Modulation of sodium-channel mRNA levels in rat skeletal muscle.

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.