Regulation of a renal urea transporter with reduced salinity in a marine elasmobranch, Raja erinacea

Physiologic responses of Cownose Rays following freshwater dips for treatment of capsalid monogeneans

OBJECTIVE

Two Cownose Rays Rhinoptera bonasus were presented for management of a severe capsalid monogenean infection Benedeniella posterocolpa in a mixed species habitat at an aquarium.

METHODS

A series of freshwater (FW) dips were elected to mitigate parasite resistance to praziquantel due to the endemic monogenean population in the system. A pretreatment blood sample was opportunistically obtained, and subsequent samples were processed due to development of clinical signs.

RESULT

While the first FW dip was tolerated well and did not induce abnormal behavior, the second led to one individual presenting with tachypnea, pallor, and lethargy. Marked hematological abnormalities requiring medical intervention occurred in both individuals after the second dip. After treatment with elasmobranch Ringer's fluid therapy, sodium bicarbonate, prednisolone acetate, and vitamin E/selenium in the more severely affected ray, and elasmobranch Ringer's alone in the second ray, hematologic derangements and symptoms resolved. Hemoconcentration, hyperproteinemia, hyperlactatemia, and hyperglycemia were attributed to a stress response. A selective loss of urea after exposure to FW resulted in decreased plasma osmolality.

CONCLUSION

Given the severity of the stress response and associated complications observed, hyposalinity treatments should be utilized with caution in this species. If this therapy is employed, the clinician is advised to be prepared to administer supportive care coinciding with the FW dip if necessary.

Effects of the environment on the evolution of the vertebrate urinary tract

Evolution of the vertebrate urinary system occurs in response to numerous selective pressures, which have been incompletely characterized. Developing research into urinary evolution led to the occurrence of clinical applications and insights in paediatric urology, reproductive medicine, urolithiasis and other domains. Each nephron segment and urinary organ has functions that can be contextualized within an evolutionary framework. For example, the structure and function of the glomerulus and proximal tubule are highly conserved, enabling blood cells and proteins to be retained, and facilitating the elimination of oceanic Ca+ and Mg+. Urea emerged as an osmotic mediator during evolution, as cells of large organisms required increased precision in the internal regulation of salinity and solutes. As the first vertebrates moved from water to land, acid-base regulation was shifted from gills to skin and kidneys in amphibians. In reptiles and birds, solute regulation no longer occurred through the skin but through nasal salt glands and post-renally, within the cloaca and the rectum. In placental mammals, nasal salt glands are absent and the rectum and urinary tracts became separate, which limited post-renal urine concentration and led to the necessity of a kidney capable of high urine concentration. Considering the evolutionary and environmental selective pressures that have contributed to renal evolution can help to gain an increased understanding of renal physiology.

Using 15N to determine the metabolic fate of dietary nitrogen in North Pacific spiny dogfish (Squalus acanthias suckleyi)

Marine elasmobranchs are ureosmotic, retaining large concentrations of urea to balance their internal osmotic pressure with that of the external marine environment. The synthesis of urea requires the intake of exogenous nitrogen to maintain whole-body nitrogen balance and satisfy obligatory osmoregulatory and somatic processes. We hypothesized that dietary nitrogen may be directed toward the synthesis of specific nitrogenous molecules in post-fed animals; specifically, we predicted the preferential accumulation and retention of labelled nitrogen would be directed towards the synthesis of urea necessary for osmoregulatory purposes. North Pacific spiny dogfish (Squalus acanthias suckleyi) were fed a single meal of 7 mmol l-1 15NH4Cl in a 2% ration by body mass of herring slurry via gavage. Dietary labelled nitrogen was tracked from ingestion to tissue incorporation and the subsequent synthesis of nitrogenous compounds (urea, glutamine, bulk amino acids, protein) in the intestinal spiral valve, plasma, liver and muscle. Within 20 h post-feeding, we found labelled nitrogen was incorporated into all tissues examined. The highest δ15N values were seen in the anterior region of the spiral valve at 20 h post-feeding, suggesting this region was particularly important in assimilating the dietary labelled nitrogen. In all tissues examined, enrichment of the nitrogenous compounds was sustained throughout the 168 h experimental period, highlighting the ability of these animals to retain and use dietary nitrogen for both osmoregulatory and somatic processes.

Investigating nitrogen movement in North Pacific spiny dogfish (Squalus acanthias suckleyi), with focus on UT, Rhp2, and Rhbg mRNA abundance

For ureosmotic marine elasmobranchs, the acquisition and retention of nitrogen is critical for the synthesis of urea. To better understand whole-body nitrogen homeostasis, we investigated mechanisms of nitrogen trafficking in North Pacific spiny dogfish (Squalus acanthias suckleyi). We hypothesized that the presence of nitrogen within the spiral valve lumen would affect both the transport of nitrogen and the mRNA abundance of a urea transporter (UT) and two ammonia transport proteins (Rhp2, Rhbg) within the intestinal epithelium. The in vitro preincubation of intestinal tissues in NH₄Cl, intended to simulate dietary nitrogen availability, showed that increased ammonia concentrations did not significantly stimulate the net uptake of total urea or total methylamine. We also examined the mRNA abundance of UT, Rhp2, and Rhbg in the gills, kidney, liver, and spiral valve of fasted, fed, excess urea fed, and antibiotic-treated dogfish. After fasting, hepatic UT mRNA abundance was significantly lower, and Rhp2 mRNA in the gills was significantly higher than the other treatments. Feeding significantly increased Rhp2 mRNA levels in the kidney and mid spiral valve region. Both excess urea and antibiotics significantly reduced Rhbg mRNA levels along all three spiral valve regions. The antibiotic treatment also significantly diminished UT mRNA abundance levels in the anterior and mid spiral valve, and Rhbg mRNA levels in the kidney. In our study, no single treatment had significantly greater influence on the overall transcript abundance of the three transport proteins compared to another treatment, demonstrating the dynamic nature of nitrogen balance in these ancient fish.

Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus (Kryptolebias marmoratus) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na+-K+-ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.

