The effect of environmental hypercapnia and salinity on the expression of NHE-like isoforms in the gills of a euryhaline fish (Fundulus heteroclitus)

Are the effects of catch-and-release angling evident in changes to mRNA abundances related to metabolism, acid-base regulation and stress in lake trout (Salvelinus namaycush) gills?

Catch-and-release (C&R) angling is a conservation-oriented practice intended to reduce the impact recreational angling has on fish populations. Even though most recreationally angled fish are released, little is known about how C&R angling impacts fish at the cellular or tissue level. As the first to explore the impacts of C&R angling on mRNA abundances, our study aimed to identify how the stress of angling influenced metabolism, acid-base regulation and cellular stress in the gills of lake trout (Salvelinus namaycush). Because gills are responsible for metabolic gas exchange, are crucial sites of acid-base homeostasis and respond to stressors quickly, we hypothesized that the relative mRNA abundance of genes related to these three physiological processes would be altered after angling. We took gill samples of live lake trout at 0, 2 or 48 h after fish were angled by rod and reel, and then used quantitative PCR (qPCR) to measure the relative abundance of nine candidate mRNA transcripts. Heat shock protein 70 (hsp70) mRNA levels significantly increased over 5-fold 2 h after angling, indicating a potential activation of a cytoprotective response. However, contrary to our hypothesis, we observed no change in the relative mRNA abundance of genes related to metabolism or acid-base regulation in response to C&R angling within a 48-h period. As C&R angling can negatively impact fish populations, further use of transcript-level studies will allow us to understand the impact C&R has on specific tissues and improve our knowledge of how C&R influences overall fish health.

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.

Molecular targets of prolactin in mummichogs (Fundulus heteroclitus): Ion transporters/channels, aquaporins, and claudins

Prolactin (Prl) was identified over 60 years ago in mummichogs (Fundulus heteroclitus) as a "freshwater (FW)-adapting hormone", yet the cellular and molecular targets of Prl in this model teleost have remained unknown. Here, we conducted a phylogenetic analysis of two mummichog Prl receptors (Prlrs), designated Prlra and Prlrb, prior to describing the tissue- and salinity-dependent expression of their associated mRNAs. We then administered ovine Prl (oPrl) to mummichogs held in brackish water and characterized the expression of genes associated with FW- and seawater (SW)-type ionocytes. Within FW-type ionocytes, oPrl stimulated the expression of Na⁺/Cl^- cotransporter 2 (ncc2) and aquaporin 3 (aqp3). Alternatively, branchial Na⁺/H⁺ exchanger 2 and -3 (nhe2 and -3) expression did not respond to oPrl. Gene transcripts associated with SW-type ionocytes, including Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1), cystic fibrosis transmembrane regulator 1 (cftr1), and claudin 10f (cldn10f) were reduced by oPrl. Isolated gill filaments incubated with oPrl in vitro exhibited elevated ncc2 and prlra expression. Given the role of Aqps in supporting gastrointestinal fluid absorption, we assessed whether several intestinal aqp transcripts were responsive to oPrl and found that aqp1a and -8 levels were reduced by oPrl. Our collective data indicate that Prl promotes FW-acclimation in mummichogs by orchestrating the expression of solute transporters/channels, water channels, and tight-junction proteins across multiple osmoregulatory organs.

Genome-wide identification of the NHE gene family in Coilia nasus and its response to salinity challenge and ammonia stress

Background

In aquatic environments, pH, salinity, and ammonia concentration are extremely important for aquatic animals. NHE is a two-way ion exchange carrier protein, which can transport Na⁺ into cells and exchange out H⁺, and also plays key roles in regulating intracellular pH, osmotic pressure, and ammonia concentration.

Results

In the present study, ten NHEs, the entire NHE gene family, were identified from Coilia nasus genome and systemically analyzed via phylogenetic, structural, and synteny analysis. Different expression patterns of C. nasus NHEs in multiple tissues indicated that expression profiles of NHE genes displayed tissue-specific. Expression patterns of C. nasus NHEs were related to ammonia excretion during multiple embryonic development stages. To explore the potential functions on salinity challenge and ammonia stress, expression levels of ten NHEs were detected in C. nasus gills under hypotonic stress, hypertonic stress, and ammonia stress. Expression levels of all NHEs were upregulated during hypotonic stress, while they were downregulated during hypertonic stress. NHE2 and NHE3 displayed higher expression levels in C. nasus larvae and juvenile gills under ammonia stress.

Conclusions

Our study revealed that NHE genes played distinct roles in embryonic development, salinity stress, and ammonia exposure. Syntenic analysis showed significant difference between stenohaline fish and euryhaline fishes. Our findings will provide insight into effects of C. nasus NHE gene family on ion transport and ammonia tolerance and be beneficial for healthy aquaculture of C. nasus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08761-9.

Role of the Basolateral Na+/H+ Exchanger-2 (NHE2) in Ionocytes of Seawater- Acclimated Medaka (Oryzias latipes)

Ionocytes in the skin and gills of seawater (SW) fishes are responsible for acid-base regulation and salt secretion. Na⁺/H⁺ exchangers (NHEs) are considered the major acid (H⁺)-secreting transporters in ionocytes of SW fishes. However, the subcellular localization and function of a specific NHE isoform (NHE2) have never clearly been revealed. In this study, we cloned and sequenced NHE2 from an SW-acclimated medaka (Oryzias latipes) and examined its functions in medaka embryos. A phylogenetic analysis showed that the evolutionary relationships of mammalian NHE2 and NHE4 are close to those of fish NHE2. A gene structure analysis showed that tetrapod NHE4 might be a tandem duplication of fish NHE2. Immunohistochemistry with a medaka-specific antibody localized NHE2 to the basolateral membrane of ionocytes. Lost-of-function experiments with photo-activated morpholino oligonucleotides showed that both H⁺ and Cl^– secretion by ionocytes were suppressed in NHE2-knockdown embryos, suggesting that the basolateral NHE2 facilitates acid and salt secretion by ionocytes of medaka in seawater.

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.

