Expression of the non-erythroid Rh glycoproteins in mammalian tissues

Ammonia transport is independent of PNH3 gradients across the gastrointestinal epithelia of the rainbow trout: A role for the stomach

Although the gastrointestinal tract (GIT) is an important site for nitrogen metabolism in teleosts, the mechanisms of ammonia absorption and transport remain to be elucidated. Both protein catabolism in the lumen and the metabolism of the GIT tissues produce ammonia which, in part, enters the portal blood through the anterior region of the GIT. The present study examined the possible roles of different GIT sections of rainbow trout (Oncorhynchus mykiss) in transporting ammonia in its unionized gas form-NH₃ -by changing the PNH₃ gradient across GIT epithelia using in vitro gut sac preparations. We also surveyed messenger RNA expression patterns of three of the identified Rh proteins (Rhbg, Rhcg1, and  Rhcg2) as potential NH₃ transporters and NKCC as a potential ammonium ion (NH₄ ⁺ ) transporter along the GIT of rainbow trout. We found that ammonia absorption is not dependent on the PNH₃ gradient despite expression of Rhbg and Rhcg2 in the intestinal tissues, and Rhcg2 in the stomach. We detected no expression of Rhbg in the stomach and no expression of Rhcg1 in any GIT tissues. There was also a lack of correlation between ammonia transport and [NH₄ ⁺ ] gradient despite NKCC expression in all GIT tissues. Regardless of PNH₃ gradients, the stomach showed the greatest absorption and net tissue consumption of ammonia. Overall, our findings suggest nitrogen metabolism zonation of GIT, with stomach serving as an important site for the absorption, handling and transport of ammonia that is independent of the PNH₃ gradient.

Post-prandial respiratory gas and acid-base profiles in the gastrointestinal tract and its venous drainage in freshwater rainbow trout (Oncorhynchus mykiss) and seawater English sole (Parophrys vetulus)

The basic respiratory gas and acid-base conditions inside the lumen of the gastrointestinal tract (GIT) and blood draining the tract are largely unestablished in teleost fishes after feeding, though there have been some recent novel discoveries on freshwater rainbow trout (Oncorhynchus mykiss) and seawater English sole (Parophrys vetulus). The present study examined in greater detail the gas (PO₂, PCO₂, PNH₃) and acid-base profiles (pH, [HCO₃^-], total [ammonia]) in the lumen of the stomach, the anterior, mid, and posterior intestine, as well as the venous drainage (subintestinal and/or hepatic portal vein) of the GIT in these two species 20 h post-feeding. Both species had high PCO₂, PNH₃, and total [ammonia], and low PO₂ (virtual anoxia) in the lumens throughout all sections of the GIT, and high [HCO₃^-] in the intestine. Total [ammonia], PNH₃, and [HCO₃^-] increased from anterior to posterior intestine in both species. P. vetulus had higher intestinal total [ammonia] and lower [HCO₃^-] than O. mykiss post feeding, but total [ammonia] was much higher in the stomach of O. mykiss. Despite the extreme conditions in the lumen, both arterial and venous blood showed relatively lower PCO₂, total [ammonia] and higher PO₂, implying limited equilibration between the two compartments. The higher [HCO₃^-] and lower total [ammonia] in the intestinal lumen of the freshwater O. mykiss than the seawater P. vetulus suggest the need for future comparative studies using conspecifics fed identical diets but acclimated to the two different salinities in order to understand the potential role of environmental salinity and associated osmoregulatory processes underlying these differences.

An in vitro analysis of intestinal ammonia transport in fasted and fed freshwater rainbow trout: roles of NKCC, K+ channels, and Na+, K+ ATPase

