Electrolyte changes of serum and muscle,and related mortalities in maturingAnguilla rostrata migrating down the St. Lawrence Estuary (Canada)

This study was made to investigate changes in serum and muscle ion concentrations and related mortalities in maturingAnguilla rostrata migrating down the St. Lawrence Estuary. Mortalities take place in the freshwater portion of the St. Lawrence. Electrolyte concentrations of moribund eels taken in freshwater were compared to those of freshwater and salt water controls. Moribund eels had a much lower serum osmolality (270 mOsm/kg) than the controls (328 and 358 mOsm/kg). This resulted from low sodium (125 mEq/l) and particularly low chloride (69 mEq/l) contents in the moribund eels compared to the freshwater controls (153 and 117 mEq/l) and the salt water controls (179 and 137 mEq/l). There was also a general decrease in muscle ion concentrations in moribund eels though the percentage water was similar to that of the freshwater controls (64.0 and 63.7%). The changes measured between the freshwater controls and the salt water controls in nature are similar to those measured onAnguilla anguilla in laboratory. These results suggest that mortalities are related to failure by some of the maturing eels to maintain their mineral balance in freshwater. Hypothesis is made that maturing eels migrating long distances in freshwater or retarded by physical or chemical barriers, start to excrete sodium and chloride under hormonal control before they have reached brackish water. In the conditions that prevail in the St. Lawrence Estuary, this results in mineral unbalance and possibly in mortalities.     

A description of rodlet cells from the alimentary canal of Anguilla anguilla and their relationship with parasitic helminths

Rodlet cells in intestinal epithelia of infected and uninfected European eels Anguilla anguilla from brackish and fresh water were studied by light and electron microscopy. Deropristis inflata (Trematoda) was found in eels from brackish water, whereas eels from fresh water were infected with Acanthocephalus clavula (Acanthocephala). In a comparison between uninfected and infected eels from brackish water, a higher number of rodlet cells was recorded in the intestinal epithelia of infected fish. Evidence is presented that rodlet cells secrete their contents in a holocrine manner into the lumen of the eel intestine. The occurrence of organelles within the mature rodlet cell was rare. ? 1998 The Fisheries Society of the British Isles     

Osmoregulation in Rhizobium meliloti: inhibition of growth by salts

A defined medium of low osmolarity was developed permitting growth of Rhizobium meliloti with generation times of approximately 2.8 h doubling^-1. The effects of sodium, potassium, magnesium, ammonium, chloride, sulfate, phosphate, bicarbonate and acetate ions on the growth rate of R. meliloti were determined. Sodium, potassium and ammonium ions had little effect on growth at concentrations of 100 mEq or less; magnesium ion inhibited growth severely at concentrations of 50 mEq (25 mM). Of the anions, chloride and sulfate appeared to have little effect while phosphate, bicarbonate, and acetate inhibited growth at concentrations of as little as 25 mEq. The addition of proline, glutamate, or betaine to cells growing in inhibitory concentrations of NaCl did not relieve the inhibition. When grown in the presence of inhibitory levels of NaCl, the intracellular concentration of glutamate but not of proline or gamma amino butyric acid increased 5-fold.     

Stable isotope analyses provide new insights into ecological plasticity in a mixohaline population of European eel

Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can display extreme plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1‰ to ~30‰ in Lough Ahalia, a tidal Atlantic lake system. Significant differences were recorded in mean δ¹³C, δ¹⁵N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW), brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes.     

Plasma homeostasis and cloacal urine composition inCrocodylus porosus caught along a salinity gradient

Summary Juveniles of the Estuarine or Saltwater Crocodile,Crocodylus porosus, maintain both osmotic pressure and plasma electrolyte homeostasis along a salinity gradient from fresh water to the sea. In fresh water (FW) the cloacal urine is a clear solution rich in ammonium and bicarbonate and containing small amounts of white precipitated solids with high concentrations of calcium and magnesium. In salt water (SW) the cloacal urine has a much higher proportion of solids, cream rather than white in colour, which are the major route for excretion of potassium in addition to calcium and magnesium. Neither liquid nor solid fractions of the cloacal urine represent a major route for excretion of sodium chloride. The solids are urates and uric acid, and their production probably constitutes an important strategy for water conservation byC. porosus in SW. These data, coupled with natural history observations and the recent identification of lingual salt glands, contribute to the conclusion thatC. porosus is able to live and breed in either fresh or salt water and may be as euryhaline as any reptile.     

