AGE-RELATED-CHANGES IN DIAPHRAGM MUSCLE CONTRACTILE PROPERTIES AND MYOSIN HEAVY-CHAIN ISOFORMS

The present study sought to examine the effects of aging on the isomet ric contractile and fatigue properties as well as the myosin heavy cha in (MCH) isoform composition of the rat diaphragm muscle. Male Fischer 344 (F344) specific pathogen-free rats 6 and 24 mo old were used in t he study. Peak twitch force was similar to 23% lower (p < 0.05) in the senescent diaphragm compared with the young. Time to peak twitch forc e and one-half relaxation time of twitch force did not differ between groups. There was a significant decrease (15 to 18%, p < 0.05) in the specific force (N/cm(2)) of the senescent diaphragm at all stimulation frequencies (10 to 100 Hz) examined. In addition, the fatigability of the diaphragm did not significantly differ between the two groups. No significant changes in the distribution of MHC 1 and 2A isoforms were observed with aging. However, the contribution of MHC 2X significantl y decreased with senescence (young, 37.5%; senescent, 30.5%), whereas the contribution of MHC 28 in the senescent diaphragm was significantl y higher (young, 6.5%; senescent, 15.0%; p < 0.05). We conclude that t he age-related decline in diaphragm muscle specific force is caused by intrinsic factors other than changes in MHC composition.