The effect of growth hormone on hepatic ornithine decarboxylase (L-ornithine carboxy- lyase, EC 188.8.131.52) (ORD) activity was studied in adult and weanling rats. Single injections of growth hormone produced up to 30-fold increases in ORD activity in both adults and weanlings. However, with lower doses of growth hormone, a greater increase in ORD activity occurred in weanlings than in adults. Both adult and weanling rats showed the same time course of ORD activity enhancement after growth hormone with peak enzymatic activity at 4 hr and a decline to basal levels by 8 hr after hormone administration. In weanling rats, cycloheximide completely prevented the elevation of ORD activity at 4 hr after growth hormone if administered any time up to 3 hr after the hormone. If given simultaneously with growth hormone, actinomycin D almost totally supressed the enhanced ORD activity 4 hr after growth hormone. However, the ability of actinomycin D to prevent increased ORD activity decreased as the time after growth hormone administration increased. When cycloheximide was administered to weanling rats 3 hr after growth hormone, ORD activity decreased rapidly with a half-life of about 15 min, indicating an extremely rapid turnover rate for ORD. The half-life calculated from the natural decline of growth hormone-induced ORD activity was about 24 min.
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