EFFECT OF TRAINING AND GROWTH-HORMONE SUPPRESSION ON INSULIN-LIKE GROWTH-FACTOR-I MESSENGER-RNA IN YOUNG-RATS

The growth hormone (GH)-insulin-like growth factor I (IGF-I) axis play s a role in the adaptation to exercise training, but IGF-I gene expres sion in response to exercise training and GH suppression has not been studied. Twenty female rats underwent a 4-wk treadmill training progra m begun in the prepubertal period (day 14 of life). In 10 of the train ing rats, GH production was suppressed by anti-GH-releasing hormone an tibodies (GH suppressed). IGF-I mRNA and protein levels were measured in liver and hindlimb skeletal muscle. GH suppression reduced IGF-I mR NA expression in the liver to a much greater extent than in the muscle . In the GH control rats, training induced significant increases in he patic exon 1-derived IGF-I mRNA (mean increase 30%; P < 0.05) and musc le exon 2-derived mRNA (mean increase 35%; P < 0.05). In the GH-suppre ssed rats, only muscle exon 1-derived transcripts were significantly i ncreased by training (55%; P < 0.05) and this was associated with a si gnificant increase in muscle IGF-I protein levels (P < 0.05). We specu late that the ana belie response to training may involve both GH-depen dent increases in IGF-I mRNA in the liver and GH-independent increases in the muscle.