Intestinal base excretion in the seawater-adapted rainbow trout: a role in acid-base balance?

Carbonate precipitates and bicarbonate secretion in the intestine of sea bass, Dicentrarchus labrax

The aim of this paper was to study the chemical composition of the precipitates found in the intestine of Dicentrarchus labrax and the source of HCO(3)(-) secreted into the intestinal lumen. The chemical analysis was performed by employing the potentiometric double titration method and by means of an electron microscope coupled with a spectrometer and X-ray powder diffraction. The results obtained suggest the presence of very insoluble intestinal precipitates, presumably formed by a mixture of CaCO(3) and MgCO(3), with a higher quantity of the former with respect to the latter. HCO(3)(-) secretion rate was investigated with the aid of the pH stat method in isolated tissues mounted in Ussing chamber, where the transepithelial electrical parameters were also measured. When the serosal surface of the intestinal mucosa was bathed in HCO(3)(-)-Ringer bubbled with 1% CO(2) in O(2) while the serosal surface was bathed in HCO(3)(-) free Ringer solution bubbled with pure O(2), bicarbonate secretion proceeded at an almost stable rate of 0.9 ± 0.05 μeq cm(-2) h(-1) for about 3 h while I(sc) maintained a constant value of 38 ± 1.5 μA cm(-2). The carbonic anhydrase inhibitor ethoxyzolamide elicited a progressive reduction of HCO(3)(-) secretion that was about 75% of the initial value after 80 min. When serosal HCO(3)(-)-CO(2) saline was substituted with Hepes-O(2) saline base secretion progressively declined reaching a value of about 20% of the initial value. It was also strongly inhibited when Na(+) was substituted with the impermeant cation choline and when either DIDS or ouabain were added to the basolateral side. These results suggest that most of the bicarbonate secreted is of extracellular source and is probably transported across the basolateral membrane by both Na(+) independent mechanism and Na(+) dependent transporter, presumably a NaHCO(3) cotransport.

Acid-base responses to feeding and intestinal Cl- uptake in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus, an agastric euryhaline teleost

Marine teleosts generally secrete basic equivalents (HCO(3)(-)) and take up Na(+) and Cl(-) in the intestine so as to promote absorption of H(2)O. However, neither the integration of these functions with feeding nor the potential role of the gut in ionoregulation and acid-base balance in freshwater have been well studied. The euryhaline killifish (Fundulus heteroclitus) is unusual in lacking both an acid-secreting stomach and a mechanism for Cl(-) uptake at the gills in freshwater. Responses to a satiation meal were evaluated in both freshwater- and seawater-acclimated killifish. In intact animals, there was no change in acid or base flux to the external water after the meal, in accord with the absence of any post-prandial alkaline tide in the blood. Indeed, freshwater animals exhibited a post-prandial metabolic acidosis ('acidic tide'), whereas seawater animals showed no change in blood acid-base status. In vitro gut sac experiments revealed a substantially higher rate of Cl(-) absorption by the intestine in freshwater killifish, which was greatest at 1-3 h after feeding. The Cl(-) concentration of the absorbate was higher in preparations from freshwater animals than from seawater killifish and increased with fasting. Surprisingly, net basic equivalent secretion rates were also much higher in preparations from freshwater animals, in accord with the 'acidic tide'; in seawater preparations, they were lowest after feeding and increased with fasting. Bafilomycin (1 micromol l(-1)) promoted an 80% increase in net base secretion rates, as well as in Cl(-) and fluid absorption, at 1-3 h post-feeding in seawater preparations only, explaining the difference between freshwater and seawater fish. Preparations from seawater animals at 1-3 h post-feeding also acidified the mucosal saline, and this effect was associated with a marked rise in P(CO(2)), which was attenuated by bafilomycin. Measurements of chyme pH from intact animals confirmed that intestinal fluid (chyme) pH and basic equivalent concentration were lowest after feeding in seawater killifish, whereas P(CO(2)) was greatly elevated (80-95 Torr) in chyme from both seawater and freshwater animals but declined to lower levels (13 Torr) after 1-2 weeks fasting. There were no differences in pH, P(CO(2)) or the concentrations of basic equivalents in intestinal fluid from seawater versus freshwater animals at 12-24 h or 1-2 weeks post-feeding. The results are interpreted in terms of the absence of gastric HCl secretion, the limitations of the gills for acid-base balance and Cl(-) transport, and therefore the need for intestinal Cl(-) uptake in freshwater killifish, and the potential for O(2) release from the mucosal blood flow by the high P(CO(2)) in the intestinal fluids. At least in seawater killifish, H(+)-ATPase running in parallel to HCO(3)(-):Cl(-) exchange in the apical membranes of teleost enterocytes might reduce net base secretion and explain the high P(CO(2)) in the chyme after feeding.

Effects of waterborne silver in a marine teleost, the gulf toadfish (Opsanus beta): effects of feeding and chronic exposure on bioaccumulation and physiological responses

Marine teleosts drink seawater, and the digestive tract is a key organ of osmoregulation. The gastro-intestinal tract therefore offers a second site for the potential uptake and toxicity of waterborne metals, but how these processes might interact with the digestive functions of the tract has not been investigated previously. We therefore compared the responses of adult gulf toadfish (Opsanus beta, collected from the wild) to a chronic 22d exposure to waterborne Ag (nominally 200 microg L(-1)=1.85 micromol L(-1)), in the presence or absence of daily satiation feeding (shrimp). Ag exposure did not affect voluntary feeding rate. Feeding reduced the net whole body accumulation of Ag by >50%, with reductions in liver concentrations (high) and white muscle concentrations (relatively low) playing the largest quantitative roles. Feeding also protected against Ag buildup in the esophagus-stomach and kidney, and increased biliary and urinary Ag concentrations. The gill was the only tissue to show the opposite response. Although terminal plasma Na(+), Cl(-), and Mg(2+) concentrations were unaffected, there were complex interactive effects on osmoregulatory functions of the gastro-intestinal tract, including drinking rate, gut fluid volumes, and intestinal base secretion rates. At the end of the exposure, the plasma clearance kinetics of an arterially injected tracer dose of (110 m)Ag were faster in toadfish that had been chronically exposed to waterborne Ag, and (110 m)Ag accumulation in their red blood cells was reduced. After 32 h, higher amounts of (110 m)Ag were found in bile and urine, but lower amounts in the intestine of the Ag-exposed toadfish; there were no other differences in tissue-specific distribution. The results suggest that feeding reduces waterborne Ag uptake through the digestive tract and alters physiological responses, while chronic exposure enhances regulatory functions. The time-dependent actions of the liver in Ag scavenging and detoxification are highlighted.

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.

Acquisition of Ca(2+) and HCO3(-)/CO3(2-) for shell formation in embryos of the common pond snail Lymnaea stagnalis

Embryos of the freshwater common pond snail Lymnaea stagnalis develop to hatch within 10 days under control conditions (22°C, Miami-Dade tap water) and this development is impaired by removal of ambient calcium. In contrast, embryos did not exhibit dependence upon an ambient HCO₃⁻/CO₃²⁻ source, developing and hatching in HCO₃⁻/CO₃²⁻-free water at rates comparable to controls. Post-metamorphic, shell-laying embryos exhibited a significant saturation-type calcium uptake as a function of increasing ambient calcium concentration. However, changes in ambient bicarbonate concentration did not influence calcium or apparent titratable alkalinity uptake. There was a distinct shift from no significant flux in pre-metamorphic embryos to net uptake of calcium in post-metamorphic stages as indicated by an increased uptake from the micro-environment surrounding the egg mass and increased net uptake in 24-h, whole egg mass flux measurements. Furthermore, HCO₃⁻/CO₃²⁻ acquisition as measured by titratable alkalinity flux is at least partially attributable to an endogenous carbonate source that is associated with acid extrusion. Thus, calcium requirements for embryonic shell formation are met via uptake but HCO₃⁻/CO₃²⁻, which is also necessary for shell formation is acquired in part from endogenous sources with no detectable correlation to ambient HCO₃⁻/CO₃²⁻ availability.

