Figure 5.

Depolarization-induced Ca²⁺ transients in dysgenic myotubes expressing the wild-type or Y366S mutant α_1S subunit. Transfected cultures were loaded with the fluorescent Ca²⁺ indicator fluo-3 AM and depolarized by passing a 2-ms, 30-V pulse (dot underneath traces) and a 2-s train of pulses at 20 Hz (bar underneath traces) across the culture chamber. Both, α_1S- and α_1S-Y366S-transfected myotubes responded with a transient increase in cytoplasmic free [Ca²⁺] to individual and tetanic stimulation. After changing to a bath solution not containing Ca²⁺, the same myotubes still responded with a [Ca²⁺] transient to depolarization (lower traces). Ca²⁺-sensitive fluorescent signals were normalized to baseline values (ΔF/F); duration of trace was 6 s.