Mechanisms and regulation of Na(+) uptake by freshwater fish

Respirometry reveals major lineage-based differences in the energetics of osmoregulation in aquatic invertebrates

All freshwater organisms are challenged to control their internal balance of water and ions in strongly hypotonic environments. We compared the influence of external salinity on the oxygen consumption rates (ṀO2) of three species of freshwater insects, one snail and two crustaceans. Consistent with available literature, we found a clear decrease in ṀO2 with increasing salinity in the snail Elimia sp. and crustaceans Hyalella azteca and Gammarus pulex (r5=-0.90, P=0.03). However, we show here for the first time that metabolic rate was unchanged by salinity in the aquatic insects, whereas ion transport rates were positively correlated with higher salinities. In contrast, when we examined the ionic influx rates in the freshwater snail and crustaceans, we found that Ca uptake rates were highest under the most dilute conditions, while Na uptake rates increased with salinity. In G. pulex exposed to a serially diluted ion matrix, Ca uptake rates were positively associated with ṀO2 (r5=-0.93, P=0.02). This positive association between Ca uptake rate and ṀO2 was also observed when conductivity was held constant but Ca concentration was manipulated (1.7-17.3 mg Ca l-1) (r5=0.94, P=0.05). This finding potentially implicates the cost of calcium uptake as a driver of increased metabolic rate under dilute conditions in organisms with calcified exoskeletons and suggests major phyletic differences in osmoregulatory physiology. Freshwater insects may be energetically challenged by higher salinities, while lower salinities may be more challenging for other freshwater taxa.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is reversible following depuration in clean water

Triphenyl phosphate (TPHP) - a widely used organophosphate-based flame retardant - blocks cardiac looping during zebrafish development in a concentration-dependent manner, a phenotype that is dependent on disruption of embryonic osmoregulation and pericardial edema formation. However, it's currently unclear whether (1) TPHP-induced effects on osmoregulation are driven by direct TPHP-induced injury to the embryonic epidermis and (2) whether TPHP-induced pericardial edema is reversible or irreversible following cessation of exposure. Therefore, the objectives of this study were to determine whether TPHP-induced pericardial edema is reversible and whether TPHP causes injury to the embryonic epidermis by quantifying the number of DAPI-positive epidermal cells and analyzing the morphology of the yolk sac epithelium using scanning electron microscopy. First, we found that exposure to 5 μM TPHP from 24-72 h post-fertilization (hpf) did not increase prolactin - a hormone that regulates ions and water levels - in embryonic zebrafish, whereas high ionic strength exposure media was associated with elevated levels of prolactin. Second, we found that exposure to 5 μM TPHP from 24-72 hpf did not decrease DAPI-positive epidermal cells within the embryonic epithelium, and that co-exposure with 2.14 μM fenretinide - a synthetic retinoid that promotes epithelial wound repair - from 24-72 hpf did not mitigate the prevalence of TPHP-induced epidermal folds within the yolk sac epithelium when embryos were exposed within high ionic strength exposure media. Finally, we found that the pericardial area and body length of embryos exposed to 5 μM TPHP from 24-72 hpf were similar to vehicle-treated embryos at 120 hpf following transfer to clean water and depuration of TPHP from 72-120 hpf. Overall, our findings suggest that (1) the ionic strength of exposure media may influence the baseline physiology of zebrafish embryos; (2) TPHP does not cause direct injury to the embryonic epidermis; and (3) TPHP-induced effects on pericardial area and body length are reversible 48 h after transferring embryos to clean water.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is dependent on the ionic strength of exposure media

Pericardial edema is commonly observed in zebrafish embryo-based chemical toxicity screens, and a mechanism underlying edema may be disruption of embryonic osmoregulation. Therefore, the objective of this study was to identify whether triphenyl phosphate (TPHP) - a widely used aryl phosphate ester-based flame retardant - induces pericardial edema via impacts on osmoregulation within embryonic zebrafish. In addition to an increase in TPHP-induced microridges in the embryonic yolk sac epithelium, an increase in ionic strength of exposure media exacerbated TPHP-induced pericardial edema when embryos were exposed from 24 to 72 h post-fertilization (hpf). However, there was no difference in embryonic sodium concentrations in situ within TPHP-exposed embryos relative to embryos exposed to vehicle (0.1% DMSO) from 24 to 72 hpf. Interestingly, increasing the osmolarity of exposure media with mannitol (an osmotic diuretic which mitigates TPHP-induced pericardial edema) and increasing the ionic strength of the exposure media (which exacerbates TPHP-induced pericardial edema) did not affect embryonic doses of TPHP, suggesting that TPHP uptake was not altered under these varying experimental conditions. Overall, our findings suggest that TPHP-induced pericardial edema within zebrafish embryos is dependent on the ionic strength of exposure media, underscoring the importance of further standardization of exposure media and embryo rearing protocols in zebrafish-based chemical toxicity screening assays.

Disturbing ion regulation and excretion in medaka (Oryzias melastigma) gills by microplastics: Insights from the gut-gill axis

The accumulation of microplastics (MPs) in fish gills has been widely recognized, however, whether such stress could thereby impact the physiological responses of fish gills is still unknown. Here, we investigated the impacts of three sizes (400 nm, 4 μm, 20 μm) of polystyrene (PS) MPs on (Na⁺, K⁺, Cl^-) ions regulation and ammonia excretion in medaka Oryzias melastigma. Significantly increased net Na⁺ and K⁺ flux rates were observed transiently during 0-3 h and 3-9 h, but not during 9-24 h. Such results suggest that the physiological resilience of fish gills regarding Na⁺ and K⁺ regulation was unaffected upon the exposure to PS-MPs, probably evidenced by the increased secretion of mucus. However, Cl^- regulation and ammonia excretion were significantly impaired, partly in consistent with the damages of ionocytes. The adverse impacts of PS-MPs on Cl^- regulation and ammonia excretion were size-dependent, with significant disturbances observed in 4 μm and 20 μm treated group for Cl^- regulation, but only in 20 μm treated group for ammonia excretion. The specific enrichment of Shinella and lower abundance of function profiles related to ion transport and metabolism might be responsible for the specific disturbance of Cl^- regulation found in the 4 μm treated group. The enrichment of Gemmobacter also accounted for the disturbances of ammonia excretion in 20 μm treated group. Our results highlighted the impacts of PS-MPs on the physiological functions in fish gills.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Repeated Genetic Targets of Natural Selection Underlying Adaptation of Fishes to Changing Salinity

Ecological transitions across salinity boundaries have led to some of the most important diversification events in the animal kingdom, especially among fishes. Adaptations accompanying such transitions include changes in morphology, diet, whole-organism performance, and osmoregulatory function, which may be particularly prominent since divergent salinity regimes make opposing demands on systems that maintain ion and water balance. Research in the last decade has focused on the genetic targets underlying such adaptations, most notably by comparing populations of species that are distributed across salinity boundaries. Here, we synthesize research on the targets of natural selection using whole-genome approaches, with a particular emphasis on the osmoregulatory system. Given the complex, integrated and polygenic nature of this system, we expected that signatures of natural selection would span numerous genes across functional levels of osmoregulation, especially salinity sensing, hormonal control, and cellular ion exchange mechanisms. We find support for this prediction: genes coding for V-type, Ca2+, and Na+/K+-ATPases, which are key cellular ion exchange enzymes, are especially common targets of selection in species from six orders of fishes. This indicates that while polygenic selection contributes to adaptation across salinity boundaries, changes in ATPase enzymes may be of particular importance in supporting such transitions.

Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology

During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.

Comparative Studies of Renin-Null Zebrafish and Mice Provide New Functional Insights

BACKGROUND

The renin-angiotensin system is highly conserved across vertebrates, including zebrafish, which possess orthologous genes coding for renin-angiotensin system proteins, and specialized mural cells of the kidney arterioles, capable of synthesising and secreting renin.

METHODS

We generated zebrafish with CRISPR-Cas9-targeted knockout of renin (ren^-/-) to investigate renin function in a low blood pressure environment. We used single-cell (10×) RNA sequencing analysis to compare the transcriptome profiles of renin lineage cells from mesonephric kidneys of ren^-/- with ren^+/+ zebrafish and with the metanephric kidneys of Ren1^c-/- and Ren1^c+/+ mice.

RESULTS

The ren^-/- larvae exhibited delays in larval growth, glomerular fusion and appearance of a swim bladder, but were viable and withstood low salinity during early larval stages. Optogenetic ablation of renin-expressing cells, located at the anterior mesenteric artery of 3-day-old larvae, caused a loss of tone, due to diminished contractility. The ren^-/- mesonephric kidney exhibited vacuolated cells in the proximal tubule, which were also observed in Ren1^c-/- mouse kidney. Fluorescent reporters for renin and smooth muscle actin (tg(ren:LifeAct-RFP; acta2:EGFP)), revealed a dramatic recruitment of renin lineage cells along the renal vasculature of adult ren^-/- fish, suggesting a continued requirement for renin, in the absence of detectable angiotensin metabolites, as seen in the Ren1YFP Ren1^c-/- mouse. Both phenotypes were rescued by alleles lacking the potential for glycosylation at exon 2, suggesting that glycosylation is not essential for normal physiological function.

CONCLUSIONS

Phenotypic similarities and transcriptional variations between mouse and zebrafish renin knockouts suggests evolution of renin cell function with terrestrial survival.

Rapid blood acid-base regulation by European sea bass (Dicentrarchus labrax) in response to sudden exposure to high environmental CO2

Fish in coastal ecosystems can be exposed to acute variations in CO₂ of between 0.2 and 1 kPa CO₂ (2000–10,000 µatm). Coping with this environmental challenge will depend on the ability to rapidly compensate for the internal acid–base disturbance caused by sudden exposure to high environmental CO₂ (blood and tissue acidosis); however, studies about the speed of acid–base regulatory responses in marine fish are scarce. We observed that upon sudden exposure to ∼1 kPa CO₂, European sea bass (Dicentrarchus labrax) completely regulate erythrocyte intracellular pH within ∼40 min, thus restoring haemoglobin–O₂ affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ∼2 h, which is one of the fastest acid–base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO₃⁻ in blood, which increased from ∼4 to ∼22 mmol l⁻¹. While the abundance and intracellular localisation of gill Na⁺/K⁺-ATPase (NKA) and Na⁺/H⁺ exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid–base regulation was completely prevented when the same high CO₂ exposure occurred in seawater with experimentally reduced HCO₃⁻ and pH, probably because reduced environmental pH inhibited gill H⁺ excretion via NHE3. The rapid and robust acid–base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO₂ fluctuations that naturally occur in coastal environments.

Summary: European sea bass exposed to 1 kPa (10,000 µatm) CO₂ regulate blood and red cell pH within 2 h and 40 min, respectively, protecting O₂ transport capacity, via enhanced gill acid excretion.

Ion Transporter Gene Families as Physiological Targets of Natural Selection During Salinity Transitions in a Copepod

Salinity is a key factor that structures biodiversity on the planet. With anthropogenic change, such as climate change and species invasions, many populations are facing rapid and dramatic changes in salinity throughout the globe. Studies on the copepod Eurytemora affinis species complex have implicated ion transporter gene families as major loci contributing to salinity adaptation during freshwater invasions. Laboratory experiments and population genomic surveys of wild populations have revealed evolutionary shifts in genome-wide gene expression and parallel genomic signatures of natural selection during independent salinity transitions. Our results suggest that balancing selection in the native range and epistatic interactions among specific ion transporter paralogs could contribute to parallel freshwater adaptation. Overall, these studies provide unprecedented insights into evolutionary mechanisms underlying physiological adaptation during rapid salinity change.

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

Salinization of freshwater is occurring throughout the world, affecting freshwater biota that inhabit rivers, streams, ponds, marshes and lakes. There are many freshwater insects, and these animals are important for ecosystem health. These insects have evolved physiological mechanisms to maintain their internal salt and water balance based on a freshwater environment that has comparatively little salt. In these habitats, insects must counter the loss of salts and dilution of their internal body fluids by sequestering salts and excreting water. Most of these insects can tolerate salinization of their habitats to a certain level; however, when exposed to salinization they often exhibit markers of stress and impaired development. An understanding of the physiological mechanisms for controlling salt and water balance in freshwater insects, and how these are affected by salinization, is needed to predict the consequences of salinization for freshwater ecosystems. Recent research in this area has addressed the whole-organism response, but the purpose of this Review is to summarize the effects of salinization on the osmoregulatory physiology of freshwater insects at the molecular to organ level. Research of this type is limited, and pursuing such lines of inquiry will improve our understanding of the effects of salinization on freshwater insects and the ecosystems they inhabit.

Acute and chronic temperature dependence of Na+/H+ exchange activity of Pimephales promelas gills

Na⁺/H⁺ exchangers (NHE) mediate at least part of Na⁺ entry into gill epithelia via Na⁺/NH₄⁺ exchange. For homeostasis, Na⁺ entry into and exit via Na⁺/K⁺ ATPase from gill epithelia must balance. Na⁺/K⁺ ATPase activity is reduced in cold- compared to warm-acclimated freshwater temperate fish. We hypothesized gill NHE activity is greater in warm- than cold-acclimated fish when measured at acclimation temperatures, and NHE activity displays a temperature dependence similar to Na⁺/K⁺ ATPase. Since NHE mRNA expression does not differ, we measured the Na⁺-dependence of pH-induced Na⁺ fluxes in gill vesicles from warm- and cold-acclimated fathead minnows at 20^o and 7 °C, and calculated maximum transport rates (V_max) and Na⁺ K_1/2s. We also measured NH₄⁺-induced Na⁺ fluxes and Na⁺-induced H⁺ fluxes. In vesicles from warm-acclimated fish, NHE V_maxs were 278 ± 33 and 149 ± 23 arbitrary unit/s (au/s) and Na⁺ K_1/2s were 12 ± 4 and 6 ± 4 mmol/l when assayed at 20^o and 7 °C (p < 0.004), respectively. In vesicles from cold-acclimated fish, V_maxs were 288 ± 35 and 141 ± 13 au/s and Na⁺ K_1/2s 17 ± 5 and 7 ± 2 mmol/l when assayed at 20^o and 7 °C (p < 0.002), respectively. Na⁺-induced H⁺ fluxes were 98 ± 8 and 104 ± 26 au/s in warm- and cold-acclimated fish assayed at 20 °C, respectively. Na⁺/NH₄⁺ exchange was 120 ± 11 and 158 ± 13 au/s in warm- and cold-acclimated fish, respectively. Conclusions: Gill NHE activity was greater in warm- than cold-acclimated fish assayed at acclimation temperatures. The temperature dependence of NHE activity was similar in both groups, but differed from that reported for Na⁺/K⁺ ATPase suggesting complex mechanisms to maintain Na⁺ homeostasis.