Water chemistry alters gene expression and physiological end points of chronic waterborne copper exposure in zebrafish, Danio rerio

This is the first study to implement a genomic approach to ascertain both transcriptional and functional end points of chronic Cu toxicity in fish associated with experimentally manipulated water chemistries. Over 21 d, zebrafish acclimated to softwater (Na(+) = 0.06 mM, Ca(2+) = 0.08 mM, Mg(2+) = 0.03 mM) were exposed to the following: soft-water (Ctrl); 12 microg L(-1) Cu (Cu); 3.3 mM Na(+) (Na); 3.3 mM Na(+) + 12 microg L(-1) Cu (Na + Cu); 3.3 mM Ca(2+) (Ca); or 3.3 mM Ca(2+) + 12 microg L(-1) Cu (Ca + Cu). Although effective at reducing Cu load in all tissues, Na(+) in the presence of Cu did not decrease the degree of oxidative damage, particularly in the gill and gut. In contrast, Ca + Cu treatment decreased Cu accumulation in gill, but not liver or gut, with no reduction in oxidative damage. Transcriptional analysis of candidate genes (atp7a, ctr1, ECaC, esr1) showed principally a down regulation of transcripts with the Cu only treatment, while Ca + Cu treatment restored some of the genes to control levels. Conversely, the Na + Cu treatment had a strong, opposing affect when compared to that of Cu alone. Zebrafish Affymetrix GeneChips revealed significantly clustered patterns of expression. Changes in expression induced by Cu appeared to be opposite to the majority of the other treatments. Our data on the preventative or enhancing effects of Na(+) and Ca(2+) both alone and in the presence of Cu, may, in the future, facilitate the incorporation of gene expression end points into a biotic ligand model predicting chronic Cu toxicity in this tropical model species of genomic importance.

The intestinal response to feeding in seawater gulf toadfish, Opsanus beta, includes elevated base secretion and increased epithelial oxygen consumption

Intestinal HCO3- secretion is essential to marine teleost fish osmoregulation and comprises a considerable source of base efflux attributable to both serosal HCO3- and endogenous CO2 hydration. The role of intestinal HCO3- secretion in dynamic acid-base balance regulation appears negligible in studies of unfed fish, but evidence of high intestinal fluid [HCO3-] in fed marine teleosts led us to investigate the source of this HCO3- and its potential role in offsetting the postprandial 'alkaline tide' commonly associated with digestion. Specifically, we hypothesized that elevated metabolic rate and thus endogenous CO2 production by intestinal tissue as well as increased transepithelial intestinal HCO3- secretion occur post-feeding and offset a postprandial alkaline tide. To test these hypotheses changes in HCO3- secretion and O2 consumption by gulf toadfish (Opsanus beta) isolated intestine were quantified 0, 3, 6, 12, 24 and 48 h post-feeding. Intestinal tissue of unfed fish in general showed high rates of HCO3- secretion (15.5 mumol g(-1) h(-1)) and O2 consumption (8.9 mumol g(-1) h(-1)). Furthermore, postprandial increases in both intestinal HCO3- secretion and O2 consumption (1.6- and 1.9-fold peak increases, respectively) were observed. Elevated intestinal HCO3- secretion rates preceded and outlasted those of O2 consumption, and occurred at a magnitude and duration sufficient to account for the lack of alkaline tide. The dependence of these high rates of postprandial intestinal base secretion on serosal HCO3- indicates transepithelial HCO3- transport increases disproportionately more than endogenous CO2 production. The magnitude of postprandial intestinal HCO(3)(-) secretion indicates the intestine certainly is capable of postprandial acid-base balance regulation.

HCO (3)(-) secretion and CaCO3 precipitation play major roles in intestinal water absorption in marine teleost fish in vivo

The intestine of marine teleosts must effectively absorb fluid from ingested seawater to avoid dehydration. This fluid transport has been almost exclusively characterized as driven by NaCl absorption. However, an additional feature of the osmoregulatory role of the intestine is substantial net HCO(3)(-) secretion. This is suggested to drive additional fluid absorption directly (via Cl(-)/HCO(3)(-) exchange) and indirectly by precipitating ingested Ca(2+) as CaCO(3), thus creating the osmotic gradient for additional fluid absorption. The present study tested this hypothesis by perfusing the intestine of the European flounder in vivo with varying [Ca(2+)]: 10 (control), 40, and 90 mM. Fractional fluid absorption increased from 47% (control) to 73% (90 mM Ca(2+)), where almost all secreted HCO(3)(-) was excreted as CaCO(3). This additional fluid absorption could not be explained by NaCl cotransport. Instead, a significant positive relationship between Na(+)-independent fluid absorption and total HCO(3)(-) secretion was consistent with the predicted roles for anion exchange and CaCO(3) precipitation. Further analysis suggested that Na(+)-independent fluid absorption could be accounted for by net Cl(-) and H(+) absorption (from Cl(-)/HCO(3)(-) exchange and CO(2) hydration, respectively). There was no evidence to suggest that CaCO(3) alone was responsible for driving fluid absorption. However, by preventing the accumulation of luminal Ca(2+) it played a vital role by dynamically maintaining a favorable osmotic gradient all along the intestine, which permits substantially higher rates of solute-linked fluid absorption. To overcome the resulting hyperosmotic and highly acidic absorbate, it is proposed that plasma HCO(3)(-) buffers the absorbed H(+) (from HCO(3)(-) production), and consequently reduces the osmolarity of the absorbed fluid entering the body.

Post-prandial metabolic alkalosis in the seawater-acclimated trout: the alkaline tide comes in

The consequences of feeding and digestion on acid-base balance and regulation in a marine teleost (seawater-acclimated steelhead trout; Oncorhynchus mykiss) were investigated by tracking changes in blood pH and [HCO3-], as well as alterations in net acid or base excretion to the water following feeding. Additionally the role of the intestine in the regulation of acid-base balance during feeding was investigated with an in vitro gut sac technique. Feeding did not affect plasma glucose or urea concentrations, however, total plasma ammonia rose during feeding, peaking between 3 and 24 h following the ingestion of a meal, three-fold above resting control values (approximately 300 micromol ml(-1)). This increase in plasma ammonia was accompanied by an increase in net ammonia flux to the water (approximately twofold higher in fed fish versus unfed fish). The arterial blood also became alkaline with increases in pH and plasma [HCO3-] between 3 and 12 h following feeding, representing the first measurement of an alkaline tide in a marine teleost. There was no evidence of respiratory compensation for the measured metabolic alkalosis, as Pa CO2 remained unchanged throughout the post-feeding period. However, in contrast to an earlier study on freshwater-acclimated trout, fed fish did not exhibit a compensating increase in net base excretion, but rather took in additional base from the external seawater, amounting to approximately 8490 micromol kg(-1) over 48 h. In vitro experiments suggest that at least a portion of the alkaline tide was eliminated through increased HCO3- secretion coupled to Cl- absorption in the intestinal tract. This did not occur in the intestine of freshwater-acclimated trout. The marked effects of the external salinity (seawater versus freshwater) on different post-feeding patterns of acid-base balance are discussed.

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.

High rates of HCO3- secretion and Cl- absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H+-pump metabolon?

Anion exchange contributes significantly to intestinal Cl(-) absorption in marine teleost fish and is thus vital for successful osmoregulation. This anion exchange process leads to high luminal HCO(3)(-) concentrations (up to approximately 100 mmol l(-1)) and high pH and results in the formation of CaCO(3) precipitates in the intestinal lumen. Recent advances in our understanding of the transport processes involved in intestinal anion exchange in marine teleost fish include the demonstration of a role for the H(+)-pump (V-ATPase) in apical H(+) extrusion and the presence of an electrogenic (nHCO(3)(-)/Cl(-)) exchange protein (SLC26a6). The H(+)-V-ATPase defends against cellular acidification, which might otherwise occur as a consequence of the high rates of base secretion. In addition, apical H(+) extrusion probably maintains lower HCO(3)(-) concentrations in the unstirred layer at the apical surface than in the bulk luminal fluids and thus facilitates continued anion exchange. Furthermore, H(+)-V-ATPase activity hyperpolarizes the apical membrane potential that provides the driving force for apical electrogenic nHCO(3)(-)/Cl(-) exchange, which appears to occur against both Cl(-) and HCO(3)(-) electrochemical gradients. We propose that a similar coupling between apical H(+) extrusion and nHCO(3)(-)/Cl(-) exchange accounts for Cl(-) uptake in freshwater fish and amphibians against very steep Cl(-) gradients.

