We describe the isolation and characterization of a cDNA encoding the α subunit of a new voltage-sensitive sodium channel, μl, from rat skeletal muscle. The 1840 amino acid μl peptide is homologous to α subunits from rat brain, but, like the protein from eel electroplax, lacks an extended (∼200) amino acid segment between homologous domains I and II. Northern blot analysis indicates that the 8.5 kb μl transcript is preferentially expressed in skeletal muscle. Sodium channels expressed in Xenopus oocytes from synthetic RNA encoding μl are blocked by tetrodotoxin and μ-conotoxin at concentrations near 5 nM. The expressed sodium channels have gating kinetics similar to the native channels in rat muscle fibers, except that inactivation occurs more slowly.
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