Regulation of Thyroxine 5′-Deiodinase Activity by 3,5,3′-Triiodothyronine in Cultured Rat Anterior Pituitary Cells*

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Abstract
The influence of the medium T3 [triiodothyronine] concentration on iodothyronine 5''-deiodinase activity was studied in cultured anterior pituitary cells derived from chronically hypothyroid rats. Type II (propylthiouracil-insensitive) enzyme activity, measured with thyroxine as substrate, was reduced by T3 in a dose-dependent manner, with an ED50 of .apprx. 1.4 .times. 10-10 M free T3. Density gradient centrifugation was used to obtain populations of pituitary cells relatively enriched in thyrotrophs, somatotrophs, mammotrophs, or gonadotrophs, and the effect of T3 on type II 5''-deiodinase activity was evaluated in each of these 4 populations. In the absence of T3, the enzyme activity was 1.5- to 2-fold greater in the somatotroph- and mammotroph-enriched cell pools than in the thyrotroph- and gonadotroph-enriched pools. In contrast, when the cells were cultured in the presence of T3, enzyme activity was reduced to the same low level in all 4 enriched pools. The increase in whole pituitary type II 5''-deiodinase activity associated with hypothyroidism is due largely or totally to increases occurring within somatotrophs and mammotrophs. The intrinsic responsiveness of the deiodinase to hypothyroidism is greater in somatotrophs and mammotrophs than in other anterior pituitary cells.