Active proton and urea transport by amphibian skin

Transcriptomes reveal the genetic mechanisms underlying ionic regulatory adaptations to salt in the crab-eating frog

The crab-eating frog, Fejervarya cancrivora, is the only frog that lives near seas. It tolerates increased environmental concentrations of sodium, chloride and potassium partly by raising ion and urea levels in its blood plasma. The molecular mechanism of the adaptation remains rarely documented. Herein, we analyze transcriptomes of the crab-eating frog and its closely related saline-intolerant species, F. limnocharis, to explore the molecular basis of adaptations to such extreme environmental conditions. Analyses reveal the potential genetic mechanism underlying the adaptation to salinity for the crab-eating frog. Genes in categories associated with ion transport appear to have evolved rapidly in F. cancrivora. Both positively selected and differentially expressed genes exhibit enrichment in the GO category regulation of renal sodium excretion. In this category, the positively selected sites of ANPEP and AVPR2 encode CD13 and V2 receptors, respectively; they fall precisely on conserved domains. More differentially expressed rapidly evolved genes occur in the kidney of F. cancrivora than in F. limnocharis. Four genes involved in the regulation of body fluid levels show signs of positive selection and increased expression. Significant up-regulation occurs in several genes of F. cancrivora associated with renin-angiotensin system and aldosterone-regulated sodium reabsorption pathways, which relate to osmotic regulation.

Identification and Expression of a Putative Facilitative Urea Transporter in Three Species of True Frogs (Ranidae): Implications for Terrestrial Adaptation

Urea transporters (UTs) help mediate the transmembrane movement of urea and therefore are likely important in amphibian osmoregulation. Although UTs contribute to urea reabsorption in anuran excretory organs, little is known about the protein’s distribution and functions in other tissues, and their importance in the evolutionary adaptation of amphibians to their environment remains unclear. To address these questions, we obtained a partial sequence of a putative UT and examined relative abundance of this protein in tissues of the wood frog (Rana sylvatica), leopard frog (R. pipiens), and mink frog (R. septentrionalis), closely related species that are adapted to different habitats. Using immunoblotting techniques, we found the protein to be abundant in the osmoregulatory organs but also present in visceral organs, suggesting that UTs play both osmoregulatory and nonosmoregulatory roles in amphibians. UT abundance seems to relate to the species’ habitat preference, as levels of the protein were higher in the terrestrial R. sylvatica, intermediate in the semiaquatic R. pipiens, and quite low in the aquatic R. septentrionalis. These findings suggest that, in amphibians, UTs are involved in various physiological processes, including solute and water dynamics, and that they have played a role in adaptation to the osmotic challenges of terrestrial environments.

Active urea transport in lower vertebrates and mammals

Some unicellular organisms can take up urea from the surrounding fluids by an uphill pumping mechanism. Several active (energy-dependent) urea transporters (AUTs) have been cloned in these organisms. Functional studies show that active urea transport also occurs in elasmobranchs, amphibians, and mammals. In the two former groups, active urea transport may serve to conserve urea in body fluids in order to balance external high ambient osmolarity or prevent desiccation. In mammals, active urea transport may be associated with the need to either store and/or reuse nitrogen in the case of low nitrogen supply, or to excrete nitrogen efficiently in the case of excess nitrogen intake. There are probably two different families of AUTs, one with a high capacity able to establish only a relatively modest transepithelial concentration difference (renal tubule of some frogs, pars recta of the mammalian kidney, early inner medullary collecting duct in some mammals eating protein-poor diets) and others with a low capacity but able to maintain a high transepithelial concentration difference that has been created by another mechanism or in another organ (elasmobranch gills, ventral skin of some toads, and maybe mammalian urinary bladder). Functional characterization of these transporters shows that some are coupled to sodium (symports or antiports) while others are sodium-independent. In humans, only one genetic anomaly, with a mild phenotype (familial azotemia), is suspected to concern one of these transporters. In spite of abundant functional evidence for such transporters in higher organisms, none have been molecularly identified yet.

Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog

We investigated hibernation physiology and freeze tolerance in a population of the wood frog, Rana sylvatica, indigenous to Interior Alaska, USA, near the northernmost limit of the species' range. Winter acclimatization responses included a 233% increase in the hepatic glycogen depot that was subsidized by fat body and skeletal muscle catabolism, and a rise in plasma osmolality that reflected accrual of urea (to 106±10 μmol ml−1) and an unidentified solute (to ~73 μmol ml−1). In contrast, frogs from a cool-temperate population (southern Ohio, USA) amassed much less glycogen, had a lower uremia (28±5 μmol ml−1) and apparently lacked the unidentified solute. Alaskan frogs survived freezing at temperatures as low as −16°C, some 10–13°C below those tolerated by southern conspecifics, and endured a 2-month bout of freezing at −4°C. The profound freeze tolerance is presumably due to their high levels of organic osmolytes and bound water, which limits ice formation. Adaptive responses to freezing (−2.5°C for 48 h) and subsequent thawing (4°C) included synthesis of the cryoprotectants urea and glucose, and dehydration of certain tissues. Alaskan frogs differed from Ohioan frogs in retaining a substantial reserve capacity for glucose synthesis, accumulating high levels of cryoprotectants in brain tissue, and remaining hyperglycemic long after thawing. The northern phenotype also incurred less stress during freezing/thawing, as indicated by limited cryohemolysis and lactate accumulation. Post-glacial colonization of high latitudes by R. sylvatica required a substantial increase in freeze tolerance that was at least partly achieved by enhancing their cryoprotectant system.

Cutaneous nitrogen excretion in the African clawed frog Xenopus laevis: effects of high environmental ammonia (HEA)

Ammonia is a highly toxic molecule and often introduced in considerable amounts into aquatic environments due to anthropogenic activities. Many aquatic and semi-aquatic amphibians utilize, in addition to their kidneys, the skin for osmoregulation and nitrogen excretion. In the present study the effects of prolonged (7-21 days) exposure to high environmental ammonia (HEA, 1 mmol l(-1) NH4Cl) on cutaneous nitrogen excretion and gene expression of key-transporters involved in nitrogen excretion and acid-base regulation were investigated in the fully aquatic African clawed frog, Xenopus laevis. The study revealed that X. laevis excretes predominately ammonia of which approximately 50% is excreted via the skin. Both the ventral and dorsal skin were capable to generate a net ammonia efflux, which was significantly activated by 10 mmol l(-1) of the phosphodiesterase blocker theophylline. The obtained data further suggest that the ammonia efflux was promoted by an acidification of the unstirred boundary layer, likely generated by an apical localized V-ATPase, with NH3 being transported via cutaneous expressed ammonia transporters, Rhbg and Rhcg. Prolonged HEA exposure did significantly reduce the net-flux rates over the ventral skin with Vmax changing from 256 nmol cm(-2) h(-1) in control frogs to 196 nmol cm(-2) h(-1) in HEA exposed animals. Further, prolonged HEA exposure caused a decrease in mRNA expression levels of the ammonia transporter Rhbg, Na(+)/K(+)-ATPase (α-subunit) and V-ATPase (subunit H) in the ventral and dorsal skin and the kidney. In contrast, Rhcg expression levels were unaffected by HEA in skin tissues.

New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism

The mechanism by which urine is concentrated in the mammalian kidney remains incompletely understood. Urea is the dominant urinary osmole in most mammals and may be concentrated a 100-fold above its plasma level in humans and even more in rodents. Several facilitated urea transporters have been cloned. The phenotypes of mice with deletion of the transporters expressed in the kidney have challenged two previously well-accepted paradigms regarding urea and sodium handling in the renal medulla but have provided no alternative explanation for the accumulation of solutes that occurs in the inner medulla. In this review, we present evidence supporting the existence of an active urea secretion in the pars recta of the proximal tubule and explain how it changes our views regarding intrarenal urea handling and UT-A2 function. The transporter responsible for this secretion could be SGLT1, a sodium-glucose cotransporter that also transports urea. Glucagon may have a role in the regulation of this secretion. Further, we describe a possible transfer of osmotic energy from the outer to the inner medulla via an intrarenal Cori cycle converting glucose to lactate and back. Finally, we propose that an active urea transporter, expressed in the urothelium, may continuously reclaim urea that diffuses out of the ureter and bladder. These hypotheses are all based on published findings. They may not all be confirmed later on, but we hope they will stimulate further research in new directions.

