Immunolocalization of chloride transporters to gill epithelia of euryhaline teleosts with opposite salinity-induced Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase responses

Opposite patterns of branchial Na⁺/K⁺-ATPase (NKA) responses were found in euryhaline milkfish (Chanos chanos) and pufferfish (Tetraodon nigroviridis) upon salinity challenge. Because the electrochemical gradient established by NKA is thought to be the driving force for transcellular Cl⁻ transport in fish gills, the aim of this study was to explore whether the differential patterns of NKA responses found in milkfish and pufferfish would lead to distinct distribution of Cl⁻ transporters in their gill epithelial cells indicating different Cl⁻ transport mechanisms. In this study, immunolocalization of various Cl⁻ transport proteins, including Na⁺/K⁺/2Cl⁻ cotransporter (NKCC), cystic fibrosis transmembrane conductance regulator (CFTR), anion exchanger 1 (AE1), and chloride channel 3 (ClC-3), were double stained with NKA, the basolateral marker of branchial mitochondrion-rich cells (MRCs), to reveal the localization of these transporter proteins in gill MRC of FW- or SW-acclimated milkfish and pufferfish. Confocal microscopic observations showed that the localization of these transport proteins in the gill MRCs of the two studied species were similar. However, the number of gill NKA-immunoreactive (IR) cells in milkfish and pufferfish exhibited to vary with environmental salinities. An increase in the number of NKA-IR cells should lead to the elevation of NKA activity in FW milkfish and SW pufferfish. Taken together, the opposite branchial NKA responses observed in milkfish and pufferfish upon salinity challenge could be attributed to alterations in the number of NKA-IR cells. Furthermore, the localization of these Cl⁻ transporters in gill MRCs of the two studied species was identical. It depicted the two studied euryhaline species possess the similar Cl⁻ transport mechanisms in gills.     

Acclimation of brackish water pearl spot (Etroplus suratensis) to various salinities: relative changes in abundance of branchial Na+/K+-ATPase and Na+/K+/2Cl− co-transporter in relation to osmoregulatory parameters

The present study was conducted to elucidate the osmoregulatory ability of the fish pearl spot (Etroplus suratensis) to know the scope of this species for aquaculture under various salinities. Juvenile pearl spot were divided into three groups and acclimated to freshwater (FW), brackish water (BW) or seawater (SW) for 15 days. The fish exhibited effective salinity tolerance under osmotic challenges. Although the plasma osmolality and Na⁺, K⁺ and Cl^? levels increased with the increasing salinities, the parameters remained within the physiological range. The muscle water contents were constant among FW-, BW- and SW-acclimated fish. Two Na+/K+-ATPase α-isoforms (NKA α) were expressed in gills during acclimation in FW, BW and SW. Abundance of one isoform was up-regulated in response to seawater acclimation, suggesting its role in ion secretion similar to NKA α1b, while expression of another isoform was simultaneously up-regulated in response to both FW and SW acclimation, suggesting the presence of isoforms switching phenomenon during acclimation to different salinities. Nevertheless, NKA enzyme activities in the gills of the SW and FW individuals were higher (p < 0.05) than in BW counterparts. Immunohistochemistry revealed that Na⁺/K⁺-ATPase immunoreactive (NKA-IR) cells were mainly distributed in the interlamellar region of the gill filaments in FW groups and in the apical portion of the filaments in BW and SW groups. The number of NKA-IR cells in the gills of the FW-acclimated fish was almost similar to that of SW individuals, which exceeded that of the BW individuals. The NKA-IR cells of BW and SW were bigger in size than their FW counterparts. Besides, the relative abundance of branchial Na⁺/K⁺/2Cl^? co-transporter showed stronger evidence in favor of involvement of this protein in hypo-osmoregulation, requiring ion secretion by the chloride cells. To the best of our knowledge, this is the first study reporting the wide salinity tolerance of E. suratensis involving differential activation of ion transporters and thereby suggesting its potential as candidate for fish farming under different external salinities.     

