Facilitated diffusion of urate in avian brush-border membrane vesicles

Avian renal proximal tubule epithelium urate secretion is mediated by Mrp4

Birds are uricotelic and, like humans, maintain high plasma urate concentrations (∼300 μM). The majority of their urate waste, as in humans, is eliminated by renal proximal tubular secretion; however, the mechanism of urate transport across the brush-border membrane of the intact proximal tubule epithelium during secretion is uncertain. The dominance of secretory urate transport in the bird provides a convenient model for examining this process. The present study shows that short hairpin RNA interference (shRNAi) effectively knocked down gene expression of multidrug resistance protein 4 (Mrp4; 25% of control) in primary monolayer cultures of isolated chicken proximal tubule epithelial cells (cPTCs). Control and Mrp4-shRNAi-treated cPTCs were mounted in Ussing chambers and unidirectional transepithelial fluxes of urate were measured. To detect nonspecific effects, transepithelial electrical resistance (TER) and sodium-dependent glucose transport (Iglu) were monitored throughout experiments. Knocking down Mrp4 expression resulted in a reduction of transepithelial urate secretion to 35% of control with no effects on TER or Iglu. Although electrical gradient-driven urate transport in isolated brush-border membrane vesicles was confirmed, potassium-induced depolarization of the plasma membrane in intact cPTCs failed to inhibit active transepithelial urate secretion. However, electrical gradient-dependent vesicular urate transport was inhibited by the MRP4 inhibitor MK-571 also known to inhibit active transepithelial urate transport by cPTCs. Based on these data, direct measure of active transepithelial urate secretion in functional avian proximal tubule epithelium indicates that Mrp4 is the dominant apical membrane exit pathway from cell to lumen.

Transepithelial urate transport by avian renal proximal tubule epithelium in primary culture

Birds are uricotelic, and because they excrete urate by renal tubular secretion, they provide a convenient model for examination of this process. Primary monolayer cultures of the isolated renal proximal tubule epithelium from the domestic chicken, Gallus gallus L., were mounted in Ussing chambers where several substrates/inhibitors of renal organic anion transporters were tested for the sidedness and specificity of their effects on transepithelial urate transport. Transepithelial electrical resistance, electrical potential and sodium-dependent glucose current were monitored to detect nonspecific effects. Under control short-circuited conditions the ratio of unidirectional fluxes of [(14)C]urate was found to be 3:1. Active net secretion was specifically inhibited by 1 mmol l(-1) probenecid and 10 mmol l(-1) para-aminohippuric acid (PAH). Bromocresol Green, cimetidine, nocodozole, cytochalasin D and ouabain also inhibited secretion but were toxic. Interstitial-side lithium (5 mmol l(-1)) and glutarate (1 mmol l(-1)) specifically blocked transport, but 10-100 micromol l(-1) glutarate had no effect. Interstitial estrone sulfate (ES) stimulated urate secretion at 10 micromol l(-1) but was inhibitory at 500 micromol l(-1). Active PAH secretion (5:1 flux ratio) was inhibited 34% by 330 micromol l(-1) urate. ES (500 micromol l(-1)) blocked the remainder. From the lumen side, glucose-free, Cl(-)-free and high K(+) (30 mmol l(-1)) solutions, or an alkaline pH of 7.7 had no effect on urate transport and neither did several compounds known to be uricosuric. Lumen-side methotrexate (500 micromol l(-1)) and MK571 (20 micromol l(-1)) strongly inhibited urate secretion. MK571 had no effect from the interstitial side. RT-PCR revealed mRNA for OAT1-, OAT3-, MRP2- and MRP4-like organic anion transporters in chicken proximal epithelium.

Thyroid hormone deiodinases: physiology and clinical disorders

Thyroid hormone action is achieved through the binding of 3,5,3'-triiodothyronine to its nuclear receptor, which results in alterations in gene expression. An impairment in thyroid hormone action during vertebrate development results in severe, irreversible abnormalities in tissue growth, maturation, and function. The deiodinases are a family of selenoproteins expressed in a number of fetal and adult tissues that catalyze the activation and inactivation of thyroid hormones. Their unique biochemical characteristics and tissue and developmental expression patterns suggest that deiodinases may control the concentration of active thyroid hormone available to specific tissues or cell types at certain stages of development. The deiodinases thus appear to play an important role in regulating thyroid hormone action at a prereceptor level. Current research focusing on a better understanding of the biochemistry, regulation, and physiologic role of these enzymes is the focus of this review.

The role of extrinsic and intrinsic factors in the evolution of the control of pulmonary surfactant maturation during development in the amniotes

Pulmonary surfactant is a mixture of lipids and proteins that is secreted by alveolar Type II cells. It reduces alveolar surface tension and hence the work of breathing. Despite the tremendous diversity of lung structures amongst the vertebrates, the composition of surfactant is highly conserved. Conserved elements of the surfactant system amongst distantly related species are likely to be crucial factors for successful lung development. Understanding the mechanisms by which the surfactant system becomes operational in animals with dramatically different birthing strategies and in distantly related species will provide important information about the role of the surfactant system in the commencement of air breathing and the processes regulating surfactant maturation and secretion. In mammals, the embryonic maturation of the surfactant system is controlled by a host of factors, including glucocorticoids, thyroid hormones, and autonomic neurotransmitters. Here we review the mechanisms controlling the maturation of surfactant production, including birthing strategy, phylogeny, lung structure, and posthatching environment. Using four species of egg-laying amniote (chicken, dragon lizard, sea turtle, and crocodile) previously described in detail and the large amount of information available for mammals, we examine the hypothesis that the control of surfactant production is dependent on glucocorticoids (dexamethasone [Dex]), thyroid hormones (T3), and autonomic neurotransmitters (epinephrine and carbachol). We also examine whether the overall intrinsic pattern of the control of surfactant maturation is conserved throughout the vertebrate radiation and then how the environment (extrinsic factors) may account for the observed differences in the patterns of development. We also discuss the utility of a coculture system of embryonic Type II cells and fibroblasts to determine the evolutionary pattern behind the control of surfactant and to demonstrate that the surfactant system matures under multihormonal control. We demonstrate that Dex and T3 are stimulators of surfactant production during embryonic development, but they lose their efficacy closer to hatching or birth. Epinephrine stimulates surfactant secretion beyond 75% of development and also after hatching or birth. Carbachol stimulates surfactant secretion in the bearded dragon and saltwater crocodile but not in the sea turtle, chicken, or mammals. It is likely that the differences in control of surfactant development are likely to be primarily related to metabolic activity and the duration of incubation (i.e., the "speed" of development). Moreover, the hormones examined appear important in promoting development and therefore appear conserved within the amniotes. However, the autonomic neurotransmitters induced different responses in different species. Hence, some factors are crucial for the proper maturation of the surfactant system, whereas others vary throughout evolution without being detrimental to the overall function of the system.