Characterization of developmental Na(+) uptake in rainbow trout larvae supports a significant role for Nhe3b

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Rainbow Trout (Oncorhynchus mykiss) Na+/H+ Exchangers tNhe3a and tNhe3b Display Unique Inhibitory Profiles Dissimilar from Mammalian NHE Isoforms

Freshwater fishes maintain an internal osmolality of ~300 mOsm, while living in dilute environments ranging from 0 to 50 mOsm. This osmotic challenge is met at least partially, by Na⁺/H⁺ exchangers (NHE) of fish gill and kidney. In this study, we cloned, expressed, and pharmacologically characterized fish-specific Nhes of the commercially important species Oncorhynchus mykiss. Trout (t) Nhe3a and Nhe3b isoforms from gill and kidney were expressed and characterized in an NHE-deficient cell line. Western blotting and immunocytochemistry confirmed stable expression of the tagged trout tNhe proteins. To measure NHE activity, a transient acid load was induced in trout tNhe expressing cells and intracellular pH was measured. Both isoforms demonstrated significant activity and recovered from an acute acid load. The effect of the NHE transport inhibitors amiloride, EIPA (5-(N-ethyl-N-isopropyl)-amiloride), phenamil, and DAPI was examined. tNhe3a was inhibited in a dose-dependent manner by amiloride and EIPA and tNhe3a was more sensitive to amiloride than EIPA, unlike mammalian NHE1. tNhe3b was inhibited by high concentrations of amiloride, while even in the presence of high concentrations of EIPA (500 µM), some activity of tNhe3b remained. Phenamil and DAPI were ineffective at inhibiting tNhe activity of either isoform. The current study aids in understanding the pharmacology of fish ion transporters. Both isoforms display inhibitory profiles uniquely different from mammalian NHEs and show resistance to inhibition. Our study allows for more direct interpretation of past, present, and future fish-specific sodium transport studies, with less reliance on mammalian NHE data for interpretation.

Enhanced osmoregulatory ability marks the smoltification period in developing chum salmon (Oncorhynchus keta)

The freshwater (FW) life of chum salmon is short, as they migrate to the ocean soon after emergence from the substrate gravel of natal waters. The alevins achieve seawater (SW) acclimating ability at an early developmental stage and the details of smoltification are not clear. We examined the stage-dependent SW acclimating ability in chum salmon alevins and found a sharp increase in SW tolerance during development that resembles the physiological parr-smolt transformation seen in other salmonids. Perturbation of plasma Na⁺ after SW exposure was prominent from the hatched embryo stage to emerged alevins, but the plasma Na⁺ became highly stable and more resistant to perturbation soon after complete absorption of yolk. Marker gene expression for SW-ionocytes including Na/K-ATPase (NKA α1b), Na-K-Cl cotransporter 1a (NKCC1a), Na/H exchanger 3a (NHE3a), cystic fibrosis transmembrane conductance regulators (CFTR I and CFTR II) were all upregulated profoundly at the same stage when the alevins were challenged by SW, suggesting that the stability of plasma Na⁺ concentration was partly a result of elevated osmoregulatory capability. FW-ionocyte markers including NKA α1a and NHE3b were consistently downregulated independent of stage by SW exposure, suggesting that embryos at all stages respond to salinity challenge, but the increase in SW osmoregulatory capability is restricted to the developmental stage after emergence. We propose that the "smoltification period" is condensed and integrated into the early development of chum salmon, and our results can be extrapolated to the future studies on hormonal controls and developmental triggers for smoltification in salmonids.

Pharmacological evidence that DAPI inhibits NHE2 in Fundulus heteroclitus acclimated to freshwater

Ionoregulation in the euryhaline killifish Fundulus heteroclitus has been intensively studied over the last two decades using a variety of techniques. However, there has been limited use of pharmacological inhibitors to identify proteins involved in ion transport for this species. In this study, we used a range of pharmacological inhibitors (EIPA, DAPI, ethoxzolamide, bumetanide, bafilomycin, phenamil, hydrochlorothiazide) to investigate the proteins involved in Na⁺ transport in freshwater (1 mM Na⁺) acclimated F. heteroclitus. Our results indicate that Na⁺ uptake under these conditions is sensitive to both EIPA (NHE-specific inhibitor) and DAPI (putative ASIC-specific inhibitor), but not to any of the other inhibitors. Results for EIPA are consistent with previous studies indicating F. heteroclitus relies solely on NHE2 for Na⁺ transport across the apical membrane of ionocytes. In contrast, results for DAPI are surprising given previous studies that have indicated the H⁺-ATPase is basolaterally located in F. heteroclitus and so cannot contribute to Na⁺ uptake via ASIC. The lack of bafilomycin sensitivity in the current study is consistent with a basolaterally located H⁺-ATPase. This suggests that DAPI is not an ASIC-specific inhibitor as has been previously assumed, and that it may also inhibit NHE2. Finally, we did not observe Na⁺ uptake to be sensitive to ethoxzolamide, suggesting that carbonic anhydrase may not be involved in generating the H⁺ needed to maintain NHE activity in freshwater as has been previously proposed.

