Effect of age on skeletal muscle myofibrillar mRNA abundance: relationship to myosin heavy chain protein synthesis rate.

Age-related changes in myosin heavy chain (MHC) phenotype impact both the quantity and functional character of skeletal muscle. The present study was undertaken to examine the hypothesis that age-related changes in MHC mRNA abundance underlie alterations in protein synthesis rates and content. We measured the abundance of mRNA for MHC isoforms (MHC I, MHC IIa, IIx) and actin by RT-PCR in 6 young (mean +/- SE; 29 +/- 3) and 12 elderly (73 +/- 1 yr; P<0.01) volunteers and examined their association to MHC protein synthesis rates and their respective protein products. We found no differences between young and elderly volunteers in MHC isoform or actin transcript levels. Because of the absence of age effects, data were pooled for correlation analyses. Although total MHC mRNA levels were not related to MHC fractional synthesis rates, the relative abundance of MHC I and MHC IIa mRNA were positively (r = 0.450; P = 0.06) and negatively (r = -0.493; P<0.05) associated with MHC protein synthesis rates, respectively. MHC mRNA levels were positively correlated to their respective protein products (range of r-values: 0.551-0.727; P<0.05 to P<0.01), but were not related to skeletal muscle IGF-I mRNA abundance, circulating IGF-I or markers of immune activation. Our results argue against the notion that changes in MHC protein synthesis rates with age are related to altered MHC mRNA abundance, although our findings do suggest that possibility that individual variability in MHC protein synthesis rates is related to the relative abundance of MHC I versus MHC IIa transcripts.