Mechanism of sodium uptake in PNA negative MR cells from rainbow trout, Oncorhynchus mykiss as revealed by silver and copper inhibition

The rate of acid-stimulated and phenamil-sensitive sodium (Na(+)) uptake was measured in three different cell lineages: pavement cells (PVC), total mitochondrion-rich (MR) cell populations, and peanut lectin agglutinin-negative mitochondrion-rich cells (PNA(-) MR) isolated from the rainbow trout gill epithelium. Despite the presence of basal levels of Na(+) uptake in PVC, this transport was not enhanced by acidification, nor was it inhibited by independent treatment with bafilomycin (i.e., a V-type H(+)-ATPase inhibitor), phenamil (i.e., a specific inhibitor of ENaC), or Ag (a specific inhibitor of active Na(+) transport in fish). In contrast, Na(+) uptake in PNA(-) MR cells was increased by ~220% above basal levels following acidification of near 0.4 pH units in the presence of 1.0 mM external Na(+). Acid-stimulated Na(+) transport was entirely inhibited by both phenamil and bafilomycin. Silver (Ag) and copper (Cu), which are known to interfere with active Na(+) transport in fish, were also responsible for inhibiting acid stimulated Na(+) uptake in PNA(-) MR cells, but by themselves had no effect on basal Na(+) transport. Thus, we demonstrate that Ag specifically prevented acid-stimulated Na(+) uptake in PNA(-) MR cells in a dose-dependent manner. We also demonstrate rapid (<1 min) and significant inhibition of carbonic anhydrase (CA) by Ag in PNA(-) MR cells, but not in PVC. These data lend further support to the idea of a PNA(-) MR cell type as the primary site for Na(+) uptake in the freshwater (FW) gill phenotype of rainbow trout. Moreover, these findings provide support for the importance of intracellular protons in regulating the movement of Na(+) across the apical surface of the fish gill.

Embryonic frog epidermis: a model for the study of cell-cell interactions in the development of mucociliary disease

Specialised epithelia such as mucociliary, secretory and transporting epithelia line all major organs, including the lung, gut and kidney. Malfunction of these epithelia is associated with many human diseases. The frog embryonic epidermis possesses mucus-secreting and multiciliated cells, and has served as an excellent model system for the biogenesis of cilia. However, ionic regulation is important for the function of all specialised epithelia and it is not clear how this is achieved in the embryonic frog epidermis. Here, we show that a third cell type develops alongside ciliated and mucus-secreting cells in the tadpole skin. These cells express high levels of ion channels and transporters; therefore, we suggest that they are analogous to ionocytes found in transporting epithelia such as the mammalian kidney. We show that frog ionocytes express the transcription factor foxi1e, which is required for the development of these cells. Depletion of ionocytes by foxi1e knockdown has detrimental effects on the development of multiciliated cells, which show fewer and aberrantly beating cilia. These results reveal a newly identified role for ionocytes and suggest that the frog embryonic skin is a model system that is particularly suited to studying the interactions of different cell types in mucociliary, as well as in secretory and transporting, epithelia.

