Intestinal anion exchange in marine fish osmoregulation

Impact of transport conditions and underlying disease on post-stocking survival of juvenile Lates calcarifer

Diseases caused by pathogens commonly occurring in the aquatic environment or those that are non-host specific are prevalent and threaten the rapid growth of tropical aquaculture. This study investigates causes of mortality in 12 batches of newly stocked juvenile Lates calcarifer from three different hatcheries. Cytology based on Diff-Quik™-stained tissue and blood smears provides rapid diagnosis of possible causes of mortality, while histopathology and haematology provide a better understanding of how prolonged transport and fish with existing chronic disease are more likely to experience elevated mortality post-stocking. Our findings showed that accumulation of ammonia during prolonged transport causes extensive damage to epithelial barriers in gastrointestinal tracts and depressed immunity due to marked hypoglycaemia, predisposing fish to acute Streptococcosis. Lates calcarifer with chronic bacterial enteritis developed severe hypoglycaemia, had low circulating total plasma protein, and suffered high mortality within 24 hours post-stocking. Hypoglycaemia and low circulating blood proteins disrupt osmoregulation and exacerbate dehydration, which is fatal in fish in sea water. Dying L. calcarifer tested PCR positive for scale drop disease virus (SDDV) at 28 days post-stocking showed a 10-fold elevation of white blood cell counts, severe vasculitis, and obstruction of blood supply to major organs. Destruction of important immune organs such as spleen is a hallmark of SDDV infection that explains high incidences of opportunistic Vibrio harveyi infections in 61% of fish with SDDV. Overall, this study reiterates the importance of stocking disease-free fish and reducing transport stress.

HepG2 cells undergo regulatory volume decrease by mechanically induced efflux of water and solutes

This study challenges the conventional belief that animal cell membranes lack a significant hydrostatic gradient, particularly under anisotonic conditions, as demonstrated in the human hepatoma cell line HepG2. The Boyle van't Hoff (BvH) relation describes volumetric equilibration to anisotonic conditions for many cells. However, the BvH relation is simple and does not include many cellular components such as the cytoskeleton and actin cortex, mechanosensitive channels, and ion pumps. Here we present alternative models that account for mechanical resistance to volumetric expansion, solute leakage, and active ion pumping. We found the BvH relation works well to describe hypertonic volume equilibration but not hypotonic volume equilibration. After anisotonic exposure and return isotonic conditions cell volumes were smaller than their initial isotonic volume, indicating solutes had leaked out of the cell during swelling. Finally, we observed HepG2 cells undergo regulatory volume decrease at both 20 °C and 4 °C, indicating regulatory volume decrease to be a relatively passive phenomenon and not driven by ion pumps. We determined the turgor-leak model, which accounts for mechanical resistance and solute leakage, best fits the observations found in the suite of experiments performed, while other models were rejected.

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals ('ichthyocarbonates') within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

Metabolic cost of osmoregulation by the gastro-intestinal tract in marine teleost fish

Introduction: Although dozens of studies have attempted to determine the metabolic cost of osmoregulation, mainly by comparing standard metabolic rates (SMR) in fish acclimated to different salinities, consensus is still lacking. Methods: In the present study, using the Gulf toadfish, Opsanus beta, we aimed to determine the metabolic cost of esophageal and intestinal osmoregulatory processes by estimating ATP consumption from known ion transport rates and pathways and comparing these estimates with measurements on isolated tissues. Further, we performed whole animal respirometry on fish acclimated to 9, 34 and 60 ppt. Results and Discussion: Our theoretical estimates of esophageal and intestinal osmoregulatory costs were in close agreement with direct measurements on isolated tissues and suggest that osmoregulation by these tissues amounts to ∼2.5% of SMR. This value agrees well with an earlier attempt to estimate osmoregulation cost from ion transport rates and combined with published measurements of gill osmoregulatory costs suggests that whole animal costs of osmoregulation in marine teleosts is ∼7.5% of SMR. As in many previous studies, our whole animal measurements were variable between fish and did not seem suited to determine osmoregulatory costs. While the esophagus showed constant metabolic rate regardless of acclimation salinity, the intestine of fish acclimated to higher salinities showed elevated metabolic rates. The esophagus and the intestine had 2.1 and 3.2-fold higher metabolic rates than corresponding whole animal mass specific rates, respectively. The intestinal tissue displays at least four different Cl- uptake pathways of which the Na+:Cl-:2 K+ (NKCC) pathway accounts for 95% of the Cl- uptake and is the most energy efficient. The remaining pathways are via apical anion exchange and seem to primarily serve luminal alkalinization and the formation of intestinal CaCO3 which is essential for water absorption.

Increased reliance on coronary perfusion for cardiorespiratory performance in seawater-acclimated rainbow trout

Salmonid ventricles are composed of spongy and compact myocardium, the latter being perfused via a coronary circulation. Rainbow trout (Oncorhynchus mykiss) acclimated to sea water have higher proportions of compact myocardium and display stroke volume-mediated elevations in resting cardiac output relative to freshwater-acclimated trout, probably to meet the higher metabolic needs of osmoregulatory functions. Here, we tested the hypothesis that cardiorespiratory performance of rainbow trout in sea water is more dependent on coronary perfusion by assessing the effects of coronary ligation on cardiorespiratory function in resting and exhaustively exercised trout acclimated to fresh water or sea water. While ligation only had minor effects on resting cardiorespiratory function across salinities, cardiac function after chasing to exhaustion was impaired, presumably as a consequence of atrioventricular block. Ligation reduced maximum O2 consumption rate by 33% and 17% in fish acclimated to sea water and fresh water, respectively, which caused corresponding 41% and 17% reductions in aerobic scope. This was partly explained by different effects on cardiac performance, as maximum stroke volume was only significantly impaired by ligation in sea water, resulting in 38% lower maximum cardiac output in seawater compared with 28% in fresh water. The more pronounced effect on respiratory performance in sea water was presumably also explained by lower blood O2 carrying capacity, with ligated seawater-acclimated trout having 16% and 17% lower haemoglobin concentration and haematocrit, respectively, relative to ligated freshwater trout. In conclusion, we show that the coronary circulation allows seawater-acclimated trout to maintain aerobic scope at a level comparable to that in fresh water.

The role of environmental salinity on Na+-dependent intestinal amino acid uptake in rainbow trout (Oncorhynchus mykiss)

Na⁺/K⁺-ATPases (NKA) in the basolateral membrane of the intestinal enterocytes create a Na⁺-gradient that drives both ion-coupled fluid uptake and nutrient transport. Being dependent on the same gradient as well as on the environmental salinity, these processes have the potential to affect each other. In salmonids, L-lysine absorption has been shown to be higher in freshwater (FW) than in seawater (SW) acclimated fish. Using electrophysiology (Ussing chamber technique), the aim was to explore if the decrease in L-lysine transport was due to allocation of the Na⁺-gradient towards ion-driven fluid uptake in SW, at the cost of amino acid transport. Intestinal NKA activity was higher in SW compared to FW fish. Exposure to ouabain, an inhibitor of NKA, decreased L-lysine transport. However, exposure to bumetanide and hydrochlorothiazide, inhibitors of Na⁺, K⁺, 2Cl^--co-transporter (NKCC) and Na⁺, Cl^--co-transporter (NCC) respectively, did not affect the rate of intestinal L-lysine transport. In conclusion, L-lysine transport is Na⁺-dependent in rainbow trout and the NKA activity and thus the available Na⁺-gradient increases after SW acclimation. This increased Na⁺-gradient is most likely directed towards osmoregulation, as amino acid transport is not compromised in SW acclimated fish.

Na+/HCO3- cotransporter 1 (nbce1) isoform gene expression during smoltification and seawater acclimation of Atlantic salmon

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na⁺/HCO₃^- cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na⁺/HCO₃^- cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na⁺ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na⁺/K⁺-ATPase activity, Na⁺/K⁺/2Cl^- cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater.

