EFFECT OF CHRONIC GROWTH-HORMONE ADMINISTRATION ON SKELETAL-MUSCLE INDOGS

Administration of growth hormone (GH) results in increased body weight gain in dogs. Increased body weight gain is believed to be a result o f the trophic effect of GH on the musculoskeletal system. However, ede ma is one of the side effects described in man following exogenous GH administration. Thus, the objective of this study was to determine if the expected increased weight gain in GH-treated dogs is a result of i ncreased muscle mass. Porcine growth hormone (pGH), administered subcu taneously to beagle dogs at doses of 0.025, 0.1, and 1 IU/kg/day for 1 4 wk, resulted in elevated serum GH and insulin-like growth factor-1 ( IGF-1) levels (see accompanying paper, Prahalada et al). This was asso ciated with a significant increase in body weight gain and weights of the cranial tibialis muscle in both male and female dogs. The increase d muscle mass likely contributed to the significant increase in body w eight gain seen in both sexes. Quantitative analysis of skeletal muscl e sections stained for ATPase activity showed increases in type I (slo w twitch) and type II (fast twitch) myofiber sizes in mid-and high-dos e males and in high-dose females. The ratio of type I and type II musc le fibers remained unchanged. Hypertrophic myofibers were enlarged but had a normal histologic and ultrastructural organization when observe d by light and transmission electron microscopy. The results of this s tudy have demonstrated that increased muscle mass in pGH-treated dogs is related to hypertrophy of muscle fibers and not due to edema. Exoge nous GH administration has an anabolic effect on skeletal muscle in do gs.