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

Time domains of the hypoxic ventilatory response in ectothermic vertebrates

Over a decade has passed since Powell et al. (Respir Physiol 112:123-134, 1998) described and defined the time domains of the hypoxic ventilatory response (HVR) in adult mammals. These time domains, however, have yet to receive much attention in other vertebrate groups. The initial, acute HVR of fish, amphibians and reptiles serves to minimize the imbalance between oxygen supply and demand. If the hypoxia is sustained, a suite of secondary adjustments occur giving rise to a more long-term balance (acclimatization) that allows the behaviors of normal life. These secondary responses can change over time as a function of the nature of the stimulus (the pattern and intensity of the hypoxic exposure). To add to the complexity of this process, hypoxia can also lead to metabolic suppression (the hypoxic metabolic response) and the magnitude of this is also time dependent. Unlike the original review of Powell et al. (Respir Physiol 112:123-134, 1998) that only considered the HVR in adult animals, we also consider relevant developmental time points where information is available. Finally, in amphibians and reptiles with incompletely divided hearts the magnitude of the ventilatory response will be modulated by hypoxia-induced changes in intra-cardiac shunting that also improve the match between O(2) supply and demand, and these too change in a time-dependent fashion. While the current literature on this topic is reviewed here, it is noted that this area has received little attention. We attempt to redefine time domains in a more 'holistic' fashion that better accommodates research on ectotherms. If we are to distinguish between the genetic, developmental and environmental influences underlying the various ventilatory responses to hypoxia, however, we must design future experiments with time domains in mind.

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.

Lack of arterial PO2 downregulation in Atlantic salmon (Salmo salar L.) during long-term normoxia and hyperoxia

Regulation of arterial partial pressure of O2 (PaO2) in Atlantic salmon (Salmo salar) was investigated during resting conditions in normoxic and hyperoxic water. Dorsal aorta cannulated adult Atlantic salmon (1.2-1.6 kg, n=8) were exposed to 2 week sequential periods of normoxia [16.7±1.1 kPa (mean±SD)] and hyperoxia (34.1±4.9 kPa) in individual tanks containing seawater (33.7±0.2 ppt) at stable temperature conditions (8.7±0.7°C) and a light regime of L:D=12:12. Tank design and sampling procedures were optimized to provide suitable shelter and current for the fish, and to allow repeated, undisturbed sampling of blood from free-swimming fish. Fish were sampled regularly through the experimental period. PwO2, PaO2, blood ion composition (Na+, K+, Cl-), acid-base status (pH, PCO2, HCO3-), haematocrit and glucose were measured. The most frequently observed PaO2 values were in the range of 60-80% of PwO2, both during normoxia and hyperoxia, and PaO2 values were significantly lower during normoxia than during hyperoxia. Blood pH, PCO2 and HCO3- were significantly elevated during hyperoxia, while, Na+, Cl- and Hct were significantly lower. K+ and glucose showed no significant differences. This study demonstrates a lack PaO2 regulation in Atlantic salmon to low partial pressures, in contrast to previous reports for many aquatic gill breathing animals. Both during normoxia and hyperoxia, PaO2 reflects PwO2, and alterations in external PO2 consequently result in proportional arterial PO2 changes. Physiological adaptation to hyperoxia, as illustrated by changes in several blood parameters, does not include down-regulation of PaO2 in Atlantic salmon. The lack of PaO2 regulation may make Atlantic salmon vulnerable to the oxidative stress caused by increased free radical formation in hyperoxic conditions.

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

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

Impact of ocean acidification on energy metabolism of oyster, Crassostrea gigas--changes in metabolic pathways and thermal response