Rainbow Trout (Oncorhynchus mykiss) Na+/H+ Exchangers tNhe3a and tNhe3b Display Unique Inhibitory Profiles Dissimilar from Mammalian NHE Isoforms

Freshwater fishes maintain an internal osmolality of ~300 mOsm, while living in dilute environments ranging from 0 to 50 mOsm. This osmotic challenge is met at least partially, by Na⁺/H⁺ exchangers (NHE) of fish gill and kidney. In this study, we cloned, expressed, and pharmacologically characterized fish-specific Nhes of the commercially important species Oncorhynchus mykiss. Trout (t) Nhe3a and Nhe3b isoforms from gill and kidney were expressed and characterized in an NHE-deficient cell line. Western blotting and immunocytochemistry confirmed stable expression of the tagged trout tNhe proteins. To measure NHE activity, a transient acid load was induced in trout tNhe expressing cells and intracellular pH was measured. Both isoforms demonstrated significant activity and recovered from an acute acid load. The effect of the NHE transport inhibitors amiloride, EIPA (5-(N-ethyl-N-isopropyl)-amiloride), phenamil, and DAPI was examined. tNhe3a was inhibited in a dose-dependent manner by amiloride and EIPA and tNhe3a was more sensitive to amiloride than EIPA, unlike mammalian NHE1. tNhe3b was inhibited by high concentrations of amiloride, while even in the presence of high concentrations of EIPA (500 µM), some activity of tNhe3b remained. Phenamil and DAPI were ineffective at inhibiting tNhe activity of either isoform. The current study aids in understanding the pharmacology of fish ion transporters. Both isoforms display inhibitory profiles uniquely different from mammalian NHEs and show resistance to inhibition. Our study allows for more direct interpretation of past, present, and future fish-specific sodium transport studies, with less reliance on mammalian NHE data for interpretation.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Na+/H+ exchanger (NHE) in Pacific white shrimp (Litopenaeus vannamei): Molecular cloning, transcriptional response to acidity stress, and physiological roles in pH homeostasis

Na⁺/H⁺ exchangers are the most common membrane proteins involved in the regulation of intracellular pH that concurrently transport Na⁺ into the cells and H⁺ out of the cells. In this study, the full-length cDNA of the Na⁺/H⁺ exchanger (NHE) from the Pacific white shrimp (Litopenaeus vannamei) was cloned. The LvNHE cDNA is 3167 bp long, contains a 5’-untranslated region (UTR) of 74 bp and a 3’-UTR of 456 bp and an open reading frame (ORF) of 2637 bp, coding for a protein of 878 amino acids with 11 putative transmembrane domains and a long cytoplasmic tail. LvNHE shows high sequence homology with mud crab NHE at the amino acid level. LvNHE mRNA was detected in the hepatopancreas, gill, eyestalk, skin, heart, intestine, muscle, brain and stomach, with the highest abundance in the intestine. In the shrimp intestinal fragment cultures exposed to gradually declining pH medium (from pH 8.0 to pH 6.4), the LvNHE mRNA expression was significantly stimulated, with the highest response when incubated in pH 7.0 medium for 6 h. To investigate the functional roles of LvNHE in pH regulation at the physiological and cellular levels, the LvNHE mRNA expression was silenced by siRNA knockdown. Upon low-pH challenge, the hemolymph pH was significantly reduced in the LvNHE mRNA knockdown shrimp. In addition, knockdown of LvNHE mRNA reduced the recovery capacity of intracellular pH in intestinal fragment cultures after acidification. Altogether, this study demonstrates the role of NHE in shrimp response to low pH stress and provides new insights into the acid/base homeostasis mechanisms of crustaceans.

Molecular characterization, cellular localization, and light-enhanced expression of Beta-Na+/H+ Exchanger-like in the whitish inner mantle of the giant clam, Tridacna squamosa, denote its role in light-enhanced shell formation

The fluted giant clam, Tridacna squamosa, lives in symbiosis with photosynthetic zooxanthellae, and can engage in light-enhanced growth and shell formation. Light-enhanced shell formation necessitates the elimination of excess H⁺ from the extrapallial fluid adjacent to the shell. This study aimed to clone Na⁺/H⁺Exchanger (NHE) from the whitish inner mantle adjacent to the extrapallial fluid of T. squamosa, to determine its cellular and subcellular localization, and to evaluate the effect of light exposure on its mRNA expression level and protein abundance therein. The complete coding cDNA sequence of NHE obtained was identified as a homolog of beta NHE (βNHE-like). It consisted of 2925 bp, encoding for a polypeptide of 974 amino acids and 107.1 kDa, and was expressed predominantly in the inner mantle. There, βNHE-like was localized in the apical membrane of the seawater-facing epithelium by immunofluorescence microscopy. After exposure to light for 12 h, the seawater-facing epithelium of the inner mantle displayed consistently stronger immunostaining than that of the control exposed to 12 h of darkness. Western blotting confirmed that light exposure significantly enhanced the protein abundance of βNHE-like in the inner mantle. These results denote that some of the excess H⁺ generated during light-enhanced shell formation can be excreted through the light-dependent βNHE-like of the seawater-facing epithelium to minimize the impact on the whole-body pH. Importantly, the excreted H⁺ could dehydrate exogenous HCO₃^-, and facilitate the absorption of inorganic carbon through the seawater-facing epithelium dedicated for light-enhanced shell formation due to its close proximity with the shell-facing epithelium. NUCLEOTIDE SYMBOL COMBINATIONS: Pairs: R = A/G; W = A/T; Y = C/T. Triples: D = A/G/T.