Inducible Nitric Oxide Synthase/Nitric Oxide System as a Biomarker for Stress and Ease Response in Fish: Implication on Na+ Homeostasis During Hypoxia

The cellular and organismal response to stressor-driven stimuli evokes stress response in vertebrates including fishes. Fishes have evolved varied patterns of stress response, including ionosmotic stress response, due to their sensitivity to both intrinsic and extrinsic stimuli. Fishes that experience hypoxia, a detrimental stressor that imposes systemic and cellular stress response, can evoke disturbed ion homeostasis. In addition, like other vertebrates, fishes have also developed mechanisms to recover from the impact of stress by way of shifting stress response into ease response that could reduce the magnitude of stress response with the aid of certain neuroendocrine signals. Nitric oxide (NO) has been identified as a potent molecule that attenuates the impact of ionosmotic stress response in fish, particularly during hypoxia stress. Limited information is, however, available on this important aspect of ion transport physiology that contributes to the mechanistic understanding of survival during environmental challenges. The present review, thus, discusses the role of NO in Na⁺ homeostasis in fish particularly in stressed conditions. Isoforms of nitric oxide synthase (NOS) are essential for the synthesis and availability of NO at the cellular level. The NOS/NO system, thus, appears as a unique molecular drive that performs both regulatory and integrative mechanisms of control within and across varied fish ionocytes. The activation of the inducible NOS (iNOS)/NO system during hypoxia stress and its action on the dynamics of Na⁺/K⁺-ATPase, an active Na⁺ transporter in fish ionocytes, reveal that the iNOS/NO system controls cellular and systemic Na⁺ transport in stressed fish. In addition, the higher sensitivity of iNOS to varied physical stressors in fishes and the ability of NO to lower the magnitude of ionosmotic stress in hypoxemic fish clearly put forth NO as an ease-promoting signal molecule in fishes. This further points to the signature role of the iNOS/NO system as a biomarker for stress and ease response in the cycle of adaptive response in fish.

Salinity significantly affects intestinal microbiota and gene expression in striped catfish juveniles

In the present study, juvenile striped catfish (Pangasianodon hypophthalmus), a freshwater fish species, have been chronically exposed to a salinity gradient from freshwater to 20 psu (practical salinity unit) and were sampled at the beginning (D20) and the end (D34) of exposure. The results revealed that the intestinal microbial profile of striped catfish reared in freshwater conditions were dominated by the phyla Bacteroidetes, Firmicutes, Proteobacteria, and Verrucomicrobia. Alpha diversity measures (observed OTUs (operational taxonomic units), Shannon and Faith's PD (phylogenetic diversity)) showed a decreasing pattern as the salinities increased, except for the phylogenetic diversity at D34, which was showing an opposite trend. Furthermore, the beta diversity between groups was significantly different. Vibrio and Akkermansia genera were affected differentially with increasing salinity, the former being increased while the latter was decreased. The genus Sulfurospirillium was found predominantly in fish submitted to salinity treatments. Regarding the host response, the fish intestine likely contributed to osmoregulation by modifying the expression of osmoregulatory genes such as nka1a, nka1b, slc12a1, slc12a2, cftr, and aqp1, especially in fish exposed to 15 and 20 psu. The expression of heat shock proteins (hsp) hsp60, hsp70, and hsp90 was significantly increased in fish reared in 15 and 20 psu. On the other hand, the expression of pattern recognition receptors (PRRs) were inhibited in fish exposed to 20 psu at D20. In conclusion, the fish intestinal microbiota was significantly disrupted in salinities higher than 10 psu and these effects were proportional to the exposure time. In addition, the modifications of intestinal gene expression related to ion exchange and stressful responses may help the fish to adapt hyperosmotic environment. KEY POINTS: • It is the first study to provide detailed information on the gut microbiota of fish using the amplicon sequencing method. • Salinity environment significantly modified the intestinal microbiota of striped catfish. • Intestinal responses may help the fish adapt to hyperosmotic environment.

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

Expression and Function of ABC Proteins in Fish Intestine

In fish, the intestine is fundamental for digestion, nutrient absorption, and other functions like osmoregulation, acid-base balance, and excretion of some metabolic products. These functions require a large exchange surface area, which, in turn, favors the absorption of natural and anthropogenic foreign substances (xenobiotics) either dissolved in water or contained in the food. According to their chemical nature, nutrients, ions, and water may cross the intestine epithelium cells' apical and basolateral membranes by passive diffusion or through a wide array of transport proteins and also through endocytosis and exocytosis. In the same way, xenobiotics can cross this barrier by passive diffusion or taking advantage of proteins that transport physiological substrates. The entry of toxic substances is counterbalanced by an active efflux transport mediated by diverse membrane proteins, including the ATP binding cassette (ABC) proteins. Recent advances in structure, molecular properties, and functional studies have shed light on the importance of these proteins in cellular and organismal homeostasis. There is abundant literature on mammalian ABC proteins, while the studies on ABC functions in fish have mainly focused on the liver and, to a minor degree, on the kidney and other organs. Despite their critical importance in normal physiology and as a barrier to prevent xenobiotics incorporation, fish intestine's ABC transporters have received much less attention. All the ABC subfamilies are present in the fish intestine, although their functionality is still scarcely studied. For example, there are few studies of ABC-mediated transport made with polarized intestinal preparations. Thus, only a few works discriminate apical from basolateral transport activity. We briefly describe the main functions of each ABC subfamily reported for mammals and other fish organs to help understand their roles in the fish intestine. Our study considers immunohistochemical, histological, biochemical, molecular, physiological, and toxicological aspects of fish intestinal ABC proteins. We focus on the most extensively studied fish ABC proteins (subfamilies ABCB, ABCC, and ABCG), considering their apical or basolateral location and distribution along the intestine. We also discuss the implication of fish intestinal ABC proteins in the transport of physiological substrates and aquatic pollutants, such as pesticides, cyanotoxins, metals, hydrocarbons, and pharmaceutical products.

Hypoosmotic stress induced functional alternations of intestinal barrier integrity, inflammatory reactions, and neurotransmission along gut-brain axis in the yellowfin seabream (Acanthopagrus latus)

The gut-brain axis plays a major role in multiple metabolic regulation processes, but studies regarding its responses to environmental stress in fish are still limited. In this study, we performed transcriptome sequencing analysis and enzyme-linked immunosorbent assay (ELISA) in yellowfin seabream (Acanthopagrus latus) exposed to environments with different water salinity (freshwater: 0 ppt; low-saline water: 3 ppt; brackish water: 6 ppt). According to transcriptome analysis, 707 and 1477 genes were identified as differentially expressed genes (DEGs) between freshwater and brackish water treatments in the brain and gut, respectively. Brain DEGs were significantly enriched into a set of Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with signal transduction, most of which were downregulated. Gut DEGs were enriched into a neurotransmission-relevant KEGG pathway tryptophan metabolism, and the downregulated DEGs were enriched into the KEGG pathway focal adhesion. ELISA demonstrated significant physiological responses of the brain and gut across treatments, as determined by the concentrations of tight junction protein ZO-2, interleukin 1β, and serotonin. Under hypoosmotic stress, the functions of the gut-brain axis are altered via impairment of intestinal barrier integrity, by disturbance of gut-brain neurotransmission, and through tissue-damaging inflammatory reactions. Our work identified candidate genes which showed significantly differential expression in the gut-brain axis when yellowfin seabream encountered hypoosmotic stress, which could shed lights on the understanding of the potential osmotic regulation mechanisms of the gut-brain axis in teleost.

