EFFECT OF GROWTH-HORMONE SUPPRESSION ON EXERCISE TRAINING AND GROWTH-RESPONSES IN YOUNG-RATS

Exercise training improves maximal oxygen uptake and endurance times i n adult human beings and other animals. The mechanism of this improvem ent results in part from anabolic effects of exercise and may be media ted by growth hormone (GH). Little is known about the role of GH in th e adaptation to exercise in younger, still-developing organisms. To ex amine this role, we began a 4-wk treadmill exercise training protocol in 14-d-old female rats. GH was suppressed by passive immunization wit h anti-GH releasing hormone antisera. There were four experimental gro ups: I) GH-control (normal GH secretory capacity), untrained (n = 21); 2) GH-suppressed, untrained (n = 13); 3) GH-control, trained (n = 14) ; and 4) GH-suppressed; trained (n = 11). At the end of the training p eriod, maximal oxygen uptake and treadmill endurance running time were measured. Serum GH and IGF-I were assessed using RIA, and whole hind limb musculature succinate dehydrogenase (an indicator of mitochondria l function) was measured with standard fluorometric technique. Body we ight gain was markedly reduced in GH-suppressed rats (mean, 54% of GH- controls in untrained rats and 55% in trained; p < 0.05). No apparent effect of training on linear growth was observed. As expected, serum I CF-I was markedly reduced by GH suppression, but no exercise-induced i ncrease occurred in IGF-I as a result of training in either the GH-con trol or GH-suppressed rats. In GH-control rats, maximal oxygen uptake and succinate dehydrogenase were 69% and 25% greater, respectively, in trained compared with untrained rats (p < 0.05). Despite GH inhibitio n, similar increases were found in the trained GH-suppressed rats (68% greater than controls for maximal oxygen uptake and 34% for succinate dehydrogenase, p < 0.05). Thus, marked improvement in cardiorespirato ry function occurs with training in young female rats even when normal pituitary GH function is suppressed.