The consequences of reversible gill remodelling on ammonia excretion in goldfish (Carassius auratus)

Cellular mechanisms of ion and acid-base regulation in teleost gill ionocytes

The mechanism(s) of sodium, chloride and pH regulation in teleost fishes has been the subject of intense interest for researchers over the past 100 years. The primary organ responsible for ionoregulatory homeostasis is the gill, and more specifically, gill ionocytes. Building on the theoretical and experimental research of the past, recent advances in molecular and cellular techniques in the past two decades have allowed for substantial advances in our understanding of mechanisms involved. With an increased diversity of teleost species and environmental conditions being investigated, it has become apparent that there are multiple strategies and mechanisms employed to achieve ion and acid-base homeostasis. This review will cover the historical developments in our understanding of the teleost fish gill, highlight some of the recent advances and conflicting information in our understanding of ionocyte function, and serve to identify areas that require further investigation to improve our understanding of complex cellular and molecular machineries involved in iono- and acid-base regulation.

Effects of structural remodelling on gill physiology

The complex relationships between the structure and function of fish gills have been of interest to comparative physiologists for many years. Morphological plasticity of the gill provides a dynamic mechanism to reversibly alter its structure in response to changes in the conditions experienced by the fish. The best known example of gill remodelling is the growth or retraction of cell masses between the lamellae, a rapid process that alters the lamellar surface area that is exposed to the water (i.e. the functional lamellar surface area). Decreases in environmental O2 availability and/or increases in metabolic O2 demand stimulate uncovering of the lamellae, presumably to increase the capacity for O2 uptake. This review addresses four questions about gill remodelling: (1) what types of reversible morphological changes occur; (2) how do these changes affect physiological function from the gill to the whole animal; (3) what factors regulate reversible gill plasticity; and (4) is remodelling phylogenetically widespread among fishes? We address these questions by surveying the current state of knowledge of gill remodelling in fishes, with a focus on identifying gaps in our understanding that future research should consider.

The gill monogenean Sciadicleithrum variabilum induces histomorphological alterations in the gill tissues of the discus Symphysodon aequifasciatus

High mortality is among the most serious problems and challenges in the ornamental fish trade. Examination of the discus Symphysodon aequifasciatus from ornamental fish hatchery revealed infestation with the monogenean Sciadicleithrum variabilum. Gill infestation with this monogenean induced serious damage to the gill lamellae, including clavate lamellae, vascular congestion in the peripheral blood vessels, lamellar blood sinus dilation, and other structural anomalies. Light and transmission electron microscopy showed that in all infested hosts the interlamellar cell mass (ILCM) completely filled the interlamellar space. The monogenean-associated damage combined with the ILCM led to severe impairment of respiratory efficiency of the gill. Anti-parasitic treatment was applied during breeding (hatchery), which was followed by almost complete regression of the ILCM seen in the fish. A single point of ILCM hyperplasia was observed in only one specimen at the site of parasite attachment to the gill filament. The ILCM covering the gill lamellae protected the discus against infestation with this monogenean, but considerable reduction in the gaseous exchange surface and serious damage to the gill lamellae contributed to the increased mortality of the fish in the hatchery, which reached 90%.

Conflict and Compromise: Using Reversible Remodeling to Manage Competing Physiological Demands at the Fish Gill

The structural features of the fish gill necessary for oxygen uptake also favor undesirable, passive movements of ions and water. Reversible gill remodeling is one solution to this conflict. Cell masses that limit functional surface area are lost when oxygen availability decreases in hypoxia or oxygen demand increases with exercise or high temperature. However, much remains to be learned about how widespread reversible gill remodeling is among fish species, and how and why it occurs.