Dynamics of Gene Expression Responses for Ion Transport Proteins and Aquaporins in the Gill of a Euryhaline Pupfish during Freshwater and High-Salinity Acclimation

Pupfishes (genus Cyprinodon) evolved some of the broadest salinity tolerances of teleost fishes, with some taxa surviving in conditions from freshwater to nearly 160 ppt. In this study, we examined transcriptional dynamics of ion transporters and aquaporins in the gill of the desert Amargosa pupfish (Cyprinodon nevadensis amargosae) during rapid salinity change. Pupfish acclimated to 7.5 ppt were exposed to freshwater (0.3 ppt), seawater (35 ppt), or hypersaline (55 ppt) conditions over 4 h and sampled at these salinities over 14 d. Plasma osmolality and Cl^- concentration became elevated 8 h after the start of exposure to 35 or 55 ppt but returned to baseline levels after 14 d. Osmolality recovery was paralleled by increased gill Na⁺/K⁺-ATPase activity and higher relative levels of messenger RNAs (mRNAs) encoding cystic fibrosis transmembrane conductance regulator (cftr) and Na⁺/K⁺/2Cl^- cotransporter-1 (nkcc1). Transcripts encoding one Na⁺-HCO₃^- cotransporter-1 isoform (nbce1.1) also increased in the gills at higher salinities, while a second isoform (nbce1.2) increased expression in freshwater. Pupfish in freshwater also had lower osmolality and elevated gill mRNAs for Na⁺/H⁺ exchanger isoform-2a (nhe2a) and V-type H⁺-ATPase within 8 h, followed by increases in Na⁺/H⁺ exchanger-3 (nhe3), carbonic anhydrase 2 (ca2), and aquaporin-3 (aqp3) within 1 d. Gill mRNAs for Na⁺/Cl^- cotransporter-2 (ncc2) also were elevated 14 d after exposure to 0.3 ppt. These results offer insights into how coordinated transcriptional responses for ion transporters in the gill facilitate reestablishment of osmotic homeostasis after changes in environmental salinity and provide evidence that the teleost gill expresses two Na⁺-HCO₃^- cotransporter-1 isoforms with different roles in freshwater and seawater acclimation.

Pharmacological evidence that DAPI inhibits NHE2 in Fundulus heteroclitus acclimated to freshwater

Ionoregulation in the euryhaline killifish Fundulus heteroclitus has been intensively studied over the last two decades using a variety of techniques. However, there has been limited use of pharmacological inhibitors to identify proteins involved in ion transport for this species. In this study, we used a range of pharmacological inhibitors (EIPA, DAPI, ethoxzolamide, bumetanide, bafilomycin, phenamil, hydrochlorothiazide) to investigate the proteins involved in Na⁺ transport in freshwater (1 mM Na⁺) acclimated F. heteroclitus. Our results indicate that Na⁺ uptake under these conditions is sensitive to both EIPA (NHE-specific inhibitor) and DAPI (putative ASIC-specific inhibitor), but not to any of the other inhibitors. Results for EIPA are consistent with previous studies indicating F. heteroclitus relies solely on NHE2 for Na⁺ transport across the apical membrane of ionocytes. In contrast, results for DAPI are surprising given previous studies that have indicated the H⁺-ATPase is basolaterally located in F. heteroclitus and so cannot contribute to Na⁺ uptake via ASIC. The lack of bafilomycin sensitivity in the current study is consistent with a basolaterally located H⁺-ATPase. This suggests that DAPI is not an ASIC-specific inhibitor as has been previously assumed, and that it may also inhibit NHE2. Finally, we did not observe Na⁺ uptake to be sensitive to ethoxzolamide, suggesting that carbonic anhydrase may not be involved in generating the H⁺ needed to maintain NHE activity in freshwater as has been previously proposed.

Physiological mechanism of osmoregulatory adaptation in anguillid eels

In recent years, the production of eel larvae has dramatic declines due to reductions in spawning stocks, overfishing, growth habitat destruction and access reductions, and pollution. Therefore, it is particularly important and urgent for artificial production of glass eels. However, the technique of artificial hatching and rearing larvae is still immature, which has long been regarded as an extremely difficult task. One of the huge gaps is artificial condition which is far from the natural condition to develop their capability of osmoregulation. Thus, understanding their osmoregulatory mechanisms will help to improve the breed and adapt to the changes in the environment. In this paper, we give a general review for a study progress of osmoregulatory mechanisms in eels from five aspects including tissues and organs, ion transporters, hormones, proteins, and high throughput sequencing methods.

Light-dependent expression of a Na+/H+ exchanger 3-like transporter in the ctenidium of the giant clam, Tridacna squamosa, can be related to increased H+ excretion during light-enhanced calcification

Na⁺/H⁺ exchangers (NHEs) regulate intracellular pH and ionic balance by mediating H⁺ efflux in exchange for Na⁺ uptake in a 1:1 stoichiometry. This study aimed to obtain from the ctenidium of the giant clam Tridacna squamosa (TS) the complete cDNA sequence of a NHE3‐like transporter (TSNHE3), and to determine the effect of light exposure on its mRNA expression level and protein abundance therein. The coding sequence of TSNHE3 comprised 2886 bp, encoding 961 amino acids with an estimated molecular mass of 105.7 kDa. Immunofluorescence microscopy revealed that TSNHE3 was localized to the apical membrane of epithelial cells of the ctenidial filaments and the tertiary water channels. Particularly, the apical immunofluorescence of the ctenidial filaments was consistently stronger in the ctenidium of clams exposed to 12 h of light than those of the control kept in darkness. Indeed, light induced significant increases in the transcript level and protein abundance of TSNHE3/TSNHE3 in the ctenidium, indicating that the transcription and translation of TSNHE3/TSNHE3 were light‐dependent. As light‐enhanced calcification generates H⁺, the increased expression of TSNHE3/TSNHE3 in the ctenidium could be a response to augment H⁺ excretion in pursuance of whole‐body acid‐base balance during light exposure. These results signify that shell formation in giant clams requires the collaboration between the ctenidium, which is a respiratory and iono‐regulatory organ, and the inner mantle, which is directly involved in the calcification process, and provide new insights into the mechanisms of light‐enhanced calcification in giant clams.

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.