A chloride-activated Na(+)/HCO(3)(-)-coupled transport activity in corneal endothelial membranes

Investigations of corneal endothelium were made to resolve the apparent contradiction of the presence of sodium/bicarbonate cotransporter (NBC) in fresh and cultured cells and NBC's reported absence in isolated plasma membrane vesicles. Gradient-driven ion fluxes into the vesicles were measured. Short-term incubations (0-30 s) showed the presence of a bicarbonate-dependent inward sodium flux (BDSF), which was active when the insides of the vesicles were preloaded with chloride ions. The BDSF was absent if chloride was present only externally to the vesicles. Chloride at concentrations between 30 and 40 mM inside the vesicle had its maximum effect on BDSF. Other anions (acetate, thiocyanate, or gluconate) inside the vesicles did not mimic the chloride effect. Associated with the net inward sodium flux was a net inward bicarbonate flux. Hill plots of sodium influx with respect to external bicarbonate concentrations indicated that the stoichiometry of the net transfer was 1.7 +/- 0.2 (mean +/- standard error, n = 5) bicarbonate ions for each sodium ion transported. There was no net chloride flux found across the membrane vesicles. The finding of a novel chloride-activated NBC activity fully resolves the apparent contradiction between whole-cell and membrane vesicle preparations.     

The influence of water salinity and stage in life history on ion concentration of fish blood serum

(1) The Na, K, Ca, Mg concentration of blood serum have been studied in nearly 60 fish species from various waterbodies (the Neva, Lake Dal'nee, the Volga, Lake Baikal, the White Sea, the Black Sea, the Barents Sea etc.). In fishes (Cyprinidae, Percidae, Siluridae etc.) from fresh water (Lake Dal'nee, the Baikal, the Neva) with low 0.18–0.24 mEq/1 Na concentration Na content in blood serum was lower than in closely related species from fresh water (the Volga, Lyutoga River) with larger Na concentration–nearly 0.5–1 mEq 1. Blood serum of marine teleosts Na concentration (156–210 mEq/1) was higher than in migrating and freshwater species (90–174) mEq/1) and it did not depend on seawater salinity: Barents Sea–465, White Sea–255, Black Sea–227 mEq/1.
(2) Rather a considerable specific variation of ion concentration was found in blood serum of fishes from different families living in the same water body (Lake Baikal: Cyprinidae 90–95 mEq/1, Percidae 114 mEq/1, Salmonidae 126–129 mEq/1 etc.; the Volga: Siluridae 139 mEq/1, Percidae 142–150 mEq/1, Clupeidae 174 mEq/1).
(3) No relation has been detected between K, Ca and Mg concentrations in blood serum of fishes and the concentrations of these ions in the environment.
(4) Na concentration in the blood serum of sockeye salmon juveniles (101 mEq/1) migrating seaward was less than in adult fishes (132 mEq/1) returning to the lake to spawn. Na concentration in the blood of non-migrating salmonids ( Oncorhynchus nerka adult residual 104 mEq/1, Salvelinus alpinus juvenile 140 mEq/1, adult residual 141 mEq/1) changed insig- nificantly during life history. Notable hypopotassemia was observed in spawning time in Salmonidae.
(5) The comparison of blood serum ion concentration in males and females of fishes from different taxonomic groups (Acipenseridae, Salmonidae, Cyprinidae) showed no significant variation.     

Osmoregulatory mechanisms of the Australian freshwater crocodile, Crocodylus johnstoni, in freshwater and estuarine habitats

The estuary of the Limmen Bight River in Australia's Northern Territory is home to an unusual salt water-adapted population of the Australian `freshwater' crocodile, Crocodylus johnstoni. Crocodiles were captured from tidal reaches of the estuary ranging in salinity from 0.5–24‰ and from several permanent fresh water reaches more or less remote from saline waters. C. johnstoni is an effective osmoregulator in moderately saline waters and has osmoregulatory mechanisms very similar to its more marine-adapted relative, the estuarine crocodile Crocodylus porosus. Fasted C. johnstoni in brackish water appear to lose little sodium in cloacal urine, relying on their lingual salt glands for excretion of excess sodium chloride. The lingual glands show clear evidence of short-term and long-term acclimation to salt water. Like estuarine crocodiles, C. johnstoni drinks fresh water and will not drink sea water. Gross sodium and water fluxes in brackish water are very similar to those in other crocodilians, suggesting differences in integumental permeability are not a major influence on osmoregulatory differences between crocodilians. The data reinforce the hypothesis that crocodylids differ fundamentally from alligatorids in the structure and function of the renal-cloacal-salt gland complex and are of interest in current debate over the evolutionary and zoogeographical history of the eusuchian crocodilians. Accepted: 25 February 1999     

