Rectal gland morphology of freshwater and seawater acclimated bull sharks Carcharhinus leucas

To compare rectal gland morphology of bull sharks Carcharhinus leucas , animals captured in the freshwater reaches of the Brisbane River, Australia, were acclimated to sea water over 17 days with 1 week in the final salinity. A control group was left in fresh water for 17 days. Animals in fresh water and sea water were strongly hyper- and hypo-ionic with respect to plasma Na⁺ and Cl^–, respectively. This difference necessitates NaCl secretion by the rectal gland in sea water and conservation of NaCl in fresh water. Structural differences in the rectal gland of freshwater and seawater acclimated bull sharks were limited. There was no difference in rectal gland cross-sectional area, lumen area, rectal gland vein area, number of secretory tubules or secretory cells per secretory tubule in freshwater and seawater acclimated animals. At a cellular level, there was no difference between the degree of basolateral and lateral folding, number of mitochondria or number of desmosomes per tight junction. Tight junction width was significantly greater in seawater acclimated animals. The number of red blood cells in the interstitial tissue was also significantly higher in seawater acclimated animals, possibly as a result of increased blood perfusion of the secretory epithelia. The lack of major structural changes in the rectal glands of bull sharks acclimated to fresh water and sea water most likely represents the salinity gradient in the Brisbane River where animals are found throughout the river and can experience large fluctuations in salinity over short distances. Differences in rectal gland morphology of bull sharks in fresh water and sea water are discussed in terms of their relevance to osmoregulation in elasmobranchs.     

Freshwater to seawater acclimation of juvenile bull sharks (<Emphasis Type="Italic">Carcharhinus leucas</Emphasis>): plasma osmolytes and Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase activity in gill,rectal gland,kidney and intestine

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l^–1 kg^–1) were acclimated to SW (980–1,000 mOsm l^–1 kg^–1) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l^–1 kg^–1. In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l^–1 kg^–1) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na⁺, Cl^–, K⁺, Mg²⁺, Ca²⁺, urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na⁺/K⁺-ATPase activity. Na⁺/K⁺-ATPase activity in the gills and intestine was less than 1 mmol Pi mg^–1 protein h^–1 and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na⁺/K⁺-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na⁺/K⁺-ATPase activity was 5.6±0.8 and 9.2±0.6 mmol Pi mg^–1 protein h^–1, respectively. Na⁺/K⁺-ATPase activity in the kidney of FW and SW acclimated animals was 8.4±1.1 and 3.3±1.1 Pi mg^–1 protein h^–1, respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.     

Expression and distribution of Na, K-ATPase in gill and kidney of the spotted green pufferfish, Tetraodon nigroviridis, in response to salinity challenge

Freshwater (FW) spotted green pufferfish (Tetraodon nigroviridis) were transferred directly from a local aquarium to fresh water (FW; 0 per thousand ), brackish water (BW; 15 per thousand ), and seawater (SW; 35 per thousand ) conditions in the laboratory and reared for at least two weeks. No mortality was found. To investigate the efficient mechanisms of osmoregulation in the euryhaline teleost, distribution and expression of Na,K-ATPase (NKA) in gill and kidney of the pufferfish were examined and the osmolality, [Na+] and [Cl-] of the blood were assayed. The lowest levels of both relative protein abundance and activity were found to be exhibited in the BW group, and higher levels in the SW group than FW group. In all salinities, branchial NKA immunoreactivity was found in epithelial cells of the interlamellar region of the filament and not on the lamellae. Relative abundance of kidney NKA alpha-subunit, as well as the NKA activity, was found to be higher in the FW pufferfish than fish in BW or SW. Renal NKA appeared in the epithelial cells of distal tubules, proximal tubules, and collecting tubules, but not in glomeruli, in fish groups of various salinities. Plasma osmolality and chloride levels were significantly lower in FW pufferfish than those in BW and SW, whereas plasma sodium did not differ among the groups. Although identical distributions of NKA were found in either gill or kidney of FW-, BW- or SW-acclimated spotted green pufferfish, differential NKA expression in fish of various salinity groups was associated with physiological homeostasis (stable blood osmolality), and illustrated the impressive osmoregulatory ability of this freshwater and estuarine species in response to salinity challenge.     