Physiological implications of ocean acidification for marine fish: emerging patterns and new insights

Ocean acidification (OA) is an impending environmental stress facing all marine life, and as such has been a topic of intense research interest in recent years. Numerous detrimental effects have been documented in marine fish, ranging from reduced mortality to neurosensory impairment, and the prevailing opinions state that these effects are largely the downstream consequences of altered blood carbon dioxide chemistry caused by respiratory acid-base disturbances. While the respiratory acid-base disturbances are consistent responses to OA across tested fish species, it is becoming increasingly clear that there is wide variability in the degree of downstream impairments between species. This can also be extended to intraspecies variability, whereby some individuals have tolerant physiological traits, while others succumb to the effects of OA. This review will synthesize relevant literature on marine fish to highlight consistent trends of impairment, as well as observed interspecies variability in the responses to OA, and the potential routes of physiological acclimation. In all cases, whole animal responses are linked to demonstrated or proposed physiological impairments. Major topics of focus include: (1) respiratory acid-base disturbances; (2) early life survival and growth; (3) the implications for metabolic performance, activity, and reproduction; and (4) emerging physiological theories pertaining to neurosensory impairment and the role of GABA_A receptors. Particular emphasis is placed on the importance of understanding the underlying physiological traits that confer inter- and intraspecies tolerance, as the abundance of these traits will decide the long-term outlook of marine fish.

Carbon dioxide induced plasticity of branchial acid-base pathways in an estuarine teleost

Anthropogenic CO₂ is expected to drive ocean pCO₂ above 1,000 μatm by 2100 – inducing respiratory acidosis in fish that must be corrected through branchial ion transport. This study examined the time course and plasticity of branchial metabolic compensation in response to varying levels of CO₂ in an estuarine fish, the red drum, which regularly encounters elevated CO₂ and may therefore have intrinsic resilience. Under control conditions fish exhibited net base excretion; however, CO₂ exposure resulted in a dose dependent increase in acid excretion during the initial 2 h. This returned to baseline levels during the second 2 h interval for exposures up to 5,000 μatm, but remained elevated for exposures above 15,000 μatm. Plasticity was assessed via gene expression in three CO₂ treatments: environmentally realistic 1,000 and 6,000 μatm exposures, and a proof-of-principle 30,000 μatm exposure. Few differences were observed at 1,000 or 6,000 μatm; however, 30,000 μatm stimulated widespread up-regulation. Translocation of V-type ATPase after 1 h of exposure to 30,000 μatm was also assessed; however, no evidence of translocation was found. These results indicate that red drum can quickly compensate to environmentally relevant acid-base disturbances using baseline cellular machinery, yet are capable of plasticity in response to extreme acid-base challenges.

Functional classification of gill ionocytes and spatiotemporal changes in their distribution after transfer from seawater to fresh water in Japanese seabass

Spatiotemporal changes in branchial ionocyte distribution were investigated following transfer from seawater (SW) to fresh water (FW) in Japanese seabass. The mRNA expression levels of cystic fibrosis transmembrane conductance regulator (CFTR) and Na+/K+/2Cl− cotransporter 1a (NKCC1a) in the gills rapidly decreased after transfer to FW, whereas Na+/H+ exchanger 3 (NHE3) and Na+/Cl− cotransporter 2 (NCC2) expressions were upregulated following the transfer. By quadruple-color whole-mount immunofluorescence staining with anti-Na+/K+-ATPase, anti-NHE3, anti-CFTR and T4 (anti-NKCC1a/NCC2) antibodies, we classified ionocytes into one SW-type and two FW-types; NHE3 cell and NCC2 cell. Time-course observation after transfer revealed an intermediate type between SW-type and FW-type NHE3 ionocytes, suggesting functional plasticity of ionocytes. Finally, on the basis of the ionocyte classification of Japanese seabass, we observed the location of ionocyte subtypes on frozen sections of the gill filaments stained by triple-color immunofluorescence staining. Our observation indicated that SW-type ionocytes transformed into FW-type NHE3 ionocytes and at the same time shifted their distribution from filaments to lamellae. On the other hand, FW-specific NCC2 ionocytes appeared mainly in the filaments. Taken together, these findings indicated that ionocytes originated from undifferentiated cells in the filaments and expanded their distribution to the lamellae during FW acclimation.

Ocean warming and acidification modulate energy budget and gill ion regulatory mechanisms in Atlantic cod (Gadus morhua)

Ocean warming and acidification are threatening marine ecosystems. In marine animals, acidification is thought to enhance ion regulatory costs and thereby baseline energy demand, while elevated temperature also increases baseline metabolic rate. Here we investigated standard metabolic rates (SMR) and plasma parameters of Atlantic cod (Gadus morhua) after 3–4 weeks of exposure to ambient and future PCO₂ levels (550, 1200 and 2200 µatm) and at two temperatures (10, 18 °C). In vivo branchial ion regulatory costs were studied in isolated, perfused gill preparations. Animals reared at 18 °C responded to increasing CO₂ by elevating SMR, in contrast to specimens at 10 °C. Isolated gills at 10 °C and elevated PCO₂ (≥1200 µatm) displayed increased soft tissue mass, in parallel to increased gill oxygen demand, indicating an increased fraction of gill in whole animal energy budget. Altered gill size was not found at 18 °C, where a shift in the use of ion regulation mechanisms occurred towards enhanced Na⁺/H⁺-exchange and HCO₃⁻ transport at high PCO₂ (2200 µatm), paralleled by higher Na⁺/K⁺-ATPase activities. This shift did not affect total gill energy consumption leaving whole animal energy budget unaffected. Higher Na⁺/K⁺-ATPase activities in the warmth might have compensated for enhanced branchial permeability and led to reduced plasma Na⁺ and/or Cl⁻ concentrations and slightly lowered osmolalities seen at 18 °C and 550 or 2200 µatm PCO₂ in vivo. Overall, the gill as a key ion regulation organ seems to be highly effective in supporting the resilience of cod to effects of ocean warming and acidification.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3− that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.

Osmoregulation and branchial plasticity after acute freshwater transfer in red drum, Sciaenops ocellatus

Red drum, Sciaenops ocellatus, is an estuarine-dependent fish species commonly found in the Gulf of Mexico and along the coast of the southeastern United States. This economically important species has demonstrated freshwater tolerance; however, the physiological mechanisms and costs related to freshwater exposure remain poorly understood. The current study therefore investigated the physiological response of red drum using an acute freshwater transfer protocol. Plasma osmolality, Cl⁻, Mg²⁺ and Ca²⁺ were all significantly reduced by 24h post-transfer; Cl⁻ and Mg²⁺ recovered to control levels by 7days post-transfer. No effect of transfer was observed on muscle water content; however, muscle Cl⁻ was significantly reduced. Interestingly, plasma and muscle Na⁺ content was unaffected by freshwater transfer. Intestinal fluid was absent by 24h post-transfer indicating cessation of drinking. Branchial gene expression analysis showed that both CFTR and NKCC1 exhibited significant down-regulation at 8 and 24h post-transfer, respectively, although transfer had no impact on NHE2, NHE3 or Na⁺, K⁺ ATPase (NKA) activity. These general findings are supported by immunohistochemical analysis, which revealed no apparent NKCC containing cells in the gills at 7days post transfer while NKA cells localization was unaffected. The results of the current study suggest that red drum can effectively regulate Na⁺ balance upon freshwater exposure using already present Na⁺ uptake pathways while also down-regulating ion excretion mechanisms.

