The role of branchial carbonic anhydrase in acid-base regulation in rainbow trout (Oncorhynchus mykiss)

Population genomics analysis to identify ion and water transporter genes involved in the adaptation of Tibetan naked carps to brackish water

Understanding how evolutionary processes shape the genetic variations and influence the response of species to environmental alterations is critical for biodiversity conservation and molecular breeding. Gymnocypris przewalskii przewalskii is the only known cyprinid fish that dwells in the brackish water of Lake Qinghai on the Qinghai-Tibetan Plateau. To reveal the genetic basis of its adaptation to high salinity and alkalinity, whole-genome sequencing was performed in G. p. przewalskii and its freshwater relatives Gymnocypris eckloni and Gymnocypris przewalskii ganzihonensis. Compared with freshwater species, lower genetic diversity and higher linkage disequilibrium were observed in G. p. przewalskii. Selective sweep analysis identified 424 core-selective genes enriched in transport activities. Transfection analysis showed that genetic changes in the positively selected gene aquaporin 3 (AQP3) improved cell viability after salt treatment, suggesting its involvement in brackish water adaptation. Our analysis indicates that ion and water transporter genes experienced intensive selection, which might have contributed to the maintenance of high osmolality and ion content in G. p. przewalskii. The current study identified key molecules involved in the adaptation of fish to brackish water, providing valuable genomic resources for the molecular breeding of salt-tolerant fish.

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.

Effects of salinity stress on antioxidant status and inflammatory responses in females of a "Near Threatened" economically important fish species Notopterus chitala: a mechanistic approach

In the present study, acute stress responses of adult female Notopterus chitala were scrutinized by antioxidant status and inflammation reaction in the gill and liver at five different salinity exposures (0, 3, 6, 9, 12 ppt). Oxidative defense was assessed by determining superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase, and glutathione reductase activities, while malondialdehyde (MDA), glutathione, and xanthine oxidase levels were determined as indicators of oxidative load. Pro-inflammatory cytokines (IL-1β, IL-6, IL-10, and TNFα) and caspase 1 levels were also analyzed. Expression levels of transcription factors (NRF2 and NF-κB) and molecular chaperons (HSF, HSP70, and HSP90) were estimated to evaluate their relative contribution to overcome salinity stress. MDA showed a significant (P < 0.05) increase (gill, + 25.35-90.14%; liver, + 23.88-80.59%) with salinity; SOD (+ 13.72-45.09%) and CAT (+ 12.73-33.96%) exhibited a sharp increase until 9 ppt, followed by a decrease at the highest salinity (12 ppt) (gill, - 3.92%; liver, - 2.18%). Levels of cytokines were observed to increase (+ 52.8-127.42%) in a parallel pattern with increased salinity. HSP70 and HSP90 expressions were higher in gill tissues than those in liver tissues. NRF2 played pivotal role in reducing salinity-induced oxidative load in both the liver and gills. Serum cortisol and carbonic anhydrase were measured and noted to be significantly (P < 0.05) upregulated in salinity stressed groups. Gill Na⁺-K⁺-ATPase activity decreased significantly (P < 0.05) in fish exposed to 6, 9, and 12 ppt compared to control. Present study suggests that a hyperosmotic environment induces acute oxidative stress and inflammation, which in turn causes cellular death and impairs tissue functions in freshwater fish species such as Notopterus chitala.

Rapid evolution fuels transcriptional plasticity to ocean acidification

Ocean acidification (OA) is postulated to affect the physiology, behavior, and life-history of marine species, but potential for acclimation or adaptation to elevated pCO₂ in wild populations remains largely untested. We measured brain transcriptomes of six coral reef fish species at a natural volcanic CO₂  seep and an adjacent control reef in Papua New Guinea. We show that elevated pCO₂ induced common molecular responses related to circadian rhythm and immune system but different magnitudes of molecular response across the six species. Notably, elevated transcriptional plasticity was associated with core circadian genes affecting the regulation of intracellular pH and neural activity in Acanthochromis polyacanthus. Gene expression patterns were reversible in this species as evidenced upon reduction of CO₂ following a natural storm-event. Compared with other species, Ac. polyacanthus has a more rapid evolutionary rate and more positively selected genes in key functions under the influence of elevated CO₂ , thus fueling increased transcriptional plasticity. Our study reveals the basis to variable gene expression changes across species, with some species possessing evolved molecular toolkits to cope with future OA.

Protective Effects of Sal B on Oxidative Stress-Induced Aging by Regulating the Keap1/Nrf2 Signaling Pathway in Zebrafish

The models of oxidative damage-induced aging were established by adding ethanol (C2H5OH), hydrogen peroxide (H2O2) and 6-hydroxydopamine (6-OHDA) to zebrafish embryos in this research. To find effective protective drugs/foods, Salvianolic acid B (Sal B) was added after the embryos were treated by these oxidative reagents. After being treated with ethanol, H2O2 and 6-OHDA, the morphological changes were obvious and the deformities included spinal curvature, heart bleeding, liver bleeding, yolk sac deformity and pericardial edema, and the expression of oxidative stress-related genes Nrf2b, sod1 and sod2 and aging-related genes myl2a and selenbp1 were significantly up-regulated compared to the control group. While after adding 0.05 μg/mL and 0.5 μg/mL Sal B to the ethanol-treated group, death rates and MDA levels decreased, the activity of antioxidant enzyme (SOD, CAT and GSH-Px) changed and Nrf2b, sod1, sod2, myl2a, selenbp1, p53 and p21 were down-regulated compared to the ethanol-treated group. The bioinformatics analysis also showed that oxidative stress-related factors were associated with a variety of cellular functions and physiological pathways. In conclusion, Sal B can protect against aging through regulating the Keap1/Nrf2 pathway as well as antioxidative genes and enzyme activity.

