9 Proton Pumps in Fish Gills

Effect of sublethal concentrations of glyphosate-based herbicides (Roundup Active®) on skin of the tropical frog (Dendropsophus molitor)

In Colombia, glyphosate (GP) is used to control weeds, with Roundup Active® being the most widely used. This use has affected aquatic ecosystems, causing malformations in amphibians. The Savannah frog (D. molitor) is a tropical frog inhabiting the mountain of Colombia. In the present study, we determined the effect of sublethal concentrations of GP (Roundup Active®) on the skin of D. molitor. Twenty-four tadpoles were exposed to concentrations of GP (T1: 0, T2: 1.4, T3: 3.6, and T4: 5.6 a.e mg/L) during 31 days. In 10 individuals per treatment, two skin regions were evaluated: dorsal cranial and caudal ventral to determine histopathological alterations. Morphometric analysis of the layers of the skin was performed: epidermis, dermis, and hypodermis-muscular. T1 did not present histopathological alterations. Since T2 was identified, glandular cell hyperplasia and hypertrophy increased melanophores and melanin accumulations in the highest concentrations of GP. The ultrastructure revealed an increase in excretory glands in the dermis. In the other layers, an increase of melanophores and melanocyte clusters was observed accompanied by vacuolization of basal cells. The morphometry showed an increase in the thickness of the dermis in the dorso-cranial region in T2 compared to the other treatments, while the ventral caudal region exhibited a variation in the thickness of the dermis from T2 and a decrease in T4. Despite evaluating sublethal concentrations, the skin of D. molitor tadpoles presents histopathological, ultrastructural, and morphometric alterations that could affect the survival of the species in the natural environment.

Transport and Barrier Functions in Rainbow Trout Trunk Skin Are Regulated by Environmental Salinity

The mechanisms underpinning ionic transport and barrier function have been relatively well characterised in amphibians and fish. In teleost fish, these processes have mostly been characterised in the gill and intestine. In contrast, these processes remain much less clear for the trunk skin of fish. In this study, we measured barrier function and active transport in the trunk skin of the rainbow trout, using the Ussing chamber technique. The effects of epithelial damage, skin region, salinity, and pharmacological inhibition were tested. Skin barrier function decreased significantly after the infliction of a superficial wound through the removal of scales. Wound healing was already underway after 3 h and, after 24 h, there was no significant difference in barrier function towards ions between the wounded and control skin. In relation to salinity, skin permeability decreased drastically following exposure to freshwater, and increased following exposure to seawater. Changes in epithelial permeability were accompanied by salinity-dependent changes in transepithelial potential and short-circuit current. The results of this study support the idea that barrier function in rainbow trout trunk skin is regulated by tight junctions that rapidly respond to changes in salinity. The changes in transepithelial permeability and short circuit current also suggest the presence of an active transport component. Immunostaining and selective inhibition suggest that one active transport component is an apical V-ATPase. However, further research is required to determine the exact role of this transporter in the context of the trunk skin.

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.

Toxicidad aguda de cloruro de mercurio (HGCL2) en Cachama blanca; <i>Piaractus brachypomus</i> (Cuvier, 1818)

La disposición inapropiada de sustancias que contienen mercurio puede causar toxicidad y acumulación en tejidos de peces. En el presente estudio se registra la concentración letal cincuenta (CL50) a 96 horas del mercurio para la cachama blanca [Piaractus brachypomus (Cuvier, 1818)]. Se calculó a través de una prueba semiestática de toxicidad aguda utilizando cloruro de mercurio (HgCl2) como fuente del metal. El experimento fue realizado en el Laboratorio de Toxicología de la Universidad del Tolima (Colombia), en condiciones controladas (28,09 ± 1,86 °C) y un fotoperiodo 12:12 (luz:oscuridad). Los alevinos (6,55 ± 1,3 g) fueron mantenidos en acuarios de vidrio con aireación constante, sin filtro y la alimentación fue suprimida 24 horas antes del inicio del experimento. Se emplearon 4 concentraciones de mercurio (Hg), con su respectiva replica, incluyendo un grupo control. Las concentraciones fueron: 0,45, 0,55, 0,65 y 0,75 mg Hg/l. Se realizó un análisis histopatológico con dos peces de cada tratamiento tomando muestras de branquias, cerebro y hígado. Los especímenes expuestos a las concentraciones más bajas (0,45 y 0,55 mg Hg/l) mostraron hiperactividad, a diferencia de los de las concentraciones más altas (0,65 y 0,75 mg Hg/l) los cuales evidenciaron disminución de su actividad. El análisis histopatológico mostró anomalías en branquias e hígado, como hiperplasia interlamelar y vaculizaciones lipídicas respectivamente, en respuesta a procesos de detoxificación. El valor de la CL50-96 h fue estimado utilizando el programa TSK (Trimmed-Spearman-Karber) y presentó un valor de 0,56 mg Hg/l. Este valor es cercano a lo registrado en otras especies de peces y representa el primer registro de toxicidad aguda para el mercurio en cachama blanca.

