Effect of transverse-tubular chloride conductance on excitability in skinned skeletal muscle fibres of rat and toad.
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.
Coonan
J R
JR
School of Zoology, La Trobe University, Bundoora, Victoria 3083, Australia.
Lamb
G D
GD
D000818:Animals; D000873:Anthracenes / Q000494:pharmacology; D002024:Bufo marinus; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000502:physiology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D009222:Myotonia / Q000503:physiopathology; D011188:Potassium / Q000494:pharmacology; D051381:Rats
D000873:Anthracenes; D018118:Chloride Channels; D002712:Chlorides; C010030:9-anthroic acid; D011188:Potassium