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

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

Mechanisms of acid-base regulation following respiratory alkalosis in red drum (Sciaenops ocellatus)

Respiratory acidosis and subsequent metabolic compensation are well-studied processes in fish exposed to elevated CO₂ (hypercapnia). Yet, such exposures in the marine environment are invariably accompanied by a return of environmental CO₂ to atmospheric baselines. This understudied phenomenon has the potential to cause a respiratory alkalosis that would necessitate base excretion. Here we sought to explore this question and the associated physiological mechanisms that may accompany base excretions using the red drum (Sciaenops ocellatus). As expected, when high pCO₂ (15,000 μatm CO₂) acclimated red drum were transferred to normal pCO₂, their net H⁺ excretion shifted from positive (0.157 ± 0.044 μmol g^-1 h^-1) to negative (-0.606 ± 0.116 μmol g^-1 h^-1) in the 2 h post-transfer period. Net H⁺ excretion returned to control rates during the 3 to 24 h flux period. Gene expression and enzyme activity assays demonstrated that while the acidosis resulted in significant changes in several relevant transporters, no significant changes accompanied the alkalosis phase. Confocal microscopy was used to assess alkalosis-stimulated translocation of V-type H⁺ ATPase to the basolateral membrane previously seen in other marine species; however, no apparent translocation was observed. Overall, these data demonstrate that fluctuations in environmental CO₂ result in both acidic and alkalotic respiratory disturbances; however, red drum maintain sufficient regulatory capacity to accommodate base excretion. Furthermore, this work does not support a role for basolateral VHA translocation in metabolic compensation from a systemic alkalosis in teleosts.

Identification and expression of carbonic anhydrase 2, myosin regulatory light chain 2 and selenium-binding protein 1 in zebrafish Danio rerio: Implication for age-related biomarkers

Proteomic study has determined age-related changes in synthesis of carbonic anhydrase 2, myosin regulatory light chain 2 and selenium-binding protein 1 in muscle of post-menopausal women. However, little information is available regarding the expression and role of these proteins in early development and life span. In this study we showed that zebrafish ca2, myl2a, myl2b and selenbp1 were highly identical to their mammalian counterparts in primary and tertiary structures as well as genomic organization and syntenic map. They displayed distinct spatiotemporal expression patterns in embryos and larvae of zebrafish. Moreover, their transcription levels in the respective tissues were obviously remodeled in an age-dependent fashion, i.e. some mRNA levels were increased, while others remained unchanged or even decreased, suggesting that CA2, MYL2a, MYL2b and SELENBP1 can be used as aging biomarkers. Our study also lays a foundation for further illumination of the functions of these genes in early development and aging processes.

Carbonic anhydrase expression in the branchial ionocytes of rainbow trout

Rainbow trout (Oncorhynchus mykiss) exposed to acid-base challenges activate branchial mechanisms for the excretion of acid-base equivalents. Current models of branchial acid-base excretion in freshwater rainbow trout propose two main ionocyte types; the peanut lectin agglutinin-positive (PNA+) mitochondrion-rich cell or ionocyte is believed to secrete HCO3− in exchange for Cl−, whereas H+ secretion is thought to occur across PNA− ionocytes in exchange for Na+. Both HCO3− and H+ are supplied by intracellular hydration of CO2 catalyzed by cytosolic carbonic anhydrase (CAc). Immunohistochemical approaches revealed that under control conditions, CAc was detectable in 92.3±1.0% (N=11) of PNA− ionocytes, and the abundance of PNA− ionocytes increased in response to systemic acidosis elicited by 72 h exposure to water of low pH (nominally pH 4.5), hypercapnia (1% CO2, nominally 7.6 Torr) or hyperoxia (achieved by gassing water with pure O2), as did the abundance of PNA− ionocytes that exhibited immunofluorescence for CAc. However, just 4.3 ± 0.6% (N=11) of PNA+ ionocytes expressed detectable CAc under control conditions. Marked increases in the abundance of CAc-positive PNA+ ionocytes were detected following exposure of trout to a base load via recovery from hypercapnia, or base infusion (72 h infusion with 140 mmol L−1 NaHCO3). The percentage of CAc-positive PNA+ ionocytes also was increased in trout treated with cortisol (10 mg kg−1 hydrocortisone 21-hemisuccinate daily for 7 d). These results suggest that regulation of CA within PNA+ ionocytes and/or the abundance of CAc-positive PNA+ ionocytes plays a role in activating base secretion in response to systemic alkalosis.

A shell-formation related carbonic anhydrase in Crassostrea gigas modulates intracellular calcium against CO2 exposure: Implication for impacts of ocean acidification on mollusk calcification

Ocean acidification (OA) could decrease the shells and skeletons formation of mollusk by reducing the availability of carbonate ions at calcification sites. Carbonic anhydrases (CAs) convert CO₂ to HCO₃^- and play important roles in biomineralization process from invertebrate to vertebrate. In the present study, a CA (designated as CgCA) was identified and characterized in Pacific oyster C. gigas. The cDNA of CgCA was of 927bp encoding a predicted polypeptide of 308 amino acids with a signal peptide and a CA catalytic function domain. The mRNA transcripts of CgCA were constitutively expressed in all tested tissues with the highest levels in mantle and hemocytes. During the early development period, the mRNA transcripts of CgCA could be detected in all the stages with the highest level in D-veliger larvae. Elevated CO₂ increased the mRNA transcripts of CgCA in muscle, mantle, hepatopancreas, gill and hemocytes significantly (p<0.05) and induced the translocation of CgCA in hemocytes and mantle. Moreover, elevated CO₂ also caused the decrease of intracellular Ca²⁺ in hemocytes (p<0.05). The inhibition of CA by acetazolamide and suppression of CgCA gene via RNA interference could increase the intracellular Ca²⁺ in hemocytes (p<0.05). Besides, the decrease of intracellular Ca²⁺ content caused by Ca²⁺ reagent ionomycin could affect localization of CgCA in mantle tissue. The results indicated CgCA played essential roles in calcification and elevated CO₂ accelerated the mutual modulation between calcium and CgCA, implying reduced calcification rate and dissolved shells under OA.

