Identification of the putative goldfish (Carassius auratus) magnesium transporter SLC41a1 and functional regulation in the gill, kidney, and intestine in response to dietary and environmental manipulations

Role of the kidneys in acid-base regulation and ammonia excretion in freshwater and seawater fish: implications for nephrocalcinosis

Maintaining normal pH levels in the body fluids is essential for homeostasis and represents one of the most tightly regulated physiological processes among vertebrates. Fish are generally ammoniotelic and inhabit diverse aquatic environments that present many respiratory, acidifying, alkalinizing, ionic and osmotic stressors to which they are able to adapt. They have evolved flexible strategies for the regulation of acid-base equivalents (H⁺, NH₄ ⁺, OH^- and HCO₃ ^-), ammonia and phosphate to cope with these stressors. The gills are the main regulatory organ, while the kidneys play an important, often overlooked accessory role in acid-base regulation. Here we outline the kidneys role in regulation of acid-base equivalents and two of the key 'urinary buffers', ammonia and phosphate, by integrating known aspects of renal physiology with recent advances in the molecular and cellular physiology of membrane transport systems in the teleost kidneys. The renal transporters (NHE3, NBC1, AE1, SLC26A6) and enzymes (V-type H⁺ATPase, CAc, CA IV, ammoniagenic enzymes) involved in H⁺ secretion, bicarbonate reabsorption, and the net excretion of acidic and basic equivalents, ammonia, and inorganic phosphate are addressed. The role of sodium-phosphate cotransporter (Slc34a2b) and rhesus (Rh) glycoproteins (ammonia channels) in conjunction with apical V-type H⁺ ATPase and NHE3 exchangers in these processes are also explored. Nephrocalcinosis is an inflammation-like disorder due to the precipitation of calcareous material in the kidneys, and is listed as one of the most prevalent pathologies in land-based production of salmonids in recirculating aquaculture systems. The causative links underlying the pathogenesis and etiology of nephrocalcinosis in teleosts is speculative at best, but acid-base perturbation is probably a central pathophysiological cause. Relevant risk factors associated with nephrocalcinosis are hypercapnia and hyperoxia in the culture water. These raise internal CO₂ levels in the fish, triggering complex branchial and renal acid-base compensations which may promote formation of kidney stones. However, increased salt loads through the rearing water and the feed may increase the prevalence of nephrocalcinosis. An increased understanding of the kidneys role in acid-base and ion regulation and how this relates to renal diseases such as nephrocalcinosis will have applied relevance for the biologist and aquaculturist alike.

Magnesium transport in the aglomerular kidney of the Gulf toadfish (Opsanus beta)

Despite having an aglomerular kidney, Gulf toadfish can survive in water ranging from nearly fresh up to 70 parts per thousand salinity. In hyperosmotic environments, the major renal function is to balance the passive Mg²⁺ load from the environment with an equal excretion. However, the molecular transporters involved in Mg²⁺ secretion are poorly understood. We investigated whether environmental MgCl₂ alone or in combination with elevated salinity affected transcriptional regulation of genes classically involved in renal Mg²⁺ secretion (slc41a1, slc41a3, cnnm3) together with three novel genes (trpm6, trpm7, claudin-19) and two isoforms of the Na⁺/K⁺-ATPase α-subunit (nka-α1a, nka-α1b). First, toadfish were acclimated to 5, 9, 35, or 60 ppt water (corresponding to ~ 7, 13, 50 and 108 mmol L^-1 ambient [Mg²⁺], respectively) and sampled at 24 h or 9 days. Next, the impact of elevated ambient [Mg²⁺] was explored by exposing toadfish to control (50 mmol L^-1 Mg²⁺), or elevated [Mg²⁺] (100 mmol L^-1) at a constant salinity for 7 days. Mg²⁺ levels in this experiment corresponded with levels in control and hypersaline conditions in the first experiment. A salinity increase from 5 to 60 ppt stimulated the level of all investigated transcripts in the kidney. In Mg²⁺-exposed fish, we observed a 14-fold increase in the volume of intestinal fluids and elevated plasma osmolality and [Mg²⁺], suggesting osmoregulatory challenges. However, none of the renal gene targets changed expression compared with the control group. We conclude that transcriptional regulation of renal Mg²⁺ transporters is induced by elevated [Mg²⁺] in combination with salinity rather than elevated ambient [Mg²⁺] alone.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Transcriptome profiling of the low-salinity stress responses in the gills of the juvenile Pseudopleuronectes yokohamae

