FIGURE 7.

α₂δ₁ knockdown myotubes fail to maintain their Ca²⁺ transient amplitude with electrical pulse trains >1 Hz. (A) Cells were stimulated by sequential bipolar electrical pulse trains lasting 120 s at frequencies ranging from 0.17 to 20 Hz with a 1 min rest period between each frequency tested. The rate of decline of the peak Ca²⁺ transient amplitude was proportional to the stimulus frequency. These data are representative of measurements acquired from n = 3 wt and n = 8 α₂δ₁ knockdown myotubes using the indicated stimulus sequence protocol. (B) Ca²⁺ transient amplitudes were resistant to rundown in wt, but not α₂δ₁ knockdown, myotubes stimulated with electrical pulse trains similar to those commonly used to elicit fatigue in adult fibers, as described in Materials and Methods. The data are representative of n = 20 wt and n = 36 knockdown cells.