Mechanisms of seawater acclimation in a primitive, anadromous fish, the green sturgeon

Relatively little is known about salinity acclimation in the primitive groups of fishes. To test whether physiological preparative changes occur and to investigate the mechanisms of salinity acclimation, anadromous green sturgeon, Acipenser medirostris (Chondrostei) of three different ages (100, 170, and 533 dph) were acclimated for 7 weeks to three different salinities (<3, 10, and 33 ppt). Gill, kidney, pyloric caeca, and spiral intestine tissues were assayed for Na(+), K(+)-ATPase activity; and gills were analyzed for mitochondria-rich cell (MRC) size, abundance, localization and Na(+), K(+)-ATPase content. Kidneys were analyzed for Na(+), K(+)-ATPase localization and the gastro-intestinal tract (GIT) was assessed for changes in ion and base content. Na(+), K(+)-ATPase activities increased in the gills and decreased in the kidneys with increasing salinity. Gill MRCs increased in size and decreased in relative abundance with fish size/age. Gill MRC Na(+), K(+)-ATPase content (e.g., ion-pumping capacity) was proportional to MRC size, indicating greater abilities to regulate ions with size/age. Developmental/ontogenetic changes were seen in the rapid increases in gill MRC size and lamellar length between 100 and 170 dph. Na(+), K(+)-ATPase activities increased fourfold in the pyloric caeca in 33 ppt, presumably due to increased salt and water absorption as indicated by GIT fluids, solids, and ion concentrations. In contrast to teleosts, a greater proportion of base (HCO(3) (-) and 2CO(3) (2-)) was found in intestinal precipitates than fluids. Green sturgeon osmo- and ionoregulate with similar mechanisms to more-derived teleosts, indicating the importance of these mechanisms during the evolution of fishes, although salinity acclimation may be more dependent on body size.

Contribution of fish to the marine inorganic carbon cycle

Oceanic production of calcium carbonate is conventionally attributed to marine plankton (coccolithophores and foraminifera). Here we report that marine fish produce precipitated carbonates within their intestines and excrete these at high rates. When combined with estimates of global fish biomass, this suggests that marine fish contribute 3 to 15% of total oceanic carbonate production. Fish carbonates have a higher magnesium content and solubility than traditional sources, yielding faster dissolution with depth. This may explain up to a quarter of the increase in titratable alkalinity within 1000 meters of the ocean surface, a controversial phenomenon that has puzzled oceanographers for decades. We also predict that fish carbonate production may rise in response to future environmental changes in carbon dioxide, and thus become an increasingly important component of the inorganic carbon cycle.

The effect of elevated salinity on 'California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) metabolism

California Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) are extremely saline tolerant and have been previously shown to reduce whole-animal oxygen consumption rate (MO(2)) upon exposures to salinities greater than that of seawater (SW). In this study tilapia were acclimated to 15, 30, 45, 60 and 75 g/L salinity for 1, 5, 14, or 28 days. There was little change in plasma osmolality or muscle water content in salinities below 60 g/L, and branchial Na(+), K(+)-ATPase (NKA) activity was low in 15 and 30 g/L relative to 60 and 75 g/L. When tilapia were exposed to 75 g/L, plasma osmolality and NKA activity were significantly increased within 5 days of exposure relative to those in 15 and 30 g/L, and remained elevated over the entire 28 days acclimation, indicating that short term salinity challenges (i.e., 5 days) are predictive of longer exposure durations in this species. MO(2) following transfer to 15 and 30 g/L was elevated, reflecting the high energy demand required for switching from a hyper- to a hypo-osmoregulatory strategy. The MO(2) of 60 g/L-exposed fish was significantly reduced at 1, 5, and 14 days, relative to 30 g/L-exposed fish; however by 28 days there were no significant differences. We investigated the potential for a metabolic basis for the salinity-induced MO(2) reduction, using forward stepwise linear regression to correlate enzyme activities of brain, liver, and kidney with MO(2). Brain NKA was correlated with MO(2) after 5 days (p<0.01, r(2)=0.944) and both brain NKA and hepatic total ATPase were correlated with the reduced MO(2) at 14 days (p=0.027, r(2)=0.980 and p=0.025, r(2)=0.780, respectively). These results may indicate a tissue-level metabolic suppression, which has not been previously described as a response to hypersaline exposure in fishes.

Effects of salinity on intestinal bicarbonate secretion and compensatory regulation of acid-base balance in Opsanus beta

Marine teleosts have extracellular fluids less concentrated than their environment, resulting in continual water loss, which is compensated for by drinking, with intestinal water absorption driven by NaCl uptake. Absorption of Cl(-) occurs in part by apical Cl(-)/HCO(3)(-) exchange, with HCO(3)(-) provided by transepithelial transport and/or by carbonic anhydrase-mediated hydration of endogenous epithelial CO(2). Hydration of CO(2) also liberates H(+), which is transported across the basolateral membrane. In this study, gulf toadfish (Opsanus beta) were acclimated to 9, 35 and 50 ppt. Intestinal HCO(3)(-) secretion, water and salt absorption, and the ensuing effects on acid-base balance were examined. Rectal fluid excretion greatly increased with increasing salinity from 0.17+/-0.05 ml kg(-1) h(-1) in 9 ppt to 0.70+/-0.19 ml kg(-1) h(-1) in 35 ppt and 1.46+/-0.22 ml kg(-1) h(-1) in 50 ppt. Rectal fluid composition and excretion rates allowed for estimation of drinking rates, which increased with salinity from 1.38+/-0.30 to 2.60+/-0.92 and 3.82+/-0.58 ml kg(-1) h(-1) in 9, 35 and 50 ppt, respectively. By contrast, the fraction of imbibed water absorbed decreased from 85.9+/-3.8% in 9 ppt to 68.8+/-3.2% in 35 ppt and 61.4+/-1.0% in 50 ppt. Despite large changes in rectal base excretion from 9.3+/-2.7 to 68.2+/-20.4 and 193.2+/-64.9 mumol kg(-1) h(-1) in 9, 35 and 50 ppt, respectively, acute or prolonged exposure to altered salinities was associated with only modest acid-base balance disturbances. Extra-intestinal, presumably branchial, net acid excretion increased with salinity (62.0+/-21.0, 229.7+/-38.5 and 403.1+/-32.9 mumol kg(-1) h(-1) at 9, 35 and 50 ppt, respectively), demonstrating a compensatory response to altered intestinal base secretion associated with osmoregulatory demand.

Identification of intestinal bicarbonate transporters involved in formation of carbonate precipitates to stimulate water absorption in marine teleost fish

Marine teleost fish precipitate divalent cations as carbonate deposits in the intestine to minimize the potential for excessive Ca2+ entry and to stimulate water absorption by reducing luminal osmotic pressure. This carbonate deposit formation, therefore, helps maintain osmoregulation in the seawater (SW) environment and requires controlled secretion of HCO3(-) to match the amount of Ca2+ entering the intestinal lumen. Despite its physiological importance, the process of HCO3(-) secretion has not been characterized at the molecular level. We analyzed the expression of two families of HCO3(-) transporters, Slc4 and Slc26, in fresh-water- and SW-acclimated euryhaline pufferfish, mefugu (Takifugu obscurus), and obtained the following candidate clones: NBCe1 (an Na+-HCO3(-) cotransporter) and Slc26a6A and Slc26a6B (putative Cl(-)/HCO3(-) exchangers). Heterologous expression in Xenopus oocytes showed that Slc26a6A and Slc26a6B have potent HCO3(-)-transporting activity as electrogenic Cl(-)/nHCO3(-) exchangers, whereas mefugu NBCe1 functions as an electrogenic Na+-nHCO3(-) cotransporter. Expression of NBCe1 and Slc26a6A was highly induced in the intestine in SW and expression of Slc26a6B was high in the intestine in SW and fresh water, suggesting their involvement in HCO3(-) secretion and carbonate precipitate formation. Immunohistochemistry showed staining on the apical (Slc26a6A and Slc26a6B) and basolateral (NBCe1) membranes of the intestinal epithelial cells in SW. We therefore propose a mechanism for HCO3(-) transport across the intestinal epithelial cells of marine fish that includes basolateral HCO3(-) uptake (NBCe1) and apical HCO3(-) secretion (Slc26a6A and Slc26a6B).