A field and laboratory study of the responses of cytoprotection and osmoregulation to salinity stress in mosquitofish (<Emphasis Type="Italic">Gambusia affinis</Emphasis>)

The mosquitofish (Gambusia affinis) naturally inhabits freshwater (FW; 1–3‰) and seawater (SW; 28–33‰) ponds in constructed wetland. To explore the physiological status and molecular mechanisms for salinity adaptation of the mosquitofish, cytoprotective responses and osmoregulation were examined. In the field study, activation of protein quality control (PQC) mechanism through upregulation of the abundance of heat shock protein (HSP) 90 and 70 and ubiquitin-conjugated proteins was found in the mosquitofish gills from SW pond compared to the individuals of FW pond. The levels of aggregated proteins in mosquitofish gills had no significant difference between FW and SW ponds. Furthermore, the osmoregulatory responses revealed that the body fluid osmolality and muscle water contents of the mosquitofish from two ponds were maintained within a physiological range while branchial Na⁺/K⁺-ATPase (NKA) expression was higher in the individuals from SW than FW ponds. Subsequently, to further clarify whether the cellular stress responses and osmoregulation were mainly induced by hypertonicity, a laboratory salinity acclimation experiment was conducted. The results from the laboratory experiment were similar to the field study. Branchial PQC as well as NKA responses were induced by SW acclimation compared to FW-acclimated individuals. Taken together, induction of gill PQC and NKA responses implied that SW represents an osmotic stress for mosquitofish. Activation of PQC was suggested to provide an osmoprotection to prevent the accumulation of aggregated proteins. Moreover, an increase in branchial NKA responses for osmoregulatory adjustment was required for the physiological homeostasis of body fluid osmolality and muscle water content.     

Phenotypic plasticity in gene expression and physiological response in red drum <Emphasis Type="Italic">Sciaenops ocellatus</Emphasis> exposed to a long-term freshwater environment

Red drum (Sciaenops ocellatus) is a euryhaline fish commonly found in the Gulf of Mexico and along the Atlantic coast of North America. Because of high commercial demand and its euryhaline characteristics, aquaculture of this species has diversified from marine to low-salinity aquaculture systems. In recent years, interest in the feasibility of producing red drum in inland freshwater systems has grown and this prompted us to investigate its osmoregulatory capacity after rearing for 8 months in a freshwater aquaculture system. We compared the activities of several genes and enzymes involved in the osmoregulatory process in freshwater-acclimatized (FW) and seawater (SW) red drum. The gene expression profiles were variable: the expression of genes encoding Na⁺/K⁺-ATPase (NKA) and the cystic fibrosis transmembrane regulator (CFTR) was slightly higher in SW than FW fish, while phosphoenolpyruvate carboxykinase (PEPCK) and the glucocorticoid receptor messenger RNA (mRNA) levels were higher in FW red drum. The total plasma K concentration was 60.3% lower, and gill NKA activity was 63.5% lower in FW than in SW fish. PEPCK activity was twofold higher in FW than in SW red drum. Similarly, liver glycogen was 60% higher in FW fish. In summary, both gene expression and the enzyme activity data support the phenotypic plasticity of red drum and suggest that the limited capacity for ion homeostasis observed, in particular the low plasma K concentration, was due to the composition of freshwater and does not necessarily reflect a physiological inability to osmoregulate.     

Effects of low environmental salinity on the cellular profiles and expression of Na+, K+-ATPase and Na+, K+, 2Cl? cotransporter 1 of branchial mitochondrion-rich cells in the juvenile marine fish Monodactylus argenteus

