Ionic and acid–base regulation

Functional divergence of teleost carbonic anhydrase 4

The functional role of membrane-bound carbonic anhydrases (CAs) has been of keen interest in the past decade, and in particular, studies have linked CA in red muscle, heart, and eye to enhanced tissue oxygen extraction in bony fishes (teleosts). However, the number of purported membrane-bound CA isoforms in teleosts, combined with the imperfect system of CA isoform nomenclature, present roadblocks for ascribing physiological functions to particular CA isoforms across different teleost lineages. Here we developed an organizational framework for membrane-bound CAs in teleosts, providing the latest phylogenetic analysis of extant CA4 and CA4-like isoforms. Our data confirm that there are three distinct isoforms of CA4 (a, b, and c) that are conserved across major teleost lineages, with the exception of CA4c gene being lost in salmonids. Tissue distribution analyses suggest CA4a functions in oxygen delivery across teleost lineages, while CA4b may be specialized for renal acid-base balance and ion regulation. This work provides an important foundation for researchers to elucidate the functional significance of CA4 isoforms in fishes.

Effects of Bicarbonate Stress on Serum Ions and Gill Transporters in Alkali and Freshwater Forms of Amur Ide (Leuciscus waleckii)

The Amur ide (Leuciscus waleckii) is a fish in the Cyprinidae family. Compared with other Amur ide living in freshwater ecosystems, the Amur ide population in Lake Dali Nor of China is famous for its high tolerance to the alkaline conditions of 54 mM (pH 9.6). Yet, surprisingly, the ionoregulatory mechanism responsible for this remarkable alkaline adaptation remains unclear. Therefore, this study sought to investigate how bicarbonate affects the acid-base balancing and ionoregulatory responses of this animal. Here, using a comparative approach, the alkali form of Amur ide and its ancestral freshwater form living in other freshwater basins were each exposed to 50 mM (pH 9.59 ± 0.09), a level close to the alkalinity of Lake Dali Nor, and their physiological (AE1) adjustment of ions and acid-base regulation were investigated. This study highlighted differences in blood pH and serum ions (e.g., Na⁺, K⁺, Cl⁻, and Ca²⁺), Na⁺/K⁺ ATPase (NKA) activity and its mRNA level, and mRNA expression of gill transporters (Na⁺/H⁺ exchanger member 2 and/or 3, Na⁺/HCO3- cotransporter (NBC1), Cl⁻/HCO3- exchanger, Na⁺/Cl⁻ cotransporter (NCC), Na⁺/K⁺/2Cl⁻ (NKCC1), SLC26A5, and SLC26A6) for alkalinity adaptation between the two forms of Amur ide differing in alkalinity tolerance. Specifically, close relationships among the serum Na⁺ and mRNA levels of NCC, NKCC1, and NHE, and also NKA and NBC1, in addition to serum Cl⁻ and bicarbonate transporters (e.g., SLC26A5 and SLC26A6), characterized the alkali form of Amur ide. We propose that this ecotype can ensure its transepithelial Cl⁻ and Na⁺ uptake/base secretions are highly functional, by its basolateral NKA with NBC1 and apical ionic transporters, and especially NCC incorporated with other transporters (e.g., SLC26). This suggests an evolved strong ability to maintain an ion osmotic and acid-base balance for more effectively facilitating its adaptability to the high alkaline environment. This study provides new insights into the physiological responses of the alkaline form of the Amur ide fish for adapting to extreme alkaline conditions. This information could be used as a reference to cultivating alkaline-tolerant fish species in abandoned alkaline waters.

Basolateral Na+/Ca2+ exchanger 1 and Na+/K+-ATPase, which display light-enhanced gene and protein expression levels, could be involved in the absorption of exogenous Ca2+ through the ctenidium of the giant clam, Tridacna squamosa