A Carbonic Anhydrase Serves as an Important Acid-Base Regulator in Pacific Oyster Crassostrea gigas Exposed to Elevated CO2: Implication for Physiological Responses of Mollusk to Ocean Acidification

Carbonic anhydrases (CAs) have been demonstrated to play an important role in acid-base regulation in vertebrates. However, the classification and modulatory function of CAs in marine invertebrates, especially their responses to ocean acidification remain largely unknown. Here, a cytosolic α-CA (designated as CgCAII-1) was characterized from Pacific oyster Crassostrea gigas and its molecular activities against CO₂ exposure were investigated. CgCAII-1 possessed a conserved CA catalytic domain, with high similarity to invertebrate cytoplasmic or mitochondrial α-CAs. Recombinant CgCAII-1 could convert CO₂ to HCO₃^- with calculated activity as 0.54 × 10³ U/mg, which could be inhibited by acetazolamide (AZ). The mRNA transcripts of CgCAII-1 in muscle, mantle, hepatopancreas, gill, and hemocytes increased significantly after exposure to elevated CO₂. CgCAII-1 could interact with the hemocyte membrane proteins and the distribution of CgCAII-1 protein became more concentrated and dense in gill and mantle under CO₂ exposure. The intracellular pH (pHi) of hemocytes under CO₂ exposure increased significantly (p < 0.05) and CA inhibition reduced the pHi value. Besides, there was no increase in CA activity in gill and mantle after CO₂ exposure. The impact of CO₂ exposure on CA activity coupled with the mRNA expression level and protein translocation of CgCAII-1 provided evidences that CgCAII-1 could respond to ocean acidification and participate in acid-base regulation. Such cytoplasmic CA-based physiological regulation mechanism might explain other physiological responses of marine organisms to OA.

Phosphorylation increases the catalytic activity of rainbow trout gill cytosolic carbonic anhydrase

Cytoplasmic carbonic anhydrase (CAc) in the gill of teleost fish contributes to ionic regulation and acid–base balance by catalyzing the reversible reaction of CO2 and water, CO2 + H2O ↔ H(+) + HCO3(-). Regulation of CAc abundance and activity therefore is expected to fine-tune responses to ionic or acid–base challenges. The present study investigated the potential for gill CAc of rainbow trout, Oncorhynchus mykiss (tCAc), to undergo reversible phosphorylation. The activity of tCAc was approximately doubled by phosphorylation achieved through in vitro stimulation of endogenous protein kinases; kinase stimulation doubled phospho-threonine content from that observed in tCAc isolated under conditions where both kinases and protein phosphatases were inhibited. In vitro incubation to preferentially stimulate specific kinases implicated protein kinase G (PKG) in mediating the increase in tCAc activity. The kinetic parameters of turnover number (k cat) and substrate affinity (K m) were similarly affected by stimulation of either kinase or phosphatase action. However, phosphorylation via kinase stimulation significantly increased the efficiency of tCAc (V max /K m), and this factor may have contributed to the elevation of tCAc activity. In addition, phosphorylation of tCAc by kinase stimulation significantly increased the inhibition constant (K i) for acetazolamide. These results demonstrate that tCAc is subject to reversible phosphorylation; future work should focus on identifying the physiological situation(s) in which phosphorylation of trout branchial CAc occurs.

A cytosolic carbonic anhydrase molecular switch occurs in the gills of metamorphic sea lamprey

Carbonic anhydrase plays a key role in CO2 transport, acid-base and ion regulation and metabolic processes in vertebrates. While several carbonic anhydrase isoforms have been identified in numerous vertebrate species, basal lineages such as the cyclostomes have remained largely unexamined. Here we investigate the repertoire of cytoplasmic carbonic anhydrases in the sea lamprey (Petromyzon marinus), that has a complex life history marked by a dramatic metamorphosis from a benthic filter-feeding ammocoete larvae into a parasitic juvenile which migrates from freshwater to seawater. We have identified a novel carbonic anhydrase gene (ca19) beyond the single carbonic anhydrase gene (ca18) that was known previously. Phylogenetic analysis and synteny studies suggest that both carbonic anhydrase genes form one or two independent gene lineages and are most likely duplicates retained uniquely in cyclostomes. Quantitative PCR of ca19 and ca18 and protein expression in gill across metamorphosis show that the ca19 levels are highest in ammocoetes and decrease during metamorphosis while ca18 shows the opposite pattern with the highest levels in post-metamorphic juveniles. We propose that a unique molecular switch occurs during lamprey metamorphosis resulting in distinct gill carbonic anhydrases reflecting the contrasting life modes and habitats of these life-history stages.

A multibiomarker evaluation of urban, industrial, and agricultural exposure of small characins in a large freshwater basin in southern Brazil

Iguaçu River is the second most polluted river of Brazil. It receives agrochemicals and contaminants of urban and industrial sources along its course. A multibiomarker approach was employed here to evaluate the health of a small characin (Astyanax spp.) at two sites along the river, sampled during a dry (autumn) and a rainy (spring) season. Biomarkers were condition factor and somatic indices (gonads and liver); genetic damage (comet assay and micronucleus test); enzyme activities such as hepatic catalase (CAT) and glutathione S-transferase (GST), lipoperoxidation (LPO), branchial and renal carbonic anhydrase (CA), acetylcholinesterase (AChE) in the muscle and the brain, histopathology of the liver and gills, and concentrations of polycyclic aromatic hydrocarbons (PAHs) in bile. There were no consistent differences in biomarker responses between the two study sites. Some biomarkers revealed greater potential impact in the rainy season, when increased amounts of contaminants are washed into the river (combined CAT inhibition and LPO increase, CA upregulation). Other biomarkers, however, revealed potential greater impact in the dry season, when contaminants potentially concentrate (GST induction, AChE inhibition, and liver histopathological alterations). Although of a complex nature, field experiments such as this provide rich data for monitoring protocols and assessment of general risk of exposure to pollutants of river systems.

The Evolutionary History of Daphniid α-Carbonic Anhydrase within Animalia

Understanding the mechanisms that drive acid-base regulation in organisms is important, especially for organisms in aquatic habitats that experience rapidly fluctuating pH conditions. Previous studies have shown that carbonic anhydrases (CAs), a family of zinc metalloenzymes, are responsible for acid-base regulation in many organisms. Through the use of phylogenetic tools, this present study attempts to elucidate the evolutionary history of the α-CA superfamily, with particular interest in the emerging model aquatic organism Daphnia pulex. We provide one of the most extensive phylogenies of the evolution of α-CAs, with the inclusion of 261 amino acid sequences across taxa ranging from Cnidarians to Homo sapiens. While the phylogeny supports most of our previous understanding on the relationship of how α-CAs have evolved, we find that, contrary to expectations, amino acid conservation with bacterial α-CAs supports the supposition that extracellular α-CAs are the ancestral state of animal α-CAs. Furthermore, we show that two cytosolic and one GPI-anchored α-CA in Daphnia genus have homologs in sister taxa that are possible candidate genes to study for acid-base regulation. In addition, we provide further support for previous findings of a high rate of gene duplication within Daphnia genus, as compared with other organisms.

