Prolactin cell activity and sodium fluxes in tilapia (Oreochromis mossambicus) after long-term acclimation to acid water

Extra-gastric expression of the proton pump H+/K+-ATPase in the gills and kidney of the teleost Oreochromis niloticus

Potassium regulation is essential for the proper functioning of excitable tissues in vertebrates. The H+/K+-ATPase (HKA), which is composed of the HKα1 (gene: atp4a) and HKβ (gene: atp4b) subunits, has an established role in potassium and acid-base regulation in mammals and is well known for its role in gastric acidification. However, the role of HKA in extra-gastric organs such as the gill and kidney is less clear, especially in fishes. In the present study in Nile tilapia, Oreochromis niloticus, uptake of the K+ surrogate flux marker rubidium (Rb+) was demonstrated in vivo; however, this uptake was not inhibited with omeprazole, a potent inhibitor of the gastric HKA. This contrasts with gill and kidney ex vivo preparations, where tissue Rb+ uptake was significantly inhibited by omeprazole and SCH28080, another gastric HKA inhibitor. The cellular localization of this pump in both the gill and kidney was demonstrated using immunohistochemical techniques with custom-made antibodies specific for Atp4a and Atp4b. Antibodies against the two subunits showed the same apical ionocyte distribution pattern in the gill and collecting tubules/ducts in the kidney. Atp4a antibody specificity was confirmed by western blotting. RT-PCT was used to confirm the expression of both subunits in the gill and kidney. Taken together, these results indicate for the first time K+ (Rb+) uptake in O. niloticus and that HKA is implicated, as shown through the ex vivo uptake inhibition by omeprazole and SCH28080, verifying a role for HKA in K+ absorption in the gill's ionocytes and collecting tubule/duct segments of the kidney.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Osmoregulation in zebrafish: ion transport mechanisms and functional regulation

Fish, like mammals, have to maintain their body fluid ionic and osmotic homeostasis through sophisticated iono-/osmoregulation mechanisms, which are conducted mainly by ionocytes of the gill (the skin in embryonic stages), instead of the renal tubular cells in mammals. Given the advantages in terms of genetic database availability and manipulation, zebrafish is an emerging model for research into regulatory and integrative physiology. At least five types of ionocytes, HR, NaR, NCC, SLC26, and KS cells, have been identified to carry out Na(+) uptake/H(+) secretion/NH4 (+) excretion, Ca(2+) uptake, Na(+)/Cl(-) uptake, K(+) secretion, and Cl(-) uptake/HCO3 (-) secretion, respectively, through distinct sets of transporters. Several hormones, namely isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyorid hormone 1, catecholamines, and the renin-angiotensin-system, have been demonstrated to positively or negatively regulate ion transport through specific receptors at different ionocytes stages, at either the transcriptional/translational or posttranslational level. The knowledge obtained using zebrafish answered many long-term contentious or unknown issues in the field of fish iono-/osmoregulation. The homology of ion transport pathways and hormone systems also means that the zebrafish model informs studies on mammals or other animal species, thereby providing insights into related fields.

The physiology of fish at low pH: the zebrafish as a model system

Ionic regulation and acid–base balance are fundamental to the physiology of vertebrates including fish. Acidification of freshwater ecosystems is recognized as a global environmental problem, and the physiological responses to acid exposure in a few fish species are well characterized. However, the underlying mechanisms promoting ionic and acid–base balance for most fish species that have been investigated remain unclear. Zebrafish (Danio rerio) has emerged as a powerful model system to elucidate the molecular basis of ionic and acid–base regulation. The utility of zebrafish is related to the ease with which it can be genetically manipulated, its suitability for state-of-the-art molecular and cellular approaches, and its tolerance to diverse environmental conditions. Recent studies have identified several key regulatory mechanisms enabling acclimation of zebrafish to acidic environments, including activation of the sodium/hydrogen exchanger (NHE) and H+-ATPase for acid secretion and Na+ uptake, cortisol-mediated regulation of transcellular and paracellular Na+ movements, and ionocyte proliferation controlled by specific cell-fate transcription factors. These integrated physiological responses ultimately contribute to ionic and acid–base homeostasis in zebrafish exposed to acidic water. In the present review, we provide an overview of the general effects of acid exposure on freshwater fish, the adaptive mechanisms promoting extreme acid tolerance in fishes native to acidic environments, and the mechanisms regulating ionic and acid–base balance during acid exposure in zebrafish.

