Growth hormone restores protein synthesis in skeletal muscle of old male rats.

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 less than .05). Treatment of old rats with bovine GH increased the rate of protein synthesis by 55% when compared with vehicle-treated old rats (p less than .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.