Evidence of prezygotic isolation,but not assortative mating,between locally adapted populations of Fundulus heteroclitus across a salinity gradient

Selection along environmental gradients can drive reproductive isolation and speciation. Among fishes, salinity is a major factor limiting species distributions, and despite its importance in generating species diversity, speciation events between marine and freshwater are rare. Here, we tested for mechanisms of reproductive isolation between locally adapted freshwater and brackish water-native populations of killifish, Fundulus heteroclitus, from either side of a hybrid zone along a salinity gradient. There was evidence for pre-zygotic endogenous reproductive isolation with reduced fertilization success between crosses of freshwater-native males and brackish water-native females. Exogenous pre-zygotic isolation was also present where females had highest fertilization in their native salinity. We used a replicated mass spawning design to test for mate choice in both brackish and fresh water. After genotyping 187 parents and 2523 offspring at 2347 SNPs across the genome, 85% of offspring were successfully assign to their parents. However, no reinforcing mate choice was observed. These results therefore demonstrate emerging, yet limited, reproductive isolation and incipient speciation across a marine to freshwater salinity gradient and suggest that both endogenous and exogenous mechanisms, but not assortative mating, contribute to divergence.     

Active NaCl absorption across split lamellae of posterior gills of Chinese crabs (Eriocheir sinensis) adapted to different salinities

Split lamellae of posterior gills of Eriocheir sinensis adapted to fresh water, brackish waters (9 or 18‰) or seawater (36‰) were mounted in Ussing chambers, and transepithelial short-circuit currents and conductances were measured with salines, containing approximately in vivo-like NaCl concentrations. Active sodium and chloride absorption (I_Na and I_Cl), the transcellular conductances and the leak conductance were identified with external amiloride and/or DIDS. Split gill lamellae of crabs adapted to fresh water displayed similar magnitudes of I_Na and I_Cl with 10 mmol l⁻¹ NaCl in the external medium (internally haemolymph-like NaCl saline). Augmenting external NaCl (50 mmol l⁻¹) resulted in an increase of I_Cl, whereas I_Na decreased. Split gill lamellae of crabs adapted to brackish waters (external NaCl of 125 and 225 mmol l⁻¹, respectively) showed lower currents than preparations of freshwater crabs (50 mmol l⁻¹ external NaCl). With split gill lamellae of seawater crabs no currents were detected (450 mmol l⁻¹ NaCl on both sides). The transcellular conductances showed similar changes as the currents. The leak conductance of split gill lamellae of crabs adapted to fresh or brackish waters was low (0.3–0.8 mS cm⁻²), whereas it was much higher (7 mS cm⁻²) with preparations of seawater crabs.     

Dog Red Blood Cells : Adjustment of salt and water content in vitro

Dog red blood cells (RBC) lack a ouabain-sensitive sodium pump, and yet they are capable of volume regulation in vivo. The present study was designed to find in vitro conditions under which dog RBC could transport sodium outward, against an electrochemical gradient. Cells were first loaded with sodium chloride and water by preincubation in hypertonic saline. They were then incubated at 37°C in media containing physiologic concentrations of sodium, potassium, chloride, bicarbonate, glucose, and calcium. The cells returned to a normal salt and water content in 16–20 h. Without calcium in the medium the cells continued slowly to accumulate sodium. Removal of glucose caused rapid swelling and lysis, whether or not calcium was present. The net efflux of sodium showed a close relationship to medium calcium over a concentration range from 0 to 5 mM. Extrusion of salt and water was also demonstrated in fresh RBC (no hypertonic preincubation) when calcium levels in the media were sufficiently raised. The ion and water movements in these experiments were not influenced by ouabain or by removal of extracellular potassium. Magnesium could not substitute for calcium. It is concluded that dog RBC have an energy-dependent mechanism for extruding sodium chloride which requires external calcium and is quite distinct from the sodium-potassium exchange pump.     