Giant clams perform light-enhanced shell formation (calcification) and therefore need to increase the uptake of exogenous Ca²⁺ during illumination. The ctenidium of the fluted giant clam, Tridacna squamosa, is involved in light-enhanced Ca²⁺ uptake. It expresses the pore-forming voltage-gated calcium channel (VGCC) subunit alpha 1 (CACNA1) in the apical membrane of the epithelial cells, and the protein expression level of CACNA1 is upregulated in the ctenidium during illumination. This study aimed to elucidate the mechanism involved in the transport of the absorbed Ca²⁺ across the basolateral membrane of the ctenidial epithelial cells into the hemolymph. We obtained a homolog of Na⁺/Ca²⁺exchanger 1 (NCX1-like) from the ctenidium of T. squamosa, which comprised 2418 bp, encoding a protein of 806 amino acids (88.9 kDa). NCX1-like had a basolateral localization in the epithelial cells of the ctenidial filaments and tertiary water channels. Illumination resulted in significant increases in the transcript and protein levels of NCX1-like/NCX1-like in the ctenidium. Hence, NCX1-like could operate in conjunction with VGCC to increase the transport of Ca²⁺ from the ambient seawater into the hemolymph during illumination. Illumination also resulted in the upregulation of the gene and protein expression levels of Na⁺/K⁺-ATPase (NKA) α-subunit (NKAα/NKAα) in the ctenidium of T. squamosa. As light-enhanced extrusion of Ca²⁺ into the hemolymph through NCX1-like would lead to a greater influx of extracellular Na⁺, the increased expression of the basolateral NKA was required to augment the capacity of intracellular Na⁺ homeostasis.

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.

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.

Dynamic changes in nitric oxide synthase expression are involved in seawater acclimation of rainbow trout Oncorhynchus mykiss

Recent research has shown that nitric oxide (NO) produced by nitric oxide synthases (NOS) is an inhibitor of ion transporter activity and a modulator of epithelial ion transport in fish, but little is known on changes in the NOS/NO system during osmotic stress. We hypothesized that the NOS/NO system responds to salinity changes as an integrated part of the acclimation process. Expression and localization of nos1/Nos1 and nos2/Nos2 were investigated in gill, kidney, and intestine of freshwater (FW)- and seawater (SW)-transferred trout using quantitative PCR, Western blotting, and immunohistochemistry, along with expressional changes of major ion transporters in the gill. The classical branchial ion transporters showed expected expressional changes upon SW transfer, there among a rapid decrease in Slc26a6 mRNA, coding a branchial Cl^-/[Formula: see text] exchanger. There was a major downregulation of nos1/ nos2/Nos2 expression in the gill during SW acclimation. A significant decrease in plasma nitrite supported an overall decreased Nos activity and NO production. In the middle intestine, Nos1 was upregulated during SW acclimation, whereas no changes in nos/Nos expression were observed in the posterior intestine and the kidney. Nos1 was localized along the longitudinal axis of the gill filament, beneath smooth muscle fibers of the intestine wall and in blood vessel walls of the kidney. Nos2 was localized within the epithelium adjacent to the gill filament axis and in hematopoietic tissues of the kidney. We conclude that downregulation of branchial NOS is integrated to the SW acclimation process likely to avoid the inhibitory effects of NO on active ion extrusion.

Insights into molecular and cellular mechanisms of hormonal actions on fish ion regulation derived from the zebrafish model

Fish have sophisticated mechanisms of ionic and acid-base regulation for maintaining body fluid homeostasis. Many hormones have been proposed to control the ionic and acid-base regulation mechanisms in fishes; however, lots of the proposed actions lack convincing cellular/molecular evidence. With the advantages of available genetic databases and molecular manipulation techniques, zebrafish has become an emerging model for research into ion transport physiology and functional regulation. Different types of ionocytes were found to transport ions through various sets of ion transporters, and the molecular mechanisms of ionocyte proliferation and differentiation have also been dissected, providing a competent platform with which to precisely study the ion transport pathways and ionocytes targeted by hormones, including isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyroid hormone 1, catecholamines, the renin-angiotensin-system, estrogen-related receptor α, and calcitonin gene-related peptide, which have been demonstrated to positively or negatively regulate ion transport through specific receptors at different molecular levels (transcriptional, translational, or posttranslational) or at different developmental stages of ionocytes (proliferation or differentiation). The knowledge obtained in zebrafish not only enhances our understanding of the hormonal control of fish ion regulation, but also informs studies on other animal species, thereby providing insights into related fields.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Ion transport in the zebrafish kidney from a human disease angle: possibilities, considerations, and future perspectives