Use of a carbonic anhydrase Ca17a knockout to investigate mechanisms of ion uptake in zebrafish (Danio rerio)

In fishes, branchial cytosolic carbonic anhydrase (CA) plays an important role in ion and acid-base regulation. The Ca17a isoform in zebrafish (Danio rerio) is expressed abundantly in Na⁺-absorbing/H⁺-secreting H⁺-ATPase-rich (HR) cells. The present study aimed to identify the role of Ca17a in ion and acid-base regulation across life stages using CRISPR/Cas9 gene editing. However, in preliminary experiments, we established that ca17a knockout is lethal with ca17a^-/- mutants exhibiting a significant decrease in survival beginning at ∼12 days postfertilization (dpf) and with no individuals surviving past 19 dpf. Based on these findings, we hypothesized that ca17a^-/- mutants would display alterations in ion and acid-base balance and that these physiological disturbances might underlie their early demise. Na⁺ uptake rates were significantly increased by up to 300% in homozygous mutants compared with wild-type individuals at 4 and 9 dpf; however, whole body Na⁺ content remained constant. While Cl^- uptake was significantly reduced in ca17a^-/- mutants, Cl^- content was unaffected. Reduction of CA activity by Ca17a morpholino knockdown or ethoxzolamide treatments similarly reduced Cl^- uptake, implicating Ca17a in the mechanism of Cl^- uptake by larval zebrafish. H⁺ secretion, O₂ consumption, CO₂ excretion, and ammonia excretion were generally unaltered in ca17a^-/- mutants. In conclusion, while the loss of Ca17a caused marked changes in ion uptake rates, providing strong evidence for a Ca17a-dependent Cl^- uptake mechanism, the underlying causes of the lethality of this mutation in zebrafish remain unclear.

Rapid evolution of genome-wide gene expression and plasticity during saline to freshwater invasions by the copepod Eurytemora affinis species complex

Saline migrants into freshwater habitats constitute among the most destructive invaders in aquatic ecosystems throughout the globe. However, the evolutionary and physiological mechanisms underlying such habitat transitions remain poorly understood. To explore the mechanisms of freshwater adaptation and distinguish between adaptive (evolutionary) and acclimatory (plastic) responses to salinity change, we examined genome-wide patterns of gene expression between ancestral saline and derived freshwater populations of the Eurytemora affinis species complex, reared under two different common-garden conditions (0 versus 15 PSU). We found that evolutionary shifts in gene expression (between saline and freshwater inbred lines) showed far greater changes and were more widespread than acclimatory responses to salinity (0 versus 15 PSU). Most notably, 30-40 genes showing evolutionary shifts in gene expression across the salinity boundary were associated with ion transport function, with inorganic cation transmembrane transport forming the largest Gene Ontology category. Of particular interest was the sodium transporter, the Na⁺ /H⁺ antiporter (NHA) gene family, which was discovered in animals relatively recently. Thirty key ion regulatory genes, such as NHA paralogue #7, demonstrated concordant evolutionary and plastic shifts in gene expression, suggesting the evolution of ion transporter function and plasticity during rapid invasions into novel salinities. Moreover, freshwater invasions were associated with the evolution of reduced plasticity in the freshwater population, again for the same key ion transporters, consistent with the predicted evolution of canalization following adaptation to stressful conditions. Our results have important implications for understanding evolutionary and physiological mechanisms of range expansions by some of the most widespread invaders in aquatic habitats.

Expression of ion transport genes in ionocytes isolated from larval zebrafish (Danio rerio) exposed to acidic or Na+-deficient water

In zebrafish (Danio rerio), a specific ionocyte subtype, the H⁺-ATPase-rich (HR) cell, is presumed to be a significant site of transepithelial Na⁺ uptake/acid secretion. During acclimation to environments differing in ionic composition or pH, ionic and acid-base regulations are achieved by adjustments to the activity level of HR cell ion transport proteins. In previous studies, the quantitative assessment of mRNA levels for genes involved in ionic and acid-base regulations relied on measurements using homogenates derived from the whole body (larvae) or the gill (adult). Such studies cannot distinguish whether any differences in gene expression arise from adjustments of ionocyte subtype numbers or transcriptional regulation specifically within individual ionocytes. The goal of the present study was to use fluorescence-activated cell sorting to separate the HR cells from other cellular subpopulations to facilitate the measurement of gene expression of HR cell-specific transporters and enzymes from larvae exposed to low pH (pH 4.0) or low Na⁺ (5 μM) conditions. The data demonstrate that treatment of larvae with acidic water for 4 days postfertilization caused cell-specific increases in H⁺-ATPase (atp6v1aa), ca17a, ca15a, nhe3b, and rhcgb mRNA in addition to increases in mRNA linked to cell proliferation. In fish exposed to low Na⁺, expression of nhe3b and rhcgb was increased owing to HR cell-specific regulation and elevated numbers of HR cells. Thus, the results of this study demonstrate that acclimation to low pH or low Na⁺ environmental conditions is facilitated by HR cell-specific transcriptional control and by HR cell proliferation.

Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio)

Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters and are enriched with mitochondria; therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na+ uptake evoked by acidic or low Na+ rearing conditions would result in corresponding changes in whole-body oxygen consumption (ṀO2 ) and/or cutaneous oxygen flux (JO2 ), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they displayed a lower ṀO2  and no difference in cutaneous JO2 Despite a higher Na+ uptake capacity in larvae reared under low Na+ conditions, there were no differences in ṀO2  and JO2  at 4 dpf. Furthermore, although Na+ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, ṀO2 and JO2  were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous JO2 Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na+, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost.

Cortisol is an osmoregulatory and glucose-regulating hormone in Atlantic sturgeon, a basal ray-finned fish

Our current understanding of the hormonal control of ion regulation in aquatic vertebrates comes primarily from studies on teleost fishes, with relatively little information on more basal fishes. We investigated the role of cortisol in regulating seawater tolerance and its underlying mechanisms in an anadromous chondrostean, the Atlantic sturgeon (Acipenser oxyrinchus). Exposure of freshwater-reared Atlantic sturgeon to seawater (25 ppt) resulted in transient (1-3 day) increases in plasma chloride, cortisol and glucose levels and long-term (6-14 day) increases in the abundance of gill Na⁺/K⁺/2Cl^- cotransporter (NKCC), which plays a critical role in salt secretion in teleosts. The abundance of gill V-type H⁺-ATPase, which is thought to play a role in ion uptake in fishes, decreased after exposure to seawater. Gill Na⁺/K⁺-ATPase activity did not increase in 25 ppt seawater, but did increase in fish gradually acclimated to 30 ppt. Treatment of Atlantic sturgeon in freshwater with exogenous cortisol resulted in dose-dependent increases in cortisol, glucose and gill NKCC and H⁺-ATPase abundance. Our results indicate that cortisol has an important role in regulating mechanisms for ion secretion and uptake in sturgeon and provide support for the hypothesis that control of osmoregulation and glucose by corticosteroids is a basal trait of jawed vertebrates.