A review of reductionist methods in fish gastrointestinal tract physiology

A holistic understanding of a physiological system can be accomplished through the use of multiple methods. Our current understanding of the fish gastrointestinal tract (GIT) and its role in both nutrient handling and osmoregulation is the result of the examination of the GIT using multiple reductionist methods. This review summarizes the following methods: in vivo mass balance studies, and in vitro gut sac preparations, intestinal perfusions, and Ussing chambers. From Homer Smith's initial findings of marine fish intestinal osmoregulation in the 1930s through to today's research, we discuss the methods, their advantages and pitfalls, and ultimately how they have each contributed to our understanding of fish GIT physiology. Although in vivo studies provide substantial information on the intact animal, segment specific functions of the GIT cannot be easily elucidated. Instead, in vitro gut sac preparations, intestinal perfusions, or Ussing chamber experiments can provide considerable information on the function of a specific tissue and permit the delineation of specific transport pathways through the use of pharmacological agents; however, integrative inputs (e.g. hormonal and neuronal) are removed and only a fraction of the organ system can be studied. We conclude with two case studies, i) divalent cation transport in teleosts and ii) nitrogen handling in the elasmobranch GIT, to highlight how the use of multiple reductionist methods contributes to a greater understanding of the organ system as a whole.

Molecular characterization of two Rhesus glycoproteins from the euryhaline freshwater white-rimmed stingray, Himantura signifer, and changes in their transcript levels and protein abundance in the gills, kidney, and liver during brackish water acclimation

Himantura signifer is a freshwater stingray which inhabits rivers in Southeast Asia. It is ammonotelic in fresh water, but retains the capacities of urea synthesis and ureosmotic osmoregulation to survive in brackish water. This study aimed to elucidate the roles of Rhesus glycoproteins (Rhgp), which are known to transport ammonia, in conserving nitrogen (N) in H. signifer during brackish water acclimation when N became limited resulting from increased hepatic urea synthesis. The complete coding sequence of rhbg from H. signifer consisted of 1383 bp, encoding 460 amino acids with an estimated molecular mass of 50.5 kDa, while that of rhcg comprised 1395 bp, encoding for 464 amino acids with an estimated molecular mass of 50.8 kDa. The deduced amino sequences of Rhbg and Rhcg contained ammonia binding sites, which could recruit NH₄⁺ to be deprotonated, and a hydrophobic pore with two histidine residues, which could mediate the transport of NH₃. Our results indicated for the first time that brackish water acclimation resulted in significant decreases in the expression levels of rhbg/Rhbg and rhcg/Rhcg in the gills of H. signifer, which offered a mechanistic explanation of brackish water-related decreased ammonia excretion reported elsewhere. Furthermore, rhbg/Rhbg expression levels increased significantly in the liver of H. signifer during brackish water acclimation, indicating that the ammonia produced by extra-hepatic tissues and released into the blood could be channeled into the liver for increased urea synthesis. Overall, these results lend support to the proposition that H. signifer becomes N-limited upon utilizing urea as an osmolyte in brackish water.

Widespread Natural Occurrence of Hydroxyurea in Animals

Here we report the widespread natural occurrence of a known antibiotic and antineoplastic compound, hydroxyurea in animals from many taxonomic groups. Hydroxyurea occurs in all the organisms we have examined including invertebrates (molluscs and crustaceans), fishes from several major groups, amphibians and mammals. The species with highest concentrations was an elasmobranch (sharks, skates and rays), the little skate Leucoraja erinacea with levels up to 250 μM, high enough to have antiviral, antimicrobial and antineoplastic effects based on in vitro studies. Embryos of L. erinacea showed increasing levels of hydroxyurea with development, indicating the capacity for hydroxyurea synthesis. Certain tissues of other organisms (e.g. skin of the frog (64 μM), intestine of lobster (138 μM) gills of the surf clam (100 μM)) had levels high enough to have antiviral effects based on in vitro studies. Hydroxyurea is widely used clinically in the treatment of certain human cancers, sickle cell anemia, psoriasis, myeloproliferative diseases, and has been investigated as a potential treatment of HIV infection and its presence at high levels in tissues of elasmobranchs and other organisms suggests a novel mechanism for fighting disease that may explain the disease resistance of some groups. In light of the known production of nitric oxide from exogenously applied hydroxyurea, endogenous hydoxyurea may play a hitherto unknown role in nitric oxide dynamics.

Physiological and molecular responses of the spiny dogfish shark (Squalus acanthias) to high environmental ammonia: scavenging for nitrogen

In teleosts, a branchial metabolon links ammonia excretion to Na(+) uptake via Rh glycoproteins and other transporters. Ureotelic elasmobranchs are thought to have low branchial ammonia permeability, and little is known about Rh function in this ancient group. We cloned Rh cDNAs (Rhag, Rhbg and Rhp2) and evaluated gill ammonia handling in Squalus acanthias. Control ammonia excretion was <5% of urea-N excretion. Sharks exposed to high environmental ammonia (HEA; 1 mmol(-1) NH4HCO3) for 48 h exhibited active ammonia uptake against partial pressure and electrochemical gradients for 36 h before net excretion was re-established. Plasma total ammonia rose to seawater levels by 2 h, but dropped significantly below them by 24-48 h. Control ΔP(NH3) (the partial pressure gradient of NH3) across the gills became even more negative (outwardly directed) during HEA. Transepithelial potential increased by 30 mV, negating a parallel rise in the Nernst potential, such that the outwardly directed NH4(+) electrochemical gradient remained unchanged. Urea-N excretion was enhanced by 90% from 12 to 48 h, more than compensating for ammonia-N uptake. Expression of Rhp2 (gills, kidney) and Rhbg (kidney) did not change, but branchial Rhbg and erythrocytic Rhag declined during HEA. mRNA expression of branchial Na(+)/K(+)-ATPase (NKA) increased at 24 h and that of H(+)-ATPase decreased at 48 h, while expression of the potential metabolon components Na(+)/H(+) exchanger2 (NHE2) and carbonic anhydrase IV (CA-IV) remained unchanged. We propose that the gill of this nitrogen-limited predator is poised not only to minimize nitrogen loss by low efflux permeability to urea and ammonia but also to scavenge ammonia-N from the environment during HEA to enhance urea-N synthesis.