Unique experimental advantages, such as its embryonic/larval transparency, high-throughput nature, and ease of genetic modification, underpin the rapid emergence of the zebrafish ( Danio rerio) as a preeminent model in biomedical research. Particularly in the field of nephrology, the zebrafish provides a promising model for studying the physiological implications of human solute transport processes along consecutive nephron segments. However, although the zebrafish might be considered a valuable model for numerous renal ion transport diseases and functional studies of many channels and transporters, not all human renal electrolyte transport mechanisms and human diseases can be modeled in the zebrafish. With this review, we explore the ontogeny of zebrafish renal ion transport, its nephron structure and function, and thereby demonstrate the clinical translational value of this model. By critical assessment of genomic and amino acid conservation of human proteins involved in renal ion handling (channels, transporters, and claudins), kidney and nephron segment conservation, and renal electrolyte transport physiology in the zebrafish, we provide researchers and nephrologists with an indication of the possibilities and considerations of the zebrafish as a model for human renal ion transport. Combined with advanced techniques envisioned for the future, implementation of the zebrafish might expand beyond unraveling pathophysiological mechanisms that underlie distinct genetic or environmentally, i.e., pharmacological and lifestyle, induced renal transport deficits. Specifically, the ease of drug administration and the exploitation of improved genetic approaches might argue for the adoption of the zebrafish as a model for preclinical personalized medicine for distinct renal diseases and renal electrolyte transport proteins.

The Control of Calcium Metabolism in Zebrafish (Danio rerio)

Zebrafish is an emerging model for the research of body fluid ionic homeostasis. In this review, we focus on current progress on the regulation of Ca²⁺ uptake in the context of Ca²⁺ sensing and hormonal regulation in zebrafish. Na⁺-K⁺-ATPase-rich cells (NaRCs), the specialized ionocytes in the embryonic skin and adult gills, play a dominant role in Ca²⁺ uptake in zebrafish. Transepithelial Ca²⁺ transport in NaRC, through apical epithelial Ca²⁺ channels (ECaC), basolateral plasma membrane Ca²⁺-ATPase (PMCA), and Na⁺/Ca²⁺ exchanger (NCX), is analogous to mammalian renal and intestinal Ca²⁺-absorption cells. Several hormones were demonstrated to differentially regulate Ca²⁺ uptake through modulating the expression of Ca²⁺ transporters and/or the proliferation/differentiation of NaRC in zebrafish. In addition, the counterbalance among these hormones is associated with the maintenance of body fluid Ca²⁺ homeostasis. Calcium-sensing receptor (CaSR) is expressed in several hormone-secreting tissues in zebrafish, and activated CaSR differentially controls calciotropic hormones. The major principles of Ca²⁺ transport and the hormonal control appear to be conserved from zebrafish to other vertebrates including mammals. The new knowledge gained from zebrafish studies provides new insights into the related issues in vertebrates.

Alternate Immersion in an External Glucose Solution Differentially Affects Blood Sugar Values in Older Versus Younger Zebrafish Adults

Recently, zebrafish have been used to examine hyperglycemia-induced complications (retinopathy and neuropathy), as would occur in individuals with diabetes. Current models to induce hyperglycemia in zebrafish include glucose immersion and streptozotocin injections. Both are effective, although neither is reported to elevate blood sugar values for more than 1 month. In this article, we report differences in hyperglycemia induction and maintenance in young (4-11 months) versus old (1-3 years) zebrafish adults. In particular, older fish immersed in an alternating constant external glucose solution (2%) for 2 months displayed elevated blood glucose levels for the entire experimental duration. In contrast, younger adults displayed only transient hyperglycemia, suggesting the fish were acclimating to the glucose exposure protocol. However, modifying the immersion protocol to include a stepwise increasing glucose concentration (from 1% → 2%→3%) resulted in maintained hyperglycemia in younger zebrafish adults for up to 2 months. Glucose-exposed younger fish collected after 8 weeks of exposure also displayed a significant decrease in wet weight. Taken together, these data suggest different susceptibilities to hyperglycemia in older and younger fish and that stepwise increasing glucose concentrations of 1% are required for maintenance of hyperglycemia in younger adults, with higher concentrations of glucose resulting in greater increases in blood sugar values.

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.