Antagonistic bioaccumulation of waterborne Cu(II) and Cd(II) in common carp (Cyprinus carpio) and effects on ion-homeostasis and defensive mechanisms

In the aquatic environment, metals are present as mixtures, therefore studies on mixture toxicity are crucial to thoroughly understand their toxic effects on aquatic organisms. Common carp (Cyprinus carpio) were used to assess the effects of short-term Cu(II) and Cd(II) mixtures, using a fixed concentration of one of the metals, representing 25 % of its individual 96h-LC₅₀ (concentration lethal for 50 % of the population) combined with a variable concentration of the other metal corresponding to 10, 25 or 50 % of its 96h-LC₅₀, and vice versa. Our results showed a fast Cu and Cd bioaccumulation, with the percentage of increase in the order gill > liver > carcass. An inhibitory effect of Cu on Cd uptake was observed; higher Cu concentrations at fixed Cd levels resulted in a decreased accumulation of Cd. The presence of the two metal ions resulted in losses of total Na, K and Ca. Fish tried to compensate for the Na loss through the induction of the genes coding for Na⁺/K⁺-ATPase and H⁺-ATPase. Additionally, a counterintuitive induction of the gene encoding the high affinity copper transporter (CTR1) occurred, while a downregulation was expected to prevent further metal ion uptake. An induction of defensive mechanisms, both metal ion binding protein and anti-oxidant defences, was observed. Despite the metal accumulation and electrolyte loss, the low mortality suggest that common carp is able to cope with these metal levels, at least during a one-week exposure.

Cellular oxygen consumption, ROS production and ROS defense in two different size-classes of an Amazonian obligate air-breathing fish (Arapaima gigas)

In air-breathing fish a reduction of gill surface area reduces the danger of losing oxygen taken up in the air-breathing organ (ABO) to hypoxic water, but it also reduces the surface area available for ion exchange, so that ion regulation may at least in part be transferred to other organs, like the kidney or the gut. In the air-breathing Arapaima gigas, gill lamellae regress as development proceeds, and starting as a water-breathing embryo Arapaima turns into an obligate air-breathing fish with proceeding development, suggesting that ion regulation is shifted away from the gills as the fish grows. In Arapaima the kidney projects medially into the ABO and thus, probably a unique situation among fishes, is in close contact to the gas of the ABO. We therefore hypothesized that the kidney would be predestined to adopt an increased importance for ion homeostasis, because the elevated ATP turnover connected to ion transport can easily be met by aerobic metabolism based on the excellent oxygen supply directly from the ABO. We also hypothesized that in gill tissue the reduced ion regulatory activity should result in a reduced metabolic activity. High metabolic activity and exposure to high oxygen tensions are connected to the production of reactive oxygen species (ROS), therefore the tissues exposed to these conditions should have a high ROS defense capacity. Using in vitro studies, we assessed metabolic activity and ROS production of gill, kidney and ABO tissue, and determined the activity of ROS degrading enzymes in small (~ 5g, 2–3 weeks old) and larger (~ 670 g, 3–4 months old) A. gigas. Comparing the three tissues revealed that kidney tissue oxygen uptake by far exceeded the uptake measured in gill tissue or ABO. ROS production was particularly high in gill tissue, and all three tissues had a high capacity to degrade ROS. Gill tissue was characterized by high activities of enzymes involved in the glutathione pathway to degrade ROS. By contrast, the tissues of the ABO and in particular the kidney were characterized by high catalase activities, revealing different, tissue-specific strategies in ROS defense in this species. Overall the differences in the activity of cells taken from small and larger fish were not as pronounced as expected, while at the tissue level the metabolic activity of kidney cells by far exceeded the activity of ABO and gill cells.

Immunohistochemical characterization and change in location of branchial ionocytes after transfer from freshwater to seawater in the euryhaline obscure puffer, Takifugu obscurus

The obscure puffer Takifugu obscurus is a euryhaline fish species suitable for studying the molecular mechanism of osmoregulation. The distributional changes of branchial ionocytes were detected following the transfer from freshwater (FW) to seawater (SW) based on two main ion transporters, Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/ 2Cl^- cotransporter 1 (NKCC1). The mRNA and protein expression levels of NKA and NKCC1 in the gills all increased rapidly in the first four days after transfer to SW. Double immunofluorescence staining showed that NKCC1 and NKA were colocalized in the branchial ionocytes and the immunoreaction of NKCC1 was stronger after transfer. Moreover, following transfer to SW, the number of lamellar ionocytes in the gills is reduced and the number of filament ionocytes is increased significantly. Taken together, these findings indicated that SW transfer of obscure puffer promotes the changes of distribution, function and size of branchial ionocytes.

Inter-individual variability in freshwater tolerance is related to transcript level differences in gill and posterior kidney of European sea bass

Acclimation to low salinities is a vital physiological challenge for euryhaline fish as the European sea bass Dicentrarchus labrax. This species undertakes seasonal migrations towards lagoons and estuaries where a wide range of salinity variations occur along the year. We have previously reported intraspecific differences in freshwater tolerance, with an average 30% mortality rate. In this study, we bring new evidence of mechanisms underlying freshwater tolerance in sea bass at gill and kidney levels. In fresh water (FW), intraspecific differences in mRNA expression levels of several ion transporters and prolactin receptors were measured. We showed that the branchial Cl^-/HCO₃^- anion transporter (slc26a6c) was over-expressed in freshwater intolerant fish, probably as a compensatory response to low blood chloride levels and potential metabolic alkalosis. Moreover, prolactin receptor a (prlra) and Na⁺/Cl^- cotransporter (ncc1) but not ncc-2a expression seemed to be slightly increased and highly variable between individuals in freshwater intolerant fish. In the posterior kidney, freshwater intolerant fish exhibited differential expression levels of slc26 anion transporters and Na⁺/K⁺/2Cl^- cotransporter 1b (nkcc1b). Lower expression levels of prolactin receptors (prlra, prlrb) were measured in posterior kidney which probably contributes to the failure in ion reuptake at the kidney level. Freshwater intolerance seems to be a consequence of renal failure of ion reabsorption, which is not sufficiently compensated at the branchial level.

Assessing intracellular pH regulation in H+-ATPase-rich ionocytes in zebrafish larvae using in vivo ratiometric imaging

The H+-ATPase-rich (HR) cells of zebrafish larvae are a sub-type of ion-transporting cell located on the yolk sac epithelium that are responsible for Na+ uptake and H+ extrusion. Current models of HR cell ion transport mechanisms in zebrafish larvae are well established, but little is known about the involvement of the various ion transport pathways in regulating intracellular acid-base status. Here, a ratiometric imaging technique was developed and validated to monitor intracellular pH (pHi) continuously in larval zebrafish HR cells in vivo Gene knockdown or CRISPR/Cas9 knockout approaches were used to evaluate the roles of the two principal apical membrane acid excretory pathways, the Na+/H+ exchanger (NHE3b; slc9a3.2) and the H+-ATPase (atpv1aa). Additionally, the role of HR cell cytosolic carbonic anhydrase (CAc) was investigated because of its presumed role in providing H+ for Na+/H+ exchange and H+-ATPase. The temporal pattern and extent of intracellular acidification during exposure of fish to 1% CO2 and the extent of post-CO2 alkalisation were altered markedly in fish experiencing knockdown/knockout of CAc, NHE3b or H+-ATPase. Although there were slight differences among the three knockdown/knockout experiments, the typical response was a greater degree of intracellular acidification during CO2 exposure and a reduced capacity to restore pHi to baseline levels post-hypercapnia. The metabolic alkalosis and subsequent acidification associated with 20 mmol l-1 NH4Cl exposure and its washout were largely unaffected by gene knockdown. Overall, the results suggest markedly different mechanisms of intracellular acid-base regulation in zebrafish HR cells depending on the nature of the acid-base disturbance.