Effects of high ammonia concentrations on three cyprinid fish: Acute and whole-ecosystem chronic tests

A number of studies have revealed ammonia to be toxic to aquatic organisms; however, little is known about its effects under natural conditions. To elucidate the role of ammonia, we conducted 96-h acute toxicity tests as well as a whole-ecosystem chronic toxicity test for one year in ten 600-m² ponds. Three common cyprinids, silver carp Hypophthalmichthys molitrix Val. (H.m.), bighead carp Aristichthys nobilis Richardson (A.n.), and gibel carp Carassius auratus gibelio Bloch (C.g.), were used as test organisms. The 96-h LC₅₀ values of un-ionized ammonia (NH₃) for H.m., A.n., and C.g. were 0.35, 0.33, and 0.73mgL^-1, respectively. In the ponds, annual mean NH₃ ranged between 0.01 and 0.54mgL^-1, with 4 ponds having a NH₃ higher than the LC₅₀ of A.n. (lowest LC₅₀ in this study). No fish were found dead in the high-nitrogen ponds, but marked histological changes were found in livers and gills. Despite these changes, the specific growth rate of H.m. and A.n. increased significantly with NH₃. Our pond results suggest that fish might be more tolerant to high ammonia concentrations in natural aquatic ecosystems than under laboratory conditions. Our finding from field experiments thus suggests that the existing regulatory limits for reactive nitrogen (NH₃) established from lab toxicity tests might be somewhat too high at the ecosystem conditions. Field-scale chronic toxicity tests covering full life histories of fish and other aquatic organisms are therefore encouraged in order to optimize determination of the effects of ammonia in natural environments.

Molecular characterization of three Rhesus glycoproteins from the gills of the African lungfish, Protopterus annectens, and effects of aestivation on their mRNA expression levels and protein abundance

African lungfishes are ammonotelic in water. They can aestivate for long periods on land during drought. During aestivation, the gills are covered with dried mucus and ammonia excretion ceases. In fishes, ammonia excretion through the gills involves Rhesus glycoproteins (RhGP/Rhgp). This study aimed to obtain the complete cDNA coding sequences of rhgp from the gills of Protopterus annectens, and to determine their branchial mRNA and protein expression levels during the induction, maintenance and arousal phases of aestivation. Three isoforms of rhgp (rhag, rhbg and rhcg) were obtained in the gills of P. annectens. Their complete cDNA coding sequences ranged between 1311 and 1398 bp, coding for 436 to 465 amino acids with estimated molecular masses between 46.8 and 50.9 kDa. Dendrogramic analyses indicated that Rhag was grouped closer to fishes, while Rhbg and Rhcg were grouped closer to tetrapods. During the induction phase, the protein abundance of Rhag, but not its transcript level, was down-regulated in the gills, suggesting that there could be a decrease in the release of ammonia from the erythrocytes to the plasma. Furthermore, the branchial transcript levels of rhbg and rhcg decreased significantly, in preparation for the subsequent shutdown of gill functions. During the maintenance phase, the branchial expression levels of rhag/Rhag, rhbg/Rhbg and rhcg/Rhcg decreased significantly, indicating that their transcription and translation were down-regulated. This could be part of an overall mechanism to shut down branchial functions and save metabolic energy used for transcription and translation. It could also be regarded as an adaptive response to stop ammonia excretion. During the arousal phase, it is essential for the lungfish to regain the ability to excrete ammonia. Indeed, the protein abundance of Rhag, Rhbg and Rhcg recovered to the corresponding control levels after 1 day or 3 days of recovery from 6 months of aestivation.

