Effect of 3,5,3'-L-triiodothyronine on hepatic ornithine decarboxylase and thymidine kinase activity in rat after partial hepatectomy.

Partial hepatectomy (60%) led to the biphasic increase (first one at 4 h and second one at 48 h) of ornithine decarboxylase activity in the remnant rat liver. Daily administration of 3,5,3'-L-triiodothyronine (T3) (10 micrograms/100 g) to rats for 7 days before partial hepatectomy had little effect on the enzyme activity. At five days, ornithine decarboxylase activity declined to control level (sham operated controls) and its activity was significantly enhanced by T3. Ornithine decarboxylase gene expression in the rat liver (examined by Northern blot analysis using poly A+ mRNA) started to increase 4 h after partial hepatectomy, remained elevated for 48 h and decreased after 5 days. Its activity was not altered by T3 treatment. The activity of thymidine kinase increased progressively after partial hepatectomy, but its peak value was delayed by T3 administration. Plasma prolactin levels increased within 5-15 min after liver resection, then declined to control values and increased 24 h after the surgery. The data demonstrate that the changes in ornithine decarboxylase activity in the rat liver after partial hepatectomy might result from the direct effect of prolactin on the activity of enzyme rather than from its induction by hormone. Triiodothyronine administration altered both ornithine decarboxylase and thymidine kinase activities suggesting that T3 appears to regulate ornithine decarboxylase activity at post-translational level.