Store-Operated Ca2+ Entry is a Tonic and Phasic Signal in Skeletal Muscle

Abstract

Store-operated Ca2+ entry (SOCE) is activated very rapidly in skeletal muscle upon depletion of [Ca2+]SR below an activation threshold due to a prepositioning of Orai1 and STIM1L uniformly throughout the junctional membranes (Edwards et al 2010, Cell Calcium, Darbellay et al, 2011, J Cell Biol). Its physiological role appears to be signalling, not refilling the sarcoplasmic reticulum (SR) (Launikonis et al 2010, Pflugers Arch). If this is the case then the rate, amplitude and frequency of SOCE during the release of SR Ca2+ is important. To examine SOCE kinetics during SR Ca2+ release we imaged cytoplasmic rhod-2 with fluo-5N inside t-system or SR of skinned fibres from mouse fast-twitch muscle, to measure Ca2+ release with [Ca2+]t-sys or [Ca2+]SR, respectively. Ca2+ release was induced by lowering [Mg2+]cyto, causing a slow Ca2+ release. Step changes in -d[Ca2+]SR/dt and -d[Ca2+]SR/dt during cell-wide Ca2+ transients and waves were observed. Imaging of SR fluo-5N indicated three states of SR Ca2+-buffering, dropping in a stepwise manner because the SR Ca2+-buffer calsequestrin (CSQ) reduces its aggregation state and affinity for Ca2+ with the progressively lowered total SR calcium, allowing [Ca2+]SR to change more freely with each step (Launikonis et al 2006, PNAS). SOCE was active in the phases of reduced CSQ aggregation. SOCE was active in the two latter phases and its rate was always proportional to -d[Ca2+]SR/dt. There was a tonic response during the cell-wide transient, then a fast, phasic SOCE response during repetitive Ca2+ waves. Phasic SOCE signal amplitude could vary by a factor of 10 but was always proportional to depletion of [Ca2+]SR. Our results show that the kinetics of SOCE signals are tightly regulated by [Ca2+]SR and we predict that these signals can be decoded by the muscle for gene expression.