Potassium Regulation in Medaka (Oryzias latipes) Larvae Acclimated to Fresh Water: Passive Uptake and Active Secretion by the Skin Cells

Investigation of verapamil-induced cardiorenal dysfunction and compensatory ion regulation in zebrafish embryos

The purpose of the present study was to investigate the development of verapamil-induced cardiorenal failure and the response of epidermal ionocytes in zebrafish embryos to this syndrome. Zebrafish embryos were exposed to verapamil for 24 h at different developmental stages (48, 72, and 96 h post-fertilization). The exposure resulted in the generation of edema in the pericardial and yolk sac regions, with more-pronounced effects observed in later-stage embryos. Cardiac parameters showed a suppressed heart rate at all stages, with a more-significant effect appearing in later stages. Verapamil also affected cardiac parameters including the end-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF), and cardiac output (CO), indicating negative overall effects on cardiac performance. mRNA levels of heart failure markers (nppa and nppb genes) were upregulated in verapamil-exposed embryos at all stages. Renal function was impaired as FITC-dextran excretion was suppressed. A whole-embryo ion content analysis revealed significant increases in sodium and calcium contents in verapamil-exposed embryos. The density of epidermal ionocytes increased, and the apical membrane of ionocytes was enlarged, indicating upregulation of ion uptake. In addition, mRNA levels of several ion transporter genes (rhcg1, slc9a3, atp6v1a, atp2b1a, trpv6, and slc12a10.2) were significantly upregulated in verapamil-exposed embryos. In summary, prolonged exposure to verapamil can induce cardiorenal failure which triggers compensatory upregulation of ionocytes in zebrafish embryos.

Effects of Acute and Subchronic Waterborne Thallium Exposure on Ionoregulatory Enzyme Activity and Oxidative Stress in Rainbow Trout (Oncorhynchus mykiss)

The mechanisms of acute (96-hour) and subchronic (28-day) toxicity of the waterborne trace metal thallium (Tl) to rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, effects on branchial and renal ionoregulatory enzymes (sodium/potassium adenosine triphosphatase [ATPase; NKA] and proton ATPase) and hepatic oxidative stress endpoints (protein carbonylation, glutathione content, and activities of catalase and glutathione peroxidase) were examined. Fish (19-55 g) were acutely exposed to 0 (control), 0.9 (regulatory limit), 2004 (half the acute median lethal concentration), or 4200 (acute median lethal concentration) µg Tl L-1 or subchronically exposed to 0, 0.9, or 141 (an elevated environmental concentration) µg Tl L-1 . The only effect following acute exposure was a stimulation of renal H+ -ATPase activity at the highest Tl exposure concentration. Similarly, the only significant effect of subchronic Tl exposure was an inhibition of branchial NKA activity at 141 µg Tl L-1 , an effect that may reflect the interaction of Tl with potassium ion handling. Despite significant literature evidence for effects of Tl on oxidative stress, there were no effects of Tl on any such endpoint in rainbow trout, regardless of exposure duration or exposure concentration. Elevated basal levels of antioxidant defenses may explain this finding. These data suggest that ionoregulatory perturbance is a more likely mechanism of Tl toxicity than oxidative stress in rainbow trout but is an endpoint of relevance only at elevated environmental Tl concentrations. Environ Toxicol Chem 2024;43:87-96. © 2023 SETAC.

Sublethal effects of methylmercury on lateral line sensory and ion-regulatory functions in zebrafish embryos

Methylmercury can interfere with the normal functioning of the nervous system, causing a variety of behavioral and physiological changes in fish. However, the influence of MeHg on the lateral line sensory and ion-regulatory functions of fish is not clear. Zebrafish embryos were utilized as a model to address this question. After exposure to water-borne MeHg (5, 10, 50, or 100 ppb) for 96 h (4-100 h post-fertilization), the survival rate declined by ca. 50 % at 100 ppb. However, MeHg at sublethal concentrations delayed hatching and decreased heart rates and body length. As to effects on the lateral line sensory system, MeHg at ≥10 ppb decreased the number of hair cells and impaired hair bundles and Ca²⁺-mediated mechanical transduction. As to ion regulation, MeHg at ≥10 ppb decreased the densities of skin stem cells and ionocytes, leading to declines in ion (Na⁺, K⁺, and Ca²⁺) contents and H⁺/NH₄⁺ excretion levels. A gene expression analysis also revealed declines in messenger RNA levels of several ion-regulatory genes (ncc2b, trpv6v1a, trpv5/6, ncx1b, and rhcg1). This study demonstrated that the lateral line sensory and ion regulatory functions of fish are extremely sensitive to MeHg.