Reassessing the contribution of the Na+/H+ exchanger Nhe3b to Na+ uptake in zebrafish (Danio rerio) using CRISPR/Cas9 gene editing

Freshwater fishes absorb Na+ from their dilute environment using ion-transporting cells. In larval zebrafish (Danio rerio), Na+ uptake is coordinated by (1) Na+/H+-exchanger 3b (Nhe3b) and (2) H+-ATPase-powered electrogenic uptake in H+-ATPase-rich (HR) cells and by (3) Na+-Cl−­­-cotransporter (Ncc) expressed in NCC cells. The present study aimed to better understand the roles of these 3 proteins in Na+ uptake by larval zebrafish under ‘normal’ (800 µmol/L) and ‘low’ (10 µmol/L) Na+ conditions. We hypothesized that Na+ uptake would be reduced by CRISPR/Cas9 knockout (KO) of slc9a3.2 (encoding Nhe3b), particularly in low Na+ where Nhe3b is believed to play a dominant role. Contrary to this hypothesis, Na+ uptake was sustained in nhe3b KO larvae under both Na+ conditions, which led to the exploration of whether compensatory regulation of H+-ATPase or Ncc was responsible for maintaining Na+ uptake in nhe3b KO larvae. mRNA expression of the genes encoding H+-ATPase and Ncc were not altered in nhe3b KO. Moreover, morpholino knockdown of H+-ATPase, which significantly reduced H+ flux by HR cells, did not reduce Na+ uptake in nhe3b KO larvae, nor did rearing larvae in chloride-free conditions, thereby eliminating any driving force for Na+-Cl−-cotransport via Ncc. Finally, simultaneously treating nhe3b KO larvae with H+-ATPase morpholino and chloride-free conditions did not reduce Na+ uptake under normal or low Na+. These findings highlight the flexibility of the Na+ uptake system and demonstrate that Nhe3b is expendable to Na+ uptake in zebrafish and that our understanding of Na+ uptake mechanisms in this species is incomplete.

Investigating the RAS can be a fishy business: interdisciplinary opportunities using Zebrafish

The renin-angiotensin system (RAS) is highly conserved, and components of the RAS are present in all vertebrates to some degree. Although the RAS has been studied since the discovery of renin, its biological role continues to broaden with the identification and characterization of new peptides. The evolutionarily distant zebrafish is a remarkable model for studying the kidney due to its genetic tractability and accessibility for in vivo imaging. The zebrafish pronephros is an especially useful kidney model due to its structural simplicity yet complex functionality, including capacity for glomerular and tubular filtration. Both the pronephros and mesonephros contain renin-expressing perivascular cells, which respond to RAS inhibition, making the zebrafish an excellent model for studying the RAS. This review summarizes the physiological and genetic tools currently available for studying the zebrafish kidney with regards to functionality of the RAS, using novel imaging techniques such as SPIM microscopy coupled with targeted single cell ablation and synthesis of vasoactive RAS peptides.

Mechanisms of toxic action of copper and copper nanoparticles in two Amazon fish species: Dwarf cichlid (Apistogramma agassizii) and cardinal tetra (Paracheirodon axelrodi)

Copper oxide nanoparticles (nCuO) are widely used in boat antifouling paints and are released into the environment, potentially inducing toxicity to aquatic organisms. The present study aimed to understand the effects of nCuO and dissolved copper (Cu) on two ornamental Amazon fish species: dwarf cichlid (Apistogramma agassizii) and cardinal tetra (Paracheirodon axelrodi). Fish were exposed to 50% of the LC₅₀ for nCuO (dwarf cichlid 58.31μgL^-1 and cardinal tetra 69.6μgL^-1) and Cu (dwarf cichlid 20μgL^-1 and cardinal tetra 22.9μgL^-1) for 24, 48, 72 and 96h. Following exposure, aerobic metabolic rate (ṀO₂), gill osmoregulatory physiology and mitochondrial function, oxidative stress markers, and morphological damage were evaluated. Our results revealed species specificity in metabolic stress responses. An increase of ṀO₂ was noted in cardinal tetra exposed to Cu, but not nCuO, whereas ṀO₂ in dwarf cichlid showed little change with either treatment. In contrast, mitochondria from dwarf cichlid exhibited increased proton leak and a resulting decrease in respiratory control ratios in response to nCuO and Cu exposure. This uncoupling was directly related to an increase in reactive oxygen species (ROS) levels. Our findings reveal different metabolic responses between these two species in response to nCuO and Cu, which are probably caused by the differences between species natural histories, indicating that different mechanisms of toxic action of the contaminants are associated to differential osmoregulatory strategies among species.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Na+/H+ Exchanger 3 Is Expressed in Two Distinct Types of Ionocyte, and Probably Augments Ammonia Excretion in One of Them, in the Gills of the Climbing Perch Exposed to Seawater

The freshwater climbing perch, Anabas testudineus, is an euryhaline teleost and an obligate air-breather with the ability to actively excrete ammonia. Members of the Na+/H+ exchanger (NHE) family help maintain intracellular pH homeostasis and ionic balance through the electroneutral exchange of Na+ and H+. This study aimed to obtain, from the gills of A. testudineus, the full cDNA coding sequence of nhe3, and to determine the effects of exposure to seawater or 100 mmol l-1 of NH4Cl in fresh water on its mRNA and protein expression levels. Efforts were also made to elucidate the type of ionocyte that Nhe3 was associated with in the branchial epithelium of A. testudineus. The transcript level and protein abundance of nhe3/Nhe3 were very low in the gills of freshwater A. testudineus, but they increased significantly in the gills of fish acclimated to seawater. In the gills of fish exposed to seawater, Nhe3 was expressed in two distinct types of seawater-inducible Na+/K+-ATPase (Nka)-immunoreactive ionocytes. In Nkaα1b-immunoreactive ionocytes, Nhe3 had an apical localization. As these ionocytes also expressed apical Rhcg1 and basolateral Rhcg2, which are known to transport ammonia, they probably participated in proton-facilitated ammonia excretion in A. testudineus during seawater acclimation. In Nkaα1c-immunoreactive ionocytes, Nhe3 was atypically expressed in the basolateral membrane, and its physiological function is uncertain. For A. testudineus exposed to NH4Cl in fresh water, the transcript and protein expression levels of nhe3/Nhe3 remained low. In conclusion, the branchial Nhe3 of A. testudineus plays a greater physiological role in passive ammonia transport and acid-base balance during seawater acclimation than in active ammonia excretion during environmental ammonia exposure.