Effects of environmental dilution on body fluid regulation in the yellow stingray, Urolophus jamaicensis

Adult yellow stingrays (Urolophus jamaicensis), collected off the southeast Florida coast, were maintained in filtered and re-circulated synthetic sea water (33 per thousand) for 5-13 days at 30 degrees C. Animals exposed to 82%, 74% and 66% SW in gradual steps exhibited rapid and significant weight gains followed by recovery to pre-dilution levels in 2-6 days. Acclimated animals at each salinity [100% (N=12), 82% (7), 74% (4) and 66% SW (3)] were anesthetized (MS222) and bled from the caudal vein. In 100% SW, stingray plasma was slightly hypo-osmotic to the external medium. Plasma osmolality decreased with stepwise dilutions, but became increasingly hyperosmotic to the bathing media. Plasma [Na] and [Cl] each decreased by approximately 13%, 23% and 16%, respectively, in 82%, 74% and 66% SW. Plasma [urea] decreased by 21%, 25% and 59%, respectively. Changes in plasma [K] and [Ca] were minor. Mean corpuscular [Hb] measurements suggest that stingray red cells swelled less at each dilution than predicted for a passive erythrocyte osmometer. RBC [K] decreased by 12%, 36% and 29%, respectively, in 82%, 74% and 66% SW. Quantitatively, the other measured electrolytes (Cl, Na and Ca) changed by lesser amounts. Results suggest that for mild and moderate dilutions (82% and 74% SW), yellow stingrays release both ions and urea from intracellular and extracellular compartments. With further dilution (66% SW), the elasmobranchs retain electrolytes at the expense of urea.     

Heat shock protein (Hsp70) induced by a mild heat shock slightly moderates plasma osmolarity increases upon salinity transfer in rainbow trout (Oncorhynchus mykiss)

We have investigated whether mild heat shock, and resulting Hsp70 expression, can confer cross-protection against the stress associated with transfer from freshwater (FW) to seawater (SW) in juvenile rainbow trout (Oncorhynchus mykiss). In experimental Series I, juvenile trout reared in FW were transferred from 13.5 degrees C to 25.5 degrees C in FW, held for 2 h, returned to 13.5 degrees C for 12 h, and then transferred to 32 ppt SW at 13.5 degrees C. Branchial Hsp70 increased approximately 10-fold in the heat-shocked fish relative to the control by the end of recovery and remained high 2, 8, and 24 h post-salinity transfer. However, no clear differences could be detected in blood parameters (blood hemoglobin, hematocrit, MCHC, plasma Na(+) and plasma osmolarity) or muscle water content between heat-shocked and sham-shocked fish in SW at any sampling interval (0, 2, 8, 24, 48, 120, 240 and 360 h post-SW transfer). In experimental Series II, trout acclimated to 8 degrees C were heat-shocked at 22 degrees C for 2 h, allowed to recover 18 h, and exposed to a more severe salinity transfer (either 36 or 45 ppt) than in Series I. Branchial Hsp70 levels increased approximately 6-fold in heat-shocked fish, but had declined to baseline after 120 h in SW. Plasma osmolarity and chloride increased in both groups upon transfer to 36 ppt; however, the increase was significantly less in heat-shocked fish when compared to the increase observed in sham-shocked fish at 24 h. No significant differences could be detected in branchial Na(+)/K(+)-ATPase activity or Na(+)/K(+)-ATPase alpha1a and alpha1b mRNA expression between the two groups. Our data indicate that a mild temperature shock has only modest effects on the ability of rainbow trout to resist osmotic stress during FW to SW transfer.     