Exposure to silver impairs the osmoregulatory capability of euryhaline medaka (Oryzias latipes) subjected to salinity changes

The widespread use of silver in nanomaterials has led to increases in environmental contamination, which poses a threat to aquatic animals. Euryhaline fish, which live in environments with fluctuating salinity levels, have strong osmotic regulatory abilities to cope with such changes. This study attempted to investigate how silver affects the osmoregulatory capabilities of euryhaline fish, using medaka (Oryzias latipes) embryos as a model. The embryos were exposed to AgNO₃ for 7 d in either fresh water (FW) or seawater (SW), and their mortality, heart rate, morphology, and ionocytes were examined. Results showed that the toxicity of AgNO₃ was higher in FW than in SW (50% lethal concentrations (LC₅₀) were 0.17 vs. 1.01 ppm). Although AgNO₃ (0.05 and 0.1 ppm) did not significantly change the morphology of embryos, it impaired ionocytes and elevated heart rates in FW. While, AgNO₃ (0.1 and 0.5 ppm) did not affect the morphology, ionocytes, or heart rate in SW, it impaired the hypo-osmoregulatory capability and elevated the mortality of embryos that were transferred from FW to SW. At 12 h after SW transfer, ionocytes were severely impaired, and water-drinking behavior was suppressed, resulting in body dehydration and sodium overload. In contrast, AgNO₃ did not elevate the mortality of embryos that were transferred from SW to FW. To sum up, the presence of silver in FW during the developmental stage of euryhaline fish could potentially endanger their survival during SW adaptation.

Effects of Single and Combined Ciprofloxacin and Lead Treatments on Zebrafish Behavior, Oxidative Stress, and Elements Content

Even though the toxic effects of antibiotics and heavy metals have been extensively studied in the last decades, their combined adverse impact on aquatic organisms is poorly understood. Therefore, the objective of this study was to assess the acute effects of a ciprofloxacin (Cipro) and lead (Pb) mixture on the 3D swimming behavior, acetylcholinesterase (AChE) activity, lipid peroxidation level (MDA-malondialdehyde), activity of some oxidative stress markers (SOD-superoxide dismutase and GPx-glutathione peroxidase), and the essential elements content (Cu-copper, Zn-zinc, Fe-iron, Ca-calcium, Mg-magnesium, Na-sodium and K-potassium) in the body of zebrafish (Danio rerio). For this purpose, zebrafish were exposed to environmentally relevant concentrations of Cipro, Pb, and a mixture for 96 h. The results revealed that acute exposure to Pb alone and in mixture with Cipro impaired zebrafish exploratory behavior by decreasing swimming activity and elevating freezing duration. Moreover, significant deficiencies of Ca, K, Mg, and Na contents, as well as an excess of Zn level, were observed in fish tissues after exposure to the binary mixture. Likewise, the combined treatment with Pb and Cipro inhibited the activity of AChE and increased the GPx activity and MDA level. The mixture produced more damage in all studied endpoints, while Cipro had no significant effect. The findings highlight that the simultaneous presence of antibiotics and heavy metals in the environment can pose a threat to the health of living organisms.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Acute exposure to polystyrene nanoplastics impairs skin cells and ion regulation in zebrafish embryos

The presence of nanoplastics in aquatic environments is a global problem. Accumulating evidence shows that nanoplastics can accumulate in fish and influence internal organs. However, it is still unknown if nanoplastics can impair skin cells (keratinocytes and ionocytes), which play critical roles in maintaining body fluid homeostasis. In the present study, zebrafish embryos were exposed to polystyrene nanoplastics (PS-NPs; 25 nm in size, at 0, 10, 25, and 50 mg/L) for 96 h to test the effects of PS-NPs on skin functions. After exposure to 50 mg/L, the survival rate, ion (Na⁺, K⁺, and Ca²⁺) contents, and acid/ammonia excretion by skin cells of embryos significantly declined. The apical structure of skin keratinocytes was damaged at 10, 25, and 50 mg/L. The number and mitochondrial activity of ionocytes were reduced at 25 and 50 mg/L. Reactive oxygen species (ROS) levels indicated by CellROX staining showed that both ionocytes and keratinocytes were under oxidative stress. PS-NPs reduced the mRNA expression of antioxidant genes (sod1, sod2, cat, and gpx1a), and promoted apoptosis-related genes (casp3a). Taken together, this study suggests that PS-NPs might suppress antioxidative reactions and induce oxidative stress, leading to mitochondrial damage and cell death of ionocytes, eventually impairing skin functions including ion uptake, pH regulation, and ammonia excretion.

