The role of environmental sodium chloride relative to calcium in gill morphology of freshwater salmonid fish

Impact of untreated and microbially treated equalization tank effluent of textile industry on freshwater fish Channa punctata using haematological, biochemical, histopathological and ultrastructural analysis

The unregulated expulsion of untreated or partially treated industrial effluents poses serious threat to the aquatic ecosystem. Therefore, in the present study fish Channa punctata were exposed to untreated and microbially treated equalization tank effluent of textile industry and toxicity studies were carried out for 45 days. The study was planned to analyze the toxicity proffered by textile effluents through haematological, biochemical, histopathological and ultrastructural analysis in blood, liver and gill tissues of fish. While comparing untreated and microbially treated effluent exposed groups haematological parameters were significantly (P ≤ 0.05) less in the untreated effluent exposed group whereas White blood cell count was highly escalated. However, in the microbially treated groups, the alterations were less severe. Increased malondialdehyde content indicating oxidative stress, reduced Catalase (CAT) and Superoxide dismutase (SOD) activity showing a weakened antioxidant defence system and increased glutathione activity was also perceived in untreated effluent exposed groups in comparison to microbially treated groups. Histopathological alterations in gill (telangiectasia, lamellae fusion, breakage, vacuolization and bending of lamellae) and liver (sinusoid dilations, fusion, necrosis and congestion) were more pronounced and severe in the untreated effluent exposed group as compared to microbially treated group. The results observed in histopathology were further reaffirmed by scanning electron microscopy. The study clearly highlights less alterations and deformities in microbially treated effluent groups in comparison to untreated effluent groups. These findings, therefore, necessitate the search for more effective microbial inocula for the better treatment of effluents in order to protect the aquatic life as well as human beings.

Highlights

Channa punctata exposed for 15, 30 and 45 days to untreated and microbially treated equalization tank effluent of textile industry.Untreated and microbially treated effluent exposed fish elicited alterations in blood, liver and gill tissuesHaematology, biochemical, histopathology and ultrastructural analysis resulted in massive pathologies in groups subjected to untreated effluent inducing maximum damage after 45 days of exposure.Less pronounced toxicity in fish C. punctata was observed in fish exposed to microbially treated effluent indicating its efficacy in toxicity reduction.

Lophiosilurus alexandri, a sedentary bottom fish, adjusts its physiological parameters to survive in hypoxia condition

We investigated blood gas, hematological and biochemical parameters, and gill morphology of Lophiosilurus alexandri juveniles submitted to hypoxia for 48 h, followed by recovery for 48 h. A total of 48 juveniles (360.0 ± 141.6 g) were distributed among eight tanks (120 L) and subjected to hypoxia condition (water with dissolved oxygen at 2.12 ± 0.90 mg L^-1) or normoxia (at 5.60 ± 0.31 mg L^-1). Blood gas values (pH, PvCO₂, PvO₂, sO₂, HCO₃^-, stHCO₃^-, and base excess) in hypoxia were significantly different from normoxia, while for lactate and the electrolytes (K⁺, Na⁺, Cl^-, and Ca²⁺) there was no significant change among treatments. The erythrocytes differed significantly between normoxia and hypoxia at 72 h (24 h of recovery), while for hemoglobin and hematocrit there were no significant differences. There was a significant difference in glucose, triglycerides, and cholesterol for both normoxia and hypoxia, while plasma protein remained unchanged. All gill components (epithelial cells, erythrocytes, pillar cells, mucous cells, ionocytes, undifferentiated cells, and blood capillary lumen) differed significantly between normoxia and hypoxia. A reduction in the length of the primary lamella was observed in the hypoxia and recovery treatments, when compared to normoxia. The secondary branchial lamella showed no significant difference for both treatments. Juveniles of Lophiosilurus alexandri adapted well to hypoxia for 48 h, as they were able to adjust most of their physiological variables to survive this stress condition. After 48 h of hypoxia recovery, fish showed parameters similar to animals in normoxia. Thus, the present study shows that the tolerance to hypoxia conditions of L. alexandri, together with other important beneficial characteristics of the species, such as the high meat quality and high commercial value, demonstrates its great potential for production among regional species.

Differential Expression of Hypothalamic and Gill-crh System With Osmotic Stress in the Euryhaline Black Porgy, Acanthopagrus schlegelii

The local gill production of corticotropin releasing hormone (crh) and crh-receptor (crhr) is hypothesized to play important roles during seawater (SW) and freshwater (FW) acclimation in euryhaline black porgy (Acanthopagrus schlegelii). The mRNA expression of crh, crhr, and Na +/K + -ATPase (a-nka) was examined in SW and FW diencephalon (Dien) and in the gills at different exposure time by Q-PCR analysis. The in situ hybridization results indicate that crh mRNA hybridization signals were more abundant in FW fish in the gigantocellular (PMgc) and parvocellular (PMpc) part of the magnocellular preoptic nucleus versus SW fish. The crh and crhr-expressing cells were located in basal cells of gill filament. Furthermore, in vitro dexamethasone (DEX) treatment could increase the crh-system in the gill. Increased transcripts of the crh-system in the gill via in vitro and in vivo CRH treatments suggest that CRH may regulate the system in a local manner. The a-Nka cells were localized in the filament and secondary lamellae mitochondria rich cells (MRCs) of FW fish at 8 h and 1 day. a-Nka cells were seen in both filament and lamellae in the FW but much less in SW fish indicating that gills play key roles in black porgy osmoregulation. Gill crh and crhr play important roles in the response to salinity stress.

Growth, feed conversion, sexual maturation, and cataract formation in coho (Oncorhynchus kisutch) and Atlantic (Salmo salar) salmon post-smolts reared at different salinities in recirculating aquaculture systems for over one year

There is increasing interest in defining optimal conditions for rearing salmon to market size in recirculating aquaculture systems (RAS). Salinity, however, as a parameter that can be potentially manipulated, has been poorly studied. To address this knowledge gap, we reared coho (Oncorhynchus kisutch (Walbaum, 1792)) and Atlantic (Salmo salar Linnaeus, 1758) salmon from smolt to market size over ∼460 days at five different salinities (0, 5, 10, 20, and 30 ppt) and examined their growth performance. We found that both species reared at intermediate salinities (5–10 ppt) started to show significantly larger body mass than fish reared either in freshwater or full-strength seawater at around days 250–300. These growth-enhancing effects were maintained until day 460. The higher growth in Atlantic salmon at intermediate salinities was associated with a reduced economic feed conversion ratio, but this was not observed in coho salmon. The all-female coho salmon showed no incidence of sexual maturation and negligible cataract formation, while the mixed-sex Atlantic salmon showed high levels of sexual maturation (up to 50%) and presence of cataracts (up to 60%). Our results indicate that all-female coho salmon may be better suited to RAS than Atlantic salmon in some aspects. However, in both species, long-term rearing at intermediate salinities improved growth in RAS.

