Denervation-induced changes in myosin heavy chain expression in the rat diaphragm muscle

Unilateral denervation (Dnv) of the rat diaphragm muscle (Dia(m)) markedly alters expression of myosin heavy chain (MHC) isoforms. After 2 wk of Dia(m) Dnv, MHC content per half-sarcomere decreases in fibers expressing MHC2X and MHC2B. We hypothesized that changes in MHC protein expression parallel changes in MHC mRNA expression. Relative MHC isoform mRNA levels were determined by Northern analysis after 1, 3, 7, and 14 days of Dnv of the rat Diam. MHC protein expression was determined by SDS-PAGE. Changes in MHC isoform protein and mRNA expression were not concurrent. Expression of MHCSlow and MHC2X mRNA isoforms decreased dramatically by 3 days of Dnv, whereas that of MHC2A and MHC2B did not change. Expression of all MHC protein isoforms decreased by 3 days of Dnv. We observed a differential effect of rat Diam Dnv on MHC isoform protein and mRNA expression. The time course of the changes in MHC isoform mRNA and protein expression suggests a predominant effect of altered protein turnover rates on MHC protein expression instead of altered transcription after Dnv.