Compensation for hypercapnia by a euryhaline elasmobranch: effect of salinity and roles of gills and kidneys in fresh water

Evolving views of ionic, osmotic and acid-base regulation in aquatic animals

The regulation of ionic, osmotic and acid-base (IOAB) conditions in biological fluids is among the most fundamental functions in all organisms; being surrounded by water uniquely shapes the IOAB regulatory strategies of water-breathing animals. Throughout its centennial history, Journal of Experimental Biology has established itself as a premier venue for publication of comparative, environmental and evolutionary studies on IOAB regulation. This Review provides a synopsis of IOAB regulation in aquatic animals, some of the most significant research milestones in the field, and evolving views about the underlying cellular mechanisms and their evolutionary implications. It also identifies promising areas for future research and proposes ideas for enhancing the impact of aquatic IOAB research.

TEMPORAL STABILITY OF IN VITRO VENOUS BLOOD GAS, pH, AND LACTATE VALUES OF COWNOSE RAYS (RHINOPTERA BONASUS) AND RED-EARED SLIDER TURTLES (PSEUDEMYS SCRIPTA ELEGANS)

This study assessed the in vitro temporal changes that occur in blood pH and lactate concentrations for an elasmobranch species and a chelonian species, as well as blood gases (partial pressures of carbon dioxide [pCO₂] and oxygen [pO₂]) for a chelonian species, with a portable clinical point-of-care analyzer. Blood samples were collected from 10 cownose rays (Rhinoptera bonasus) and 10 red-eared sliders (Pseudemys scripta elegans), stored on ice, and serially analyzed at six time points up to 90 min postcollection. Results indicate that analysis should be conducted as soon as possible after blood collection for these species, with immediate analysis being preferred. However, if analysis must be delayed, syringes may be capped, placed on ice, and analyzed at a later time. Analysis within 90 min provided clinically acceptable results for pH and lactate in both species and for pCO₂ in red-eared sliders, whereas substantial artifactual increases of pO₂ were seen in red-eared sliders.

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

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

Expression of a novel isoform of Na(+)/H(+) exchanger 3 in the kidney and intestine of banded houndshark, Triakis scyllium

Na(+)/H(+) exchanger 3 (NHE3) provides one of the major Na(+) absorptive pathways of the intestine and kidney in mammals, and recent studies of aquatic vertebrates (teleosts and elasmobranchs) have demonstrated that NHE3 is expressed in the gill and plays important roles in ion and acid-base regulation. To understand the role of NHE3 in elasmobranch osmoregulatory organs, we analyzed renal and intestinal expressions and localizations of NHE3 in a marine elasmobranch, Japanese banded houndshark (Triakis scyllium). mRNA for Triakis NHE3 was most highly expressed in the gill, kidney, spiral intestine, and rectum. The kidney and intestine expressed a transcriptional isoform of NHE3 (NHE3k/i), which has a different amino terminus compared with that of NHE3 isolated from the gill (NHE3g), suggesting that NHE3k/i and NHE3g arise from a single gene by alternative promoter usage. Immunohistochemical analyses of the Triakis kidney demonstrated that NHE3k/i is expressed in the apical membrane of a part of the proximal and late distal tubules in the sinus zone. In the bundle zone of the kidney, NHE3k/i was expressed in the apical membrane of the early distal tubules known as the diluting segment. In the spiral intestine and rectum, NHE3k/i was localized toward the apical membrane of the epithelial cells. The transcriptional levels of NHE3k/i were increased in the kidney when Triakis was acclimated in 130% seawater, whereas those in the spiral intestine were increased in fish acclimated in diluted seawater. These results suggest that NHE3 is involved in renal Na(+) reabsorption, urine acidification, and intestinal Na(+) absorption in elasmobranchs.

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.

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.

Endothelin and endothelin converting enzyme-1 in the fish gill:evolutionary and physiological perspectives

