Ornithine Decarboxylase Induction in Partially Hepatectomized Rat Liver and Modes of Its Stimulation by Glucagon and Insulin

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Abstract
Hepatic ornithine decarboxylase (ODC) activity increases after partial hepatectomy and this activity is further stimulated by pharmacologic doses of glucagon and insulin. We now present data suggesting that glucagon and insulin stimulate ODC activity by distinct mechanisms. ODC activity increased progressively after partial hepatectomy and reached an initial peak at 4 h. Activity decreased to 50% of its peak value at 6 and 8 h and then rose progressively to a maximum at 12h. Enzymatic activity was well correlated with the amount of hepatic immunoreactive ODC protein, thus suggesting that increased enzyme activity was due to increased amount of enzyme protein. Hepatic ODC mRNA increased gradually and continuously, reaching the maximal value by 12 h. In rats receiving glucagon after partial hepatectomy, ODC mRNA increased significantly by 2 h and enzyme immunoreactive protein and activity by 2 to 4 h as compared to controls. In contrast, insulin administration only induced a significant increase in enzyme immunoreactive protein and activity 10 to 12 h after partial hepatectomy. No significant changes in ODC mRNA level were observed. Our data suggest that the regulation mechanism of ODC induction following partial hepatectomy differs depending on the time after operation. Our data also suggest that while glucagon appears to regulate ODC activity by a transcriptional mechanism, insulin appears to operate at a post-transcriptional level.

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