FIGURE 3.

Restoration of action potential-induced Ca²⁺ transients by GFP-α_1D in dysgenic myotubes. Field stimulation at low frequency (30 V, 1 ms, 0.3 Hz/0.5 Hz) evoked rapid Ca²⁺ transients in myotubes expressing GFP-α_1D, GFP-α_1C, or GFP-α_1S. Bath application of 0.5 mM Cd²⁺/0.1 mM La³⁺ immediately stopped the activation of transients in myotubes expressing GFP-α_1D (A) and GFP-α_1C (B), indicating that the transients were dependent on Ca²⁺ influx through the L-type channels. Addition of 6 mM caffeine (A, right trace) triggered a strong Ca²⁺ transient, proving that SR Ca²⁺ release was still functional during the Cd²⁺/La³⁺ block. (C) Persistent Ca²⁺ transients after addition of Cd²⁺/La³⁺ in myotubes expressing GFP-α_1S characterizes skeletal-type EC-coupling, which is independent of Ca²⁺ influx. In contrast, GFP-α_1D restored action potential-induced Ca²⁺ transients with cardiac characteristics. Marks underneath the traces indicate the approximate times of stimulation; bars indicate periods of Cd²⁺/La³⁺ and caffeine application.