Hormonal and ion regulatory response in three freshwater fish species following waterborne copper exposure

We evaluated effects of sublethal copper exposure in 3 different freshwater fish: rainbow trout (Oncorhynchus mykiss), common carp (Cyprinus carpio) and gibel carp (Carassius auratus gibelio). In a first experiment we exposed these fishes to an equally toxic Cu dose, a Cu level 10 times lower than their 96 h LC50 value: 20, 65, and 150 microg/L Cu. In a second series we exposed them to the same Cu concentration (50 microg/L). Na+/K+-ATPase activity in gill tissue was disturbed differently in rainbow trout then in common and gibel carp. Rainbow trout showed a thorough disruption of plasma ion levels at the beginning of both exposures, whereas common carp and gibel carp displayed effects only after 3 days. Rainbow trout and common carp thyroid hormones experienced adverse effects in the beginning of the exposure. The involvement of prolactin in handling metal stress was reflected in changes of mRNA prolactin receptor concentrations in gill tissue, with an up regulation of this mRNA in rainbow trout and a down regulation in gibel carp, which was more pronounced in the latter. Overall, rainbow trout appeared more sensitive in the beginning of the exposure, however, when it overcame this first challenge, it handled copper exposure in a better manner then common and gibel carp as they showed more long term impacts of Cu exposure.

Effects of Acid Water Exposure on Plasma Cortisol, Ion Balance, and Immune Functions in the “Cobalt” Variant of Rainbow Trout

This study was undertaken to examine physiological responses to acidification of environmental water in the "cobalt" variant of rainbow trout (Oncorhynchus mykiss), which exhibits malformation of the pituitary, by following changes in plasma levels of cortisol and electrolytes, blood pH, gill Na(+), K(+)-ATPase activity, and immune functions after exposure to acid water (pH 4.5). Resting levels of plasma cortisol and lysozyme were significantly lower in the cobalt variant than in the normal trout, whereas plasma ceruloplasmin was significantly higher in the cobalt variant, suggesting that some endocrine factors, lacking or deficient in the cobalt variant, are important for the regulation of its immune functions. Blood pH was slightly but significantly lower in the cobalt variant at rest. After exposure to acid water for 24 h, both the normal trout and cobalt variant showed a significant elevation in plasma cortisol, although the increased level in the cobalt variant was still lower than that in the normal trout transferred to neutral water. No differences were seen in blood pH, plasma electrolytes, and gill Na(+), K(+)-ATPase activity between the normal trout and the cobalt variant, indicating that the cobalt variant regulates ion balance when exposed to acid water, despite malformation of the pituitary. Although the normal trout showed a reduction in plasma lysozyme level after acid exposure, there was no significant change in the cobalt trout. Adverse effects of pituitary malformation on ion balance and immune functions may be compensated by extrapituitary factors in the cobalt variant when it is exposed to acid water.

Effects of prolactin and growth hormone on strategies of hypoosmotic adaptation in a marine teleost, Sparus sarba

Silver seabream (Sparus sarba) held in seawater (33 per thousand) or acclimated to a hypoosmotic environment of 6 per thousand were given intraperitoneal injections of saline (0.8% NaCl), recombinant bream growth hormone (rbGH, 1 microg/g), or ovine prolactin (oPRL, 6microg/g) for 7 consecutive days. Serum Na+ levels were unaffected by hypoosmotic acclimation and rbGH and oPRL treatment. Treatment of seawater fish with oPRL resulted in hyperchloremia. In 6 per thousand, saline-treated fish exhibited elevated branchial chloride cell (CC) numbers and exposure indices, all of which were markedly reduced by oPRL. CC numbers and morphometrics were unaffected by oPRL in seawater fish. In contrast, rbGH treatment of seawater fish resulted in elevated CC numbers, apical area, and fractional area and, in 6 per thousand fish, elevated CC fractional area and exposure numbers. Branchial Na+-K+-ATPase activity reduced in saline-treated fish adapted to 6% but was unaffected by rbGH regardless of salinity. oPRL reduced activity in both seawater and 6 per thousand-adapted fish. Neither hypoosmotic adaptation nor oPRL had any effect on renal Na+-K+-ATPase activity whereas rbGH reduced activity in both 33 and 6 per thousand. Saline-treated fish adapted to 6 per thousand exhibited reduced Na+-K+-ATPase activity in most regions of the intestine. Treatment with rbGH did not change intestinal Na+-K+-ATPase activity of seawater fish but elevated activity in the anterior regions (esophagus and stomach) of 6 per thousand-adapted fish. Treatment with oPRL elevated Na+-K+-ATPase activity throughout the gastrointestinal tract of seawater fish and in the anterior reaches of 6 per thousand-adapted fish. The data indicated that the as yet uncharacterized osmoregulatory roles of PRL and GH in seabream may warrant further attention as the present study connoted differing responses to that of other teleosts studied.