Effects of fresh and seawater ingestion on osmoregulation in Atlantic bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) are marine mammals with body water needs challenged by little access to fresh water and constant exposure to salt water. Osmoregulation has been studied in marine mammals for a century. Research assessing the effects of ingested fresh water or seawater in dolphins, however, has been limited to few animals and sampling times. Nine 16- to 25-h studies were conducted on eight adult dolphins to assess the hourly impact of fresh water, seawater, and seawater with protein ingestion on plasma and urine osmolality, urine flow rate (ufr), urinary and plasma solute concentrations, and solute clearance rates. Fresh water ingestion increased ufr. Fresh water ingestion also decreased plasma and urine osmolality, sodium and chloride urine concentrations, and solute excretion rates. Seawater ingestion resulted in increased ufr, sodium, chloride, and potassium urine concentrations, sodium excretion rates, and urine osmolality. Seawater with protein ingestion was associated with increased ufr, plasma osmolality, sodium excretion, and sodium, chloride, potassium, and urea urine concentrations. In conclusion, bottlenose dolphins appear to maintain water and plasma solute balance after ingesting fresh water or seawater by altering urine osmolality and solute clearance. Ingestion of protein with seawater appears to further push osmoregulation limits and urine solute concentrations in dolphins.     

A novel sperm adaptation to evolutionary constraints on reproduction: Pre‐ejaculatory sperm activation in the beach spawning capelin (Osmeridae)

Reproduction of external fertilizing vertebrates is typically constrained to either fresh or salt water, not both. For all studied amphibians and fishes, this constraint includes immotile sperm that are activated after ejaculation only by the specific chemistry of the fertilizing medium in which the species evolved (fresh, brackish, or salt water). No amphibians can reproduce in the sea. Although diadromous fishes may migrate between salt and fresh water, they are shackled to their natal environment for spawning in part because of sperm activation. Here, we report for the first time among all documented external fertilizing vertebrates, that in the absence of any external media, sperm are motile at ejaculation in a marine spawning fish (Osmeridae, capelin, Mallotus villosus). To illuminate why, we evaluated sperm behavior at different salinities in M. villosus as well as the related freshwater spawning anadromous rainbow smelt (Osmerus mordax). Surprisingly, sperm performance was superior in fresh water for both species. M. villosus spend their entire life at sea but our results show that their sperm are deactivated by sea water, suggesting a freshwater ancestry. By circumventing constraining water chemistry, we interpret the unique pre‐ejaculatory sperm activation in this species as a novel adaptation that enables fertilization in the marine environment. These findings also contribute to understanding the persistence of anadromy, despite great energetic costs to adult fishes.     

Lack of the Sodium-Driven Chloride Bicarbonate Exchanger NCBE Impairs Visual Function in the Mouse Retina

Regulation of ion and pH homeostasis is essential for normal neuronal function. The sodium-driven chloride bicarbonate exchanger NCBE (Slc4a10), a member of the SLC4 family of bicarbonate transporters, uses the transmembrane gradient of sodium to drive cellular net uptake of bicarbonate and to extrude chloride, thereby modulating both intracellular pH (pH_i) and chloride concentration ([Cl⁻]_i) in neurons. Here we show that NCBE is strongly expressed in the retina. As GABA_A receptors conduct both chloride and bicarbonate, we hypothesized that NCBE may be relevant for GABAergic transmission in the retina. Importantly, we found a differential expression of NCBE in bipolar cells: whereas NCBE was expressed on ON and OFF bipolar cell axon terminals, it only localized to dendrites of OFF bipolar cells. On these compartments, NCBE colocalized with the main neuronal chloride extruder KCC2, which renders GABA hyperpolarizing. NCBE was also expressed in starburst amacrine cells, but was absent from neurons known to depolarize in response to GABA, like horizontal cells. Mice lacking NCBE showed decreased visual acuity and contrast sensitivity in behavioral experiments and smaller b-wave amplitudes and longer latencies in electroretinograms. Ganglion cells from NCBE-deficient mice also showed altered temporal response properties. In summary, our data suggest that NCBE may serve to maintain intracellular chloride and bicarbonate concentration in retinal neurons. Consequently, lack of NCBE in the retina may result in changes in pH_i regulation and chloride-dependent inhibition, leading to altered signal transmission and impaired visual function.     

