Ultra-rapid activation and deactivation of store-operated Ca2+ entry in skeletal muscle

Skeletal muscle is highly specialized for the rapid delivery of Ca2+ to the contractile apparatus during excitation–contraction coupling (EC coupling). Previous studies have shown the presence of a relatively fast-activated store-operated Ca2+ entry (SOCE) mechanism (<1 s) to be present in skeletal muscle, unlike the situation occurring in non-excitable cells. We simultaneously imaged [Ca2+] in the t-system and cytoplasm in mechanically skinned fibers during SR Ca2+ release and observed both cell-wide Ca2+ release and Ca2+ waves. SOCE activation followed cell-wide Ca2+ release from high sarcoplasmic reticulum (SR) [Ca2+] ([Ca2+]SR) by seconds, consistent with depletion of [Ca2+]SR to an absolute threshold for SOCE and an unformed SOCE complex at high [Ca2+]SR. Ca2+ waves occurred at low [Ca2+]SR, close to the threshold for SOCE, minimizing the time between Ca2+ release and Ca2+ influx. Local activation of SOCE during Ca2+ waves occurred in ∼27 ms following local initiation of SR depletion indicating a steep relationship between [Ca2+]SR and SOCE activation. Most of this delay was due to slow release of Ca2+ from SR, leaving only milliseconds at most for the activation of Ca2+ entry following store depletion. SOCE was also observed to deactivate effectively instantly during store refilling at low [Ca2+]SR. These rapid kinetics of SOCE persisted as subsequent Ca2+ waves propagated along the fiber. Thus we show for the first time millisecond activation and deactivation of SOCE during low amplitude [Ca2+]SR oscillations at low [Ca2+]SR. To account for the observed Ca2+ movements we propose the SOCE complex forms during the progressive depletion of [Ca2+]SR prior to reaching the activation threshold of SOCE and this complex remains stable at low [Ca2+]SR.