In euryhaline fishes like the killifish (Fundulus heteroclitus)that experience daily fluctuations in environmental salinity, endothelin 1(EDN1) may be an important regulator molecule necessary to maintain ion homeostasis. The purpose of this study was to determine if EDN1 and the endothelin converting enzyme (ECE1; the enzyme necessary for cleaving the precursor proendothelin-1 to EDN1) are present in the killifish, to determine if environmental salinity regulates their expression, and to examine the phylogenetic relationships among the EDNs and among the ECEs. We sequenced killifish gill cDNA for two EDN1 orthologues, EDN1A and EDN1B, and also sequenced a portion of ECE1 cDNA. EDN1A and ECE1 mRNA are expressed ubiquitously in the killifish while EDN1B mRNA has little expression in the killifish opercular epithelium or gill. Using in situ hybridization and immunohistochemistry, EDN1 was localized to large round cells adjacent to the mitochondrion-rich cells of the killifish gill, and to lamellar pillar cells. In the gill, EDN1A and EDN1B mRNA levels did not differ with acute (<24 h) or chronic (30 days) acclimation to seawater (SW); however, EDN1B levels increased threefold post SW to freshwater (FW) transfer,and ECE1 mRNA levels significantly increased twofold over this period. ECE1 mRNA levels also increased sixfold over 24 h post FW to SW transfer. Chronic exposure to SW or FW had little effect on ECE1mRNA levels. Based upon our cellular localization studies, we modeled EDN1 expression in the fish gill and conclude that it is positioned to act as a paracrine regulator of gill functions in euryhaline fishes. It also may function as an autocrine on pillar cells, where it is hypothesized to regulate local blood flow in the lamellae. From our phylogenetic analyses, ECE is predicted to have an ancient origin and may be a generalist endoprotease in non-vertebrate organisms, while EDNs are vertebrate-specific peptides and may be key characters in vertebrate evolution.

Postprandial acid-base balance and ion regulation in freshwater and seawater-acclimated European flounder, Platichthys flesus

The effects of feeding on both acid-base and ion exchange with the environment, and internal acid-base and ion balance, in freshwater and seawater-acclimated flounder were investigated. Following voluntary feeding on a meal of 2.5-5% body mass and subsequent gastric acid secretion, no systemic alkaline tide or respiratory compensation was observed in either group. Ammonia efflux rates more than doubled from 489 +/- 35 and 555 +/- 64 mumol kg(-1) h(-1) under control conditions to 1,228 +/- 127 and 1,300 +/- 154 mumol kg(-1) h(-1) post-feeding in freshwater and seawater-acclimated fish, respectively. Based on predictions of gastric acid secreted during digestion, we calculated net postprandial internal base gains (i.e., HCO (3) (-) secreted from gastric parietal cells into the blood) of 3.4 mmol kg(-1) in seawater and 9.1 mmol kg(-1 )in freshwater-acclimated flounder. However, net fluxes of ammonia, titratable alkalinity, Na(+) and Cl(-) indicated that branchial Cl(-)/HCO (3) (-) and Na(+)/H(+) exchange played minimal roles in counteracting these predicted base gains and cannot explain the absence of alkaline tide. Instead, intestinal Cl(-)/HCO (3) (-) exchange appears to be enhanced after feeding in both freshwater and seawater flounder. This implicates the intestine rather than the gills as a potential route of postprandial base excretion in fish, to compensate for gastric acid secretion.

The putative mechanism of Na+ absorption in euryhaline elasmobranchs exists in the gills of a stenohaline marine elasmobranch, Squalus acanthias

We recently cloned an NHE3 orthologue from the gills of the euryhaline Atlantic stingray (Dasyatis sabina), and generated a stingray NHE3 antibody to unequivocally localize the exchanger to the apical side of epithelial cells that are rich with Na(+)/K(+)-ATPase (A MRC). We also demonstrated an increase in NHE3 expression when stingrays are in fresh water, suggesting that NHE3 is responsible for active Na(+) absorption. However, the vast majority of elasmobranchs are only found in marine environments. In the current study, immunohistochemistry with the stingray NHE3 antibody was used to localize the exchanger in the gills of the stenohaline marine spiny dogfish shark (Squalus acanthias). NHE3 immunoreactivity was confined to the apical side of cells with basolateral Na(+)/K(+)-ATPase and was excluded from cells with high levels of vacuolar H(+)-ATPase. Western blots detected a single protein of 88 kDa in dogfish gills, the same size as NHE3 in stingrays and mammals. These immunological data demonstrate that the putative cell type responsible for active Na(+) absorption in euryhaline elasmobranchs is also present in stenohaline marine elasmobranchs, and suggest that the inability of most elasmobranchs to survive in fresh water is not due to a lack of the gill ion transporters for Na(+) absorption.