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.

Immunocytochemical localization of carbonic anhydrase in the pseudobranch tissue of the rainbow trout Oncorhynchus mykiss

Pseudobranch function has long interested scientists, but its role has yet to be elucidated. Several studies have suggested that pseudobranchs serve respiratory, osmoregulatory, and sensory functions. This work investigated the immunolocalization of pseudobranch carbonic anhydrase (CA) in the teleost fish species rainbow trout (Oncorhynchus mykiss) to clarify its physiological function. CA was purified from rainbow trout gills O. mykiss and specific antibodies were raised. Immunoblotting between tissue homogenates of pseudobranch and gill CA antibodies showed specific immunostaining with only one band corresponding to CA in the pseudobranch homogenate. Results of immunohistochemical technique revealed that CA was distributed within pseudobranch cells and more precisely in the apical parts (anti-vascular) of cells. The basal (vascular) parts of cells, tubular system, blood capillaries, and pillar cells were not immunostained. Immunocytochemistry confirmed these results and showed that some CA enzyme was cytoplasmic and the remainder was linked to membranous structures. The results also showed that the lacunar tissue layers did not display immunoperoxidase activity. Our results indicated that pseudobranch CA may have a function related to the extracellular medium wherein CA intervenes with the mechanism of stimulation of afferent nerve fibers.

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.

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-base balance during social interactions in rainbow trout (Oncorhynchus mykiss)

Socially subordinate rainbow trout (Oncorhynchus mykiss) experience chronic stress that impacts upon a variety of physiological functions, including Na(+) regulation. Owing to the tight coupling between Na(+) and Cl(-) uptake and, respectively, H(+) and HCO(3)(-) loss at the gill, ionoregulatory changes associated with social status may affect acid-base regulation. The present study assessed the responses of dominant, subordinate and control trout to hypercapnia (1% CO(2)) to test this hypothesis. Social status appeared to impact net acid excretion (J(net)H(+)) as subordinate individuals failed to increase net acid flux in response to hypercapnia. However, blood acid-base status was found to be unaffected by social status before or during hypercapnic exposure, indicating that subordinate fish were as effective as dominant or control trout in achieving compensation for the acid-base disturbance induced by hypercapnic exposure. Compensation in all groups involved decreasing Cl(-) uptake in response to hypercapnia. The branchial activities of both Na(+),K(+)-ATPase (NKA) and V-type H(+)-ATPase were affected by social interactions and/or exposure to hypercapnia. Branchial NKA activity was higher but V-ATPase activity was lower in control fish than in dominant or subordinate trout. In addition, control and subordinate but not dominant trout exposed to 24h of hypercapnia exhibited significantly higher branchial V-ATPase activity than fish maintained in normocapnia. Collectively, the data suggest that subordinate trout are able to regulate blood pH during a respiratory acidosis.

Does the presence of a seawater gill morphology induced by dietary salt loading affect Cl(-) uptake and acid-base regulation in freshwater rainbow trout Oncorhynchus mykiss

The goal of this study was to determine the effect of the changes in gill morphology induced by dietary salt feeding on several aspects of gill function in rainbow trout Oncorhynchus mykiss maintained in fresh water with specific emphasis on Cl(-) uptake (J(IN)Cl(-)) and acid-base regulation. The addition of 11% NaCl to the diet caused J(IN)Cl(-) to be reduced by c. 45% from 214·4 ± 26·7 to 117·3 ± 17·4 µmol kg(-1) h(-1) (mean ± s.e.). Rates of Cl(-) efflux (J(OUT)Cl(-)), net Cl(-) flux (J(NET)Cl(-)), J(NET) Na(+) and plasma levels of Na(+) or Cl(-) were unaffected by salt feeding. On the basis of significant effect of the salt diet on decreasing the maximal uptake rate of Cl(-)(J(MAX)Cl(-)), it would appear that internal salt loading caused a decrease in the number of functional ion transport proteins involved in Cl(-) uptake (e.g. Cl(-) -HCO(3)(-) exchangers) and decreased the transporting capacity of existing proteins. The acid-base regulating capacity of control fish and salt-loaded fish was assessed by monitoring arterial blood acid-base status [partial pressure of CO(2) (PCO(2)), pH and HCO(3)(-)] during exposure to external hypercapnia (nominally 7·5 mm Hg). Both groups of fish exhibited typical compensatory responses to sustained hypercapnia consisting of the gradual accumulation of plasma HCO(3) (-) and thus metabolic restoration within 24 h of the initial respiratory acidosis elicited by hypercapnia. Overall, the results demonstrate that while Cl(-) uptake capacity is reduced in salt-fed fish, there is no associated alteration in acid-base regulating capability.

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.

Proteome profiles in medaka (Oryzias melastigma) liver and brain experimentally exposed to acute inorganic mercury

Mercury is a widespread and persistent pollutant occurring in a variety of forms in freshwater and marine ecosystems. Using the proteomic approach, this study examined the protein profiles of the medaka (Oryzias melastigma) liver and brain exposed to an acute mercuric chloride (HgCl(2)) concentration (1000μg/L) for 8h. The results showed that acute exposure of medaka to inorganic mercury enhanced metal accumulation in both the liver and brain, and a higher content of mercury was detected in the latter. Comparison of the two-dimensional electrophoresis protein profiles of HgCl(2)-exposed and non-exposed group revealed that altered protein expression was quantitatively detected in 20 spots in the brain and 27 in the liver. The altered protein spots were subjected to matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis, with the resultant identification of 46 proteins. The proteins identified were involved in oxidative stress, cytoskeletonal assembly, signal transduction, protein modification, metabolism and other related functions (e.g. immune response, ionoregulation and transporting), highlighting the fact that inorganic mercury toxicity in fish seems to be complex and diverse. This study provided basic information to aid our understanding of the possible molecular mechanisms of acute inorganic mercury toxicity in aquatic organisms, as well as potential protein biomarker candidates for aquatic environmental monitoring.

