The rapid transient stimulation of both cytoplasmic and mitochondrial ornithine decarboxylases in the liver of thyroidectomized rats by 3,5,3'-triiodo-L-thyronine.

Evidence is presented that liver from thyroidectomized rats has ornithine decarboxylase (ODC) in both mitochondrial and soluble fractions. The cytosolic activity was stimulated 7-fold and the mitochondrial activity 3-fold 15 min after the administration of triiodothyronine (T3). While the rapid transient stimulation of ODC could represent a direct intracellular response to T3, an incidental effect on a very-rapidly-turning-over enzyme seems more likely; this suggests that in vivo ODC may be controlled by a demand for polyamines.