Charge movements in intact amphibian skeletal muscle fibres in the presence of cardiac glycosides

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Abstract
1 Intramembrane charge movements were examined in intact voltage‐clamped amphibian muscle fibres following treatment with cardiac glycosides in the hypertonic gluconate‐containing solutions hitherto reported to emphasise the features of qγ at the expense of qβ charge. 2 The application of chlormadinone acetate (CMA) at concentrations known selectively to block Na+‐K+‐ATPase conserved the steady‐state voltage dependence of intramembrane charge, contributions from delayed (qγ) charging transients, and their inactivation characteristics brought about by shifts in holding potential. 3 The addition of either ouabain (125, 250 or 500 nm) or digoxin (5 nm) at concentrations previously reported additionally to influence excitation‐contraction coupling similarly conserved the steady‐state charge‐voltage relationships, Q(V), in fully polarised fibres to give values of maximum charge, Qmax, transition voltage, V*, and steepness factor, k, that were consistent with a persistent qγ component as reported on earlier occasions (Qmax≈ 25‐27 nC μF−1, V*≈ ‐45 to ‐50 mV, k≈ 7‐9 mV). 4 In both cases shifts in holding potential from ‐90 to ‐50 mV produced a partial inactivation that separated steeply and more gradually voltage‐dependent charge components in agreement with previous characterisations. 5 However, charge movements that were observed in the presence of either digoxin or ouabain were monotonic decays in which delayed (qγ) transients could not be distinguished from the early charging records. These features persisted despite the further addition of chlormadinone acetate over a 10‐fold concentration range (5‐50 μm) known to displace ouabain from the Na+‐K+‐ATPase. 6 Ouabain (500 nm) restored the steady‐state charge movement that was previously abolished by the addition of 2.0 mm tetracaine in common with previous results of using ryanodine receptor (RyR)‐specific agents. 7 Perchlorate (8.0 mm) restored the delayed ‘on’ relaxations and increased the prominence of the ‘off’ decays produced by qγ charge following treatment with cardiac glycosides. This was accompanied by a negative (≈10‐15 mV) shift in the steady‐state charge‐voltage relationship but an otherwise conserved maximum charge, Qmax, and steepness factor, k, in parallel with previously reported effects of perchlorate following treatments with RyR‐specific agents. 8 The features of cardiac glycoside action thus parallel those of other agents that act on RyR‐Ca2+ release channels yet influence the kinetics but spare the steady‐state properties of intramembrane charge.