Effect of combined stress (salinity and temperature) in European sea bass Dicentrarchus labrax osmoregulatory processes

European sea bass Dicentrarchus labrax undertake seasonal migrations to estuaries and lagoons that are characterized by fluctuations in environmental conditions. Their ability to cope with these unstable habitats is undeniable, but it is still not clear how and to what extent salinity acclimation mechanisms are affected at temperatures higher than in the sea. In this study, juvenile sea bass were pre-acclimated to seawater (SW) at 18°C (temperate) or 24°C (warm) for 2weeks and then transferred to fresh water (FW) or SW at the respective temperature. Transfer to FW for two weeks resulted in decreased blood osmolalities and plasma Cl^- at both temperatures. In FW warm conditions, plasma Na⁺ was ~15% lower and Cl^- was ~32% higher than in the temperate-water group. Branchial Na⁺/K⁺-ATPase (NKA) activity measured at the acclimation temperature (V_apparent) did not change according to the conditions. Branchial Na⁺/K⁺-ATPase activity measured at 37°C (V_max) was lower in warm conditions and increased in FW compared to SW conditions whatever the considered temperature. Mitochondrion-rich cell (MRC) density increased in FW, notably due to the appearance of lamellar MRCs, but this increase was less pronounced in warm conditions where MRC's size was lower. In SW warm conditions, pavement cell apical microridges are less developed than in other conditions. Overall gill morphometrical parameters (filament thickness, lamellar length and width) differ between fish that have been pre-acclimated to different temperatures. This study shows that a thermal change affects gill plasticity affecting whole-organism ion balance two weeks after salinity transfer.

Gill remodelling and growth rate of striped catfish Pangasianodon hypophthalmus under impacts of hypoxia and temperature

Gill morphometric and gill plasticity of the air-breathing striped catfish (Pangasianodon hypophthalmus) exposed to different temperatures (present day 27°C and future 33°C) and different air saturation levels (92% and 35%) during 6weeks were investigated using vertical sections to estimate the respiratory lamellae surface areas, harmonic mean barrier thicknesses, and gill component volumes. Gill respiratory surface area (SA) and harmonic mean water - blood barrier thicknesses (HM) of the fish were strongly affected by both environmental temperature and oxygen level. Thus initial values for 27°C normoxic fish (12.4±0.8g) were 211.8±21.6mm²g^-1 and 1.67±0.12μm for SA and HM respectively. After 5weeks in same conditions or in the combinations of 33°C and/or PO₂ of 55mmHg, this initial surface area scaled allometrically with size for the 33°C hypoxic group, whereas branchial SA was almost eliminated in the 27°C normoxic group, with other groups intermediate. In addition, elevated temperature had an astounding effect on growth with the 33°C group growing nearly 8-fold faster than the 27°C fish.

Cold acclimation of NaCl secretion in a eurythermic teleost: mitochondrial function and gill remodeling

Active chloride secretion, measured as short-circuit current (Isc) in ionocytes of opercular epithelia (OE) in the eurythermic, euryoxic, and euryhaline killifish or mummichog (Fundulus heteroclitus) was studied in cold (5°C) and warm (20°C) acclimated fish to determine if homeoviscous adaptation aided chloride secretion in the cold. Isolated opercular epithelia were cooled from 30°C to 0.2°C for warm and cold acclimated fish; from 30 to 8°C, Isc decreased with Q10=1.68 for warm and Q10=1.56 for cold acclimated tissues. By Arrhenius plots, there is a critical temperature, 8°C, below which aerobic Isc decreased sharply (Q10=6.90 for warm and 4.23 for cold acclimated tissues), suggesting a shift in mitochondrial efficiency of oxidative phosphorylation. In anaerobic conditions (0.5mM NaCN; N2 saturation), chloride transport continued at a lower rate, and Isc decrease with cooling below 8°C was less pronounced (Q10=2.95 for warm and 3.08 for cold), suggesting a shift in transporter function in plasma membrane. Under anaerobic conditions, NaCl secretion at 20°C was reversibly inhibited by hypotonic shock, indicating normal regulation of transport. Chloride secretion in warm-acclimated fish was supported mostly (75% at 20°C) by aerobic metabolism, whereas that for cold-acclimated fish was lower (55% at 20°C), suggesting a greater reliance on anaerobic metabolism in the cold. Once acclimated to cold, ionocytes may be temporarily incapable of increasing their aerobic ATP supply, even when warmed to 30°C. In cold acclimated fish there was increased polyunsaturated fatty acid composition of gill epithelium (consistent with homeoviscous adaptation) and gill remodeling, wherein epithelial cells filled the interlamellar space (interlamellar cell mass, ILCM) by as much as 70%, thus increasing diffusion distance against passive ion gain. Most ionocytes in these thickened epithelial masses became taller, still connecting basal lamina with the environment, consistent with the continuing transport rates at low temperatures. Whereas the low aerobic scope of cold-acclimated fish and thickened gill epithelium is appropriate to winter inactivity, metabolic depression and anaerobiosis, the large aerobic scope of warm-acclimated fish favors active foraging at high temperatures.