The physiology of fish at low pH: the zebrafish as a model system

Ionic regulation and acid–base balance are fundamental to the physiology of vertebrates including fish. Acidification of freshwater ecosystems is recognized as a global environmental problem, and the physiological responses to acid exposure in a few fish species are well characterized. However, the underlying mechanisms promoting ionic and acid–base balance for most fish species that have been investigated remain unclear. Zebrafish (Danio rerio) has emerged as a powerful model system to elucidate the molecular basis of ionic and acid–base regulation. The utility of zebrafish is related to the ease with which it can be genetically manipulated, its suitability for state-of-the-art molecular and cellular approaches, and its tolerance to diverse environmental conditions. Recent studies have identified several key regulatory mechanisms enabling acclimation of zebrafish to acidic environments, including activation of the sodium/hydrogen exchanger (NHE) and H+-ATPase for acid secretion and Na+ uptake, cortisol-mediated regulation of transcellular and paracellular Na+ movements, and ionocyte proliferation controlled by specific cell-fate transcription factors. These integrated physiological responses ultimately contribute to ionic and acid–base homeostasis in zebrafish exposed to acidic water. In the present review, we provide an overview of the general effects of acid exposure on freshwater fish, the adaptive mechanisms promoting extreme acid tolerance in fishes native to acidic environments, and the mechanisms regulating ionic and acid–base balance during acid exposure in zebrafish.

Calcium-sensing receptor mediates Ca(2+) homeostasis by modulating expression of PTH and stanniocalcin

Regulation of the synthesis and/or secretion of hypocalcemic and hypercalcemic hormones by the calcium-sensing receptor (CaSR) is believed to be a major pathway for maintaining Ca(2+) homeostasis in vertebrates, based primarily on findings in mammals. However, understanding the evolution of this physiological process requires that it be described in nonmammalian species. Here, we describe the use of zebrafish as a model to investigate whether CaSR contributes to body fluid Ca(2+) homeostasis by regulating synthesis of hypercalcemic (PTH1 and PTH2) and hypocalcemic (stanniocalcin [STC]) hormones. We report that PTH1, but not PTH2, increases Ca(2+) uptake through stimulation of the expression of the gene encoding the epithelial Ca(2+) channel (ecac). Furthermore, we demonstrate that CaSR, as a Ca(2+) sensor, may affect stc-1 and pth1 expressions differently, thereby suppressing ecac expression and Ca(2+) uptake. Finally, we show that CaSR knockdown has time-dependent effects on STC-1 and PTH1 expression, and these 2 hormones have mutual effects on the expression, thus forming a possible counterbalance. These findings enhance our understanding of CaSR-PTH-STC control of Ca(2+) homeostasis in vertebrates.

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.

Extracellular Ca2+ and Mg2+ modulate aminoglycoside blockade of mechanotransducer channel-mediated Ca2+ entry in zebrafish hair cells: an in vivo study with the SIET

Zebrafish lateral-line hair cells are an in vivo model for studying hair cell development, function, and ototoxicity. However, the molecular identification and properties of the mechanotransducer (MET) channel in hair cells are still controversial. In this study, a noninvasive electrophysiological method, the scanning ion-electrode technique (SIET), was applied for the first time to investigate properties of MET channels in intact zebrafish embryos. With the use of a Ca2+-selective microelectrode to deflect hair bundles and simultaneously record the Ca2+ flux, the inward Ca2+ flux was detected at stereocilia of hair cells in 2- to ∼4-day postfertilization embryos. Ca2+ influx was blocked by MET channel blockers (BAPTA, La3+, Gd3+, and curare). In addition, 10 μM aminoglycoside antibiotics (neomycin and gentamicin) were found to effectively block Ca2+ influx within 10 min. Elevating the external Ca2+ level (0.2–2 mM) neutralized the effects of neomycin and gentamicin. However, elevating the Mg2+ level up to 5 mM neutralized blockade by gentamicin but not by neomycin. This study demonstrated MET channel-mediated Ca2+ entry at hair cells and showed that the SIET to be a sensitive approach for functionally assaying MET channels in zebrafish.

Glucocorticoid receptor, but not mineralocorticoid receptor, mediates cortisol regulation of epidermal ionocyte development and ion transport in zebrafish (danio rerio)