Sodium and calcium balance in Mozambique tilapia, Oreochromis mossambicus, raised at different salinities

Mozambique tilapia, Oreochromis mossambicus, born and raised in five salinities, viz. (relatively soft) fresh water, 25, 50, 75% and full-strength sea-water, were analyzed for ionoregulatory performance (in particular sodium and calcium handling) and growth. This tilapia regulates its blood serum mineral composition rather effectively; however, in sea-water serum concentrations of sodium, chloride and calcium (in males only) were increased, as was the serum osmolarity. In sea-water, the total body sodium pool was significantly enlarged. With increasing salinity, sodium turnover increased. Serum calcium levels and the total body calcium pool were more strictly controlled than those of sodium. The lowest density of chloride cells in opercular epithelium and the lowest branchial Na+-K+-ATPase activity were observed in 50% sea-water; these values were higher in fish kept in waters of lower or higher salinities. Fish grew more rapidly in brackish water. Fish kept in brackish water appeared to depend on food-related calcium for growth as branchial calcium uptake provides no more than 20% of growth related Ca-accumulation.

Effect of confinement stress on circulating levels of growth hormone and two prolactins in freshwater-adapted tilapia (Oreochromis niloticus)

The aim of the present study was to assess a potential link between confinement stress and prolactin (PRL), the hormone responsible for adaptation to a hypoosmotic environment in freshwater-adapted tilapia (Oreochromis niloticus). The effect of stress on plasma levels of the two tilapia PRL forms, tiPRLI (or tiPRL188) and tiPRLII (or tiPRL177), was examined along with the effects on plasma levels of cortisol and growth hormone (GH). In a preliminary study, various sampling protocols (immediate sampling; sampling one by one; anesthesia at 0.5, 1, 2 ml/liter phenoxyethanol) were tested for their ability to modify basal plasma PRL and cortisol. In fish sampled within 1 min of capture (immediate sampling), no changes in the plasma levels of these hormones were observed, whereas when fish were sampled one at a time, PRL levels did not change but cortisol levels were modified. The immediate sampling protocol was used to study the effects of 1 hr confinement stress, which induced a large increase in plasma cortisol levels as well as increases tiPRLI and tiPRLII levels with kinetics similar to those of cortisol. In contrast, plasma tiGH levels significantly decreased after 1 hr confinement. When this stress situation was removed, plasma cortisol and tiPRL levels decreased and plasma GH levels increased. Two and one-half hours later, values were not significantly different from those measured in control fish. In tilapia exposed to 24 hr confinement stress, similar changes in hormone levels were observed. However, after 24 hr confinement, only cortisol levels were significantly different from those measured in control fish. None of these stress conditions significantly changed plasma chloride levels. Together, these results indicate that both PRL and GH have important roles in the adaptive response of freshwater-adapted tilapia to confinement stress.

Ionoregulatory and respiratory responses of brown trout, Salmo trutta, exposed to lethal and sublethal aluminium in acidic soft waters