Climate change with increasing temperature and ocean acidification (OA) poses risks for marine ecosystems. According to Pörtner and Farrell, synergistic effects of elevated temperature and CO₂-induced OA on energy metabolism will narrow the thermal tolerance window of marine ectothermal animals. To test this hypothesis, we investigated the effect of an acute temperature rise on energy metabolism of the oyster, Crassostrea gigas chronically exposed to elevated CO₂ levels (partial pressure of CO₂ in the seawater ~0.15 kPa, seawater pH ~ 7.7). Within one month of incubation at elevated PCo₂ and 15 °C hemolymph pH fell (pH(e) = 7.1 ± 0.2 (CO₂-group) vs. 7.6 ± 0.1 (control)) and P(e)CO₂ values in hemolymph increased (0.5 ± 0.2 kPa (CO₂-group) vs. 0.2 ± 0.04 kPa (control)). Slightly but significantly elevated bicarbonate concentrations in the hemolymph of CO₂-incubated oysters ([HCO₃⁻](e) = 1.8 ± 0.3 mM (CO₂-group) vs. 1.3 ± 0.1 mM (control)) indicate only minimal regulation of extracellular acid-base status. At the acclimation temperature of 15 °C the OA-induced decrease in pH(e) did not lead to metabolic depression in oysters as standard metabolism rates (SMR) of CO₂-exposed oysters were similar to controls. Upon acute warming SMR rose in both groups, but displayed a stronger increase in the CO₂-incubated group. Investigation in isolated gill cells revealed a similar temperature dependence of respiration between groups. Furthermore, the fraction of cellular energy demand for ion regulation via Na+/K+-ATPase was not affected by chronic hypercapnia or temperature. Metabolic profiling using ¹H-NMR spectroscopy revealed substantial changes in some tissues following OA exposure at 15 °C. In mantle tissue alanine and ATP levels decreased significantly whereas an increase in succinate levels was observed in gill tissue. These findings suggest shifts in metabolic pathways following OA-exposure. Our study confirms that OA affects energy metabolism in oysters and suggests that climate change may affect populations of sessile coastal invertebrates such as mollusks.

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.

The phylogeny of central chemoreception

Respiratory chemoreceptors responsive to changes in CO(2)/H(+) appear to be present in all vertebrates from fish to birds and mammals. They appear to have arisen first in the periphery sensitive to the external environment. Thus, in most fish CO(2)/H(+) chemoreceptors reside primarily in the gills and respond to changes in aquatic rather than arterial P(CO)₂ . In the air-breathing tetrapods (amphibians, mammals, reptiles and birds), the branchial arches regress developmentally and the derivatives of the branchial arteries are now exclusively internal. The receptors associated with these arteries now sense only arterial (not environmental) P(CO)₂/pH . Central CO(2)/H(+) chemoreception also appears to have arisen with the advent of air breathing, presumably as a second line of defense. These receptors may have arisen multiple times in association with several (but not all) of the independent origins of air breathing in fishes. There is strong evidence for multiple central sites of CO(2)/H(+) sensing, at least in amphibians and mammals, suggesting that it may not only have originated multiple times in different species but also multiple times within a single species.

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.

Localization of ion-regulatory epithelia in embryos and hatchlings of two cephalopods

The tissue distribution and ontogeny of Na(+)/K(+)-ATPase has been examined as an indicator for ion-regulatory epithelia in whole animal sections of embryos and hatchlings of two cephalopod species: the squid Loligo vulgaris and the cuttlefish Sepia officinalis. This is the first report of the immunohistochemical localization of cephalopod Na(+)/K(+)-ATPase with the polyclonal antibody alpha (H-300) raised against the human alpha1-subunit of Na(+)/K(+)-ATPase. Na(+)/K(+)-ATPase immunoreactivity was observed in several tissues (gills, pancreatic appendages, nerves), exclusively located in baso-lateral membranes lining blood sinuses. Furthermore, large single cells in the gill of adult L. vulgaris specimens closely resembled Na(+)/K(+)-ATPase-rich cells described in fish. Immunohistochemical observations indicated that the amount and distribution of Na(+)/K(+)-ATPase in late cuttlefish embryos was similar to that found in juvenile and adult stages. The ion-regulatory epithelia (e.g., gills, excretory organs) of the squid embryos and paralarvae exhibited less differentiation than adults. Na(+)/K(+)-ATPase activities for whole animals were higher in hatchlings of S. officinalis (157.0 +/- 32.4 micromol g (FM) (-1) h(-1)) than in those of L. vulgaris (31.8 +/- 3.3 micromol g (FM) (-1) h(-1)). S. officinalis gills and pancreatic appendages achieved activities of 94.8 +/- 18.5 and 421.8 +/- 102.3 micromol(ATP) g (FM) (-1) h(-1), respectively. High concentrations of Na(+)/K(+)-ATPase in late cephalopod embryos might be important in coping with the challenging abiotic conditions (low pH, high pCO(2)) that these organisms encounter inside their eggs. Our results also suggest a higher sensitivity of squid vs. cuttlefish embryos to environmental acid-base disturbances.

