Differential effects of sarcoplasmic reticular Ca2+-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)-Ca2+ release channels as opposed to one driven by release of intracellularly stored Ca2+ would predict that such charging phenomena should persist in skeletal muscle fibres unable to release stored Ca2+. Charge movement components were accordingly investigated in intact voltage-clamped amphibian fibres treated with known sarcoplasmic reticular (SR) Ca2+-ATPase inhibitors. Cyclopiazonic acid (CPA) pretreatment abolished Ca2+ 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(gamma)) 'hump' components into simpler decays. However, steady-state charge-voltage characteristics were conserved to values (maximum charge, Q(max) approximate to 20-25 nC muF(-1); transition voltage, V* = -40 to -50 mV; steepness factor, k approximate to 6-9 mV; holding voltage -90 mV) indicating persistent q(gamma) charge. The features of charge inactivation similarly suggested persistent q(beta) and q(gamma) charge contributions in CPA-treated fibres. Perchlorate (8.0 mm) restored the delayed kinetics shown by 'on' q(gamma) charge movements, prolonged their 'off' decays, conserved both Q(max) and k, yet failed to restore the capacity of such CPA-treated fibres for Ca2+ release. Introduction of perchlorate (8.0 mm) or caffeine (0.2 mm) to tetracaine (2.0 mm)-treated fibres, also known to restore q(gamma) charge, similarly failed to restore Ca2+ transients. Steady-state intrantembrane q(gamma) 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 Ca2+. It is thus unlikely that q(gamma) charge movement is a consequence of SR Ca2+ release rather than changes in tubular membrane potential.