Uptake of triiodothyronine sulfate and suppression of thyrotropin secretion in cultured anterior pituitary cells.

To investigate the uptake of triiodothyronine sulfate (T3S) and its effect on thyrotropin-releasing hormone (TRH)-induced thyrotropin (TSH) secretion, anterior pituitary cells were isolated from euthyroid rats and cultured for 3 days in medium containing 10% fetal calf serum. Incubation was performed at 37 degrees C in medium containing 0.5% bovine serum albumin (BSA). Exposure of the pituitary cells to TRH (0.1 mumol/L) for 2 hours stimulated TSH secretion by 176%. This effect was reduced by approximately 45% after a 2-hour preincubation with T3 (0.001 to 1 mumol/L). A significant inhibitory effect of T3S on TRH-induced TSH release was only observed at a concentration of 1 mumol/L. The uptake of [125I]T3 after 1 hour of incubation was reduced by 40% +/- 4% (P < .001) by simultaneous addition of 10 nmol/L unlabeled T3, whereas 1 mumol/L T3S was required to obtain a reduction of the [125I]T3 uptake by 34% +/- 2% (P < .001). The amount of T3 present in the unlabeled T3S preparation was 0.25% as determined by radioimmunoassay. When pituitary cells were incubated for 1 hour with [125I]T3S or [125I]T3 (both 50,000 cpm/0.25 mL), the uptake of [125I]T3S expressed as a percentage of the dose was 0.04% +/- 0.02% (mean +/- SE, n = 4), whereas that of [125I]T3 amounted to 3.0% +/- 0.4% (n = 4). In contrast, when hepatocytes were incubated for 1 hour with [125I]T3S, the uptake amounted to 5.1% +/- 0.8% (n = 9), whereas that of [125I]T3 was 22.1% +/- 1.7% (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)