The involvement of SLC26 anion transporters in chloride uptake in zebrafish (Danio rerio) larvae

After demonstrating phylogenetic relatedness to orthologous mammalian genes, tools were developed to investigate the roles of three members (A3, A4 and A6c) of the SLC26 anion exchange gene family in Cl- uptake and HCO3 excretion in embryos and larvae of zebrafish (Danio rerio). Whole-mount in situ hybridization revealed the presence of SLC26 mRNA in gill primordia, mesonephros and heart (slc26a3 and a4 only) at 5-9 days postfertilization (d.p.f.). SLC26A3 protein was highly expressed in lateral line neuromasts and within the gill, was localized to a sub-population of epithelial cells, which often (but not always) coexpressed Na+/K+-ATPase. SLC26 mRNA levels increased with developmental age, peaking at 5-10 d.p.f.; the largest increases in rates of Cl- uptake (JinCl-) preceded the mRNA spike, occurring at 2-5 d.p.f. Raising zebrafish in water with a low [Cl-] caused marked increases in JinCl- at 3-10 d.p.f. and was associated with increased levels of SLC26 mRNA. Raising fish in water of high [Cl-] was without effect on JinCl- or SLC26 transcript abundance. Selective gene knockdown using morpholino antisense oligonucleotides demonstrated a significant role for SLC26A3 in Cl- uptake in larval fish raised in control water and roles for A3, A4 and A6c in fish raised in water with low [Cl-]. Prolonged (7 days) or acute (24 h) exposure of fish to elevated (2 or 5 mmol l(-1)) ambient [HCO3-] caused marked increases in Cl- uptake when determined in water of normal [HCO3-] that were accompanied by elevated levels of SLC26 mRNA. The increases in JinCl- associated with high ambient [HCO3-] were not observed in the SLC26 morphants (significant only at 5 mmol l(-1) HCO3- for A4 and 2 mmol l(-1) HCO3- for A6c). Net base excretion was markedly inhibited in the slc26a3 and a6c morphants thereby implicating these genes in Cl-/HCO3- exchange. The results suggest that under normal conditions, Cl- uptake in zebrafish larvae is mediated by SLC26A3 Cl-/HCO3- exchangers but under conditions necessitating higher rates of high affinity Cl- uptake, SlC26A4 and SLC26A6c may assume a greater role.

Bicarbonate-sensing soluble adenylyl cyclase is an essential sensor for acid/base homeostasis

pH homeostasis is essential for life, yet it remains unclear how animals sense their systemic acid/base (A/B) status. Soluble adenylyl cyclase (sAC) is an evolutionary conserved signaling enzyme that produces the second messenger cAMP in response to bicarbonate ions (HCO(3)(-)). We cloned the sAC ortholog from the dogfish, a shark that regulates blood A/B by absorbing and secreting protons (H(+)) and HCO(3)(-) at its gills. Similar to mammalian sAC, dogfish soluble adenylyl cyclase (dfsAC) is activated by HCO(3)(-) and can be inhibited by two structurally and mechanistically distinct small molecule inhibitors. dfsAC is expressed in the gill epithelium, where the subset of base-secreting cells resides. Injection of inhibitors into animals under alkaline stress confirmed that dfsAC is essential for maintaining systemic pH and HCO(3)(-) levels in the whole organism. One of the downstream effects of dfsAC is to promote the insertion of vacuolar proton pumps into the basolateral membrane to absorb H(+) into the blood. sAC orthologs are present throughout metazoans, and mammalian sAC is expressed in A/B regulatory organs, suggesting that systemic A/B sensing via sAC is widespread in the animal kingdom.

Ion uptake and acid secretion in zebrafish (Danio rerio)

Transepithelial transport is one of the major processes involved in the mechanism of homeostasis of body fluids in vertebrates including fish. The current models of ion regulation in fish gill ionocytes have been proposed mainly based on studies in traditional model species like salmon, trout, tilapia, eel and killifish, but the mechanisms are still being debated due to the lack of convincing molecular physiological evidence. Taking advantage of plentiful genetic databases for zebrafish, we studied the molecular/cellular mechanisms of ion regulation in fish skin/gills. In our recently proposed model, there are at least three subtypes of ionocytes in zebrafish skin/gills: Na(+)-K(+)-ATPase-rich (NaR), Na(+)-Cl(-) cotransporter (NCC) and H(+)-ATPase-rich (HR) cells. Specific isoforms of transporters and enzymes have been identified as being expressed by these ionocytes: zECaC, zPMCA2 and zNCX1b by NaR cells; zNCC gill form by NCC cells; and zH(+)-ATPase, zNHE3b, zCA2-like a and zCA15a by HR cells. Serial molecular physiological experiments demonstrated the distinct roles of these ionocytes in the transport of various ions: HR, NaR and NCC cells are respectively responsible for acid secretion/Na(+) uptake, Ca(2+) uptake and Cl(-) uptake. The expression, regulation and function of transporters in HR and NaR cells are much better understood than those in NCC cells. The basolateral transport pathways in HR and NCC cells are still unclear, and the driving forces for the operations of apical NHE and NCC are another unresolved issue. Studies on zebrafish skin/gill ionocytes are providing new insights into fish ion-regulatory mechanisms, but the zebrafish model cannot simply be applied to other species because of species differences and a lack of sufficient molecular physiological evidence in other species.