Assessing the role of the acid-sensing ion channel ASIC4b in sodium uptake by larval zebrafish

Na⁺ uptake in larval zebrafish (Danio rerio) is coordinated by three mechanisms: Na⁺/H⁺-exchanger 3b (NHE3b) expressed in H⁺-ATPase-rich (HR) cells, an unidentified Na⁺ channel coupled to electrogenic H⁺-ATPase expressed in HR cells, and Na⁺-Cl^--cotransporter (NCC) expressed in NCC cells. Recently, acid-sensing ion channels (ASICs) were proposed to be the putative Na⁺ channel involved in H⁺-ATPase-mediated Na⁺ uptake in adult zebrafish and rainbow trout. In the present study, we hypothesized that ASICs also play this role in Na⁺ uptake in larval zebrafish. In support of this hypothesis, immunohistochemical analyses revealed that ASIC4b was expressed in HR cells on the yolk sac skin at 4 days post-fertilization (dpf). However, neither treatment with the ASIC-specific blocker 4,6-diamidino-2-phenylindole (DAPI) nor morpholino knockdown of ASIC4b reduced Na⁺ uptake in circumneutral conditions at 4 dpf. However, because ASIC4b knockdown led to significant increases in the mRNA expression of nhe3b and ncc and a significant increase in HR cell density, it is possible that Na⁺ influx was sustained by increased participation of non-ASIC4b pathways. Moreover, when fish were reared in acidic water (pH = 4), ASIC4b knockdown led to a stimulation of Na⁺ uptake at 3 and 4 dpf, results which also were inconsistent with an essential role for ASIC-mediated Na⁺ uptake, even under conditions known to constrain Na⁺ uptake via NHE3b. Thus, while ASIC4b clearly is expressed in HR cells, the current functional experiments cannot confirm its involvement in Na⁺ uptake in larval zebrafish.

Ammonia-independent sodium uptake mediated by Na+ channels and NHEs in the freshwater ribbon leech Nephelopsis obscura

Freshwater organisms actively take up ions from their environment to counter diffusive ion losses due to inhabiting hypo-osmotic environments. The mechanisms behind active Na⁺ uptake are quite well understood in freshwater teleosts; however, the mechanisms employed by invertebrates are not. Pharmacological and molecular approaches were used to investigate Na⁺ uptake mechanisms and their link to ammonia excretion in the ribbon leech Nephelopsis obscura At the molecular level, we identified a Na⁺ channel and a Na⁺/H⁺ exchanger (NHE) in the skin of N. obscura, where the NHE was up-regulated when acclimated to extremely low [Na⁺] (0.05 mmol l^-1, pH 5) conditions. Additionally, we found that leeches in dilute freshwater environments use both a vacuolar-type H⁺-ATPase (VHA)-assisted uptake via a Na⁺ channel and a NHE-based mechanisms for Na⁺ uptake. Immunolocalization of VHA and Na⁺/K⁺-ATPase (NKA) indicated at least two cell types present within leech skin, VHA⁺ and VHA^- cells, where the VHA⁺ cells are probably involved in Na⁺ uptake. NKA was present throughout the epithelium. We also found that increasing ammonia excretion by decreasing water pH, ammonia loading leeches or exposing leeches to high environmental ammonia does not affect Na⁺ uptake, providing indications that an NHE-Rh metabolon is not present and that ammonia excretion and Na⁺ uptake are not coupled in N. obscura To our knowledge, this is the first study showing the mechanisms of Na⁺ uptake and their links to ammonia excretion in a freshwater invertebrate, where results suggest an ammonia-independent Na⁺ uptake mechanism relying on both Na⁺ channels and NHEs.

Different mechanisms of Na+ uptake and ammonia excretion by the gill and yolk sac epithelium of early life stage rainbow trout

In rainbow trout, the dominant site of Na+ uptake (JNain) and ammonia excretion (Jamm) shifts from the skin to the gills over development. Post-hatch (PH; 7 days post-hatch) larvae utilize the yolk sac skin for physiological exchange, whereas by complete yolk sac absorption (CYA; 30 days post-hatch), the gill is the dominant site. At the gills, JNain and Jamm occur via loose Na+/NH4+ exchange, but this exchange has not been examined in the skin of larval trout. Based on previous work, we hypothesized that, contrary to the gill model, JNain by the yolk sac skin of PH trout occurs independently of Jamm. Following a 12-h exposure to high environmental ammonia (HEA; 0.5 mmol l−1 NH4HCO3; [Na+]=600 µmol l−1; pH=8), Jamm by the gills of CYA trout and the yolk sac skin of PH larvae, which were isolated using divided chambers, increased significantly. However, this was coupled to an increase in JNain across the gills only, supporting our hypothesis. Moreover, gene expression of proteins involved in JNain (Na+/H+-exchanger-2 (NHE2) and H+-ATPase) increased in response to HEA only in the CYA gills. We further identified expression of the apical Rhesus (Rh) proteins Rhcg2 in putative pavement cells and Rhcg1 (co-localized with apical NHE2 and NHE3b and Na+/K+-ATPase) in putative peanut lectin agglutinin-positive (PNA+) ionocytes in gill sections. Similar Na+/K+-ATPase-positive cells expressing Rhcg1 and NHE3b, but not NHE2, were identified in the yolk sac epithelium. Overall, our findings suggest that the mechanisms of JNain and Jamm by the dominant exchange epithelium at two distinct stages of early development are fundamentally different.