Soft water acclimated (Ca(2+) 0.02 mM; Na(+) 0.03 mM; K(+) 0.01 mM; pH 7.0), cannulated brown trout (Salmo trutta) were exposed to various pH and aluminium (Al) regimes (pH 7.0, pH 5.0, pH 5.0 plus Al: 50, 25, and 12.5 μg l(-1)) for up to 5 days in order to determine (i) the sublethal concentration of Al at pH 5.0 for this species (ii) their ionoregulatory and respiratory status. No mortality or physiological disturbances were evident at pH 7.0 or pH 5.0. All trout died within 48 h at pH 5.0 in the presence of Al at 50 μg l(-1) and 67% died over the 5 day period at pH 5.0 in the presence of Al at 25 μg l(-1). Fish at these lethal Al concentrations showed significant decreases in arterial blood oxygen content (CaO2) but no changes in plasma osmolarity or the concentrations of plasma Na(+), K(+) and Cl(-). Physiological disturbance was more marked at the 50 μg l(-1) Al concentration. The surviving fish at 25 μg l(-1) showed few signs of physiological recovery while continually exposed to this regime. No fish died during the exposure to water of pH 5.0 containing 12.5 μg l(-1) Al, but physiological disturbance was still apparent. These sublethally-stressed trout showed a transient decline in the plasma concentrations of Na(+) and Cl(-1). Although CaO2 decreased, recovery was evident. The data suggest that in the brown trout, environmental Al concentration is as important as pH and calcium concentration in determining the physiological status of the fish.

Actions of cadmium on basolateral plasma membrane proteins involved in calcium uptake by fish intestine

The inhibition of Ca(2+)-ATPase, (Na+ + K+)-ATPase and Na+/Ca2+ exchange by Cd2+ was studied in fish intestinal basolateral plasma membrane preparations. ATP driven 45Ca2+ uptake into inside-out membrane vesicles displayed a Km for Ca2+ of 88 +/- 17 nM, and was extremely sensitive to Cd2+ with an IC50 of 8.2 +/- 3.0 pM Cd2+, indicating an inhibition via the Ca2+ site. (Na+ + K+)-ATPase activity was half-maximally inhibited by micromolar amounts of Cd2+, displaying an IC50 of 2.6 +/- 0.6 microM Cd2+. Cd2+ ions apparently compete for the Mg2+ site of the (Na+ + K+)-ATPase. The Na+/Ca2+ exchanger was inhibited by Cd2+ with an IC50 of 73 +/- 11 nM. Cd2+ is a competitive inhibitor of the exchanger via an interaction with the Ca2+ site (Ki = 11 nM). Bepridil, a Na+ site specific inhibitor of Na+/Ca2+ exchange, induced an additional inhibition, but did not change the Ki of Cd2+. Also, Cd2+ is exchanged against Ca2+, albeit to a lesser extent than Ca2+. The exchanger is only partly blocked by the binding of Cd2+. In vivo cadmium that has entered the enterocyte may be shuttled across the basolateral plasma membrane by the Na+/Ca2+ exchanger. We conclude that intracellular Cd2+ ions will inhibit plasma membrane proteins predominantly via a specific interaction with divalent metal ion sites.

Prolactin cell activity and sodium balance in the acid-tolerant mudminnow Umbra pygmaea in acid and neutral water

In East-American mudminnows, obtained from lakes and bogs of different water pH levels, prolactin cell size and ultrastructure reflect higher secretory activity in neutral water than in water of pH 3.5-6.5. This contrasts with observations on other species in which prolactin cell activity is higher at low water pH. Laboratory experiments involving acute exposure of mudminnows from pH 5.5 for 48 hr to different pH levels showed that prolactin secretion increased in water below pH 3 and above pH 6.5, which could be correlated with losses of blood electrolytes. No osmoregulatory stress was noticeable in the pH range of 3.5 to 5.5. Acclimation of fish for 5 to 6 months to either pH 4.5 or 7.2 confirmed that prolactin cell activity, as estimated with ultrastructural morphometry, was significantly higher in water of neutral pH than of pH 4.5. The growth rate was significantly higher at the lower pH. Determination of whole body sodium fluxes, with 24Na as a tracer, showed that both groups had a positive sodium balance. However, total body Na+ influx as well as efflux values were slightly but significantly reduced at pH 7.0 when compared to pH 4.5. The reduction of Na+ efflux at pH 7.0 is in line with increased secretion of prolactin since this hormone is known to limit the branchial permeability to water and ions, including Na+. The results show that the mudminnow is extremely acid tolerant, having an optimum water pH range of about 3.5 to 6.0, which is consistent with ecological observations. Prolactin secretion is at a minimum in this range. Exposure to neutral water represents osmoregulatory stress for the mudminnow, in contrast to all other teleost species examined so far.