Immunohistochemical localization of urea and ammonia transporters in two confamilial fish species, the ureotelic gulf toadfish (Opsanus beta) and the ammoniotelic plainfin midshipman (Porichthys notatus)

This study aims to illustrate potential transport mechanisms behind the divergent approaches to nitrogen excretion seen in the ureotelic toadfish (Opsanus beta) and the ammoniotelic plainfin midshipman (Porichthys notatus). Specifically, we wish to confirm the expression of a urea transporter (UT), which is found in the gill of the toadfish and which is responsible for the unique "pulsing" nature of urea excretion and to localize the transporter within specific gill cells and at specific cellular locations. Additionally, the localization of ammonia transporters (Rhesus glycoproteins; Rhs) within the gill of both the toadfish and midshipman was explored. Toadfish UT (tUT) was found within Na(+)-K(+)-ATPase (NKA)-enriched cells, i.e., ionocytes (probably mitochondria-rich cells), especially along the basolateral membrane and potentially on the apical membrane. In contrast, midshipman UT (pnUT) immunoreactivity did not colocalize with NKA immunoreactivity and was not found along the filaments but instead within the lamellae. The cellular location of Rh proteins was also dissimilar between the two fish species. In toadfish gills, the Rh isoform Rhcg1 was expressed in both NKA-reactive cells and non-reactive cells, whereas Rhbg and Rhcg2 were only expressed in the latter. In contrast, Rhbg, Rhcg1 and Rhcg2 were expressed in both NKA-reactive and non-reactive cells of midshipman gills. In an additional transport epithelium, namely the intestine, the expression of both UTs and Rhs was similar between the two species, with only subtle differences being observed.

Seven things fish know about ammonia and we don't

In this review we pose the following seven questions related to ammonia and fish that represent gaps in our knowledge. 1. How is ammonia excretion linked to sodium uptake in freshwater fish? 2. How much does branchial ammonia excretion in seawater teleosts depend on Rhesus (Rh) glycoprotein-mediated NH(3) diffusion? 3. How do fish maintain ammonia excretion rates if branchial surface area is reduced or compromised? 4. Why does high environmental ammonia change the transepithelial potential across the gills? 5. Does high environmental ammonia increase gill surface area in ammonia tolerant fish but decrease gill surface area in ammonia intolerant fish? 6. How does ammonia contribute to ventilatory control? 7. What do Rh proteins do when they are not transporting ammonia? Mini reviews on each topic, which are able to present only partial answers to each question at present, are followed by further questions and/or suggestions for research approaches targeted to uncover answers.

New insights into the plasticity of gill structure

The ability of some fishes to reversibly remodel their gill morphology has become a focus of research after the discovery of extreme morphological gill plasticity in crucian carp and goldfish-both members of the cyprinid genus Carassius. Their lamellae are largely embedded in an interlamellar cell mass (ILCM) during normoxic conditions in cold water. The ILCM regresses in hypoxia, warm water, and during exercise, whereby the respiratory surface area and the capacity for oxygen uptake are greatly increased. There may be several reasons for covering the lamellae when oxygen needs are low. Reducing osmoregulatory costs have been suggested as an advantage of gill remodeling, but this has been difficult to show, putting the importance of the osmo-respiratory compromise into question. Other reasons could be to limit uptake of toxic substances and to reduce the risks for infections. In support for the latter, we present evidence showing that crucian carp infected by gill flukes maintain their ILCM when exposed to hypoxia. So far, gill remodeling in response to oxygen needs has been seen in several cyprinids, killifish and eel. In response to other environmental factors it may also occur in salmonids and anabantoids, revealing a phylogenetically widespread occurrence among teleosts.