Further photophysical and photochemical characterization of flavins associated with single-shelled vesicles

Summary In order to investigate whether the loop diuretic sensitive, sodium-chloride cotransport system described previously in shark rectal gland is in fact a sodium-potassium chloride cotransport system, plasma membrane vesicles were isolated from rectal glands ofSqualus acanthias and sodium and rubidium uptake were measured by a rapid filtration technique. In addition, the binding of N-methylfurosemide to the membranes was investigated. Sodium uptake into the vesicles in the presence of a 170mm KCl gradient was initially about five-fold higher than in the presence of a 170mm KNO₃ gradient. In the presence of chloride, sodium uptake was inhibited 56% by 0.4mm bumetanide and 40% by 0.8mm N-methylfurosemide. When potassium chloride was replaced by choline chloride or lithium chloride, sodium uptake decreased to the values observed in the presence of potassium nitrate. Replacement of potassium chloride by rubidium chloride, however, did not change sodium uptake. Initial rubidium uptake into the membrane vesicles was about 2.5-fold higher in the presence of a 170mm NaCl gradient than in the presence of a 170mm NaNO₃ gradient. The effect of chloride was completely abolished by 0.4mm bumetanide. Replacement of the sodium chloride gradient by a lithium chloride gradient decreased rubidium uptake by about 40%; replacement by a choline chloride gradient reduced the uptake even further. Rubidium uptake was also strongly inhibited by potassium. Sodium chloride dependence and bumetanide inhibition of rubidium flux were also found in tracer exchange experiments in the absence of salt gradients. The isolated plasma membranes bound³[H]-N-methylfurosemide in a dose-dependent manner. In Scatchard plots, one saturable component could be detected with an apparentK _D of 3.5×10^–6 m and a number of sitesn of 104 pmol/mg protein. At 0.8 m, N-methylfurosemide binding decreased 51% when sodium-free or low-potassium media were used. The same decrease was observed when the chloride concentration was increased from 200 to 600mm or when 1mm bumetanide or furosemide were added to the incubation medium. These studies indicate that the sodium-chloride cotransport system described previously in the rectal gland is in fact a sodium-potassium chloride cotransport system. It is postulated that this transport system plays an essential role in the secondary active chloride secretion of the rectal gland.     

Salt excretion by the perfused head of trout adapted to sea water and its inhibition by adrenaline

Summary Isolated heads of trout (Salmo gairdneri) were used to study the unidirectional flux of sodium and chloride across the gills in salt water.Two perfusion techniques were employed. Under constant pressure perfusion, the addition of adrenaline during the perfusion causes an increase in the flow-rate. Under constant flow-rate conditions, adrenaline provokes a decrease in pressure. A comparison of influx determination made with these two techniques of perfusion shows that variations in flow-rate of perfusion do not affect the assessment of these fluxes.A net efflux of sodium, but not of chloride, is demonstrated in sea water. The effluxes of sodium and chloride observed in sea water are decreasedd about 50% during a rapid transfer to fresh water. The addition of potassium to this medium stimulates the effluxes of sodium and chloride, suggesting a Na/K exchange participating in the chloride excretion.Adrenaline causes an inhibition of sodium and chloride efflux in sea water which persists after transfer to fresh water and the addition of potassium. Only the influx of chloride is inhibited at a concentration of 10^–5 M whereas the sodium influx is unaffected. The presence of adrenaline results in a net influx of both sodium and chloride.The differential action of adrenaline on the influxes of sodium and chloride suggests that the hemodynamic modifications provoked by this catecholamine occur independently of its aforementioned ion exchange effects.     