The ontogeny of Na+ uptake in larval rainbow trout reared in waters of different Na+ content

Teleost fish have a remarkable capacity to maintain ion homeostasis against diffusion gradients in hypo-ionic freshwater. In adult teleosts the gills are the primary site for ion uptake; however, in larvae, the gills are underdeveloped, and as ion-regulation is primarily cutaneous, branchial mechanisms of plasticity are not yet available. In larval rainbow trout, the gills become the primary site for Na⁺ uptake at ~ 15 days post hatch (dph). To address how Na⁺ uptake develops in response to differences in water [Na⁺], the present study characterised the ontogeny of Na⁺ uptake in rainbow trout larvae, at a time when ion regulation transitions from being a primarily cutaneous to a primarily branchial process. Results indicate that initially (0-15 dph), when ion-regulation is cutaneous, low-[Na⁺] reared larvae had a higher Na⁺ affinity (lower K_m) compared to the high-[Na⁺] treatment. In addition, larvae reared in low-[Na⁺] water had a lower internal Na⁺ content, despite similar Na⁺-uptake rates ([Formula: see text]) across treatments. But, once the gills became the dominant site for ion-regulation (> 15 dph), larvae in all treatments maintained the same Na⁺ content, despite large differences in [Formula: see text], indicating plasticity in those mechanisms that control Na⁺ efflux ([Formula: see text]). The mechanisms of Na⁺ uptake in larval rainbow trout showed plasticity during all stages of development. However, in young larvae that relied on cutaneous Na⁺ uptake, the internal Na⁺ content was significantly affected by the [Na⁺] in the water, perhaps revealing challenges to ion homeostasis and a period of heightened vulnerability to external stressors during early larval development.

Cytotoxic and genotoxic affects of acid mine drainage on fish Channa punctata (Bloch)

The investigation deals with the effects of Acid Mine Drainage (AMD) of coal mine on fish Channa punctata (Bloch) by examining the incidence of haematological, morphological, histological changes and DNA fragmentation in tissues of C. punctata in laboratory condition. For this study fishes were exposed to 10% of AMD for a period of 30 days. The fusion of the primary and secondary gill lamellae, distortion, loss of alignment, deposition of worn out tissues and mucous on the surface of the lamella in the gills; degeneration of morphological architecture, loss of alignment of tubules, mucous deposition in the kidney; cellular damage, cellular necrosis, extraneous deposition on the surface, pore formation in the liver are some important changes detected by scanning electron microscopy. Fishes of AMD treated group showed gradual significant decrease in TEC, Hb and, increase in TLC and DLC as compared to that of the control. DNA fragmentation observed in kidney of fishes from treated group indicates an intricate pollutant present in the AMD. The high incidence of morphological and histological alterations, haematological changes along with DNA breakage in C. punctata is an evidence of the cytotoxic and genotoxic potential of AMD of coal mines.

Immunolocalization of chloride transporters to gill epithelia of euryhaline teleosts with opposite salinity-induced Na+/K+-ATPase responses

Opposite patterns of branchial Na(+)/K(+)-ATPase (NKA) responses were found in euryhaline milkfish (Chanos chanos) and pufferfish (Tetraodon nigroviridis) upon salinity challenge. Because the electrochemical gradient established by NKA is thought to be the driving force for transcellular Cl(-) transport in fish gills, the aim of this study was to explore whether the differential patterns of NKA responses found in milkfish and pufferfish would lead to distinct distribution of Cl(-) transporters in their gill epithelial cells indicating different Cl(-) transport mechanisms. In this study, immunolocalization of various Cl(-) transport proteins, including Na(+)/K(+)/2Cl(-) cotransporter (NKCC), cystic fibrosis transmembrane conductance regulator (CFTR), anion exchanger 1 (AE1), and chloride channel 3 (ClC-3), were double stained with NKA, the basolateral marker of branchial mitochondrion-rich cells (MRCs), to reveal the localization of these transporter proteins in gill MRC of FW- or SW-acclimated milkfish and pufferfish. Confocal microscopic observations showed that the localization of these transport proteins in the gill MRCs of the two studied species were similar. However, the number of gill NKA-immunoreactive (IR) cells in milkfish and pufferfish exhibited to vary with environmental salinities. An increase in the number of NKA-IR cells should lead to the elevation of NKA activity in FW milkfish and SW pufferfish. Taken together, the opposite branchial NKA responses observed in milkfish and pufferfish upon salinity challenge could be attributed to alterations in the number of NKA-IR cells. Furthermore, the localization of these Cl(-) transporters in gill MRCs of the two studied species was identical. It depicted the two studied euryhaline species possess the similar Cl(-) transport mechanisms in gills.

Gill transcriptome response to changes in environmental calcium in the green spotted puffer fish

Background

Calcium ion is tightly regulated in body fluids and for euryhaline fish, which are exposed to rapid changes in environmental [Ca²⁺], homeostasis is especially challenging. The gill is the main organ of active calcium uptake and therefore plays a crucial role in the maintenance of calcium ion homeostasis. To study the molecular basis of the short-term responses to changing calcium availability, the whole gill transcriptome obtained by Super Serial Analysis of Gene Expression (SuperSAGE) of the euryhaline teleost green spotted puffer fish, Tetraodon nigroviridis, exposed to water with altered [Ca²⁺] was analysed.

Results

Transfer of T. nigroviridis from 10 ppt water salinity containing 2.9 mM Ca²⁺ to high (10 mM Ca²⁺ ) and low (0.01 mM Ca²⁺) calcium water of similar salinity for 2-12 h resulted in 1,339 differentially expressed SuperSAGE tags (26-bp transcript identifiers) in gills. Of these 869 tags (65%) were mapped to T. nigroviridis cDNAs or genomic DNA and 497 (57%) were assigned to known proteins. Thirteen percent of the genes matched multiple tags indicating alternative RNA transcripts. The main enriched gene ontology groups belong to Ca²⁺ signaling/homeostasis but also muscle contraction, cytoskeleton, energy production/homeostasis and tissue remodeling. K-means clustering identified co-expressed transcripts with distinct patterns in response to water [Ca²⁺] and exposure time.

