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      Contraction-stimulated glucose transport in muscle is controlled by AMPK and mechanical stress but not sarcoplasmatic reticulum Ca 2+ release

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          Abstract

          Understanding how muscle contraction orchestrates insulin-independent muscle glucose transport may enable development of hyperglycemia-treating drugs. The prevailing concept implicates Ca 2+ as a key feed forward regulator of glucose transport with secondary fine-tuning by metabolic feedback signals through proteins such as AMPK. Here, we demonstrate in incubated mouse muscle that Ca 2+ release is neither sufficient nor strictly necessary to increase glucose transport. Rather, the glucose transport response is associated with metabolic feedback signals through AMPK, and mechanical stress-activated signals. Furthermore, artificial stimulation of AMPK combined with passive stretch of muscle is additive and sufficient to elicit the full contraction glucose transport response. These results suggest that ATP-turnover and mechanical stress feedback are sufficient to fully increase glucose transport during muscle contraction, and call for a major reconsideration of the established Ca 2+ centric paradigm.

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          Most cited references 54

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          A role for AMP-activated protein kinase in contraction- and hypoxia-regulated glucose transport in skeletal muscle.

          Eukaryotic cells possess systems for sensing nutritional stress and inducing compensatory mechanisms that minimize the consumption of ATP while utilizing alternative energy sources. Such stress can also be imposed by increased energy needs, such as in skeletal muscle of exercising animals. In these studies, we consider the role of the metabolic sensor, AMP-activated protein kinase (AMPK), in the regulation of glucose transport in skeletal muscle. Expression in mouse muscle of a dominant inhibitory mutant of AMPK completely blocked the ability of hypoxia or AICAR to activate hexose uptake, while only partially reducing contraction-stimulated hexose uptake. These data indicate that AMPK transmits a portion of the signal by which muscle contraction increases glucose uptake, but other AMPK-independent pathways also contribute to the response.
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            AMP-activated protein kinase (AMPK) beta1beta2 muscle null mice reveal an essential role for AMPK in maintaining mitochondrial content and glucose uptake during exercise.

            AMP-activated protein kinase (AMPK) β1 or β2 subunits are required for assembling of AMPK heterotrimers and are important for regulating enzyme activity and cellular localization. In skeletal muscle, α2β2γ3-containing heterotrimers predominate. However, compensatory up-regulation and redundancy of AMPK subunits in whole-body AMPK α2, β2, and γ3 null mice has made it difficult to determine the physiological importance of AMPK in regulating muscle metabolism, because these models have normal mitochondrial content, contraction-stimulated glucose uptake, and insulin sensitivity. In the current study, we generated mice lacking both AMPK β1 and β2 isoforms in skeletal muscle (β1β2M-KO). β1β2M-KO mice are physically inactive and have a drastically impaired capacity for treadmill running that is associated with reductions in skeletal muscle mitochondrial content but not a fiber-type switch. Interestingly, young β1β2M-KO mice fed a control chow diet are not obese or insulin resistant but do have impaired contraction-stimulated glucose uptake. These data demonstrate an obligatory role for skeletal muscle AMPK in maintaining mitochondrial capacity and contraction-stimulated glucose uptake, findings that were not apparent in mice with single mutations or deletions in muscle α, β, or γ subunits.
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              Knockout of the alpha2 but not alpha1 5'-AMP-activated protein kinase isoform abolishes 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranosidebut not contraction-induced glucose uptake in skeletal muscle.

              We investigated the importance of the two catalytic alpha-isoforms of the 5'-AMP-activated protein kinase (AMPK) in 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) and contraction-induced glucose uptake in skeletal muscle. Incubated soleus and EDL muscle from whole-body alpha2- or alpha1-AMPK knockout (KO) and wild type (WT) mice were incubated with 2.0 mm AICAR or electrically stimulated to contraction. Both AICAR and contraction increased 2DG uptake in WT muscles. KO of alpha2, but not alpha1, abolished AICAR-induced glucose uptake, whereas neither KO affected contraction-induced glucose uptake. AICAR and contraction increased alpha2- and alpha1-AMPK activity in wild type (WT) muscles. During AICAR stimulation, the remaining AMPK activity in KO muscles increased to the same level as in WT. During contraction, the remaining AMPK activity in alpha2-KO muscles was elevated by 100% probably explained by a 2-3-fold increase in alpha1-protein. In alpha1-KO muscles, alpha2-AMPK activity increased to similar levels as in WT. Both interventions increased total AMPK activity, as expressed by AMPK-P and ACCbeta-P, in WT muscles. During AICAR stimulation, this was dramatically reduced in alpha2-KO but not in alpha1-KO, whereas during contraction, both measurements were essentially similar to WT in both KO-muscles. The results show that alpha2-AMPK is the main donor of basal and AICAR-stimulated AMPK activity and is responsible for AICAR-induced glucose uptake. In contrast, during contraction, the two alpha-isoforms seem to substitute for each other in terms of activity, which may explain the normal glucose uptake despite the lack of either alpha2- or alpha1-AMPK. Alternatively, neither alpha-isoform of AMPK is involved in contraction-induced muscle glucose uptake.
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                Author and article information

                Contributors
                Journal
                Mol Metab
                Mol Metab
                Molecular Metabolism
                Elsevier
                2212-8778
                28 July 2014
                28 July 2014
                October 2014
                : 3
                : 7
                : 742-753
                Affiliations
                Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, University of Copenhagen, 2100 Copenhagen, Denmark
                Author notes
                []Corresponding author. Tel.: +45 30593437. TEJensen@ 123456ifi.ku.dk
                [1]

                Present address: Joint Division Molecular Metabolic Control, DKFZ-ZMBH Alliance, German Cancer Research Center (DKFZ) Heidelberg, Center for Molecular Biology (ZMBH) and University Hospital, University of Heidelberg, 69120 Heidelberg, Germany.

                [2]

                Present address: Novo Nordisk, Clinical Pharmacology Diabetes Unit, Søborg, Denmark.

                Article
                S2212-8778(14)00131-8
                10.1016/j.molmet.2014.07.005
                4209358
                25353002
                © 2014 The Authors

                This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

                Categories
                Original Article

                exercise, skeletal muscle, ca2+, ampk, stretch

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