We examined mechanisms of ammonia handling in the anterior, mid, and posterior intestine of unfed and fed freshwater rainbow trout (Oncorhynchus mykiss), with a focus on the Na⁺:K⁺:2Cl^- co-transporter (NKCC), Na⁺:K ⁺-ATPase (NKA), and K⁺ channels. NKCC was localized by immunohistochemistry to the mucosal (apical) surface of enterocytes, and NKCC mRNA was upregulated after feeding in the anterior and posterior segments. NH₄⁺ was equally potent to K⁺ in supporting NKA activity in all intestinal sections. In vitro gut sac preparations were employed to examine mucosal ammonia flux rates (Jm_amm, disappearance from the mucosal saline), serosal ammonia flux rates (Js_amm, appearance in the serosal saline), and total tissue ammonia production rates (Jt_amm = Js_amm - Jm_amm). Bumetanide (10^-4 mol L^-1), a blocker of NKCC, inhibited Js_amm in most preparations, but this was largely due to reduction of Jt_amm; Jm_amm was significantly inhibited only in the anterior intestine of fed animals. Ouabain (10^-4 mol L^-1), a blocker of NKA, generally reduced both Jm_amm and Js_amm without effects on Jt_amm in most preparations, though the anterior intestine was resistant after feeding. Barium (10^-2 mol L^-1), a blocker of K⁺ channels, inhibited Jm_amm in most preparations, and Js_amm in some, without effects on Jt_amm. These pharmacological results, together with responses to manipulations of serosal and mucosal Na⁺ and K⁺ concentrations, suggest that NKCC is not as important in ammonia absorption as previously believed. NH₄⁺ appears to be taken up through barium-sensitive K⁺ channels on the mucosal surface. Mucosal NH₄⁺ uptake via both NKCC and K⁺ channels is energized by basolateral NKA, which plays an additional role in scavenging NH₄⁺ on the serosal surface to possibly minimize blood toxicity or enhance ion uptake and amino acid synthesis following feeding. Together with recent findings from other studies, we have provided an updated model to describe the current understanding of intestinal ammonia transport in teleost fish.

Expression of the rhesus glycoproteins, ammonia transporter family members, RHCG and RHBG in male reproductive organs

The rhesus glycoproteins, Rh B glycoprotein (RHBG) and Rh C glycoprotein (RHCG), are recently identified ammonia transporters. Rhcg expression is necessary for normal male fertility, but its specific cellular expression is unknown, and Rhbg has not been reported to be expressed in the male reproductive tract. This study sought to determine the specific cellular expression of Rhcg, to determine whether Rhbg is expressed in the male reproductive tract, and, if so, to determine which cells express Rhbg using real-time RT-PCR, immunoblot analysis, and immunohistochemistry. Both Rhbg and Rhcg were expressed throughout the male reproductive tract. In the testis, high levels of Rhbg were expressed in Leydig cells, and Rhcg was expressed in spermatids during the later stages of their maturation (steps 13-16) in stages I-VIII of the seminiferous epithelium cycle. In the epididymis, basolateral Rhbg was present in narrow cells in the initial segment, in principal cells in the upper corpus, and in clear cells throughout the epididymis. Apical Rhcg immunolabel was present in principal cells in the caput and upper corpus epididymidis and in clear cells in the middle and lower corpus and cauda epididymidis. In the vas deferens, apical Rhcg immunolabel and basolateral Rhbg immunolabel were present in some principal cells and colocalized with H(+)-ATPase immunolabel. We conclude that both Rhbg and Rhcg are highly expressed in specific cells in the male reproductive tract where they can contribute to multiple components of male fertility.

Renal ammonia excretion in response to hypokalemia: effect of collecting duct-specific Rh C glycoprotein deletion

The Rhesus factor protein, Rh C glycoprotein (Rhcg), is an ammonia transporter whose expression in the collecting duct is necessary for normal ammonia excretion both in basal conditions and in response to metabolic acidosis. Hypokalemia is a common clinical condition associated with increased renal ammonia excretion. In contrast to basal conditions and metabolic acidosis, increased ammonia excretion during hypokalemia can lead to an acid-base disorder, metabolic alkalosis, rather than maintenance of acid-base homeostasis. The purpose of the current studies was to determine Rhcg's role in hypokalemia-stimulated renal ammonia excretion through the use of mice with collecting duct-specific Rhcg deletion (CD-Rhcg-KO). In mice with intact Rhcg expression, a K(+)-free diet increased urinary ammonia excretion and urine alkalinization and concurrently increased Rhcg expression in the collecting duct in the outer medulla. Immunohistochemistry and immunogold electron microscopy showed hypokalemia increased both apical and basolateral Rhcg expression. In CD-Rhcg-KO, a K(+)-free diet increased urinary ammonia excretion and caused urine alkalinization, and the magnitude of these changes did not differ from mice with intact Rhcg expression. In mice on a K(+)-free diet, CD-Rhcg-KO increased phosphate-dependent glutaminase (PDG) expression in the outer medulla. We conclude that hypokalemia increases collecting duct Rhcg expression, that this likely contributes to the hypokalemia-stimulated increase in urinary ammonia excretion, and that adaptive increases in PDG expression can compensate for the absence of collecting duct Rhcg.

Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney

Hypokalemia is a common electrolyte disorder that increases renal ammonia metabolism and can cause the development of an acid-base disorder, metabolic alkalosis. The ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), are expressed in the distal nephron and collecting duct and mediate critical roles in acid-base homeostasis by facilitating ammonia secretion. In the current studies, the effect of hypokalemia on renal Rhbg and Rhcg expression was examined. Normal Sprague-Dawley rats received either K(+)-free or control diets for 2 wk. Rats receiving the K(+)-deficient diet developed hypokalemia and metabolic alkalosis associated with significant increases in both urinary ammonia excretion and urine pH. Rhcg expression increased in the outer medullary collecting duct (OMCD). In OMCD intercalated cells, hypokalemia resulted in more discrete apical Rhcg expression and a marked increase in apical plasma membrane immunolabel. In principal cells, in the OMCD, hypokalemia increased both apical and basolateral Rhcg immunolabel intensity. Cortical Rhcg expression was not detectably altered by immunohistochemistry, although there was a slight decrease in total expression by immunoblot analysis. Rhbg protein expression was decreased slightly in the cortex and not detectably altered in the outer medulla. We conclude that in rat OMCD, hypokalemia increases Rhcg expression, causes more polarized apical expression in intercalated cells, and increases both apical and basolateral expression in the principal cell. Increased plasma membrane Rhcg expression in response to hypokalemia in the rat, particularly in the OMCD, likely contributes to the increased ammonia excretion and thereby to the development of metabolic alkalosis.

Molecular physiology of the Rh ammonia transport proteins

PURPOSE OF REVIEW

Recent studies have identified a new family of ammonia-specific transporters, Rh glycoproteins, which enable NH3-specific transport. The purpose of this review is to summarize recent evidence regarding the role of Rh glycoproteins in renal ammonia transport.

RECENT FINDINGS

The Rh glycoproteins, RhAG/Rhag, RhBG/Rhbg and RhCG/Rhcg, transport ammonia in the form of molecular NH3, although there is some evidence suggesting the possibility of NH4 transport. RhAG/Rhag is expressed only in erythrocytes, and not in the kidney. Rhbg and Rhcg are expressed in distal nephron sites, from the distal convoluted tubule through the inner medullary collecting duct, with basolateral Rhbg expression and both apical and basolateral Rhcg expression. Whether Rhbg contributes to renal ammonia transport remains controversial. Rhcg expression parallels ammonia excretion in multiple experimental models and genetic deletion studies, both global and collecting duct-specific, demonstrate a critical role for Rhcg in both basal and acidosis-stimulated renal ammonia excretion. X-ray crystallography has defined critical structural elements in Rh glycoprotein-mediated ammonia transport. Finally, Rh glycoproteins may also function as CO2 transporters.

SUMMARY

No longer can NH3 transport be considered to occur only through diffusive NH3 movement. Transporter-mediated NH3 movement is fundamental to ammonia metabolism.

Cloning and functional expression of Rh50-like glycoprotein, a putative ammonia channel, in Aedes albopictus mosquitoes

Evidence has shown that female mosquitoes can deaminate more than 80% of the ingested bloodmeal protein amino acids, and thus lead to a massive amount of ammonia production. Ammonia transport is a critical step for detoxifying ammonia in organisms. Here we characterized a putative ammonia channel gene, Rhesus (Rh) 50 glycoprotein, from Aedes albopictus (AalRh50) and determined the difference of its expression profile in different tissues at both message and protein levels as well as its response to a blood meal. We showed that AalRh50 shares a low identity with E. coli ammonia transporter (EcoAmtB), but higher identities with human RhBG and Drosophila Rh50 genes. The analysis of ammonia-conductance sites indicates that AalRh50 has residue substitutions of S237L (equivalent to S219 in AmtB) in the external vestibule, F127I (equivalent to F107 in AmtB) in the pore entrance, and S281N (equivalent to S263 in AmtB) in the internal vestibule, which could alter or reduce ammonia-conductance activity. The results from quantitative real-time-PCR and immunohistochemistry revealed that AalRh50 is expressed at significantly higher levels in the head, Malpighian tubules, and thorax of the non-blood-fed females, suggesting that AalRh50 might play roles in maintaining normal neurotransmitter metabolism, acid-base balance, and flight energy production in different tissues of mosquitoes at the non-blood-fed condition. A blood meal significantly increases AalRh50 expression in midgut, fat body, and Malpighian tubules from 3 or 6 to 24h post feeding, indicating that AalRh50 plays an important role in detoxification of excess systemic ammonia of female adults during the gonotrophic cycle.