A mesenchymal to epithelial switch in Fgf10 expression specifies an evolutionary-conserved population of ionocytes in salivary glands

Fibroblast growth factor 10 (FGF10) is well established as a mesenchyme-derived growth factor and a critical regulator of fetal organ development in mice and humans. Using a single-cell RNA sequencing (RNA-seq) atlas of salivary gland (SG) and a tamoxifen inducible Fgf10^CreERT2:R26-tdTomato mouse, we show that FGF10^pos cells are exclusively mesenchymal until postnatal day 5 (P5) but, after P7, there is a switch in expression and only epithelial FGF10^pos cells are observed after P15. Further RNA-seq analysis of sorted mesenchymal and epithelial FGF10^pos cells shows that the epithelial FGF10^pos population express the hallmarks of ancient ionocyte signature Forkhead box i1 and 2 (Foxi1, Foxi2), Achaete-scute homolog 3 (Ascl3), and the cystic fibrosis transmembrane conductance regulator (Cftr). We propose that epithelial FGF10^pos cells are specialized SG ionocytes located in ducts and important for the ionic modification of saliva. In addition, they maintain FGF10-dependent gland homeostasis via communication with FGFR2b^pos ductal and myoepithelial cells.

Mauduit et al. identified unique FGF10-expressing ionocytes in salivary glands. FGF10 expression shifts from fibroblasts to epithelial ionocytes during postnatal development. Ionocytes play a dual role in salivary gland homeostasis; they maintain specific ion composition in saliva and act as niche cells, providing growth factor support for other epithelial cells.

Expression of ion transport proteins and routine metabolism in juveniles of tropical gar (Atractosteus tropicus) exposed to ammonia

Tropical gar (Atractosteus tropicus) thrives in aquatic habitats with high levels of total nitrogen (TAN) and unionized ammonia (NH₃). However, the tolerance of TAN and NH₃, the excretion mechanisms involved, and the effects of these chemicals on routine metabolism are still unknown. Therefore, our objectives were to assess the acute toxicity of TAN and NH₃ in A. tropicus juveniles after a 96-h exposure (LC50-96 h) to NH₄Cl and after chronic exposure to two concentrations (15% and 30% of LC50-96 h TAN) for 12 days, as well as to evaluate the transcriptional effects associated with Rhesus proteins (rhag, rhbg, rhcg) and ion transporters (NHE, NKA, NKCC, and CFTR) in gills and skin; and to determine the effects of TAN and NH3 on routine metabolism through oxygen consumption (μM g^-1 h^-1) and gill ventilation frequency (beats min^-1). LC50-96 h values were 100.20 ± 11.21 mg/L for TAN and 3.756 ± 0.259 mg/L for NH₃. The genes encoding Rhesus proteins and ion transporters in gills and skin showed a differential expression according to TAN concentrations and exposure time. Oxygen consumption on day 12 showed significant differences between treatments with 15% and 30% TAN. Gill ventilation frequency on day 12 was higher in fish exposed to 30% TAN. In conclusion, A. tropicus juveniles are highly tolerant to TAN, showing upregulation of the genes involved in TAN excretion through gills and skin, which affects routine oxygen consumption and energetic cost. These findings are relevant for understanding adaptations in the physiological response of a tropical ancestral air-breathing fish.

A Systematic Analysis of Metal and Metalloid Concentrations in Eight Zebrafish Recirculating Water Systems

Metals and metalloids are integral to biological processes and play key roles in physiology and metabolism. Nonetheless, overexposure to some metals or lack of others can lead to serious health consequences. In this study, eight zebrafish facilities collaborated to generate a multielement analysis of their centralized recirculating water systems. We report a first set of average concentrations for 46 elements detected in zebrafish facilities. Our results help to establish an initial baseline for trouble-shooting purposes, and in general for safe ranges of metal concentrations in recirculating water systems, supporting reproducible scientific research outcomes with zebrafish.

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.

Effects of Dissolved Potassium on Growth Performance, Body Composition, and Welfare of Juvenile African Catfish (Clarias gariepinus)

Optimal crop production in aquaponics is influenced by water pH and potassium concentrations. The addition of potassium hydroxide (KOH) into the recirculating aquaculture system (RAS) may benefit aquaponics by increasing the water pH for better biofilter activity and supplementing K for better plant growth and quality. We investigated the growth, feed conversion, body composition and welfare indicators of juvenile African catfish (Clarias gariepinus) treated with four concentrations of K (K0 = 2, K200 = 218, K400 = 418, and K600 = 671 mg L−1). While growth, feed conversion and final body composition were unaffected, the feeding time and individual resting significantly increased with increasing K+. The swimming activity and agonistic behavior were reduced significantly under increased concentrations of K+. Leftover feed and the highest number of skin lesions were observed under K600. We suggest that K+ concentrations between 200 and 400 mg L−1 can improve the welfare status of juvenile African catfish. This enables the application of KOH in RAS to supply alkalinity to achieve optimum nitrification at minimum water exchange and improve the nutritional profile of the process water with benefits for the welfare status of African catfish and aquaponics plant production and quality.

