RESISTANCE EXERCISE AND GROWTH-HORMONE AS COUNTERMEASURES FOR SKELETAL-MUSCLE ATROPHY IN HINDLIMB-SUSPENDED RATS

Unweighting of rat hindlimb muscles results in skeletal muscle atrophy , decreased protein synthesis, and reduced growth hormone (GH) secreti on. Resistance exercise (ladder climbing) and GH treatment partially a ttenuate skeletal muscle atrophy in hypophysectomized hindlimb-suspend ed rats. It was hypothesized that a combination of multiple bouts of d aily resistance exercise and GH (1 mg.kg(-1).day(-1)) would prevent sk eletal muscle atrophy in growing nonhypophysectomized hindlimb-suspend ed rats. Hindlimb suspension decreased the absolute (mg/pair) and rela tive (mg/100 g body wt) weights of the soleus, a slow-twitch plantar f lexor, by 30 and 21%, respectively, and the absolute and relative weig hts of the gastrocnemius, a predominantly fast-twitch plantar flexor, by 20 and 11%, respectively (P < 0.05). Exercise did not increase sole us mass but attenuated loss of relative wet weight in the gastrocnemiu s muscles of hindlimb-suspended rats (P < 0.05). Hindlimb suspension d ecreased gastrocnemius myofibrillar protein content and synthesis (mg/ day) by 26 and 64%, respectively (P < 0.05). The combination of exerci se and GH attenuated loss of gastrocnemius myofibrillar protein conten t and synthesis by 70 and 23%, respectively (P < 0.05). Results of the present investigation indicate that a combination of GH and resistanc e exercise attenuates atrophy of unweighted fast-twitch skeletal muscl es.