Growth Hormone Restores Protein Synthesis in Skeletal Muscle of Old Male Rats

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Abstract
The present study was undertaken to determine if the decline in skeletal muscle protein synthesis with age results from diminished secretion of growth hormone (GH). Young rats (3 to 4 months) were injected with vehicle and old rats (19 to 21 months) with bovine GH (2 mg/kg/day), L-dopa (100 mg/kg/day), or vehicle for 8 days, and the rates of phenylalanine incorporated into total diaphragm muscle protein/ug DNA/unit time were compared. After correction for free phenylalanine specific activity, rates of protein synthesis were determined to be reduced by 26% in old as compared with young rats (p < .05). Treatment of old rats with bovine GH increased the rate of protein synthesis by 55% when compared with vehicletreated old rats (p < .05). L-dopa increased protein synthesis by 22% when compared with vehicle-treated old rats, although this was not statistically different from young or old vehicle-treated rats. These data indicate that (a) there is diminished capacity by skeletal muscle of old rats to synthesize protein; (b) this decrease is related to the reduced GH secretion because protein synthesis can be restored by GH administration; and (c) the dose of l-dopa given, although it increased the levels of circulating GH, did not completely restore protein synthetic capacity