The interactive effects of exercise and gill remodeling in goldfish (Carassius auratus)

Gill remodeling in goldfish (Carassius auratus) is accomplished by the appearance or retraction of a mass of cells (termed the interlamellar cell mass or ILCM) between adjacent lamellae. Given the presumed effects of gill remodeling on diffusing capacity, the goals of the current study were (1) to determine the consequences of increased aerobic O(2) demand (swimming) on gill remodelling and (2) to assess the consequences of the presence or absence of the ILCM on aerobic swimming capacity. Fish acclimated to 7 °C exhibited a marked increase in the ILCM which occupied, on average, 70.0 ± 4.1% of the total interlamellar channel area in comparison to an average ILCM area of only 28.3 ± 0.9% in fish acclimated to 25 °C. Incrementally increasing swimming velocity in fish at 7 °C to achieve a maximum aerobic swimming speed (U (CRIT)) within approximately 3 h resulted in a marked loss of the ILCM area to 44.8 ± 3.5%. Fish acclimated to 7 °C were subjected to 35 min swimming trials at 30, 60 or 80% U (CRIT) revealing that significant loss of the ILCM occurred at swimming speeds exceeding 60% U (CRIT). Prior exposure of cold water-acclimated fish to hypoxia to induce shedding of the ILCM did not affect swimming performance when assessed under normoxic conditions (control fish U (CRIT) = 2.34 ± 0.30 body lengths s(-1); previously hypoxic fish U (CRIT) = 2.99 ± 0.14 body lengths s(-1)) or the capacity to raise rates of O(2) consumption with increasing swimming speeds. Because shedding of ILCM during U (CRIT) trials complicated the interpretation of experiments designed to evaluate the impact of the ILCM on swimming performance, additional experiments using a more rapid 'ramp' protocol were performed to generate swimming scores. Neither prior hypoxia exposure nor a previous swim to U (CRIT) (both protocols are known to cause loss of the ILCM) affected swimming scores (the total distance swum during ramp U (CRIT) trials). However, partitioning all data based on the extent of ILCM coverage upon cessation of the swimming trial revealed that fish with less than 40% ILCM coverage exhibited a significantly greater swimming score (539 ± 86 m) than fish with greater than 50% ILCM coverage (285 ± 70 m). Thus, while loss of the ILCM at swimming speeds exceeding 60% U (CRIT) confounds the interpretation of experiments designed to assess the impact of the ILCM on swimming performance, we suggest that the shedding of the ILCM, in itself, coupled with improved swimming scores in fish exhibiting low ILCM coverage (<40%), provide evidence that the ILCM in goldfish acclimated to cold water (7 °C) is indeed an impediment to aerobic swimming capacity.

The relationship between NMDA receptor function and the high ammonia tolerance of anoxia-tolerant goldfish

