Exercise‐induced modulation of calcineurin activity parallels the time course of myofibre transitions

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Abstract
This study establishes a causal link between the limitation of myofibre transitions and modulation of calcineurin activity, during different exercise paradigms. We have designed a new swimming‐based training protocol in order to draw a comparison between a high frequency and amplitude exercise (swimming) and low frequency and amplitude exercise (running). We initially analysed the time course of muscle adaptations to a 6‐ or 12‐week swimming‐ or running‐based training exercise program, on two muscles of the mouse calf, the slow‐twitch soleus and the fast‐twitch plantaris. The magnitude of exercise‐induced muscle plasticity proved to be dependent on both the muscle type and the exercise paradigm. In contrast to the running‐based training which generated a continuous increase of the slow phenotype throughout a 12‐week training program, swimming induced transitions to a slower phenotype which ended after 6 weeks of training. We then compared the time course of the exercise‐induced changes in calcineurin activity during muscle adaptation to training. Both exercises induced an initial activation followed by the inhibition of calcineurin. In the muscles of animals submitted to a 12‐week swimming‐based training, this inhibition was concomitant with the end of myofibre transition. Calcineurin inhibition was a consequence of the inhibition of its catalytic subunit gene expression on one hand, and of the expression increase of the modulatory calcineurin interacting proteins 1 gene (MCIP1), on the other. The present study provides the first experimental cues for an interpretation of muscle phenotypic variation control. J. Cell. Physiol. 214:126–135, 2008.