Sharpey-Schafer lecture: gas channels

The traditional dogma has been that all gases diffuse through all membranes simply by dissolving in the lipid phase of the membrane. Although this mechanism may explain how most gases move through most membranes, it is now clear that some membranes have no demonstrable gas permeability, and that at least two families of membrane proteins, the aquaporins (AQPs) and the Rhesus (Rh) proteins, can each serve as pathways for the diffusion of both CO₂ and NH₃. The knockout of RhCG in the renal collecting duct leads to the predicted consequences in acid–base physiology, providing a clear-cut role for at least one gas channel in the normal physiology of mammals. In our laboratory, we have found that surface-pH (pH_S) transients provide a sensitive approach for detecting CO₂ and NH₃ movement across the cell membranes of Xenopus oocytes. Using this approach, we have found that each tested AQP and Rh protein has its own characteristic CO₂/NH₃ permeability ratio, which provides the first demonstration of gas selectivity by a channel. Our preliminary AQP1 data suggest that all the NH₃ and less than half of the CO₂ move along with H₂O through the four monomeric aquapores. The majority of CO₂ takes an alternative route through AQP1, possibly the central pore at the four-fold axis of symmetry. Preliminary data with two Rh proteins, bacterial AmtB and human erythroid RhAG, suggest a similar story, with all the NH₃ moving through the three monomeric NH₃ pores and the CO₂ taking a separate route, perhaps the central pore at the three-fold axis of symmetry. The movement of different gases via different pathways is likely to underlie the gas selectivity that these channels exhibit.

Expression of ammonia transporter family members, Rh B glycoprotein and Rh C glycoprotein, in the developing rat kidney

Ammonia metabolism is a primary component of acid-base homeostasis but is incompletely developed at time of birth. Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg) are recently recognized ammonia transporter family members expressed in the mammalian kidney. This study's purpose was to establish the expression and localization of Rhbg and Rhcg during kidney development. We examined kidneys from fetal days 16 (E16), 18 (E18), and 20 (E20), and from the first 21 days of postnatal development. Rhbg was expressed initially at E18, with expression only in the connecting tubule (CNT); at E20, Rhbg was expressed in both the CNT and the medullary collecting duct (MCD). In contrast, Rhcg was first expressed at E16 with basal expression in the ureteric bud; at E18, it was expressed in a subset of CNT cells with an apical pattern, followed by apical and basolateral expression in the MCD at E20. In the cortex, Rhbg and Rhcg expression increased in the CNT before expression in the cortical collecting duct during fetal development. In the MCD, both Rhbg and Rhcg expression was initially in cells in the papillary tip, with gradual removal from the tip during the late fetal period and transition during the early neonatal period to an adult pattern with predominant expression in the outer MCD and only rare expression in cells in the initial inner MCD. Double-labeling with intercalated cell-specific markers identified that Rhbg and Rhcg were expressed initially in CNT cells, CNT A-type intercalated cells and non-A, non-B intercalated cells, and in MCD A-type intercalated cells. We conclude that expression of Rhbg and Rhcg parallels intercalated cell development and that immature Rhbg and Rhcg expression at birth contributes to incomplete ammonia excretion capacity.

Functional reconstitution into liposomes of purified human RhCG ammonia channel

BACKGROUND

Rh glycoproteins (RhAG, RhBG, RhCG) are members of the Amt/Mep/Rh family which facilitate movement of ammonium across plasma membranes. Changes in ammonium transport activity following expression of Rh glycoproteins have been described in different heterologous systems such as yeasts, oocytes and eukaryotic cell lines. However, in these complex systems, a potential contribution of endogenous proteins to this function cannot be excluded. To demonstrate that Rh glycoproteins by themselves transport NH(3), human RhCG was purified to homogeneity and reconstituted into liposomes, giving new insights into its channel functional properties.

METHODOLOGY/PRINCIPAL FINDINGS

An HA-tag introduced in the second extracellular loop of RhCG was used to purify to homogeneity the HA-tagged RhCG glycoprotein from detergent-solubilized recombinant HEK293E cells. Electron microscopy analysis of negatively stained purified RhCG-HA revealed, after image processing, homogeneous particles of 9 nm diameter with a trimeric protein structure. Reconstitution was performed with sphingomyelin, phosphatidylcholine and phosphatidic acid lipids in the presence of the C(12)E(8) detergent which was subsequently removed by Biobeads. Control of protein incorporation was carried out by freeze-fracture electron microscopy. Particle density in liposomes was a function of the Lipid/Protein ratio. When compared to empty liposomes, ammonium permeability was increased two and three fold in RhCG-proteoliposomes, depending on the Lipid/Protein ratio (1/300 and 1/150, respectively). This strong NH(3) transport was reversibly inhibited by mercuric and copper salts and exhibited a low Arrhenius activation energy.