Branchial osmoregulatory response to salinity in the gilthead sea bream, Sparus auratus

The branchial osmoregulatory response of gilthead sea bream (Sparus auratus L.) to short-term (2-192 hr) and long-term (2 weeks) exposure to different environmental salinities (5 per thousand, 15 per thousand, 25 per thousand, 38 per thousand and 60 per thousand) was investigated. A "U-shaped" relationship was observed between environmental salinity and gill Na+,K+ -ATPase activity in both long- and short-term exposure to altered salinity, with the increase in activity occurring between 24 and 96 hr after the onset of exposure. Plasma osmolality and plasma ions (sodium, chloride, calcium and potassium) showed a tendency to increase in parallel with salinity. These variables only differed significantly (P<0.05) in fish adapted to 60 per thousand salinity with respect to fish adapted to full-strength sea-water (SW). Plasma glucose remained unchanged whereas plasma lactate was elevated at 5 per thousand and 60 per thousand. Muscle water content (MWC) was significantly lower in fish adapted to 60 per thousand. Chloride cells (CC) were only present on the surface of the gill filaments and absent from the secondary lamellae. CC distribution was not altered by external salinity. However, the number and size of CC were significantly increased at salinity extremes (5 per thousand and 60 per thousand), whereas fish exposed to intermediate salinities (15 per thousand and 25 per thousand) had fewer and smaller cells. Furthermore, the CC of fish exposed to diluted SW became rounder whereas they were more elongated in fish in full-strength and hypersaline SW. This is consistent with previous reports indicating the existence of two CC types in euryhaline fish. At likely environmental salinities, gilthead sea bream show minor changes in plasma variables and the effective regulation of gill Na+,K+ -ATPase. However, at very low salinities both haemodilution and up-regulation of gill Na+,K+ -ATPase predict a poor adaptation most likely related to deficiency or absence of specific components of the CC important for ion xuptake.     

Plasma and erythrocyte solute properties of juvenile bull sharks, Carcharhinus leucas, acutely exposed to increasing environmental salinity

Plasma and erythrocyte solute properties were examined in freshwater (FW) acclimated juvenile Carcharhinus leucas following acute transfer to 75% seawater (SW), and 100% SW. Blood samples were taken at 0, 12 and 96 h following transfer to 75% SW and 24 and 72 h after transfer to 100% SW. A control group in FW was subjected to the same sampling regime. Upon transfer of C. leucas to 75% and 100% SW, plasma Na⁺, Cl⁻, K⁺, Mg²⁺, Ca²⁺, urea and TMAO concentrations all increased significantly but disproportionately. Plasma Na⁺ and Cl⁻ increased immediately, followed by an increase in plasma urea. Erythrocyte urea and TMAO concentrations increased significantly following transfer to 75% and 100% SW; however, changes in erythrocyte inorganic ion concentrations were insignificant. Haematocrit, haemoglobin and mean cell haematocrit did not differ significantly after transfer to seawater; however, plasma water was slightly reduced after 24 and 72 h in 100% SW. Red blood cell (RBC) water content was elevated 24 h after transfer to 100% SW but returned to FW levels after 72 h. These results demonstrate that the transfer from 75% to 100% SW presents C. leucas with a greater osmoregulatory challenge than transfer from FW to 75% SW, despite the larger concentration gradient in the latter. In summary, C. leucas tolerate rapid and significant increases in salinity by rapidly increasing plasma osmolality to be hyperosmotic to the environment whilst maintaining a tight regulation of their intracellular fluid environment.     

Nitrogen metabolism and excretion in the aquatic chinese soft-shelled turtle, Pelodiscus sinensis, exposed to a progressive increase in ambient salinity

This study aimed to determine effects of 6-day progressive increase in salinity from 1 per thousand to 15 per thousand on nitrogen metabolism and excretion in the soft-shelled turtle, Pelodiscus sinensis. For turtles exposed to 15 per thousand water on day 6, the plasma osmolality and concentrations of Na+, Cl- and urea increased significantly, which presumably decreased the osmotic loss of water. Simultaneously, there were significant increases in contents of urea, certain free amino acids (FAAs) and water-soluble proteins that were involved in cell volume regulation in various tissues. There was an apparent increase in proteolysis, releasing FAAs as osmolytes. In addition, there might be an increase in catabolism of certain amino acids, producing more ammonia. The excess ammonia was retained as indicated by a significant decrease in the rate of ammonia excretion on day 4 in 15 per thousand water, and a major portion of it was converted to urea. The rate of urea synthesis increased 1.4-fold during the 6-day period, although the capacity of the hepatic ornithine urea cycle remained unchanged. Urea was retained for osmoregulation because there was a significant decrease in urea excretion on day 4. Increased protein degradation and urea synthesis implies greater metabolic demands, and indeed turtles exposed to 15 per thousand water had significantly higher O2 consumption rate than the freshwater (FW) control. When turtles were returned from 15 per thousand water to FW on day 7, there were significant increases in ammonia (probably released through increased amino acid catabolism) and urea excretion, confirming that FAAs and urea were retained for osmoregulatory purposes in brackish water.     