Osmoregulation, ionoregulation and acid-base regulation by the gastrointestinal tract after feeding in the elasmobranch (Squalus acanthias)

In order to study the physiological consequences of voluntary feeding in the gastrointestinal tract of a ureotelic marine elasmobranch, dogfish (fasted for 96 h) were sampled at various times up to 360 h after consuming a 5-6% ration of teleost fish (hake) under natural feeding conditions. Digestion and absorption were completed between 120 and 360 h post-feeding. The tissue masses of different segments of the gastrointestinal tract increased and decreased markedly as the chyme moved through, mainly because of fluid engorgement rather than hyperplasia. In fasted dogfish, the cardiac and pyloric stomachs contained only small volumes of highly acidic fluid (pH 1.77+/-1.12, 2.05+/-0.08) similar in composition to seawater. Feeding resulted in gastric pHs of 3.20+/-0.31 and 3.95+/-0.40 at 6 h, followed by slow declines through 60 h. An alkaline tide in the blood also occurred at 6 h. In the face of large changing masses of highly acidic chyme in the stomachs, the pH (6.50+/-0.10), ionic composition and volume of chyme in the intestine (spiral valve) were precisely regulated from 6 to 60 h post-feeding at very different values from those in the stomachs, and intestinal HCO3(-) remained low (5.12+/-0.83 mmol l(-1)). The colon was usually empty and its pH constant at 7.20+/-0.16 at all times. Despite the ingestion of strongly hypo-osmotic teleost tissue, the osmolality of the chyme remained in equilibrium with that of the blood plasma in all segments at all times after feeding. Much of the osmotic equilibration was because of the secretion of urea into the chyme, particularly in the intestine. After feeding, gastric fluid concentrations of Na(+) and Mg(2+) declined, K(+) and Ca(2+) increased, whereas Cl(-) exhibited little change, indicating that additional drinking of seawater was minimal. Na(+), K(+), water and especially Cl(-) were absorbed in the intestine, whereas Mg(2+) and Ca(2+) were largely excluded. Our results illustrate the complex integration of digestive and ionoregulatory function in the elasmobranch digestive tract, and marked differences from the teleost pattern.

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.

Intestinal anion exchange in teleost water balance

Simultaneous measurements of all major electrolytes including HCO3(-) and H+ as well as water demonstrated that fluids absorbed by the anterior intestine of the marine gulf toadfish under in vivo-like conditions on an overall net basis are hypertonic at 380 mOsm and acidic ([H+] = 27 mM). This unusual composition of fluids absorbed across the intestinal epithelium is due to the unusual intestinal fluid chemistry resulting from seawater ingestion and selective ion and water absorption along the gastro-intestinal tract. Measurement under near symmetrical conditions with high NaCl concentrations and low MgSO4 concentrations revealed absorption of iso-osmotic and much less acidic fluids by the intestinal epithelium, a situation resembling that of other water absorbing leaky vertebrate epithelia. Reduced luminal NaCl concentrations seen in vivo results in lower absolute water absorption rates but higher Cl-/HCO3(-) exchange rates which are associated with higher net H+ absorption rates. It appears that apical anion exchange is important for net Cl- uptake by the marine teleost intestine especially when luminal NaCl concentrations are low and/or when MgSO4 concentrations are high. Observations indicate that fluid absorption from solutions of low NaCl but high MgSO4 concentrations is energetically more demanding than absorption from NaCl rich solutions at the level of the intestinal epithelium. Furthermore, the high luminal MgSO4 concentration which is an unavoidable consequence of seawater ingestion projects a demand for renal and branchial compensation for intestinal MgSO4 uptake and absorption of hypertonic and acidic fluid by the intestine.

Intestinal anion exchange in marine fish osmoregulation

Despite early reports, dating back three quarters of a century, of high total CO(2) concentrations in the intestinal fluids of marine teleost fishes, only the past decade has provided some insight into the functional significance of this phenomenon. It is now being recognized that intestinal anion exchange is responsible for high luminal HCO(3)(-) and CO(3)(2-) concentrations while at the same time contributing substantially to intestinal Cl(-) and thereby water absorption, which is vital for marine fish osmoregulation. In species examined to date, the majority of HCO(3)(-) secreted by the apical anion exchange process is derived from hydration of metabolic CO(2) with the resulting H(+) being extruded via a Na(+):H(+) exchange mechanism in the basolateral membrane. The basolateral H(+) extrusion is critical for the apical anion exchange and relies on the Na(+) gradient established by the Na(+)-K(+)-ATPase. This enzyme thereby ultimately fuels the secondary active transport of HCO(3)(-) and Cl(-) by the apical anion exchanger. High cellular HCO(3)(-) concentrations (>10 mmol l(-1)) are required for the anion exchange process and could be the result of both a high metabolic activity of the intestinal epithelium and a close association of the anion exchange protein and the enzyme carbonic anhydrase. The anion exchange activity in vivo is likely most pronounced in the anterior segment and results in net intestinal acid absorption. In contrast to other water absorbing vertebrate epithelia, the marine teleost intestine absorbs what appears to be a hypertonic fluid to displace diffusive fluid loss to the marine environment.

A review of causal factors and control measures for bloat in farmed salmonids with a suggested mechanism for the development of the condition

A mechanism for the development of bloat in salmonids is proposed and explained in terms of the physiology of digestion and osmoregulation in fish. Understanding the causal factors for bloat enables control measures to be identified. In farmed salmonids, the chyme produced during the digestion of nutrient-rich pelleted foods that rapidly disintegrate in the stomach will be a potent activator of a negative feedback mechanism (enterogastric control), which slows stomach emptying to protect the small intestine from nutrient overload. In saline environments salmonids continuously drink sea water to replace fluid lost across the gills. Fluid loss is increased during periods of stress caused by factors such as low oxygen levels, elevated temperature or high salinity. When ingestion of nutrient-rich food results in prolonged activation of enterogastric control, slowed stomach emptying leads to decreased absorption of water, thirst and increased drinking. This further exacerbates stomach distention. The proposed mechanism for the development of bloat is supported by on-farm experience where measures to control bloat include reducing food intake, altering the composition of the diet and using appropriate strategies to reduce stress.

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.

Tribute to R. G. Boutilier: acid-base transfer across fish gills

The gills are the major site of acid-base regulation in most fish. Acid-base transfer across fish gills is dominated by carbon dioxide and ammonia excretion, especially the former. Bicarbonate buffering in the blood is less than that found in mammals; regulation of ventilation has little effect on CO(2) levels in the blood and control of ventilation is not used to regulate body pH in fish. Proton ATPase (freshwater fish), Na(+)/H(+) exchangers (marine fish) and anion exchangers (marine and freshwater fish) are located in the gills. These transporters contribute to the regulation of internal pH, but little is known about how this is done in fish. Fish kept in confined water volumes acidify their environment, largely due to CO(2). This acidification augments ammonia excretion and reduces ammonia toxicity. The possible involvement of ammonia recycling in acid excretion is also discussed.