Morphological and functional characteristics of the kidney of cartilaginous fishes: with special reference to urea reabsorption

For adaptation to high-salinity marine environments, cartilaginous fishes (sharks, skates, rays, and chimaeras) adopt a unique urea-based osmoregulation strategy. Their kidneys reabsorb nearly all filtered urea from the primary urine, and this is an essential component of urea retention in their body fluid. Anatomical investigations have revealed the extraordinarily elaborate nephron system in the kidney of cartilaginous fishes, e.g., the four-loop configuration of each nephron, the occurrence of distinct sinus and bundle zones, and the sac-like peritubular sheath in the bundle zone, in which the nephron segments are arranged in a countercurrent fashion. These anatomical and morphological characteristics have been considered to be important for urea reabsorption; however, a mechanism for urea reabsorption is still largely unknown. This review focuses on recent progress in the identification and mapping of various pumps, channels, and transporters on the nephron segments in the kidney of cartilaginous fishes. The molecules include urea transporters, Na(+)/K(+)-ATPase, Na(+)-K(+)-Cl(-) cotransporters, and aquaporins, which most probably all contribute to the urea reabsorption process. Although research is still in progress, a possible model for urea reabsorption in the kidney of cartilaginous fishes is discussed based on the anatomical features of nephron segments and vascular systems and on the results of molecular mapping. The molecular anatomical approach thus provides a powerful tool for understanding the physiological processes that take place in the highly elaborate kidney of cartilaginous fishes.

Genes and proteins of urea transporters

A urea transporter protein in the kidney was first proposed in 1987. The first urea transporter cDNA was cloned in 1993. The SLC14a urea transporter family contains two major subgroups: SLC14a1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14a2, the UT-A group originally isolated from kidney inner medulla. Slc14a1, the human UT-B gene, arises from a single locus located on chromosome 18q12.1-q21.1, which is located close to Slc14a2. Slc14a1 includes 11 exons, with the coding region extending from exon 4 to exon 11, and is approximately 30 kb in length. The Slc14a2 gene is a very large gene with 24 exons, is approximately 300 kb in length, and encodes 6 different isoforms. Slc14a2 contains two promoter elements: promoter I is located in the typical position, upstream of exon 1, and drives the transcription of UT-A1, UT-A1b, UT-A3, UT-A3b, and UT-A4; while promoter II is located within intron 12 and drives the transcription of UT-A2 and UT-A2b. UT-A1 and UT-A3 are located in the inner medullary collecting duct, UT-A2 in the thin descending limb and liver, UT-A5 in testis, UT-A6 in colon, UT-B1 primarily in descending vasa recta and erythrocytes, and UT-B2 in rumen.

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.

The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney

The Chinese soft-shelled turtle, Pelodiscus sinensis, is well adapted to aquatic environments, including brackish swamps and marshes. It is ureotelic, and occasionally submerges its head into puddles of water during emersion, presumably for buccopharyngeal respiration. This study was undertaken to test the hypothesis that the buccophyaryngeal cavity constitutes an important excretory route for urea in P. sinensis. Results indicate that a major portion of urea was excreted through the mouth instead of the kidney during immersion. When restrained on land, P. sinensis occasionally submerged their head into water (20-100 min), during which urea excretion and oxygen extraction occurred simultaneously. These results indicate for the first time that buccopharyngeal villiform processes (BVP) and rhythmic pharyngeal movements were involved in urea excretion in P. sinensis. Urea excretion through the mouth was sensitive to phloretin inhibition, indicating the involvement of urea transporters (UTs). In addition, saliva samples collected from the buccopharyngeal surfaces of P. sinensis injected intraperitoneally with saline contained ~36 mmol N l(-1) urea, significantly higher than that (~2.4 mmol N l(-1)) in the plasma. After intraperitoneal injection with 20 μmol urea g(-1) turtle, the concentration of urea in the saliva collected from the BVP increased to an extraordinarily high level of ~614 μmol N ml(-1), but the urea concentration (~45 μmol N ml(-1)) in the plasma was much lower, indicating that the buccopharyngeal epithelium of P. sinensis was capable of active urea transport. Subsequently, we obtained from the buccopharyngeal epithelium of P. sinensis the full cDNA sequence of a putative UT, whose deduced amino acid sequence had ~70% similarity with human and mouse UT-A2. This UT was not expressed in the kidney, corroborating the proposition that the kidney had only a minor role in urea excretion in P. sinensis. As UT-A2 is known to be a facilitative urea transporter, it is logical to deduce that it was localized in the basolateral membrane of the buccopharyngeal epithelium, and that another type of primary or secondary active urea transporter yet to be identified was present in the apical membrane. The ability to excrete urea through the mouth instead of the kidney might have facilitated the ability of P. sinensis and other soft-shelled turtles to successfully invade the brackish and/or marine environment.

Seasonal variation and response to osmotic challenge in urea transporter expression in the dehydration- and freeze-tolerant wood frog, Rana sylvatica

Urea accumulation is a universal response to osmotic challenge in anuran amphibians, and facilitative urea transporters (UTs) seem to play an important role in this process by acting in the osmoregulatory organs to mediate urea retention. Although UTs have been implicated in urea reabsorption in anurans, little is known about the physiological regulation of UT protein abundance. We examined seasonal variation in and effects of osmotic challenge on UT protein and mRNA levels in kidney and urinary bladder of the wood frog (Rana sylvatica), a terrestrial species that tolerates both dehydration and tissue freezing. Using immunoblotting techniques to measure relative UT abundance, we found that UT numbers varied seasonally, with a low abundance prevailing in the fall and winter, and higher levels occurring in the spring. Experimental dehydration of frogs increased UT protein abundance in the urinary bladder, whereas experimental urea loading decreased the abundance of UTs in kidney and bladder. Experimental freezing, whether or not followed by thawing, had no effect on UT numbers. UT mRNA levels, assessed using quantitative real-time polymerase chain reaction, did not change seasonally nor in response to any of our experimental treatments. These findings suggest that regulation of UTs depends on the nature and severity of the osmotic stress and apparently occurs posttranscriptionally in response to multiple physiological factors. Additionally, UTs seem to be regulated to meet the physiological need to accumulate urea, with UT numbers increasing to facilitate urea reabsorption and decreasing to prevent retention of excess urea.