Hepatic urea biosynthesis in the euryhaline elasmobranch Carcharhinus leucas

Plasma urea levels and hepatic urea production in the euryhaline bull shark, Carcharhinus leucas, acclimated to freshwater and seawater environments were measured. It was found that plasma urea concentration increased with salinity and that this increase was, in part, the result of a significant increase in hepatic production of urea. This study provides direct evidence that hepatic production of urea plays an important role in the osmoregulatory strategy of C. leucas.     

Response of prolactin cells to environmental calcium in the eel (Anguilla anguilla L.)

Prolactin (PRL) cell activity was investigated in eels kept in fresh water (FW), deionized water (DW) supplemented or not with Ca (2 mM), in Ca-enriched FW (10 mM), in normal (Ca 3.4 mM) or Ca-free 1/3 sea water (SW), and in SW (Ca 10.2 mM) or Ca-free SW (Ca 0.15 mM). Light-microscopic studies, including measurement of the nuclear area and cell height, showed that PRL cell activity, reduced in DW, is not affected by Ca supplementation. Activity is reduced in Ca-enriched FW, in 1/3 SW and in SW, conditions inducing an increase in the plasma sodium level. The lack of calcium in saline environments partly suppresses the nuclear atrophy occurring in SW. There is no significant correlation between external or total plasma calcium concentration and PRL cell activity. In artificial Ca-free SW, eels show a rapid increase in plasma osmolarity and sodium levels; there is a significant negative correlation between these two plasma values and the nuclear area or cell height of PRL cells. As in some other teleosts, plasma osmolarity and plasma sodium seem to play a more important role than external or internal calcium in controlling PRL secretion. This correlation is not apparent in eels kept in SW, having unstimulated PRL cells but active calcium-sensitive (Ca-s) cells in the pars intermedia.     

Salinity regulates claudin mRNA and protein expression in the teleost gill

The teleost gill carries out NaCl uptake in freshwater (FW) and NaCl excretion in seawater (SW). This transformation with salinity requires close regulation of ion transporter capacity and epithelial permeability. This study investigates the regulation of tight-junctional claudins during salinity acclimation in fish. We identified claudin 3- and claudin 4-like immunoreactive proteins and examined their expression and that of select ion transporters by performing Western blot in tilapia (Oreochromis mossambicus) gill during FW and SW acclimation. Transfer of FW tilapia to SW increased plasma osmolality, which was corrected after 4 days, coinciding with increased gill Na+-K+-ATPase and Na+-K+-2Cl(-) cotransporter expression. Gill claudin 3- and claudin 4-like proteins were reduced with exposure to SW. Transfer to FW increased both claudin-like proteins. Immunohistochemistry shows that claudin 3-like protein was localized deep in the FW gill filament, whereas staining was found apically in SW gill. Claudin 4-like proteins are localized predominantly in the filament outer epithelial layer, and staining appears more intense in the gill of FW versus SW fish. In addition, tilapia claudin 28a and 30 genes were characterized, and mRNA expression was found to increase during FW acclimation. These studies are the first to detect putative claudin proteins in teleosts and show their localization and regulation with salinity in gill epithelium. The data indicate that claudins may be important in permeability changes associated with salinity acclimation and possibly the formation of deeper tight junctions in FW gill. This may reduce ion permeability, which is a critical facet of FW osmoregulation.     