Transcriptional responses to thermal acclimation in the eurythermal fish Gillichthys mirabilis (Cooper 1864)

Thermal acclimation (acclimatization) capacity may be critical for determining how successfully an ectotherm can respond to temperature change, and adaptive shifts in gene expression may be pivotal for mediating these acclimatory responses. Using a cDNA microarray, we examined transcriptional profiles in gill tissue of a highly eurythermal goby fish, Gillichthys mirabilis , following 4 wk of acclimation to 9°C, 19°C, or 28°C. Overall, gill transcriptomes were not strikingly different among acclimation groups. Of the 1,607 unique annotated genes on the array, only 150 of these genes (9%) were significantly different in expression among the three acclimation groups (ANOVA, false discovery rate < 0.05). Principal component analysis revealed that 59% of the variation in expression among these genes was described by an expression profile that is upregulated with increasing acclimation temperature. Gene ontology analysis of these genes identified protein biosynthesis, transport, and several metabolic categories as processes showing the greatest change in expression. Our results suggest that energetic costs of macromolecular turnover and membrane-localized transport rise with acclimation temperature. The upregulation of several classes of stress-related proteins, e.g., heat shock proteins, seen in the species' response to acute thermal stress was not observed in the long-term 28°C-acclimated fish. The transcriptional differences found among the acclimation groups thus may reflect an acclimation process that has largely remedied the effects of acute thermal stress and established a new steady-state condition involving changes in relative energy costs for different processes. This pattern of transcriptional alteration in steady-state acclimated fish may be a signature of eurythermy.

Branchial ammonia excretion in the Asian weatherloach Misgurnus anguillicaudatus

The weatherloach, Misgurnus anguillicaudatus, is a freshwater, facultative air-breathing fish that lives in streams and rice paddy fields, where it may experience drought and/or high environmental ammonia (HEA) conditions. The aim of this study was to determine what roles branchial Na(+)/K(+)-ATPase, H(+)-ATPase, and Rhcg have in ammonia tolerance and how the weatherloach copes with ammonia loading conditions. The loach's high ammonia tolerance was confirmed as was evident from its high 96 h LC(50) value and high tissue tolerance to ammonia. The weatherloach does not appear to make use of Na(+)/NH(4)(+)-ATPase facilitated transport to excrete ammonia when exposed to HEA or to high environmental pH since no changes in activity were observed. Using immunofluorescence microscopy, distinct populations of vacuolar (V)-type H(+)-ATPase and Na(+)/K(+)-ATPase immunoreactive cells were identified in branchial epithelia, with apical and basolateral staining patterns, respectively. Rhesus C glycoprotein (Rhcg1), an ammonia transport protein, immunoreactivity was also found in a similar pattern as H(+)-ATPase. Rhcg1 (Slc42a3) mRNA expression also increased significantly during aerial exposure, although not significantly under ammonia loading conditions. The colocalization of H(+)-ATPase and Rhcg1 to the similar non-Na(+)/K(+)-ATPase immunoreactive cell type would support a role for H(+)-ATPase in ammonia excretion via Rhcg by NH(4)(+) trapping. The importance of gill boundary layer acidification in net ammonia excretion was confirmed in this fish; however, it was not associated with an increase in H(+)-ATPase expression, since tissue activity and protein levels did not increase with high environmental pH and/or HEA. However the V-ATPase inhibitor, bafilomycin, did decrease net ammonia flux whereas other ion transport inhibitors (amiloride, SITS) had no effect. H(+)-ATPase inhibition also resulted in a consequent elevation in plasma ammonia levels and a decrease in the net acid flux. In gill, aerial exposure was also associated with a significant increase in membrane fluidity (or increase in permeability) which would presumably enhance NH(3) permeation through the plasma membrane. Taken together, these results indicate the gill of the weatherloach is responsive to aerial conditions that would aid ammonia excretion.

