Temperature and RyR1 Regulate the Activation Rate of Store-Operated Ca2+ Entry Current in Myotubes

Store-operated calcium entry (SOCE) is an important Ca2+ entry pathway in skeletal muscle. However, direct electrophysiological recording and full characterization of the underlying SOCE current in skeletal muscle cells (I-SkCRAC) has not been reported. Here, we characterized the biophysical properties, pharmacological profile, and molecular identity of I-SkCRAC in skeletal myotubes, as well as the regulation of its rate of activation by temperature and the type I ryanodine receptor (RyR1). I-SkCRAC exhibited many hallmarks of Ca2+ release activated Ca2+ currents (I-CRAC): store dependence, strong inward rectification, positive reversal potential, limited cesium permeability, and sensitivity to SOCE channel blockers. I-SkCRAC was reduced by siRNA knockdown of stromal interaction molecule 1 and expression of dominant negative Orai1. Average I-SkCRAC current density at 80mV was 1.00 +/- 0.05 pA/pF. In the presence of 20 mM intracellular EGTA, I-SkCRAC activation occurred over tens of seconds during repetitive depolarization at 0.5Hz and was inhibited by treatment with 100 mM ryanodine. The rate of SOCE activation was reduced threefold in myotubes from RyR1-null mice and increased 4.6-fold at physiological temperatures (35-37 degrees C). These results show that I-SkCRAC exhibits similar biophysical, pharmacological, and molecular properties as I-CRAC in nonexcitable cells and its rate of activation during repetitive depolarization is strongly regulated by temperature and RyR1 activity.