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Proteomic and physiological responses of leopard sharks (Triakis semifasciata) to salinity change

Partially euryhaline elasmobranchs may tolerate physiologically challenging, variable salinity conditions in estuaries as a trade-off to reduce predation risk or to gain access to abundant food resources. To further understand these trade-offs and to evaluate the underlying mechanisms, we examined the responses of juvenile leopard sharks to salinity changes using a suite of measurements at multiple organizational levels: gill and rectal gland proteomes (using 2-D gel electrophoresis and tandem mass spectrometry), tissue biochemistry (Na+/K+-ATPase, caspase 3/7 and chymotrypsin-like proteasome activities), organismal physiology (hematology, plasma composition, muscle moisture) and individual behavior. Our proteomics results reveal coordinated molecular responses to low salinity – several of which are common to both rectal gland and gill – including changes in amino acid and inositol (i.e. osmolyte) metabolism, energy metabolism and proteins related to transcription, translation and protein degradation. Overall, leopard sharks employ a strategy of maintaining plasma urea, ion concentrations and Na+/K+-ATPase activities in the short-term, possibly because they rarely spend extended periods in low salinity conditions in the wild, but the sharks osmoconform to the surrounding conditions by 3 weeks. We found no evidence of apoptosis at the time points tested, while both tissues exhibited proteomic changes related to the cytoskeleton, suggesting that leopard sharks remodel existing osmoregulatory epithelial cells and activate physiological acclimatory responses to solve the problems posed by low salinity exposure. The behavioral measurements reveal increased activity in the lowest salinity in the short-term, while activity decreased in the lowest salinity in the long-term. Our data suggest that physiological/behavioral trade-offs are involved in using estuarine habitats, and pathway modeling implicates tumor necrosis factor α (TNFα) as a key node of the elasmobranch hyposmotic response network.

The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15‰) water

This study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine—urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15‰) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15‰ water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na+] and [Cl−] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15‰ water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15‰ water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10‰ water with fasting and could not survive well in 15‰ water without food.

Subcellular distribution of urea transporter in the collecting tubule of shark kidney is dependent on environmental salinity

In the kidney of marine elasmobranchs, urea reabsorption from filtered urine is essential for maintaining high levels of urea in the body. In the kidney of the houndshark, Triakis scyllium, we previously found that a facilitative urea transporter (UT) is localized to a specific nephron segment, the collecting tubule, suggesting that the collecting tubule has an important role in the urea reabsorption process. To elucidate the roles of UT, we transferred T. scyllium to high (130%) and low (30%) salinity, and examined UT mRNA levels and UT distribution patterns in the kidney using real-time PCR and semi-quantitative fluorescence immunohistochemistry, respectively. Following transfer to low and high salinity, houndshark decreased and increased plasma urea concentrations, respectively, in order to control plasma osmolality. The abundance of UT mRNA did not differ among the experimental groups, whereas that of UT protein in the collecting tubule was significantly decreased in 30% seawater (SW). Furthermore, the subcellular UT distribution was dramatically changed. UT in the apical plasma membrane of collecting tubule almost disappeared in 30% SW, whereas it slightly increased in 130% SW compared with 100% SW. Conversely, reverse transfer of fish from 30 to100% SW restored UT in the apical membrane. These results indicate that the accumulation of UT to the apical plasma membrane of the collecting tubule of Triakis is an important factor for regulating urea reabsorption in the kidney.

Multiple urea transporter proteins in the kidney of holocephalan elephant fish (Callorhinchus milii)

Reabsorption of filtered urea by the kidney is essential for retaining high levels of urea in marine cartilaginous fish. Our previous studies on the shark facilitative urea transporter (UT) suggest that additional UT(s) comprising the urea reabsorption system could exist in the cartilaginous fish kidney. Here, we isolated three cDNAs encoding UTs from the kidney of elephant fish, Callorhinchus milii, and termed them efUT-1, efUT-2 and efUT-3. efUT-1 is orthologous to known elasmobranch UTs, while efUT-2 and efUT-3 are novel UTs in cartilaginous fish. Two variants were found for efUT-1 and efUT-2, in which the NH(2)-terminal intracellular domain was distinct between the variants. Differences in potential phosphorylation sites were found in the variant-specific NH(2)-terminal domains. When expressed in Xenopus oocytes, all five UT transcripts including the efUT-1 and efUT-2 variants induced more than a 10-fold increase in [(14)C] urea uptake. Phloretin inhibited dose-dependently the increase of urea uptake, suggesting that the identified UTs are facilitative UTs. Molecular phylogenetic analysis revealed that efUT-1 and efUT-2 had diverged in the cartilaginous fish lineage, while efUT-3 is distinct from efUT-1 and efUT-2. The present finding of multiple UTs in elephant fish provides a key to understanding the molecular mechanisms of urea reabsorption system in the cartilaginous fish kidney.

Gastro-intestinal handling of water and solutes in three species of elasmobranch fish, the white-spotted bamboo shark, Chiloscyllium plagiosum, little skate, Leucoraja erinacea and the clear nose skate Raja eglanteria

The present study reports aspects of GI tract physiology in the white-spotted bamboo shark, Chiloscyllium plagiosum, little skate, Leucoraja erinacea and the clear nose skate, Raja eglanteria. Plasma and stomach fluid osmolality and solute values were comparable between species, and stomach pH was low in all species (2.2 to 3.4) suggesting these elasmobranchs may maintain a consistently low stomach pH. Intestinal osmolality, pH and ion values were comparable between species, however, some differences in ion values were observed. In particular Ca(2+) (19.67+/-3.65mM) and Mg(2+) (43.99+/-5.11mM) were high in L. erinacea and Mg(2+) was high (130.0+/-39.8mM) in C. palgiosum which may be an indication of drinking. Furthermore, intestinal fluid HCO(3)(-) values were low (8.19+/-2.42 and 8.63+/-1.48mM) in both skates but very high in C. plagiosum (73.3+/-16.3mM) suggesting ingested seawater may be processed by species-specific mechanisms. Urea values from the intestine to the colon dropped precipitously in all species, with the greatest decrease seen in C. plagiosum (426.0+/-8.1 to 0mM). This led to the examination of the molecular expression of both a urea transporter and a Rhesus like ammonia transporter in the intestine, rectal gland and kidney in L. erinacea. Both these transporters were expressed in all tissues; however, expression levels of the Rhesus like ammonia transporter were orders of magnitude higher than the urea transporter in the same tissue. Intestinal flux rates of solutes in L. erinacea were, for the most part, in an inward direction with the notable exception of urea. Colon flux rates of solutes in L. erinacea were all in an outward direction, although absolute rates were considerably lower than the intestine, suggestive of a much tighter epithelia. Results are discussed in the context of the potential role of the GI tract in salt and water, and nitrogen, homeostasis in elasmobranchs.