TEP on the tide in killifish (<Emphasis Type="Italic">Fundulus heteroclitus</Emphasis>): effects of progressively changing salinity and prior acclimation to intermediate or cycling salinity

Transepithelial potentials (TEP) were measured in killifish, acclimated to freshwater (FW), seawater (SW), 33% SW or cycling salinities relevant to tidal cycles in an estuary, and subsequently subjected to salinity changes in progressive or random order. Random compared to progressive salinity changes in an upward or downward direction in FW- and SW-acclimated fish, respectively, did not greatly influence responses to salinity change. Fish acclimated to SW or 33% SW as well as those acclimated to cycling salinities behaved similarly (TEP more positive than +15 mV in 100% SW, decreasing to ~0 mV at 20–40% SW, and more negative than −30 mV in FW). In contrast, FW-acclimated fish displayed a less pronounced TEP response to salinity (0 mV in FW through 20% SW, increasing thereafter to values more positive than +10 mV at 100% SW). We conclude that when evaluated under estuarine tidal conditions, the killifish gill exhibits adaptive electrical characteristics, opposing Na⁺ loss at low salinity and favouring Na⁺ extrusion at high salinity, changes explained at least in part by the Cl⁻ to Na⁺ permeability ratio. Thus animals living in the estuaries can move to lower and higher salinities for short periods with little physiological disturbance, but this ability is lost after acclimation to FW.     

Stimulation of non-specific immune functions in seawater-acclimated rainbow trout, Oncorhynchus mykiss, with reference to the role of growth hormone

The influence of acclimation to seawater (SW) and growth hormone (GH) administration on immune functions was examined in the rainbow trout (Oncorhynchus mykiss). After 3 days acclimation to dilute SW (12 parts per thousand, ppt), an increase in plasma lysozyme activity was observed compared to the fish kept in fresh water (FW). No change was seen in plasma immunoglobulin M (IgM) levels. When they were transferred from dilute SW to full-strength SW (29 ppt) after a single intra-peritoneal injection of ovine or salmon GH, plasma sodium levels of GH-treated fish were significantly lower than those of the control fish injected with Ringer's solution 24 h after the transfer. The plasma level of IgM was not influenced by GH injection in the fish kept in FW nor in those transferred to SW. The administration of GH increased plasma lysozyme activity in the fish in FW, but no further increase was seen after SW transfer. The production of superoxide anions in peripheral blood leucocytes was stimulated by GH in both FW and SW. These results suggest that GH is involved in the stimulation of the non-specific immune functions in SW-acclimated salmonids.     

Exchange diffusion effect and euryhalinity in teleosts

The sea water (SW)-adapted euryhaline Platichthys flesus, and the marine Serranus exchange about 50% of their internal sodium with the external sodium per hour. This rate of exchange decreases with decreasing salinity of the adaptation medium. When the flounder is transferred from SW to FW an instantaneous 90% reduction of the Na and Cl outflux is observed. About 30 min later a second, progressive, reduction occurs. The outflux reductions appear to result from two types of regulatory mechanisms reducing gill permeability and preventing excessive salt loss. The first reduction corresponds to independent "Na- and Cl-free effects" as shown by transfers to artificial media containing either Na or Cl with an impermeant co-ion. The pattern of simultaneous rapid variations of Na influx and outflux for a range of salinity changes in flounder adapted to SW, 1/2 SW, or 1/4 SW has been studied. The data are compatible with the hypothesis of an exchange diffusion mechanism characterized by a coupling of both unidirectional fluxes. The affinity of the exchange diffusion carrier for sodium has been measured (Km approximately equal to 400 mM). The delayed reduction would result from a progressive diminution of the quantity of carrier available but without modification of its affinity for sodium. When the stenohaline marine perch is transferred from SW to FW, a 40% reduction of the outflux is observed. But it is not the result of an exchange diffusion effect as it is related to the external osmolarity change and not to the NaCl concentration change. Furthermore no delayed reduction is observed after transfer into FW. This transfer is accompanied by a heavy loss of electrolytes resulting in a rapid decline of the plasma electrolyte level and death. A comparative survey of the relative importance of these regulatory mechanisms has been made.     