Ouabain-sensitive bicarbonate secretion and acid absorption by the marine teleost fish intestine play a role in osmoregulation

The gulf toadfish (Opsanus beta) intestine secretes base mainly in the form of HCO3- via apical anion exchange to serve Cl- and water absorption for osmoregulatory purposes. Luminal HCO3- secretion rates measured by pH-stat techniques in Ussing chambers rely on oxidative energy metabolism and are highly temperature sensitive. At 25 degrees C under in vivo-like conditions, secretion rates averaged 0.45 micromol x cm(-2) x h(-1), of which 0.25 micromol x cm(-2) x h(-1) can be accounted for by hydration of endogenous CO2 partly catalyzed by carbonic anhydrase. Complete polarity of secretion of HCO3- and H+ arising from the CO2 hydration reaction is evident from equal rates of luminal HCO3- secretion via anion exchange and basolateral H+ extrusion. When basolateral H+ extrusion is partly inhibited by reduction of serosal pH, luminal HCO3- secretion is reduced. Basolateral H+ secretion occurs in exchange for Na+ via an ethylisopropylamiloride-insensitive mechanism and is ultimately fueled by the activity of the basolateral Na+-K+-ATPase. Fluid absorption by the toadfish intestine to oppose diffusive water loss to the concentrated marine environment is accompanied by a substantial basolateral H+ extrusion, intimately linking osmoregulation and acid-base balance.

Evolutionary aspects of intestinal bicarbonate secretion in fish

Experiments compared intestinal HCO3- secretion in the intestine of marine teleost Gulf toadfish, Opsanus beta, to representatives of early chondrostean and chondrichthyan fishes, the Siberian sturgeon, Acipenser baerii, and white-spotted bamboo shark, Chiloscyllium plagiosum, respectively. As seen in marine teleosts, luminal HCO3- concentrations were 10-fold plasma levels in all species when exposed to hyperosmotic conditions. While intestinal water absorption left Mg2+ and SO4(2-) concentrated in intestinal fluids up to four-fold ambient seawater concentrations, HCO3- was concentrated up to 50 times ambient levels as a result of intestinal HCO3- secretion. Reduced luminal Cl- concentrations in the intestine of all species suggest that HCO3- secretion also occurs via Cl-/HCO3- exchange in chondrostean and chondrichthyan fishes. Sturgeon began precipitating carbonates from the gut after only 3 days at 14 per thousand, a mechanism utilized by marine teleosts to reduce intestinal fluid osmolality and maintain calcium homeostasis. Analysis of published intestinal fluid composition in the cyclostome Lampetra fluviatilis reveals that this species likely also utilize intestinal HCO3- secretion for osmoregulation. Analysis of existing cyclostome data and our results indicate that intestinal Cl-/HCO3- exchange plays an integral role in maintaining hydromineral balance not only in teleosts, but in all fish (and perhaps other animals) with a need to drink seawater.

Physiological responses to hyper-saline waters in sailfin mollies (Poecilia latipinna)

We examined the ionoregulatory physiology and biochemistry of the teleost sailfin molly (Poecilia latipinna), an inhabitant of salt marshes along the gulf coast, during exposure to hyper-saline waters (salinity range 35-95 ppt). Mollies were able to tightly control plasma Na(+) and Cl(-) concentrations and tissue water levels up to 65 ppt, but at higher salinities plasma ion levels began to rise and muscle water content dropped. Still, even at the highest salinity (90 ppt) plasma Na(+) and Cl(-) levels were only 32% and 39%, respectively, above levels at 35 ppt. Drinking rates at 60 ppt climbed 35%, while gut Na(+)/K(+)-ATPase (NAK) activity rose 70% and branchial NAK activity jumped 200%. The relatively small rise in drinking rate, in the face of a more than doubling of the osmotic gradient, suggests that a reduction in branchial water permeability significantly limited water loss and associated salt load. At 80 ppt, a salinity where plasma ion levels just begin to rise, drinking rate rose more rapidly, but gut and gill NAK activity did not, suggesting that mollies employed other pathways (perhaps renal) of salt excretion. At higher salinities, plasma ion levels continued to rise and muscle water content fell slightly indicating the beginnings of internal osmotic disturbances. To evaluate the energetic costs of hyper-salinity on mollies we measured the rate of O(2) consumption and found it rose with salinity, in sharp contrast to virtually all species previously examined. Interestingly, despite higher metabolism, growth was unaffected by hyper-salinity.

Osmoregulatory physiology of pyloric ceca: regulated and adaptive changes in chinook salmon

Functions of the anatomically obvious, yet peculiar, pyloric ceca of the fish gut have been a source of conjecture for over two millennia since Aristotle hypothesized on digestive utilities. Here, we demonstrate regulated and adaptive changes in osmoregulatory physiology of ceca from chinook salmon (Onchorhynchus tshawytscha). Transfer of salmon from freshwater to seawater (both short- and long-term) significantly stimulated both fluid uptake from 5.1 to 8.3-9.3 microl/cm2/hr and also Na+/K+ -ATPase from 6.5 to 8.3-9.6 micromol/ADP/mg protein/hr. Similar changes were induced with implants of cortisol, which resulted in high physiological cortisol levels in plasma. Ceca, which can number about 200 in chinook salmon, were estimated to account for the majority of fluid uptake capacity of the intestine and, after long-term seawater adaptation, the proportion of uptake capacity was sixfold higher. Transport physiology of ceca is thus under environmental and endocrine control indicative of an important role in salt and water homeostasis.

Bicarbonate secretion plays a role in chloride and water absorption of the European flounder intestine

Experiments performed on isolated intestinal segments from the marine teleost fish, the European flounder (Platichthys flesus), revealed that the intestinal epithelium is capable of secondary active HCO3(-) secretion in the order of 0.2-0.3 micromol x cm(-2) x h(-1) against apparent electrochemical gradient. The HCO3(-) secretion occurs via anion exchange, is dependent on mucosal Cl(-), results in very high mucosal HCO3(-) concentrations, and contributes significantly to Cl(-) and fluid absorption. This present study was conducted under in vivo-like conditions, with mucosal saline resembling intestinal fluids in vivo. These conditions result in a transepithelial potential of -16.2 mV (serosal side negative), which is very different from the -2.2 mV observed under symmetrical conditions. Under these conditions, we found a significant part of the HCO3(-) secretion is fueled by endogenous epithelial CO2 hydration mediated by carbonic anhydrase because acetazolamide (10(-4) M) was found to inhibit HCO3(-) secretion and removal of serosal CO(2) was found not to influence HCO3(-) secretion. Reversal of the epithelial electrochemical gradient for Cl(-) (removal of serosal Cl(-)) and elevation of serosal HCO3(-) resulted in enhanced HCO3(-) secretion and enhanced Cl(-) and fluid absorption. Cl(-) absorption via an anion exchange system appears to partly drive fluid absorption across the intestine in the absence of net Na(+) absorption.

Effects of prolonged copper exposure in the marine gulf toadfish (Opsanus beta) II: copper accumulation, drinking rate and Na+/K+ -ATPase activity in osmoregulatory tissues

Gulf toadfish were exposed to sublethal levels of copper (12.8 or 55.2 microM) for 30 days. Drinking in control fish averaged 1 ml kg(-1)h(-1) but exposure to 55.2 microM copper resulted in a complex biophasic pattern with initial (3 h and 1 day) inhibition of drinking rate, followed by an elevation of drinking rate from day 3 onwards. Drinking led to copper accumulation in the intestinal fluids at levels three to five times higher than the ambient copper concentrations, which in turn resulted in intestinal copper accumulation. The gill exhibited more rapid accumulation of copper than the intestine and contributed to early copper uptake leading to accumulation in internal organs. Muscle, spleen and plasma exhibited little if any disturbance of copper homeostasis while renal copper accumulation was evident at both ambient copper concentrations. The liver exhibited the highest copper concentrations and the greatest copper accumulation of all examined internal organs during exposure to 55.2 microM. Elevated biliary copper excretion was evident from measurements of gall bladder bile copper concentrations and appeared to protect partially against internal accumulation in fish exposed to 12.8 microM copper. No inhibition of Na+/K+ -ATPase activity in either gills or intestine was seen despite copper accumulation in these organs. Calculations of inorganic copper speciation suggest that Cu(CO3)(2)2- complexes which dominate in seawater and intestinal fluids are of limited availability for uptake while the low levels of ionic Cu2+, CuOH+ and CuCO3 may be the forms taken up by the gill and the intestinal epithelium.

