Abstract
Action potentials in many types of excitable cells result from changes in permeability to Na ions. Although these permeability changes in nerve and muscle are mediated by voltage-gated Na channels that are functionally similar, we found that the Na-channel gene expressed in skeletal muscle is different from the genes coding for two Na channels (type I and type II) in brain. Despite the structural differences between muscle and brain Na-channel genes, a cDNA clone derived from rat brain hybridizes to skeletal muscle Na-channel mRNA of approximately 9.5 kilobases. We used this cDNA probe to measure changes in Na-channel mRNA levels in skeletal muscle during development and following denervation. By blot hybridization analysis of electrophoretically fractionated RNA, we found that Na-channel mRNA can be detected as early as embryonic day 17 and that mRNA levels increase 2-fold between birth and postnatal day 35. Denervation of adult muscle causes a further 2- to 3-fold increase in muscle Na-channel mRNA levels, suggesting that expression of Na-channel genes in fast-twitch muscle may be regulated by the state of innervation.