The expression of gill Na,K-ATPase in milkfish,Chanos chanos,acclimated to seawater,brackish water and fresh water

Juvenile milkfish Chanos chanos (Forssk?l, 1775) were transferred from a local fish farm to fresh water (FW; 0 per thousand ), brackish water (BW; 10 per thousand, 20 per thousand ) and seawater (SW; 35 per thousand ) conditions in the laboratory and reared for at least two weeks. The blood and gill of the fish adapted to various salinities were analyzed to determine the osmoregulatory ability of this euryhaline species. No significant difference was found in plasma osmolality, sodium or chloride concentrations of milkfish adapted to various salinities. In FW, the fish exhibited the highest specific activity of Na, K-ATPase (NKA) in gills, while the SW group was found to have the lowest. Relative abundance of branchial NKA alpha-subunit revealed similar profiles. However, in contrary to other euryhaline teleosts, i.e. tilapia, salmon and eel, the naturally SW-dwelling milkfish expresses higher activity of NKA in BW and FW. Immunocytochemical staining has shown that most Na, K-ATPase immunoreactive (NKIR) cells in fish adapted to BW and SW were localized to the filaments with very few on the lamellae. Moreover, in FW-adapted milkfish, the number of NKIR cells found on the lamellae increased significantly. Such responses as elevated NKIR cell number and NKA activity are thought to improve the osmoregulatory capacity of the milkfish in hyposaline environments.     

Lipids as energy source during salinity acclimation in the euryhaline crab Chasmagnathus granulata dana, 1851 (crustacea-grapsidae)

Lipids seem to be the major energy store in crustaceans. Moreover, they are extremely important in maintaining structural and physiological integrity of cellular and sub cellular membranes. During salinity adaptation, energy-demanding mechanisms for hemolymph osmotic and ionic regulation are activated. Thus, the main goal of this work was to verify the possible involvement of lipids as an energy source in the osmotic adaptation process. The estuarine crab Chasmagnathus granulata was captured and acclimated to salt water at 20 per thousand salinity and 20 +/- 2 degrees C, for 30 days. After acclimation, crabs were divided into groups of ten and transferred to fresh water (0 per thousand ), salt water at 40 per thousand salinity, or maintained in salt water at 20 per thousand salinity (control group), without feeding. Before and seven days after the salinity change, wet weight and lipid concentration in gills, muscle, hepatopancreas, and hemolymph were determined according to the colorimetric assay of sulphophosphovanilin. Results show that hepatopancreas lipids were not mobilized during osmotic stress regulation. Gill and muscle lipids were significantly lower in crabs subjected to hypo-osmotic stress than those subjected to the hyper-osmotic stress or maintained at the control salinity. Our results point to the occurrence of lipid mobilization and involvement of these compounds in the osmotic acclimation process in C. granulata, but with differences between tissues and the osmotic shock (hypo or hyper) considered.     

Osmoregulation and expression of ion transport proteins and putative claudins in the gill of southern flounder (Paralichthys lethostigma)

The southern flounder is a euryhaline teleost that inhabits ocean, estuarine, and riverine environments. We investigated the osmoregulatory strategy of juvenile flounder by examining the time-course of homeostatic responses, hormone levels, and gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein expression after salinity challenge. Transfer of freshwater (FW)-acclimated flounder to sea water (SW) induced an increase in plasma osmolality and cortisol and a decrease in muscle water content, plasma insulin-like growth factor I (IGF-I) and hepatic IGF-I mRNA, all returning to control levels after 4 days. Gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein levels were elevated in response to SW after 4 days. Transfer of SW-acclimated flounder to FW reduced gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein, increased plasma IGF-I, but did not alter hepatic IGF-I mRNA or plasma cortisol levels. Gill claudin-3 and claudin-4 immunoreactive proteins were elevated in FW versus SW acclimated flounder. The study demonstrates that successful acclimation of southern flounder to SW or FW occurs after an initial crisis period and that the salinity adaptation process is associated with changes in branchial expression of ion transport and putative tight junction claudin proteins known to regulate epithelial permeability in mammalian vertebrates.     