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.

Measuring titratable alkalinity by single versus double endpoint titration: An evaluation in two cyprinodont species and implications for characterizing net H+ flux in aquatic organisms

In this study, Na(+) uptake and acid-base balance in the euryhaline pupfish Cyprinodon variegatus variegatus were characterized when fish were exposed to pH 4.5 freshwater (7mM Na(+)). Similar to the related cyprinodont, Fundulus heteroclitus, Na(+) uptake was significantly inhibited when exposed to low pH water. However, it initially appeared that C. v. variegatus increased apparent net acid excretion at low pH relative to circumneutral pH. This result is opposite to previous observations for F. heteroclitus under similar conditions where fish were observed to switch from apparent net H(+) excretion at circumneutral pH to apparent net H(+) uptake at low pH. Further investigation revealed disparate observations between these studies were the result of using double endpoint titrations to measure titratable alkalinity fluxes in the current study, while the earlier study utilized single endpoint titrations to measure these fluxes (i.e.,. Cyprinodon acid-base transport is qualitatively similar to Fundulus when characterized using single endpoint titrations). This led to a comparative investigation of these two methods. We hypothesized that either the single endpoint methodology was being influenced by a change in the buffer capacity of the water (e.g., mucus being released by the fish) at low pH, or the double endpoint methodology was not properly accounting for ammonia flux by the fish. A series of follow-up experiments indicated that buffer capacity of the water did not change significantly, that excretion of protein (a surrogate for mucus) was actually reduced at low pH, and that the double endpoint methodology does not properly account for NH(3) excretion by fish under low pH conditions. As a result, it overestimates net H(+) excretion during low pH exposure. After applying the maximum possible correction for this error (i.e., assuming that all ammonia is excreted as NH(3)), the double endpoint methodology indicates that net H(+) transport was reduced to effectively zero in both species at pH 4.5. However, significant differences between the double endpoint (no net H(+) transport at low pH) and single endpoint titrations (net H(+) uptake at low pH) remain to be explained.

Branchial ionocyte organization and ion-transport protein expression in juvenile alewives acclimated to freshwater or seawater

The alewife (Alosa pseudoharengus) is a clupeid that undergoes larval and juvenile development in freshwater preceding marine habitation. The purpose of this study was to investigate osmoregulatory mechanisms in alewives that permit homeostasis in different salinities. To this end, we measured physiological, branchial biochemical and cellular responses in juvenile alewives acclimated to freshwater (0.5 p.p.t.) or seawater (35.0 p.p.t.). Plasma chloride concentration was higher in seawater-acclimated than freshwater-acclimated individuals (141 mmol l(-1) vs 134 mmol l(-1)), but the hematocrit remained unchanged. In seawater-acclimated individuals, branchial Na(+)/K(+)-ATPase (NKA) activity was higher by 75%. Western blot analysis indicated that the abundance of the NKA α-subunit and a Na(+)/K(+)/2Cl(-) cotransporter (NKCC1) were greater in seawater-acclimated individuals by 40% and 200%, respectively. NKA and NKCC1 were localized on the basolateral surface and tubular network of ionocytes in both acclimation groups. Immunohistochemical labeling for the cystic fibrosis transmembrane conductance regulator (CFTR) was restricted to the apical crypt of ionocytes in seawater-acclimated individuals, whereas sodium/hydrogen exchanger 3 (NHE3) labeling was present on the apical surface of ionocytes in both acclimation groups. Ionocytes were concentrated on the trailing edge of the gill filament, evenly distributed along the proximal 75% of the filamental axis and reduced distally. Ionocyte size and number on the gill filament were not affected by salinity; however, the number of lamellar ionocytes was significantly lower in seawater-acclimated fish. Confocal z-series reconstructions revealed that mature ionocytes in seawater-acclimated alewives occurred in multicellular complexes. These complexes might reduce paracellular Na(+) resistance, hence facilitating Na(+) extrusion in hypo-osmoregulating juvenile alewives after seaward migration.

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

Mechanisms of ion uptake by freshwater (FW) fish have received considerable attention over the past 80 years. Through an assortment of techniques incorporating whole animal physiology, electrophysiology and molecular biological approaches, three models have been proposed to account for Na(+) uptake. (1) Direct exchange of Na(+) and H(+) via one or more types of Na(+)/H(+) exchanger (slc9), (2) uptake of Na(+) through epithelial Na(+) channels energized by an electrical gradient created by H(+)-ATPase and (3) Na(+)/Cl(-) co-transport (slc12). While each mechanism is supported at least in part by theoretical or experimental data, there are several outstanding questions that have not yet been fully resolved. Furthermore, there are few details concerning how these Na(+) uptake mechanisms are fine tuned in response to the fluctuating FW environments. In this review, we summarize the current understanding of these three Na(+) uptake mechanisms and discuss their regulation by endocrine (cortisol and prolactin) and neurohumoral (catecholamines) factors.