Continuous gastric saline perfusion elicits cardiovascular responses in freshwater rainbow trout (Oncorhynchus mykiss)

When in seawater, rainbow trout (Oncorhynchus mykiss) drink to avoid dehydration and display stroke volume (SV) mediated elevations in cardiac output (CO) and an increased proportion of CO is diverted to the gastrointestinal tract as compared to when in freshwater. These cardiovascular alterations are associated with distinct reductions in systemic and gastrointestinal vascular resistance (R_Sys and R_GI, respectively). Although increased gastrointestinal blood flow (GBF) is likely essential for osmoregulation in seawater, the sensory functions and mechanisms driving the vascular resistance changes and other associated cardiovascular changes in euryhaline fishes remain poorly understood. Here, we examined whether internal gastrointestinal mechanisms responsive to osmotic changes mediate the cardiovascular changes typically observed in seawater, by comparing the cardiovascular responses of freshwater-acclimated rainbow trout receiving continuous (for 4 days) gastric perfusion with half-strength seawater (½ SW, ~ 17 ppt) to control fish (i.e., no perfusion). We show that perfusion with ½ SW causes significantly larger increases in CO, SV and GBF, as well as reductions in R_Sys and R_GI, compared with the control, whilst there were no significant differences in blood composition between treatments. Taken together, our data suggest that increased gastrointestinal luminal osmolality is sensed directly in the gut, and at least partly, mediates cardiovascular responses previously observed in SW acclimated rainbow trout. Even though a potential role of mechano-receptor stimulation from gastrointestinal volume loading in eliciting these cardiovascular responses cannot be excluded, our study indicates the presence of internal gastrointestinal milieu-sensing mechanisms that affect cardiovascular responses when environmental salinity changes.

The digestive tract as an essential organ for water acquisition in marine teleosts: lessons from euryhaline eels

Adaptation to a hypertonic marine environment is one of the major topics in animal physiology research. Marine teleosts lose water osmotically from the gills and compensate for this loss by drinking surrounding seawater and absorbing water from the intestine. This situation is in contrast to that in mammals, which experience a net osmotic loss of water after drinking seawater. Water absorption in fishes is made possible by (1) removal of monovalent ions (desalinization) by the esophagus, (2) removal of divalent ions as carbonate (Mg/CaCO₃) precipitates promoted by HCO₃^- secretion, and (3) facilitation of NaCl and water absorption from diluted seawater by the intestine using a suite of unique transporters. As a result, 70-85% of ingested seawater is absorbed during its passage through the digestive tract. Thus, the digestive tract is an essential organ for marine teleost survival in the hypertonic seawater environment. The eel is a species that has been frequently used for osmoregulation research in laboratories worldwide. The eel possesses many advantages as an experimental animal for osmoregulation studies, one of which is its outstanding euryhalinity, which enables researchers to examine changes in the structure and function of the digestive tract after direct transfer from freshwater to seawater. In recent years, the molecular mechanisms of ion and water transport across epithelial cells (the transcellular route) and through tight junctions (the paracellular route) have been elucidated for the esophagus and intestine. Thanks to the rapid progress in analytical methods for genome databases on teleosts, including the eel, the molecular identities of transporters, channels, pumps and junctional proteins have been clarified at the isoform level. As 10 y have passed since the previous reviews on this subject, it seems relevant and timely to summarize recent progress in research on the molecular mechanisms of water and ion transport in the digestive tract in eels and to compare the mechanisms with those of other teleosts and mammals from comparative and evolutionary viewpoints. We also propose future directions for this research field to achieve integrative understanding of the role of the digestive tract in adaptation to seawater with regard to pathways/mechanisms including the paracellular route, divalent ion absorption, metabolon formation and cellular trafficking of transporters. Notably, some of these have already attracted practical attention in laboratories.

Salinity-dependent expression of calcium-sensing receptors in Atlantic salmon (Salmo salar) tissues

Multiple reports suggest that calcium-sensing receptors (CaSRs) are involved in calcium homeostasis, osmoregulation, and/or salinity sensing in fish (Loretz 2008, Herberger and Loretz 2013). We have isolated three unique full-length CaSR cDNAs from Atlantic salmon (Salmo salar) kidney that share many features with other reported CaSRs. Using anti-CaSR antibodies and PCR primers specific for individual salmon CaSR transcripts we show expression in osmoregulatory, neuroendocrine and sensory tissues. Furthermore, CaSRs are expressed in different patterns in salmon tissues where mRNA and protein expression are modified by freshwater or seawater acclimation. For example, in seawater, CaSR mRNA and protein expression is increased significantly in kidney as compared to freshwater. Electrophysiological recordings of olfactory responses produced upon exposure of salmon olfactory epithelium to CaSR agonists suggest a role for CaSRs in chemoreception in this species consistent with other freshwater, anadromous, and marine species where similar olfactory responses to divalent and polyvalent cations have been reported. These data provide further support for a role of CaSR proteins in osmoregulatory and sensory functions in Atlantic salmon, an anadromous species that experiences a broad range of environmental salinities in its life history.

Corticosteroid control of Na+/K+-ATPase in the intestine of the sea lamprey (Petromyzon marinus)

Anadromous sea lamprey (Petromyzon marinus) larvae undergo a months-long true metamorphosis during which they develop seawater (SW) tolerance prior to downstream migration and SW entry. We have previously shown that intestinal Na⁺/K⁺-ATPase (NKA) activity increases during metamorphosis and is critical to the osmoregulatory function of the intestine in SW. The present study investigated the role of 11-deoxycortisol (S) in controlling NKA in the anterior (AI) and posterior (PI) intestine during sea lamprey metamorphosis. In a tissue profile, nka mRNA and protein were most abundant in the gill, kidney, and AI. During metamorphosis, AI nka mRNA increased 10-fold, whereas PI nka mRNA did not change. Specific corticosteroid receptors were found in the AI, which had a higher binding affinity for S compared to 11-deoxycorticosterone (DOC). In vivo administration of S in mid-metamorphic lamprey upregulated NKA activity 3-fold in the AI and PI, whereas administration of DOC did not affect intestinal NKA activity. During a 24 h SW challenge test, dehydration of white muscle moisture was rescued by prior treatment with S, which was associated with increased intestinal nka mRNA and NKA activity. These results indicate that intestinal osmoregulation in sea lamprey is a target for control by S during metamorphosis and the development of SW tolerance.

Freshening effect on the osmotic response of the Antarctic spiny plunderfish Harpagifer antarcticus

Global warming is having a significant impact around the world, modifying environmental conditions in many areas, including in zones that have been thermally stable for thousands of years, such as Antarctica. Stenothermal sedentary intertidal fish species may suffer due to warming, notably if this causes water freshening from increased freshwater inputs. Acute decreases in salinity, from 33 down to 5, were used to assess osmotic responses to environmental salinity fluctuations in Antarctic spiny plunderfish Harpagifer antarcticus, in particular to evaluate if H. antarcticus is able to cope with freshening and to describe osmoregulatory responses at different levels (haematological variables, muscle water content, gene expression, NKA activity). H. antarcticus were acclimated to a range of salinities (33 as control, 20, 15, 10 and 5) for 1 week. At 5, plasma osmolality and calcium concentration were both at their lowest, while plasma cortisol and percentage muscle water content were at their highest. At the same salinity, gill and intestine Na⁺ -K⁺ -ATPase (NKA) activities were at their lowest and highest, respectively. In kidney, NKA activity was highest at intermediate salinities (15 and 10). The salinity-dependent NKA mRNA expression patterns differed depending on the tissue. Marked changes were also observed in the expression of genes coding membrane proteins associated with ion and water transport, such as NKCC2, CFTR and AQP8, and in the expression of mRNA for the regulatory hormone prolactin (PRL) and its receptor (PRLr). Our results demonstrate that freshening causes osmotic imbalances in H. antarcticus, apparently due to reduced capacity of both transport and regulatory mechanisms of key organs to maintain homeostasis. This has implications for fish species that have evolved in stable environmental conditions in the Antarctic, now threatened by climate change.

