Calcineurin Does Not Mediate Exercise-Induced Increase in Muscle GLUT4

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Abstract
Exercise induces a rapid increase in expression of the GLUT4 isoform of the glucose transporter in skeletal muscle. One of the signals responsible for this adaptation appears to be an increase in cytosolic Ca2+. Myocyte enhancer factor 2A (MEF2A) is a transcription factor that is involved in the regulation of GLUT4 expression. It has been reported that the Ca2+-regulated phosphatase calcineurin mediates the activation of MEF2 by exercise. It has also been shown that the expression of activated calcineurin in mouse skeletal muscle results in an increase in GLUT4. These findings suggest that increases in cytosolic Ca2+ induce increased GLUT4 expression by activating calcineurin. However, we have obtained evidence that this response is mediated by a Ca2+-calmodulin−dependent protein kinase. The purpose of this study was to test the hypothesis that calcineurin is involved in mediating exercise-induced increases in GLUT4. Rats were exercised on 5 successive days using a swimming protocol. One group of swimmers was given 20 mg/kg body weight of cyclosporin, a calcineurin inhibitor, 2 h before exercise. A second group was given vehicle. GLUT4 protein was increased ∼80%, GLUT4 mRNA was increased ∼2.5-fold, MEF2A protein was increased twofold, and hexokinase II protein was increased ∼2.5-fold 18 h after the last exercise bout. The cyclosporin treatment completely inhibited calcineurin activity but did not affect the adaptive increases in GLUT4, MEF2A, or hexokinase expression. We conclude that calcineurin activation does not mediate the adaptive increase in GLUT4 expression induced in skeletal muscle by exercise.