Cellular mechanisms of Cl- transport in trout gill mitochondrion-rich cells

Exposure to pyrogallol impacts the hemato-biochemical endpoints in catfish (Clarias gariepinus)

Pyrogallol is widely used in several industrial applications and can subsequently contaminate aquatic ecosystems. Here, we report for the first time the presence of pyrogallol in wastewater in Egypt. Currently, there is a complete lack of toxicity and carcinogenicity data for pyrogallol exposure in fish. To address this gap, both acute and sub-acute toxicity experiments were conducted to determine the toxicity of pyrogallol in catfish (Clarias gariepinus). Behavioral and morphological endpoints were evaluated, in addition to blood hematological endpoints, biochemical indices, electrolyte balance, and the erythron profile (poikilocytosis and nuclear abnormalities). In the acute toxicity assay, it was determined that the 96 h median-lethal concentration (96 h-LC50) of pyrogallol for catfish was 40 mg/L. In sub-acute toxicity experiment, fish divided into four groups; Group 1 was the control group. Group 2 was exposed to 1 mg/L of pyrogallol, Group 3 was exposed to 5 mg/L of pyrogallol, and Group 4 was exposed to 10 mg/L of pyrogallol. Fish showed morphological changes such as erosion of the dorsal and caudal fins, skin ulcers, and discoloration following exposure to pyrogallol for 96 h. Exposure to 1, 5, or 10 mg/L pyrogallol caused a significant decrease in hematological indices, including red blood cells (RBCs), hemoglobin, hematocrit, white blood cells (WBC), thrombocytes, and large and small lymphocytes in a dose-dependent manner. Several biochemical parameters (creatinine, uric acid, liver enzymes, lactate dehydrogenase, and glucose) were altered in a concentration dependent manner with short term exposures to pyrogallol. Pyrogallol exposure also caused a significant concentration-dependent rise in the percentage of poikilocytosis and nuclear abnormalities of RBCs in catfish. In conclusion, our data suggest that pyrogallol should be considered further in environmental risk assessments of aquatic species.

Disturbance in Na+ regulation in cells rich in mitochondria isolated from gills of the yellow clam Mesodesma mactroides exposed to copper under different osmotic conditions

Cells rich in mitochondria were isolated from gills of the seawater clam Mesodesma mactroides, incubated in isosmotic saline solution (840 mOsmol/kg H₂O), and exposed (3 h) to environmentally realistic Cu concentrations (nominally: 0, 5, 9 and 20 μg/L). In cells exposed to 20 μg Cu/L, Cu accumulation, Na⁺ content reduction and carbonic anhydrase (CA) activity inhibition were observed, without significant changes in cell viability and Na⁺,K⁺-ATPase (NKA) activity. In the absence of Cu, cell viability and Cu content were reduced in hyposmotic media respect with the control, without changes in Na⁺ content and enzyme (CA and NKA) activities. In the presence of 5 or 9 μg/L Cu, cell Cu content was increased, especially at 670 mOsmol/kg H₂O. Cell Na⁺ content and NKA activity were reduced after exposure to 20 μg/L Cu at 670 mOsmol/kg H₂O. In turn, CA activity was dependent on Cu concentration, being significantly reduced in cells exposed to 9 and 20 μg/L Cu in both hyposmotic conditions. These findings indicate that Cu also negatively affects Na⁺ regulation in gill cells of the seawater clam M. mactroides, with Cu toxicity increasing at hyposmotic conditions. Also, they indicate that physiology is more important than water chemistry in predicting Cu toxicity in environments of changing salinity, pointing out CA activity as a potential biomarker of Cu exposure.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

An in vitro investigation of gastrointestinal Na(+) uptake mechanisms in freshwater rainbow trout

In vitro gut-sac preparations of all four sections (stomach, anterior, mid, and posterior intestine) of the gastrointestinal tract (GIT) of freshwater rainbow trout, together with radiotracer ((22)Na) techniques, were used to study unidirectional Na(+) uptake rates (UR, mucosal → blood space) and net absorptive fluid transport rates (FTR) under isosmotic conditions (mucosal = serosal osmolality). On an area-specific basis, unidirectional Na(+) UR was highest in the mid-intestine, but when total gut area was taken into account, the three intestinal sections contributed equally, with very low rates in the stomach. The theoretical capacity for Na(+) uptake across the whole GIT is sufficient to supply all of the animal's nutritive requirements for Na(+). Transport occurs by low affinity systems with apparent K m values 2-3 orders of magnitude higher than those in the gills, in accord with comparably higher Na(+) concentrations in chyme versus fresh water. Fluid transport appeared to be Na(+)-dependent, such that treatments which altered unidirectional Na(+) UR generally altered FTR in a comparable fashion. Pharmacological trials (amiloride, EIPA, phenamil, bafilomycin, furosemide, hydrochlorothiazide) conducted at a mucosal Na(+) concentration of 50 mmol L(-1) indicated that GIT Na(+) uptake occurs by a variety of apical mechanisms (NHE, Na(+) channel/H(+) ATPase, NCC, NKCC) with relative contributions varying among sections. However, at a mucosal Na(+) concentration of 10 mmol L(-1), EIPA, phenamil, bafilomycin, and hydrochlorothiazide were no longer effective in inhibiting unidirectional Na(+) UR or FTR, suggesting the contribution of unidentified mechanisms under low Na(+) conditions. A preliminary model is presented.

