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

J Physiol. 1998 Jun 1;509 ( Pt 2)(Pt 2):551-64. doi: 10.1111/j.1469-7793.1998.551bn.x.

Abstract

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 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. 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 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 microM 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 Ca2+ release if the myoplasmic [K+] was reduced by 50 %. In toad fibres, 100 microM 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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Anthracenes / pharmacology
  • Bufo marinus
  • Chloride Channels / antagonists & inhibitors
  • Chloride Channels / physiology*
  • Chlorides / metabolism
  • Chlorides / pharmacology
  • In Vitro Techniques
  • Membrane Potentials / physiology
  • Muscle Contraction / drug effects
  • Muscle Contraction / physiology*
  • Muscle Fibers, Skeletal / drug effects
  • Muscle Fibers, Skeletal / physiology*
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / physiology*
  • Myotonia / physiopathology
  • Potassium / pharmacology
  • Rats

Substances

  • Anthracenes
  • Chloride Channels
  • Chlorides
  • 9-anthroic acid
  • Potassium