Conclusions

The generated transcript expression patterns provide a framework of novel water calcium-responsive genes in the gill during the initial response after transfer to different [Ca²⁺]. This molecular response entails initial perception of alterations, activation of signaling networks and effectors and suggests active remodeling of cytoskeletal proteins during the initial acclimation process. Genes related to energy production and energy homeostasis are also up-regulated, probably reflecting the increased energetic needs of the acclimation response. This study is the first genome-wide transcriptome analysis of fish gills and is an important resource for future research on the short-term mechanisms involved in the gill acclimation responses to environmental Ca²⁺ changes and osmoregulation.

Chloride transport in mitochondrion-rich cells of euryhaline tilapia (Oreochromis mossambicus) larvae

A noninvasive scanning ion-selective electrode technique (SIET) was applied to measure Cl- transport at individual mitochondrion-rich cells (MRCs) in the skin of euryhaline tilapia (Oreochromis mossambicus) larvae. In seawater (SW)-acclimated larvae, outward Cl- gradients (20-80 mM higher than the background) were measured at the surface, indicating a secretion of Cl- from the skin. By serial probing over the surface of MRCs and adjacent keratinocytes (KCs), a significant outward flux of Cl- was detected at the apical opening (membrane) of MRCs. Treatment with 100 microM ouabain or bumetanide inhibited the Cl- secretion by approximately 75%. In freshwater (FW)-acclimated larvae, a lower level of outward Cl- gradients (0.2-1 mM) was measured at the skin surface. Low-Cl- water (<0.005 mM) acclimation increased the apical Na+-Cl- cotransporter (NCC) immunoreactivity of MRCs in the larval skin. An inward flux of Cl- was detected when probing the exterior surface of a group of MRCs (convex-MRCs) that express the NCC. An NCC inhibitor (100 microM metolazone) reduced the flux by approximately 90%. This study provides direct and convincing evidence for Cl- transport by MRCs of SW- and FW-acclimated euryhaline tilapia and the involvement of an apical NCC in Cl- uptake of MRCs of FW-acclimated fish.

Appearance of cuboidal cells in relation to salinity in gills of Fundulus heteroclitus, a species exhibiting branchial Na+ but not Cl− uptake in freshwater

Fundulus heteroclitus (killifish) is a model organism for ionoregulatory studies, particularly because of its opercular epithelium, although the gills are the major sites of ion exchange. Whereas Na+ and Cl- are excreted through the gills in seawater (SW), the killifish is unusual in taking up only Na+ and not Cl- at the gills in freshwater (FW). We describe morphological changes in the branchial epithelium following transfer from an acclimation medium of 10% SW to 100% SW or FW. In 10% SW, mitochondria-rich cells resemble typical seawater chloride cells (SWCCs) with accessory cells. After transfer to 100% SW, no change occurs in pavement cell (PVC) morphology or mitotic rate (measured by bromo-deoxyuridine technique), although the density of SWCC apertures increases several fold because of the uncovering of buried SWCCs by PVCs, in accord with increased rates of Na+ and Cl- efflux. After transfer to FW, PVC morphology remains unchanged, but SWCCs and accessory cells are quickly covered by PVCs, with many undergoing apoptosis or necrosis. The mitotic rate doubles by 10-14 h but typical freshwater chloride cells (FWCCs) do not appear. Instead, a wedge-shaped cell type that is moderately rich in apically oriented mitochondria, with a large ovoid nucleus, thin cytoplasmic layer, paucity of vesicular-tubular network, and variably villous surface rapidly (by 3 h) and progressively appears in the filament epithelium, by both uncovering and mitosis. This cell type is similar to that recently identified as the site of Na+ uptake in the FW trout gill. We propose the new term "cuboidal cell" for this cell, based on its morphology, to avoid confusion with traditional terminology (of PVC). We hypothesize that the cuboidal cells are the sites of active Na+ uptake in FW F. heteroclitus and suggest that the lack of Cl- uptake is attributable to the absence of typical FWCCs previously described in teleosts.

Physiological action of dissolved organic matter in rainbow trout in the presence and absence of copper: sodium uptake kinetics and unidirectional flux rates in hard and softwater

We investigated the physiological effects of dissolved organic matter (DOM) on sodium (Na+) transport in juvenile Oncorhynchus mykiss (approximately 2.5 g) in the presence and absence of simultaneous acute exposure to copper (Cu2+; 0, 70, and 300 microg l(-1)). Trout were acclimated in either hardwater (approximately 1000 microM Ca2+) or softwater (approximately 100 microM Ca2+), and DOM was tested at approximately 8 mg C l(-1) using a natural (NOM) and a commercial (AHA) source. Ion transport was evaluated based on kinetics estimates (maximum Na+ uptake rates, Jmax; substrate affinity, Km) and unidirectional flux measurements (Jin, Jout, Jnet). Jmax was higher and unidirectional flux rates were greater in softwater-acclimated trout. Fish exposed to DOM alone in hardwater exhibited an increased Na+ transport capacity indicated by both the kinetics (67% higher Jmax for AHA) and Jin measurements (153% higher for AHA and 125% higher for NOM). In softwater, the effects of DOM alone on kinetic parameters and unidirectional flux rates were negligible. Cu2+ affected Na+ uptake by a mixed-type inhibition (both non-competitive and competitive). In hardwater, only Km was increased (i.e., affinity decreased), whereas in softwater, Km was increased and Jmax was decreased, with more marked effects at the higher Cu2+ level. In hardwater, the stimulatory effect of AHA on Jmax persisted even in the presence of 300 microg l(-1) Cu2+, whereas both AHA and NOM prevented the increase in Km caused by Cu2+; these effects were reflected in Jin measurements. In softwater, AHA helped to protect against the increased Km caused by high Cu2+, but there was no protection against the inhibition of Jmax. Unidirectional flux measurements indicated that in softwater, Cu2+ inhibited Jin at 70 microg l(-1), whereas at 300 microg l(-1) Cu2+, Jout was also stimulated. Fish were more affected by Cu2+ in softwater, as indicated by the inability to control diffusive losses of Na+ and a reduced ability to take up Na+, but in the presence of DOM, losses were better controlled at the end of 6 h exposure. We conclude that DOM has direct effects on the gills, as well as protecting fish against acute Cu2+ toxicity. This occurs because DOM complexes Cu2+, and because it acts on the transport and permeability properties of the gills. These effects differ depending on both water hardness and the nature of the DOM source.

