Fast voltage gating of Ca2+ release in frog skeletal muscle revealed by supercharging pulses

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Abstract
In single frog skeletal muscle fibres, we utilized supercharging voltage clamp command pulses to boost the rate of depolarization in the transverse tubular system (T‐system) such that 95 % of steady‐state potential is achieved in < 2 ms (as indicated by fluorescent potentiometric dye signals detected from a global illumination region). Signals detected near the edge of muscle fibres indicate that peripheral regions of the T‐system are not significantly overcompensated under these conditions. We explored the impact of accelerating T‐system depolarization on voltage‐dependent events of excitation‐contraction (E‐C) coupling by measuring charge movement currents (CMCs) and Ca2+ fluorescence transients in response to both supercharging and conventional step pulses. When compared with CMCs elicited by step pulses, supercharging CMCs are larger, and their kinetics more closely resemble those of gating current records reported for ionic channels. Furthermore, they decay bi‐exponentially (τfast range, 1.3‐1.8 ms; τslow range, 7.3‐11.9 ms), whereas step CMCs fall with a single exponential time course (τ range, 12.5‐26.7 ms). Similarly, supercharging produces a distinct acceleration in Ca2+ release transients, which show little evidence of the voltage‐dependent onset latencies previously encountered using step pulses. The use of this novel methodology in skeletal muscle unveils a previously undetected component of charge movement, the rapid, voltage‐dependent recruitment of which may provide the basis for understanding the fast gating of physiological E‐C coupling.