Cortisol is the major endogenous glucocorticoid (GC) both in human and fish, mediated by corticosteroid receptors. Due to the absence of aldosterone production in teleost fish, cortisol is also traditionally accepted to function as mineralocorticoid (MC); but whether it acts through the glucocorticoid receptor (GR) or the mineralocorticoid receptor (MR) remains a subject of debate. Here, we used loss-of-function and rescue assays to determine whether cortisol affects zebrafish epidermal ionocyte development and function via the GR and/or the MR. GR knockdown morphants displayed a significant decrease in the major ionocytes, namely Na(+)-K(+)-ATPase-rich cells (NaRCs) and H(+)-ATPase-rich cells (HRCs), as well as other cells, including epidermal stem cells (ESCs), keratinocytes, and mucus cells; conversely, cell numbers were unaffected in MR knockdown morphants. In agreement, GR morphants, but not MR morphants, exhibited decreased NaRC-mediated Ca(2+) uptake and HRC-mediated H(+) secretion. Rescue via GR capped mRNA injection or exogenous cortisol incubation normalized the number of epidermal ionocytes in GR morphants. We also provide evidence for GR localization in epidermal cells. At the transcript level, GR mRNA is ubiquitously expressed in gill sections and present in both NaRCs and HRCs, supporting the knockdown and functional assay results in embryo. Altogether, we have provided solid molecular evidence that GR is indeed present on ionocytes, where it mediates the effects of cortisol on ionocyte development and function. Hence, cortisol-GR axis performs the roles of both GC and MC in zebrafish skin and gills.

Development in a naturally acidified environment: Na+/H+-exchanger 3-based proton secretion leads to CO2 tolerance in cephalopod embryos

BACKGROUND

Regulation of pH homeostasis is a central feature of all animals to cope with acid-base disturbances caused by respiratory CO2. Although a large body of knowledge is available for vertebrate and mammalian pH regulatory systems, the mechanisms of pH regulation in marine invertebrates remain largely unexplored.

RESULTS

We used squid (Sepioteuthis lessoniana), which are known as powerful acid-base regulators to investigate the pH regulatory machinery with a special focus on proton secretion pathways during environmental hypercapnia. We cloned a Rhesus protein (slRhP), V-type H+-ATPase (slVHA) and the Na+/H+ exchanger 3 (slNHE3) from S. lessoniana, which are hypothesized to represent key players in proton secretion pathways among different animal taxa. Specifically designed antibodies for S. lessoniana demonstrated the sub-cellular localization of NKA, VHA (basolateral) and NHE3 (apical) in epidermal ionocytes of early life stages. Gene expression analyses demonstrated that slNHE3, slVHA and slRhP are up regulated in response to environmental hypercapnia (pH 7.31; 0.46 kPa pCO2) in body and yolk tissues compared to control conditions (pH 8.1; 0.045 kPa pCO2). This observation is supported by H+ selective electrode measurements, which detected increased proton gradients in CO2 treated embryos. This compensatory proton secretion is EIPA sensitive and thus confirms the central role of NHE based proton secretion in cephalopods.

CONCLUSION

The present work shows that in convergence to teleosts and mammalian pH regulatory systems, cephalopod early life stages have evolved a unique acid-base regulatory machinery located in epidermal ionocytes. Using cephalopod molluscs as an invertebrate model this work provides important insights regarding the unifying evolutionary principles of pH regulation in different animal taxa that enables them to cope with CO2 induced acid-base disturbances.

The role of aquaporin and tight junction proteins in the regulation of water movement in larval zebrafish (Danio rerio)

Teleost fish living in freshwater are challenged by passive water influx; however the molecular mechanisms regulating water influx in fish are not well understood. The potential involvement of aquaporins (AQP) and epithelial tight junction proteins in the regulation of transcellular and paracellular water movement was investigated in larval zebrafish (Danio rerio). We observed that the half-time for saturation of water influx (K_u) was 4.3±0.9 min, and reached equilibrium at approximately 30 min. These findings suggest a high turnover rate of water between the fish and the environment. Water influx was reduced by the putative AQP inhibitor phloretin (100 or 500 μM). Immunohistochemistry and confocal microscopy revealed that AQP1a1 protein was expressed in cells on the yolk sac epithelium. A substantial number of these AQP1a1-positive cells were identified as ionocytes, either H⁺-ATPase-rich cells or Na⁺/K⁺-ATPase-rich cells. AQP1a1 appeared to be expressed predominantly on the basolateral membranes of ionocytes, suggesting its potential involvement in regulating ionocyte volume and/or water flux into the circulation. Additionally, translational gene knockdown of AQP1a1 protein reduced water influx by approximately 30%, further indicating a role for AQP1a1 in facilitating transcellular water uptake. On the other hand, incubation with the Ca²⁺-chelator EDTA or knockdown of the epithelial tight junction protein claudin-b significantly increased water influx. These findings indicate that the epithelial tight junctions normally act to restrict paracellular water influx. Together, the results of the present study provide direct in vivo evidence that water movement can occur through transcellular routes (via AQP); the paracellular routes may become significant when the paracellular permeability is increased.