Morphological study on the gills of two species of fresh water fishes: Oreochromis niloticus and Clarias gariepinus

The gross and microscopic morphology of the gills of both Nile tilabia (Oreochromis niloticus) and sharptooth catfish (Clarias gariepinus) were described including light and scanning electron microscopy. The anatomical differences between the gill system of the two species were basically related to the geometry of the head and opercular cavities. Each species had four pairs of gills which were connected in a median interbranchial septum. In addition, a fifth rudimentary gill without gill filaments was found in catfish. The lengths and gaps between the gill arches in both species decreased medialwards. The gill rakers of tilabia were generally short and widely spaced compared to the long and narrow spaced rakers of the catfish. The gill filaments and lamellae of tilabia were longer than those of the catfish indicating a greater gill surface area in the former species. The catfish was additionally supplied by modified gills in the form of branched bulbous dendritic structures originating from the second and fourth gill arches. Scanning electron microscopy revealed that the surface of both gill filaments and lamellae was covered by polyhedral cells in tilabia and oval or elongated cells in catfish. These cells carried numerous microplicae which were better developed in tilabia. All gill parts, as revealed by light microscope, were nearly covered by mucous epithelium, however, the mucous cells were present on the lamellae of the catfish only. The rakers of both species demonstrated many taste buds and infiltrating lymphocytes in the epithelial covering. In catfish, in particular, several alarm substance cells were also scattered in this epithelium.

Fish gill morphology: inside out

In this short review of fish gill morphology we cover some basic gross anatomy as well as in some more detail the microscopic anatomy of the branchial epithelia from representatives of the major extant groups of fishes (Agnathans, Elasmobranchs, and Teleosts). The agnathan hagfishes have primitive gill pouches, while the lampreys have arch-like gills similar to the higher fishes. In the lampreys and elasmobranchs, the gill filaments are supported by a complete interbranchial septum and water exits via external branchial slits or pores. In contrast, the teleost interbranchial septum is much reduced, leaving the ends of the filaments unattached, and the multiple gill openings are replaced by the single caudal opening of the operculum. The basic functional unit of the gill is the filament, which supports rows of plate-like lamellae. The lamellae are designed for gas exchange with a large surface area and a thin epithelium surrounding a well-vascularized core of pillar cell capillaries. The lamellae are positioned for the blood flow to be counter-current to the water flow over the gills. Despite marked differences in the gross anatomy of the gill among the various groups, the cellular constituents of the epithelium are remarkably similar. The lamellar gas-exchange surface is covered by squamous pavement cells, while large, mitochondria-rich, ionocytes and mucocytes are found in greatest frequency in the filament epithelium. Demands for ionoregulation can often upset this balance. There has been much study of the structure and function of the branchial mitochondria-rich cells. These cells are generally characterized by a high mitochondrial density and an amplification of the basolateral membrane through folding or the presence of an intracellular tubular system. Morphological subtypes of MRCs as well as some methods of MRC detection are discussed.

Branchial chloride cells in sea bass (Dicentrarchus labrax) adapted to fresh water, seawater, and doubly concentrated seawater

Branchial chloride cells (CC) were studied in sea bass (Dicentrarchus labrax) maintained in seawater (SW: 35 per thousand) or gradually adapted to and subsequently maintained in fresh water (0.2 per thousand) or doubly concentrated seawater (DSW: 70 per thousand). Changes were observed in the location, number, and structure of CCs, that were discriminated by light, scanning, and transmission electron microscopy, as well as by immunofluorescence on the basis of their high Na(+)/K(+)-ATPase antigen content. The number of CCs increased in both fresh water and doubly concentrated seawater compared to control fish maintained in SW. In both experimental conditions, these cells were found on the gill filament (as in control fish) and even on the lamellae, especially in hypersaline conditions. Structural changes concerned the shapes and sizes of CCs and their apical outcrops and particularly the structures of their functional complexes (mitochondria, tubular system, and endoplasmic reticulum), which developed significantly in DSW adapted fish. The changes in the expression of the Na(+)/K(+)-ATPase were evaluated by assessing the enzyme's density at the ultrastructural level following immunogold labeling. This parameter was significantly higher in doubly concentrated seawater. The adaptative significance of the quantitative and morphofunctional changes in branchial chloride cells is discussed in relation to the original osmoregulatory strategy of this marine euryhaline teleost.

Ionoregulatory strategies and the role of urea in the Magadi tilapia (Alcolapia grahami)

The unique ureotelic tilapia Alcolapia grahami lives in the highly alkaline and saline waters of Lake Magadi, Kenya (pH ~10.0, alkalinity ~380 mmol·L–1, Na+~350 mmol·L–1, Cl–~110 mmol·L–1, osmolality ~580 mosmol·kg–1). In 100% lake water, the Magadi tilapia maintained plasma Na+, Cl–, and osmolality at levels typical of marine teleosts and drank the medium at 8.01 ± 1.29 mL·kg–1·h–1. Gill chloride cells were predominantly of the sea water type (recessed, with apical pits) but a few freshwater-type chloride cells (surficial, with flat apical exposure) were also present. Whole-body Na+and Cl–concentrations were relatively high and exhibited larger relative changes in response to salinity transfers than did plasma ions. All fish succumbed upon acute transfer to 1% lake water, but tolerated acute transfer to 10% lake water well, and gradual long-term acclimation to both 10 and 1% lake water without change in plasma cortisol. Plasma osmolytes were here maintained at levels typical of freshwater teleosts. Curiously, drinking continued at the same rate in fish adapted to 1% lake water, but chloride cells were now exclusively of the freshwater type. Significant mortality and elevated cortisol occurred after acute transfer to 200% lake water. However, the fish survived well during gradual adaptation to 200% lake water, although plasma cortisol remained chronically elevated. Urea levels accounted for only 2–3% of internal osmolality in 100% lake water but responded to a greater extent than plasma ions during exposure to 10 and 200% lake water, decreasing by 28–42% in the former and increasing by over 500% in the latter relative to simultaneous-control values. Urea thereby played a small but significant role (up to 8% of internal osmolality) in osmoregulation.