Cortisol-sensitive urea transport across the gill basolateral membrane of the gulf toadfish (Opsanus beta)

Gulf toadfish ( Opsanus beta) use a unique pulsatile urea excretion mechanism that allows urea to be voided in large pulses via the periodic insertion or activation of a branchial urea transporter. The precise cellular and subcellular location of the facilitated diffusion mechanism(s) remains unclear. An in vitro basolateral membrane vesicle (BLMV) preparation was used to test the hypothesis that urea movement across the gill basolateral membrane occurs through a cortisol-sensitive carrier-mediated mechanism. Toadfish BLMVs demonstrated two components of urea uptake: a linear element at high external urea concentrations, and a phloretin-sensitive saturable constituent ( Km = 0.24 mmol/l; Vmax = 6.95 μmol·mg protein−1·h−1) at low urea concentrations (<1 mmol/l). BLMV urea transport in toadfish was unaffected by in vitro treatment with ouabain, N-ethylmaleimide, or the absence of sodium, conditions that are known to inhibit sodium-coupled and proton-coupled urea transport in vertebrates. Transport kinetics were temperature sensitive with a Q10 > 2, further suggestive of carrier-mediated processes. Our data provide evidence that a basolateral urea facilitated transporter accelerates the movement of urea between the plasma and gills to enable the pulsatile excretion of urea. Furthermore, in vivo infusion of cortisol caused a significant 4.3-fold reduction in BLMV urea transport capacity in lab-crowded fish, suggesting that cortisol inhibits the recruitment of urea transporters to the basolateral membrane, which may ultimately affect the size of the urea pulse event in gulf toadfish.

Increased gene expression of a facilitated diffusion urea transporter in the skin of the African lungfish (Protopterus annectens) during massively elevated post-terrestrialization urea excretion

The full-length cDNA sequence of a putative urea transporter (lfUT) of the facilitated diffusion UT-A type has been cloned from the African lungfish Protopterus annectens. The lfUT cDNA is 1990 bp in length and its open reading frame encodes a 409 amino acid long protein, with a calculated molecular mass of 44,723 Da. The sequence is closest to those of amphibians ( approximately 65% amino acid homology), followed by mammals and elasmobranchs ( approximately 60%), and then teleosts ( approximately 50%). lfUT was clearly expressed in gill, kidney, liver, skeletal muscle and skin. Upon re-immersion in water after 33 days of air exposure ('terrestrialization'), lungfish exhibited a massive rise in urea-N excretion which peaked at 12-30 h with rates of 2000-5000 micromol-N kg(-1) h(-1) (versus normal aquatic rates of <130 micromol-N kg(-1) h(-1)) and persisted until 70 h. This appears to occur mainly through the skin. Total 'excess' urea-N excretion amounted to approximately 81,000-91,000 micromol-N kg(-1) over 3 days. By real-time PCR, there was no difference in lfUT expression in the ventral abdominal skin between aquatic ammoniotelic controls and terrestrialized lungfish immediately after return to water (0 h), and no elevation of urea-N excretion at this time. However, skin biopsies revealed a significant 2.55-fold elevation of lfUT expression at 14 h, coincident with peak urea-N excretion. At 48 h, there was no longer any significant difference in lfUT mRNA levels from those at 0 and 14 h, or from aquatic fed controls. In accordance with earlier studies, which identified elevated urea-N excretion via the skin of P. dolloi with pharmacology typical of UT-A carriers, these results argue that transcriptional activation of a facilitated diffusion type urea transporter (lfUT) occurs in the skin during re-immersion. This serves to clear the body burden of urea-N accumulated during terrestrialization.

Carrier-mediated urea transport across the mitochondrial membrane of an elasmobranch (Raja erinacea) and a teleost (Oncorhynchus mykiss) fish

In osmoregulating teleost fish, urea is a minor nitrogen excretory product, whereas in osmoconforming marine elasmobranchs it serves as the major tissue organic solute and is retained at relatively high concentrations (∼400 mmol/l). We tested the hypothesis that urea transport across liver mitochondria is carrier mediated in both teleost and elasmobranch fishes. Intact liver mitochondria in rainbow trout ( Oncorhynchus mykiss) demonstrated two components of urea uptake, a linear component at high concentrations and a phloretin-sensitive saturable component [Michaelis constant ( Km) = 0.58 mmol/l; maximal velocity ( Vmax) = 0.12 μmol·h−1·mg protein−1] at lower urea concentrations (<5 mmol/l). Similarly, analysis of urea uptake in mitochondria from the little skate ( Raja erinacea) revealed a phloretin-sensitive saturable transport ( Km= 0.34 mmol/l; Vmax= 0.054 μmol·h−1·mg protein−1) at low urea concentrations (<5 mmol/l). Surprisingly, urea transport in skate, but not trout, was sensitive to a variety of classic ionophores and respiration inhibitors, suggesting cation sensitivity. Hence, urea transport was measured in the reverse direction using submitochondrial particles in skate. Transport kinetics, inhibitor response, and pH sensitivity were very similar in skate submitochondrial particle submitochondrial particles ( Km= 0.65 mmol/l, Vmax= 0.058 μmol·h−1·mg protein−1) relative to intact mitochondria. We conclude that urea influx and efflux in skate mitochondria is dependent, in part, on a bidirectional proton-sensitive mechanism similar to bacterial urea transporters and reminiscent of their ancestral origins. Rapid equilibration of urea across the mitochondrial membrane may be vital for cell osmoregulation (elasmobranch) or nitrogen waste excretion (teleost).

The physiology and evolution of urea transport in fishes

This review summarizes what is currently known about urea transporters in fishes in the context of their physiology and evolution within the vertebrates. The existence of urea transporters has been investigated in red blood cells and hepatocytes of fish as well as in renal and branchial cells. Little is known about urea transport in red blood cells and hepatocytes, in fact, urea transporters are not believed to be present in the erythrocytes of elasmobranchs nor in teleost fish. What little physiological evidence there is for urea transport across fish hepatocytes is not supported by molecular evidence and could be explained by other transporters. In contrast, early findings on elasmobranch renal urea transporters were the impetus for research in other organisms. Urea transport in both the elasmobranch kidney and gill functions to retain urea within the animal against a massive concentration gradient with the environment. Information on branchial and renal urea transporters in teleost fish is recent in comparison but in teleosts urea transporters appear to function for excretion and not retention as in elasmobranchs. The presence of urea transporters in fish that produce a copious amount of urea, such as elasmobranchs and ureotelic teleosts, is reasonable. However, the existence of urea transporters in ammoniotelic fish is curious and could likely be due to their ability to manufacture urea early in life as a means to avoid ammonia toxicity. It is believed that the facilitated diffusion urea transporter (UT) gene family has undergone major evolutionary changes, likely in association with the role of urea transport in the evolution of terrestriality in the vertebrates.