Acute ammonia toxicity in vertebrates is thought to be characterized by a cascade of deleterious events resembling those associated with anoxic/ischemic injury in the central nervous system. A key event is the over-stimulation of neuronal N-methyl-D-aspartate (NMDA) receptors, which leads to excitotoxic cell death. The similarity between the responses to acute ammonia toxicity and anoxia suggests that anoxia-tolerant animals such as the goldfish (Carassius auratus Linnaeus) may also be ammonia tolerant. To test this hypothesis, the responses of goldfish were compared with those of the anoxia-sensitive rainbow trout (Oncorhynchus mykiss Walbaum) during exposure to high external ammonia (HEA). Acute toxicity tests revealed that goldfish are ammonia tolerant, with 96 h median lethal concentration (LC(50)) values of 199 μmol l(-1) and 4132 μmol l(-1) for NH(3) and total ammonia ([T(Amm)]=[NH(3)]+[NH(4)(+)]), respectively. These values were ~5-6 times greater than corresponding NH(3) and T(Amm) LC(50) values measured in rainbow trout. Further, the goldfish readily coped with chronic exposure to NH(4)Cl (3-5 mmol l(-1)) for 5 days, despite 6-fold increases in plasma [T] to ~1300 μmol l(-1) and 3-fold increases in brain [T(Amm)] to 6700 μmol l(-1). Muscle [T(Amm)] increased by almost 8-fold from ~900 μmol kg(-1) wet mass (WM) to greater than 7000 μmol kg(-1) WM by 48 h, and stabilized. Although urea excretion rates (J(Urea)) increased by 2-3-fold during HEA, the increases were insufficient to offset the inhibition of ammonia excretion that occurred, and increases in urea were not observed in the brain or muscle. There was a marked increase in brain glutamine concentration at HEA, from ~3000 μmol kg(-1) WM to 15,000 μmol kg(-1) WM after 48 h, which is consistent with the hypothesis that glutamine production is associated with ammonia detoxification. Injection of the NMDA receptor antagonists MK801 (0.5-8 mg kg(-1)) or ethanol (1-8 mg kg(-1)) increased trout survival time by 1.5-2.0-fold during exposure to 2 mmol l(-1) ammonia, suggesting that excitotoxic cell death contributes to ammonia toxicity in this species. In contrast, similar doses of MK801 or ethanol had no effect on ammonia-challenged (8-9.5 mmol l(-1) T(Amm)) goldfish survival times, suggesting that greater resistance to excitotoxic cell death contributes to the high ammonia-tolerance of the goldfish. Whole-cell recordings measured in isolated brain slices of goldfish telencephalon during in vitro exposure to 5 mmol l(-1) or 10 mmol l(-1) T(Amm) reversibly potentiated NMDA receptor currents. This observation suggested that goldfish neurons may not be completely resistant to ammonia-induced excitotoxicity. Subsequent western blot and densitometric analyses revealed that NMDA receptor NR1 subunit abundance was 40-60% lower in goldfish exposed to 3-5 mmol l(-1) T(Amm) for 5 days, which was followed by a restoration of NR1 subunit abundance after 3 days recovery in ammonia-free water. We conclude that the goldfish brain may be protected from excitotoxicity by downregulating the abundance of functional NMDA receptors during periods when it experiences increased internal ammonia.

Branchial and extra-branchial ammonia excretion in goldfish (Carassius auratus) following thermally induced gill remodeling

Under cold acclimated conditions, goldfish (Carassius auratus) express an interlamellar cell mass (ILCM) which limits diffusive ion loss but may also impede branchial ammonia excretion (J(amm)). In the present study, goldfish were subjected to a 2-week 5 or 25 °C acclimation in order to modulate the degree of ILCM gill coverage and determine potential effects on J(amm). 25 °C-fish displayed gill coverage which was significantly lower than the 5 °C-fish, though the ILCM was not completely absent in these fish. 5 °C-fish demonstrated J(amm) values approximately 60% lower than those of 25 °C-fish. The magnitude of anterior (branchial) J(amm) strongly correlated with gill coverage (r(2)=0.83), suggesting that the ILCM may impede branchial J(amm). Divided chamber experiments demonstrated that relative to the 25 °C-fish, 5 °C-fish relied more upon posterior routes of excretion. In response to high external ammonia (HEA; 1.5mM NH(4)HCO(3)) exposures, 25 °C-fish displayed ammonia uptake while 5 °C-fish maintained excretion against HEA, suggesting that the ILCM may act as a barrier preventing ammonia uptake. In summary, the ILCM appears to impede branchial J(amm), such that 5 °C-rely more on extra-branchial routes of excretion. We hypothesize that gill remodeling in these fish may be intimately tied to physiological adjustments on the whole-body scale.

