Evidence for carrier-mediated uptake of triiodothyronine in cultured anterior pituitary cells of euthyroid rats

T₃ uptake and TSH secretion were investigated in anterior pituitary cells isolated from adult fed Wistar rats and cultured for 3 days in medium containing 10% fetal calf serum. TSH release during culture increased linearly with the number of cells in the range of 80, 000800, 000 cells/well. Uptake and incubation experiments were performed at 37 C in medium containing 0.5% BSA. Incubation with TRH (0.1 μm) for 2 h stimulated TSH release 2.6-fold, and this effect was partly (-45%) suppressed by preexposure for 2 h to T₃ (0.01-1 μm) or T₄(1 μM). Similar concentrations of T₃ and T₄reduced the cellular uptake of [¹²⁵I]T₃ (50 pM) during 1 h of incubation by 55%. After 15 min of incubation, [¹²⁵I]T₃uptake (percent dose) amounted to 1.26 ± 0.05% (mean ± SE; n = 9)/500, 000 cells. The major part (75%) of the [¹²⁵I]T₃ was found in the extranuclear fraction. Simultaneous incubation with unlabeled T₃ (1 or 10 μm) reduced [¹²⁵I]T₃ uptake by 43% (n = 3; P < 0.001) and 52% (n = 6; P < 0.001), respectively. Reduction of the temperature to 20 C diminished the T₃-suppressible fraction of [^l26I]T₃uptake approximately 3-fold. After preincubation (30 min) and incubation (15 min) with monodansylcadaverine (100 μM), the uptake of [¹²⁵I]T₃was reduced by 32% (n = 3; P < 0.01). When the Na⁺ gradient was reduced by preincubation and incubation with ouabain (0.5 mM) or monensin (10 or 100 μm), T₃ uptake was inhibited by 25% (n = 5; P < 0.01), 37% (n = 6; P < 0.001), and 61% (n = 3; P < 0.001), respectively. It is concluded that 1) T₃ is taken up by the pituitary by a carrier-mediated mechanism, and 2) this uptake is at least partly dependent on the Na⁺ gradient.