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.

Role of cardiac natriuretic peptides in seawater adaptation of medaka embryos as revealed by loss-of-function analysis

Cardiac natriuretic peptides (atrial natriuretic peptide, ANP; b-type natriuretic peptide, BNP; ventricular natriuretic peptide, VNP) and their direct ancestor C-type natriuretic peptide 3 (CNP3) exert potent osmoregulatory actions in fish. However, very little is known about their roles in embryonic osmoregulation. In this study, we performed loss-of-function analysis using euryhaline medaka (Oryzias latipes), which has lost ANP and VNP during evolution and thus possesses only BNP and CNP3. We found that the maintenance of whole-body osmolality in seawater embryos was impaired by the knockdown of BNP+OLGC7 (BNP receptor) or CNP3 alone from 1 day postfertilization, and the CNP3 knockdown was accompanied by greater water loss. The impaired osmoregulation in the knockdown embryos was not due to the suppressed expression of major transporters for NaCl excretion via ionocytes or of key enzyme genes for metabolic water production, but to the impaired blood circulation to the yolk-sac membrane caused by abnormal heart development. We detected a strong positive correlation between impaired blood circulation and increased body fluid osmolality and pharmacological blockade of blood flow increased body fluid osmolality in seawater embryos. We also found that the exaggerated water loss in CNP3 knockdown embryos is related to the failure to suppress aquaporin (AQP3, AQP4, and AQP9) gene expression. These results show that CNP3 decrease water permeability of body surfaces and that both BNP and CNP3 ensure sufficient blood flow to the yolk-sac membrane for efficient salt excretion by ionocytes and sufficient water production by yolk metabolism to promote seawater adaptation during early development in medaka.

Cortisol regulates Na+ uptake in zebrafish, Danio rerio, larvae via the glucocorticoid receptor

Unlike other freshwater fish previously examined, zebrafish are capable of increasing their rate of Na(+) uptake during chronic exposure to acidic water (pH 4). In the present study, the potential role of cortisol in the induction of Na(+) uptake during acid-exposure was investigated. When zebrafish larvae (4 days post-fertilization) were treated with waterborne cortisol, the rate of Na(+) uptake was significantly increased; this effect was blocked by co-incubating larvae with RU-486, an antagonist selective for the glucocorticoid receptor (GR). A similar induction in Na(+) uptake, which was also blocked by RU-486, was observed when larvae were treated with dexamethasone, a selective GR agonist. Conversely, treating larvae with aldosterone, a selective agonist for the mineralocorticoid receptor (MR) had no effect on Na(+) uptake. Acid-exposure increased whole body cortisol levels and translational knockdown of GR using antisense morpholinos prevented the full induction of Na(+) uptake during exposure to acidic water, further confirming the role of cortisol and GR in Na(+) uptake stimulation. Using immunohistochemistry, GR was localized to ionocytes known to be responsible for Na(+) uptake (HR-cells). Knockdown of Rhcg1, an apical membrane ammonia channel or Na(+)/H(+) exchanger 3b (NHE3b), proteins known to play an important role in facilitating Na(+) uptake in acidic water, prevented the stimulatory effects of cortisol treatment on Na(+) uptake, suggesting that cortisol regulates Na(+) uptake by stimulating an Rhcg1-NHE3b "functional metabolon".

β-Adrenergic regulation of Na+ uptake by larval zebrafish Danio rerio in acidic and ion-poor environments