Cloning and functional characterization of a second urea transporter from the kidney of the Atlantic stingray, Dasyatis sabina

The cloning of cDNAs encoding facilitated urea transporters (UTs) from the kidneys of the elasmobranchs indicates that in these fish renal urea reabsorption occurs, at least in part, by passive processes. The previously described elasmobranch urea transporter clones from shark (shUT) and stingray (strUT-1) differ from each other primarily because of the COOH-terminus of the predicted strUT-1 translation product being extended by 51-amino acid residues compared with shUT. Previously, we noted multiple UT transcripts were present in stingray kidney. We hypothesized that a COOH terminally abbreviated UT isoform, homologous to shUT, would also be present in stingray kidney. Therefore, we used 5'/3' rapid amplification of cDNA ends to identify a 3'UTR-variant (strUT-1a) of the cDNA that encodes (strUT-1), as well as three, 3'UTR-variant cDNAs (strUT-2a,b,c) that encode a second phloretin-sensitive, urea transporter (strUT-2). The 5'UTR and the first 1,132 nucleotides of the predicted coding region of the strUT-2 cDNAs are identical to the strUT-1 cDNAs. The remainder of the coding region contains only five novel nucleotides. The strUT-2 cDNAs putatively encode a 379-amino acid protein, the first 377 amino acids identical to strUT-1 plus 2 additional amino acids. We conclude that 1) a second UT isoform is expressed in the Atlantic stingray and that this isoform is similar in size to the UT previously cloned from the kidney of the dogfish shark, and 2) at least five transcripts encoding the 2 stingray UTs are derived from a single gene product through alternative splicing and polyadenylation.

Exposure to brackish water, upon feeding, leads to enhanced conservation of nitrogen and increased urea synthesis and retention in the Asian freshwater stingray Himantura signifer

The white-edge freshwater whip ray Himantura signifer is ammonotelic in freshwater, but retains the capacities of urea synthesis and ureosmotic osmoregulation to survive in brackish water. The first objective of this study was to examine whether exposure to brackish water would lead to increases in food intake, and/or conservation of nitrogen in H. signifer upon daily feeding. Results obtained showed that a progressive increase in ambient salinity, from 1 per thousand to 15 per thousand over a 10-day period, did not lead to an increase in daily food intake. However, there were significant reductions in daily rates of ammonia and urea excretion in H. signifer during salinity changes, especially between day 5 (in 10 per thousand water) and day 10 (in 15 per thousand water) when compared to those of the control kept in 1 per thousand water. Consequently, there was a significant decrease in the percentage of nitrogen (N) from the food being excreted as nitrogenous waste (ammonia-N+urea-N) during this period. On day 10, the tissue urea contents in fish exposed to 15 per thousand water were significantly greater than those of fish kept in 1 per thousand water, and the excess urea-N accumulated in the former fish could totally account for the cumulative deficit in excretion of urea-N+ammonia-N during the 10-day period. Thus, it can be concluded that H. signifer is N-limited, and conserved more N from food when exposed to brackish water. The conserved N was converted to urea, which was retained in tissues for osmoregulation. The second objective of this study was to elucidate whether the retention of the capacity of N conservation in H. signifer would lead to an accumulation of urea in fish exposed to not only 15 per thousand water, but also 1 per thousand water, upon feeding. For fish pre-acclimated to 1 per thousand water or 15 per thousand water for 10 days and then fasted for 48 h, the rate of ammonia excretion in fish exposed to 15 per thousand water was consistently lower than that of fish exposed to 1 per thousand water, throughout the 36-h post-feeding period. In addition, the hourly rate of urea excretion in the former was significantly lower than that of the latter between hours 12 and 36. There were postprandial increases in ammonia contents in the muscle, liver, stomach, intestine, brain and plasma of fish kept in 1 per thousand water; but postprandial increases in ammonia occurred only in the liver and brain of fish exposed to 15 per thousand water, and the magnitudes of increases in the latter were smaller than those in the former. Indeed, postprandial increases in tissue urea contents occurred in both groups of fish, but the greatest increase in urea content was observed in the muscle of fish exposed to 15 per thousand water. Taken together, these results indicate that H. signifer in freshwater could be confronted with postprandial osmotic stress because of its capacity of conserving N and increasing urea synthesis upon feeding.

The dogfish shark (Squalus acanthias) increases both hepatic and extrahepatic ornithine urea cycle enzyme activities for nitrogen conservation after feeding

Urea not only is utilized as a major osmolyte in marine elasmobranchs but also constitutes their main nitrogenous waste. This study investigated the effect of feeding, and thus elevated nitrogen intake, on nitrogen metabolism in the Pacific spiny dogfish Squalus acanthias. We determined the activities of ornithine urea cycle (O-UC) and related enzymes in liver and nonhepatic tissues. Carbamoyl phosphate synthetase III (the rate-limiting enzyme of the O-UC) activity in muscle is high compared with liver, and the activities in both tissues increased after feeding. The contribution of muscle to urea synthesis in the dogfish body appears to be much larger than that of liver when body mass is considered. Furthermore, enhanced activities of the O-UC and related enzymes (glutamine synthetase, ornithine transcarbamoylase, arginase) were seen after feeding in both liver and muscle and were accompanied by delayed increases in plasma urea, trimethylamine oxide, total free amino acids, alanine, and chloride concentrations, as well as in total osmolality. The O-UC and related enzymes also occurred in the intestine but showed little change after feeding. Feeding did not change the rate of urea excretion, indicating strong N retention after feeding. Ammonia excretion, which constituted only a small percentage of total N excretion, was raised in fed fish, while plasma ammonia did not change, suggesting that excess ammonia in plasma is quickly ushered into synthesis of urea or protein. In conclusion, we suggest that N conservation is a high priority in this elasmobranch and that feeding promotes ureogenesis and growth. Furthermore, exogenous nitrogen from food is converted into urea not only by the liver but also by the muscle and to a small extent by the intestine.