WNK1 and p38-MAPK distribution in ionocytes and accessory cells of euryhaline teleost fish implies ionoregulatory function

Ionocytes of euryhaline teleost fish secrete NaCl, under regulation by serine and threonine kinases, including with-no-lysine kinase (WNK1) and p38 mitogen-activated protein kinase (MAPK). Mummichogs (Fundulus heteroclitus L.) were acclimated to freshwater (FW), full strength seawater (SW) and hypersaline conditions (2SW). Immunocytochemistry of ionocytes in opercular epithelia of fish acclimated to SW and 2SW revealed that WNK1-anti-pT58 phosphoantibody localized strongly to accessory cells and was present in the cytosol of ionocytes, close to cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane and the sodium potassium 2 chloride cotransporter (NKCC) in the basolateral membrane. In FW acclimated fish, WNK1 localized to a sub-apical zone, did not colocalize with apical membrane-located sodium chloride cotransporter (NCC), and typically was present in one cell of paired ionocytes and in some single ionocytes. Forskolin treatment (10 μM, 30 min) increased WNK1 immunofluorescence in SW ionocytes only, while hypertonicity had little effect, compared to controls. Anti-p38-MAPK antibody localized to the cytosolic compartment. The distribution of WNK1 and p38MAPK is consistent with a proximal position in regulatory cascades, rather than directly affecting transporters. The strong staining of accessory cells by WNK1 phosphoantibody infers an osmoregulatory function for WNK.

Summary: Fish opercular epithelium ionocytes and accessory cells have WNK family kinases that may regulate paracellular and transcellular ion transport.

Ammonia-independent sodium uptake mediated by Na+ channels and NHEs in the freshwater ribbon leech Nephelopsis obscura

Freshwater organisms actively take up ions from their environment to counter diffusive ion losses due to inhabiting hypo-osmotic environments. The mechanisms behind active Na⁺ uptake are quite well understood in freshwater teleosts; however, the mechanisms employed by invertebrates are not. Pharmacological and molecular approaches were used to investigate Na⁺ uptake mechanisms and their link to ammonia excretion in the ribbon leech Nephelopsis obscura At the molecular level, we identified a Na⁺ channel and a Na⁺/H⁺ exchanger (NHE) in the skin of N. obscura, where the NHE was up-regulated when acclimated to extremely low [Na⁺] (0.05 mmol l^-1, pH 5) conditions. Additionally, we found that leeches in dilute freshwater environments use both a vacuolar-type H⁺-ATPase (VHA)-assisted uptake via a Na⁺ channel and a NHE-based mechanisms for Na⁺ uptake. Immunolocalization of VHA and Na⁺/K⁺-ATPase (NKA) indicated at least two cell types present within leech skin, VHA⁺ and VHA^- cells, where the VHA⁺ cells are probably involved in Na⁺ uptake. NKA was present throughout the epithelium. We also found that increasing ammonia excretion by decreasing water pH, ammonia loading leeches or exposing leeches to high environmental ammonia does not affect Na⁺ uptake, providing indications that an NHE-Rh metabolon is not present and that ammonia excretion and Na⁺ uptake are not coupled in N. obscura To our knowledge, this is the first study showing the mechanisms of Na⁺ uptake and their links to ammonia excretion in a freshwater invertebrate, where results suggest an ammonia-independent Na⁺ uptake mechanism relying on both Na⁺ channels and NHEs.

Osmoregulation in zebrafish: ion transport mechanisms and functional regulation

Fish, like mammals, have to maintain their body fluid ionic and osmotic homeostasis through sophisticated iono-/osmoregulation mechanisms, which are conducted mainly by ionocytes of the gill (the skin in embryonic stages), instead of the renal tubular cells in mammals. Given the advantages in terms of genetic database availability and manipulation, zebrafish is an emerging model for research into regulatory and integrative physiology. At least five types of ionocytes, HR, NaR, NCC, SLC26, and KS cells, have been identified to carry out Na(+) uptake/H(+) secretion/NH4 (+) excretion, Ca(2+) uptake, Na(+)/Cl(-) uptake, K(+) secretion, and Cl(-) uptake/HCO3 (-) secretion, respectively, through distinct sets of transporters. Several hormones, namely isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyorid hormone 1, catecholamines, and the renin-angiotensin-system, have been demonstrated to positively or negatively regulate ion transport through specific receptors at different ionocytes stages, at either the transcriptional/translational or posttranslational level. The knowledge obtained using zebrafish answered many long-term contentious or unknown issues in the field of fish iono-/osmoregulation. The homology of ion transport pathways and hormone systems also means that the zebrafish model informs studies on mammals or other animal species, thereby providing insights into related fields.

Cloning and expression of the epithelial sodium channel and its role in osmoregulation of aquatic and estivating African lungfish Protopterus annectens

The epithelial sodium channel (ENaC) is a sodium (Na(+))-selective aldosterone-stimulated ion channel involved in Na(+) transport homeostasis of tetrapods. We examined full-length cDNA sequences and tissue distributions of ENaCα, ENaCβ, and ENaCγ subunits in the African lungfish Protopterus annectens. Protopterus ENaC (pENaC) comprises 3 subunits: pENaCα, pENaCβ, and pENaCγ. pENaCα, pENaCβ, and pENaCγ subunits are closely related to α, β, and γ subunits of the Australian lungfish Neoceratodus forsteri ENaC (nENaC), respectively. Three ENaC subunit mRNAs were highly expressed in the gills and moderately expressed in the kidney and rectum of P. annectens. During estivation for 2-4weeks and 2-3months, plasma Na(+) concentration was relatively stable, but plasma urea concentration significantly increased in comparison with the control fish kept in a freshwater environment. Plasma aldosterone concentration and mRNA expression of the ENaCα subunit gradually and significantly decreased in the gills and kidney after 2months of estivation. Thus, aldosterone-dependent Na(+) absorption via ENaC probably exists in the epithelial cells of osmoregulatory organs of lungfish kept in fresh water, whereas plasma Na(+) concentration may be maintained by a mechanism independent of aldosterone-ENaC axis during estivation in lungfish.

An in vitro investigation of gastrointestinal Na(+) uptake mechanisms in freshwater rainbow trout

In vitro gut-sac preparations of all four sections (stomach, anterior, mid, and posterior intestine) of the gastrointestinal tract (GIT) of freshwater rainbow trout, together with radiotracer ((22)Na) techniques, were used to study unidirectional Na(+) uptake rates (UR, mucosal → blood space) and net absorptive fluid transport rates (FTR) under isosmotic conditions (mucosal = serosal osmolality). On an area-specific basis, unidirectional Na(+) UR was highest in the mid-intestine, but when total gut area was taken into account, the three intestinal sections contributed equally, with very low rates in the stomach. The theoretical capacity for Na(+) uptake across the whole GIT is sufficient to supply all of the animal's nutritive requirements for Na(+). Transport occurs by low affinity systems with apparent K m values 2-3 orders of magnitude higher than those in the gills, in accord with comparably higher Na(+) concentrations in chyme versus fresh water. Fluid transport appeared to be Na(+)-dependent, such that treatments which altered unidirectional Na(+) UR generally altered FTR in a comparable fashion. Pharmacological trials (amiloride, EIPA, phenamil, bafilomycin, furosemide, hydrochlorothiazide) conducted at a mucosal Na(+) concentration of 50 mmol L(-1) indicated that GIT Na(+) uptake occurs by a variety of apical mechanisms (NHE, Na(+) channel/H(+) ATPase, NCC, NKCC) with relative contributions varying among sections. However, at a mucosal Na(+) concentration of 10 mmol L(-1), EIPA, phenamil, bafilomycin, and hydrochlorothiazide were no longer effective in inhibiting unidirectional Na(+) UR or FTR, suggesting the contribution of unidentified mechanisms under low Na(+) conditions. A preliminary model is presented.