The goal of this study was to determine the osmoregulatory ability of a juvenile marine fish, silver moony (Monodactylus argenteus), for the purpose of developing a new experimental species for ecophysiological research. In this study, M. argenteus was acclimated to freshwater (FW), brackish water (BW), or seawater (SW). The salinity tolerance of this euryhaline species was effective, and the fish survived well upon osmotic challenges. The largest apical surface of mitochondrion-rich cells was found in the FW individuals. Immunohistochemical staining revealed that Na(+), K(+)-ATPase immunoreactive (NKA-IR) cells were distributed in the interlamellar region of the gill filaments of the silver moony in all experimental groups. In addition to the filaments, NKA-IR cells were also found in the lamellae of the FW individuals. The number of NKA-IR cells in the gills of the FW individuals exceeded that of the BW and SW individuals. The NKA-IR cells of FW and SW individuals exhibited bigger size than that of BW fish. The NKA activities and protein expression of the NKA α-subunit in the gills of the FW individuals were significantly higher than in the BW and SW groups. Additionally, the relative amounts of Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were salinity-dependent in the gills. Immunofluorescent signals of NKCC1 were localized to the basolateral membrane of NKA-IR cells in all groups. In the gills of the FW individuals, however, some NKA-IR cells did not exhibit a basolateral NKCC1 signal. In conclusion, the present study illustrated the osmoregulatory mechanisms of this easy- and economic-to-rear marine teleost with euryhaline capacity and proved the silver moony to be a good experimental animal.     

Comparative time-course study on pituitary and branchial response to salinity challenge in Mozambique tilapia (<Emphasis Type="Italic">Oreochromis mossambicus</Emphasis>) and Nile tilapia (<Emphasis Type="Italic">O</Emphasis>. <Emphasis Type="Italic">niloticus</Emphasis>)

The physiological response of Mozambique and Nile tilapia transferred from fresh to brackish (15 ppt) water was compared during a one-week time course. Response in the pituitary was measured by the gene expression pattern of prolactin (PRL I), growth hormone (GH), and calcium-sensing receptor (CaSR), while the response in the gills was measured by the gene expression pattern of the prolactin receptor (PRL-R), Na⁺/K⁺/2Cl⁻ cotransporter (NKCC) and Na⁺/Cl⁻ cotransporter (NCC), and by activity and expression of Na⁺/K⁺-ATPase (NKA). The time-course curves of plasma osmolality levels indicate a rapid elevation 24 h after transfer, which later decreased and maintained at stable level. PRL I expression decreased in both species, but with stronger response in the Nile tilapia, while no differences were found in the slightly elevated levels of GH mRNA. The branchial response demonstrated a faster up-regulation of NKA and NKCC in the Mozambique tilapia, but similar levels after a week, while Nile tilapia had stronger and constant down-regulation of NCC. The time-course response of the measured osmoregulatory parameters indicate that 24 h after transfer is a critical time point for brackish-water adaptation. The differences in responses to saltwater challenge between Mozambique and Nile tilapia shown in this study may be associated with the differences in saltwater tolerance between these two tilapiine species.     

Physiology of seawater acclimation in the striped bass,Morone saxatilis (Walbaum)

Several experiments were performed to investigate the physiology of seawater acclimation in the striped bass, Morone saxatilis. Transfer of fish from fresh water (FW) to seawater (SW; 31–32 ppt) induced only a minimal disturbance of osmotic homeostasis. Ambient salinity did not affect plasma thyroxine, but plasma cortisol remained elevated for 24h after SW transfer. Gill and opercular membrane chloride cell density and Na⁺,K⁺-ATPase activity were relatively high and unaffected by salinity. Average chloride cell size, however, was slightly increased (16%) in SW-acclimated fish. Gill succinate dehydrogenase activity was higher in SW-acclimated fish than in FW fish. Kidney Na⁺, K⁺-ATPase activity was slightly lower (16%) in SW fish than in FW fish. Posterior intestinal Na⁺,K⁺-ATPase activity and water transport capacity (J_v) did not change upon SW transfer, whereas middle intestinal Na⁺,K⁺-ATPase activity increased 35% after transfer and was correlated with an increase in J_v (110%). As salinity induced only minor changes in the osmoregulatory organs examined, it is proposed that the intrinsic euryhalinity of the striped bass may be related to a high degree of “preparedness” for hypoosmoregulation that is uncommon among teleosts studied to data.     