Evidence that SLC26 anion transporters mediate branchial chloride uptake in adult zebrafish (Danio rerio)

Experiments were performed to test the hypothesis that three members of the SLC26 anion transporter gene family (SLC26a3, A4, and A6; hereafter termed za3, za4, and za6) mediate branchial Cl(-)/HCO(3)(-) exchange in adult zebrafish (Danio rerio). Real-time RT-PCR demonstrated that the gill expressed relatively high levels of za6 mRNA; za3 and za4 mRNA, while present, were less abundant. Also, za4 and za6 were expressed at relatively high levels in the kidney. The results of in situ hybridization or immunocytochemistry (za3 only) experiments performed on gill sections revealed that the SLC26 transporters were predominantly expressed on the filament epithelium (especially within the interlamellar regions) and to a lesser extent on the lamellar epithelium at the base of lamellae. This distribution pattern suggests that the SLC26 anion transporters are localized to mitochondrion-rich cells (ionocytes). Transferring fish to water containing low [Cl(-)] (0.02 mmol/l) resulted in significant increases in branchial SLC26 mRNA expression after 5-10 days of exposure relative to fish raised in normal water [Cl(-)] (0.4 mmol/l); transferring fish to Cl(-)-enriched water (2.0 mmol/l) was without effect on mRNA levels. Transferring fish to water containing elevated levels of NaHCO(3) (10-12.5 mmol/l) caused marked increases in branchial SLC26 mRNA expression between 3 and 10 days of transfer that was associated with a significant 40% increase in Cl(-) uptake (as measured upon return to normal water after 7 days). A decrease in whole body net acid excretion (equivalent to an increase in net base excretion) in fish previously maintained in high [NaHCO(3)] water, concurrent with increases in Cl(-) uptake and SLC26 mRNA levels, suggests a role for these anion transporters in Cl(-) uptake and acid-base regulation owing to their Cl(-)/HCO(3)(-) exchange activities.

Ammonia as a stimulant to ventilation in rainbow trout Oncorhynchus mykiss

Ammonia is the third most important respiratory gas in ammoniotelic fish after oxygen and carbon dioxide. We here investigated the effects of elevated plasma ammonia on ventilation in freshwater rainbow trout. Intact trout fitted with indwelling dorsal aortic catheters were given injections (over 5 min) of Cortland saline, isotonic high ammonia solutions (NH(4)HCO(3), (NH(4))(2)SO(4), NH(4)OH at pH 8.0, and NH(4)OH at pH 9.0), and other solutions as controls for acid-base effects, while ventilatory rate (VR) and buccal pressure amplitude (DeltaP(buccal)) were recorded. All high ammonia solutions resulted in immediate elevations of plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a), and increases in ventilatory DeltaP(buccal) and VR to different degrees. However, while Pa(O2) remained constant, in every case there was a confounding change in one or more components of acid-base status (decreases in pH(a) or increases in [HCO(3)(-)](a) or Pa(CO2) in different treatments), although the ventilatory responses to ammonia injections were generally larger than could be explained by changes in acid-base status. Therefore a series was performed in which normal blood perfusion of the gills was replaced by ventral aortic perfusion with either Cortland saline or Cortland saline plus high ammonia in which pH, [HCO(3)(-)], P(CO2), and P(O2) remained unchanged. Although ventilation was depressed in these anaesthetized, spontaneously ventilating preparations, perfusion with high ammonia saline increased DeltaP(buccal). In a final series, trout were infused for 24h with Cortland saline, isotonic NH(4)HCO(3), or isotonic (NH(4))(2)SO(4) solutions. The two ammonia solutions both caused persistent elevations in VR and DeltaP(buccal), together with similar large increases in plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a). As there was no changes in Pa(O2), pH(a), Pa(CO2), or [HCO(3)(-)](a) in the (NH(4))(2)SO(4) infusion series, this, together with the ventral aortic perfusion experiment, provides the most convincing evidence that ammonia stimulates ventilation. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O(2) uptake, and propose that ammonia-induced hyperventilation may also facilitate ammonia excretion in rainbow trout.

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.

