Age and Exercise Effects on Myosin Heavy Chain Isoform mRNA Expression in Human Muscle

Abstract

0389 Aging is associated with changes in the contractile and metabolic properties of human skeletal muscle. Among these changes is a shift in fiber type towards a predominance of Type I fibers. It was reported that mRNA levels of myosin heavy chain (MHC) isoforms, the determinant of contractile phenotype, are altered with age in a manner consistent with fiber type results, although two studies found no changes in MHC mRNA expression. Further, a predominance of Type I fibers has been observed in highly aerobically-trained athletes but the effect of aerobic training on MHC mRNA expression and the potential interaction with age remains to be determined. PURPOSE: To determine the effect of aging and a moderate intensity aerobic exercise training program on mRNA levels of MHC isoforms in human muscle. METHODS: Previously sedentary, healthy men and women aged 22–88 y were studied before and after 16 weeks of bicycle training (30–45 min at 70–80% peak heart rate, 3–4 days/week, N = 62) or control (stretching, N = 37) activity. Muscle biopsies of the vastus lateralis were analyzed for MHC mRNA isoforms I, IIA and IIX using real-time PCR. RESULTS: At baseline MHC-I mRNA was unchanged with age, while levels of IIA (r = −0.45) and IIX (r = −0.34) declined by 14% and 10% per decade, respectively (p < 0.001). After training, the mRNA levels of MHC-I and IIA increased 61 and 99%, respectively, while IIX decreased 50% (all p < 0.001). The increase in MHC-I mRNA was positively associated with age (r = 0.30, p < 0.01). There was no interaction between age and exercise responses for MHC-IIA or IIX, and MHC mRNA did not change in the control group. CONCLUSION: This study demonstrates that MHC isoform expression changes with aging consistent with reported changes in fiber type. Despite baseline differences, older muscle retains the ability to alter expression of MHC isoforms at the mRNA level in response to aerobic training. This agrees with previous data from this study demonstrating that training-induced enhancement of mitochondrial genes and functions are similar among younger and older people. Thus, skeletal muscle plasticity, at least at the transcript level, remains intact in older people. Future work will determine if training-induced changes in MHC mRNA are also reflected at the protein level. Supported by NIH AG-RO109531, MO1-RR00585 and Mayo Foundation.