A molecular mechanism for frequency dependent skeletal muscle fiber plasticity

Abstract

Frequency of stimulation determines slow or fast fiber type gene expression in muscle.Fibers isolated from mice flexor digitorium brevis were stimulated at 20 and 90 Hz (270, 0. 3 ms pulses). We analyzed gene expression (qPCR), ATP release (luciferase), IP3 production (ELISA) and calcium signals in Fluo‐4 loaded fibers.At 20 Hz there was an important, biphasic release of ATP from myofibers which was inhibited by blockers of pannexin channels and dihydropyridine receptors. This stimulation pattern induced a sequential biphasic IP3 increase; mRNA levels for slow troponin I (TnIs) increased and fast troponin I (TnIf) decreased. These changes were inhibited by apyrase, carbenoxolone and IP3R blockers (xestospongin B). We also saw an increase of citrate synthase and a decrease of enolase mRNAs, not inhibited by xestospongin B. At 90 Hz there was no ATP release and IP3 production was much reduced. In these conditions there was an IP3R‐independent increase in TnIf mRNA levels and a decrease in TnIs.Upon potassium depolarization, IP3‐dependent calcium signals at membrane potential values more negative than those described for calcium release from RyR, associate to an increase in mRNA of the slow isoform of TroponinI.Our data supports a mechanism involving frequency‐dependent ATP release and activation of P2Y receptors to induce the slow fiber phenotype in skeletal muscle.FONDECYT 11108467, FONDAP 15010006