Figure 1. Muscle differentiation is associated with increased expression of STIM1 and redistribution of STIM1.

A) Differentiating C₂C₁₂ cells were harvested at the indicated times and protein lysates were separated by SDS-PAGE and immunoblotted for STIM1 and SERCA1 using specific antibodies. Complete scans of these gels are shown in supplemental information (supplemental fig 6). B) STIM1 expression in C₂C₁₂ myoblasts (MB, scale bar = 10 μm) and C) in C₂C₁₂ cells allowed to differentiate into myotubes (MT, scale bar = 20 μm). STIM1 aggregation and redistribution to the cellular periphery occurs during myogenesis. Arrows represent peripherally localized STIM1. D) Store-operated calcium entry was greater in myotubes than in myoblasts. Fura-2 loaded C₂C₁₂ myoblasts and myotubes were placed in zero calcium media, and treated with thapsigargin to induce store depletion and verapamil to inhibit L-type Ca²⁺ channels. Once cytoplasmic Ca²⁺ returned to baseline, barium was added to the extracellular medium as a surrogate for Ca²⁺. Representative average tracings of individual myoblasts and myotubes showed a significant increase in store-operated influx in myotubes. E) The rate of store-operated barium influx, calculated by the first derivative of the 340/380 nm ratio in the first 100 seconds of influx, was 5.57 × 10⁻⁴ ± 4 × 10⁻⁵ arbitrary unit/sec (n = 23) in myoblasts, and 2.72 × 10⁻³ ± 3 × 10⁻⁴ arbitrary unit/sec (n = 6) in myotubes (p<0.001). The data shown represent the mean ± SE.