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Effect of transverse‐tubular chloride conductance on excitability in skinned skeletal muscle fibres of rat and toad
Author(s) -
Coonan J. R.,
Lamb G. D.
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.551bn.x
Subject(s) - toad , biophysics , skeletal muscle , conductance , chemistry , chloride , skeletal muscle fibers , anatomy , endocrinology , biology , physics , condensed matter physics , organic chemistry
1 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 Ca 2+ 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. 2 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 Ca 2+ 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 Ca 2+ 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 − . 3 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 Ca 2+ 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. 4 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. 5 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.