The Changes of [Ca2+] in Sr and Ca2+ Release Flux during Fatiguing Activation of Mouse Skeletal Muscle Fibers

Abstract

Sustained activity leads to muscle fatigue, cellular correlates of which include depression in Ca2+ released per action potential (Gyorke, JPhysiol, 1993; Westerblad, JAppPhysiol, 1993). This decrease is also observed in cells activated with voltage clamp pulse trains (Royer, JGP, 2010), which points at effectors downstream from the voltage sensor, including depletion of SR Ca2+ (Allen, JAppPhysiol, 2011). In mouse FDB cells expressing the biosensor D4cpv-calsequestrin loaded with X-rhod1-AM or in mice constitutively expressing the troponin-based FRET biosensor TnXX (Mank, NatureM, 2008) we measure [Ca2+]SR, [Ca2+]cyto and release flux. Fatiguing activation is produced by 0.5s trains of action potentials at 2s intervals. Concentrations were measured before the tetanus (rest) or at its end (tet) in the rested condition (t0), upon 1 or 3 min of stimulation (t1,t3) and after 10 min recovery (A10). Flux was calculated as amount released/tetanus duration. Further analysis of this quantitative picture of Ca2+ handling during fatigue indicates that the reduction in Ca2+ release is the outcome of interconnected decreases in SR load, Ca2+ release permeability and SR Ca2+ buffering power.Funded by NIAMS and EU Framework7 Program.View Large Image | View Hi-Res Image | Download PowerPoint Slide