Peanut lectin binds to a subpopulation of mitochondria-rich cells in the rainbow trout gill epithelium

Fluorescently labeled peanut lectin agglutinin (PNA-FITC) was used to identify a subtype of mitochondria-rich (MR) cells in the gills of freshwater rainbow trout. In situ binding of PNA-FITC was visualized by inverted fluorescence microscopy and found to bind to cells on the trailing edge of the filament epithelium as demonstrated by differential interference contrast optics. The amount of PNA-FITC binding on the filament epithelium increased with cortisol pretreatment concomitant with an increased chloride cell fractional area as demonstrated by scanning electron microscopy. Dispersed gill cells were isolated by trypsinization and separated using a discontinuous Percoll density gradient. Cells migrating to the 1.06-1.09 g/ml interface were found to be MR as demonstrated by staining with the vital mitochondrial dye 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide and transmission electron microscopy (TEM). However, only approximately 40% of the MR cells were found to bind PNA-FITC. Cortisol pretreatment increased the relative numbers of MR cells isolated from the dispersed gill cell population, but the relative proportions of PNA binding cells remained unchanged. Ultrastructural analysis of isolated cells in the TEM demonstrated that the MR cell fraction was comprised of a mixed population of chloride cells and pavement cells.

Developmental and environmental regulation of chloride cells in young American shad, Alosa sapidissima

Location, abundance, and morphology of gill chloride cells were quantified during changes in osmoregulatory physiology accompanying early development in American shad, Alosa sapidissima. During the larval-juvenile transition of shad, gill chloride cells increased 3.5-fold in abundance coincident with gill formation, increased seawater tolerance, and increased Na(+),K(+)-ATPase activity. Chloride cells were found on both the primary filament and secondary lamellae in pre-migratory juveniles. Chloride cells on both the primary filament and secondary lamellae increased in abundance (1.5- to 2-fold) and size (2- to 2.5-fold) in juveniles held in fresh water from August 31 to December 1 (the period of downstream migration) under declining temperature. This proliferation of chloride cells was correlated with physiological changes associated with migration (decreased hyperosmoregulatory ability and increased gill Na(+),K(+)-ATPase activity). Increases in chloride cell size and number of fish in fresh water were delayed and of a lower magnitude when shad were maintained at constant temperature (24 degrees C). When juveniles were acclimated to seawater, chloride cell abundance on the primary filament did not (though size increased 1.5- to 2-fold), but cells on the secondary lamellae disappeared. Na(+),K(+)-ATPase was immunolocalized to chloride cells in both fresh water and seawater acclimated fish. The disappearance of chloride cells on the secondary lamellae upon seawater acclimation is evidence that their role is confined to fresh water. The proliferation of chloride cells in fresh water during the migratory-associated loss of hyperosmoregulatory ability is likely to be a compensatory mechanism for increasing ion uptake. J. Exp. Zool. 290:73-87, 2001.

Modification of morphology and function of integument mitochondria-rich cells in tilapia larvae (Oreochromis mossambicus) acclimated to ambient chloride levels

Similar to those of the gills of adults, three types of mitochondria-rich (MR) cells with different morphologies of apical surfaces (wavy convex, shallow basin, and deep hole) were identified on the integument of freshwater-acclimated tilapia larvae (Oreochromis mossambicus). The object of this study is to test the hypothesis that these subtype cells may represent MR cells equipped with variable efficiencies in Cl(-) uptake. Larvae acclimated to low-Cl(-) =0.001-0.007 mM) water developed higher densities of MR cells than those acclimated to high-Cl(-) =7.3-7.9 mM) water. The percentage of wavy-convex-type cells in total MR cells was higher in low-Cl(-)-acclimated larvae than in high-Cl(-)-acclimated larvae, which displayed only deep-hole type. In addition, Cl(-) influx rates of whole larva measured with (36)Cl(-) showed a coincident correlation with MR cell densities, that is, low-Cl(-) larvae displayed higher Cl(-) influx rates than did high-Cl(-) larva, suggesting that tilapia larvae develop a higher density of MR cells with larger apical surfaces (wavy-convex type) to boost Cl(-) uptake in Cl(-)-deficient water. The distinct types of apical surfaces may represent different phases of MR cells that possess different efficiencies of Cl(-) uptake. Increased apical membrane surface areas of MR cells may provide larvae with rapid regulation of Cl(-) before new MR cells differentiate.

Epithelial gill cells in the armored catfish, Hypostomus cf. plecostomus (Loricariidae)

Epithelial gill cell morphology and distribution were investigated in the armored catfish, Hypostomus cf. plecostomus, which lives in soft ion-poor Brazilian freshwaters. Pavement cells are the most abundant type of cell on both filament and lamellar epithelia and there are a great number of mucous and chloride cells between them. Mucous cells are almost covered by adjacent pavement cells and have large packed granules showing electrondense differences. No mucous cells were found on the lamellar epithelium. Chloride cell were distributed throughout both epithelia and usually have large apical surface facing the external medium and may exhibit short and sparsely distributed microvilli. The presence of chloride cells on the lamellar epithelium may be an adaptation to low ion concentrations in the water, allowing for improved ion-transport capacity of the gill. The large size of these cells increases the water-blood barrier and may affect the transference of respiratory gases. However, the negative effect on the respiratory process may be minimized by this species' ability to resort to atmospheric air to fulfill its oxygen requirements.

NaCl uptake by the branchial epithelium in freshwater teleost fish: an immunological approach to ion-transport protein localization

Teleost fishes, living in fresh water, engage in active ion uptake to maintain ion homeostasis. Current models for NaCl uptake involve Na(+) uptake via an apical amiloride-sensitive epithelial Na(+) channel (ENaC), energized by an apical vacuolar-type proton pump (V-ATPase) or alternatively by an amiloride-sensitive Na(+)/H(+) exchange (NHE) protein, and apical Cl(−) uptake mediated by an electroneutral, SITS-sensitive Cl(−)/HCO(3-) anion-exchange protein. Using non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models. Na(+)/K(+)-ATPase was used as a cellular marker for differentiating branchial epithelium mitochondria-rich (MR) cells from pavement cells. In both the freshwater tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-ATPase and ENaC-like immunoreactivity co-localized to pavement cells, although apical labelling was also found in MR cells in the trout. In the freshwater tilapia, apical anion-exchanger-like immunoreactivity is found in the MR cells. Thus, a freshwater-type MR chloride cell exists in teleost fishes. The NHE-like immunoreactivity is associated with the accessory cell type and with a small population of pavement cells in tilapia.