Elevated seawater Pco2 differentially affects branchial acid-base transporters over the course of development in the cephalopod Sepia officinalis

The specific transporters involved in maintenance of blood pH homeostasis in cephalopod molluscs have not been identified to date. Using in situ hybridization and immunohistochemical methods, we demonstrate that Na+/K+-ATPase ( soNKA), a V-type H+-ATPase ( soV-HA), and Na+/HCO3− cotransporter ( soNBC) are colocalized in NKA-rich cells in the gills of Sepia officinalis. mRNA expression patterns of these transporters and selected metabolic genes were examined in response to moderately elevated seawater Pco2 (0.16 and 0.35 kPa) over a time course of 6 wk in different ontogenetic stages. The applied CO2 concentrations are relevant for ocean acidification scenarios projected for the coming decades. We determined strong expression changes in late-stage embryos and hatchlings, with one to three log2-fold reductions in soNKA, soNBCe, socCAII, and COX. In contrast, no hypercapnia-induced changes in mRNA expression were observed in juveniles during both short- and long-term exposure. However, a transiently increased ion regulatory demand was evident during the initial acclimation reaction to elevated seawater Pco2. Gill Na+/K+-ATPase activity and protein concentration were increased by ∼15% during short (2–11 days) but not long-term (42-days) exposure. Our findings support the hypothesis that the energy budget of adult cephalopods is not significantly compromised during long-term exposure to moderate environmental hypercapnia. However, the downregulation of ion regulatory and metabolic genes in late-stage embryos, taken together with a significant reduction in somatic growth, indicates that cephalopod early life stages are challenged by elevated seawater Pco2.

Roles of cortisol and carbonic anhydrase in acid-base compensation in rainbow trout, Oncorhynchus mykiss

Fish compensate for acid-base disturbances primarily by modulating the branchial excretion of acid-base equivalents, with a supporting role played by adjustment of urinary acid excretion. The present study used metabolic acid-base disturbances in rainbow trout, Oncorhynchus mykiss, to evaluate the role played by cortisol in stimulating compensatory responses. Trout infused with acid (an iso-osmotic solution of 70 mmol L(-1) HCl), base (140 mmol L(-1) NaHCO(3)) or saline (140 mmol L(-1) NaCl) for 24 h exhibited significant elevation of circulating cortisol concentrations. Acid infusion significantly increased both branchial (by 328 μmol kg(-1) h(-1)) and urinary (by 5.9 μmol kg(-1) h(-1)) net acid excretion, compensatory responses that were eliminated by pre-treatment of trout with the cortisol synthesis inhibitor metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone). The significant decrease in net acid excretion (equivalent to enhanced base excretion) of 203 μmol kg(-1) h(-1) detected in base-infused trout was unaffected by metyrapone treatment. Acid- and base-infusions also were associated with significant changes in the relative mRNA expression of branchial and renal cytosolic carbonic anhydrase (tCAc) and renal membrane-linked CA IV (tCA IV). Cortisol treatment caused changes in CA gene expression that tended to parallel those observed with acid but not base infusion. For example, significant increases in renal relative tCA IV mRNA expression were detected in both acid-infused (~2x) and cortisol-treated (~10x) trout, whereas tCA IV mRNA expression was significantly reduced (~5x) in base-infused fish. Despite changes in CA gene expression in acid- or base-infused fish, neither acid nor base infusion affected CAc protein levels in the gill, but both caused significant increases in branchial CA activity. Cortisol treatment similarly increased branchial CA activity in the absence of an effect on branchial CAc protein expression. Taken together, these findings provide support for the hypothesis that in rainbow trout, cortisol is involved in mediating acid-base compensatory responses to a metabolic acidosis, and that cortisol exerts its effects at least in part through modulation of CA.

Acid-base regulation in the plainfin midshipman (Porichthys notatus): an aglomerular marine teleost

The plainfin midshipman (Porichthys notatus) possesses an aglomerular kidney and like other marine teleosts, secretes base into the intestine to aid water absorption. Each of these features could potentially influence acid-base regulation during respiratory acidosis either by facilitating or constraining HCO(3)(-) accumulation, respectively. Thus, in the present study, we evaluated the capacity of P. notatus to regulate blood acid-base status during exposure to increasing levels of hypercapnia (nominally 1-5% CO(2)). Fish exhibited a well-developed ability to increase plasma HCO(3)(-) levels with values of 39.8 ± 2.8 mmol l(-1) being achieved at the most severe stage of hypercapnic exposure (arterial blood PCO(2) = 21.9 ± 1.7 mmHg). Consequently, blood pH, while lowered by 0.15 units (pH = 7.63 ± 0.06) during the final step of hypercapnia, was regulated far above values predicted by chemical buffering (predicted pH = 7.0). The accumulation of plasma HCO(3)(-) during hypercapnia was associated with marked increases in branchial net acid excretion (J (NET)H(+)) owing exclusively to increases in the titratable alkalinity component; total ammonia excretion was actually reduced during hypercapnia. The increase in J (NET)H(+) was accompanied by increases in branchial carbonic anhydrase (CA) enzymatic activity (2.8×) and CA protein levels (1.6×); branchial Na(+)/K(+)-ATPase activity was unaffected. Rectal fluids sampled from control fish contained on average HCO(3)(-) concentrations of 92.2 ± 4.8 mmol l(-1). At the highest level of hypercapnia, rectal fluid HCO(3)(-) levels were increased significantly to 141.8 ± 7.4 mmol l(-1) but returned to control levels during post-hypercapnia recovery (96.0 ± 13.2 mmol l(-1)). Thus, the impressive accumulation of plasma HCO(3)(-) to compensate for hypercapnic acidosis occurred against a backdrop of increasing intestinal HCO(3)(-) excretion. Based on in vitro measurements of intestinal base secretion in Ussing chambers, it would appear that P. notatus did not respond by minimizing base loss during hypercapnia; the increases in base flux across the intestinal epithelium in response to alterations in serosal HCO(3)(-) concentration were similar in preparations obtained from control or hypercapnic fish. Fish returned to normocapnia developed profound metabolic alkalosis owing to unusually slow clearance of the accumulated plasma HCO(3)(-). The apparent inability of P. notatus to effectively excrete HCO(3)(-) following hypercapnia may reflect its aglomerular (i.e., non-filtering) kidney coupled with the normally low rates of urine production in marine teleosts.

