Uptake of triiodothyroacetic acid and its effect on thyrotropin secretion in cultured anterior pituitary cells.
The uptake of [125I]triiodothyroacetic acid ([125I]Triac) in anterior pituitary cells was investigated and compared with that of [125I]T3. Furthermore, the effects of Triac, T3, and T4 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 [125I]Triac (100,000 cpm; 120 pM) or [125I]T3 (50,000 cpm; 50 pM) in medium with 0.5% BSA. In this medium, the ratio of the free fractions of Triac, T3, and T4 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 microM), Triac and T3 were equally effective in reducing this response, and both were 10-fold more effective than T4. The time course (15 min to 4 h) of [125I]Triac uptake was similar to that of [125I]T3, showing equilibrium after 1 h. Unlabeled Triac (1 microM) reduced the uptake of [125I]Triac and [125I]T3 at all time intervals. Expressed per pM free hormone, the cellular and nuclear uptake of [125I]Triac were twice those of [125I]T3. The 15-min uptake of [125I]Triac was reduced by incubation with 10 nM unlabeled Triac (35%; P < 0.001). Maximum inhibition (56%; P < 0.001) was found with 10 microM Triac. A similar effect was seen with 10 microM T3, T4, or 3,3',5,5'-tetraiodothyroacetic acid. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced the cellular ATP content by 51% (P < 0.001), [125I]T3 uptake by 77% (P < 0.001), and [125I]Triac uptake by only 25% (P < 0.001). The temperature dependence of [125I]Triac and [125I]T3 uptake was the same. Preincubation and incubation with 10 microM monensin (reduces the Na+ gradient) or 10 microM monodansylcadaverine (inhibits receptor-mediated endocytosis) reduced 15-min [125I] 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 T3 or T4 in suppressing TSH secretion; and 2) the rapid uptake of [125I]Triac by the anterior pituitary occurs by a carrier-mediated mechanism that is only partially dependent on ATP or the Na+ gradient.
Everts
M E
ME
Department of Internal Medicine, Erasmus University Medical School, Rotterdam, The Netherlands.
Visser
T J
TJ
Moerings
E P
EP
Docter
R
R
van Toor
H
H
Tempelaars
A M
AM
de Jong
M
M
Krenning
E P
EP
Hennemann
G
G
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D008297:Male; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013972:Thyrotropin / Q000378:metabolism*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology; D013997:Time Factors; D014284:Triiodothyronine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology
D014284:Triiodothyronine; C010642:3,3',5-triiodothyroacetic acid; D013973:Thyrotropin-Releasing Hormone; D013972:Thyrotropin