Vincristine exposure impairs skin keratinocytes, ionocytes, and lateral-line hair cells in developing zebrafish embryos

Environmental contamination by anticancer pharmaceuticals has been widely reported. These drugs are not readily biodegradable, and their parent compounds and/or metabolites have been detected in surface waters and groundwater throughout the world. Adverse effects of anticancer drugs occur frequently in cancer patients, and a large body of clinical knowledge has accumulated. However, the effects of these drugs on aquatic organisms have not been thoroughly studied. This study aimed to investigate the effects of acute exposure to a common anticancer drug, vincristine (VCR), on zebrafish embryonic development and skin function. After 96 h of VCR exposure (0, 1, 10, 15, and 25 mg/L), significant teratogenic effects were observed, including growth retardation, pericardial edema, spine, tail, and yolk sac malformations (VCR ≥ 15 mg/L), a decreased heart rate, and ocular malformations (VCR ≥ 10 mg/L). The value of the half lethal concentration for zebrafish embryos was 20.6 mg/L. At ≥10 mg/L VCR, systemic ion contents and acid secretion in the skin over the yolk-sac decreased, and these findings were associated with decreases in skin ionocytes (H⁺-ATPase-rich cells and Na⁺-K⁺-ATPase-rich cells). Also, the microridge-structure of skin keratinocytes was significantly damaged. The number of lateral line hair cells was reduced when VCR was ≥10 mg/L, and functional impairment was detected when VCR was as low as 1 mg/L. Results of this in vivo study in zebrafish embryos indicate that acute exposure to VCR can lead to developmental defects, impairment of skin functions, and even fish death.

Exposure to copper nanoparticles impairs ion uptake, and acid and ammonia excretion by ionocytes in zebrafish embryos

The potential toxicity of copper nanoparticles (CuNPs) to early stages of fishes is not fully understood, and little is known about their effects on ionocytes and associated functions. This study used zebrafish embryos as a model to investigate the toxic effects of CuNPs on two subtypes of ionocytes. Zebrafish embryos were exposed to 0.1, 1, and 3 mg L^-1 CuNPs for 96 h. After exposure, whole-body Na⁺ and Ca²⁺ contents were significantly reduced at ≥0.1 mg L^-1, while the K⁺ content had decreased at ≥1 mg L^-1. H⁺ and NH₄⁺ excretion by the skin significantly decreased at ≥1 mg L^-1. The number of living ionocytes labeled with rhodamine-123 had significantly decreased with ≥0.1 mg L^-1 CuNPs. The ionocyte subtypes of H⁺-ATPase-rich (HR) and Na⁺/K⁺-ATPase-rich (NaR) cells were labeled by immunostaining and had decreased with ≥1 mg L^-1. Shrinkage of the apical opening of ionocytes was revealed by scanning electronic microscopy. Functional impairment was also reflected by changes in gene expressions, including ion transporters/channels and Ca²⁺-regulatory hormones. This study shows that CuNP exposure can impair two subtypes of ionocytes and their associated functions, including Na⁺/Ca²⁺ uptake and H⁺/NH₄⁺ excretion in zebrafish embryos.

Ammonia exposure impairs lateral-line hair cells and mechanotransduction in zebrafish embryos

Ammonia (including NH₃ and NH₄⁺) is a major pollutant of freshwater environments. However, the toxic effects of ammonia on the early stages of fish are not fully understood, and little is known about the effects on the sensory system. In this study, we hypothesized that ammonia exposure can cause adverse effects on embryonic development and impair the lateral line system of fish. Zebrafish embryos were exposed to high-ammonia water (10, 15, 20, 25, and 30 mM NH₄Cl; pH 7.0) for 96 h (0-96 h post-fertilization). The body length, heart rate, and otic vesicle size had significantly decreased with ≥15 mM NH₄Cl, while the number and function of lateral-line hair cells had decreased with ≥10 mM NH₄Cl. The mechanoelectrical transduction (MET) channel-mediated Ca²⁺ influx was measured with a scanning ion-selective microelectrode technique to reveal the function of hair cells. We found that NH₄⁺ (≥5 mM NH₄Cl) entered hair cells and suppressed the Ca²⁺ influx of hair cells. Neomycin and La³⁺ (MET channel blockers) suppressed NH₄⁺ influx, suggesting that NH₄⁺ enters hair cells via MET channels in hair bundles. In conclusion, this study showed that ammonia exposure (≥10 mM NH₄Cl) can cause adverse effects in zebrafish embryos, and lateral-line hair cells are sensitive to ammonia exposure.