Morphological changes in gill mitochondria-rich cells in cultured Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> acclimated to a wide range of environmental salinity

Morphological changes in gill mitochondria-rich (MR) cells were examined in cultured Japanese eel acclimated to deionized freshwater (DFW), freshwater (FW), 30%-diluted seawater (DSW), and seawater (SW). The gill Na⁺/K⁺-ATPase activity was higher in SW-acclimated eel than in those acclimated to DFW, FW, and DSW. Immunocytochemical observations revealed that MR cells in the gill filaments were most developed in SW, whereas MR cells in the lamellae were preferentially observed in DFW, suggesting that filament and lamellar MR cells are responsible for ion secretion and absorption, respectively. In scanning electron microscopic observations, the apical membrane of lamellar MR cells appeared as a flat or slightly projecting disk with a mesh-like structure on its surface. In contrast, the apical membrane of filament MR cells showed a slightly concave surface. Whole-mount immunocytochemistry revealed that most MR cells showed cystic fibrosis transmembrane conductance regulator immunoreaction in their apical region in fish in DSW and SW, but not in those in DFW and FW, indicating that MR cells developed in DSW and SW function as an ion-secreting site. In addition to MR cells, distinct Na⁺/K⁺-ATPase immunoreaction was observed in the outermost layer of gill epithelia, suggesting that pavement cells are an additional site of ion uptake in the gills.     

Physiological effects of salinity on Delta Smelt, <Emphasis Type="Italic">Hypomesus transpacificus</Emphasis>

Abiotic factors like salinity are relevant to survival of pelagic fishes of the San Francisco Bay Estuary. We tested the effects of 4 parts per thousand (ppt) salinity increases on Delta Smelt (DS) in a laboratory experiment simulating salinity increases that might occur around the low-salinity zone (LSZ) (<6 ppt). Adult DS, fed 2 % body mass per day, starting at 0.5 ppt [freshwater (FW)], were exposed to weekly step-increases of 4 ppt to a maximum of 10 ppt saltwater (SW) over 19 days, and compared to FW controls. DS (n = 12/treatment per sampling) were sampled at 24, 72, and 96 h (1, 3, and 4 days) post-salinity increase for analyses of hematocrit, plasma osmolality, muscle water content, gill chloride cell (CC) Na⁺/K⁺-ATPase (NKA) and apoptosis after being weighed and measured (n = 3 tanks per treatment). No apparent increase in length or weight occurred nor did a difference in survival. Following step-increases in SW, hematocrit increased over time. Other fish responses generally showed a pattern; specifically plasma osmolality became elevated at 1 day and diminished over 4 days in SW. Percent muscle water content (%) did not show significant changes. CCs showed increased NKA, cell size and apoptosis over time in SW, indicating that CCs turnover in DS. The cell renewal process takes days, at least over 19 days. In summary, DS are affected by salinities of the LSZ and ≤10 ppt, though they employ physiological strategies to acclimate.     

Enhanced hypoosmoregulatory response to growth hormone after cortisol treatment in immature rainbow trout,Salmo gairdneri

The growth-independent effect of ovine growth hormone (oGH) and oGH + cortisol treatment on seawater (SW) adaptation in immature rainbow trout, Salmo gairdneri was investigated. Fish were injected every second day with saline, 2.0 μg oGH/g or 2.0 μg oGH + 8.0 μg cortisol/g for a maximum of 8 injections in freshwater (FW). Subgroups were transferred to 28‰ SW after 4 or 8 injections, and changes in plasma Na⁺ and Cl⁻, muscle water content and gill Na⁺/K⁺-ATPase activity were measured. In both of the hormone-treated groups retained in FW, gill Na⁺/K⁺-ATPase activity and interlamellar chloride cell density increased. The effects were most pronounced in the oGH + cortisol group after 2 weeks of treatment. After transfer to SW most of the control fish died due to the osmotic stress, whereas in the hormone-treated groups, mortality was low and there was a positive correlation between pretransfer gill Na⁺/K⁺-ATPase and the ability to maintain ionic-osmotic homeostasis after SW transfer. After two weeks of oGH + cortisol treatment, gill Na⁺/K⁺-ATPase activity was maximal. In contrast, after SW transfer, Na⁺/K⁺-ATPase activity increased further in the oGH-treated group. This group regulated ionic-osmotic parameters less effectively than the oGH + cortisol-treated group. The data indicate that GH and cortisol are important hormones in the regulation of hypoosmoregulatory mechanisms in S. gairdneri.     