CONCLUSIONS/SIGNIFICANCE

This study allowed the determination of ammonia permeability per RhCG monomer, showing that the apparent Punit(NH3) (around 1x10(-3) microm(3)xs(-1)) is close to the permeability measured in HEK293E cells expressing a recombinant human RhCG (1.60x10(-3) microm(3)xs(-1)), and in human red blood cells endogenously expressing RhAG (2.18x10(-3) microm(3)xs(-1)). The major finding of this study is that RhCG protein is active as an NH(3) channel and that this function does not require any protein partner.

Expression of the gas-transporting proteins, Rh B glycoprotein and Rh C glycoprotein, in the murine lung

A family of gas-transporting proteins, the Mep/Amt/Rh glycoprotein family, has been identified recently. These are integral membrane proteins, are widely expressed in sites of gas transport, and are known to transport the gaseous molecule, NH(3), and recent evidence indicates they can transport CO(2). Because the mammalian lung is a critical site for gas transport, the current studies examine the expression of the nonerythroid members of this extended family, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), in the normal mouse lung. Real-time RT-PCR and immunoblot analysis demonstrated both Rhbg and Rhcg mRNA and protein expression, respectively. Immunohistochemistry demonstrated both Rhbg and Rhcg were expressed in bronchial and bronchiolar epithelial cells. Rhbg was expressed by Clara cells, specifically, whereas all bronchial/bronchiolar epithelial cells, with the exception of goblet cells, expressed Rhcg. Rhbg expression was basolateral, whereas Rhcg exhibited apical and intracellular immunolabel, polarized expression similar to that observed in Rhbg- and Rhcg-expressing epithelial cells in other organs. There was no detectable expression of either Rhbg or Rhcg in alveolar endothelial or epithelial cells, in pneumocytes or in vascular tissue. In vitro studies using cultured bronchial epithelial cells confirm Rhbg and Rhcg expression, demonstrate that saturable, not diffusive, transport is the primary mechanism of ammonia/methylammonia transport, and show that the saturable transport mechanism has kinetics similar to those demonstrated previously for Rhbg and Rhcg. These findings suggest Rhbg and Rhcg may contribute to bronchial epithelial cell ammonia metabolism and suggest that they do not contribute to pulmonary CO(2) transport.

Basolateral expression of the ammonia transporter family member Rh C glycoprotein in the mouse kidney

Ammonia metabolism and transport are critical for acid-base homeostasis. The ammonia transporter family member Rh C glycoprotein (Rhcg) is expressed in distal renal tubular segments, and its expression is regulated in parallel with renal ammonia metabolism. However, there are inconsistencies in its reported subcellular distribution, with both apical and basolateral Rhcg reported in rat and human kidney and only apical expression in mouse kidney. Because the membrane location of Rhcg is critical for understanding its physiological role, we reassessed mouse Rhcg localization using refined immunolocalization methods. Two antibodies directed against different Rhcg-specific epitopes identified both apical and basolateral Rhcg immunolabel in mouse kidney. Immunogold electron microscopy both confirmed basolateral plasma membrane Rhcg expression and showed that apical immunolabel represented expression in both the apical plasma membrane and in subapical cytoplasmic vesicles. Immunoblots and Northern blots identified similar bands in Balb/c and C57BL/6 kidneys, suggesting basolateral Rhcg may result from alternative trafficking. Basolateral Rhcg intensity was strain dependent, with less basolateral Rhcg expression in the Balb/c mouse compared with the C57BL/6 mouse. In mice with collecting duct-specific Rhcg gene deletion, generated using Cre-loxP techniques, neither apical nor basolateral Rhcg immunolabel was identified in the collecting duct, confirming that basolateral Rhcg was the product of the same gene product as apical Rhcg. Although basolateral Rhcg expression differed between C57BL/6 and Balb/c mice, Rh B glycoprotein, which is exclusively basolateral, was expressed at similar levels in the two strains. We conclude that Rhcg is present in both the apical and basolateral plasma membrane in the mouse kidney, where it is likely to contribute to renal ammonia metabolism.