Effect of low environmental salinity on plasma composition and renal function of the Atlantic stingray, a euryhaline elasmobranch

Marine elasmobranchs maintain internal osmolality higher than their external environment, resulting in an osmotic gradient for branchial water uptake. This gradient is markedly increased in low-salinity habitats. The subsequent increase in water uptake presents a challenge to volume homeostasis. The Atlantic stingray is a marine elasmobranch that inhabits a remarkable range of environmental salinities. We hypothesized that the ability of these stingrays to regulate fluid volume in low-salinity environments is due primarily to a renal glomerular and tubular functional reserve. We tested this hypothesis by measuring renal excretory function after a rapid and sustained 50% reduction in the osmolality of the external medium. Atlantic stingrays were maintained in harbor water [control salinity (CS) ∼850 mosmol/kgH2O] for 1 wk. Rays were then either transferred to diluted harbor water [low salinity (LS) ∼440 mosmol/kgH2O] or maintained in CS for a further 24 h. Renal excretory function was markedly higher in the rays subjected to low salinity. Glomerular filtration rate was threefold higher and urine flow rate ninefold higher in the LS group. The clearance of solute-free water was greater, and solute-free water comprised a significantly larger proportion of the urine output for the stingrays transferred to dilute harbor water. We conclude that 1) the kidneys of Atlantic stingrays have a remarkable glomerular and tubular functional reserve, and 2) the marked increase in renal function attenuates the increase in fluid volume when these fish move into low-salinity habitats.

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models

Carbon dioxide (CO(2)) excretion and acid-base regulation in fish are linked, as in other animals, though the reversible reactions of CO(2) and the acid-base equivalents H(+) and HCO(3)(-): CO(2)+H(2)O<-->H(+)+HCO(3)(-). These relationships offer two potential routes through which acid-base disturbances may be regulated. Respiratory compensation involves manipulation of ventilation so as to retain CO(2) or enhance CO(2) loss, with the concomitant readjustment of the CO(2) reaction equilibrium and the resultant changes in H(+) levels. In metabolic compensation, rates of direct H(+) and HCO(3)(-) exchange with the environment are manipulated to achieve the required regulation of pH; in this case, hydration of CO(2) yields the necessary H(+) and HCO(3)(-) for exchange. Because ventilation in fish is keyed primarily to the demands of extracting O(2) from a medium of low O(2) content, the capacity to utilize respiratory compensation of acid-base disturbances is limited and metabolic compensation across the gill is the primary mechanism for re-establishing pH balance. The contribution of branchial acid-base exchanges to pH compensation is widely recognized, but the molecular mechanisms underlying these exchanges remain unclear. The relatively recent application of molecular approaches to this question is generating data, sometimes conflicting, from which models of branchial acid-base exchange are gradually emerging. The critical importance of the gill in acid-base compensation in fish, however, has made it easy to overlook other potential contributors. Recently, attention has been focused on the role of the kidney and particularly the molecular mechanisms responsible for HCO(3)(-) reabsorption. It is becoming apparent that, at least in freshwater fish, the responses of the kidney are both flexible and essential to complement the role of the gill in metabolic compensation. Finally, while respiratory compensation in fish is usually discounted, the few studies that have thoroughly characterized ventilatory responses during acid-base disturbances in fish suggest that breathing may, in fact, be adjusted in response to pH imbalances. How this is accomplished and the role it plays in re-establishing acid-base balance are questions that remain to be answered.

Organochlorine concentrations, reproductive physiology, and immune function in unique populations of freshwater Atlantic stingrays (Dasyatis sabina) from Florida's St. Johns River

Within the past decade, reproductive and health disorders have been reported to occur in unique populations of Atlantic stingrays (Dasyatis sabina) inhabiting certain components of Florida's St. Johns River. Since these irregularities are consistent with the alleged effects of organochlorine (OC) contaminant exposure in other Florida wildlife, the goal of this study was to examine possible associations between OC concentrations and reproduction and/or immune function in stingrays from this river system. Liver concentrations of 30 OC pesticides/pesticide metabolites and total polychlorinated biphenyls (PCBs) were measured and compared in D. sabina collected from four central Florida lakes of the St. Johns River: Lake George, Lake Harney, Lake Jesup, and Lake Monroe. Reproductive biology, serum testosterone and 17beta-estradiol concentrations, and circulating white blood cell counts were examined and compared in stingrays from lakes that were determined to contain low (Lake George), intermediate (Lake Monroe), and high (Lake Jesup) levels of pesticide contamination, based on the results of liver OC assessments. Successful breeding occurred in Lake Jesup stingrays, indicating that the degree of OC accumulation in these animals is not high enough to cause reproductive impairment. However, elevated serum steroid concentrations and white blood cell counts were observed in Lake Jesup stingrays, suggesting that endocrine and immune function may be altered in these animals due to OC exposure and/or other, as yet unknown, ecological factors. Inconsistencies in the reproductive success of Lake Monroe stingrays were observed, confirming earlier reports of reproductive complications in this sub-population. Based on these findings, previous occurrences of reproductive failure in St. Johns River stingrays may be due to environmental factors other than OC exposure.