Marine, freshwater and aerially acclimated mangrove rivulus (Kryptolebias marmoratus) use different strategies for cutaneous ammonia excretion

Rhesus (Rh) glycoproteins are ammonia gas (NH(3)) channels known to be involved in ammonia transport in animals. Because of the different osmoregulatory and ionoregulatory challenges faced by teleost fishes in marine and freshwater (FW) environments, we hypothesized that ammonia excretion strategies would differ between environments. Also, we hypothesized that cutaneous NH(3) volatilization in air-acclimated fish is facilitated by base secretion. To test these hypotheses, we used the skin of the euryhaline amphibious mangrove rivulus (Kryptolebias marmoratus). The skin excretes ammonia and expresses Rh glycoproteins. Serosal-to-mucosal cutaneous ammonia flux was saturable (0-16 mmol/l ammonia, K(m) of 6.42 mmol/l). In FW, ammonia excretion increased in response to low mucosal pH but decreased with pharmacological inhibition of Na(+)/H(+) exchangers (NHE) and H(+) ATPase. Conversely, in brackish water (BW), lowering the mucosal pH significantly decreased ammonia excretion. Inhibitors of NHE also decreased ammonia excretion in BW fish. Immunofluorescence microscopy demonstrated that both the Rh isoform, Rhcg1, and NHE3 proteins colocalized in Na(+)/K(+) ATPase expressing mitochondrion-rich cells in the gills, kidney, and skin. We propose that the mechanisms of cutaneous ammonia excretion in FW K. marmoratus are consistent with the model for branchial ammonia excretion in FW teleost fish. NH(4)(+) excretion appeared to play a stronger role in BW. NH(4)(+) excretion in BW may be facilitated by apical NHE and/or diffuse through paracellular pathways. In aerially acclimated fish, inhibition of NHE and H(+) ATPase, but not the Cl(-)/HCO(3)(-) exchanger, significantly affected cutaneous surface pH, suggesting that direct base excretion is not critical for NH(3) volatilization. Overall, K. marmoratus use different strategies for excreting ammonia in three different environments, FW, BW, and air, and Rh glycoproteins and NHE are integral to all.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na(+) uptake/acid secretion/NH(4)(+) excretion, Ca(2+) uptake, and Cl(-) uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.

Mechanism of sodium uptake in PNA negative MR cells from rainbow trout, Oncorhynchus mykiss as revealed by silver and copper inhibition

The rate of acid-stimulated and phenamil-sensitive sodium (Na(+)) uptake was measured in three different cell lineages: pavement cells (PVC), total mitochondrion-rich (MR) cell populations, and peanut lectin agglutinin-negative mitochondrion-rich cells (PNA(-) MR) isolated from the rainbow trout gill epithelium. Despite the presence of basal levels of Na(+) uptake in PVC, this transport was not enhanced by acidification, nor was it inhibited by independent treatment with bafilomycin (i.e., a V-type H(+)-ATPase inhibitor), phenamil (i.e., a specific inhibitor of ENaC), or Ag (a specific inhibitor of active Na(+) transport in fish). In contrast, Na(+) uptake in PNA(-) MR cells was increased by ~220% above basal levels following acidification of near 0.4 pH units in the presence of 1.0 mM external Na(+). Acid-stimulated Na(+) transport was entirely inhibited by both phenamil and bafilomycin. Silver (Ag) and copper (Cu), which are known to interfere with active Na(+) transport in fish, were also responsible for inhibiting acid stimulated Na(+) uptake in PNA(-) MR cells, but by themselves had no effect on basal Na(+) transport. Thus, we demonstrate that Ag specifically prevented acid-stimulated Na(+) uptake in PNA(-) MR cells in a dose-dependent manner. We also demonstrate rapid (<1 min) and significant inhibition of carbonic anhydrase (CA) by Ag in PNA(-) MR cells, but not in PVC. These data lend further support to the idea of a PNA(-) MR cell type as the primary site for Na(+) uptake in the freshwater (FW) gill phenotype of rainbow trout. Moreover, these findings provide support for the importance of intracellular protons in regulating the movement of Na(+) across the apical surface of the fish gill.

Evidence that SLC26 anion transporters mediate branchial chloride uptake in adult zebrafish (Danio rerio)

Experiments were performed to test the hypothesis that three members of the SLC26 anion transporter gene family (SLC26a3, A4, and A6; hereafter termed za3, za4, and za6) mediate branchial Cl(-)/HCO(3)(-) exchange in adult zebrafish (Danio rerio). Real-time RT-PCR demonstrated that the gill expressed relatively high levels of za6 mRNA; za3 and za4 mRNA, while present, were less abundant. Also, za4 and za6 were expressed at relatively high levels in the kidney. The results of in situ hybridization or immunocytochemistry (za3 only) experiments performed on gill sections revealed that the SLC26 transporters were predominantly expressed on the filament epithelium (especially within the interlamellar regions) and to a lesser extent on the lamellar epithelium at the base of lamellae. This distribution pattern suggests that the SLC26 anion transporters are localized to mitochondrion-rich cells (ionocytes). Transferring fish to water containing low [Cl(-)] (0.02 mmol/l) resulted in significant increases in branchial SLC26 mRNA expression after 5-10 days of exposure relative to fish raised in normal water [Cl(-)] (0.4 mmol/l); transferring fish to Cl(-)-enriched water (2.0 mmol/l) was without effect on mRNA levels. Transferring fish to water containing elevated levels of NaHCO(3) (10-12.5 mmol/l) caused marked increases in branchial SLC26 mRNA expression between 3 and 10 days of transfer that was associated with a significant 40% increase in Cl(-) uptake (as measured upon return to normal water after 7 days). A decrease in whole body net acid excretion (equivalent to an increase in net base excretion) in fish previously maintained in high [NaHCO(3)] water, concurrent with increases in Cl(-) uptake and SLC26 mRNA levels, suggests a role for these anion transporters in Cl(-) uptake and acid-base regulation owing to their Cl(-)/HCO(3)(-) exchange activities.