The uptake of [I-125]triiodothyroacetic acid ([I-125]Triac) in anterior pituitary cells was investigated and compared with that of [I-125]T-3. Furthermore, the effects of Triac, T-3, and T-4 on TSH release were compared. Cells isolated from adult male Wistar rats were cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C with [I-125]Triac (100,000 cpm; 120 pM) or [I-125]T-3 (50,000 cpm; 50 pM) in medium with 0.5% BSA. In this medium, the ratio of the free fractions of Triac, T-3, and T-4 was 1:8:1. Exposure of cells to 100 nM TRH for 2 h stimulated TSH release by 80-110% (P < 0.001). Comparing total hormone levels (1 nM to 1 mu M), Triac and T-3 were equally effective in reducing this response, and both were 10-fold more effective than T-4. The time course (15 min to 4 h) of [I-125]Triac uptake was similar to that of [I-125]Ts showing equilibrium after 1 h. Unlabeled Triac (1 mu M) reduced the uptake of [I-125]Triac and [I-125]T-3 at all time intervals. Expressed per pM free hormone, the cellular and nuclear uptake of [I-125]Triac were twice those of [I-125]T-3, The 15-min uptake of [I-125]Triac was reduced by incubation with 10 nM unlabeled Triac (35%; P < 0.001). Maximum inhibition (56%; P < 0.001) was found with 10 mu M Triac. A similar effect was seen with 10 mu M T-3, T-4, or 3,3',5,5'-tetraiodothyroacetic acid. Preincubation (30 min) and incubation (15 min) with 10 mu M oligomycin reduced the cellular ATP content by 51% (P < 0.001), [I-125]T-3 uptake by 77% (P < 0.001), and [I-125]Triac uptake by only 25% (P < 0.001). The temperature dependence of [I-125]Triac and [I-125]T-3 uptake was the same. Preincubation and incubation with 10 mu M monensin (reduces the Na+ gradient) or 10 mu M monodansylcadaverine (inhibits receptor-mediated endocytosis) reduced 15-min [I-125] Triac uptake by 15% (P < 0.005) and 19% (P < 0.005), respectively. The data show that 1) Triac, on the basis of the free hormone concentration, is more potent than T-3 or T-4 in suppressing TSH secretion; and 2) the rapid uptake of [I-125]Triac by the anterior pituitary occurs by a carrier-mediated mechanism that is only partially dependent on ATP or the Na+ gradient.