Carbonic anhydrase in the gills of seawater- and freshwater-acclimated flounders Platichthys flesus: purification, characterization, and immunohistochemical localization

Flounders Platichthys flesus were investigated with respect to isolation, purification, and cellular localization of carbonic anhydrase (CA) in the respiratory system. CA was purified from gills and erythrocytes and was shown to exclusively represent a soluble enzyme with an apparent molecular weight of 30 kD. Inhibition constants (KI) towards acetazolamide (ACTZ) were 8.4.10(-9) M for erythrocyte CA and 7.6.10(-9) M for gill CA, indicating a high sensitivity to sulfonamides, as exhibited by human CA II. Specific CA activity did not differ significantly in seawater- and freshwater-acclimated fish. Antibodies were raised against purified gill and erythrocyte CA. Both antisera crossreacted and were used to localize CA in the gills of seawater and freshwater flounders at the light microscopic level. Independent of the salinity, a positive reaction of variable intensity was found in the following cell types: pavement cells (PVCs), forming the gill epithelial surface layer; mucous cells (MCs); pillar cells (PCs), bordering the vascular channels of the secondary lamellae; and chloride cells (CCs), mitochondria-rich cells located in the primary epithelium, the interlamellar regions, and at the bases of the secondary lamellae.(J Histochem Cytochem 47:43-50, 1999)

The adrenergic stress response in fish: control of catecholamine storage and release

In fish, the catecholamine hormones adrenaline and noradrenaline are released into the circulation, from chromaffin cells, during numerous 'stressful' situations. The physiological and biochemical actions of these hormones (the efferent adrenergic response) have been the focus of numerous investigations over the past several decades. However, until recently, few studies have examined aspects involved in controlling/modulating catecholamine storage and release in fish. This review provides a detailed account of the afferent limb of the adrenergic response in fish, from the biosynthesis of catecholamines to the exocytotic release of these hormones from the chromaffin cells. The emphasis is on three particular topics: (1) catecholamine biosynthesis and storage within the chromaffin cells including the different types of chromaffin cells and their varying arrangement amongst species; (2) situations eliciting the secretion of catecholamines (e.g. hypoxia, hypercapnia, chasing); (3) cholinergic and non-cholinergic (i.e. serotonin, adrenocorticotropic hormone, angiotensin, adenosine) control of catecholamine secretion. As such, this review will demonstrate that the control of catecholamine storage and release in fish chromaffin cells is a complex processes involving regulation via numerous hormones, neurotransmitters and second messenger systems.

Localization of cortisol receptor in branchial chloride cells in chum salmon fry

To clarify the involvement of cortisol in functional differentiation of branchial chloride cells, cellular gene expression and localization of cortisol receptor were examined in chum salmon (Oncorhynchus keta) fry in freshwater (FW) and those adapted to seawater (SW) by in situ hybridization and immunocytochemical staining. Sodium-potassium adenosinetriphosphatase (Na+,K(+)-ATPase) activity in the whole gill homogenate was significantly higher in SW fish than in FW fish. There were no significant differences in plasma cortisol levels nor in the expression of cortisol receptor mRNA, examined by Northern blot analysis, between SW and FW fish. The receptor gene expression, examined by in situ hybridization with biotin-labeled synthetic oligonucleotide probe, and specific immunostaining with anticortisol receptor serum were found in two types of chloride cells distributed in the gill filaments and lamellae, which were also labeled with anti-Na+,K(+)-ATPase serum, indicating that cortisol may be one of the important factors regulating chloride cell functions. Gene expression of cortisol receptor in filament chloride cells, which were highly activated in SW-adapted fry, was significantly greater in the fry adapted to SW than in FW-adapted fry, reflecting their specific role in salt secretion in SW. Cortisol receptors were also present in undifferentiated cells in the interlamellar regions adjacent to the central venous sinus, also suggesting the involvement of cortisol in the functional differentiation of chloride cells.

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.

Relationships between branchial chloride cells and gas transfer in freshwater fish

The gill lamellar epithelium is composed of two predominant cell types, pavement cells and mitochondria-rich chloride cells. The chloride cells play a vital role in ionic regulation because they are the sites of Ca2+ and Cl- uptake from water. Consequently, lamellar chloride cell proliferation occurs in response to ionoregulatory challenges so as to increase the ion-transporting capacity of the gill. It has been argued that such chloride cell proliferation might increase the thickness of the blood-to-water diffusion barrier and thereby impede gas diffusion. This review focuses on the potential negative consequences of chloride cell proliferation on gas transfer and possible compensatory mechanisms that might minimise the extent of respiratory impairment. Two approaches were used to evoke chloride cell proliferation in rainbow trout, hormone treatment (growth hormone/cortisol) and exposure to soft water. In all cases, chloride cell proliferation was associated with a pronounced thickening of the lamellar diffusion barrier. The thickening of the diffusion barrier was associated with a significant impairment of gas transfer. Subsequent studies revealed that several compensatory physiological responses occurred concurrently with the chloride cell proliferation to alleviate or reduce the detrimental consequences of the thickened diffusion barrier. These included hyperventilation, an increased affinity of haemoglobin-oxygen binding and earlier onset of catecholamine release during acute hypoxia.

Changes in gill morphology of Atlantic salmon (Salmo salar) smolts due to addition of acid and aluminum to stream water

One-year-old Atlantic salmon smolts were held in three artificial channels adjacent to a softwater (mean sp. cond. 30 microS cm(-1), circumneutral stream. Water in one channel was untreated (mean pH 6.25); the others received additions of acid (to mean pH 5.6), or acid plus aluminum (to mean pH 5.5; mean exchangeable Al 158 microg litre(-1)). Gills were sampled after 16 and 23 days of exposure for morphometric examination. On primary lamellae, chloride cells were more numerous in both experimental treatments than in controls. In contrast, numbers of chloride cells on secondary lamellae were elevated only in fish exposed to acid without added Al. Chloride cell size and shape also varied with time and treatment. Fewer gill mucous cells were found in fish exposed to acid plus Al than in controls. Chloride cell proliferation and structural changes may represent an attempt to compensate for increased ionic effluxes with low pH stress by increasing uptake. However, if Al concentrations are high, chloride cells do not proliferate along the secondary lamellae, or proliferating cells are damaged and lost. This may limit the potential to increase ionic uptake.

