Lewis, Michael I., Thomas J. LoRusso, and Mario Fournier. Anabolic influences of insulin-like growth factor I and/or growth hormone on the diaphragm of young rats. J. Appl. Physiol. 82(6): 1972-1978, 1997.It is controversial whether insulin-like growth factor I (IGF-I), growth hormone (GH), or their combination might enhance body growth and/or tissue anabolism in the well-fed animal with an intact somatotrophic axis. To assess this further, we studied four groups of adolescent rats: 1) control (Ctr), 2) GH, 3) IGF-I, and 4) GH/IGF-I. IGF-I was given via an osmotic minipump, whereas GH was injected subcutaneously for a period of 72 h. Diaphragm (Dia) contractile and fatigue properties were determined in vitro. Quantitative histochemical and morphometric analyses were performed on Dia fibers. Total serum IGF-I levels were significantly increased in the groups receiving growth factors. Although body weight increased to a greater extent in the animals receiving growth factors, a further synergistic effect was noted in the GH/IGF-I animals compared with either GH or IGF-I groups. Costal Dia mass was greater in the groups receiving growth factors. The Dia of GH/IGF-I animals was more fatigue resistant than the Dia in Ctr. The cross-sectional area of types IIa and IIx fibers were increased to a similar extent in all groups receiving growth factors compared with Ctr. Succinate dehydrogenase activity of type IIa fibers was significantly greater in the GH/IGF-I animals compared with the other groups. We conclude that the short-term provision of growth factors to well-nourished, normally growing adolescent rats can accelerate body growth and promote selective hypertrophy of predominantly type II Dia fibers.