Relationship between the thyroid hormone transport system and the Na(+)-H+ exchanger in cultured rat brain astrocytes.

The entry of T3 and T4 into rat cultured astrocytes is mediated by a sterospecific saturable transport system. This study examines the effect of inhibiting the Na(+)-H+ exchanger and intracellular acidification on the initial velocity of [125I]T3 and [125I]T4 uptake. The resting intracellular pH (pHi) was approximately 7.15 in astrocytes exposed to CO2/HCO3(-)-free medium buffered with HEPES at pH 7.40 at 22 C. Isoosmotic replacement of extracellular sodium by mannitol or choline decreased the pHi by 0.15 pH unit and reduced uptake by about 20%. Replacing sodium with lithium had no effect on uptake. Amiloride, a specific blocker of the Na(+)-H+ exchanger, reduced pHi, as described above, and inhibited T3 and T4 uptake by about 35%. Acid loading the cells with a NH4+ pulse decreased the pHi by up to 1.2 pH units and the uptake of T3 and T4 by up to 50%. The maximum velocity of uptake was decreased, whereas the Km was unchanged. An isoosmotic increase in the extracellular K+ concentration to 50 mM had no effect on T3 uptake. The initial velocity of T3 uptake by acid-loaded cells was gradually restored by increasing the extracellular Na+ concentration. These results indicate that thyroid hormone transport into rat cultured astrocytes involves a mechanism linked to the activity of the Na(+)-H+ exchanger and the H+ concentration inside the cells.

Identification by photoaffinity labelling of a pyridine nucleotide-dependent tri-iodothyronine-binding protein in the cytosol of cultured astroglial cells.

High-affinity 3,3',5-tri-iodo-L-thyronine (T3) binding (Kd approximately 0.3 nM) to the cytosol of cultured rat astroglial cells was strongly activated in the presence of pyridine nucleotides. A 35 kDa pyridine nucleotide-dependent T3-binding polypeptide (35K-TBP) was photoaffinity labelled using underivatized [125I]T3 in the presence of pyridine nucleotides and the free-radical scavenger dithiothreitol. Maximum activations of T3 binding and 35K-TBP photolabelling were obtained at approx. 1 x 10(-7) M NADP+ or NADPH, or 1 x 10(-4) M NADH. NAD+ and other nucleotides were without effect. NADPH is the form which activates T3 binding and 35K-TBP photolabelling, since cytosol contains NADP(+)-reducing activity, and the activation of both processes in the presence of NADPH and NADP+ was prevented by an exogenous NADPH oxidation system. NADPH behaved as an allosteric activator of T3 binding. The NADPH oxidation system promoted the release of bound T3 in the absence of any change in the total concentration of the hormone. The 35K-TBP photolabelling and [125I]T3 binding were similarly inhibited by non-radioactive T3 (half-maximum effect at 0.5-1.0 nM T3). The concentrations of iodothyronine analogues that inhibited both processes were correlated (3,3',5-tri-iodo-D-thyronine > or = T3 > L-thyroxine > tri-iodothyroacetic acid > 3,3'5'-tri-iodo-L-thyronine). Molecular sieving and density-gradient centrifugation of cytosol identified a 65 kDa T3-binding entity, which included the 35K-TBP. These results indicate that 35K-TBP is the cytosolic entity involved in the pyridine nucleotide-dependent T3 binding, and suggest that the sequestration and release of intracellular thyroid hormones are regulated by the redox state of astroglial cell compartment(s).

Triiodothyronine is a high-affinity inhibitor of amino acid transport system L1 in cultured astrocytes.

The relationship between the transport of thyroid hormones and that of amino acids was examined by measuring the uptake of amino acids that are characteristic substrates of systems L, A, and N, and the effect of 3,3',5-triiodo-L-thyronine (T3) on this uptake, in cultured astrocytes. Tryptophan and leucine uptakes were rapid, Na(+)-independent, and efficiently inhibited by T3 (half-inhibition at approximately 2 microM). Two Na(+)-independent L-like systems (L1 and L2), common to leucine and aromatic amino acids, were characterized kinetically. System L2 had a low affinity for leucine and tryptophan (Km = 0.3-0.9 mM). The high-affinity system L1 (Km approximately 10 microM for both amino acids) was competitively inhibited by T3 with a Ki of 2-3 microM (close to the T3 transport Km). Several T3 analogues inhibited system L1 and the T3 transport system similarly. Glutamine uptake and alpha-(methylamino)isobutyric acid uptake were, respectively, two and 200 times lower than tryptophan and leucine uptakes. T3 had little effect on the uptakes of glutamine and alpha -(methylamino)isobutyric acid. The results indicate that the T3 transport system and system L1 are related.