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

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

Silver nanoparticle exposure impairs ion regulation in zebrafish embryos

The potential toxicity of silver nanoparticles (AgNPs) to the early stages of fish is still unclear. This study used zebrafish embryos as a model to investigate the toxic effects of AgNPs on ion regulation by skin ionocytes. Zebrafish embryos were exposed to AgNPs for 96 h (4-100 h post-fertilization (hpf)) or 4 h (96-100 hpf). After 96 h of exposure to 5 and 10 mg/L AgNPs, survival rates had decreased to 42% and 0%, respectively; the body length had also significantly decreased at 5 mg/L. Whole-body Na⁺ and K⁺ contents significantly decreased at 1 and 3 mg/L, while Ca²⁺ contents decreased at ≥0.1 mg/L. H⁺ secretion by the skin significantly decreased at 1 mg/L. The density of skin ionocytes labeled with rhodamine 123 (a mitochondrion marker) decreased by 25% and 55% at 1 and 3 mg/L, respectively; and 54% of ionocytes (at 3 mg/L) were deformed from an oval to a spinous shape. After 4 h of exposure to 1 and 5 mg/L, whole-body Na⁺ and Ca²⁺ contents, H⁺ secretion, and density of ionocytes had also significantly decreased. This study revealed the toxicity of AgNPs to skin ionocytes and ion regulation in the early stages of zebrafish embryos.

Novel discoveries in acid-base regulation and osmoregulation: A review of selected hormonal actions in zebrafish and medaka

Maintenance of internal ionic and acid-base homeostasis is critical for survival in all biological systems. Similar to mammals, aquatic fishes have developed sophisticated homeostatic mechanisms to mitigate metabolic or environmental disruptions in ionic and acid-base status of systemic body fluids via hormone-controlled transport of ions or acid equivalents. The present review summarizes newly discovered actions of several hormones in zebrafish (Danio rerio) and medaka (Oryzias latipes) that have greatly contributed to our overall understanding of ionic/acid-base regulation. For example, isotocin and cortisol were reported to enhance transport of various ions by stimulating the proliferation and/or differentiation of ionocyte progenitors. Meanwhile, stanniocalcin-1, a well-documented hypocalcemic hormone, was found to suppress ionocyte differentiation and thus downregulate secretion of H⁺ and uptake of Na⁺ and Cl^-. Estrogen-related receptor and calcitonin gene-related peptide also regulate the differentiation of certain types of ionocytes to either stimulate or suppress H⁺ secretion and Cl^- uptake. On the other hand, endothelin and insulin-like growth factor 1 activate the respective secretion of H⁺ and Na⁺/Cl through fast actions. These new findings enhance our understanding of how hormones regulate fish ionic and acid-base regulation while further providing new insights into vertebrate evolution, mammalian endocrinology and human disease-related therapeutics.

Cisplatin exposure impairs ionocytes and hair cells in the skin of zebrafish embryos

This study aimed to assess the sublethal effects of a platinum-based compound, cisplatin, using a zebrafish model. Zebrafish embryos were incubated in different concentrations of cisplatin at 0-96 h post-fertilization. Using a non-invasive, scanning ion-selective electrode technique (SIET), we measured the functions of hair cells (Ca²⁺ influx) and ionocytes ([H⁺] gradients). The survival rate, hatching rate, phenotype, body length, whole-body ion (Na⁺, Cl^-, and Ca²⁺) and Pt contents were also determined. The effects of cisplatin on zebrafish embryos were demonstrated as first impairing hair cell function (at 1 μM of cisplatin), the hair cell number, and body ion content of Cl^- (at 10 μM of cisplatin), then decreasing ionocyte acid secretion and overall body ion contents of Na⁺ and Ca²⁺ (at 50 μM of cisplatin). The body length and ionocyte density decreased at 100 μM of cisplatin, and survival decreased at 500 μM of cisplatin. As the cisplatin concentration increased, the accumulation of Pt in fish embryos also increased. These results revealed that hair cells are significantly more susceptible to cisplatin toxicity than ionocytes. By determining the lowest observed effective concentration of cisplatin that caused in vivo functional alterations of zebrafish hair cells and skin ionocytes, this model demonstrated 500-fold greater sensitivity than by detecting changes in survival, for early assessment of the effects of platinum-based chemotherapeutic drugs on fish.