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1 Department of Cell Biology and Physiology, 2 Magee Womens Research Institute, 3 Children's Hospital of Pittsburgh, and 4 Division of Pulmonary, Allergy, and Critical Care Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261
The effects of
growth hormone (GH) on diaphragm muscle myosin heavy chain (MHC)
composition and mechanical performance were investigated in Fischer 344 male rats aged to senescence (24.5 mo of age). Chronic undernutrition
(UN), refeeding (RF), and RF+GH were compared with ad libitum feeding
by using a model of UN that produced a 50% decrease in body weight
over a 12-mo period. The effect of aging was assessed by comparing MHC
composition of ad libitum-fed rats at 12 and 24.5 mo of age. At
senescence, significant decreases in slow type I (
23%) and fast
type IIA (31%) MHC had occurred with aging. Conversely, UN over
this aging period increased types I (32-73%) and IIA
(22-23%) MHC and decreased fast types IIB (32-54%) and IIX
(30-31%) MHC. RF and RF+GH reversed these shifts back toward
control values. At senescence, maximal specific force, maximal
velocity, and specific power capacity were not different across
treatment groups. During repetitive isotonic contraction trials, the
diaphragms of UN rats maintained power production over time (54% of
initial power at 60 s), whereas the power production of diaphragms of
ad libitum-fed rats fell to 0% (P < 0.05). In comparison with UN rats, the diaphragms of RF and RF+GH rats
produced 23 (not significant) and 11%
(P < 0.05) of initial power,
respectively, suggesting that RF+GH treatment restored performance
characteristics after UN. We conclude that RF+GH can reverse
alterations in MHC composition and mechanical performance produced by
chronic UN in the aged rat diaphragm.
Fischer 344 rats; fatigue; force; power; velocity
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