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Transport of thyroxine (T(4)) into the liver is inhibited in fasting and by bilirubin, a compound often accumulating in the serum of critically ill patients. We tested the effects of chronic and acute energy deprivation, bilirubin and its precursor biliverdin on the 15-min uptake of [(125)I]tri-iodothyronine ([(125)I]T(3)) and [(125)I]T(4) and on TSH release in rat anterior pituitary cells maintained in primary culture for 3 days. When cells were cultured and incubated in medium without glucose and glutamine to induce chronic energy deprivation, the ATP content was reduced by 45% (P<0. 05) and [(125)I]T(3) uptake by 13% (NS), but TSH release was unaltered. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced ATP content by 51% (P<0.05) and 53% (P<0. 05) under energy-rich and energy-poor culture conditions respectively; [(125)I]T(3) uptake was reduced by 66% (P<0.05) and 64% (P<0.05). Neither bilirubin nor biliverdin (both 1-200 microM) affected uptake of [(125)I]T(3) or [(125)I]T(4). Bilirubin (1-50 microM) did not alter basal or TRH-induced TSH release. In conclusion, the absence of inhibitory effects of chronic energy deprivation and bilirubin on thyroid hormone uptake by pituitary cells supports the view that the transport is regulated differently than that in the liver.

Cellular and nuclear uptake of [125I]tri-iodothyronine (T3) and [125I]triiodothyroacetic acid (Triac) were compared in cardiomyocytes of 2-3 day old rats, and the effect of thyroid hormone analogs on cellular T(3) uptake was measured. Cells (5-10 x 10(5) per well) were cultured in DMEM-M199 with 5% horse serum and 5% FCS. Incubations were performed for from 15 min to 24 h at 37 degrees C in the same medium, 0.5% BSA and [125I]T3 (100 pM), or [125I]Triac (240 pM). Expressed as % dose, T(3) uptake was five times Triac uptake, but expressed as fmol/pM free hormone, Triac uptake was at least 30% (P<0.001) greater than T3 uptake, whereas the relative nuclear binding of the two tracers was comparable. The 15 min uptake of [125I]T3 was competitively inhibited by 10 microM unlabeled T3 (45-52%; P<0.001) or 3,3'- diiodothyronine (T2) (52%; P<0.001), and to a smaller extent by thyroxine (T(4)) (27%; 0.05<0.1). In contrast, 10 microM 3,5-T2, Triac, or tetraiodothyroacetic acid (Tetrac) did not affect T3 uptake after 15 min or after 24 h. Diiodothyropropionic acid (DITPA) (10 microM) reduced 15-min T3 uptake by about 24% (P<0.05), but it had a greater effect after 4 h (56%; P<0.001). Exposure to 10 nM DITPA during culture reduced cellular T3 uptake, as did 10 nM T3, suggesting down-regulation of the plasma membrane T3 transporters. We conclude that i) Triac is taken up by cardiomyocytes; ii) 3,3'-T2 and, to a lesser extent, DITPA and T4 interfere with plasma membrane transport of T3, whereas 3,5-T2, Triac, or Tetrac do not; iii) the transport mechanism for Triac is probably different from that for T3.