Time course of extracellular acid-base adjustments under hypo- or hyperosmotic conditions in the euryhaline fish Platichthys flesus

In order to better understand the basis for the euryhalinity of the flounder, Platichthys flesus , which tolerates large variations in water salinity, experiments have been designed to characterize the time course of extracellular ionic and acid-base adjustments under hypo- or hyperosmotic conditions. Abrupt transfer from sea water (SW) to fresh water (FW) provokes a transient decrease in the plasma osmolality (Posm) and a concomitant transient metabolic alkalosis (whole blood pH 7.78 in SW and 8.04 five days after FW transfer) associated with a marked, persistent hypercapnia. After 33 days in FW, Posm and whole blood pH are not significantly different from those in SW, but whole blood Pco₂ and plasma bicarbonate concentration are always higher than SW values. Opposite transitory fluctuations, i.e. a metabolic acidosis associated with a respiratory alkalosis, occur when flounder long-acclimated to FW are again exposed to SW. The mechanisms involved in these salinity-dependent acid-base disturbances are rather complex and remain to be elucidated. These observations attest to the importance of the extracellular acid-base changes that may be (i) linked to extracellular anisosmotic regulation and/or to cellular metabolic adjustments, and (ii) compensated partially by ventilatory adjustments.     

Effects of environmental salinity and temperature on osmoregulatory ability, organic osmolytes, and plasma hormone profiles in the Mozambique tilapia (Oreochromis mossambicus)

The Mozambique tilapia, Oreochromis mossambicus, is capable of surviving a wide range of salinities and temperatures. The present study was undertaken to investigate the influence of environmental salinity and temperature on osmoregulatory ability, organic osmolytes and plasma hormone profiles in the tilapia. Fish were acclimated to fresh water (FW), seawater (SW) or double-strength seawater (200% SW) at 20, 28 or 35 degrees C for 7 days. Plasma osmolality increased significantly as environmental salinity and temperature increased. Marked increases in gill Na(+), K(+)-ATPase activity were observed at all temperatures in the fish acclimated to 200% SW. By contrast, Na(+), K(+)-ATPase activity was not affected by temperature at any salinity. Plasma glucose levels increased significantly with the increase in salinity and temperature. Significant correlations were observed between plasma glucose and osmolality. In brain and kidney, content of myo-inositol increased in parallel with plasma osmolality. In muscle and liver, there were similar increases in glycine and taurine, respectively. Glucose content in liver decreased significantly in the fish in 200% SW. Plasma prolactin levels decreased significantly after acclimation to SW or 200% SW. Plasma levels of cortisol and growth hormone were highly variable, and no consistent effect of salinity or temperature was observed. Although there was no significant difference among fish acclimated to different salinity at 20 degrees C, plasma IGF-I levels at 28 degrees C increased significantly with the increase in salinity. Highest levels of IGF-I were observed in SW fish at 35 degrees C. These results indicate that alterations in gill Na(+), K(+)-ATPase activity and glucose metabolism, the accumulation of organic osmolytes in some organs as well as plasma profiles of osmoregulatory hormones are sensitive to salinity and temperature acclimation in tilapia.     

The effect of environmental hypercapnia and salinity on the expression of NHE-like isoforms in the gills of a euryhaline fish (Fundulus heteroclitus)

The current models for branchial acid excretion in fishes include Na(+)/H(+) exchange and the electrogenic excretion of H+ via H+-ATPase. The predominant route of acid excretion in some freshwater fishes is thought to be via the H+-ATPase/Na+ channel system. The euryhaline Fundulus heteroclitus may not fit this profile even when adapted to freshwater (FW). We hypothesize that the Na+/H+ exchanger (NHE) in this species may play a predominant role in acid-base regulation for both marine and FW adapted animals. Acidosis induced by ambient hypercapnia (1% CO2 in air), resulted in an increase in net H+ excretion to the water in F. heteroclitus pre-adapted to FW, brackish (isoosmotic; BW) and seawater (SW). Both FW and SW adapted mummichogs were tested for NHE protein expression using mammalian NHE antibodies, and we identified NHE-like immunoreactive proteins in gill membrane preparations from both groups. Hypercapnia induced a approximately three-fold elevation in gill NHE2-like protein in FW animals but SW adapted fish showed inconsistent NHE3-like protein expression. There was no change in NHE-1 levels in FW fish. In contrast, SW animals demonstrated a significant increase in both NHE1 and NHE3-like proteins following hypercapnia but limited expression of the NHE2 protein. We hypothesize that different isoforms of NHE may be preferentially expressed depending on the salinity to which the animals are adapted. Net H+ transfers during acidosis may be driven, at least in part by the action of these transporters.