Growth hormone improves body mass recovery with refeeding after chronic undernutrition-induced muscle atrophy in aging male rats

The effects of growth hormone (GH) administration and refeeding after chron ic undernutrition (UN) were investigated in Fischer 344 male rats aging int o senescence (24.5 mo of age) during UN initiated at 12.5 mo of age that pr oduced muscle atrophy and a 50% decrease in body mass. Muscle mass, protein , myosin heavy-chain (MHC) composition and circulating testosterone levels were measured and compared to controls with free access to food. Within 9 w k, refeeding + GH restored body mass to control levels, whereas it was stil l decreased with refeeding alone. By 24.5 mo of age, refeeding alone restor ed body mass, while addition of GH resulted in overshoot. UN uniformly decr eased mass of the gastrocnemius, extensor digitorum longus, soleus and diap hragm muscles to 50-60% of controls. Refeeding and refeeding + GH restored these losses with some overshoot of gastrocnemius muscle suggesting hypertr ophy. UN move than doubled slow Type I MHC composition and approximately ha lved fast Type IIB and IIX MHC in the deep gastrocnemius muscle while it in creased Type IIA MHC in the diaphragm. Refeeding and refeeding + GH reverse d these shifts. MHC shifts in the extensor digitorum longus and soleus musc les were not statistically significant, whereas UN increased fast Type IIA MHC followed by decrease with refeeding + GH. UN decreased testosterone lev els to nearly zero followed by restoration with refeeding and refeeding + G H. We conclude that the phenotype of mixed-MHC muscles such as the gastrocn emius and diaphragm are most affected by chronic UN, which is reversible wi th refeeding and refeeding + GH. These alterations were associated with cha nges in circulating testosterone, which may be a key regulatory element in these processes.