Previous studies in vivo have shown that the activity of ornithine decarboxylase (ODC), the rate-controlling enzyme in polyamine biosynthesis, is markedly decreased in muscle of diabetic rats and is restored to normal by insulin therapy. Also, muscle ODC is diminished by starvation and increased by refeeding. To investigate the basis for these findings, the regulation of ODC was studied in vitro using rat soleus and extensor digitorum longus muscles. Incubation of muscles from fed rats in Krebs-Henseleit solution resulted in a 75% decrease in ODC activity within 1 h. Addition of insulin and amino acids had no effect; however, 50% rat serum increased ODC activity four- to sevenfold after the initial decrease. Rat serum also increased ODC in muscles from starved rats. The effect of serum was blocked by both cycloheximide and actinomycin D. Serum from diabetic rats was only 50% as effective as serum from normal rats in increasing ODC activity. Addition of physiologic levels of insulin to diabetic serum had no effect; however, treatment of diabetic rats with insulin in vivo restored serum activity to normal. These findings suggest that insulin modulates the synthesis of ODC via production of a second circulating factor, the activity of which is diminished in serum of diabetic rats. They also suggest that the stimulation of polyamine biosynthesis by this factor may be an integral component of the growth-promoting effect of insulin on muscle in vivo.

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