Diet influences salinity preference of an estuarine fish, the killifish Fundulus heteroclitus

Understanding the interplay among the external environment, physiology and adaptive behaviour is crucial for understanding how animals survive in their natural environments. The external environment can have wide ranging effects on the physiology of animals, while behaviour determines which environments are encountered. Here, we identified changes in the behavioural selection of external salinity in Fundulus heteroclitus, an estuarine teleost, as a consequence of digesting a meal. Fish that consumed high levels of dietary calcium exhibited a higher preferred salinity compared with unfed fish, an effect that was exaggerated by elevated dietary sodium chloride. The mean swimming speed (calculated as a proxy of activity level) was not affected by consuming a diet of any type. Constraining fish to water of 22 p.p.t. salinity during the digestion of a meal did not alter the amount of calcium that was absorbed across the intestine. However, when denied the capacity to increase their surrounding salinity, the compromised ability to excrete calcium to the water resulted in significantly elevated plasma calcium levels, a potentially hazardous physiological consequence. This study is the first to show that fish behaviourally exploit their surroundings to enhance their ionoregulation during digestion, and to pinpoint the novel role of dietary calcium and sodium in shaping this behaviour. We conclude that in order to resolve physiological disturbances in ion balance created by digestion, fish actively sense and select the environment they inhabit. Ultimately, this may result in transient diet-dependent alteration of the ecological niches occupied by fishes, with broad implications for both physiology and ecology.

Structures and immunolocalization of Na+, K+ -ATPase, Na+ /H+ exchanger 3 and vacuolar-type H+ -ATPase in the gills of blennies (Teleostei: Blenniidae) inhabiting rocky intertidal areas

The structure and immunolocalization of the ion transporters Na(+) ,K(+) -ATPase (NKA), Na(+) /H(+) exchanger (NHE3) and vacuolar-type H(+) -ATPase (VHA) were examined in the gills of teleosts of the family Blenniidae, which inhabit rocky shores with vertical zonation in subtropical seas. These features were compared among the following species with different ecologies: the amphibious rockskipper blenny Andamia tetradactylus, the intertidal white-finned blenny Praealticus tanegasimae and the purely marine yaeyama blenny Ecsenius yaeyamaensis. Light and electron microscopic observations indicated that thick gill filaments were arranged close to each other and alternately on two hemibranches of a gill arch in the opercular space of A. tetradactylus. Many mucous cells (MC) and mitochondrion-rich cells (MRC) were present in the interlamellar regions of the gill filament. An immunohistochemical study demonstrated that numerous NKA, NHE3 and some VHA were located predominantly on presumed MRCs of gill filaments and at the base of the lamellae. Analyses using serial (mirror image) sections of the gills indicated that only a few NKA immunoreactive cells (IRC) were colocalized with VHA on some MRCs in the filaments. In the gills of P. tanegasimae, NKA- and NHE3-IRCs were observed in the interlamellar region of the filaments and at the base of the lamellae. VHA-IRCs were located sparsely on the lamellae and filaments. In the gills of E. yaeyamaensis, the lamellae and filaments were thin and straight, respectively. MCs were located at the tip as well as found scattered in the interlamellar region of gill filaments. NKA-, NHE3- and VHA-IRCs were moderately frequently observed in the filaments and rarely on the lamellae. This study shows that the structure and distribution of ion transporters in the gills differ among the three blennid species, presumably reflecting their different ecologies.

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.

Intestinal anion exchange in marine teleosts is involved in osmoregulation and contributes to the oceanic inorganic carbon cycle

Marine teleost fish osmoregulation involves seawater ingestion and intestinal fluid absorption. Solute coupled fluid absorption by the marine teleost fish intestine has long been believed to be the product of Na(+) and Cl(-) absorption via the Na(+) :K(+) :2Cl(-) co-transporter (NKCC2). However, the past decade has revealed that intestinal anion exchange contributes significantly to Cl(-) absorption, in exchange for HCO(3) (-) secretion, and that this process is important for intestinal water absorption. In addition to contributing to solute coupled water absorption intestinal anion exchange results in luminal precipitation of CaCO(3) which acts to reduce luminal osmotic pressure and thus assist water absorption. Most recently, activity of apical H(+) -pumps, especially in distal segments of the intestine have been suggested to not only promote anion exchange, but also to reduce luminal osmotic pressure by preventing excess HCO(3)(-) concentrations from accumulating in intestinal fluids, thereby aiding water absorption. The present review summarizes and synthesizes the most recent advances in our view of marine teleosts osmoregulation, including our emerging understanding of epithelial transport of acid-base equivalents in the intestine, the consequences for whole organism acid-base balance and finally the impact of piscine CaCO(3) formation on the global oceanic carbon cycle.

Freshwater fish gill ion transport: August Krogh to morpholinos and microprobes

August Krogh proposed that freshwater fishes (and other freshwater animals) maintain body NaCl homoeostasis by extracting these ions from the environment via separate Na(+) /NH(4)(+) and Cl(-) /HCO(3)(-) exchangers in the gill epithelium. Subsequent data from other laboratories suggested that Na(+) uptake was more probably coupled to H(+) secretion via a vesicular proton pump (V-ATPase) electrically coupled to a Na(+) channel. However, despite uncertainty about electrochemical gradients, evidence has accrued that epithelial Na(+) /H(+) exchange indeed may be an alternative pathway for Na(+) uptake. The specific pathways for Na(+) uptake may be species and environment specific. An apical Cl(-) /HCO(3)(-) exchanger is generally accepted for most species (some species do not extract Cl(-) from freshwater), but the relative roles of anion exchanger-like (SLC4A1) vs. pendrin-like (SLC26Z4) exchangers are unknown, and also may be species specific. Most recently, data have supported the presence of an apical Na(+) + Cl(-) cotransporter (NCC-type), despite thermodynamic uncertainty. Ammonia extrusion may be via NH(3) diffusing through the paracellular junctions or NH(4) (+) substitution on both basolateral and apical ionic exchangers (Na(+) + K(+) -ATPase; Na(+) + K(+) + Cl(-) - cotransporter; and Na(+) /H(+) exchanger), but recent evidence suggests that Rhesus-glycoproteins mediate both basolateral and apical movement of ammonia.

