STIMULATION OF MYOFIBRILLAR PROTEIN-SYNTHESIS IN HINDLIMB SUSPENDED RATS BY RESISTANCE EXERCISE AND GROWTH-HORMONE

The objective of this study was to determine the ability of a single b out of resistance exercise alone or in combination with recombinant hu man growth hormone (rhGH) to stimulate myofibrillar protein synthesis (Ks) in hindlimb suspended (HLS) adult female rats. Plantar flexor mus cles were stimulated with resistance exercise, consisting of 10 repeti tions of ladder climbing on a Im grid (85 degrees), carrying an additi onal 50% of their body weight attached to their tails. Saline or rhGH (1 mg/kg) was administered 30' prior to exercise, and Ks was determine d with a constant infusion of H-3-Leucine at 15', 60', 180', and 360' following exercise. Three days of HLS depressed Ks similar to 65% and 30-40% in the soleus and gastrocnemius muscles, respectively (p less t han or equal to 0.05). Exercise increased soleus Ks in saline-treated rats 149% 60' following exercise (p less than or equal to 0.05), decay ing to that of non-exercised animals during the next 5 hours. Relative to suspended, non-exercised rats rhGH+exercise increased soleus Ks 84 %, 108%, and 72% at 15', 60' and 360' following exercise (p less than or equal to 0.05). Gastrocnemius Ks was not significantly increased by exercise or the combination of rhGH and exercise up to 360' post-exer cise. Results from this study indicate that resistance exercise stimul ated Ks 60' post-exercise in the soleus of HLS rats, with no apparent effect of rhGH to enhance or prolong exercise-induced stimulation. Res ults suggests that exercise frequency may be important to maintenance of the slow-twitch soleus during non-weightbearing, but that the abili ty of resistance exercise to maintain myofibriliar protein content in the gastrocnemius of hindlimb suspended rats cannot be explained by ac ute stimulation of synthesis.