Acute toxicity and histopathological alterations of Roundup® herbicide on "cachama blanca" (Piaractus brachypomus)

Acute toxicity of the glyphosate -N (phosphonomethyl) glycine- herbicide, Roundup®, in juveniles of cachama blanca, (Piaractus brachypomus), was evaluated and the histopathological lesions were assessed. The 96 h lethal concentration 50 was 97.47mg.L-1 (P<0.05). In the gill, necrotic and proliferative lesions were detected. In the liver, congestion, degenerative foci, hyaline droplets and lipidic vacuolization of the hepatocytes were observed. In the stomach mild hyperplasia of mucous cells was detected, which was also observed in the skin. In this latter tissue, a large increase in the thickness of the epidermis with necrotic lesions, infiltration of leukocytes and melanin pigment were observed. In the brain, degenerative foci of neuronal bodies in the telencephalon associated with gliosis and infiltration of eosinophilic granule cells/mast cells were shown. In conclusion, gills, liver, skin and brain are susceptible to Roundup®. Moreover, effects on the central nervous system could affect olfaction as well as individual and group behavior, the reproductive performance of the fish and hence have repercussions at the population level.

Strategies for regulation of hemolymph pH in acidic and alkaline water by the larval mosquito Aedes aegypti (L.) (Diptera; Culicidae)

The responses of larval Aedes aegypti to media of pH 4, 7 and 11 provide evidence for pH regulatory strategies. Drinking rates in pH 4 media were elevated 3- to 5-fold above those observed in pH 7 or 11. Total body water was elevated during acute exposure to acidic media. During chronic exposure, total body water was decreased and Malpighian tubule mitochondrial luminosity, quantified using Mitotracker Green FM, increased. Malpighian tubule secretion rates and energy demands thus appear to increase dramatically during acid exposure. In alkaline media, drinking rates were quite low. Larvae in pH 11 media excreted net acid (0.12 nequiv H(+) g(-1) h(-1)) and the pH indicators azolitmin and bromothymol blue revealed that the rectal lumen is acidic in vivo at all ambient pH values. The anal papillae (AP) were found to be highly permeant to acid-base equivalents. Ambient pH influenced the length, and the mass-specific length, of the AP in the presence of NaCl (59.9 mmol l(-1)). In contrast, the length and mass-specific length of AP were not influenced by ambient pH in low NaCl conditions. Mitochondrial luminosity was reduced in AP of larvae reared in acidic media, and was not elevated in alkaline media, relative to that of larvae reared in neutral media. These data suggest that the AP may compromise acid-base balance in acidic media, and may also be an important site of trade-offs between H(+) homeostasis and NaCl uptake in dilute, acidic media.

Tribute to R. G. Boutilier: acid-base transfer across fish gills

The gills are the major site of acid-base regulation in most fish. Acid-base transfer across fish gills is dominated by carbon dioxide and ammonia excretion, especially the former. Bicarbonate buffering in the blood is less than that found in mammals; regulation of ventilation has little effect on CO(2) levels in the blood and control of ventilation is not used to regulate body pH in fish. Proton ATPase (freshwater fish), Na(+)/H(+) exchangers (marine fish) and anion exchangers (marine and freshwater fish) are located in the gills. These transporters contribute to the regulation of internal pH, but little is known about how this is done in fish. Fish kept in confined water volumes acidify their environment, largely due to CO(2). This acidification augments ammonia excretion and reduces ammonia toxicity. The possible involvement of ammonia recycling in acid excretion is also discussed.