Carbonic anhydrase expression and CO2 excretion during early development in zebrafish Danio rerio

Carbonic anhydrase (CA) is critical for CO2 excretion in adult fish, but little is known of the expression or function of CA during early development. The present study examined the hypothesis that, as rates of CO2 production increased during early development in zebrafish (Danio rerio), CA would become necessary for effective CO2 excretion, and that the pattern of CA expression during early development would reflect this transition. Real-time RT-PCR was used to examine the mRNA expression of the two main intracellular CA isoforms over a time course of early development ranging from 0 to 120 h post fertilization (h.p.f.). The mRNA expression of zCAb was generally higher than that of zCAc, particularly during the earliest stages of development. Rates of CO2 excretion increased approximately 15-fold from 24 to 48 h.p.f. whereas rates of O2 uptake increased only 6.7-fold over the same period, indicating a relative stimulation of CO2 excretion over O2 uptake. Treatment of 48 h.p.f. larvae with the CA inhibitor acetazolamide resulted in CO2 excretion rates that were 52% of the value in control larvae, a significant difference that occurred in the absence of any effect on O2 uptake. Antisense morpholino oligonucleotides were used to selectively knock down one or both of the main intracellular CA isoforms. Subsequent measurement of gas transfer rates at 48 h.p.f. indicated that CA knockdown caused a significant relative inhibition of CO2 excretion over O2 uptake, regardless of which cytosolic CA isoform was targeted for knockdown. These results suggest that between 24 h.p.f. and 48 h.p.f., developing zebrafish begin to rely on CA to meet requirements for increased CO2 excretion.

Zebrafish (Danio rerio) gill neuroepithelial cells are sensitive chemoreceptors for environmental CO2

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO(2) levels as low as 0.13% ( mmHg), more than an order of magnitude lower than the typical arterial levels (40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO(2) levels is not clear. Here, we show that the neuroepithelial cells (NECs) of the zebrafish gill, known to sense O(2) levels, also respond to low levels of CO(2). An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO(2) levels, a background K(+) channel was inhibited, resulting in a partial pressure-dependent depolarization of the NEC. To elucidate the signalling pathway underlying K(+) channel inhibition, we used immunocytochemistry to show that these NECs express carbonic anhydrase (CA), an enzyme involved in CO(2) sensing in the mammalian carotid body. Further, the NEC response to CO(2) (magnitude of membrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by 50% by the CA-inhibitor acetazolamide. This suggests that the CO(2) detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO(2) and which does not require a change of extracellular pH. Because some cells that were responsive to increasing also responded to hypoxia with membrane depolarization, the present results demonstrate that a subset of the NECs in the zebrafish gill are bimodal sensors of CO(2) and O(2).

The involvement of H+-ATPase and carbonic anhydrase in intestinal HCO3- secretion in seawater-acclimated rainbow trout

Pyloric caeca and anterior intestine epithelia from seawater-acclimated rainbow trout exhibit different electrophysiological parameters with lower transepithelial potential and higher epithelial conductance in the pyloric caeca than the anterior intestine. Both pyloric caeca and the anterior intestine secrete HCO(3)(-) at high rates in the absence of serosal HCO(3)(-)/CO(2), demonstrating that endogenous CO(2) is the principal source of HCO(3)(-) under resting control conditions. Apical, bafilomycin-sensitive, H(+) extrusion occurs in the anterior intestine and probably acts to control luminal osmotic pressure while enhancing apical anion exchange; both processes with implications for water absorption. Cytosolic carbonic anhydrase (CAc) activity facilitates CO(2) hydration to fuel apical anion exchange while membrane-associated, luminal CA activity probably facilitates the conversion of HCO(3)(-) to CO(2). The significance of membrane-bound, luminal CA may be in part to reduce HCO(3)(-) gradients across the apical membrane to further enhance anion exchange and thus Cl(-) absorption and to facilitate the substantial CaCO(3) precipitation occurring in the lumen of marine teleosts. In this way, membrane-bound, luminal CA thus promotes the absorption of osmolytes and reduction on luminal osmotic pressure, both of which will serve to enhance osmotic gradients to promote intestinal water absorption.

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.

Morphological and functional classification of ion-absorbing mitochondria-rich cells in the gills of Mozambique tilapia

To clarify ion-absorbing functions and molecular mechanisms of mitochondria-rich (MR) cells, Mozambique tilapia (Oreochromis mossambicus) were acclimated to artificial freshwaters with normal or lowered Na+ and/or Cl- concentration: (1) normal Na+/normal Cl- (control); (2) normal Na+/low Cl-; (3) low Na+/normal Cl-; and (4) low Na+/low Cl-. Scanning electron microscopy (SEM) revealed that concave and convex apical surfaces of MR cells predominantly developed in low Na+ and low Cl- waters, respectively, whereas small apical pits predominated in control conditions. Expression of Na+/H+ exchanger-3 (NHE3) mRNA in the gills was increased in low Na+ waters (low Na+/normal Cl- and low Na+/low Cl-, whereas that of Na+/Cl- cotransporter (NCC) expression was upregulated in low Cl-, but not in low Na+/low Cl-. Immunofluorescence staining showed that enlarged NHE3-immunoreactive apical regions were concave or flat in low Na+ waters, whereas NCC-immunoreactive regions were enlarged convexly in low Cl- waters. Using SEM immunocytochemistry the distribution of NHE3/NCC was compared with SEM images obtained simultaneously, it was further demonstrated that NHE3 and NCC were confined to concave and convex apical surfaces, respectively. These results indicated that small apical pits developed into concave apical surfaces to facilitate Na+ uptake through NHE3, and into convex apical surfaces to enhance Na+/Cl- uptake through NCC. Our findings integrated morphological and functional classifications of ion-absorbing MR cells in Mozambique tilapia.