Rh glycoprotein expression is modulated in pufferfish (Takifugu rubripes) during high environmental ammonia exposure

Rhesus (Rh) protein involvement in ammonia transport processes in freshwater fish has received considerable attention; however, parallel investigations in seawater species are scant. We exposed pufferfish to high environmental ammonia (HEA; 1 and 5 mmol l–1 NH4HCO3) and evaluated the patterns of ammonia excretion and gill Rh mRNA and protein expression. Gill H+-ATPase, NHE1, NHE2, NHE3, Na+/K+-ATPase (NKA), Na+/K+/2Cl– co-transporter (NKCC1) mRNA, H+-ATPase activity, NKA protein and activity, were also quantified. Activation of NKA by NH4+ was demonstrated in vitro. The downregulation of Rhbg mRNA and simultaneous upregulations of Rhcg1, H+-ATPase, NHE3, NKA, NKCC1 mRNA, H+-ATPase activity, and NKA protein and activity levels suggested that during HEA, ammonia excretion was mediated mainly by mitochondria-rich cells (MRCs) driven by NKA with basolateral NH4+ entry via NKA and/or NKCC1, and apical NH3 extrusion via Rhcg1. Reprotonation of NH3 by NHE3 and/or H+-ATPase would minimise back flux through the Rh channels. Downregulated Rhbg and Rhag mRNA observed in the gill during HEA suggests a coordinated protective response to minimise the influx of external ammonia via the pavement cells and pillar cells, respectively, while routing ammonia excretion through the MRCs. Exposure to hypercapnia (1% CO2 in air) resulted in downregulated gill and erythrocyte Rhag mRNA. Surprisingly, Rhag, Rhbg, Rhcg1 and Rhcg2 proteins responded to both hypercapnia and HEA with changes in their apparent molecular masses. A dual NH3/CO2 transport function of the pufferfish Rh proteins is therefore suggested. The results support and extend an earlier proposed model of pufferfish gill ammonia excretion that was based on immunolocalisation of the Rh proteins. Passive processes and/or Rhbg and Rhcg2 in the pavement cells may maintain basal levels of plasma ammonia but elevated levels may require active excretion via NKA and Rhcg1 in the MRCs.

Molecular identification of Na(+)-H(+) exchanger isoforms (NHE2) in the gills of the euryhaline teleost Fundulus heteroclitus

In the current study, reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR were used to clone full-length putative Na(+)-H(+) exchanger isoforms (NHE2a) cDNA from the gills of Fundulus heteroclitus. The 2480 bp cDNA includes a coding region for a protein that shows a 57% amino acid homology to rabbit NHE2. These sequences allowed data mining of available fish genome data, which revealed at least three NHE2 subtypes in some teleost species.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.

Complete intracellular pH protection during extracellular pH depression is associated with hypercarbia tolerance in white sturgeon, Acipenser transmontanus

Sturgeons are among the most CO2 tolerant of fishes investigated to date. However, the basis of this exceptional CO2 tolerance is unknown. Here, white sturgeon, Acipenser transmontanus, were exposed to elevated CO2 to investigate the mechanisms associated with short-term hypercarbia tolerance. During exposure to 1.5 kPa Pco2, transient blood pH [extracellular pH (pHe)] depression was compensated within 24 h and associated with net plasma HCO3- accumulation and equimolar Cl- loss, and changes in gill morphology, such as a decrease in apical surface area of mitochondrial-rich cells. These findings indicate that pHe recovery at this level of hypercarbia is accomplished in a manner similar to most freshwater teleost species studied to date, although branchial mechanisms involved may differ. White sturgeon exposed to more severe hypercarbia (3 and 6 kPa Pco2) for 48 h exhibited incomplete pH compensation in blood and red blood cells. Despite pHe depression, intracellular pH (pHi) of white muscle, heart, brain, and liver did not decrease during a transient (6 h of 1.5 kPa Pco2) or prolonged (48 h at 3 and 6 kPa Pco2 blood acidosis. This pHi protection was not due to high intrinsic buffering in tissues. Such tight active cellular regulation of pHi in the absence of pHe compensation represents a unique pattern for non-air-breathing fishes, and we hypothesize that it is the basis for the exceptional CO2 tolerance of white sturgeon and, likely, other CO2 tolerant fishes. Further research to elucidate the specific mechanisms responsible for this tremendous pH regulatory capacity in tissues of white sturgeon is warranted.

The effects of CO2 and external buffering on ammonia excretion and Rhesus glycoprotein mRNA expression in rainbow trout

Rhesus (Rh) proteins were recently characterized as ammonia gas (NH(3)) channels. Studies indicate, however, that Rh proteins also facilitate CO2 transport in a green alga and in human erythrocytes. Previously, we reported that Rh mRNA expression in various rainbow trout tissues responded to high environmental ammonia. To determine whether or not Rh proteins may also be involved in CO2 transport in rainbow trout, we examined the effects of a 12 h exposure to external hypercapnia (1% CO2 in air) on Rh mRNA expression in the gill, skin and erythrocytes. External hypercapnic conditions lowered the water pH and facilitated ammonia excretion; therefore, we also studied the effects of hypercapnia and normocapnia in the presence of 10 mmol l(-1) Hepes-buffered water. Hepes treatment prevented water acidification, but resulted in elevated plasma ammonia levels and reduced ammonia excretion rates. Hypercapnia exposure without buffering did not elicit changes in Rh mRNA expression in the gill or skin. However, Rhcg2 mRNA expression was downregulated in the gills and upregulated in the skin of both normocapnia- and hypercapnia-exposed fish in Hepes-buffered water. mRNA expression of a newly cloned Rhbg2 cDNA was downregulated in the skin of fish exposed to buffered water, and Rhag mRNA expression in erythrocytes was decreased with exposure to normocapnia in buffered water but not with hypercapnia exposure in either buffered or unbuffered water. With the aid of Hepes buffering, we were able to observe the effects of both CO2 and ammonia on Rh mRNA expression. Overall, we conclude that high CO2 did not directly elicit changes in Rh mRNA transcription levels in the gill and skin, and that the changes observed probably reflect responses to high plasma ammonia, mirroring those in trout exposed to high environmental ammonia. Therefore a dual function for gill and skin Rh proteins in CO2 and ammonia transport is not evident from these results. Rhag expression, however, responded differentially to high CO2 and high ammonia, suggesting a possible dual role in the erythrocytes.