Intestinal bicarbonate secretion in marine teleost fish-source of bicarbonate, pH sensitivity, and consequences for whole animal acid-base and calcium homeostasis

Whole animal studies using seawater European flounder (Platichthys flesus) revealed that increasing intestinal [Ca(2+)] to 20 mM stimulated net HCO(3)(-) base secretion by 57%, but this was effectively balanced by an increase in net acid secretion, likely from the gills, to maintain whole animal acid-base status. Higher Ca(2+) concentrations (40 and 70 mM) in ambient seawater resulted in reduced plasma total CO(2). This indicates (1) imperfect acid-base compensation, and (2) that endogenous metabolic CO(2) is insufficient to fuel intestinal HCO(3)(-) secretion, under hyper-stimulated conditions. Bicarbonate secretion plays an important role in preventing calcium absorption by precipitating a large fraction of the imbibed calcium as CaCO(3). Indeed, under high Ca(2+) conditions (20 mM), up to 75% of the intestinal Ca(2+) is precipitated as CaCO(3) and then excreted. This is undoubtedly important in protecting the marine teleost kidney from the need for excessive calcium excretion and risk of renal stone formation. Using an in vitro pH-stat technique with the isolated intestinal epithelium, the replacement of serosal CO(2) with a HEPES buffered saline had no effect on HCO(3)(-) secretion, indicating that the endogenous supply of HCO(3)(-) from CO(2) hydration within epithelial cells is adequate for driving baseline secretion rates. Further, in vitro data demonstrated a stimulatory effect of low pH on intestinal HCO(3)(-) secretion. Thus, both luminal Ca(2+) and H(+) can regulate HCO(3)(-) secretion but the precise mechanisms and their potential interaction are currently unresolved.

Integrated responses of Na+/HCO3- cotransporters and V-type H+-ATPases in the fish gill and kidney during respiratory acidosis

Using degenerate primers, followed by 3' and 5' RACE and "long" PCR, a continuous 4050-bp cDNA was obtained and sequenced from rainbow trout (Oncorhynchus mykiss) gill. The cDNA included an open reading frame encoding a deduced protein of 1088 amino acids. A BLAST search of the GenBank protein database demonstrated that the trout gene shared high sequence similarity with several vertebrate Na(+)/HCO(3)(-) cotransporters (NBCs) and in particular, NBC1. Protein alignment revealed that the trout NBC is >80% identical to vertebrate NBC1s and phylogenetic analysis provided additional evidence that the trout NBC is indeed a homolog of NBC1. Using the same degenerate primers, a partial cDNA (404 bp) for NBC was obtained from eel (Anguilla rostrata) kidney. Analysis of the tissue distribution of trout NBC, as determined by Northern blot analysis and real-time PCR, indicated high transcript levels in several absorptive/secretory epithelia including gill, kidney and intestine and significant levels in liver. NBC mRNA was undetectable in eel gill by real-time PCR. In trout, the levels of gill NBC1 mRNA were increased markedly during respiratory acidosis induced by exposure to hypercarbia; this response was accompanied by a transient increase in branchial V-type H(+)-ATPase mRNA levels. Assuming that the branchial NBC1 is localised to basolateral membranes of gill cells and operates in the influx mode (HCO(3)(-) and Na(+) entry into the cell), it would appear that in trout, the expression of branchial NBC1 is transcriptionally regulated to match the requirements of gill pHi regulation rather than to match trans-epithelial HCO(3)(-) efflux requirements for systemic acid-base balance. By analogy with mammalian systems, NBC1 in the kidney probably plays a role in the tubular reabsorption of both Na(+) and HCO(3)(-). During periods of respiratory acidosis, levels of renal NBC1 mRNA increased (after a transient reduction) in both trout and eel, presumably to increase HCO(3)(-) reabsorption. This strategy, when coupled with increased urinary acidification associated with increased vacuolar H(+)-ATPase activity, ensures that HCO(3)(-) levels accumulate in the body fluids to restore pH.

Physiological, biochemical and morphological indicators of osmoregulatory stress in `California' Mozambique tilapia (Oreochromis mossambicus×O. urolepis hornorum) exposed to hypersaline water

The salinity tolerance of the `California' Mozambique tilapia(Oreochromis mossambicus × O. urolepis hornorum), a current inhabitant of the hypersaline Salton Sea in California, USA, was investigated to identify osmoregulatory stress indicators for possible use in developing a model of salinity tolerance. Seawater-acclimated (35 g l–1) tilapia hybrids were exposed to salinities from 35–95 g l–1, using gradual and direct transfer protocols, and physiological (plasma osmolality, [Na+],[Cl–], oxygen consumption, drinking rate, hematocrit, mean cell hemoglobin concentration, and muscle water content), biochemical(Na+, K+-ATPase) and morphological (number of mature,accessory, immature and apoptotic chloride cells) indicators of osmoregulatory stress were measured. Tilapia tolerated salinities ranging from 35 g l–1 to 65 g l–1 with little or no change in osmoregulatory status; however, in fish exposed to 75–95 g l–1 salinity, plasma osmolality, [Na+],[Cl–], Na+, K+-ATPase, and the number of apoptotic chloride cells, all showed increases. The increase in apoptotic chloride cells at salinities greater than 55 g l–1, prior to changes in physiological and biochemical parameters, indicates that it may be the most sensitive indicator of osmoregulatory stress. Oxygen consumption decreased with salinity, indicating a reduction in activity level at high salinity. Finally, `California' Mozambique tilapia have a salinity tolerance similar to that of pure Mozambique tilapia; however, cellular necrosis at 95 g l–1 indicates they may be unable to withstand extreme salinities for extended periods of time.

Physiological adaptations of the gut in the Lake Magadi tilapia, Alcolapia grahami, an alkaline- and saline-adapted teleost fish

We describe the gut physiology of the Lake Magadi tilapia (Alcolapia grahami), specifically those aspects associated with feeding and drinking while living in water of unusually high carbonate alkalinity (titratable base=245 mequiv l(-1)) and pH (9.85). Drinking of this highly alkaline lake water occurs at rates comparable to or higher than those seen in marine teleosts. Eating and drinking take place throughout the day, although drinking predominates during hours of darkness. The intestine directly intersects the esophagus at the anterior end of the stomach forming a 'T', and the pyloric sphincter, which comprises both smooth and striated muscle, is open when the stomach is empty and closed when the stomach is full. This unique configuration (a functional trifurcation) allows imbibed alkaline water to bypass the empty stomach, thereby avoiding a reactive mixing with acidic gastric fluids, and minimizes interference with a full stomach. No titratable base was present in the stomach, where the mean pH was 3.55, but the intestine was progressively more alkaline (foregut 6.96, midgut 7.74, hindgut 8.12, rectum 8.42); base levels in the intestinal fluid were comparable to those in lake water. The gut was highly efficient at absorbing water (76.6%), which accompanied the absorption of Na(+) (78.5%), titratable base (80.8%), and Cl(-) (71.8%). The majority of Na(+), base and water absorption occurred in the foregut by an apparent Na(+) plus base co-transport system. Overall, more than 70% of the intestinal flux occurred via Na(+) plus base co-transport, and less than 30% by Na(+) plus Cl(-) co-transport, a very different situation from the processes in the intestine of a typical marine teleost.

