamma2 was the most highly expressed isoform, indicating a considerable difference compared with adult skeletal muscle. Immunoblot analysis of mouse gastrocnemius and quadriceps muscle extracts precipitated with a gamma3-specific antibody showed that gamma3 was exclusively associated with the alpha2- and beta2-subunit isoforms. The observation that the AMPKgamma3 isoform is expressed primarily in white skeletal muscle, in which it is the predominant gamma-isoform, strongly suggests that gamma3 has a key role in this tissue.

The use of anterior cervical plating after one- and two-level ACDF with allograft cortical bone significantly enhanced arthrodesis. The improved fusion rate and negligible complication rate associated with anterior cervical plating are compelling factors justifying its use in the treatment of cervical spondylosis.

Neuregulin, a growth factor involved in myogenesis, has rapid effects on muscle metabolism. In a manner analogous to insulin and exercise, neuregulins stimulate glucose transport through recruitment of glucose transporters to surface membranes in skeletal muscle. Like muscle contraction, neuregulins have additive effects with insulin on glucose uptake. Therefore, we examined whether neuregulins are involved in the mechanism by which muscle contraction regulates glucose transport. We show that caffeine-induced increases in cytosolic Ca 2؉ mediate a metalloproteinase-dependent release of neuregulins, which stimulates tyrosine phosphorylation of ErbB4 receptors. Activation of ErbB4 is necessary for Ca 2؉ -derived effects on glucose transport. Furthermore, blockage of ErbB4 abruptly impairs contraction-induced glucose uptake in slow twitch muscle fibers, and to a lesser extent, in fast twitch muscle fibers. In conclusion, we provide evidence that contraction-induced activation of neuregulin receptors is necessary for the stimulation of glucose transport and a key element of energetic metabolism during muscle contraction.

Aims/hypothesis: AMP-activated protein kinase (AMPK) regulates metabolic adaptations in skeletal muscle. The aim of this study was to investigate whether AMPK modulates the expression of skeletal muscle genes that have been implicated in lipid and glucose metabolism under fed or fasting conditions. Methods: Two genetically modified animal models were used: AMPK γ3 subunit knockout mice (Prkag3 −/− ) and skeletal musclespecific transgenic mice (Tg-Prkag3225Q ) that express a mutant (R225Q) γ3 subunit. Levels of mRNA transcripts of genes involved in lipid and glucose metabolism in white gastrocnemius muscles of these mice (under fed or 16-h fasting conditions) were assessed by quantitative real-time PCR. Results: Wild-type mice displayed a coordinated increase in the transcription of skeletal muscle genes encoding proteins involved in lipid/oxidative metabolism (lipoprotein lipase, fatty acid transporter, carnitine palmitoyl transferase-1 and citrate synthase) and glucose metabolism (glycogen synthase and lactate dehydrogenase) in response to fasting. In contrast, these fasting-induced responses were impaired in Prkag3 −/− mice. The transcription of genes involved in lipid and oxidative metabolism was increased in the skeletal muscle of Tg-Prkag3 225Q mice compared with that in wild-type mice. Moreover, the expression of the genes encoding hexokinase II and 6-phosphofrucktokinase was decreased in Tg-Prkag3 225Q mice after fasting. Conclusions/ interpretation: AMPK is involved in the coordinated transcription of genes critical for lipid and glucose metabolism in white glycolytic skeletal muscle.

Cr supplementation augments repeated sprint cycle performance in the heat without altering thermoregulatory responses.