Cortisol-induced changes in oxygen consumption and ionic regulation in coastal cutthroat trout (Oncorhynchus clarki clarki) parr

The influence of cortisol on oxygen consumption and osmoregulatory variables was examined in coastal cutthroat trout (Oncorhynchus clarki clarki) parr kept in fresh water (FW) and transferred to seawater (SW). Intraperitoneal implants containing cortisol (50 g g^–1) in vegetable oil resulted in elevated plasma cortisol titres similar to those observed in fish following a 24h SW exposure. Cortisol treatment significantly increased the oxygen consumption and plasma glucose levels of trout in FW, consistent with the glucocorticoid role of cortisol. Cortisol treatment did not cause any changes in plasma ion concentrations or gill Na⁺,K⁺-ATPase activity in FW after 10 days. Cortisol-implanted fish exposed to SW for 24h showed slightly improved ion regulatory ability compare to non-implanted controls. The results of this study suggest that during SW transfer in juvenile salmonids, increases in cortisol may act as both a mineralocorticoid and a glucocorticoid, depending on the developmental state of the fish (e.g., smolt versus parr). Furthermore, the relative energetic costs of osmoregulation and that of the stress associated SW transfer cannot be discerned using whole-animal oxygen consumption rates.     

Effect of exposure to low salinity water on plasma ion regulation and survival rates in artificially wounded devil stinger <Emphasis Type="Italic">Inimicus japonicus</Emphasis>

We investigated the effect of exposure to low salinity water on plasma ion regulation and survival rates in artificially wounded devil stinger Inimicus japonicus. All fishes survived in 33% seawater (SW), while survival rate in 100% SW was 5.1% at 24 h. In 100% SW, plasma Na⁺, K⁺, Mg²⁺, and Ca²⁺ concentrations significantly increased to 238?±?49.9, 9.6?±?2.4, 15.1?±?3.5 and 5.0?±?0.7 mmol/l at 6 h, respectively; the gill Na⁺/K⁺–ATPase (NKA) activity was almost stable, although only one fish survived to 24 h. In 33% SW, plasma Na⁺ and K⁺ concentrations remained at the same level, and plasma Mg²⁺ and Ca²⁺ concentrations gradually increased to 16.2?±?0.7 and 4.5?±?0.2 mmol/l until 24 h, respectively. The NKA activity significantly increased to 5.1?±?1.1 µmol ADP/mg protein per h at 6 h. A positive correlation was observed between the wound surface area against body weight and the plasma ion concentrations, although no difference was observed in the restoration rate of the wounded area between 100 and 33% SW. These results indicate that exposure of wounded fish to low salinity water improves survivability by favoring plasma ion regulation without influencing the restoration rate.     

Stereological Analysis of Blood Space and Tissue Types in the Pseudobranch of the Rainbow Trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>)

The teleost pseudobranch, a reduced mandibular gill arch, morphologically closely resembles a gill hemibranch. It consists of several filaments bearing numerous lamellae. The vascular system of the pseudobranch also matches that of a gill, with afferent and efferent filamental arteries and the lamellar blood space. Despite the pseudobranch’s gill-like appearance, however, a gas exchange function is excluded in many teleosts by overlying opercular epithelium, effectively eliminating diffusional exchange with the external medium. The physiological function of the pseudobranch is still unknown, though its conspicuously close association with the ocular choroid rete mirabile, a structure supporting elevated oxygen partial pressures in the eye, suggests a role related to the process of oxygen secretion into the eyes of teleosts. In this paper we present new stereological data on the pseudobranch of rainbow trout (average weight: 478 ± 114 g, x ± SD, n = 12). The total Cavalieri volume of the pseudobranch was 46.4 μl (97 μl kg⁻¹), consisting of lamellae 49%, connective tissue 43.8%, large blood vessels 4.5%, filamental cartilage 2.3%, and central venous sinus 1.3%. Lamellae were comprised of pillar and pseudobranchial cells 81%, blood space 12.3%, and lacunar tissue 6.4%. The inner surface of the blood space in the lamellae was 12.2 cm² (25.5 cm² kg⁻¹), and its volume was 2.81 μl (5.88 μl kg⁻¹). The results demonstrate the pseudobranch’s potential for physiological functions requiring a high surface area or surface-to-volume ratio, i.e. ion transfer, gas exchange and other regulatory processes. More detailed analysis of the function of the pseudobranch, however, has to remain subject of future morphological and physiological studies.     