Nitric oxide synthase (NOS) inhibitor L-NAME activates inducible NOS/NO system and drives multidimensional regulation of Na+ /K+ -ATPase in ionocyte epithelia of immersion-stressed air-breathing fish (Anabas testudineus Bloch)

Nitric oxide (NO) has been implicated in Na⁺  homeostatic control in water-breathing fishes. It is, however, uncertain whether air-breathing fish relies on NO to coordinate Na⁺ /K⁺ -ATPase (NKA)-driven Na⁺  transport during acute hypoxemia. We, thus, examined the action of nitric oxide synthase (NOS) inhibitor, L-NAME on NO availability, inducible NOS (iNOS) protein abundance and the regulatory dynamics of NKA in osmoregulatory epithelia of Anabas testudineus kept at induced hypoxemia. As expected in nonstressed fish, in vivo L-NAME (100 ng g^-1 ) challenge for 30 min declined NO production in serum (40%) and osmoregulatory tissues (average 51.6%). Surprisingly, the magnitude of such reduction was less in hypoxemic fish after L-NAME challenge due to the net gain of NO (average 23.7%) in these tissues. Concurrently, higher iNOS protein abundance was found in branchial and intestinal epithelia of these hypoxemic fish. In nonstressed fish, L-NAME treatment inhibited the NKA activity in branchial and intestinal epithelia while stimulating its activity in renal epithelia. Interestingly in hypoxemic fish, L-NAME challenge restored the hypoxemia-inhibited NKA activity in branchial and renal epithelia. Similar recovery response was evident in the NKAα protein abundance in immunoblots and immunofluorescence images of branchial epithelia of these fish. Analysis of Nkaα1 isoform transcript abundance (Nkaα1a, α1b, α1c) also showed spatial and preferential regulation of Nkaα1 isoform switching. Collectively, the data indicate that L-NAME challenge activates iNOS/NO system in the branchial ionocyte epithelia of hypoxemia-stressed Anabas and demands multidimensional regulation of NKA to restore the Na⁺  transport rate probably to defend against acute hypoxemia.

Cloning and Expression of Four Aquaporin Homologs from the Chinese Black Sleeper (Bostrychus sinensis): The Effects of Salinity Acclimation

Several fish species are known to possess mechanisms that allow them to adapt to environments with different salinities. The aim of this study was to investigate the effects of salinity on the expression of aquaporins (aqp1a, aqp3a, aqp8a, and aqp9a) in the gills and intestines of Chinese black sleeper. After 30 days of acclimation, the expression of aqp1a, aqp3a, and aqp9a in the gills was significantly higher in fish transferred to 5 ppt than in those transferred to 40 ppt seawater, whereas aqp8 expression was lower. In contrast, aqp1a, aqp3a, and aqp8a expression in the intestines was higher in fish acclimated in 40 ppt than in those acclimated in 5 ppt. During abrupt salinity acclimation, the levels of aqp1a and aqp9a in the gills varied over time in fish acclimated in 5 ppt, but not in 40 ppt. The aqp3a levels in gills were higher in the 5 ppt group after 24 h than in the 40 ppt. The expression level of aqp8a in gills was higher in 40 ppt than in 5 ppt, except for that at 12 h. In the intestines, expression level of aqp1a and aqp8a were significantly upregulated from 12 to 48 h following acclimation in 40 ppt and aqp3a was higher in 40 ppt group than in 5 ppt, while aqp9a expression exhibited an opposite trend. These findings suggest that aqp1a, aqp3a, aqp8a and aqp9a may play a major osmoregulatory role in water transport in the gills and intestines during acclimation to different salinity environment.

The osmotic response capacity of the Antarctic fish Harpagifer antarcticus is insufficient to cope with projected temperature and salinity under climate change

Over the last decades, climate change has intensified. Temperatures have increased and seawater has become "fresher" in Antarctica, affecting fish such as Harpagifer antarcticus. Thus, this study aimed to evaluate changes in the osmoregulatory response of the Antarctic notothenioid fish Harpagifer antarcticus and evaluate how it will cope with the future climate change and environmental conditions in the Antarctic, and in the hypothetical case that its geographical distribution will be extended to the Magellanes region. The present study was undertaken to determine the interaction between temperature and salinity tolerance (2 °C and 33 psu as the control group, the experimental groups were 5, 8, and 11 °C and 28 and 23 psu) and their effect on the osmoregulatory status of H. antarcticus. We evaluated changes in gill-kidney-intestine NKA activity, gene expression of NKAα, NKCC, CFTR, Aquaporins 1 and 8 in the same tissues, muscle water percentage, and plasma osmolality to evaluate osmoregulatory responses. Plasma osmolality decreased with high temperature, also the gill-kidney-intestine NKA activity, gene expression of NKA α, NKCC, CFTR, Aquaporins 1, and 8 were modified by temperature and salinity. We demonstrated that H. antarcticus can not live in the Magallanes region, due to its incapacity to put up with temperatures over 5 °C and with over 8 °C being catastrophic.

Response of gut microbiota and immune function to hypoosmotic stress in the yellowfin seabream (Acanthopagrus latus)

Osmotic stress is associated with heightened immune functions and altered microbiota in the fish intestine. In this study, we explored the effects of hypoosmotic stress on the intestine of euryhaline yellowfin seabream (Acanthopagrus latus) after acute exposure to brackish water, low-saline water, and freshwater environments. The results showed that hypoosmotic stress reshaped the composition of the microbial community and altered the gene expression in the intestine. Probiotics Lactobacillus and Pseudomonas showed higher relative abundance in a brackish water environment, whereas pathogenic bacteria, including Vibrio and Aeromonas, were more abundant in the freshwater environment. At the transcriptional level, osmoregulation-related genes were identified as up/down regulated differentially expressed genes (DEGs) as well as a series of immune-related DEGs associated with pathogen recognition, antimicrobial ability, pro-inflammatory cytokines, cell apoptosis, and antioxidant defense. Physiological analysis showed that Na⁺ K⁺-ATPase activity was significantly inhibited by hypoosmotic stress in freshwater. Meanwhile, the intestinal antioxidant defense system of yellowfin seabream was challenged. Correlation network analysis demonstrated the close interactions among intestinal microbes, differentially expressed genes, and physiological parameters. This study provides the critical insights into the function of the intestine fish encountering hypoosmotic stress.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

A marine teleost, Opsanus beta, compensates acidosis in hypersaline water by H+ excretion or reduced HCO3- excretion rather than HCO3- uptake

Increases in ambient salinity demand parallel increases in intestinal base secretion for maintenance of osmoregulatory status, which is likely the cause of a transient acidosis following transfer of euryhaline fish from freshwater to seawater. It was predicted that transfer of the marine Gulf toadfish (Opsanus beta) from seawater (35 ppt) to hypersaline (60 ppt) seawater (HSW) would lead to a transient acidosis that would be compensated by increases in branchial acid excretion to offset the acid-base disturbance. Toadfish exposed to HSW showed a significant decrease in blood pH and [HCO₃^-] but no increase in pCO₂, followed by a full recovery after 48-96 h. A similar metabolic acidosis and recovery was found when fish were exposed to 60-ppt HCO₃^--free seawater (HEPES-buffered), which may suggest that compensation for intestinal base loss during hypersaline treatment is from gill H⁺ excretion rather than gill HCO₃^- uptake. However, we cannot rule out that reduced branchial HCO₃^- excretion contributed to an increase in net acid excretion. Since colchicine prevents full compensation, translocation of H⁺ and/or HCO₃^- transporters between cytosolic compartments and plasma membrane fractions might be involved in compensating for the hypersalinity-induced acidosis. Translocation of transporters rather than de novo synthesis may represent a faster and less energetically demanding response to rapidly fluctuating and high salinities encountered by toadfish in their natural environment.

Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus)

Lampreys are the most basal vertebrates with an osmoregulatory strategy. Previous research has established that the salinity tolerance of sea lamprey increases dramatically during metamorphosis, but underlying changes in the gut have not been examined. In the present work, we examined changes in intestinal function during metamorphosis and seawater exposure of sea lamprey (Petromyzon marinus). Fully metamorphosed juvenile sea lamprey had 100% survival after direct exposure to 35 parts per thousand seawater (SW) and only slight elevations in plasma chloride (Cl^-) levels. Drinking rates of sea lamprey juveniles in seawater were 26-fold higher than juveniles in freshwater (FW). Na⁺-K⁺-ATPase (NKA) activity in the anterior and posterior intestine increased 12- and 3-fold, respectively, during metamorphosis, whereas esophageal NKA activity was lower than in the intestine and did not change with development. Acclimation to SW significantly enhanced NKA activity in the posterior intestine but did not significantly change NKA activity in the anterior intestine, which remained higher than that in the posterior intestine. Intestinal Cl^- and water uptake, which were observed in ex vivo preparations of anterior and posterior intestine under both symmetric and asymmetric conditions, were higher in juveniles than in larvae and were similar in magnitude of those of teleost fish. Inhibition of NKA by ouabain in ex vivo preparations inhibited intestinal water absorption by 64%. Our results indicate drinking and intestinal ion and water absorption are important to osmoregulation in SW and that preparatory increases in intestinal NKA activity are important to the development of salinity tolerance that occurs during sea lamprey metamorphosis.

Hemodynamic responses to warming in euryhaline rainbow trout: implications of the osmo-respiratory compromise

In seawater, rainbow trout (Oncorhynchus mykiss) drink and absorb water through the gastrointestinal tract to compensate for water passively lost to the hyperosmotic environment. Concomitantly, they exhibit elevated cardiac output and a doubling of gastrointestinal blood flow to provide additional O₂ to the gut and increase convective flux of absorbed ions and water. Yet, it is unknown how warming waters, which elevate tissue O₂ demand and the rate of diffusion of ions and water across the gills (i.e. the osmo-respiratory compromise), affects these processes. We measured cardiovascular and blood variables of rainbow trout acclimated to freshwater and seawater during acute warming from 11 to 17°C. Relative to freshwater-acclimated trout, cardiac output was 34% and 55% higher in seawater-acclimated trout at 11 and 17°C, respectively, which allowed them to increase gastrointestinal blood flow significantly more during warming (increases of 75% in seawater vs. 31% in freshwater). These adjustments likely served to mitigate the impact of warming on osmotic balance, as changes in ionic and osmotic blood composition were minor. Furthermore, seawater-acclimated trout seemingly had a lower tissue O₂ extraction, explaining why trout acclimated to freshwater and seawater often exhibit similar metabolic rates, despite a higher cardiac output in seawater. Our results highlight a novel role of gastrointestinal blood perfusion in the osmo-respiratory compromise in fish, and improve our understanding of the physiological changes euryhaline fishes must undergo when faced with interacting environmental challenges such as transient warming events.

Peptide Transporters in the Primary Gastrointestinal Tract of Pre-Feeding Mozambique Tilapia Larva

Fish larvae differ greatly from the adult form in their morphology and organ functionality. The functionality of the gastrointestinal tract depends on the expression of various pumps, transporters, and channels responsible for feed digestion and nutrients absorption. During the larval period, the gastrointestinal tract develops from a simple closed tube, into its complex form with differentiated segments, crypts and villi, as found in the adult. In this study, we characterized the expression of three peptide transporters (PepT1a, PepT1b, and PepT2) in the gastrointestinal tract of Mozambique tilapia (Oreochromis mossambicus) larvae along 12 days of development, from pre-hatching to the completion of yolk sac absorption. Gene expression analysis revealed differential and complimentary time-dependent expression of the PepT1 variants and PepT2 along the larval development period. Immunofluorescence analysis showed differential protein localization of the three peptide transporters (PepTs) along the gastrointestinal tract, in a similar pattern to the adult. In addition, PepT1a was localized in mucosal cells in the larvae esophagus, in much higher abundance than in the adults. The results of this study demonstrate specialization of intestinal sections and absorbance potential of the enterocytes prior to the onset of active exogenous feeding, thus pointing to an uncharacterized function and role of the gastrointestinal tract and its transporters during the larval period.

Carbonic Anhydrase as a Biomarker of Global and Local Impacts: Insights from Calcifying Animals

The emission of greenhouse gases has grown in unprecedented levels since the beginning of the industrial era. As a result, global climate changes, such as heightened global temperature and ocean acidification, are expected to negatively impact populations. Similarly, industrial and urban unsustainable development are also expected to impose local impacts of their own, such as environmental pollution with organic and inorganic chemicals. As an answer, biomarkers can be used in environmental programs to assess these impacts. These tools are based in the quantification of biochemical and cellular responses of target species that are known to respond in a sensitive and specific way to such stresses. In this context, carbonic anhydrase has shown to be a promising biomarker candidate for the assessment of global and local impacts in biomonitoring programs, especially in marine zones, such as coral reefs, considering the pivotal role of this enzyme in the calcification process. Therefore, the aim of this review is to show the recent advances in the carbonic anhydrase research and the reasons why it can be considered as a promising biomarker to be used for calcifying organisms.

Increased intestinal carbonate precipitate abundance in the sea bream (Sparus aurata L.) in response to ocean acidification

Marine fish contribute to the carbon cycle by producing mineralized intestinal precipitates generated as by-products of their osmoregulation. Here we aimed at characterizing the control of epithelial bicarbonate secretion and intestinal precipitate presence in the gilthead sea bream in response to predicted near future increases of environmental CO₂. Our results demonstrate that hypercapnia (950 and 1800 μatm CO₂) elicits higher intestine epithelial HCO₃^- secretion ex vivo and a subsequent parallel increase of intestinal precipitate presence in vivo when compared to present values (440 μatm CO₂). Intestinal gene expression analysis in response to environmental hypercapnia revealed the up-regulation of transporters involved in the intestinal bicarbonate secretion cascade such as the basolateral sodium bicarbonate co-transporter slc4a4, and the apical anion transporters slc26a3 and slc26a6 of sea bream. In addition, other genes involved in intestinal ion uptake linked to water absorption such as the apical nkcc2 and aquaporin 1b expression, indicating that hypercapnia influences different levels of intestinal physiology. Taken together the current results are consistent with an intestinal physiological response leading to higher bicarbonate secretion in the intestine of the sea bream paralleled by increased luminal carbonate precipitate abundance and the main related transporters in response to ocean acidification.

Molecular and functional regionalization of bicarbonate secretion cascade in the intestine of the European sea bass (Dicentrarchus labrax)

In marine fish the intestinal HCO₃^- secretion is the key mechanism to enable luminal aggregate formation and water absorption. Using the sea bass (Dicentrarchus labrax), the present study aimed at establishing the functional and molecular organization of different sections of the intestine concerning bicarbonate secretion and Cl^- movements. The proximal intestinal regions presented similar HCO₃^- secretion rates, while differences were detected in the molecular expression of the transporters involved and on regional HCO₃^- concentrations. The anterior region presented significantly higher Na⁺/K⁺-ATPase activity, Cl^- transepithelial transport and basolateral slc4a4, apical slc26a6 and slc26a3 expression levels. In the mid intestine, the total HCO₃^- content was significantly increased in the fluid as in the carbonate aggregates. In the rectum no HCO₃^- secretion was observed and was characterized by the diminished HCO₃^- total content, residual molecular expression of slc4a4, slc26a6 and slc26a3, higher H⁺-ATPase activity and expression, suggesting the existence of a different bicarbonate handling mechanism. The possible regulation of HCO₃^- secretion by extracellular HCO₃^- and increased intracellular cAMP levels were also investigated. cAMP did not affect HCO₃^- secretion, although Cl^- secretion was enhanced by cftr. HCO₃^- secretion rise due to the HCO₃^- basolateral increment showed that at resting levels slc4a4 was not a limiting step for secretion. The transcellular/intracellular dependence of apical HCO₃^- secretion differed between the proximal regions. In conclusion, intestinal HCO₃^- secretion has a functional region-dependent organization that was not reflected by the anterior-posterior regionalization on HCO₃^- secretion and expression profiles of chloride/water absorption related genes.