Mitochondria-rich cells in amphibian skin epithelium: Relationship of immuno- and peanut lectin labeling pattern and transport functions

Mitochondria-rich cells are an integral component of the epidermis of amphibian skin and play a functional role. Whereas the principal cell compartment of the epithelium is specialized almost exclusively for active uptake of sodium, the mitochondria-rich cells perform other diverse ion-transport functions, including transport of Cl(-), H(+), HCO(3)(-) and organic molecules. These transporting functions differ in different species. Antibodies, such as those directed against band 3, H(+)-ATPase, and also peanut lectin (PNA), bind specifically to the mitochondria-rich cells, but do so differently in various species. Examination of these immunolocalizations and lectin labeling in the skin of over 10 amphibian species, including both Anurans and Urodeles, illustrate species-specific differences. The binding pattern and the transport capabilities of the skin in the various species do not show a universal correlation, they appear to be species specific and do not permit construction of a general scheme common to all the species studied. The mitochondria-rich cells of heterocellular epithelia and their roles in ion transport remain a subject that requires further studies to elucidate their particular functions within the framework of the whole epithelium.

Theoretical considerations underlying Na(+) uptake mechanisms in freshwater fishes

Ion and acid-base regulating mechanisms have been studied at the fish gill for almost a century. Original models proposed for Na(+) and Cl(-) uptake, and their linkage with H(+) and HCO(3)(-) secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill, two dominant mechanisms for Na(+) uptake from dilute environments have persisted in the literature. The use of an apical Na(+)/H(+) exchanger driven by a basolateral Na(+)/K(+)-ATPase versus an apical Na(+) channel electrogenically coupled to an apical H(+)-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill. However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on the recent molecular advances for Na(+) uptake in freshwater fish. Emphasis will be placed on thermodynamic constraints that prevent electroneutral apical NHE function in most freshwater environments. By combining recent advances in molecular and functional physiology of fish gills with thermodynamic considerations of ion transport, our knowledge in the field should continue to grow in a logical manner.

Hormonal regulation of ion and water transport in anuran amphibians

Amphibians occupy a wide variety of ecological habitats, and their adaptation is made possible through the specialization of the epithelia of their osmoregulatory organs, such as the skin, kidney, and urinary bladder, which control the hydromineral and acid-base balance of their internal medium. Amphibians can change drastically plasma Na+, Cl-, and urea levels and excretion rates in response to environmental stimuli such as acute desiccation and changes in external salinity. Several hormones and the autonomic nervous system act to control osmoregulation. Several ion channels including an epithelial sodium channel (ENaC), a urea transporter (UT), and water channels (AQPs) are found in epithelial tissues of their osmoregulatory organs. This mini review examines the currents status of our knowledge about hormone receptors for arginine vasotocin, angiotensin II and aldosterone, and membrane ion channels and transporters, such as ENaC, UT, and AQPs in amphibians.

Effect of osmotic stress on expression of a putative facilitative urea transporter in the kidney and urinary bladder of the marine toad, Bufo marinus

Anuran amphibians accumulate a large amount of urea in their extracellular fluids to avoid a severe dehydration under dry and hyper-saline environments. To clarify the mechanisms of urea retention, we examined structure and distribution of the urea transporter (UT) in the kidney of the marine toad (Bufo marinus), and its expression in the kidney and urinary bladder following exposure to dry and hyper-saline conditions by means of cDNA cloning, semi-quantitative RT-PCR, immunoblot analysis and immunohistochemistry. The Bufo UT cDNA cloned from the kidney encodes a 390-amino-acid residue protein, which is 80% identical to Rana esculenta UT with the functional characteristics of a urea transporter. The Bufo UT mRNA was abundantly expressed in the kidney and urinary bladder, but not in the skin. In immunoblot analysis using a specific antibody raised against the Bufo UT, a 52 kDa protein similar to the glycosylated forms of mammalian UT-A2 ( approximately 55 kDa) was detected in extracts from plasma membrane fractions of the kidney and urinary bladder. When toads were acclimated to dry and hyper-saline environments for 7 days, UT mRNA expression was upregulated in the kidney and urinary bladder and there was an elevated plasma urea concentration and osmolality. Immunohistochemistry showed that the UT was specifically localized on the apical membrane of the early distal tubule, known to be the diluting segment, in the kidney and the epithelial cells of urinary bladder. Immunoreactive cells were not detected along the late distal tubule, the connecting tubule or the collecting duct in the kidney. The present findings suggest that the Bufo UT probably contributes to urea transport in the kidney and urinary bladder in response to hyperosmotic stresses such as body fluid hypertonicity and dehydration.