Physiological characterisation of a pH- and calcium-dependent sodium uptake mechanism in the freshwater crustacean,Daphnia magna

Daphnia are highly sensitive to sodium metabolism disruption caused by aquatic acidification and ionoregulatory toxicants, due to their finely balanced ion homeostasis. Nine different water chemistries of varying pH (4, 6 and 8) and calcium concentration (0, 0.5 and 1 mmol l–1) were used to delineate the mechanism of sodium influx in Daphnia magna. Lowering water pH severely inhibited sodium influx when calcium concentration was high, but transport kinetic analysis revealed a stimulated sodium influx capacity (Jmax) when calcium was absent. At low pH increasing water calcium levels decreased Jmax and raised Km (decreased sodium influx affinity), while at high pH the opposite pattern was observed (elevated Jmax and reduced Km). These effects on sodium influx were mirrored by changes in whole body sodium levels. Further examination of the effect of calcium on sodium influx showed a severe inhibition of sodium uptake by 100 μmol l–1 calcium gluconate at both low (50 μmol l–1) and high (1000 μmol l–1) sodium concentrations. At high sodium concentrations,stimulated sodium influx was noted with elevated calcium levels. These results, in addition to data showing amiloride inhibition of sodium influx(Ki=180 μmol l–1), suggest a mechanism of sodium influx in Daphnia magna that involves the electrogenic 2Na+/1H+ exchanger.

Vacuolar-type proton pump in the basolateral plasma membrane energizes ion uptake in branchial mitochondria-rich cells of killifish Fundulus heteroclitus, adapted to a low ion environment

We examined the involvement of mitochondria-rich (MR) cells in ion uptake through gill epithelia in freshwater-adapted killifish Fundulus heteroclitus, by morphological observation of MR cells and molecular identification of the vacuolar-type proton pump (V-ATPase). MR cell morphology was compared in fish acclimated to defined freshwaters with different NaCl concentrations: low (0.1 mmol l(-1))-, mid (1 mmol l(-1))- and high (10 mmol l(-1))-NaCl environments. MR cells, mostly located on the afferent-vascular side of the gill filaments, were larger in low- and mid-NaCl environments than in the high-NaCl environment. Electron-microscopic observation revealed that the apical membrane of well-developed MR cells in low- and mid-NaCl environments was flat or slightly projecting, and equipped with microvilli to expand the surface area exposed to these environments. On the other hand, in the high-NaCl environment, the apical membrane was invaginated to form a pit, and MR cells often formed multicellular complexes with accessory cells, although the NaCl concentration was much lower than that in plasma. We cloned and sequenced a cDNA encoding the A-subunit of killifish V-ATPase. The deduced amino acid sequence showed high identity with V-ATPase A-subunits from other vertebrate species. Light-microscopic immunocytochemistry, using a homologous antibody, revealed V-ATPase-immunoreactivity in Na(+)/K(+)-ATPase-immunoreactive MR cells in low-NaCl freshwater, whereas the immunoreactivity was much weaker in higher NaCl environments. Furthermore, immuno-electron microscopy revealed V-ATPase to be located in the basolateral membrane of MR cells. These findings indicate that MR cells are the site responsible for active ion uptake in freshwater-adapted killifish, and that basolaterally located V-ATPase is involved in the Na(+) and/or Cl(-) absorbing mechanism of MR cells.

Dietary sodium inhibits aqueous copper uptake in rainbow trout (Oncorhynchus mykiss)