Salinity is an important environmental factor that affects the life cycle of fish, including their growth, development and reproduction. The marbled flounder, Pseudopleuronectes yokohamae, is an important economic resource and serves as a good model to investigate osmoregulation, as it can adapt to a wide range of salinity levels. However, the lack of genomic resources for this species has hampered the understanding of the mechanisms underlying its salinity tolerance. In this study, RNA-Seq analysis was conducted to identify genes related to salinity adaptation and osmotic regulation in the gill tissue of marbled flounder exposed to different concentrations of environmental salinity (6 and 30 ppt). After de novo assembly, 19,265 genes were annotated by the Nr database. A comparison of expression between the two salinity groups revealed 673 differentially expressed genes, of which 180 were upregulated and 493 were downregulated in the low salinity group relative to the high salinity group. The related molecular biological processes were explored from several important perspectives, and potential functions were determined by enrichment analyses, including those of metabolites in ion transportation, energy metabolism and protein synthesis, and immune responses. This study is the first transcriptomic study conducted on marbled flounder, and it revealed many novel sequences for further biological analyses. In addition, the candidate genes identified in the gene expression analysis provided insights into responses to salinity change and molecular mechanisms underlying osmoregulation in the gills of marbled flounder.

Quantification of Mg2+, Ca2+ and H+ transport by the gastrointestinal tract of the goldfish, Carassius auratus, using the Scanning Ion-selective Electrode Technique (SIET)

An in vitro gut-sac technique and the scanning ion-selective electrode technique (SIET) were used to characterize Mg²⁺, Ca²⁺, and H⁺ transport at both the mucosal and serosal surfaces of non-everted and everted gastrointestinal tissues obtained from Carassius auratus. As part of the study, two magnesium ionophores were compared (II vs. VI). Unfed animals displayed uniform transport of all ions along the intestine. Feeding resulted in elevated Mg²⁺ and Ca²⁺ transport when the gut lumen contained chyme however, under symmetrical conditions this increased transport rate was absent. Furthermore, zonation of divalent cation transport was present for both Ca²⁺ and Mg²⁺ under non-symmetrical conditions while the zonation remained for Ca²⁺ alone under symmetrical conditions. High dietary Mg²⁺ decreased absorption and induced secretion of Mg²⁺ in the posterior intestine. Uptake kinetics in the esophagus suggest large diffusive and/or convective components based on a linear relationship between Mg²⁺ transport and concentration and lack of inhibition by ouabain, an inhibitor of Na⁺-K⁺-ATPase. In contrast, kinetics in the rectum were suggestive of a low affinity, saturable carrier-mediated pathway. A decrease in Mg²⁺ and Ca²⁺ transport was observed in the posterior intestine (both at the mucosal and serosal surfaces) in response to ouabain. This impact was greatest for Ca²⁺ transport and when applied to the mucosal fluid and measured in everted preparations. In contrast a putative Mg²⁺ transport inhibitor, cobalt(III)hexamine-chloride, did not affect Mg²⁺ transport. This is the first study to use SIET approaches to study ion transport in the gut of teleost fish. This is also the first study to provide characterization of Mg²⁺ transport in the gut of C. auratus. Due to the limited selectivity of Magnesium ionophore II, subsequent studies of tissues bathed in physiological saline should be made using Magnesium Ionophore VI.