Molecular mechanisms for intestinal HCO3− secretion and its regulation by guanylin in seawater-acclimated eels

The intestine of marine teleosts secretes HCO3− into the lumen and precipitates Ca2+ and Mg2+ in the imbibed seawater as carbonates to decrease luminal fluid osmolality and facilitate water absorption. However, hormonal regulation of HCO3−secretion is largely unknown. Here, mucosally-added guanylin (GN) increased HCO3− secretion, measured by pH-stat, across isolated seawater-acclimated eel intestine bathed in saline at pH 7.4 (5% CO2). The effect of GN on HCO3− secretion was slower than that on the short-circuit current, and the time-course of the GN effect was similar to that of bumetanide. Mucosal bumetanide and serosal 4,4’-dinitrostilbene-2,2’-disulfonic acid (DNDS) inhibited the GN effect, suggesting an involvement of apical Na+-K+-2Cl− cotransporter (NKCC2) and basolateral Cl−/HCO3− exchanger (AE)/Na+-HCO3− cotransporter (NBC) in the GN effect. As mucosal DNDS failed to inhibit the GN effect, apical DNDS-sensitive AE may not be involved. To identify molecular species of transporters involved in the GN effect, we performed RNA-seq analyses followed by quantitative real-time PCR after transfer of eels to seawater. Among the genes upregulated after seawater transfer, AE genes, draa, b, and pat1a, c, on the apical membrane, and NBC genes, nbce1a, n1, n2a, and a AE gene, sat-1, on the basolateral membrane were candidates involved in HCO3− secretion. Judging from the slow effect of GN, we suggest that GN inhibits NKCC2b on the apical membrane and decreases cytosolic Cl− and Na+, which then activates apical DNDS-insensitive DRAs and basolateral DNDS-sensitive NBCs to enhance transcellular HCO3− flux across the intestinal epithelia of seawater-acclimated eels.

The influence of salinity and water chemistry on acute toxicity of cadmium to two euryhaline fish species

The euryhaline killifishes, Fundulus heteroclitus and Kryptolebias marmoratus inhabit estuaries that rapidly change salinity. Although cadmium (Cd) toxicity has been well characterized in fish inhabiting freshwaters, fewer studies have examined the toxic effects of Cd in estuarine and saltwater environments. Additionally, current environmental regulations do not account for organism physiology in different salinity waters even though metal sensitivity is likely to change in these environments. In this study, we investigated effects of changing salinity on acute Cd toxicity to larval (7-9 d old) F. heteroclitus and K. marmoratus. Median 96-h lethal concentrations (LC50) for Cd were calculated for both fish species at six different salinities. As salinity increased, metal toxicity decreased in both fish species up to 18 ppt salinity; and F. heteroclitus were more sensitive than K. marmoratus at salinities above 12 ppt. To determine which components of saltwater were protective against Cd toxicity, we investigated the influence of CaSO₄ (100 and 200 mg/L), CaCl₂ (200 mg/L), and MgSO₄ (300 mg/L) on Cd toxicity to K. marmoratus. The results demonstrated that both competition with calcium and complexation with chloride reduced the toxic effects of Cd to K. marmoratus. These findings could be used to improve marine/estuarine biotic ligand models for the determination of site-specific water quality criteria for Cd.

The role of salinity in the trophic transfer of 137Cs in euryhaline fish

In order to better understand the influence of changing salinity conditions on the trophic transfer of ¹³⁷Cs in marine fish that live in dynamic coastal environments, its depuration kinetics was investigated in controlled aquaria. The juvenile turbot Scophthalmus maximus was acclimated to three distinct salinity conditions (10, 25 and 38) and then single-fed with compounded pellets that were radiolabelled with ¹³⁷Cs. At the end of a 21-d depuration period, assimilation efficiencies (i.e. AEs = proportion of ¹³⁷Cs ingested that is actually assimilated by turbots) were determined from observational data acquired over the three weeks. Our results showed that AEs of ¹³⁷Cs in the turbots acclimated to the highest salinity condition were significantly lower than for the other conditions (p < 0.05). Osmoregulation likely explains the decreasing AE observed at the highest salinity condition. Indeed, observations indicate that fish depurate ingested ¹³⁷Cs at a higher rate when they increase ion excretion, needed to counterbalance the elevated salinity. Such data confirm that ambient salinity plays an important role in trophic transfer of ¹³⁷Cs in some fish species. Implications for such findings extend to seafood safety and climate change impact studies, where the salinity of coastal waters may shift in future years in response to changing weather patterns.

Dietary salt levels affect digestibility, intestinal gene expression, and the microbiome, in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is the world’s most widely cultured fish species. Therefore, its nutritional physiology is of great interest from an aquaculture perspective. Studies conducted on several fish species, including tilapia, demonstrated the beneficial effects of dietary salt supplementation on growth; however, the mechanism behind these beneficial effects is still not fully understood. The fish intestine is a complex system, with functions, such as nutrient absorption, ion equilibrium and acid-base balance that are tightly linked and dependent on each other's activities and products. Ions are the driving force in the absorption of feed components through pumps, transporters and protein channels. In this study, we examined the impact of 5% increase in dietary NaCl on protein, lipid, ash and dry matter digestibility, as well as on the expression of intestinal peptide transporters (PepTs) and ion pumps (Na⁺/K⁺-ATPase, V-H⁺-ATPase, N⁺/H⁺-Exchanger) in Nile tilapia. In addition, effects on the gut microbiome were evaluated. Our results show that dietary salt supplementation significantly increased digestibility of all measured nutritional components, peptide transporters expression and ion pumps activity. Moreover, changes in the gut microbial diversity were observed, and were associated with lipid digestibility and Na⁺/K⁺-ATPase expression.

Intestinal FXYD12 and sodium-potassium ATPase: A comparative study on two euryhaline medakas in response to salinity changes

FXYD proteins are the regulators of sodium-potassium ATPase (Na+/K+-ATPase, NKA). In teleosts, NKA is a primary driving force for the operation of many ion transport systems in the osmoregulatory organs (e.g. intestines). Hence, the purpose of this study was to determine the expression of FXYD proteins and NKA α-subunit in the intestines of two closely related medakas (Oryzias dancena and O. latipes), which came from different salinity habitats and have diverse osmoregulatory capabilities, to illustrate the association between NKA and FXYD proteins of two medaka species in response to salinity changes. The results showed that the fxyd12 mRNA was the most predominant in the intestines of both medakas. The association of FXYD12 and NKA in the intestines of the two medaka species was demonstrated via double immunofluorescent staining and co-immunoprecipitation. Upon salinity challenge, the localization of FXYD12 and NKA was similar in the intestines of the two medaka species. However, the expression profiles of intestinal FXYD12 and NKA (mRNA and protein levels), as well as NKA activity differed between the medakas. These results showed that FXYD12 may play a role in modulating NKA activity in the intestines of the two medakas following salinity changes in the maintenance of internal homeostasis. These findings contributed to knowledge of the expression and potential role of vertebrate FXYD12, the regulators of NKA, upon salinity challenge.