Carbonic anhydrase and acid–base regulation in fish

Carbonic anhydrase (CA) is the zinc metalloenzyme that catalyses the reversible reactions of CO2 with water. CA plays a crucial role in systemic acid–base regulation in fish by providing acid–base equivalents for exchange with the environment. Unlike air-breathing vertebrates, which frequently utilize alterations of breathing (respiratory compensation) to regulate acid–base status, acid–base balance in fish relies almost entirely upon the direct exchange of acid–base equivalents with the environment (metabolic compensation). The gill is the critical site of metabolic compensation, with the kidney playing a supporting role. At the gill, cytosolic CA catalyses the hydration of CO2 to H+ and HCO3– for export to the water. In the kidney, cytosolic and membrane-bound CA isoforms have been implicated in HCO3– reabsorption and urine acidification. In this review, the CA isoforms that have been identified to date in fish will be discussed together with their tissue localizations and roles in systemic acid–base regulation.

Cellular mechanisms of Cl- transport in trout gill mitochondrion-rich cells

We have studied Cl(-) transport mechanisms in freshwater rainbow trout gill mitochondrion-rich (MR) cells using intracellular pH (pH(i)) imaging. Scanning electron microscopy demonstrated maintenance of cellular polarity in isolated MR cells. MR cell subtypes were identified by Na(+) introduction to the bath, and Cl(-) transport mechanisms were subsequently examined. Cl(-)-free exposure resulted in an alkalinization of pH(i) in both MR cell subtypes, which was dependent on HCO(3)(-) in the bath and inhibited by 1 mM DIDS. Recovery of pH(i) from an acidified state in Na(+)-free conditions was also DIDS sensitive. These results are the first functional evidence for Cl(-)/HCO(3)(-) exchangers in fish gill MR cells. A direct switch from NaCl to Cl(-)-free conditions caused a pH(i) acidification in a subset of MR cells, which was enhanced in the absence of HCO(3)(-). The acidification was replaced by an alkalinization when Cl(-) removal was performed in the presence of NPPB (500 microM) or EIPA (500 microM). Finally, we found that the Na(+)-induced alkalinization of pH(i) found in a previous study is inhibited by EIPA. This inhibitor profile's results suggest the presence of a Cl(-)-dependent Na(+)/H(+) exchange mechanism.

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.

Effects of hypothermia on gene expression in zebrafish gills: upregulation in differentiation and function of ionocytes as compensatory responses

Ectothermic vertebrates are different from mammals that are sensitive to hypothermia and have to maintain core temperature for survival. Why and how ectothermic animals survive, grow and reproduce in low temperature have been for a long time a scientifically challenging and important inquiry to biologists. We used a microarray to profile the gill transcriptome in zebrafish (Danio rerio) after exposure to low temperature. Adult zebrafish were acclimated to a low temperature of 12 degrees C for 1 day and up to 30 days, and the gill transcriptome was compared with that of control fish in 28 degrees C by oligonucleotide microarray hybridization. Results showed 11 and 22 transcripts were found to be upregulated, whereas 56 and 70 transcripts were downregulated by low-temperature treatment for 1 day and 30 days, respectively. The gill transcriptome profiles revealed that ionoregulation-related genes were highly upregulated in cold-acclimated zebrafish. This paved the way to investigate the role of ionoregulatory genes in zebrafish gills during cold acclimation. Cold acclimation caused upregulation of genes that are essential for ionocyte specification, differentiation, ionoregulation, acid-base balance and the number of cells expressing these genes increased. For instance, epithelial Ca2+ channel (EcaC; an ionoregulatory protein) mRNA increased in parallel with the level of Ca2+ influx, revealing a functional compensation after long-term acclimation to cold. Phosphohistone H3 and TUNEL staining showed that the cell turnover rate was retarded in cold-acclimated gills. Altogether, these results suggest that gills may sustain their functions by producing mature ionocytes from pre-existing undifferentiated progenitors in low-temperature environments.

Theoretical considerations underlying Na(+) uptake mechanisms in freshwater fishes

Ion and acid-base regulating mechanisms have been studied at the fish gill for almost a century. Original models proposed for Na(+) and Cl(-) uptake, and their linkage with H(+) and HCO(3)(-) secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill, two dominant mechanisms for Na(+) uptake from dilute environments have persisted in the literature. The use of an apical Na(+)/H(+) exchanger driven by a basolateral Na(+)/K(+)-ATPase versus an apical Na(+) channel electrogenically coupled to an apical H(+)-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill. However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on the recent molecular advances for Na(+) uptake in freshwater fish. Emphasis will be placed on thermodynamic constraints that prevent electroneutral apical NHE function in most freshwater environments. By combining recent advances in molecular and functional physiology of fish gills with thermodynamic considerations of ion transport, our knowledge in the field should continue to grow in a logical manner.

Carbonic anhydrase 2-like a and 15a are involved in acid-base regulation and Na+ uptake in zebrafish H+-ATPase-rich cells

H(+)-ATPase-rich (HR) cells in zebrafish gills/skin were found to carry out Na+ uptake and acid-base regulation through a mechanism similar to that which occurs in mammalian proximal tubular cells. However, the roles of carbonic anhydrases (CAs) in this mechanism in zebrafish HR cells are still unclear. The present study used a functional genomic approach to identify 20 CA isoforms in zebrafish. By screening with whole mount in situ hybridization, only zca2-like a and zca15a were found to be expressed in specific groups of cells in zebrafish gills/skin, and further analyses by triple in situ hybridization and immunocytochemistry demonstrated specific colocalizations of the two zca isoforms in HR cells. Knockdown of zca2-like a caused no change in and knockdown of zca15a caused an increase in H+ activity at the apical surface of HR cells at 24 h postfertilization (hpf). Later, at 96 hpf, both the zca2-like a and zca15a morphants showed decreased H+ activity and increased Na+ uptake, with concomitant upregulation of znhe3b and downregulation of zatp6v1a (H+-ATPase A-subunit) expressions. Acclimation to both acidic and low-Na+ fresh water caused upregulation of zca15a expression but did not change the zca2-like a mRNA level in zebrafish gills. These results provide molecular physiological evidence to support the roles of these two zCA isoforms in Na+ uptake and acid-base regulation mechanisms in zebrafish HR cells.

