THE INFLUENCE OF PERCHLORATE IONS ON COMPLEX CHARGING TRANSIENTS IN AMPHIBIAN STRIATED-MUSCLE

1. The effects of perchlorate ions on intramembrane charge movements w ere examined under different conditions of ryanodine receptor (RyR) mo dification in intact voltage-clamped amphibian skeletal muscle fibres studied in the gluconate-containing solutions previously reported to e mphasize the features of q(gamma) at the expense of those of the q(bet a) charge. 2. The introduction of graded increases in perchlorate conc entration to the experimental solutions selectively shifted the thresh old of appearance of the q(gamma) 'hump' currents to more negative tes t potentials at which they actually appeared in the absence of prior q (beta) transients at perchlorate concentrations of 4.0-8.0 mM. Such fi ndings suggested that the delayed (q(gamma)) transitions can take plac e independently of any previous exponential (q(beta)) decay. 3. These kinetic effects were accompanied by hyperpolarizing shifts in the tran sition potentials (V) of the steady-state voltage dependences of eith er the overall or the isolated q(gamma) charge. These shifts were grad ed with concentration and reached their maximum effects at 4.0-8.0 mM perchlorate. However, both the total charge (Q(max)) and the steepness factor (k) remained conserved at values consistent with a system that included significant contributions from the steeply voltage-sensitive q(gamma) component (overall charge: Q(max) approximate to 19-21 nC mu F-1, k approximate to 7-9 mV; q(gamma) component alone: Q(max) approx imate to 10-12 nC mu F-1, k approximate to 4-6 mV). This contrasts wit h earlier reports on the effects of perchlorate in fibres that were st udied in sulphate- or methanesulphonate-containing extracellular solut ions. 4. Perchlorate (8.0 mM) restored the 'hump' waveform associated with q(gamma) charge movements that had previously been obliterated by the prior application of fully effective (0.1 mM) concentrations of e ither ryanodine or daunorubicin. 5. Perchlorate similarly reversed the positive shift in the transition potential of the q(gamma) component that was brought about by such RyR modification (from V approximate t o -40 mV back to V approximate to -60 mV). In contrast, the values of either Q(max) (overall charge, 19-21 nC mu F-1; q(gamma) component, 1 0-13 nC mu F-1) or k (overall charge, 7-9 mV; q(gamma) component, 4-6 mV) remained conserved through all these experimental manoeuvres. 6. T he inclusion of perchlorate also reversed the action of 2 mM tetracain e and restored delayed q(gamma) transients to an extent that was grade d with concentration (0.5-8.0 mM perchlorate). There was an accompanyi ng recovery of the steeply voltage-dependent steady-state (q(gamma)) c omponent consistent with a competitive interaction between these agent s upon the q(gamma) intramembrane charge. 7. The present findings sugg est that perchlorate exerts a specific action upon the q(gamma) charge in independent transitions that are driven by the tubular membrane fi eld. Its interactions with the known RyR inhibitors that nevertheless conserve both the charge and its voltage sensitivity suggest a primary action upon the RyR that in turn exerts reciprocal actions upon the v oltage sensor.