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 80,000-800,000 cells/well. Uptake and incubation experiments were performed at 37 C in medium containing 0.5% BSA. Incubation with TRH (0.1 mum) for 2 h stimulated TSH release 2.6-fold, and this effect was partly (approximately 45%) suppressed by preexposure for 2 h to T3 (0.01-1 mum) or T4 (1 mum). Similar concentrations of T3 and T4 reduced the cellular uptake of [I-125]T3 (50 pm) during 1 h of incubation by 55%. After 15 min of incubation, [I-125]T3 uptake (percent dose) amounted to 1.26 +/- 0.05% (mean +/- SE; n = 9)/500,000 cells. The major part (75%) of the [I-125]T3 was found in the extranuclear fraction. Simultaneous incubation with unlabeled T3 (1 or 10 mum) reduced [I-125]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 [I-125]T3 uptake approximately 3-fold. After preincubation (30 min) and incubation (15 min) with monodansylcadaverine (100 mum), the uptake of [I-125]T3 was 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 mum), 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.