Intestinal carbonic anhydrase, bicarbonate, and proton carriers play a role in the acclimation of rainbow trout to seawater

Abrupt transfer of rainbow trout from freshwater to 65% seawater caused transient disturbances in extracellular fluid ionic composition, but homeostasis was reestablished 48 h posttransfer. Intestinal fluid chemistry revealed early onset of drinking and slightly delayed intestinal water absorption that coincided with initiation of NaCl absorption and HCO(3)(-) secretion. Suggestive of involvement in osmoregulation, relative mRNA levels for vacuolar H(+)-ATPase (V-ATPase), Na(+)-K(+)-ATPase, Na(+)/H(+) exchanger 3 (NHE3), Na(+)-HCO(3)(-) cotransporter 1, and two carbonic anhydrase (CA) isoforms [a general cytosolic isoform trout cytoplasmic CA (tCAc) and an extracellular isoform trout membrane-bound CA type IV (tCAIV)], were increased transiently in the intestine following exposure to 65% seawater. Both tCAc and tCAIV proteins were localized to apical regions of the intestinal epithelium and exhibited elevated enzymatic activity after acclimation to 65% seawater. The V-ATPase was localized to both basolateral and apical regions and exhibited a 10-fold increase in enzymatic activity in fish acclimated to 65% seawater, suggesting a role in marine osmoregulation. The intestinal epithelium of rainbow trout acclimated to 65% seawater appears to be capable of both basolateral and apical H(+) extrusion, likely depending on osmoregulatory status and intestinal fluid chemistry.

Mechanisms of acid–base regulation in the African lungfishProtopterus annectens

African lungfish Protopterus annectens utilized both respiratory and metabolic compensation to restore arterial pH to control levels following the imposition of a metabolic acidosis or alkalosis. Acid infusion (3 mmol kg–1 NH4Cl) to lower arterial pH by 0.24 units increased both pulmonary (by 1.8-fold) and branchial (by 1.7-fold) ventilation frequencies significantly, contributing to 4.8-fold and 1.9-fold increases in,respectively, aerial and aquatic CO2 excretion. This respiratory compensation appeared to be the main mechanism behind the restoration of arterial pH, because even though net acid excretion(JnetH+) increased following acid infusion in 7 of 11 fish, the mean increase in net acid excretion, 184.5±118.5μmol H+ kg–1 h–1 (mean± s.e.m., N=11), was not significantly different from zero. Base infusion (3 mmol kg–1 NaHCO3) to increase arterial pH by 0.29 units halved branchial ventilation frequency, although pulmonary ventilation frequency was unaffected. Correspondingly, aquatic CO2 excretion also fell significantly (by 3.7-fold) while aerial CO2 excretion was unaffected. Metabolic compensation consisting of negative net acid excretion (net base excretion) accompanied this respiratory compensation, with JnetH+ decreasing from 88.5±75.6 to –337.9±199.4 μmol H+kg–1 h–1 (N=8). Partitioning of net acid excretion into renal and extra-renal (assumed to be branchial and/or cutaneous) components revealed that under control conditions, net acid excretion occurred primarily by extra-renal routes. Finally, several genes that are involved in the exchange of acid–base equivalents between the animal and its environment (carbonic anhydrase, V-type H+-ATPase and Na+/HCO –3 cotransporter) were cloned, and their branchial and renal mRNA expressions were examined prior to and following acid or base infusion. In no case was mRNA expression significantly altered by metabolic acid–base disturbance. These findings suggest that lungfish, like tetrapods, alter ventilation to compensate for metabolic acid–base disturbances, a mechanism that is not employed by water-breathing fish. Like fish and amphibians, however, extra-renal routes play a key role in metabolic compensation.

Biochemical Characterization of a S-glutathionylated Carbonic Anhydrase Isolated from Gills of the Antarctic Icefish Chionodraco hamatus

Gill cytoplasmic carbonic anhydrase of the haemoglobinless Antarctic icefish Chionodraco hamatus (Ice-CA) was directly sequenced and consists in 259 residues with an acetylated N-terminus. The molecular mass, deduced from the sequence, was 28.45 kDa, while mass spectrometry analysis of the native protein gave higher values. Treatment with dithiothreitol abolished this difference, indicating possible post-translational modifications. Isoelectric focusing analysis of Ice-CA suggested S-thiolation, which was identified as S-glutathionylation by immunostaining. Deglutathionylated Ice-CA maintained the anhydrase activity but showed higher susceptibility to hydrogen peroxide, suggesting that glutathione binding to Cys residues may have a role in the defence against oxidative damage. Ice-CA is characterized by lower thermal stability, higher activity and lower activation energy than its homologue gill CA of the temperate European eel, confirming the adaptation of the catalytic capacity to low temperatures. Alignment of Ice-CA with homologous enzymes from other fish shows high identity; the enzyme is grouped with a previously described fish CA monophyletic clade although Ice-CA shows several characteristics that can increase protein-solvent interaction and structural flexibility.

Interactions between Na+channels and Na+-HCO3−cotransporters in the freshwater fish gill MR cell: a model for transepithelial Na+uptake

Isolated mitochondria-rich (MR) cells from the rainbow trout gill epithelium were subjected to intracellular pH (pHi) imaging with the pH-sensitive dye BCECF-AM. MR cells were categorized into two distinct functional subtypes based on their ability to recover pHifrom an NH4Cl-induced acidification in the absence of Na+. An apparent link between resting pHiand Na+-independent pHirecovery was made. We observed a unique pHiacidification event that was induced by extracellular Na+addition. This further classified the mixed MR cell population into two functional subtypes: the majority of cells (77%) demonstrated the Na+-induced pHiacidification, whereas the minority (23%) demonstrated an alkalinization of pHiunder the same circumstances. The focus of this study was placed on the Na+-induced acidification and pharmacological analysis via the use of amiloride and phenamil, which revealed that Na+uptake was responsible for the intracellular acidification. Further experiments revealed that pHiacidification could be abolished when Na+was allowed entry into the cell, but the activity of an electrogenic Na+-HCO3−cotransporter (NBC) was inhibited by DIDS. The electrogenic NBC activity was supported by a DIDS-sensitive, Na+-induced membrane potential depolarization as observed via imaging of the voltage-sensitive dye bis-oxonol. We also demonstrated NBC immunoreactivity via Western blotting and immunohistochemistry in gill tissue. We propose a model for transepithelial Na+uptake occurring via an apical Na+channel linked to a basolateral, electrogenic NBC in one subpopulation of MR cells.