DIAPHRAGM MYOSIN HEAVY-CHAIN COMPOSITION SHIFTS IN AGING CHRONICALLY-UNDERNOURISHED FISCHER-344 MALE RATS

The effects of chronic undernutrition (UN) on respiratory muscle were investigated during UN producing a 50% decrease in body weight over a prolonged period (45 weeks) in Fischer 344 male mts. This model focuse d on progressive, aging-related changes in myosin heavy chain (MHC) pr ofile over time. in which the confounding effects of early development and late senescence were avoided. With aging toward late adulthood (6 8 weeks), MHC composition of control diaphragms was shifted, with decr eased type I (slow) and IIA MHC, and increased type IIB and IIX (fast) MHC. UN produced a divergence of this profile, with an increase in ty pe I and IIA MHC, and decreased type IIX MHC. UN diaphragms in vitro w ere more resistant to loss of active force with fatigue, during repeti tive contractions. However passive tension rose disproportionately dur ing fatigue, suggesting increased fatigability. We conclude that the o bserved changes in diaphragm mechanical Junction are consistent with t he UN-induced shifts in MHC composition; however the elevated passive tension with fatigue suggests additional UN-induced changes in mechani cal properties that are possibly detrimental to respiratory muscle fun ction. The UN-dependent divergence in phenotype and mechanical propert ies may be amplified by aging-related shifts in muscle MHC composition over time, in the control group. (Aging Clin. Exp. Res. 10: 112-119, 1998) (C)1998, Editrice Kurtis.