Digestion of a single meal affects gene expression of ion and ammonia transporters and glutamine synthetase activity in the gastrointestinal tract of freshwater rainbow trout

Experiments on freshwater rainbow trout, Oncorhynchus mykiss, demonstrated how digestion affected the transcriptional expression of gastrointestinal transporters following a single satiating meal (~3% body mass ration) after a 1-week fast. Quantitative real-time polymerase chain reaction was employed to measure the relative mRNA expression of three previously cloned and sequenced transporters [H(+)-K(+)-ATPase (HKA), Na(+)/HCO(3)(-) cotransporter (NBC), and the Rhesus glycoprotein (Rhbg1; an ammonia transporter)] over a 24-h time course following feeding. Plasma total ammonia increased about threefold from pre-feeding levels to 288 μmol l(-1), whereas total ammonia levels in chyme supernatant reached a sixfold higher value (1.8 mmol l(-1)) than plasma levels. Feeding did not appear to have a statistically significant effect on the relative mRNA expression of the gastric HKA or Rhbg1. However, the relative mRNA expression of gastric NBC was increased 24 h following the ingestion of a meal. Along the intestinal tract, feeding increased the relative mRNA expression of Rhbg1, but had no effect on the expression of NBC. Expression of the gastric HKA was undetectable in the intestinal tract of freshwater rainbow trout. Digestion increased the activity of glutamine synthetase in the posterior intestine at 12 and 24 h following feeding. This study is among the first to show that there are digestion-associated changes in gene expression and enzyme activity in the gastrointestinal tract of teleost fish illustrating the dynamic plasticity of this organ. These post-prandial changes occur over the relative short-term duration of digesting a single meal.

Ammonia excretion via Rhcg1 facilitates Na⁺ uptake in larval zebrafish, Danio rerio, in acidic water

The involvement of a Na(+)/H(+) exchanger (NHE) in mediating Na(+) uptake by freshwater fish is currently debated. Although supported indirectly by empirical molecular and pharmacological data, theoretically its operation should be constrained thermodynamically, owing to unfavorable chemical gradients. Recently, there has been an increasing focus on ammonia channels (Rh proteins) as potentially contributing to Na(+) uptake across the freshwater fish gill. In this study, we tested the hypothesis that Rhcg1, a specific apical isoform of Rh protein, is critically important in facilitating Na(+) uptake in zebrafish larvae via its interaction with NHE. Treating larvae (4 days postfertilization) with 5-(N-ethyl-N-isopropyl) amiloride (EIPA), an inhibitor of NHE, caused a significant reduction in Na(+) uptake in fish reared in acidic water (pH ∼ 4.0). A role for NHE in Na(+) uptake was further confirmed by translational knockdown of NHE3b, an isoform of NHE thought to be responsible for Na(+)/H(+) exchange in zebrafish larvae. Exposing the larvae reared in acidic water to 5 mM external ammonium sulfate or increasing the buffering capacity of the water with 10 mM HEPES caused concurrent reductions in ammonia excretion and Na(+) uptake. Furthermore, translational knockdown of Rhcg1 significantly reduced ammonia excretion and Na(+) uptake in larvae chronically (4 days) or acutely (24 h) exposed to acidic water. Unlike in sham-injected larvae, EIPA did not affect Na(+) uptake in fish experiencing Rhcg1 knockdown. Additionally, exposure of larvae to bafilomycin A1 (an inhibitor of H(+)-ATPase) significantly reduced Na(+) uptake in fish reared in acidic water. These observations suggest the existence of multiple mechanisms of Na(+) uptake in larval zebrafish in acidic water: one in which Na(+) uptake via NHE3b is linked to ammonia excretion via Rhcg1, and another facilitated by H(+)-ATPase.