The effects of thermally induced gill remodeling on ionocyte distribution and branchial chloride fluxes in goldfish (Carassius auratus)

Experiments were performed to evaluate the effects of temperature-induced changes in functional gill lamellar surface area on the distribution of ionocytes and branchial chloride fluxes in goldfish (Carassius auratus). In fish acclimated to warm water (25 degrees C), the ionocytes were scattered along the lamellae and within the interlamellar regions of the filament. In cold water (7 degrees C), the ionocytes were largely absent from the lamellae and filaments but instead were mostly confined to the outer regions of an interlamellar cell mass (ILCM) that formed within the interlamellar channels. Using a ;time-differential double fluorescent staining' technique, it was determined that in fish transferred from 25 degrees to 7 degrees C, the ionocytes on the outer edge of (and within) the ILCM originated predominantly from the migration of pre-existing ionocytes and to a lesser extent from the differentiation of progenitor cells. Despite the greater functional lamellar surface area in the warm-water-acclimated fish, there was no associated statistically significant increase in passive branchial Cl(-) efflux. Because the paracellular efflux of polyethylene glycol was increased 2.5-fold at the warmer temperature, it would suggest that goldfish specifically regulate (minimize) Cl(-) loss that otherwise would accompany the increasing functional lamellar surface area. In contrast to predictions, the numbers and sizes of individual ionocytes was inversely related to functional lamellar surface area resulting in a markedly greater ionocyte surface area in fish acclimated to cold water (5219+/-438 compared with 2103+/-180 microm(2) mm(-1) of filament). Paradoxically, the activity of Na(+)/K(+)-ATPase (as measured at room temperature) also was lower in the cold-water fish (0.43+/-0.06 compared with 1.28+/-0.15 micromol mg(-1) protein h(-1)) despite the greater numbers of ionocytes. There were no statistically significant differences in the rates of Cl(-) uptake in the two groups of fish despite the differences in ionocyte abundance. It is possible that to maintain normal rates of Cl(-) uptake, a greater ionocyte surface area is required in the cold-water fish that possess an ILCM because of the unfavorable positioning of the ionocytes on and within the ILCM, a structure lacking any obvious blood supply.

Swimming performance in Atlantic Cod (Gadus morhua) following long-term (4-12 months) acclimation to elevated seawater P(CO2)

Anthropogenic CO2 emissions lead to chronically elevated seawater CO2 partial pressures (hypercapnia). The induced ocean acidification will very likely be a relevant factor shaping future marine environments. CO2 exposure concomitantly challenges the animal's capacity of acid-base and ionic regulation as well as the ability to maintain energy metabolism and calcification. Under conditions of acute hypercapnia, numerous studies have revealed a broad range of tolerance levels displayed by various marine taxa. Thus, it is well known that, in contrast to many marine invertebrates, most teleost fish are able to fully compensate acid-base disturbances in short-term experiments (hours to several days). In order to determine whether marine fish are able to preserve aerobic scope following long-term incubation to elevated CO2, we exposed two groups of Atlantic Cod for 4 and 12 months to 0.3 and 0.6 kPa P(CO2), respectively. Measurements of standard and active metabolic rates, critical swimming speeds and aerobic scope of long-term incubated cod showed no deviations from control values, indicating that locomotory performance is not compromised by the different levels of chronic hypercapnia. While the maintenance of high activity levels is supported by a 2-fold increased Na+/K+-ATPase protein expression and 2-fold elevated Na+/K+-ATPase activity in the 12 month incubated fish (0.6 kPa P(CO2)), no such elevation in Na+/K+-ATPase activity could be observed in the group treated with 0.3 kPa P(CO2). Owing to the relevance of Na+/K+-ATPase as a general indicator for ion regulatory capacity, these results point at an adjustment of enzymatic activity to cope with the CO2 induced acid-base load at 0.6 kPa P(CO2) while under milder hypercapnic conditions the 'standard' Na+/K+-ATPase capacity might still be sufficient to maintain acid-base status.

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.

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.

Functional regulation of H+-ATPase-rich cells in zebrafish embryos acclimated to an acidic environment

It is important to maintain internal pH homeostasis in biological systems. In our previous studies, H(+)-ATPase-rich (HR) cells were found to be responsible for proton secretion in the skin of zebrafish embryos during development. In this study, zebrafish embryos were exposed to acidic and basic waters to investigate the regulation of HR cell acid secretion during pH disturbances. Our results showed that the function of HR cells on the skin of zebrafish embryos can be upregulated in pH 4 water not only by increasing the cell number but also by enlarging the acid-secreting function of single cells. We also identified an "alveolar-type" apical opening under scanning electron microscopy observations of the apical membrane of HR cells, and the density and size of the alveolar type of apical openings were also increased in pH 4 water. p63 and PCNA immunostaining results also showed that additional HR cells in pH 4 water may be differentiated not only from ionocyte precursor cells but also newly proliferating epithelial stem cells.

