Voltage-gated sodium channel (SkM1) content in dystrophin-deficient muscle

The membrane cytoskeleton is increasingly considered as both an anchor and a functional modulator for ion channels. The cytoskeletal disruptions that occur in the absence of dystrophin led us to investigate the voltage-gated sodium channel (SkM1) content in the extensor digitorum longus (EDL) muscle of the dystrophin-deficient mdx mouse. Levels of SkM1 mRNA were determined by semiquantitative reverse transcriptase polymerase chain reaction (RT-PC R). A C-terminal portion of the mouse-specific SkM1 alpha -subunit cDNA (mS cn4a) was identified first. SkM1 mRNA levels were as abundant in mdx as in normal muscle, thus suggesting that the transcriptional rate of SkMI remain s unchanged in mdx muscle. However, SkMI density in the extrajunctional sar colemma was shown to be significantly reduced in mdx muscle, using confocal immunofluorescence image analysis. This decrease was found to be associate d with a reduction in the number of SkM1-rich fast-twitch IIb fibres in mdx muscle. In addition, lowered SkM1 sarcolemmal labelling was found in all m dx fibres regardless of their metabolic type. These results suggest the exi stence of a perturbation of SkM1 anchorage to the plasma membrane. Such an alteration is likely to be related to the 50% decrease in mdx muscle of the dystrophin-associated syntrophins, which are presumed to be involved in Sk MI anchorage. However, the moderate reduction in SkMI density (-12.7%) obse rved in mdx muscle argues in favour of a non-exclusive role of syntrophins in SkM1 anchorage and suggests that other membrane-associated proteins are probably also involved.