Ammonium transport in the colonic crypt cell line, T84: role for Rhesus glycoproteins and NKCC1

Although colonic lumen NH(4)(+) levels are high, 15-44 mM normal range in humans, relatively few studies have addressed the transport mechanisms for NH(4)(+). More extensive studies have elucidated the transport of NH(4)(+) in the kidney collecting duct, which involves a number of transporter processes also present in the distal colon. Similar to NH(4)(+) secretion in the renal collecting duct, we show that the distal colon secretory model, T84 cell line, has the capacity to secrete NH(4)(+) and maintain an apical-to-basolateral NH(4)(+) gradient. NH(4)(+) transport in the secretory direction was supported by basolateral NH(4)(+) loading on NKCC1, Na(+)-K(+)-ATPase, and the NH(4)(+) transporter, RhBG. NH(4)(+) was transported on NKCC1 in T84 cells nearly as well as K(+) as determined by bumetanide-sensitive (86)Rb-uptake. (86)Rb-uptake and ouabain-sensitive current measurement indicated that NH(4)(+) is transported by Na(+)-K(+)-ATPase in these cells to an equal extent as K(+). T84 cells expressed mRNA for the basolateral NH(4)(+) transporter RhBG and the apical NH(4)(+) transporter RhCG. Net NH(4)(+) transport in the secretory direction determined by (14)C-methylammonium (MA) uptake and flux occurred in T84 cells suggesting functional RhG protein activity. The occurrence of NH(4)(+) transport in the secretory direction within a colonic crypt cell model likely serves to minimize net absorption of NH(4)(+) because of surface cell NH(4)(+) absorption. These findings suggest that we rethink the present limited understanding of NH(4)(+) handling by the distal colon as being due solely to passive absorption.

Rhesus glycoprotein gene expression in the mangrove killifish Kryptolebias marmoratus exposed to elevated environmental ammonia levels and air

The mechanism(s) of ammonia excretion in the presence of elevated external ammonia are not well understood in fish. Recent studies in other organisms have revealed a new class of ammonia transporters, Rhesus glycoprotein genes(Rh genes), which may also play a role in ammonia excretion in fish. The first objective of this study was to clone and characterize Rhgenes in a fish species with a relatively high tolerance to environmental ammonia, the mangrove killifish Kryptolebias marmoratus (formerly Rivulus marmoratus). We obtained full-length cDNAs of three Rh genes in K. marmoratus: RhBG (1736 bp), RhCG1 (1920 bp) and RhCG2 (2021 bp), which are highly homologous with other known Rh gene sequences. Hydropathy analysis revealed that all three Rh genes encode membrane proteins with 10–12 predicted transmembrane domains. RhBG, RhCG1 and RhCG2 are highly expressed in gill tissue, with RhBG also present in skin of K. marmoratus. Exposure to elevated environmental ammonia (2 mmol l–1 NH4HCO3) for 5 days resulted in a modest (+37%) increase in whole-body ammonia levels, whereas gill RhCG2 and skin RhCG1 mRNA levels were upregulated by 5.8- and 7.7-fold, respectively. RhBG mRNA levels were also increased in various tissues, with 3- to 7-fold increases in the liver and skeletal muscle. In a separate group of killifish exposed to air for 24 h, RhCG1 and RhCG2 mRNA levels were elevated by 4- to 6-fold in the skin. Thus, the multifold induction of Rh mRNA levels in excretory tissues (gills and skin) and internal tissues in response to conditions that perturb normal ammonia excretion suggests that RhBG, RhCG1 and RhCG2 may be involved in facilitating ammonia transport in this species. Furthermore, the findings support earlier studies demonstrating that the skin is an important site of ammonia excretion in K. marmoratus.

Molecular mechanisms of renal ammonia transport

Acid-base homeostasis to a great extent relies on renal ammonia metabolism. In the past several years, seminal studies have generated important new insights into the mechanisms of renal ammonia transport. In particular, the theory that ammonia transport occurs almost exclusively through nonionic NH(3) diffusion and NH(4)(+) trapping has given way to a model postulating that a variety of proteins specifically transport NH(3) and NH(4)(+) and that this transport is critical for normal ammonia metabolism. Many of these proteins transport primarily H(+) or K(+) but also transport NH(4)(+). Nonerythroid Rh glycoproteins transport ammonia and may represent critical facilitators of ammonia transport in the kidney. This review discusses the underlying aspects of renal ammonia transport as well as specific proteins with important roles in renal ammonia transport.