The transcription factor, glial cell missing 2, is involved in differentiation and functional regulation of H+-ATPase-rich cells in zebrafish (Danio rerio)

H(+)-ATPase-rich (HR) cells in zebrafish are known to be involved in acid secretion and Na(+) uptake mechanisms in zebrafish gills/skin; however, little is known about how HR cells are functionally regulated. In the present work, we studied the roles of Drosophila glial cell missing (gcm), a cell fate-related transcription factor, in the differentiation and functional regulation of zebrafish HR cells. Zebrafish gcm2 (zgcm2) was found to begin expression in zebrafish embryos at 10 h postfertilization (hpf), and to be extensively expressed in gills but only mildly so in eyes, heart, muscles, and testes. By whole mount in situ hybridization, zgcm2 mRNA signals were found in a group of cells on the zebrafish yolk sac surface initially in the tail bud stage (10 hpf); they had disappeared at 36 hpf and thereafter appeared again in the gill region from 48 hpf. Double fluorescence in situ hybridization further demonstrated specific colocalization of zgcm2 mRNA in HR cells in zebrafish embryos. Knockdown of zgcm2 with a specific morpholino oligonucleotide caused the complete disappearance of HR cells with a concomitant decrease in H(+) activity at the apical surface of HR cells, but it did not affect the occurrence of Na(+)-K(+)-ATPase-rich cells. A decrease in the H(+)-ATPase subunit A (zatp6v1a) expression and no change in zgcm2 expression in zebrafish gills were seen from 12 h to 3 days after transfer to acidic fresh water, but a compensatory stimulation in the expressions of both genes appeared 4 days post-transfer. In conclusion, functional regulation of HR cells is probably achieved by enhancing cell differentiation via zGCM2 activation.

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.

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.

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

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

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

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

Post-prandial alkaline tide in freshwater rainbow trout: effects of meal anticipation on recovery from acid–base and ion regulatory disturbances

The post-feeding alkaline tide (elevated blood pH and HCO3–) has been well characterised in air-breathing animals, but to date this phenomenon has only been demonstrated in one piscine species, a marine elasmobranch. We have investigated the acid–base and ion regulatory responses of a freshwater teleost to voluntary feeding as well as to involuntary filling of the stomach via an indwelling gastric intubation tube. One group of rainbow trout(Oncorhynchus mykiss) were fed a 1% body mass ration of homogenised food via the gastric intubation tube. Another group fed voluntarily on a 1% body mass ration. Blood samples were taken via dorsal aortic catheters from fish in both groups before feeding and over the subsequent 72 h. Trout fed via the gastric intubation tube exhibited post-prandial metabolic alkalosis of the blood (pH and plasma HCO3– increases of up to ∼0.2 pH units and 3 mmol l–1, respectively), that was more than twofold greater than the voluntary feeding fish, and took three times as long to recover (72 versus 24 h). Arterial PCO2 was unchanged in both groups indicating that freshwater trout do not retain CO2 to compensate for a post-prandial alkaline tide. Although excretion of HCO3– to the water increased post-prandially, NH4+ excretion followed a similar pattern, such that net acid equivalent fluxes were unaffected. Thus, sites other than the gills or kidney must be responsible for recovery of blood acid–base status, with intestinal HCO3–secretion being a likely candidate. In addition, fish fed via the gastric intubation tube experienced a large (17 mmol l–1) but acute (6 h) drop in plasma chloride and a very large (53%) and long lasting decline in plasma magnesium concentration, that were absent in voluntarily feeding fish. These results further indicate a potentially important role for neuro-endocrine mediated mechanisms when fish feed voluntarily, in promoting the earlier initiation of compensatory responses that regulate blood ion levels and acid–base status. This aspect should also be considered when interpreting studies on other aspects of post-prandial physiology, where force feeding by gavage is commonly used in preference to voluntary feeding.

Renal expression and localization of SLC9A3 sodium/hydrogen exchanger and its possible role in acid-base regulation in freshwater rainbow trout (Oncorhynchus mykiss)

Experiments were performed to assess the possible involvement of the Na(+)/H(+) exchanger isoform 3 (NHE3; SLC9A3) in renal acid-base regulation in adult rainbow trout (Oncorhynchus mykiss). NHE3 mRNA was expressed at high levels in the kidney relative to its paralog, NHE2. The results of in situ hybridization demonstrated an abundance of NHE3 mRNA in renal tubules. The combination of immunocytochemistry and histological staining revealed that NHE3 was confined to the apical membrane of proximal tubules, where it was colocalized with the vacuolar-type H(+)-ATPase. Levels of NHE3 protein (assessed by Western blotting) were increased during hypercapnia, likely as a result of increased transcription, as indicated by increasing levels of NHE3 mRNA (as determined by real-time PCR). Plasma cortisol concentration was increased during hypercapnia, and administration of exogenous cortisol caused a marked increase in NHE3 mRNA and protein. Thus we speculate that the elevation of plasma cortisol during hypercapnia contributes to transcriptional activation of NHE3 that ultimately promotes acid-base regulation by stimulating H(+) secretion and HCO(3)(-) reabsorption.