Urea and urine concentrating ability: new insights from studies in mice

Urea is the most abundant solute in the urine in humans (on a Western-type diet) and laboratory rodents. It is far more concentrated in the urine than in plasma and extracellular fluids. This concentration depends on the accumulation of urea in the renal medulla, permitted by an intrarenal recycling of urea among collecting ducts, vasa recta and thin descending limbs, all equipped with specialized, facilitated urea transporters (UTs) (UT-A1 and 3, UT-B, and UT-A2, respectively). UT-B null mice have been recently generated by targeted gene deletion. This review describes 1) the renal handling of urea by the mammalian kidney; 2) the consequences of UT-B deletion on urinary concentrating ability; and 3) species differences among mice, rats, and humans related to their very different body size and metabolic rate, leading to considerably larger needs to excrete and to concentrate urea in smaller species (urea excretion per unit body weight in mice is 5 times that in rats and 23 times that in humans). UT-B null mice have a normal glomerular filtration rate but moderately reduced urea clearance. They exhibit a 30% reduction in urine concentrating ability with a more severe defect in the capacity to concentrate urea (50%) than other solutes, despite a twofold enhanced expression of UT-A2. The urea content of the medulla is reduced by half, whereas that of chloride is almost normal. When given an acute urea load, UT-B null mice are unable to raise their urinary osmolality, urine urea concentration (Uurea), and the concentration of non-urea solutes, as do wild-type mice. When fed diets with progressively increasing protein content (10, 20, and 40%), they cannot prevent a much larger increase in plasma urea than wild-type mice because they cannot raise Uurea. In both wild-type and UT-B null mice, urea clearance was higher than creatinine clearance, suggesting the possibility that urea could be secreted in the mouse kidney, thus allowing more efficient excretion of the disproportionately high urea load. On the whole, studies in UT-B null mice suggest that recycling of urea by countercurrent exchange in medullary vessels plays a more crucial role in the overall capacity to concentrate urine than its recycling in the loops of Henle.

Urea transport in kidney brush-border membrane vesicles from an elasmobranch, Raja erinacea

Marine elasmobranch fishes maintain high urea concentrations and therefore must minimize urea loss to the environment in order to reduce the energetic costs of urea production. Previous studies have identified a facilitated urea transporter in the kidney of the dogfish. We examined mechanisms of urea transport in the kidney of the little skate Raja erinacea using an isolated brush-border membrane vesicle preparation. Urea uptake by brush-border membrane vesicles is by a phloretin-sensitive, non-saturable uniporter in the dorsal section and a phloretin-sensitive, sodium-linked urea transporter (Km = 0.70 mmol l(-1), Vmax = 1.18 micromol h(-1) mg(-1) protein) in the ventral section of the kidney. This provides evidence for two separate urea transporters in the dorsal versus ventral sections of the kidney. We propose that these two mechanisms of urea transport are critical for renal urea reabsorption in the little skate.

Mitochondria-rich cells as experimental model in studies of epithelial chloride channels

The mitochondria-rich (mr) cell of amphibian skin epithelium is differentiated as a highly specialised pathway for passive transepithelial transport of chloride. The apical membrane of mr cells expresses several types of Cl(-) channels, of which the function of only two types has been studied in detail. (i) One type of channel is gated by voltage and external chloride concentration. This intriguing type of regulation leads to opening of channels only if [Cl(-)](o) is in the millimolar range and if the electrical potential is of a polarity that secures an inwardly directed net flux of this ion. Reversible voltage activations of the conductance proceed with long time constants, which depend on V in such a way that the rate of conductance activation increases when V is clamped at more negative values (serosal bath grounded). The gating seems to involve processes that are dependent on F-actin localised in the submembrane domain in the neck region of the flask-shaped mr cell. (ii) The other identified Cl(-) pathway of mr cells is mediated by small-conductance apical CFTR chloride channels as concluded from its activation via beta-adrenergic receptors, ion selectivity, genistein stimulation and inhibition by glibenclamide. bbCFTR has been cloned, and immunostaining has shown that the gene product is selectively expressed in mr cells. There is cross-talk between the two pathways in the sense that activation of the conductance of the mr cell by voltage clamping excludes activation via receptor occupation, and vice versa. The mechanism of this cross-talk is unknown.