Low-salinity tolerance of juvenile fugu <Emphasis Type="Italic">Takifugu rubripes</Emphasis>

ABSTRACT:   To study the adaptability of juvenile fugu Takifugu rubripes to low-salinity environments, fish were transferred from full-strength seawater (100% SW) to freshwater (FW) and 25, 50, 75 and 100% SW, and checked for mortality over 3 days. No mortality was observed in 25–100% SW, whereas all fish died in FW. In fish transferred to 25–100% SW, blood osmolality was maintained within a physiological range. To further explore the lower limit of salinity that fugu could tolerate, fish were transferred from 100% SW to FW and 1, 5, 10, 15 and 25% SW. All fish survived in 5–25% SW, but fish died in FW and 1% SW. In fish surviving transfer to FW and 1 and 5% SW, blood osmolality was decreased to a near sublethal level of approximately 300 mOsm/kg·H₂O. Therefore, the lower limit of salinity tolerance is estimated to lie between 5 and 10% SW. Preacclimation in 25% SW for 7 days did not essentially affect the survival salinity range. Although survival rates and blood osmolality were slightly improved by preacclimation in 25% SW, blood osmolality was markedly decreased in salinities less than 10% SW, as was seen in the direct transfer. Neither chloride cell morphology nor sodium-potassium adenosinetriphosphatase activity in the gills showed a significant change following transfer to low salinities. These findings indicate that fugu can be adapted to hypoosmotic environments to some extent, exerting hyperosmoregulatory ability, although chloride cells are less likely to absorb ions in hypoosmotic environments.     

The effect of dietary sodium chloride on some osmoregulatory parameters of the teleost, Oreochromis niloticus, after transfer from freshwater to seawater

The aim of this work was to determine the effects of supplemental dietary sodium chloride on salt water acclimation of tilapia Oreochromis niloticus. Fish were fed a basal diet supplemented with NaCl (8%) during three weeks in fresh water (FW) and then transferred to salt water (SW) at 15 and 20. Changes in plasma osmolality, chloride ion concentration (Cl^–), plasma level of cortisol and gill Na⁺, K⁺-ATPase activity were measured at 6, 12, 24, 48, 72 and 168 h after transfer to 15SW, while the higher strength SW group (20) was only monitored up to 24 h. Morphological changes in the gill mitochondria-rich (MR) cells were examined in relation to environmental salinity. The changes associated with dietary NaCl were sporadic and of small magnitude. The plasma osmolality and Cl^– increased immediately after transfer up to 12–24 h, but fish fed dietary salt (S) showed lower values than the control group (C). The S group showed higher plasma levels of cortisol than the control, which maintained its initial levels during the experiment. Gill Na⁺, K⁺-ATPase activity of the S group began to increase in the first hours after transfer, reaching maximum at 12 h and returned to basal level at 24 h, while the control group maintained basal levels. The differences between gill Na⁺, K⁺-ATPase activity of S and C fish were significant (p < 0.05) at 12 h. Transmission electron microscopy (TEM) revealed that MR cells in SW show more mitochondria and a more developed tubular system arising from the basolateral membrane. The MR cells of both groups frequently formed a multicellular complex in SW, consisting of a main MR and one or more accessory cells. Such complexes are rarely observed in FW. Some MR cells of fish fed supplemented dietary salt displayed convex apical membrane in FW.     

