Effect of transverse-tubular chloride conductance on excitability in skinned skeletal muscle fibres of rat and toad

摘要

class="enumerated" style="list-style-type:decimal">The influence of the transverse-tubular (T-) system Cl⁻ conductance on membrane excitability in skeletal muscle fibres of toad and rat was examined because of conflicting conclusions of previous studies on Cl⁻ conductance. A mechanically skinned fibre preparation was used that permitted investigation of Ca²⁺ release via the normal T-system voltage-sensor mechanism after complete removal of the surface membrane, which thereby allowed estimation of the T-system potential from force measurements.When a skinned fibre was bathed in a high-[K⁺] solution, the sealed T-system became polarized and could be rapidly depolarized by replacing the K⁺ with Na⁺, thereby eliciting Ca²⁺ release from the sarcoplasmic reticulum. In rat skinned fibres, addition of 20 mM Cl⁻ to the ‘myoplasm’ (i.e. bathing solution) partially depolarized the T-system, inducing Ca²⁺ release and subsequent voltage-sensor inactivation. These effects were completely abolished with 100 μM of the Cl⁻ channel blocker 9-anthracene carboxylic acid (9-AC). Voltage-sensor inactivation increased in a graded manner over the range 3-20 mM myoplasmic Cl⁻.In toad fibres, voltage-sensor inactivation was only detectable at > 10 mM myoplasmic Cl⁻, and 20 mM Cl⁻ was only able to depolarize the T-system sufficiently to trigger Ca²⁺ release if the myoplasmic [K⁺] was reduced by 50 %. In toad fibres, 100 μM 9-AC caused little if any block of the T-system Cl⁻ conductance.It was also found that when skinned fibres were obtained from muscles that had been bathed in a zero Cl⁻ extracellular solution, the initial Na⁺ substitutions were more effective at depolarizing the T-system. This is consistent with Cl⁻ trapped in the sealed T-system exerting a polarizing effect on T-system potential.These results unequivocally demonstrate that there is a large 9-AC-sensitive Cl⁻ conductance in the T-system of rat fibres, and a smaller, though still appreciable, Cl⁻ conductance in the T-system of toad fibres, which is relatively insensitive to 9-AC. The results are important for understanding the basis of the Cl⁻ channel aberration in myotonia.