Ours is the first study to demonstrate an influence of dietary sodium on waterborne copper uptake in fish. We examined possible interactions between dietary sodium and the response of freshwater rainbow trout (Oncorhynchus mykiss) to waterborne copper in light of recent evidence of interactions between sodium and copper metabolism in the gills. Trout were maintained for 6 days on one of four diets of increasing sodium concentration (0.25 mmol g(-1), 0.51 mmol g(-1), 0.76 mmol g(-1) and 1.27 mmol g(-1), which corresponds to 0.6%, 1.2%, 1.8% and 3% sodium by mass, respectively). At the end of 7 days, fish were exposed for 6 h to waterborne copper spiked with (64)Cu to determine if the dietary sodium affected responses to a subsequent short-term waterborne copper exposure. The radiotracer allowed us to distinguish between Cu occurring in fish tissues before the experiment and 'newly accumulated' Cu arising from the experimental exposure. Dietary sodium concentrations of 1.8% or 3% reduced newly accumulated copper concentrations in gill (from 93.9 ng g(-1) in control to 38.9 ng g(-1) and 20.0 ng g(-1) in fish fed 1.8% or 3% Na(+)-supplemented diets, respectively), liver (from 64.3 ng g(-1) to 23.1 ng g(-1) and 7.5 ng g(-1), respectively), kidney (from 29.3 ng g(-1) to 11.7 ng g(-1) and 7.8 ng g(-1), respectively), plasma (from 64.7 ng g(-1) to 21.5 ng g(-1) and 10.7 ng g(-1), respectively) and gut (from 6.8 ng g(-1) to 3.4 ng g(-1) and 2.2 ng g(-1), respectively) by 50.0-88.2%. The 3% Na(+)-supplemented diets also increased plasma and gut sodium concentrations by 38.1% (from 137.1 micromol g(-1) to 189.3 micromol g(-1)) and 104.3% (from 56.5 micromol g(-1) to 115.4 micromol g(-1)), respectively, relative to fish maintained on untreated diets. Whole body uptake rates of both sodium and copper were significantly reduced, and highly correlated (r=0.97) with one another, in fish fed high-sodium diets relative to controls. Moreover, sodium efflux was 12% and 38% higher in fish fed 1.8% and 3% sodium-enriched diets, respectively. Fish fed high-sodium diets also drank more water, but the contribution of drinking to waterborne copper uptake was negligible. From these results, we speculate that, at least in part, aqueous sodium and copper share a common branchial uptake route, probably through an apical sodium channel. According to this hypothesis, as the channel is downregulated with increasing internal sodium concentrations, both sodium and copper uptake from the water are inhibited.

Localization and characterization of phenamil-sensitive Na+ influx in isolated rainbow trout gill epithelial cells

Percoll density-gradient separation, combined with peanut lectin agglutinin (PNA) binding and magnetic bead separation, was used to separate dispersed fish gill cells into sub-populations. Functional characterization of each of the sub-populations was performed to determine which displayed acid-activated phenamil- and bafilomycin-sensitive Na(+) uptake. Analysis of the mechanism(s) of (22)Na(+) influx was performed in control and acid-activated (addition of 10 mmoll(-1) proprionic acid) cells using a variety of Na(+) transport inhibitors (ouabain, phenamil, HOE-694 and bumetanide) and a V-type ATPase inhibitor (bafilomycin). We found that cells migrating to a 1.03-1.05 g ml(-1) Percoll interface [pavement cells (PVCs)] possessed the lowest rates of Na(+) uptake and that influx was unchanged during either bafilomycin (10 nmoll(-1)) treatment or internal acidification with addition of proprionic acid (10 mmoll(-1)). Mitochondria-rich (MR) cells that migrated to the 1.05-1.09 g ml(-1) interface of the Percoll gradient demonstrated acidification-activated bafilomycin and phenamil-sensitive Na(+) influx. Further separation of the MR fraction into PNA(+) and PNA(-) fractions using magnetic separation demonstrated that only the PNA(-) cells (alpha-MR cells) demonstrated phenamil-and bafilomycin-sensitive acid-activated (22)Na(+) uptake. We confirm the coupling of a V-type H(+)-ATPase with phenamil-sensitive Na(+) uptake activity and conclude that high-density alpha-MR cells function in branchial Na(+) uptake in freshwater fish.

Mechanisms of acid secretion in pseudobranch cells of rainbow trout(Oncorhynchus mykiss)

Cell suspensions of rainbow trout Oncorhynchus mykisspseudobranch, prepared by Ca2+ depletion and mechanical maceration,contained a distinct population of cells that always kept their relatively cuboidal shape and did not round up in suspension or proliferate after adhering to the surface of cell culture dishes. Phasecontrast microscopy revealed an extensive system of basal membrane invaginations, and Na+-K+-ATPase- and anionexchanger-like immunoreactivity could be localized in cell membranes. The cells were characterized by a high mitochondrial density. Using specific antibodies, V-ATPase subunit B was localized in the plasma membrane. Using a cytosensor microphysiometer, the rate of acid secretion of these cells was measured and compared with the activity of a gill cell preparation. Incubation of pseudobranch cells with bafilomycin A1 (10-6 moll-1), a specific inhibitor of V-ATPase, reduced the rate of acid secretion by about 10% under control conditions, while no effect of bafilomycin on the rate of acid secretion of gill cells was observed. Application of amiloride (5×10-5moll-1) reduced the rate of acid secretion in cells of both organs,pseudobranch and gills. Incubation of pseudobranch cells with DIDS(10-3 moll-1) resulted in a minor increase in the rate of proton secretion, but in cells prepared from the gills of rainbow trout acid secretion was reduced by about 30-40%. It is concluded that pseudobranch cells are equipped with various pathways to secrete protons, and that the anion exchange activity especially of pseudobranch cells appears to be different from that in gills.