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.

Greatly elevated urea excretion after air exposure appears to be carrier mediated in the slender lungfish (Protopterus dolloi)

Under aquatic conditions, Protopterus dolloi is ammoniotelic, excreting only small amounts of urea-N. However, upon return to water after 30 d estivation in air, the lungfish excretes only small amounts of ammonia-N but massive amounts of urea-N. A similar pattern is seen after 21-30 d of terrestrialization, a treatment in which the lungfish is air exposed but kept moist throughout. After both treatments, the time course of urea-N excretion is biphasic with an immediate increase, then a fall, and finally a second larger increase that peaks at about 12 h and may be prolonged for several days thereafter. Urea-N excretion rates during the second peak reach 2,000-6,000 micromol N kg(-1) h(-1), two to three orders of magnitude greater than rates in most fish and comparable only to rates in species known to employ UT-A type facilitated diffusion urea transporters. Divided chamber studies and measurements of the clearance rates of [3H]-PEG-4000 (a glomerular filtration and paracellular diffusion marker) and two structural analogs of urea ([14C]-acetamide and [14C]-thiourea) were performed to characterize the two peaks of urea-N excretion. The smaller first peak was almost equally partitioned between the head (including internal and external gills) and the body compartment (including urinary opening), was accompanied by only a modest increase in [14C]-acetamide clearance equal to that in [14C]-thiourea clearance, and could be accounted for by a large but short-lasting increase in [3H]-PEG-4000 clearance (to about fivefold the terrestrial rate). The delayed, much larger second peak in urea-N excretion represented an elevated efflux into both compartments but occurred mainly (72%) via the body rather than the head region. This second peak was accompanied by a substantial increase in [14C]-acetamide clearance but only a modest further rise in [14C]-thiourea clearance. The acetamide to thiourea permeability ratio was typical of UT-A type transporters in other fish. [3H]-PEG-4000 clearance was stable at this time at about double the terrestrial rate, and excretion rates of urea and its analogs were many fold greater than could be accounted for by [3H]-PEG-4000 clearance. We conclude that the first peak may be explained by elevated urinary excretion and paracellular diffusion across the gills upon resubmergence, while the second peak is attributable to a delayed and prolonged activation of a UT-A type facilitated diffusion mechanism, primarily in the skin and perhaps also in branchial epithelia.

Effects of intra-peritoneal injection with NH4Cl, urea, or NH4Cl+urea on nitrogen excretion and metabolism in the African lungfish Protopterus dolloi

This study aimed to (1) determine if ammonia (as NH(4)Cl) injected intra-peritoneally into the ureogenic slender African lungfish, Protopterus dolloi, was excreted directly rather than being converted to urea; (2) examine if injected urea was retained in this lungfish, leading to decreases in liver arginine and brain tryptophan levels, as observed during aestivation on land; and (3) elucidate if increase in internal ammonia level would affect urea excretion, when ammonia and urea are injected simultaneously into the fish. Despite being ureogenic, P. dolloi rapidly excreted the excess ammonia as ammonia within the subsequent 12 h after NH(4)Cl was injected into its peritoneal cavity. Injected ammonia was not detoxified into urea through the ornithine-urea cycle, probably because it is energetically intensive to synthesize urea and because food was withheld before and during the experiment. In addition, injected ammonia was likely to stay in extracellular compartments available for direct excretion. At hour 24, only a small amount of ammonia accumulated in the muscle of these fish. In contrast, when urea was injected intra-peritoneally into P. dolloi, only a small percentage (34%) of it was excreted during the subsequent 24-h period. A significant increase in the rate of urea excretion was observed only after 16 h. At hour 24, significant quantities of urea were retained in various tissues of P. dolloi. Injection with urea led to an apparent reduction in endogenous ammonia production, a significant decrease in the hepatic arginine content, and a significantly lower level of brain tryptophan in this lungfish. All three phenomena had been observed previously in aestivating P. dolloi. Hence, it is logical to deduce that urea synthesis and accumulation could be one of the essential factors in initiating and perpetuating aestivation in this lungfish. Through the injection of NH(4)Cl + urea, it was demonstrated that an increase in urea excretion occurred in P. dolloi within the first 12 h post-injection, which was much earlier than that of fish injected with urea alone. These results suggest that urea excretion in P. dolloi is likely to be regulated by the level of internal ammonia in its body.

Marine (Taeniura lymma) and Freshwater (Himantura signifer) Elasmobranchs Synthesize Urea for Osmotic Water Retention

The objective of this study was to elucidate whether the marine blue-spotted fantail ray, Taeniura lymma, and the freshwater white-edge whip ray, Himantura signifer, injected with NH(4)Cl intraperitoneally would excrete the majority of the excess ammonia as ammonia per se to ameliorate ammonia toxicity despite being ureogenic. To examine the roles of urea and the ornithine-urea cycle, experimental fishes were exposed to salinity changes after being injected with NH(4)Cl. The ammonia excretion rates of the marine ray, T. lymma, injected with NH(4)Cl followed by exposure to seawater (30 per thousand) or diluted seawater (25 per thousand) increased 13-fold and 10-fold, respectively, within the first 3 h. Consequently, the respective percentage of nitrogenous wastes excreted as ammonia were 55% and 65% compared with 21% of the saline-injected control, indicating that T. lymma became apparently ammonotelic after injection with NH(4)Cl. By hour 6, large portions (70%-85%) of the ammonia injected into T. lymma exposed to seawater or diluted seawater had been excreted, and T. lymma excreted much more nitrogenous wastes (135%-180%), in excess of the ammonia injected into the fish, during the 24-h period. For T. lymma exposed to seawater, a small portion (30%) of the ammonia injected into the fish was detoxified to urea during the first 6 h, but there was an apparent suppression of urea synthesis thereafter, contributing partially to the large decrease (19%) in urea contents in its muscle at hour 24. A major contributing factor to the decrease in urea content was a reduction in ammonia production, as indicated by a large deficit between urea loss in the muscle and excess ammonia accumulated plus excess nitrogen excreted in the experimental fish. The freshwater ray, H. signifer, injected with NH(4)Cl followed by exposure to freshwater (0.7 per thousand) or brackish water (10 per thousand) was capable of excreting all the ammonia injected into the body, mainly as ammonia, within 12 h. Like T. lymma, it also excreted the injected ammonia mainly as ammonia during the first 3 h postinjection. During this period, the percentage of the injected ammonia excreted in fish exposed to brackish water (28.4%+/-4.6%) was significantly lower than those exposed to freshwater (56.1%+/-8.26%). In contrast, the percentage of nitrogenous wastes being excreted as urea in the former (38.4%) was significantly greater than that in the latter (14.1%). These results suggest that a portion of the ammonia injected into the fish was turned into urea, and urea synthesis was increased transiently in fish exposed to brackish water during the initial postinjection period. However, urea was not retained effectively by H. signifer. Taken together, these results suggest that the primary function of the ornithine-urea cycle in ureogenic marine and freshwater elasmobranchs is to synthesize urea for osmotic water retention and not for ammonia detoxification.

