Requisite structural characteristics for benzodiazepine inhibition of triiodothyronine uptake into a human liver cell line

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Modulation of binding characteristics of peripheral benzodiazepine receptors in vitamin A-deficient guinea pig lung

Both vitamin A and peripheral benzodiazepine receptors (PBRs) are involved in the control of cell proliferation and differentiation. The objective of this study was to determine whether vitamin A deficiency causes any modulation in the binding characteristics of the PBRs. Forty-five weanling guinea pigs were divided into three groups (control, pair-fed control, and vitamin A-deficient). Vitamin A-deficiency status was achieved after 90 days of feeding. It caused atelectasis, hyperplasia and metaplasia of alveolar epithelium, acute inflammation of the tracheobronchial tree, and a significant increase in the number of alveolar type II cells. In comparison to the control and pair-fed control groups, the lung of vitamin A-deficient animals had 40.6 and 42.8% less PBR density, respectively. There was no significant difference in the equilibrium dissociation constant (KD) of PBRs between the control and the pair-fed control groups, whereas the KD value was 77.7 and 60% higher in the vitamin A-deficient groups than in the control and the pair-fed control groups, respectively. Furthermore, vitamin A-deficiency caused a decrease in the binding capacity of PBRs in both nuclear and mitochondrial fractions. These results may suggest a close functional relationship between vitamin A and PBRs.