Diverse mechanisms for body fluid regulation in teleost fishes

Teleost fishes are the major group of ray-finned fishes and represent more than one-half of the total number of vertebrate species. They have experienced in their evolution an additional third-round whole genome duplication just after the divergence of their lineage, which endowed them with an extra adaptability to invade various aquatic habitats. Thus their physiology is also extremely diverse compared with other vertebrate groups as exemplified by the many patterns of body fluid regulation or osmoregulation. The key osmoregulatory organ for teleosts, whose body fluid composition is similar to mammals, is the gill, where ions are absorbed from or excreted into surrounding waters of various salinities against concentration gradients. It has been shown that the underlying molecular physiology of gill ionocytes responsible for ion regulation is highly variable among species. This variability is also seen in the endocrine control of osmoregulation where some hormones have distinct effects on body fluid regulation in different teleost species. A typical example is atrial natriuretic peptide (ANP); ANP is secreted in response to increased blood volume and acts on various osmoregulatory organs to restore volume in rainbow trout as it does in mammals, but it is secreted in response to increased plasma osmolality, and specifically decreases NaCl, and not water, in the body of eels. The distinct actions of other osmoregulatory hormones such as growth hormone, prolactin, angiotensin II, and vasotocin among teleost species are also evident. We hypothesized that such diversity of ionocytes and hormone actions among species stems from their intrinsic differences in body fluid regulation that originated from their native habitats, either fresh water or seawater. In this review, we summarized remarkable differences in body fluid regulation and its endocrine control among teleost species, although the number of species is still limited to substantiate the hypothesis.

Hydrogen sulfide inhibits Na+ uptake in larval zebrafish, Danio rerio

The present study investigated the role of hydrogen sulfide (H2S) in regulating Na(+) uptake in larval zebrafish, Danio rerio. Waterborne treatment of larvae at 4 days post-fertilization (dpf) with Na2S or GYY-4137 (chemicals known to generate H2S) significantly reduced Na(+) uptake. Exposure of larvae to water enriched with NaCl (1 mM NaCl) caused a pronounced reduction in Na(+) uptake which was prevented by pharmacological inhibition of cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE), two key enzymes involved in the endogenous synthesis of H2S. Furthermore, translational gene knockdown of CSE and CBSb significantly increased the basal rate of Na(+) uptake. Waterborne treatment with Na2S significantly decreased whole-body acid excretion and reduced Na(+) uptake in larval zebrafish preexposed to acidic (pH 4.0) water (a condition shown to promote Na(+) uptake via Na(+)-H(+)-exchanger 3b, NHE3b). However, Na2S did not affect Na(+) uptake in larvae depleted of NHE3b-containing ionocytes (HR cells) after knockdown of transcription factor glial cell missing 2 (gcm2) in which Na(+) uptake occurs predominantly via Na(+)-Cl(-) co-transporter (NCC)-containing cells. These observations suggest that Na(+) uptake via NHE3b, but not NCC, is regulated by H2S. Whole-mount immunohistochemistry demonstrated that ionocytes expressing NHE3b also express CSE. These data suggests a physiologically relevant role of H2S as a mechanism to lower Na(+) uptake in zebrafish larvae, probably through its inhibitory action on NHE3b.

Identification of fxyd genes from the spotted scat (Scatophagus argus): molecular cloning, tissue-specific expression, and response to acute hyposaline stress

By interacting with Na(+), K(+)-ATPase (NKA), the FXYD domain-containing ion transport regulator (FXYD) is involved in teleost osmoregulation, but knowledge of FXYD in marine fish is limited. In the present study, fxyd11 and fxyd12 were identified from the spotted scat (Scatophagus argus), and the two members of the FXYD protein family were expressed in a tissue-specific manner. Fxyd11 mRNA was predominantly expressed in gills, whereas fxyd12 mRNA was mainly distributed in kidneys and intestines. Acute hyposaline stress altered the activity of NKA and the expression of fxyd11 and fxyd12 in gills, kidneys, and intestines. Branchial fxyd11 mRNA expression remained at a low level during freshwater acclimation, whereas NKA activity increased, showing a negative correlation that differed from previous reports. Similarly, renal expression of fxyd11 and fxyd12 mRNA was negatively correlated with NKA activity. Unlike in gills and kidneys, intestinal NKA activity and mRNA expression of fxyd11 and fxyd12 were comparably suppressed. Taken together, the salinity-dependent expression of fxyd11 and fxyd12, and correlation with NKA activity suggested that both fxyd11 and fxyd12 were involved in the response to acute hyposaline challenge in the spotted scat.

Angiotensin-II promotes Na+ uptake in larval zebrafish, Danio rerio, in acidic and ion-poor water

The contribution of the renin-angiotensin system (RAS) to Na(+) uptake was investigated in larval zebrafish (Danio rerio). At 4 days post fertilization (dpf), the level of whole-body angiotensin-II (ANG-II) was significantly increased after 1- or 3-h exposure to acidic (pH=4.0) or ion-poor water (20-fold dilution of Ottawa tapwater), suggesting rapid activation of the RAS. Long-term (24 h) treatment of 3 dpf larvae with ANG-I or ANG-II significantly increased Na(+) uptake which was accompanied by an increase in mRNA expression of the Na(+)-Cl(-) cotransporter (zslc12a10.2). Induction of Na(+) uptake by exposure to ANG-I was blocked by simultaneously treating larvae with lisinopril (an angiotensin-converting enzyme inhibitor). Acute (2 h) exposure to acidic water or ion-poor water led to significant increase in Na(+) uptake which was partially blocked by the ANG-II receptor antagonist, telmisartan. Consistent with these data, translational knockdown of renin prevented the stimulation of Na(+) uptake following exposure to acidic or ion-poor water. The lack of any effects of pharmacological inhibition (using RU486), or knockdown of glucocorticoid receptors on the stimulation of Na(+) uptake during acute exposure to acidic or ion-poor environments, indicates that the acute effects of RAS occur independently of cortisol signaling. The results of this study demonstrate that the RAS is involved in Na(+) homeostasis in larval zebrafish.

Na+/H+ and Na+/NH+4 activities of zebrafish NHE3b expressed in Xenopus oocytes

Zebrafish Na(+)/H(+) exchanger 3b (zNHE3b) is highly expressed in the apical membrane of ionocytes where Na(+) is absorbed from ion-poor fresh water against a concentration gradient. Much in vivo data indicated that zNHE3b is involved in Na(+) absorption but not leakage. However, zNHE3b-mediated Na(+) absorption has not been thermodynamically explained, and zNHE3b activity has not been measured. To address this issue, we overexpressed zNHE3b in Xenopus oocytes and characterized its activity by electrophysiology. Exposure of zNHE3b oocytes to Na(+)-free media resulted in significant decrease in intracellular pH (pH(i)) and intracellular Na(+) activity (aNa(i)). aNa(i) increased significantly when the cytoplasm was acidified by media containing CO₂-HCO₃(-) or butyrate. Activity of zNHE3b was inhibited by amiloride or 5-ethylisopropyl amiloride (EIPA). Although the activity was accompanied by a large hyperpolarization of ∼50 mV, voltage-clamp experiments showed that Na(+)/H(+) exchange activity of zNHE3b is electroneutral. Exposure of zNHE3b oocytes to medium containing NH₃/NH₄(+) resulted in significant decreases in pH(i) and aNa(i) and significant increase in intracellular NH₄(+) activity, indicating that zNHE3b mediates the Na(+)/NH₄(+) exchange. In low-Na(+) (0.5 mM) media, zNHE3b oocytes maintained aNa(i) of 1.3 mM, and Na(+)-influx was observed when pHi was decreased by media containing CO₂-HCO₃(-) or butyrate. These results provide thermodynamic evidence that zNHE3b mediates Na(+) absorption from ion-poor fresh water by its Na(+)/H(+) and Na(+)/NH₄(+) exchange activities.

