Figure 2. Depolarization-induced Ca²⁺ release.

Intracellular Ca²⁺ ([Ca²⁺]_i) of 1B5 myotubes transduced with either α-RyR or β-RyR virions was imaged using fluo-4 as described in Materials and Methods. Cells were exposed for 30 sec to varied high [K⁺]_o solutions (with the constant [K⁺]×[Cl⁻] product) to trigger DICR, and finally to 20 mM caffeine to confirm the functional expression of RyR. Ca²⁺ in the bath solution was omitted prior to and during stimuli to prevent Ca²⁺ influx. A. Representative traces of Ca²⁺ transients of individual cells by [K⁺]_o and caffeine. B. Averaged maximal change in fluo-4 fluorescence (ΔF/F₀) was plotted against [K⁺]_o concentration. Values are expressed as mean ± SE (n = 112 for α-RyR and 18 for β-RyR). α-RyR but not β-RyR exhibited Ca²⁺ transients induced by high [K⁺]_o. C. A representative trace of Ca²⁺ transients of myotubes expressing α-RyR stimulated with 80 mM [K⁺]_o before and after treatment with 10 µM nifedipine. D. Averaged maximal change in fluo-4 fluorescence (ΔF/F₀) was plotted with or without 10 µM nifedipine. Values are expressed as mean ± SE (n = 15). Nifedipine inhibited high [K⁺]_o-induced Ca²⁺ transients.