COX2 in a euryhaline teleost, Fundulus heteroclitus: primary sequence, distribution, localization, and potential function in gills during salinity acclimation

In the kidneys of mammals, cyclooxygenase type 2 (COX2) is expressed in medullary interstitial cells, the macula densa and epithelial cells of the cortical thick ascending limb where it generates prostaglandins that regulate hormone secretion, inhibit ion transport, and support cell survival during salt loading and dehydration. In teleosts, the gills are in direct contact with an aquatic environment and are the dominant site of osmoregulation. During transfers between salinities, specialized cells in the gills (chloride cells) rapidly regulate NaCl secretion for systemic osmoregulation while they simultaneously are exposed to acute osmotic shock. This study was conducted to determine if COX2 is expressed in the gills, and if so, to evaluate its function in cellular and systemic osmoregulation. Degenerate primers, reverse transcription–PCR and rapid amplification of cDNA ends were used to deduce the complete cDNA sequence of a putative COX2 enzyme from the gills of the euryhaline killifish (Fundulus heteroclitus). The 2738 base pair cDNA includes a coding region for a 610 amino acid protein that is over 70%identical to mammalian COX2. A purified antibody generated against a conserved region of mouse COX2 labeled chloride cells, suggesting that the enzyme may control NaCl secretion as an autocrine agent. Real-time PCR was then used to demonstrate that mRNA expression of the COX2 homologue was threefold greater in gills from chronic seawater killifish than in gills from chronic freshwater killifish. Expression of Na+/K+/2Cl–cotransporter and the cystic fibrosis transmembrane conductance regulator were also greater in seawater, suggesting that chronic COX2 expression in the gills is regulated in parallel to the key ion transporters that mediate NaCl secretion. Real-time PCR was also used to demonstrate that acute transfer from seawater to freshwater and from freshwater to seawater led to rapid, transient inductions of COX2 expression. Together with previous physiological evidence,the present molecular and immunological data suggest that constitutive branchial COX2 expression is enhanced in seawater, where prostaglandins can regulate NaCl secretion in chloride cells. Our data also suggest that branchial COX2 expression may play a role in cell survival during acute osmotic shock.

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

The objective of the present study was to examine the branchial distribution of the recently identified rainbow trout cytoplasmic carbonic anhydrase isoform (tCAc) and to investigate its role in the regulation of acid-base disturbances in rainbow trout (Oncorhynchus mykiss). In situ hybridization using an oligonucleotide probe specific to tCAc revealed tCAc mRNA expression in both pavement cells and mitochondria-rich cells (chloride cells). Similarly, using a homologous polyclonal antibody,tCAc immunoreactivity was localized to pavement cells and mitochondria-rich cells in the interlamellar region and along the lamellae of the gills. Exposure of rainbow trout to hypercarbia (∼0.8% CO2) for 24 h resulted in significant increases in tCAc mRNA expression (∼20-fold;quantified by real-time PCR) and protein levels (∼1.3-fold; quantified by western analysis) but not enzyme activity (assessed on crude gill homogenates using the delta-pH CA assay). Inhibition of branchial CA activity in vivo using acetazolamide reduced branchial net acid excretion significantly by 20%. This effect was enhanced to a 36% reduction in branchial net acid excretion by subjecting the trout to hypercarbia (∼0.8%CO2) for 10 h prior to acetazolamide injection, an exposure that significantly increased branchial net acid excretion. The results of the present study support the widely held premise that branchial intracellular CA activity (tCAc) plays a key role in regulating acid-base balance in freshwater teleost fish.