Regulation of Bicarbonate Secretion in Marine Fish Intestine by the Calcium-Sensing Receptor

In marine fish, high epithelial intestinal HCO₃− secretion generates luminal carbonate precipitates of divalent cations that play a key role in water and ion homeostasis. The present study was designed to expose the putative role for calcium and the calcium-sensing receptor (CaSR) in the regulation of HCO₃− secretion in the intestine of the sea bream (Sparus aurata L.). Effects on the expression of the CaSR in the intestine were evaluated by qPCR and an increase was observed in the anterior intestine in fed fish compared with unfed fish and with different regions of intestine. CaSR expression reflected intestinal fluid calcium concentration. In addition, anterior intestine tissue was mounted in Ussing chambers to test the putative regulation of HCO₃− secretion in vitro using the anterior intestine. HCO₃− secretion was sensitive to varying calcium levels in luminal saline and to calcimimetic compounds known to activate/block the CaSR i.e., R 568 and NPS-2143. Subsequent experiments were performed in intestinal sacs to measure water absorption and the sensitivity of water absorption to varying luminal levels of calcium and calcimimetics were exposed as well. It appears, that CaSR mediates HCO₃− secretion and water absorption in marine fish as shown by responsiveness to calcium levels and calcimimetic compounds.

Renoguanylin stimulates apical CFTR translocation and decreases HCO3- secretion through PKA activity in the Gulf toadfish (Opsanus beta)

The guanylin peptides - guanylin, uroguanylin and renoguanylin (RGN) - are endogenously produced hormones in teleost fish enterocytes that are activators of guanylyl cyclase-C (GC-C) and are potent modulators of intestinal physiology, particularly in seawater teleosts. Most notably, they reverse normal net ion-absorbing mechanisms that are vital to water absorption, an important process for seawater teleost survival. The role of guanylin-peptide stimulation of the intestine remains unclear, but it is hypothesized to facilitate the removal of solids from the intestine by providing fluid to enable their removal by peristalsis. The present study used one member of this group of peptides - RGN - to provide evidence for the prominent role that protein kinase A (PKA) plays in mediating the effects of guanylin-peptide stimulation in the posterior intestine of the Gulf toadfish (Opsanus beta). Protein kinase G was found to not mediate the intracellular effects of RGN, despite previous evidence showing that GC-C activation leads to higher cyclic guanosine monophosphate formation. RGN reversed the absorptive short-circuit current and increased conductance in the Gulf toadfish intestine. These effects are correlated to increased trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel to the apical membrane, which is negated by PKA inhibition. Moreover, RGN decreased HCO₃^- secretion, likely by limiting apical HCO₃^-/Cl^- exchange (possibly by reducing SLC26a6 activity), a reduction that was enhanced by PKA inhibition. RGN seems to alter PKA activity in the posterior intestine to recruit CFTR to the apical membrane and reduce HCO₃^- secretion.

Comparison of the organic matrix found in intestinal CaCO3 precipitates produced by several marine teleost species

Marine bony fish poses the unique ability to hydrate from imbibed seawater. They accomplish this, in part, by the precipitation of inorganic carbonate mineral in their intestine, which lowers luminal osmotic pressure and allows for water uptake. It has recently been described that in the Gulf toadfish (Opsanus beta) this Ca(Mg)CO₃ precipitation occurs under the regulation of an organic matrix. To date no investigations have aimed to determine if this phenomenon applies more generally to marine fish. Here, intestinally derived precipitates were collected from gray snapper (Lutjanus griseus), white grunt (Haemulon plumieri), European flounder (Platichthys flesus), as well as Gulf toadfish, and their matrices were extracted. The ability of these matrices to regulate CaCO₃ production was determined using an in vitro calcification assay, which revealed that the matrix derived from each of the tested species increased precipitation at low concentrations, while inhibiting it at higher concentrations in full agreement with the earlier studies on toadfish. Matrix extracted from European flounder precipitates was then analyzed by mass spectrometry, leading to the identification of over 50 unique proteins. When the identities of these proteins were compared to previous investigation of toadfish precipitate matrix, nearly 35% were found to overlap between the flounder and toadfish analyses, suggesting conserved mechanisms of precipitation control. The effects of using different sodium hypochlorite (NaOCl) solutions during precipitate purification on the resulting organic matrix are also discussed.

Hypoxia, but not an electrolyte-imbalanced diet, reduces feed intake, growth and oxygen consumption in rainbow trout (Oncorhynchus mykiss)

Oxygen limitation and dietary imbalances are key aspects influencing feed intake (FI) and growth performance in cultured fish. This study investigated the combined effects of hypoxia and dietary electrolyte balance on the growth performance, body composition and nutrient utilization in a rainbow trout (Oncorhynchus mykiss) isogenic line. Fish were fed ad libitum two experimental diets: electrolyte-balanced or -imbalanced diets (DEB 200 or 700 mEq kg⁻¹, respectively) and exposed to normoxia or hypoxia (7.9 or 4.5 mg O₂ l⁻¹, respectively) for 42 days. DEB did not affect FI, growth performance or body composition. Nevertheless, hypoxia had a negative impact, reducing FI (6%), growth rate (8%), oxygen consumption (19%), energy (5%) and lipid (42%) contents. Digestible energy intake and heat production were higher in normoxic fish (40% and 23%, respectively), retaining 64% more energy in lipid or protein. Hypoxia reduced the apparent digestibility of dry matter, ash, protein, lipid, carbohydrates and energy. Trout fed DEB 700 diet were energetically less efficient, reflected in higher heat production and energy requirements for maintenance. FI was inhibited by low dissolved oxygen levels, but not by electrolyte-imbalanced diet, in spite of the higher energy requirements for maintenance. This study highlights the importance that dietary-electrolyte content and DO levels have on energy balance and growth performance when fish are fed to satiation.

Effects of the acclimation to high salinity on intestinal ion and peptide transporters in two tilapia species that differ in their salinity tolerance

Tilapiine species, widely distributed across habitats with diverse water salinities, are important to aquaculture as well as a laboratory model. The effects of water salinity on two tilapia species, that differ in their salinity tolerance, was evaluated. Oreochromis niloticus reared in brackish-water, showed a significant decrease in growth and feed efficiency, whereas O. mossambicus reared in seawater did not show any significant changes. The expression and activity of Na⁺/K⁺-ATPase (NKA), V-type H⁺-ATPase (VHA) and carbonic anhydrase (CA), as well as expression levels of genes encoding two HCO₃^- and three peptide transporters (nbc1, slc26a6, slc15a1a, slc15a1b and slc15a2) were measured in three intestinal sections of these two species, grown in freshwater and brackish/sea-water. Overall, the spatial distribution along the intestine of the genes examined in this study was similar between the two species, with the exception of tcaIV. The salinity response, on the other hand, varied greatly between these species. In O. mossambicus, there was a salinity-dependent increased expression of most of the examined genes (except slc26a6 and slc15a2), while in O. niloticus the expression of most genes did not change, or even decreased (tcaIV, nbc1 and slc15a1b). This study highlighted differences in the intestinal response to salinity acclimation between closely- related species that differ in their salinity tolerance. O. mossambicus, which has a high salinity tolerance, showed expression patterns and responses similar to marine species, and differed from the low-salinity-tolerance O. niloticus, which showed a response that differed from the accepted models, that are based on marine and diadromous fishes.

Immunohistochemical analysis of the distribution of molecules involved in ionic and pH regulation in the lancelet Branchiostoma floridae (Hubbs, 1922)

The aim of present work is to analyse the distribution of carbonic anhydrase II (CAII), cystic fibrosis transmembrane regulator (CFTR), vacuolar-type H⁺-ATPase (V-H⁺-ATPase), Na⁺/K⁺ ATPase, Na⁺/H⁺ exchanger (NHE) and SLC26A6 (solute carrier family 26, member 6), also known as pendrin protein, in the lancelet Branchiostoma floridae in order to go in depth in the evolution of osmoregulation and pH regulation in Chordates. In view of their phylogenetic position, lancelets may indeed provide a critical point of reference for studies on osmoregulation evolution in Chordates. The results of present work demonstrated that, except to Na⁺/K⁺ ATPase that is strongly expressed in nephridia only, all the other studied molecules are abundantly present in skin, coelomic epithelium, renal papillae and nephridia and hepatic coecum. Thus, it is possible to hypothesize that also in lancelet, as in fish, these organs are involved in pH control and ionic regulation. In the digestive tract of B. floridae, the intestine epithelium was weakly immune-reactive to all tested antibodies, while the hepatic coecum showed an intense immunoreactivity to all molecules. Since in amphioxus the hepatic coecum functions simultaneously as stomach, liver and pancreas, these immunohistochemical results proved the secretion of H+ and HCO₃^- ions, typical of digestive process. Colocalization studies indicated a co-expression of the studied proteins in all considered organs, excluding NHE and pendrin for renal papillae, since some renal papillae are NHE immunopositive only.