Mitochondria-rich cells in the branchial epithelium of the teleost,Oreochromis mossambicus, acclimated to various hypotonic environments

Branchial mitochondria-rich (MR) cells were examined on the afferent side of gill filaments in tilapia (Oreochromis mossambicus) acclimated to different hypotonic environments, local fresh water (LFW), hard fresh water (HFW) and 5‰ salt water (SW). Scanning electron micrographs (SEM) identified three types of apical surfaces of the MR cells, wavy convex, shallow basin and deep hole. In spite of the different types of apical surfaces, light microscopic (LM) and transmission electron microscopic (TEM) studies suggested that these cells were MR cells. The relative abundance of these 3 types of branchial MR cells varied with external hypotonic milieus. Wavy-convexed MR cells were dominant in the gills of fish adapted to HFW, whereas shallow-basined MR cells were evident in LFW-adapted fish. In SW-adapted fish, most of the MR cells were deep holes. Experiments on adaptation to various hypotonic milieus revealed that the changes of the branchial MR cells were reversible and occurred within 24 hours following transfer. The morphological alterations of the MR cells correlated with ionic changes in different milieus, indicating that these distinct types of MR cells may play key roles for osmoregulation in hypotonic media.

Simultaneous recognition of ionocytes and mucous cells in the gill epithelium of turbot and in the rat stomach

A simple and rapid method is described which allows the recognition of different types of epithelial cells in one paraffin section. In the fish gill, the "chloride cells" (also called ionocytes or mitochondria-rich cells) appeared black and well contrasted after the zinc iodideosmium (ZIO) fixation, a contrast which is preserved after staining with alcian blue. All mucous cells appeared pale beige and can be clearly distinguished from the other cells but the known specificity of alcian blue for acid and sulfate mucins was lost. A complex tissue like the gill showed an interesting metachromasia. The combined ZIO fixation and alcian blue staining has also been applied on another epithelium, the rat stomach.

Ultrastructural changes in the respiratory lamellae of the catfish, Heteropneustes fossilis after sublethal exposure to malathion

Transmission electron microscopy study of the gills of Heteropneustes fossilis, exposed to 4 mg/liter of malathion (1/3 of LC50) for 24, 48, 72, and 96 h showed significant changes in its ultrastructures. Exposure to the pesticide after 24 h caused a slightly disarrayed condition in the double layered epithelial structure. Lymphatic spaces became more apparent, and a few chloride cells appeared which protruded toward the peripheral margin of the secondary lamellae. Chloride cells were exposed to the exterior by an apical pit. Pinocytosis was observed with marginal folds (MF) originating from the pillar and epithelial cells. Some vascular constrictions were also seen in the capillaries with erythrocytes. After 48 h exposure, the outer epithelial cells were stretched into a thin boundary wall and lymphatic spaces were engorged with plasma exudate. Chloride cells transversed the whole epithelium of the lamella and came into direct contact with lymphoid space and exterior to epithelial lining. Basement membrane of the capillaries became thicker. After 72 h a distorted lamellar epithelium ruptured in a few places allowing many spheroid bodies and some chloride cells come out. Marginal folds of pillar cells migrated into vascular spaces. Basement membrane of capillaries became thicker and blood channels were constricted causing vascular stasis. No erythrocytes were visible. Blood channels were filled with leukocytes and amoebocytes. After 96 h exposure to malathion narrowing of lymphatic spaces, proliferation of epithelial cells and development of pinocytotic vesicles from marginal folds of pillar cell flanges were observed. Only marginal blood channels maintained normal configuration. Vascular stasis due to thickening of the basal lamina were still evident in centrally located blood channels filled with leukocytes. Vascular stasis would likely cause a decrease in respiratory efficiency. This study has revealed that the gills of H. fossilis were affected by a sublethal dose of malathion. The ultrastructural damages to the gills were observed as early as at 24 h exposure, but the most severe damage occurred at 72 h exposure. However, signs of gill structure regeneration were seen in malathion-exposed fish after 96 h.

Gill structure of a fish from an alkaline lake: effect of short-term exposure to neutral conditions

The morphology and morphometry of the gills of Oreochromis alcalicus grahami, a unique ureogenic teleost that lives in the alkaline environment of Lake Magadi, Kenya (pH 10, [Formula: see text], temperature 30 – 40 °C) were examined by transmission electron, scanning electron and light microscopy. Fish were examined in normal Lake Magadi water and 2 – 3 or 24 h after transfer to Lake Magadi water neutralized to pH 7 with HCl (i.e., [Formula: see text] replaced with Cl−), a treatment that caused severe reductions in urea excretion and O2uptake, internal acidosis, and ionoregulatory disturbance. In Lake Magadi water, the organization of the filament epithelium of the gill was similar to that of sea water teleosts. Indeed, chloride cells were located at the bottom of pits bordered by overlying pavement cells and flanked by typical accessory cells. Total numbers of chloride cells remained unchanged after transfer to pH 7, but after 2 – 3 h, many were covered by pavement cells, restricting their communication with the external milieu. At 24 h, this trend was reversed, an observation indicative of a reactivation of chloride cells. Mucous cells were located at maximum density on the trailing edge of the filament; most of them were empty after 24 h at pH 7. The harmonic mean thickness of the lamellar epithelium (blood-to-water diffusion pathway) was very small and not altered by acute or longer term exposure to pH 7. A model of alterations in ion and acid – base transport accompanying the morphological changes is presented.