Effects of dissolved metals and other hydrominerals on in vivo intestinal zinc uptake in freshwater rainbow trout

For aquatic organisms, zinc is both an essential nutrient and an environmental contaminant. The intestine is potentially the most important route of zinc absorption, yet little is known regarding this uptake pathway for zinc in fish. A recently developed in vivo perfusion system was used to investigate the effect of luminal composition upon intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss). Perfusate cadmium and copper had specific, yet distinct, antagonistic effects upon lumen to tissue zinc movement. Copper significantly reduced the proportion of zinc taken up from the perfusate, and concomitantly limited the passage of zinc into the circulation and beyond. Conversely, cadmium decreased subepithelial zinc accumulation, with rates falling to 29 nmol g(-1) h(-1) from the control (zinc alone) values of 53 nmol g(-1) h(-1). Calcium had a similar action to copper, also reducing post-intestinal zinc accumulation from 0.06 to 0.02 nmol g(-1) h(-1), an effect attributed to interactions between calcium and the zinc uptake pathway. In addition to these effects, luminal composition also had a marked influence upon epithelial response to zinc. Calcium, copper and magnesium all greatly reduced zinc-induced mucus secretion. Cadmium, a toxic metal, significantly increased mucus secretion. It is proposed that these modifications were related to the essentiality of each element, and their potential mechanisms of uptake. Despite changes at the epithelium, the post-epithelial accumulation of zinc was dependent mainly upon the nature of the competing cation. Intestinal saline ion substitution experiments suggested a potential link of potassium ion efflux to zinc uptake. The effect of pH buffering of luminal solutions was also investigated.

Dietary sodium inhibits aqueous copper uptake in rainbow trout (Oncorhynchus mykiss)

Ours is the first study to demonstrate an influence of dietary sodium on waterborne copper uptake in fish. We examined possible interactions between dietary sodium and the response of freshwater rainbow trout (Oncorhynchus mykiss) to waterborne copper in light of recent evidence of interactions between sodium and copper metabolism in the gills. Trout were maintained for 6 days on one of four diets of increasing sodium concentration (0.25 mmol g(-1), 0.51 mmol g(-1), 0.76 mmol g(-1) and 1.27 mmol g(-1), which corresponds to 0.6%, 1.2%, 1.8% and 3% sodium by mass, respectively). At the end of 7 days, fish were exposed for 6 h to waterborne copper spiked with (64)Cu to determine if the dietary sodium affected responses to a subsequent short-term waterborne copper exposure. The radiotracer allowed us to distinguish between Cu occurring in fish tissues before the experiment and 'newly accumulated' Cu arising from the experimental exposure. Dietary sodium concentrations of 1.8% or 3% reduced newly accumulated copper concentrations in gill (from 93.9 ng g(-1) in control to 38.9 ng g(-1) and 20.0 ng g(-1) in fish fed 1.8% or 3% Na(+)-supplemented diets, respectively), liver (from 64.3 ng g(-1) to 23.1 ng g(-1) and 7.5 ng g(-1), respectively), kidney (from 29.3 ng g(-1) to 11.7 ng g(-1) and 7.8 ng g(-1), respectively), plasma (from 64.7 ng g(-1) to 21.5 ng g(-1) and 10.7 ng g(-1), respectively) and gut (from 6.8 ng g(-1) to 3.4 ng g(-1) and 2.2 ng g(-1), respectively) by 50.0-88.2%. The 3% Na(+)-supplemented diets also increased plasma and gut sodium concentrations by 38.1% (from 137.1 micromol g(-1) to 189.3 micromol g(-1)) and 104.3% (from 56.5 micromol g(-1) to 115.4 micromol g(-1)), respectively, relative to fish maintained on untreated diets. Whole body uptake rates of both sodium and copper were significantly reduced, and highly correlated (r=0.97) with one another, in fish fed high-sodium diets relative to controls. Moreover, sodium efflux was 12% and 38% higher in fish fed 1.8% and 3% sodium-enriched diets, respectively. Fish fed high-sodium diets also drank more water, but the contribution of drinking to waterborne copper uptake was negligible. From these results, we speculate that, at least in part, aqueous sodium and copper share a common branchial uptake route, probably through an apical sodium channel. According to this hypothesis, as the channel is downregulated with increasing internal sodium concentrations, both sodium and copper uptake from the water are inhibited.

Active sulfate secretion by the intestine of winter flounder is through exchange for luminal chloride

SO4(2-) transport by winter flounder intestine in Ussing chambers was characterized. With 50 mM SO4(2-) (physiological level) bathing the lumen, net absorption (lumen to blood) dominated. Under short-circuited conditions, 1 mM SO4(2-) on both sides, net active SO4(2-) secretion occurred (8.55 +/- 0.96 nmol. cm(-2). h(-1)). NaCN (10 mM), ouabain (10(-4) M), and luminal DIDS (0.2 mM) inhibited net secretion. Removal of luminal Cl- and HCO3- together (Cl--HCO3-) or Cl- alone blocked net secretion, whereas removal of luminal HCO3- alone increased net secretion. SO4(2-) uptake into foregut brush-border membrane vesicles was stimulated by a trans-Cl- gradient (in > out) and unaffected by a trans-HCO3- gradient (in > out). Short-circuiting with K+ (in = out) and valinomycin had no effect on Cl--stimulated SO4(2-) uptake, suggesting electroneutral exchange. Satiety (i.e., full stomach) stimulated the unidirectional absorptive flux, eliminating net secretion. It was concluded that the intestine is a site of SO4(2-) absorption in marine teleosts and that active SO4(2-) secretion is in exchange for luminal Cl-.

Intestinal bicarbonate secretion by marine teleost fish--why and how?

Intestinal fluids of most marine teleosts are alkaline (pH 8.4-9.0) and contain high levels of HCO(3)(-) equivalents (40-130 mM) which are excreted at a significant rate (>100 microEq kg(-1) h(-1)). Recent research reveals the following about this substantial HCO(3)(-) secretion: (1) It is not involved in acid-base regulation or neutralisation of stomach acid, but increases in parallel with drinking rate at elevated ambient salinities suggesting a role in osmoregulation; (2) In species examined so far, all sections of the intestine can secrete bicarbonate; (3) The secretion is dependent on mucosal Cl(-), sensitive to mucosal DIDS, and immuno-histochemistry indicates involvement of an apical Cl(-)/HCO(3)(-) exchanger. In addition, hydration of CO(2) via carbonic anhydrase in combination with proton extrusion appears to be essential for bicarbonate secretion. The mode of proton extrusion is currently unknown but potential mechanisms are discussed. One consequence of the luminal alkalinity and high bicarbonate concentrations is precipitation of calcium and magnesium as carbonate complexes. This precipitation is hypothesised to reduce the osmolality of intestinal fluids and thus play a potential role in water absorption and osmoregulation. The present studies on European flounder reveal that elevated luminal calcium (but not magnesium) concentrations stimulate intestinal bicarbonate secretion both acutely and chronically, in vitro and in vivo. At the whole animal level, the result of this elevated bicarbonate secretion was increased calcium precipitation with an associated reduction in the osmolality of rectal fluids and plasma. These observations suggest direct functional links between intestinal bicarbonate secretion, divalent cation precipitation and osmoregulation in marine teleost fish.

Acid-base regulation in fishes: cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na(+)/H(+) exchange (NHE) and vacuolar-type H(+)-ATPase transport H(+) from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H(+)-ATPase drives H(+) excretion and is indirectly coupled to Na(+) uptake (via Na(+) channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H(+)-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl(-)/HCO(3)(-) anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance.