An Adipokinetic Hormone Acts as a Volume Regulator in the Intertidal Gastropod Mollusk, Aplysia californica

Adipokinetic hormone (AKH) is a multifunctional neuropeptide in the gonadotropin-releasing hormone superfamily. In insects, AKH acts to mobilize energy stores during times of high energetic demand, but has been shown to have other effects. In lophotrochozoans, the presence and function of AKH are less characterized. We have previously identified an AKH in an intertidal gastropod mollusk, the California sea hare (Aplysia californica), and named it ac-AKH. Our previous data showed ac-AKH induced an acute weight loss, suggesting a role in volume regulation. The overarching goals of this study were to test the role of ac-AKH as a volume regulator and examine the mechanism by which ac-AKH induced the acute weight loss. Our results showed that ac-AKH reduced body mass, in part, through the reduction of hemolymph volume without altering hemolymph osmolality or specific osmolytes. The effect of ac-AKH on volume loss was accentuated under a hyposaline condition. We further showed that ac-akh expression was inhibited during a hyposaline challenge, and that the administration of ac-AKH partially reversed the increase in body mass, but not hemolymph osmolality change, caused by the hyposaline challenge. These data collectively show that ac-AKH is a proximate regulator controlling the fluid volume, but not osmolality, in A. californica. Importantly, our results highlight the functional divergence of this structurally conserved neuropeptide in the molluscan lineage.

Intestinal Na+, K+, 2Cl- cotransporter 2 plays a crucial role in hyperosmotic transitions of a euryhaline teleost

Euryhaline fishes, such as the red drum (Sciaenops ocellatus), must quickly transition between hyperosmotic and hypoosmotic physiological strategies. When freshwater individuals transition to seawater they are exposed to increased diffusive water loss and ion gain. To maintain osmoregulatory balance these animals must drink and absorb seawater through the intestine, followed by ion excretion at the gills. The ability of fishes to transition between strategies can limit the magnitude of osmotic shock that can be tolerated. Here, we demonstrate that red drum can tolerate direct transfer from freshwater to full‐strength seawater with marginal impacts on osmotic balance, as indicated by plasma and muscle ion concentration, as well as muscle water. Seawater transition is concurrent with a significant increase in intestinal fluid volume. Typical patterns of osmoregulatory plasticity were observed in the gill with increased expression of nkcc1 and cftr. Expression changes in the anterior intestine were observed after 24 h for nkcc2 with smaller and later responses observed for slc26a3, slc26a6, and nbc. Immunofluorescence staining demonstrated similar patterns of NKCC localization in freshwater and seawater intestines; however, reduced basolateral staining of V‐type ATPase was observed in seawater. Electrophysiological preparations demonstrated that seawater fish had increased absorptive current in the anterior intestine, which was significantly reduced in the presence of 10 μmol/L bumetanide. Overall, these results suggest that nkcc2 plays a crucial role during hyperosmotic transitions, and may be a more important complement to the well‐known bicarbonate secretion pathway than generally considered.

Exposure to seawater increases intestinal motility in euryhaline rainbow trout (Oncorhynchus mykiss)

Upon exposure to seawater, euryhaline teleosts need to imbibe and desalinate seawater to allow for intestinal ion and water absorption, as this is essential for maintaining osmotic homeostasis. Despite the potential benefits of increased mixing and transport of imbibed water for increasing the efficiency of absorptive processes, the effect of water salinity on intestinal motility in teleosts remains unexplored. By qualitatively and quantitatively describing in vivo intestinal motility of euryhaline rainbow trout (Oncorhynchus mykiss), this study demonstrates that, in freshwater, the most common motility pattern consisted of clusters of rhythmic, posteriorly propagating contractions that lasted ∼1-2 min followed by a period of quiescence lasting ∼4-5 min. This pattern closely resembles mammalian migrating motor complexes (MMCs). Following a transition to seawater, imbibed seawater resulted in a significant distension of the intestine and the frequency of MMCs increased twofold to threefold with a concomitant reduction in the periods of quiescence. The increased frequency of MMCs was also accompanied by ripple-type contractions occurring every 12-60 s. These findings demonstrate that intestinal contractile activity of euryhaline teleosts is dramatically increased upon exposure to seawater, which is likely part of the overall response for maintaining osmotic homeostasis as increased drinking and mechanical perturbation of fluids is necessary to optimise intestinal ion and water absorption. Finally, the temporal response of intestinal motility in rainbow trout transitioning from freshwater to seawater coincides with previously documented physiological modifications associated with osmoregulation and may provide further insight into the underlying reasons shaping the migration patterns of salmonids.

Similarity of osmoregulatory capacity in coastal and inland alligator gar

The alligator gar Atractosteus spatula is a primitive fish species, occupying a wide range of temperature and salinity habitats. Long-distance movements are limited, leading to genetic differentiation between inland and coastal populations. Unknown is whether physiological capacity differs between geographically separated populations, particularly for traits important to osmoregulation in saline environments. Alligator gar from inland and coastal populations were reared in a similar environment and exposed to temperature (10, 30°C) and salinity (0, 20ppt) extremes to determine whether iono- and osmoregulatory ability differed between populations. There were few differences in osmoregulatory ability between populations, with similar gill, blood and gastrointestinal tract osmoregulatory parameters. Blood plasma osmolality, ion concentrations, intestinal pH and bicarbonate base concentrations, intestinal fluid osmolality, ion concentrations and gill Na⁺, K⁺-ATPase (NKA) activity were similar between populations. Notably, gar from both populations did not osmoregulate well at low temperature and high salinity, with elevated plasma osmolality and ion concentrations, low gill NKA, and little evidence of gastrointestinal tract contribution to ionic and base regulation based on a lack of intestinal fluid and low base content. Therefore, the hypothesis that coastal gar would have improved osmotic regulatory ability in saline environments as compared to inland alligator gar was not supported, suggesting physiological capacity may be retained in primitive species possibly due to its importance to their persistence through time.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.

Kidney morphology and candidate gene expression shows plasticity in sticklebacks adapted to divergent osmotic environments

Novel physiological challenges in different environments can promote the evolution of divergent phenotypes, either through plastic or genetic changes. Environmental salinity serves as a key barrier to the distribution of nearly all aquatic organisms, and species diversification is likely to be enabled by adaptation to alternative osmotic environments. The threespine stickleback (Gasterosteus aculeatus) is a euryhaline species with populations found both in marine and freshwater environments. It has evolved both highly plastic and locally adapted phenotypes due to salinity-derived selection, but the physiological and genetic basis of adaptation to salinity is not fully understood. We integrated comparative cellular morphology of the kidney, a key organ for osmoregulation, and candidate gene expression to explore the underpinnings of evolved variation in osmotic plasticity within two populations of sticklebacks from distinct salinity zones in the Baltic Sea: the high salinity Kattegat, representative of the ancestral marine habitat, and the low salinity Bay of Bothnia. A common-garden experiment revealed that kidney morphology in the ancestral high salinity population had a highly plastic response to salinity conditions, whereas this plastic response was reduced in the low salinity population. Candidate gene expression in kidney tissue revealed a similar pattern of population-specific differences, with a higher degree of plasticity in the native high salinity population. Together these results suggest that renal cellular morphology has become canalized to low salinity, and that these structural differences may have functional implications for osmoregulation.