The role of prolactin in fish osmoregulation: a review

The protein hormone prolactin (PRL) was first discovered as an anterior pituitary factor capable of stimulating milk production in mammals. We now know that PRL has over 300 different functions in vertebrates. In fish, PRL plays an important role in freshwater osmoregulation by preventing both the loss of ions and the uptake of water. This paper will review what is currently known about the structure and evolution of fish PRL and its mechanisms of action in relation to the maintenance of hydromineral balance. Historically, functional studies of fish PRL were carried out using heterologous PRLs and the results varied greatly between experiments and species. In some cases this variability was due to the ability of these PRLs to bind to both growth hormone and PRL receptors. In fact, a recurring theme in the literature is that the actions of PRL cannot be generalized to all fish due to marked differences between species. Many of the effects of PRL on hydromineral balance are specific to euryhaline fish, which is appropriate given that they frequently experience sudden changes in environmental salinity. Much of the recent work has focused on the isolation and characterization of fish PRLs and their receptors. These studies have provided the necessary tools to obtain a better understanding of the evolution of PRL and its role in osmoregulation.

Immunolocalization of Vacuolar-Type H(+)-ATPase in the Yolk-Sac Membrane of Tilapia (Oreochromis mossambicus) Larvae

To investigate the involvement of the yolk-sac membrane in ion absorption, developmental changes in whole-body cation contents, cellular localization of vacuolar-type H(+)-ATPase (V-ATPase), and size and density of pavement and chloride cells in the yolk-sac membrane were examined in tilapia (Oreochromis mossambicus) larvae in fresh water (FW) and those transferred to seawater (SW) at 2 days before hatching (day-2). The whole-body content of Na(+) in embryos and larvae adapted to both FW and SW increased constantly from day-2 to day 10, although they were not fed through the experiment. The yolk-sac membrane of FW larvae at days 0 and 2 showed V-ATPase immunoreactivity in pavement cells, but not in chloride cells. No positive immunoreactivity was detected in SW larvae. Whole-mount immunocytochemistry showed that some pavement cells were intensively immunoreactive, whereas others were less or not immunoreactive. Electron-microscopic immunocytochemistry revealed that V-ATPase immunoreactivity was present in the apical regions of pavement cells in FW larvae, especially in their ridges. The pavement cells in FW larvae were significantly smaller in size but higher in density than those in SW. These results suggest that pavement cells are the site of active Na(+) uptake in exchange for H(+) secretion through V-ATPase in FW-adapted tilapia during early life stages.

Gill morphology and acid-base regulation in freshwater fishes

This review examines the recent advances in our understanding of the mechanisms of ion transport and acid-base regulation in the freshwater fish gill. The application of a combination of morphological, immunocytochemical and biochemical techniques has yielded considerable insight into the field. An important mechanism for regulation of Cl- uptake/base excretion is by morphological modification of the gill epithelium. During acidosis, the chloride cell associated Cl-/HCO3- exchanger is effectively removed from the apical epithelium because of a covering by adjacent pavement cells; this mechanism reduces base excretion and contributes to the compensation of the acidosis. In addition, acidosis induces changes in both the surface structure and ultrastructure of pavement cells. Evidence is accumulating to support the hypothesis that Na+ uptake/H+ excretion is accomplished by the pavement cell. Further, specific localization of a V-type H+-ATPase on the pavement cell epithelium and an increased expression during acidosis provides support for the model originally proposed, that this exchange is accomplished by an electrochemically coupled H+-ATPase/Na+ channel mechanism.