BENZODIAZEPINES INHIBIT TEMPERATURE-DEPENDENT-L-[I-125] TRIIODOTHYRONINE ACCUMULATION INTO HUMAN LIVER, HUMAN NEUROBLAST, AND RAT PITUITARY CELL-LINES

L-T3 (T3) accumulates into cells in a temperature-dependent saturable manner through a purported iodothyronine membrane carrier protein. We report energy-dependent uptake of picomolar [I-125]T3 into differentiated cell lines derived from human liver, human neuroblast, and rat pituitary malignancies. Furthermore, this cellular uptake is inhibited by classical and nonclassical benzodiazepine-type drugs (BZs); the apparent half-maximal inhibitory concentrations range from 50 nM to 50-mu-M, varying with drug and cell type. The site of this T3-BZ interaction was explored with cross-competitive radioligand binding to rat liver cell fractions. No interaction was seen in experiments cross-competing unlabeled T3 (10(-9)-10(-5) M) against [H-3]Ro5 4864, a peripheral BZ receptor ligand, for binding sites in a crude rat liver mitochondrial fraction. As well, lormetazepam and triazolam, BZs that potently inhibit cellular uptake of [I-125]T3, have no effect on [I-125]T3 binding to rat liver nuclear sites. Studies of [H-3]diazepam and [H-3]Ro5 4864 show very little temperature-dependent uptake into HepG2 cells (< 0.5% over 90 min) and no effect from coincubation of unlabeled T3 (1-mu-M). Thus, the possibility that BZs are substrates for the T3 carrier protein and are causing the reduced cellular hormonal accumulation via competitive uptake and dilution of the radiolabeled cellular T3 is unlikely. In summary, 1) drugs from the BZ class inhibit high affinity temperature-dependent cellular accumulation of thyroid hormone into cell lines from rat and human species; 2) the site of action of BZ inhibition does not involve direct antagonism of the T3 nuclear receptor, nor is it likely that the peripheral BZ receptor is the iodothyronine carrier. BZs could be interacting with the purported iodothyronine carrier protein itself to block uptake.