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Developmental Biology Unit, University of Bielefeld, Germany.

1. In mature mammalian muscle, the muscular chloride channel ClC-1 contributes about 75% of the sarcolemmal resting conductance (Gm). In mice carrying two defective alleles of the corresponding Clc1 gene, chloride conductance (GCl) is reduced to less than 10% of that of wild-type, and this causes hyperexcitability, the salient feature of the disease myotonia. Potassium conductance (GK) values in myotonic mouse muscle fibres are lowered by about 60% compared with wild-type. 2. The defective Clcadr allele causes loss of the 4.5 kb ClC-1 mRNA. Mice heterozygous for the defective Clc1adr allele contain about 50% functional mRNA in their muscles compared with homozygous wild-type mice. 3. Despite a halved functional gene dosage, heterozygous muscles display an average GCl which is not significantly different from that of homozygous wild-type animals. The GK values in heterozygotes are also indistinguishable from homozygous wild-type animals. 4. These results indicate that a regulatory mechanism acting at the post-transcriptional level limits the density of ClC-1 channels. GK is probably indirectly regulated by muscle activity.

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