RAPID STIMULATION OF TYPE-I 5'-DEIODINASE IN RAT PITUITARIES BY 3,3',5-TRIIODO-L-THYRONINE

The negative feedback control of thyrotropin production by the anterior pituitary involves focal 5'-deiodination of L-thyroxine (L-T-4) to the active thyroid hormone 3,3',5-triiodo-L-thyronine (T-3) by two 5'-deiodinase isozymes which are distinctly regulated by thyroid hormone. T-3 rapidly increased steady-state mRNA levels and activity of type I iodothyronine-5'-deiodinase (5'DI) in rat anterior pituitary and in reaggregate cultures of anterior pituitaries. Type II 5'-deiodinase activity determined in parallel with 3,3',5'triiodo-L-thyronine (rT(3)) as substrate was markedly lower than that of 5'DI. 5'DI mRNA levels and activity were higher in anterior pituitaries of female compared to male rats. Neither gender differences nor T-3 stimulation of 5'DI activity were found in the posterior part. These data demonstrate a T-3 dependent expression of 5'DI in euthyroid anterior pituitary and suggest that this isozyme serves a major function within the complex network of thyroid hormone homeostasis.