The potential role of adrenergic systems in regulating Na(+) uptake in zebrafish (Danio rerio) larvae was investigated. Treatment with isoproterenol (a generic β-adrenergic receptor agonist) stimulated Na(+) uptake, whereas treatment with phenylephrine (an α(1)-adrenergic receptor agonist) as well as clonidine (an α(2)-adrenergic receptor agonist) significantly reduced Na(+) uptake, suggesting opposing roles of α- and β-adrenergic receptors in Na(+) uptake regulation. The increase in Na(+) uptake associated with exposure to acidic water (pH = 4.0) was attenuated in the presence of the nonselective β-receptor antagonist propranolol or the β(1)-receptor blocker atenolol; the β(2)-receptor antagonist ICI-118551 was without effect. The stimulation of Na(+) uptake associated with ion-poor water (32-fold dilution of Ottawa tapwater) was unaffected by β-receptor blockade. Translational gene knockdown of β-receptors using antisense oligonucleotide morpholinos was used as a second method to assess the role of adrenergic systems in the regulation of Na(+) uptake. Whereas β(1)- or β(2B)-receptor knockdown led to significant decreases in Na(+) uptake during exposure to acidic water, only β(2A)-receptor morphants failed to increase Na(+) uptake in response to ion-poor water. In support of the pharmacology and knockdown experiments that demonstrated an involvement of β-adrenergic systems in the control of Na(+) uptake, we showed that the H(+)-ATPase-rich (HR) cell, a subtype of ionocyte known to be a site of Na(+) uptake, is innervated and appears to express β-adrenergic receptors (propranolol binding sites) at 4 days postfertilization. These data indicate an important role of adrenergic systems in regulating Na(+) uptake in developing zebrafish.

New insights into the many functions of carbonic anhydrase in fish gills

Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible reactions of carbon dioxide and water: CO(2) + H(2)O ↔ H(+) + HCO(3)(-). It has long been recognized that CA is abundant in the fish gill, with attention focused on the role of CA in catalyzing the hydration of CO(2) to provide H(+) and HCO(3)(-) for the branchial ion transport processes that underlie systemic ionic and acid-base regulation. Recent work has explored the diversity of CA isoforms in the fish gill. By linking these isoforms to different cell types in the gill, and by exploiting the diversity of fish species available for study, this work is increasing our understanding of the many roles that CA plays in the fish gill. In particular, recent work has revealed that fish utilize more than one model of CO(2) excretion, that to understand the role of CA and the gill in ionic regulation and acid-base balance means characterizing the transporter and CA complement of individual cell types, and that CA plays roles in branchial sensory mechanisms. The goal of this brief review is to summarize these new developments, while at the same time highlighting key areas in which further research is needed.

Acid secretion by mitochondrion-rich cells of medaka (Oryzias latipes) acclimated to acidic freshwater

In the present study, medaka embryos were exposed to acidified freshwater (pH 5) to investigate the mechanism of acid secretion by mitochondrion-rich (MR) cells in embryonic skin. With double or triple in situ hybridization/immunocytochemistry, the Na+/H+ exchanger 3 (NHE3) and H+-ATPase were localized in two distinct subtypes of MR cells. NHE3 was expressed in apical membranes of a major proportion of MR cells, whereas H+-ATPase was expressed in basolateral membranes of a much smaller proportion of MR cells. Gill mRNA levels of NHE3 and H+-ATPase and the two subtypes of MR cells in yolk sac skin were increased by acid acclimation; however, the mRNA level of NHE3 was remarkably higher than that of H+-ATPase. A scanning ion-selective electrode technique was used to measure H+, Na+, and NH4+ transport by individual MR cells in larval skin. Results showed that Na+ uptake and NH4+ excretion by MR cells increased after acid acclimation. These findings suggested that the NHE3/Rh glycoprotein-mediated Na+ uptake/NH4+ excretion mechanism plays a critical role in acidic equivalent (H+/NH4+) excretion by MR cells of the freshwater medaka.

Effects of elevated seawater pCO(2) on gene expression patterns in the gills of the green crab, Carcinus maenas

Background

The green crab Carcinus maenas is known for its high acclimation potential to varying environmental abiotic conditions. A high ability for ion and acid-base regulation is mainly based on an efficient regulation apparatus located in gill epithelia. However, at present it is neither known which ion transport proteins play a key role in the acid-base compensation response nor how gill epithelia respond to elevated seawater pCO₂ as predicted for the future. In order to promote our understanding of the responses of green crab acid-base regulatory epithelia to high pCO₂, Baltic Sea green crabs were exposed to a pCO₂ of 400 Pa. Gills were screened for differentially expressed gene transcripts using a 4,462-feature microarray and quantitative real-time PCR.