Effects of hypoxia-induced gill remodelling on the innervation and distribution of ionocytes in the gill of goldfish, Carassius auratus

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish acclimated to 7°C leads to preferential distribution of branchial ionocytes to the distal edges of the ILCM, where they are likely to remain in contact with the water and hence remain functional. Upon exposure to hypoxia, the ILCM retracts, and the ionocytes become localized to the lamellar surfaces and on the filament epithelium, owing to their migration and the differentiation of new ionocytes from progenitor cells. Here we demonstrate that the majority of the ionocytes receive neuronal innervation, which led us to assess the consequences of ionocyte migration and differentiation during hypoxic gill remodelling on the pattern and extent of ionocyte neuronal innervation. Normoxic 7°C goldfish (ILCM present) possessed significantly greater numbers of ionocytes/mm² (951.2 ± 94.3) than their 25°C conspecifics (ILCM absent; 363.1 ± 49.6) but a statistically lower percentage of innervated ionocytes (83.1% ± 1.0% compared with 87.8% ± 1.3%). After 1 week of exposure of goldfish to hypoxia, the pool of branchial ionocytes was composed largely of pre-existing migrating cells (555.6 ± 38.1/mm²) and to a lesser extent newly formed ionocytes (226.7 ± 15.1/mm²). The percentage of new (relative to pre-existing) ionocytes remained relatively constant (at ∼30%) after 1 or 2 weeks of normoxic recovery. After hypoxia, pre-existing ionocytes expressed a greater percentage of innervation than newly formed ionocytes in all treatment groups; however, their percentage innervation steadily decreased over 2 weeks of normoxic recovery.

The role of cAMP-mediated intracellular signaling in regulating Na+ uptake in zebrafish larvae

In the current study, the role of cAMP in stimulating Na(+) uptake in larval zebrafish was investigated. Treating larvae at 4 days postfertilization (dpf) with 10 μM forskolin or 1 μM 8-bromo cAMP significantly increased Na(+) uptake by three-fold and twofold, respectively. The cAMP-dependent stimulation of Na(+) uptake was probably unrelated to protein trafficking via microtubules because pretreatment with 200 μM colchicine or 30 μM nocodazole did not attenuate the magnitude of the response. Na(+) uptake was stimulated markedly following acute (2 h) exposure to acidic water. The acid-induced increase in Na(+) uptake was accompanied by a twofold elevation in whole body cAMP levels and attenuated by inhibiting PKA with 10 μM H-89. Knockdown of Na(+)-H(+) exchanger 3b (NHE3b) attenuated, but did not abolish, the stimulation of Na(+) uptake during forskolin treatment. In glial cell missing 2 morphants, in which the role of NHE3b in Na(+) uptake is diminished and the Na(+)-Cl(-) cotransporter (NCC) becomes the predominant route of Na(+) entry, forskolin treatment continued to increase Na(+) uptake. These data suggest that at least NHE3b and NCC are targeted by cAMP in zebrafish larvae. Staining of larvae with fluorescent forskolin and propranolol revealed the presence of transmembrane adenylyl cyclase within multiple subtypes of ionocytes expressing β-adrenergic receptors. Taken together, results of the present study demonstrate that cAMP-mediated intracellular signaling may regulate multiple Na(+) transporters and plays an important role in regulating Na(+) uptake in zebrafish larvae during acute exposure to an acidic environment.

Compensatory regulation of Na+ absorption by Na+/H+ exchanger and Na+-Cl- cotransporter in zebrafish (Danio rerio)

INTRODUCTION

In mammals, internal Na+ homeostasis is maintained through Na+ reabsorption via a variety of Na+ transport proteins with mutually compensating functions, which are expressed in different segments of the nephrons. In zebrafish, Na+ homeostasis is achieved mainly through the skin/gill ionocytes, namely Na+/H+ exchanger (NHE3b)-expressing H+-ATPase rich (HR) cells and Na+-Cl- cotransporter (NCC)-expressing NCC cells, which are functionally homologous to mammalian proximal and distal convoluted tubular cells, respectively. The present study aimed to investigate whether or not the functions of HR and NCC ionocytes are differentially regulated to compensate for disruptions of internal Na+ homeostasis and if the cell differentiation of the ionocytes is involved in this regulation pathway.

RESULTS

Translational knockdown of ncc caused an increase in HR cell number and a resulting augmentation of Na+ uptake in zebrafish larvae, while NHE3b loss-of-function caused an increase in NCC cell number with a concomitant recovery of Na+ absorption. Environmental acid stress suppressed nhe3b expression in HR cells and decreased Na+ content, which was followed by up-regulation of NCC cells accompanied by recovery of Na+ content. Moreover, knockdown of ncc resulted in a significant decrease of Na+ content in acid-acclimated zebrafish.

CONCLUSIONS

These results provide evidence that HR and NCC cells exhibit functional redundancy in Na+ absorption, similar to the regulatory mechanisms in mammalian kidney, and suggest this functional redundancy is a critical strategy used by zebrafish to survive in a harsh environment that disturbs body fluid Na+ homeostasis.

Zebrafish as an animal model to study ion homeostasis

Zebrafish (Danio rerio) possesses several advantages as an experimental organism, including the applicability of molecular tools, ease of in vivo cellular observation and functional analysis, and rapid embryonic development, making it an emerging model for the study of integrative and regulatory physiology and, in particular, the epithelial transport associated with body fluid ionic homeostasis. Zebrafish inhabits a hypotonic freshwater environment, and as such, the gills (or the skin, during embryonic stages) assume the role of the kidney in body fluid ionic homeostasis. Four types of ionocyte expressing distinct sets of transporters have been identified in these organs: H⁺-ATPase-rich, Na⁺-K⁺-ATPase-rich, Na⁺-Cl⁻ cotransporter-expressing and K⁺-secreting cells; these ionocytes perform transepithelial H⁺ secretion/Na⁺ uptake/NH₄⁺ excretion, Ca²⁺ uptake, Na⁺/Cl⁻ uptake, and K⁺ secretion, respectively. Zebrafish ionocytes are analogous to various renal tubular cells, in terms of ion transporter expression and function. During embryonic development, ionocyte progenitors develop from epidermal stem cells and then differentiate into different types of ionocyte through a positive regulatory loop of Foxi3a/-3b and other transcription factors. Several hormones, including cortisol, vitamin D, stanniocalcin-1, calcitonin, and isotocin, were found to participate in the control pathways of ionic homeostasis by precisely studying the target ion transport pathways, ion transporters, or ionocytes of the hormonal actions. In conclusion, the zebrafish model not only enhances our understanding of body fluid ion homeostasis and hormonal control in fish but also informs studies on mammals and other animal species, thereby providing new insights into related fields.