Regulation of muscle sodium channel transcripts during development and in response to denervation

We have recently described the cloning and functional expression of a new sodium channel subtype, μI, isolated from a denervated rat skeletal muscle cDNA library. In studies described here, we have used RNase protection and Northern blot analyses to examine the expression of μI mRNA in different tissues and in neonatal, adult, and adult denervated muscle. We found that μI transcripts were not expressed in brain or heart, or in the myogenic cell line L6, even after differentiation to myotubes. Transcripts for μI were present at low levels in neonatal skeletal muscle and increased to maximum levels in adult tissue, paralleling the expression of tetrodotoxin (TTX)-sensitive sodium currents. Surprisingly, denervation of adult muscle was also followed by a rise in μI mRNA, at a time when TTX-insensitive currents reappear. These results show that expression of this channel subtype is regulated by tissue type, development, and innervation.