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The influence of perchlorate ions on complex charging transients in amphibian striated muscle

Christopher L.-H. Huang

1. The effects of perchlorate ions on intramembrane charge movements were examined under different conditions of ryanodine receptor (RyR) modification in intact voltage-clamped amphibian skeletal muscle fibres studied in the gluconate-containing solutions previously reported to emphasize the features of q at the expense of those of the q charge.

2. The introduction of graded increases in perchlorate concentration to the experimental solutions selectively shifted the threshold of appearance of the q 'hump' currents to more negative test potentials at which they actually appeared in the absence of prior q transients at perchlorate concentrations of 4·0-8·0 mM. Such findings suggested that the delayed (q) transitions can take place independently of any previous exponential (q) decay.

3. These kinetic effects were accompanied by hyperpolarizing shifts in the transition potentials (V*) of the steady-state voltage dependences of either the overall or the isolated q charge. These shifts were graded with concentration and reached their maximum effects at 4·0-8·0 mM perchlorate. However, both the total charge (Qmax) and the steepness factor (k) remained conserved at values consistent with a system that included significant contributions from the steeply voltage-sensitive q component (overall charge: Q_max 19-21 nC µF^-1, k 7-9 mV; q component alone: Q_max 10-12 nC µF^-1, k 4-6 mV). This contrasts with earlier reports on the effects of perchlorate in fibres that were studied in sulphate- or methanesulphonate-containing extracellular solutions.

4. Perchlorate (8·0 mM) restored the 'hump' waveform associated with q charge movements that had previously been obliterated by the prior application of fully effective (0·1 mM) concentrations of either ryanodine or daunorubicin.

5. Perchlorate similarly reversed the positive shift in the transition potential of the q component that was brought about by such RyR modification (from V* -40 mV back to V* -60 mV). In contrast, the values of either Q_max (overall charge, 19-21 nC µF^-1; q component, 10-13 nC µF^-1) or k (overall charge, 7-9 mV; q component, 4-6 mV) remained conserved through all these experimental manoeuvres.

6. The inclusion of perchlorate also reversed the action of 2 mM tetracaine and restored delayed q transients to an extent that was graded with concentration (0·5-8·0 mM perchlorate). There was an accompanying recovery of the steeply voltage-dependent steady-state (q) component consistent with a competitive interaction between these agents upon the q intramembrane charge.

7. The present findings suggest that perchlorate exerts a specific action upon the q charge in independent transitions that are driven by the tubular membrane field. 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 voltage sensor.

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