Amphibian skin diseases

Skin diseases in amphibian species seem to carry an additional degree of seriousness compared with those of other vertebrates. Because of the skin's importance in respiration and ion transport, breaching of the integument of these animals can result in fatal septicemias or metabolic disturbances. The timely diagnosis and treatment of these skin lesions is important. A review of the diseases affecting amphibian skin is provided. This article also describes the clinical signs, appropriate diagnostic steps, and treatment and control of amphibian skin diseases.

Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch

In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregulation. Retention of urea by the gill against this huge blood-to-water diffusion gradient requires specialized adaptations to the epithelial cell membranes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Squalus acanthias. Analysis of urea uptake by gill basolateral membrane vesicles revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 micromol. h(-1). mg protein(-1)). We propose that this system actively transports urea out of the gill epithelial cells back into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributing to the highest reported cholesterol-to-phospholipid molar ratio (3.68). This unique combination of active urea transport and modification of the phospholipid bilayer membrane is responsible for decreasing the gill permeability to urea and facilitating urea retention by the gill of Squalus acanthias.

Cloning and molecular characterisation of the trout (Oncorhynchus mykiss) vacuolar H(+)-ATPase B subunit

The current model of transepithelial ion movements in the gill of freshwater fish incorporates an apically oriented vacuolar H(+)-ATPase (H(+)V-ATPase; proton pump) that is believed to facilitate both acid excretion and Na(+) uptake. To substantiate this model, we have cloned and sequenced a cDNA encoding the B subunit of the rainbow trout (Oncorhynchus mykiss) H(+)V-ATPase. The cloning of the B subunit enabled an examination by northern analysis of its tissue distribution and expression during external hypercapnia. Degenerate oligonucleotide primers to the B subunit of the H(+)V-ATPase were designed and used in a semi-nested polymerase chain reaction (PCR) to amplify an 810 base pair (bp) product from a trout gill/kidney cDNA library. This PCR product was cloned and sequenced and then used to screen the same cDNA library. The assembled 2262 bp cDNA included an open reading frame coding for a deduced protein of 502 amino acid residues. A BLAST search of the GenBank nucleotide database revealed numerous matches to other vertebrate and invertebrate H(+)V-ATPase B subunits. Protein alignment demonstrated that the trout H(+)V-ATPase B subunit is more than 85 % identical and more than 90 % similar to those in other vertebrate species. An initial analysis of H(+)V-ATPase mRNA tissue distribution revealed significant expression in blood. Although a comparison of perfused tissues (blood removed) with non-perfused tissues demonstrated no obvious contribution of the blood to total tissue H(+)-ATPase mRNA levels, all subsequent experiments were performed using perfused tissues. Levels of H(+)V-ATPase mRNA expression were high in the gill, kidney (anterior or posterior), intestine, heart and spleen, but lower in liver and white muscle. Exposure of the fish to 12 h of external hypercapnia (water P(CO2)=7. 5 mmHg; 1 kPa) was associated with a transient increase (at 2 h) in the levels of H(+)V-ATPase B subunit mRNA in gill and kidney; liver mRNA levels were unaffected. These results are consistent with the hypothesis of an apically localised plasma membrane H(+)V-ATPase in the freshwater trout gill and that the expression of this proton pump is increased during periods of acidosis, at least in part because of an increased steady-state level of H(+)V-ATPase mRNA.