Gill epithelial cells kinetics in a freshwater teleost, Oncorhynchus mykiss during adaptation to ion-poor water and hormonal treatments

The aim of this work was to determine the kinetics of the dramatic development of the gill chloride cells (CCs) during adaptation of the salmonid Oncorhynchus mykiss to an ion-poor environment.To monitor cell division, the incorporation in the mitotic cell DNA of bromo-deoxyuridine (BrdUrd) was visualized with a monoclonal antibody. The density of labelled nuclei was used as an index of cellular division (proliferation), concomitantly with morphometry of phenotypic changes monitored with SEM.In the filament epithelium, a phase of CC differentiation occurred within 12h after the transfer, followed by a delayed phase of cell proliferation (48h). In the lamellar epithelium, the present study demonstrates the absence of cell proliferation after ion-poor water transfer. The conclusion is that proliferation (mitosis) is important in the primary filament whereas differentiation and migration (from the filament) is the main mechanism for the appearance of CCs on the secondary lamellae.The present study suggests that cortisol promoted differentiation, but not division, of cells. CCs, presumably premature, were stained by anti-cortisol monoclonal antibody indicating the presence of cortisol. No mature CCs were stained.Growth hormone (oGH, ratGH) increased the rate of cell division both in lamellar and filament epithelium.

Proton pumps in fish gill pavement cells?

The gill epithelium which comprises several types of cell faces multiple functions (O2/CO2 transfer, acid-base balance and ionic regulation). Little is known of the respective cellular localization of these functions. TEM examination of the catfish gill shows, in pavement cells, cytoplasmic vesicles and apical pits, both ornamented with studs reminiscent of the proton pumps observed in H+ secretory epithelia. Ornamented apical pits are more frequently observed in acidotic fish. Taking together with our previous studies, this finding suggests that pavement cells play an important role, in addition to transfer of gas, by secreting protons. A new model of gill exchanges is proposed.

Site of acid-base relevant ion transfer in the gills of rainbow trout (Oncorhynchus mykiss) exposed to environmental hypercapnia

In order to delineate the contribution of primary vs. secondary circulatory circuits in the gill for acid-base and ionic regulation, the flow and composition of the fluids in the central venous sinus (CVS) and the systemic circuit of rainbow trout were studied by application of a previously developed microcannulation technique during normocapnic and hypercapnic conditions. The average haematocrit (Hct) of blood from dorsal aorta (DA) and sinus venosus (SV) ranged from 20.1 to 26.7%, whereas average Hct in the fluid from the branchial vein (BV), representing drainage from the central venous sinus (CVS), was in the range of 4.2 to 7.0%. Under normocapnic conditions, the largest fraction of cardiac output, 92.9%, was directed through the systemic vascular circuit, whereas the CVS circuit was perfused with 7.1 % of cardiac output. Hypercapnia did not significantly affect the blood flow distribution between the two circuits.The pattern of acid-base regulation in dorsal aortic blood reflected the characteristic response of fish exposed to environmental hypercapnia. Upon initiation of environmental hypercapnia (2% CO2), plasma PCO 2 was elevated in all three flow compartments (CVS, DA, SV), inducing an extracellular respiratory acidosis of about 0.4 pH units. pH and [HCO3 (-)] values in the DA were consistently lower, than those for both CVS and SV sites throughout the hypercapnic period. During the 8h of exposure plasma bicarbonate concentration was elevated by about 12 mM, complemented by a fall in extracellular [Cl(-)] of about 10 mM in all three compartments. The amount of HCO3 (-) gained at the CVS site during 8h of hypercapnia (3.3 mmol · kg(-1)) exceeds the amount accumulated in the extracellular space (2.1 mmol·kg(-1)), suggesting the CVS as the main site of ionic acid-base regulation in trout.

Mechanisms of ion and acid-base regulation at the gills of freshwater fish

This review examines the branchial mechanisms utilized by freshwater fish to regulate internal acid-base status and presents a model to explain the underlying basis of the compensatory processes. Rainbow trout, Oncorhynchus mykiss, and brown bullhead, Ictalurus nebulosus, were examined under a variety of experimental treatments which induced respiratory and metabolic acid-base disturbances. Acid-base regulation was achieved by appropriate adjustments of Na+ and Cl- net fluxes across the gills which, in turn, were accomplished by variable contributions of three different branchial mechanisms: 1) differential changes in Na+ and Cl- diffusive effluxes, 2) changes in internal substrate (H+, HCO3-) availability, and 3) morphological adjustments to the gill epithelium. Differential diffusive efflux of Na+ over Cl- was involved only during periods of metabolic alkalosis. The importance of internal substrate availability was demonstrated using a two-substrate model. According to the model, ionic flux rates (J(in)Cl-, J(in)Na+) are determined not only by the concentration of the external ion (Na+, Cl-) but also by the concentration of the internal counterion (H+, HCO3-). This system provides for an "automatic negative feedback" to aid in the compensation of metabolic acid-base disturbances. Morphological alteration of the gill epithelia and the associated regulation of chloride cell (CC) fractional area is an essential third mechanism which is especially important during respiratory acid-base disturbances. Specifically, fish vary the availability of the CC associated Cl-/HCO3- exchange mechanism by physical covering/uncovering of CCs by adjacent pavement cells.

Evidence for a morphological component in acid-base regulation during environmental hypercapnia in the brown bullhead (Ictalurus nebulosus)

Exposure of adult brown bullheads Ictalurus nebulosus (120-450 g) to environmental hypercapnia (2% carbon dioxide in air) and subsequent recovery caused transient changes in whole body net sodium flux (JnetNa+) and net chloride flux (JnetCl-) resulting largely from changes in whole body sodium influx (JNa+in) and chloride influx (JinCl-). Scanning electron microscopy (SEM) revealed that the fractional area of chloride cells (CCs) on the interlamellar regions was reduced by 95% during environmental hypercapnia. During post-hypercapnic recovery, gill filament CC fractional area increased. The changes in JinCl- during and after environmental hypercapnia were closely associated with the changes in CC fractional area while the changes in JinNa+ did not correspond to the changes in CC fractional area. Transmission electron microscopy (TEM) supported the SEM observations of CC surface area changes and demonstrated that these changes were caused by covering/uncovering by adjacent pavement cells (PVCs). Lamellar and filament PVC microvilli density increased during hypercapnia while there was a subsequent reduction in the posthypercapnic period. These data suggest that an important mechanism of acid-base regulation during hypercapnic acidosis is modification of the chloride cell-associated Cl-/HCO3- exchange mechanism. We suggest that bullheads vary availability, and thus functional activity, of this transporter via reversible morphological alterations of the gill epithelium. The increase in density of PVC microvilli may be associated with sodium uptake and/or acidic equivalent excretion during acidosis.