NHE3 in an ancestral vertebrate: primary sequence, distribution, localization, and function in gills

In mammals, the Na+/H+ exchanger 3 (NHE3) is expressed with Na+/K+-ATPase in renal proximal tubules, where it secretes H+ and absorbs Na+ to maintain blood pH and volume. In elasmobranchs (sharks, skates, and stingrays), the gills are the dominant site of pH and osmoregulation. This study was conducted to determine whether epithelial NHE homologs exist in elasmobranchs and, if so, to localize their expression in gills and determine whether their expression is altered by environmental salinity or hypercapnia. Degenerate primers and RT-PCR were used to deduce partial sequences of mammalian NHE2 and NHE3 homologs from the gills of the euryhaline Atlantic stingray (Dasyatis sabina). Real-time PCR was then used to demonstrate that mRNA expression of the NHE3 homolog increased when stingrays were transferred to low salinities but not during hypercapnia. Expression of the NHE2 homolog did not change with either treatment. Rapid amplification of cDNA was then used to deduce the complete sequence of a putative NHE3. The 2,744-base pair cDNA includes a coding region for a 2,511-amino acid protein that is 70% identical to human NHE3 (SLC9A3). Antisera generated against the carboxyl tail of the putative stingray NHE3 labeled the apical membranes of Na+/K+-ATPase-rich epithelial cells, and acclimation to freshwater caused a redistribution of labeling in the gills. This study provides the first NHE3 cloned from an elasmobranch and is the first to demonstrate an increase in gill NHE3 expression during acclimation to low salinities, suggesting that NHE3 can absorb Na+ from ion-poor environments.

A putative H+-K+-ATPase in the Atlantic stingray,Dasyatis sabina: primary sequence and expression in gills

In mammals, the gastric H+-K+-ATPase (HKα1) mediates acid secretion in the stomach and kidneys. Like mammals, elasmobranchs also secrete acid from their stomachs, but unlike mammals they primarily use their gills for systemic acid excretion instead of their kidneys. The purpose of this study was to determine if an HKα1 orthologue exists in an elasmobranch (Atlantic stingray, Dasyatis sabina), to determine if it is expressed in gills and, if so, to localize its expression and determine if its expression is regulated during hypercapnia or freshwater acclimation. A polyclonal antibody made against an HKα1 peptide detected HKα1 immunoreactivity in protein isolates and tissue sections of stingray stomachs and gills. Immunohistochemistry demonstrated that HKα1 immunoreactivity was present in a subpopulation of epithelial cells in both organs. Double-labeling experiments in the gills showed that HKα1 immunoreactivity occurred in Na+-K+-ATPase-rich cells and not in V-type H+-ATPase-rich cells. RT-PCRs were used to deduce the primary sequence of a putative H+-K+-ATPase from the stomach of Atlantic stingrays. The 3,421-base pair cDNA includes a coding region for a 1,025-amino acid protein that is over 80% identical to HKα1 of mammals. RT-PCRs were then used to demonstrate that this transcript is also expressed in the gills. To our knowledge, this is the first H+-K+-ATPase sequence reported for any elasmobranch and the first full-length sequence for any fish. We also provide the first evidence for its expression in the gills of any fish and demonstrate that its expression increased during freshwater acclimation but not exposure to hypercapnia.

Osmotic, sodium, carbon dioxide and acid-base state of the Port Jackson shark, Heterodontus portusjacksoni, in response to lowered salinity

In marine elasmobranch fish the consequences for CO2 and acid-base state of moving into low salinity water are not well described. Sub-adult Port Jackson sharks, Heterodontus portusjacksoni, occasionally enter brackish water and survive in 50% seawater (SW). The unidirectional Na efflux and content, plasma volume, glomerular filtration rate (GFR), body mass, as well as CO2 and acid-base state in H. portusjacksoni were investigated following transfer from 100% SW to 75% SW and then to 50% SW. A rapid water influx resulted in a doubling of the plasma volume within 24 h in sharks in 75% SW and an 11% increase in body weight. Osmotic water influx was only partially offset by a doubling of the GFR. There was a approximately 40% decrease in plasma [Na] through a transiently elevated Na clearance and haemodilution. The result was a decrease in the inward gradient for Na+ together with reductions of nearly 50% in CO2 and buffer capacity. The sharks remained hypo-natric to 50% SW by partially conforming to the decrease in external osmotic pressure and avoided the need for active Na+ uptake. The gradient for Na+ efflux would by extrapolation approach zero at approximately 27% SW which may of itself prove a lethal internal dilution. In sharks transferred to 75% SW, a small transient hypercapnia and a later temporary metabolic alkalosis were all largely explained through anaemia promoting loss of CO2 and buffer capacity. In sharks transferred to 50% SW the metabolic alkalosis persisted until the end of the 1-week trial. Within the erythrocytes, increased pH was consequent on the large decrease in haemoglobin content exhibited by the sharks, which caused a large reduction in intracellular buffer. In water as dilute as 50% SW there was no evidence of specific effects on the mechanisms of management of CO2 or H+ excretion but rather significant and indirect effects of the severe haemodilution.