Results

Crabs responded mainly through fine scale adjustment of gene expression to elevated pCO₂. However, 2% of all investigated transcripts were significantly regulated 1.3 to 2.2-fold upon one-week exposure to CO₂ stress. Most of the genes known to code for proteins involved in osmo- and acid-base regulation, as well as cellular stress response, were were not impacted by elevated pCO₂. However, after one week of exposure, significant changes were detected in a calcium-activated chloride channel, a hyperpolarization activated nucleotide-gated potassium channel, a tetraspanin, and an integrin. Furthermore, a putative syntaxin-binding protein, a protein of the transmembrane 9 superfamily, and a Cl^-/HCO₃^- exchanger of the SLC 4 family were differentially regulated. These genes were also affected in a previously published hypoosmotic acclimation response study.

Conclusions

The moderate, but specific response of C. maenas gill gene expression indicates that (1) seawater acidification does not act as a strong stressor on the cellular level in gill epithelia; (2) the response to hypercapnia is to some degree comparable to a hypoosmotic acclimation response; (3) the specialization of each of the posterior gill arches might go beyond what has been demonstrated up to date; and (4) a re-configuration of gill epithelia might occur in response to hypercapnia.

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.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na(+) uptake/acid secretion/NH(4)(+) excretion, Ca(2+) uptake, and Cl(-) uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.

Anion exchanger 1b, but not sodium-bicarbonate cotransporter 1b, plays a role in transport functions of zebrafish H+-ATPase-rich cells

Similar to mammalian proximal tubular cells, H(+)-ATPase rich (HR) cells in zebrafish skin and gills are also responsible for Na(+) uptake and acid secretion functions. However, the basolateral transport pathways in HR cells are still unclear. In the present study, we tested the hypothesis if there are specific slc4 members involved in basolateral ion transport pathways in HR cells. Fourteen isoforms were identified in the zebrafish(z) slc4 family, and the full-length cDNAs of two novel isoforms, zslc4a1b (anion exchanger, zAE1b) and zslc4a4b (Na(+)/HCO(3)(-) cotransporter, zNBCe1b), were sequenced. mRNA signals of zslc4a1b and zslc4a4b were mainly detected in certain groups of ionocytes in zebrafish skin/gills. Further double immunocytochemistry or in situ hybridization demonstrated that zAE1b, but not zNBCe1b, was localized to basolateral membranes of HR cells. Acclimation to low-Na(+) or acidic environments stimulated the mRNA expression of zslc4a1b in zebrafish gills, and loss-of-function of zslc4a1b with specific morpholinos caused significant decreases in both the whole body Na(+) content and the skin H(+) activity in the morphants. On the basis of these results, it was concluded that zAE1b, but not zNBCe1b, is involved in the basolateral transport pathways in Na(+) uptake/acid secretion mechanisms in zebrafish HR cells.

Functional analysis of the glucose transporters-1a, [corrected] -6, and -13.1 expressed by zebrafish epithelial cells

The hexose supply and subsequent metabolism are crucial for the operations of the iono- and osmoregulatory mechanisms in fish, but how hexose is transported and supplied to cells of the ionoregulatory epithelia is unknown. Three zebrafish glucose transporters (zGLUTs), zGLUT1a, -13.1, and -6, were previously found to respectively be expressed by ionocytes (Na(+)-K(+)-ATPase-rich, Na(+)-Cl(-) cotransporter-expressing, and H(+)-ATPase-rich cells) and adjacent energy-depositing cells [glycogen-rich (GR) cells] in zebrafish skin and gills (32). The present study aimed to test if the transport kinetics of these three zGLUTs differ, and if the transport functional differences are of physiological relevance to the respective functions of epithelial cells. The three zGLUTs expressed by Xenopus laevis oocytes revealed different d-glucose transport kinetics; zGLUT13.1 showed the lowest Michaelis constant (K(m)), whereas zGLUT6 had the highest K(m) and maximal velocity. In morpholino injection experiments, translational knockdown of zGLUT1a and -13.1, respectively, impaired Cl(-)/Ca(2+) and Na(+)/Ca(2+) uptake, but loss-of-function of zGLUT6 did not cause a significant effect on ion uptake functions in zebrafish. Based on these results, zGLUT1a and -13.1 appear to be superior to zGLUT6 in competing for glucose under a situation of low blood glucose due to extensive energy consumption, whereas, in a high blood glucose situation, zGLUT6 is able to absorb the excess glucose for energy deposition. The timely and sufficient supply of energy to ionocytes so that they can carry out ion regulation is definitely a more important event than storing energy in GR cells, particularly when acute environmental change disturbs the ion balance in zebrafish.