Role and regulation of urea transporters

In the past few years, significant knowledge has been gained about the physiological role and regulation of urea transporters, which have been now cloned in many species. The two major mammalian urea transporters, UT-A and UT-B, have been best studied in the kidney, where they mediate the facilitated diffusion of urea across tubular, interstitial, and vascular compartments, necessary to maintain an osmolar gradient along the renal corticomedullary axis. The genes encoding these transporters, Slc14A2 for UT-A and Slc14A1 for UT-B, have been characterized in rodents and humans, allowing identification of transcriptional mechanisms involved in the regulation of UT-A expression. The crucial role that urea transporters play in renal physiology is underscored by the phenotypic characteristics of UT-A and UT-B knockout mice, in which lack of specific urea transporters impairs the ability to concentrate urine. Expression of the UT-A and UT-B transporters has also been identified in extra-renal sites, where their physiological significance is only beginning to be elucidated. More information on the mechanisms modulating urea transporter expression is becoming available, and the possible involvement of aberrant regulation of these transporters in pathological conditions, or as a result of certain pharmacological treatments, has emerged from recent studies.

A novel type of urea transporter, UT-C, is highly expressed in proximal tubule of seawater eel kidney

A new type of urea transporter was identified by a database search and shown to be highly expressed in the renal proximal tubule cells of teleosts; proximal tubule-type urea transporters have not been describe previously. We first identified urea transporter-like sequences in the fugu genome and in an EST database of rainbow trout. Based on these pieces of sequence information, we obtained a full-length cDNA for the eel ortholog, consisting of 378 amino acid residues, and named it eUT-C. Although its sequence similarity to the known urea transporters is low (approximately 35%), its heterologous expression in Xenopus laevis oocytes indicated that it is a facilitative urea transporter sensitive to phloretin. Its activity is not dependent on Na+. Northern blot analysis showed that expression of eUT-C is highly restricted to the kidney, with weak expression in the stomach. In both tissues, eUT-C mRNA was strongly induced when eels were transferred from freshwater to seawater. Immunohistochemistry and in situ hybridization histochemistry revealed proximal tubule cell localization of eUT-C. Taking into account that 1) urea is mainly secreted from the gill where another type of urea transporter (eUT) has been identified and 2) fish excrete a very small volume of urine in seawater, we propose that eUT-C cloned here is a key component working in combination with the gill transporter to achieve an efficient urea excretory system in fish, namely, eUT-C reabsorbs urea from glomerular filtrate and sends it to the gill, through the circulation, for excretion.

The little skate Raja erinacea exhibits an extrahepatic ornithine urea cycle in the muscle and modulates nitrogen metabolism during low-salinity challenge

Urea synthesis via the hepatic ornithine urea cycle (OUC) has been well described in elasmobranchs, but it is unknown whether OUC enzymes are also present in extrahepatic tissues. Muscle and liver urea, trimethylamine oxide (TMAO), and other organic osmolytes, as well as selected OUC enzymes (carbamoyl phosphate synthetase III, ornithine transcarbamoylase, arginase, and the accessory enzyme glutamine synthetase), were measured in adult little skates (Raja erinacea) exposed to 100% or 75% seawater for 5 d. Activities of all four OUC enzymes were detected in the muscle. There were no changes in muscle OUC activities in skates exposed to 75% seawater; however, arginase activity was significantly lower in the liver, compared to controls. Urea, TMAO, and several other osmolytes were significantly lower in the muscle of little skates exposed to 75% seawater, whereas only glycerophosphorylcholine was significantly lower in the liver. Urea excretion rates were twofold higher in skates exposed to 75% seawater. Taken together, these data suggest that a functional OUC may be present in the skeletal muscle tissues of R. erinacea. As well, enhanced urea excretion rates and the downregulation of the anchor OUC enzyme, arginase, in the liver may be critical in regulating tissue urea content under dilute-seawater stress.

Renal urea transporters

PURPOSE OF REVIEW

Urea is transported across the kidney inner medullary collecting duct by urea-transporter proteins. Two urea-transporter genes have been cloned from humans and rodents: the UT-A (Slc14A2) gene encodes five protein and eight cDNA isoforms; the UT-B (Slc14A1) gene encodes a single isoform. In the past year, significant progress has been made in understanding the regulation of urea-transporter protein abundance in kidney, studies of genetically engineered mice that lack a urea transporter, identification of urea transporters outside of the kidney, cloning of urea transporters in nonmammalian species, and active urea transport in microorganisms.

RECENT FINDINGS

UT-A1 protein abundance is increased by 12 days of vasopressin, but not by 5 days. Analysis of the UT-A1 promoter suggests that vasopressin increases UT-A1 indirectly following a direct effect to increase the transcription of other genes, such as the Na(+)-K(+)-2Cl- cotransporter NKCC2/BSC1 and the aquaporin (AQP) 2 water channel, that begin to increase inner medullary osmolality. UT-A1 protein abundance is also increased by adrenalectomy, and is decreased by glucocorticoids or mineralocorticoids. However, each hormone works through its own receptor. Knockout mice that lack UT-A1 and UT-A3, or lack UT-B, have a urine-concentrating defect and a decrease in inner medullary interstitial urea content.

SUMMARY

Urea transporters play a critical role in the urine-concentrating mechanism. Their abundance is regulated by vasopressin, glucocorticoids, and mineralocorticoids. These regulatory mechanisms may be important in disease states such as diabetes because changes in urea-transporter abundance in diabetic rats require glucocorticoids and vasopressin.

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.