Head‐shape polymorphism in Japanese eels Anguilla japonica in relation to differences of somatic growth in freshwater and brackish habitats

The age, total length (L_T), head shape and skull shape were investigated for 379 Japanese eels Anguilla japonica sampled in freshwater and brackish areas of the Kojima Bay–Asahi River system, Okayama, Japan, to learn about the differentiation process of head‐shape polymorphism. The relative mouth width (ratio of mouth width to L_T) of A. japonica > 400 mm L_T collected in fresh water was significantly greater than that of fish collected in brackish water. Growth rates of mouth width and the distance from the snout to the midpoint of the eyes (the ratio of width and distance to age, respectively) were not significantly different between freshwater and brackish‐water samples, whereas the somatic growth rate (the ratio of L_T to age) of freshwater samples was significantly lower than that of brackish‐water eel samples. These results suggest that the factors affecting head and somatic growth of A. japonica are not identical. According to these results and feeding patterns in each habitat reported by another study, it is suggested that somatic growth appears to play a significant role in the differentiation process of the head‐shape polymorphism in A. japonica, with the slow‐growing fish in fresh water becoming broad‐headed and the fast‐growing fish in brackish water becoming narrow‐headed.     

Active chloride transport in rabbit thick ascending limb of Henle's loop and elasmobranch rectal gland: chloride fluxes in isolated plasma membranes

Summary To investigate directly whether a sodium-potassium-chloride cotransport system is operating in the mammalian thick ascending limb of Henle's loop (TALH) and in the elasmobranch rectal gland, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and from rectal glands ofSqualus acanthias, and chloride uptake was measured by a rapid filtration technique. Chloride uptake into TALH vesicles in the presence of a 25 mM Na₂SO₄, 25 mM K₂SO₄ gradient reached 70% of equilibrium at 2.5 min. In the presence of both sodium and potassium, the 15 s chloride uptake was inhibited 35% by 1 mM bumetanide. When either sodium or potassium was removed from the incubation medium, chloride uptake decreased to the level observed in the presence of 1 mM bumetanide. 0.5 mM SITS had no effect on chloride uptake by the plasma membrane vesicles. This sodium and potassium dependent, bumetanide sensitive chloride uptake was also observed under tracer exchange conditions. Chloride uptake into rectal gland plasma membrane vesicles in the presence of a 50 mM Na₂SO₄, 50 mM K₂SO₄ gradient reached 80% of equilibrium at 2.5 min. 1 mM bumetanide inhibited the 15 s uptake of chloride by 34% and removal of either sodium or potassium from the incubation medium reduced chloride uptake to the level observed in the presence of bumetanide under both gradient and tracer exchange conditions. These studies provide additional support for the hypothesis that a sodium-potassium-chloride cotransport system is operating in these epithelia.Abbreviations SITS 4-acetamido-4-isothiocyanato-stilbene-2,2-disulfonic acid - TALH thick ascending limb of Henle's loop     

Effects of freshwater to seawater transfer on osmoregulation,acid-base balance and respiration in river migrating whitefish (Coregonus lavaretus)

Osmoregulation, acid-base balance and respiratory parameters were investigated in whitefish following transfer from freshwater to salt water. Whitefish acclimated successfully to 25 ppt brackish water but died after direct transfer to 32 ppt sea water. Transfer to brackish water induced rapid (<6 h) and permanent increases in plasma [Na⁺], [Cl^–], total [Ca] and [Mg]. The extracellular hyperosmolality effected a transient (<3 days) muscle tissue dehydration and red blood cell shrinkage. Exposure to brackish water decreased both the arterial O₂ tension and whole body O₂ uptake. The extracellular acid-base status changed from an initial respiratory acidosis at 1 h towards a pronounced metabolic acidosis at 48 h of brackish water exposure. Red cell pH_i decreased in parallel with extracellular pH_e, but the in vivo pH_i/pH_e was only 0.26, suggesting some selective protection of red cell pH_i. Plasma cortisol concentration and gill Na⁺, K⁺-ATPase activity increased after exposure to high ambient salinity, reflecting the induction of hypo-osmoregulatory mechanisms. The physiological changes in whitefish are discussed in relation to salinity-induced effects in other salmonid fishes.Abbreviations CO₂ solubility in plasma - water O₂ capacitance coefficient - BW brackish water - C _T total CO₂ content in plasma - FW fresh water - Hb hemoglobin - Hct hematocrit - M _b body mass of fish - MCHC mean cellular hemoglobin concentration - PCO₂ carbon dioxide tension - pH _e extracellular pH - pH _i intracellular pH - PO_{2 in} oxygen tension in water flowing in - PO_{2 out} oxygen tension in water flowing out - ppt parts per thousand - RBC red blood cell(s) - SW sea water - V _m water flows through chamber - OV ₂ ml O₂ consumed per kg per hour