Differential effects of sarcoplasmic reticular Ca 2+ ‐ATPase inhibition on charge movements and calcium transients in intact amphibian skeletal muscle fibres

A hypothesis in which intramembrane charge reflects a voltage sensing process allosterically coupled to transitions in ryanodine receptor (RyR)‐Ca 2+ release channels as opposed to one driven by release of intracellularly stored Ca 2+ would predict that such charging phenomena should persist in skeletal muscle fibres unable to release stored Ca 2+ . Charge movement components were accordingly investigated in intact voltage‐clamped amphibian fibres treated with known sarcoplasmic reticular (SR) Ca 2+ ‐ATPase inhibitors. Cyclopiazonic acid (CPA) pretreatment abolished Ca 2+ transients in fluo‐3‐loaded fibres following even prolonged applications of caffeine (10 mM) or K + (122 mM). Both CPA and thapsigargin (TG) transformed charge movements that included delayed ( q γ ) ‘hump’ components into simpler decays. However, steady‐state charge‐voltage characteristics were conserved to values (maximum charge, Q max ∼ 20–25 nC μF −1 ; transition voltage, V* ∼−40 to −50 mV; steepness factor, k ∼ 6–9 mV; holding voltage −90 mV) indicating persistent q γ charge. The features of charge inactivation similarly suggested persistent q β and q γ charge contributions in CPA‐treated fibres. Perchlorate (8.0 m m ) restored the delayed kinetics shown by ‘on’ q γ charge movements, prolonged their ‘off’ decays, conserved both Q max and k , yet failed to restore the capacity of such CPA‐treated fibres for Ca 2+ release. Introduction of perchlorate (8.0 m m ) or caffeine (0.2 m m ) to tetracaine (2.0 m m )‐treated fibres, also known to restore q γ charge, similarly failed to restore Ca 2+ transients. Steady‐state intramembrane q γ charge thus persists with modified kinetics that can be restored to its normally complex waveform by perchlorate, even in intact muscle fibres unable to release Ca 2+ . It is thus unlikely that q γ charge movement is a consequence of SR Ca 2+ release rather than changes in tubular membrane potential.