Copyright © 2006 by the American Physiological Society. In this issue, Geiger and her colleagues (7) provide an excellent and comprehensive database of mRNA abundance and protein expression of myosin heavy chain (MHC) isoforms in the rat diaphragm during a period of rapid growth (i.e., postnatal days [P]-0, 14, 28 and 84 [adult]). While the authors postulated that changes in mRNA abundance would be followed by concomitant changes in MHC protein expression, this was not the case. For example, by P-28, the abundances of mRNAs for MHCSlow and MHC2X were largely unchanged, yet significant increments in protein expression of these isoforms were observed. Even expression of the neonatal isoform, MHCNeo (interpreted by the investigators as being consistent with transcriptional control) is complex. Initially, the relative abundance of MHCNeo mRNA increased significantly, while the relative expression of the protein decreased significantly. At P-28, the relative abundance of MHCNeo mRNA was 34 % with zero protein expression. Given the highly complex nature of myogenic regulation during development and growth and a multitude of factors impacting on the entire process of MHC protein synthesis and phenotypic expression
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