Cortisol treatment improves the development of hypoosmoregulatory mechanisms in the euryhaline rainbow trout,Salmo gairdneri

The effect of cortisol on osmoregulatory parameters was studied in rainbow trout, (Salmo gairdneri), kept in freshwater (FW) and/or transferred to seawater (SW). Repeated injections of 20 μg cortisol/g fish stimulated gill and gut Na⁺/K⁺-ATPase activity and reduced plasma Na⁺ and Cl⁻ levels after 2 weeks of treatment in FW-adapted fish. Cortisol doses of 0.05 and 1.0 μg/g were without effect. Repeated injections of 10 μg cortisol/g stimulated gill Na⁺/K⁺-ATPase activity and reduced plasma Na+ and Cl⁻ levels in fish in FW, and significantly improved ion regulation after their transfer to 28SW. Higher doses of cortisol (10 and 20 μg/g) induced hyperglycemia, whereas low doses (0.05 and 1.0 μg/g were without effect or induced hypoglycemia. Plasma glucose levels decreased in cortisol-treated fish transferred to SW, whereas transient hyperglycemia was seen in the control fish.     

Guanylyl cyclase stimulation by homologous and heterologous natriuretic peptides in salmonids

A range of homologous (trout ANP, trout CNP, trout VNP) and heterologous (eel ANP, eel ANP-NH₂, rat ANP, porcine CNP) NPs were tested for their effect on guanylyl cyclase in gill and kidney membrane preparations from freshwater and seawater-acclimated rainbow trout and Atlantic salmon. All NPs stimulated guanylyl cyclase at 1 μmol l⁻¹in all preparations. ANP was the most potent stimulator of kidney guanylyl cyclase and CNP the most potent stimulator of guanylyl cyclase in gills. Some differences were apparent between the potencies of homologous and heterologous peptides at 1 μmol l⁻¹: tANP was more potent than rANP in the SW trout kidney and tCNP was more potent than pCNP in FW salmon tissues. While eANP was more potent than tANP in trout gills, it was less potent than tANP in FW salmon gills. However, there was no significant difference between the potencies of eANP and eANP-NH₂ in trout or salmon gills. Salinity did not affect guanylyl cyclase activity with the exception that trout ANP at 1 μmol l⁻¹was more potent in SW trout kidneys than in FW trout kidneys. These results suggest a predomination of NPR-A in the kidney and NPR-B in the gill. It appears that salmonid NPR-A and NPR-B are relatively promiscuous in their ligand affinity, with few differences in the potencies of trout and mammalian NPs and only small differences in cGMP production where these differences do occur. This revised version was published online in July 2006 with corrections to the Cover Date.     

Salinity effects on osmoregulation and gill morphology in juvenile Persian sturgeon (<Emphasis Type="Italic">Acipenser persicus</Emphasis>)

The effect of abrupt and 5-day gradual salinity transfers from freshwater (FW) to 11 ‰ Caspian Sea brackish water (BW) was investigated in juvenile Persian sturgeon Acipenser persicus with three different weight groups: 1–2 g (1.62 ± 0.27 g), 2–3 g (2.55 ± 0.41 g) and 3–5 g (4.28 ± 0.76 g). Mortality rates, blood osmotic pressure, gill morphology and branchial Na⁺, K⁺-ATPase (NKA) activity were measured 4 and 10 days after abrupt transfer and 9 and 15 days after the initial gradual transfer (i.e. 4 and 10 days after reaching Caspian Sea salinity). Fish under 3 g could not survive increased salinity, and the blood osmotic pressure of the remaining surviving fish increased and remained elevated. However, heavier fish were able to survive and successfully acclimate, even to rapid salinity change with osmotic pressure reduced to Caspian Sea osmolality levels. At the gill level, the developmental increase in chloride cell volume and a higher NKA content most probably allow juveniles weighing more than 2 g to sharply increase NKA activity if the fish are transferred to BW. The rapid chloride cell proliferation occurring with increased salinity should strengthen this acclimation response. Therefore, a drastic physiological change occurs when fish weigh more than 2 g that allows migration to higher salinities. The triggering signal on chloride cells must be further investigated in order to optimize this functional step.