Regulation of ion transport by pH and [HCO3−] in isolated gills of the crabNeohelice(Chasmagnathus)granulata

Posterior isolated gills of Neohelice ( Chasmagnathus) granulatus were symmetrically perfused with hemolymph-like saline of varying [HCO3−] and pH. Elevating [HCO3−] in the saline from 2.5 to 12.5 mmol/l (pH 7.75 in both cases) induced a significant increase in the transepithelial potential difference ( Vte), a measure of ion transport. The elevation in [HCO3−] also induced a switch from acid secretion (−43.7 ± 22.5 μequiv·kg−1·h−1) in controls to base secretion (84.7 ± 14.4 μequiv·kg−1·h−1). The HCO3−-induced Vteincrease was inhibited by basolateral acetazolamide (200 μmol/l), amiloride (1 mmol/l), and ouabain (5 mmol/l) but not by bafilomycin (100 nmol/l). The Vteresponse to HCO3−did not take place in Cl−-free conditions; however, it was unaffected by apical SITS (2 mmol/l) or DIDS (1 mmol/l). A decrease in pH from 7.75 to 7.45 pH units in the perfusate also induced a significant increase in Vte, which was matched by a net increase in acid secretion of 67.8 ± 18.4 μequiv kg−1h−1. This stimulation was sensitive to basolateral acetazolamide, bafilomycin, DIDS, and Na+-free conditions, but it still took place in Cl−-free saline. Therefore, the cellular response to low pH is different from the HCO3−-stimulated response. We also report V-H+-ATPase- and Na+-K+-ATPase-like immunoreactivity in gill sections for the first time in this crab. Our results suggest that carbonic anhydrase (CA), basolateral Na+/H+exchangers and Na+-K+-ATPase and apical anion exchangers participate in the HCO3−-stimulated response, while CA, apical V-H+-ATPase and basolateral HCO3−-dependent cotransporters mediate the response to low pH.

Molecular detection and immunological localization of gill Na+/H+ exchanger in the dogfish (Squalus acanthias)

The dogfish (Squalus acanthias) can make rapid adjustments to gill acid-base transfers to compensate for internal acidosis/alkalosis. Branchial Na+/H+ exchange (NHE) has been postulated as one mechanism driving the excretion of H+ following acidosis. We have cloned gill cDNA that includes an open reading frame coding for a 770-residue protein most homologous (approximately 71%) to mammalian NHE2. RT-PCR revealed NHE2 transcripts predominantly in gill, stomach, rectal gland, intestine, and kidney. In situ hybridization with an antisense probe against NHE2 in gill sections revealed a strong mRNA signal from a subset of interlamellar and lamellae cells. We developed dogfish-specific polyclonal antibodies against NHE2 that detected a approximately 70-kDa protein in Western blots and immunologically recognized branchial cells having two patterns of protein expression. Cytoplasmic and apical NHE2 immunoreactivity were observed in cells coexpressing basolateral Na+-K+-ATPase. Other large ovoid cells more generally staining for NHE2 also were strongly positive for basolateral H+-ATPase. Gill mRNA levels for NHE2 and H+-ATPase did not change following systemic acidosis (as measured by quantitative PCR 2 h after a 1- or 2-meq/kg acid infusion). These data indicate that posttranslational adjustments of NHE2 and other transport systems (e.g., NHE3) following acidosis may be of importance in the short-term pH adjustment and net branchial H+ efflux observed in vivo. NHE2 may play multiple roles in the gills, involved with H+ efflux from acid-secreting cells, basolateral H+ reabsorption for pHi regulation, and in parallel with H+-ATPase for the generation of HCO3(-) in base-secreting cells.

The functional ontogeny of the teleost gill: Which comes first, gas or ion exchange?