NaCl and fluid secretion by the intestine of the teleostFundulus heteroclitus: involvement of CFTR

Sections of posterior intestine of the euryhaline killifish Fundulus heteroclitus adapted to sea water were stimulated by the calcium ionophore ionomycin (1 μmol l–1) in combination with agents to elevate intracellular cyclic AMP levels, 0.5 mmol l–1 dibutyryl-cyclic AMP (db-cAMP) with 0.1 mmol l–1 3-isobutyl-1-methylxanthine (IBMX). Intestinal bag preparations from recently fed animals (but not from overnight unfed animals) changed from fluid absorption (+18.9±8.30 μl cm–2 h–1 , N=8) in the untreated control period to net fluid secretion after stimulation (–7.43±1.30 μl cm–2 h–1, N=8, P&lt;0.01; means ± s.e.m.), indicative of the capacity of teleost intestine to undergo secretion. Posterior intestinal pieces mounted in vitro in Ussing-style membrane chambers showed net Cl– uptake (+2.245±0.633 μequiv cm–2 h–1, N=7) that turned to net secretion following stimulation by ionomycin + db-cAMP + IBMX (–3.809±1.22 μequiv cm–2 h–1, N=7, P&lt;0.01). Mucosal application of the anion channel blocker 1 mmol l–1 diphenylamine-2-carboxylate (DPC) after ionomycin + db-cAMP + IBMX treatment significantly reduced serosal-to-mucosal unidirectional Cl– flux (P&lt;0.001), net Cl– flux (P&lt;0.05), short-circuit current (Isc, P&lt;0.001) and tissue conductance (Gt, P&lt;0.001), while 0.1 mmol l–1 4,4′-diisothiocyano-2,2′-stilbene-disulphonic acid (DIDS, a blocker of anion exchange) was without effect. Stimulation by db-cAMP + IBMX (no ionomycin) significantly increased unidirectional fluxes, Isc and Gt but did not produce net Cl– secretion. Ionomycin alone produced a transient increase in Isc but had no effect on Gt and caused no significant changes in unidirectional or net Cl– fluxes. Addition of db-cAMP + IBMX after ionomycin treatment produced net secretion of Cl– and large increases in unidirectional fluxes and Gt. Cystic fibrosis transmembrane conductance regulator (CFTR) was immunocytochemically localized with a monoclonal mouse antibody to the carboxy terminus and found to be present in the cytoplasm and basolateral membranes of all enterocytes and in the brush-border membrane of some cells, whereas NKCC immunofluorescence, demonstrating the presence of the Na+/K+/2Cl– cotransporter, was present in the cytoplasm and brush-border membrane. We conclude that the teleost intestine is capable of salt and fluid secretion only if intracellular Ca2+ and cyclic AMP pathways are stimulated together and that this secretion appears to involve activation of CFTR ion channels in the apical membrane of a subpopulation of enterocytes.

Ionoregulatory strategies and the role of urea in the Magadi tilapia (Alcolapia grahami)

The unique ureotelic tilapia Alcolapia grahami lives in the highly alkaline and saline waters of Lake Magadi, Kenya (pH ~10.0, alkalinity ~380 mmol·L–1, Na+~350 mmol·L–1, Cl–~110 mmol·L–1, osmolality ~580 mosmol·kg–1). In 100% lake water, the Magadi tilapia maintained plasma Na+, Cl–, and osmolality at levels typical of marine teleosts and drank the medium at 8.01 ± 1.29 mL·kg–1·h–1. Gill chloride cells were predominantly of the sea water type (recessed, with apical pits) but a few freshwater-type chloride cells (surficial, with flat apical exposure) were also present. Whole-body Na+and Cl–concentrations were relatively high and exhibited larger relative changes in response to salinity transfers than did plasma ions. All fish succumbed upon acute transfer to 1% lake water, but tolerated acute transfer to 10% lake water well, and gradual long-term acclimation to both 10 and 1% lake water without change in plasma cortisol. Plasma osmolytes were here maintained at levels typical of freshwater teleosts. Curiously, drinking continued at the same rate in fish adapted to 1% lake water, but chloride cells were now exclusively of the freshwater type. Significant mortality and elevated cortisol occurred after acute transfer to 200% lake water. However, the fish survived well during gradual adaptation to 200% lake water, although plasma cortisol remained chronically elevated. Urea levels accounted for only 2–3% of internal osmolality in 100% lake water but responded to a greater extent than plasma ions during exposure to 10 and 200% lake water, decreasing by 28–42% in the former and increasing by over 500% in the latter relative to simultaneous-control values. Urea thereby played a small but significant role (up to 8% of internal osmolality) in osmoregulation.

In vivo characterisation of intestinal zinc uptake in freshwater rainbow trout

Knowledge of the uptake mechanisms and metabolism of metals is essential for understanding the factors governing metal toxicity, discerning means by which acclimation and homeostasis may be achieved and characterising interactions between the metal of interest and other environmental moieties. Zinc is both an important aquatic contaminant and a vital micronutrient. The physiological characterisation of dietary zinc absorption in fish has, therefore, important implications for environmental protection and aquaculture. The present study aimed to elucidate the mechanism of intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss), using an in vivo cannulation technique. Only a saturable component of zinc uptake, with a concentration giving half-maximal rate of accumulation (K0.5) of 309 μmol l–1, and a maximal rate of accumulation (Jmax) of 933 nmol kg–1 h–1, was described. This characterised the intestine as a low-affinity, high-capacity zinc absorption pathway. Physiological mechanisms appear to regulate zinc uptake. Intestinal mucus was one important regulatory locus, promoting zinc uptake at low concentrations yet buffering the animal against high luminal zinc loads. Regulatory mechanisms also seemed to limit subepithelial zinc accumulation. Experiments using ethylene glycol tetraacetic acid (EGTA) to wash the intestinal lumen following zinc perfusion exhibited a higher proportion of loosely associated zinc at higher perfused concentrations. This was attributed to saturation of the uptake process or efflux from the subepithelium. Two distinct pathways for passage of zinc across the epithelium were discerned, with post-intestinal transfer possibly mediated by sulphydryl groups, as illustrated by N-ethylmaleimide perfusion experiments. Putative roles of zinc transporters and/or intracellular-binding proteins are discussed.

Carbonic anhydrase activity in tissues of the icefish Chionodraco hamatus and of the red-blooded teleosts Trematomus bernacchii and Anguilla anguilla

Carbonic anhydrase (CA) activity was measured in blood, intestine, kidney and gill of two Antarctic teleosts, the haemoglobinless Chionodraco hamatus and the red-blooded Trematomus bernacchii, and of the temperate teleost Anguilla anguilla. In all species, the highest CA activity was in the gills, with the greatest activity in C. hamatus. CA activity in the blood was highest in A. anguilla, but none was detected in the blood of C. hamatus despite the presence of plasma CA inhibitors. The enzyme was present but its activity was low in the intestine and kidney of all three species.

The existence of very high CA activity in C. hamatus gills compared with the red-blooded species was investigated further by isolating and characterising the branchial cytosolic CA isoforms. The turnover rate of the C. hamatus isoform was significantly higher than that of T. bernacchii and A. anguilla. The isoforms from both the Antarctic species exhibited lower apparent Km (Km,app) and heat stability than those from A. anguilla. Sensitivity to sulphonamides was similar in all species and was within the range of the mammalian CA II isoform. The branchial CA isoforms of C. hamatus, T. bernacchii and A. anguilla displayed relative molecular masses of 28.9, 29.9 and 31.2 kDa, respectively.

The results suggest that the hemoglobinless teleost possesses a different branchial cytosolic CA isoform from that of red-blooded teleosts.

Intestinal iron uptake in the European flounder (Platichthys flesus)

Iron is an essential element because it is a key constituent of the metalloproteins involved in cellular respiration and oxygen transport. There is no known regulated excretory mechanism for iron, and homeostasis is tightly controlled via its uptake from the diet. This study assessed in vivo intestinal iron uptake and in vitro iron absorption in a marine teleost, the European flounder Platichthys flesus. Ferric iron, in the form 59FeCl3, was reduced to Fe2+ by ascorbate, and the bioavailability of Fe3+ and Fe2+ were compared. In vivo Fe2+ uptake was significantly greater than Fe3+ uptake and was reduced by the iron chelator desferrioxamine. Fe2+ was also more bioavailable than Fe3+ in in vitro studies that assessed the temporal pattern and concentration-dependency of iron absorption. The posterior region, when compared with the anterior and mid regions of the intestine, was the preferential site for Fe2+ uptake in vivo. In vitro iron absorption was upregulated in the posterior intestine in response to prior haemoglobin depletion of the fish, and the transport showed a Q10 value of 1.94. Iron absorption in the other segments of the intestine did not correlate with haematocrit, and Q10 values were lower. Manipulation of the luminal pH had no effect on in vitro iron absorption. The present study demonstrates that a marine teleost absorbs Fe2+ preferentially in the posterior intestine. This occurs in spite of extremely high luminal bicarbonate concentrations recorded in vivo, which may be expected to reduce the bioavailability of divalent cations as a result of the precipitation as carbonates (e.g. FeCO3).