Physiological and molecular mechanisms of osmoregulatory plasticity in killifish after seawater transfer

We have explored the molecular and physiological responses of the euryhaline killifish Fundulus heteroclitus to transfer from brackish water (10% seawater) to 100% seawater for 12 h, 3 days or 7 days. Plasma [Na+] and [Cl-] were unchanged after transfer, and plasma cortisol underwent a transient increase. Na+/K+-ATPase activity increased 1.5-fold in the gills and opercular epithelium at 7 days (significant in gills only), responses that were preceded by three- to fourfold increases in Na+/K+-ATPase alpha(1a) mRNA expression. Expression of Na+/K+/2Cl- cotransporter 1, cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, Na+/H+-exchanger 3 (significant in opercular epithelium only) and carbonic anhydrase II mRNA also increased two- to fourfold after transfer. Drinking rate increased over twofold after 12 h and remained elevated for at least 7 days. Surprisingly, net rates of water and ion absorption measured in vitro across isolated intestines decreased approximately 50%, possibly due to reduced salt demands from the diet in seawater, but water absorption capacity still exceeded the drinking rate. Changes in bulk water absorption were well correlated with net ion absorption, and indicated that slightly hyperosmotic solutions (>or=298 mmol l(-1)) were transported. There were no reductions in unidirectional influx of Na+ from luminal to serosal fluid or intestinal Na+/K+-ATPase activity after transfer. Overall, our results indicate that gill and opercular epithelia function similarly at a molecular level in seawater, in contrast to their divergent function in freshwater, and reveal unexpected changes in intestinal function. As such they provide further insight into the mechanisms of euryhalinity in killifish.

Branchial expression and localization of SLC9A2 and SLC9A3 sodium/hydrogen exchangers and their possible role in acid-base regulation in freshwater rainbow trout (Oncorhynchus mykiss)

Experiments were conducted on adult rainbow trout (Oncorhynchus mykiss) to test the hypothesis that SLC9 Na+/H+ exchangers (SLC9A2, NHE2; and SLC9A3, NHE3) on the gill epithelium are localized specifically to a subset of mitochondria-rich cells (MRCs) that are unable to bind peanut lectin agglutinin (PNA). This cell type, termed the PNA- MRC, is a sub-type of MRC believed to function in Na+ uptake and acid excretion. A technique using biotinylated PNA was used to distinguish between the PNA- and PNA+ MRCs on fixed gill sections. In contrast to expectations, both NHE2 (mRNA) and NHE3 (protein) were confined to cells enriched with Na+/K+-ATPase and capable of binding PNA. Thus, in trout, NHE2 and NHE3 are localized to PNA+ MRCs, the cells previously believed to be responsible for Cl- uptake and base excretion. Levels of mRNA for NHE2, the predominant isoform in the gill, were increased during 72 h of hypercapnic acidosis; NHE3 mRNA and protein levels were unaffected. Because plasma cortisol levels were increased during hypercapnia (from 35.3+/-9.4 to 100.1+/-30.9 ng ml(-1)), the effects of experimentally elevated cortisol levels on NHE expression were investigated. The elevation of plasma cortisol using intraperitoneal implants caused a significant increase in NHE2 mRNA expression without affecting NHE3 mRNA or protein abundance. Thus, we suggest that NHE2 contributes to acid-base regulation during hypercapnia owing to its transcriptional regulation by cortisol. The finding of NHE expression in PNA+ MRCs is discussed with reference to current models of ionic and acid-base regulation in teleost fish.

Acclimation of ion regulatory capacities in gills of marine fish under environmental hypercapnia

The preservation of ion balance and pH despite environmental fluctuations is essential for the maintenance of vital cellular functions. While several ion transporters contribute to acid-base regulation in fish, the involvement and expression of key transporters under hypercapnia remain to be established. Here, two members of the HCO(3)(-) transporter family (Na(+)/HCO(3)(-) cotransporter NBC1 and Cl(-)/HCO(3)(-) exchanger AE1) were described for the first time in gills of marine fish. Benthic eelpout Zoarces viviparus were acclimated to 10,000 ppm CO(2). Hypercapnia did not affect whole animal oxygen consumption over a period of 4 days. During a time series of 6 wk NBC1 mRNA levels first decreased by about 40% (8 to 24 h) but finally increased about threefold over control. mRNA expression of AE1 decreased transiently by 50% at day 4 but recovered to control levels only. Reduced mRNA levels were also found for two Na(+)/H(+) exchangers (NHE1A, NHE1B) during the first days (by 50-60% at days 1 and 2), followed by restoration of control levels. This pattern was mirrored in a slight decrease of NHE1 protein contents and its subsequent recovery. In contrast, Na(+)-K(+)-ATPase mRNA and protein contents, as well as maximum activity, rose steadily from the onset of hypercapnia, and reached up to twofold control levels at the end. These results indicate shifting acclimation patterns between short- and long-term CO(2) exposures. Overall, ion gradient-dependent transporter mRNA levels were transiently downregulated in the beginning of the disturbance. Upregulation of NBC1 on long timescales stresses the importance of this transporter in the hypercapnia response of marine teleosts. Long-term rearrangements include Na(+)-K(+)-ATPase at higher densities and capacities, indicating a shift to elevated rates of ion and acid-base regulation under environmental hypercapnia.

Teleost fish osmoregulation: what have we learned since August Krogh, Homer Smith, and Ancel Keys

In the 1930s, August Krogh, Homer Smith, and Ancel Keys knew that teleost fishes were hyperosmotic to fresh water and hyposmotic to seawater, and, therefore, they were potentially salt depleted and dehydrated, respectively. Their seminal studies demonstrated that freshwater teleosts extract NaCl from the environment, while marine teleosts ingest seawater, absorb intestinal water by absorbing NaCl, and excrete the excess salt via gill transport mechanisms. During the past 70 years, their research descendents have used chemical, radioisotopic, pharmacological, cellular, and molecular techniques to further characterize the gill transport mechanisms and begin to study the signaling molecules that modulate these processes. The cellular site for these transport pathways was first described by Keys and is now known as the mitochondrion-rich cell (MRC). The model for NaCl secretion by the marine MRC is well supported, but the model for NaCl uptake by freshwater MRC is more unsettled. Importantly, these ionic uptake mechanisms also appear to be expressed in the marine gill MRC, for acid-base regulation. A large suite of potential endocrine control mechanisms have been identified, and recent evidence suggests that paracrines such as endothelin, nitric oxide, and prostaglandins might also control MRC function.