For most of the last century, the need to obtain sufficient oxygen to meet the respiratory requirements of the tissues was viewed as the primary selective pressure driving gill development in teleost fish. Recently, however, it has been suggested that ionoregulatory pressures may actually be more important. This manuscript reviews the theoretical and empirical evidence dealing with the functional ontogeny of the gill in the context of the oxygen and ionoregulatory hypotheses. Gas and ion exchange are subject to similar geometric constraints in developing fish. Both initially are exclusively cutaneous but shift to the gill with tissue growth because of declining surface-to-volume ratios. Based on the appearance of mitochondria-rich cells (MRCs), ionoregulatory activity shifts to the gill in advance of gas exchange. In every species examined to date, MRCs appear on the developing gill in advance of secondary lamellae, the definitive gas exchange structure of the adult gill. Biochemical and histochemical studies indicate that these early branchial MRCs are actively involved in ion exchange. In some cases, the specific activity is many times greater than in the adult gill. In contrast, O2 microelectrode and hemoglobin ablation experiments suggest that the early gill contributes little O2 to the general systemic circulation. Any oxygen taken up appears to be consumed locally. Functional ablation experiments with zebrafish indicated that the larval gill became essential for ion balance well before it was needed for O2 uptake. Similar experiments with rainbow trout, however, found that the gill became essential in terms of gas and ion exchange at about the same time. On balance, the evidence appears to favour the ionoregulatory hypothesis but the oxygen hypothesis cannot be absolutely rejected without more information. Some of the major deficiencies in our knowledge regarding the transition from cutaneous to branchial gas and ion exchange are highlighted and potential implications of the ionoregulatory hypothesis are discussed.

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.

V-H+ -ATPase translocation during blood alkalosis in dogfish gills: interaction with carbonic anhydrase and involvement in the postfeeding alkaline tide

We investigated the involvement of carbonic anhydrase (CA) in mediating V-H(+)-ATPase translocation into the basolateral membrane in gills of alkalotic Squalus acanthias. Immunolabeling revealed that CA is localized in the same cells as V-H(+)-ATPase. Blood plasma from dogfish injected with acetazolamide [30 mg/kg at time (t) = 0 and 6 h] and infused with NaHCO(3) for 12 h (1,000 microeq.kg(-1).h(-1)) had significantly higher plasma HCO(3)(-) concentration than fish that were infused with NaHCO(3) alone (28.72 +/- 0.41 vs. 6.57 +/- 2.47 mmol/l, n = 3), whereas blood pH was similar in both treatments (8.03 +/- 0.11 vs. 8.04 +/- 0.11 pH units at t = 12 h). CA inhibition impaired V-H(+)-ATPase translocation into the basolateral membrane, as estimated from immunolabeled gill sections and Western blotting on gill cell membranes (0.24 +/- 0.08 vs. 1.00 +/- 0.28 arbitrary units, n = 3; P < 0.05). We investigated V-H(+)-ATPase translocation during a postfeeding alkalosis ("alkaline tide"). Gill samples were taken 24-26 h after dogfish were fed to satiety in a natural-like feeding regime. Immunolabeled gill sections revealed that V-H(+)-ATPase translocated to the basolateral membrane in the postfed fish. Confirming this result, V-H(+)-ATPase abundance was twofold higher in gill cell membranes of the postfed fish than in fasted fish (n = 4-5; P < 0.05). These results indicate that 1) intracellular H(+) or HCO(3)(-) produced by CA (and not blood pH or HCO(3)(-)) is likely the stimulus that triggers the V-H(+)-ATPase translocation into the basolateral membrane in alkalotic fish and 2) V-H(+)-ATPase translocation is important for enhanced HCO(3)(-) secretion during a naturally occurring postfeeding alkalosis.

The novel correlation of carbonic anhydrase II and anion exchanger 1 in gills of the spotted green pufferfish,Tetraodon nigrovirids

A novel relationship between branchial carbonic anhydrase II (CAII) and anion exchanger 1 (AE1) was investigated in the euryhaline spotted green pufferfish (Tetraodon nigroviridis). The immunoblots revealed that AE1 was only detected in the membrane fraction of gills while CAII can be probed both in the membrane and cytosol fractions of gills. CAII protein abundance in the membrane fraction is salinity dependent. Immunological detection of the membrane fraction CAII protein in gills showed 3.9-fold higher in the hyposmotic (freshwater) group than the hyperosmotic (seawater;35 per thousand) group. In contrast, there was no change in the protein level of cytosolic CAII between seawater and freshwater groups. The whole-mount immunocytochemical staining demonstrated that both AE1 and CAII were colocalized to the Na(+)/K(+)-ATPase-immunoreactive cells in gill epithelium of the pufferfish. The interaction between CAII and AE1 was further identified by co-immunoprecipitation because AE1 was detected in the immunoprecipitates of CAII and vice versa. Our results showed that in pufferfish gills CAII was not only expressed in the cytosol to produce the substrate for AE1 transport during Cl(-) influx but also associated with the plasma membrane via AE1. Obviously, it is essential for the physiological function of AE1 to interact with CAII in the membrane of gill Na(+)/K(+)-ATPase-immunoreactive cells. To our knowledge, this is the first study to demonstrate the interaction of branchial CAII and AE1 in fish. The novel correlation proposed a new model of Cl(-)/HCO(3) (-) transport in gills of the teleosts.