Branchial Chloride Cells Research Articles

Evidence for an Osmoregulatory Role of Thyroid Hormones in the Freshwater Mozambique Tilapia Oreochromis mossambicus

The existing equivocal reports on the osmoregulatory role of triiodothyronine (T3) and thyroxine (T4) in teleosts prompted a reinvestigation of their osmoregulatory function in the euryhaline teleost Oreochromis mossambicus. Evidence is presented for thyroidal involvement in hydromineral balance in freshwater tilapia. Dose- and tissue-related responses to various T3 and T4 concentrations were observed in the branchial and renal tissues. The branchial Na+,K+-ATPase activity, known to reflect sodium pump dynamics, increased significantly after the administration of low doses of T3 (20 and 40 ng · g−1) or T4 (40 and 80 ng · g−1). Higher doses of T3 and T4 (>160 ng · g−1) did not change the enzyme activity, compared to sham-injected fish. Conversely, the specific activity of renal Na+,K+-ATPase decreased significantly at all doses of T3 or T4. Further, immunoreactive Na+,K+-ATPase in T4-treated fish increased in branchial chloride cells and this was coupled with a significant increase in the size of chloride cells. T4 treatment, however, did not change branchial chloride cell density. Plasma osmolality, [Na+], and [Cl−] increased, whereas [K+] decreased following low doses of T3 or T4. As expected, plasma levels of T3 and T4 increased significantly in a dose-dependent manner after a single injection of either T3 or T4. The basal levels of T3 and T4 were 4.45 ± 0.49 and 1.25 ± 0.26 nmol · L−1, respectively. This study shows that physiological concentrations of T3 (<10.57 nmol · L−1) and T4 (<6.64 nmol · L−1) enhance branchial Na+ pump activity and chloride cell morphometric dynamics, favoring hyperosmoregulatory capacity in freshwater tilapia. These data are consistent with the hypothesis that thyroid hormones perform a role in hydromineral regulation in freshwater teleosts.

Effects of copper on cortisol receptor and metallothionein expression in gills of Oncorhynchus mykiss

Effects of waterborne Cu (2.4 μM) on the expression of glucocorticoid receptor (GR) and metallothionein (MT) in the branchial epithelium of freshwater rainbow trout (Oncorhynchus mykiss) was studied by immunocytochemistry. After 5 days of Cu exposure, the number of GR-immunoreactive (GR-ir) cells in the gill epithelium had decreased, whereas the number of MT-ir cells had increased. Localization of GR in chloride cells was achieved by double staining for Na +/K +-ATPase; other cell types were identified on the basis of their topology. GRs were present in the chloride cells in both the filaments and lamellae, in respiratory cells in the lamellae, in pavement cells, basal layer cells and undifferentiated cells in the filaments. Co-localization of Na +/K +-ATPase and MT revealed chat MT was expressed in chloride cells, both in filaments and lamellae. Occasionally, MT immunoreactivity was found in pavement cells and in undifferentiated cells. By double staining for Na +/K +-ATPase and GR, for Na +/K +-ATPase and MT and for GR and MT, we can conclude that after 5 days of Cu stress there are chloride cells that express GR and MT, GR or MT alone or neither of the two proteins. This apparent functional heterogeneity of branchial chloride cells may reflect a limited window when chloride cell subpopulations show an adaptive response to Cu.

Expression of the prolactin receptor (tiPRL-R) gene in tilapia Oreochromis niloticus: tissue distribution and cellular localization in osmoregulatory organs.

The expression of the prolactin receptor (PRL-R) gene has been investigated in various tissues of tilapia (Oreochromis niloticus) reared in fresh or brackish water. Using a cDNA probe spanning the extracellular domain of the tilapia PRL-R and Northern blot analysis, the presence of tilapia PRL-R mRNA has been confirmed in the osmoregulatory organs and has been detected in other tissues, including the skin, the brain, the reproductive organs, and the two major hematopoietic organs (spleen and head kidney), as well as circulating lymphocytes. These findings suggest a conservation of the physiological processes regulated by prolactin throughout the vertebrates, including immunity and central nervous activity. A non-radioactive in situ hybridization procedure has allowed us to detect the expression of the tilapia PRL-R in the branchial chloride cells and the intestinal mucosal layer of fresh water animals, confirming the direct control exerted by prolactin on the water and ionic exchanges in tilapia. In all the tissues examined one unique PRL-R transcript has been detected with a similar size (3.2 kb) whatever the salinity conditions. Thus, the transcriptional expression of the tilapia PRL-R strongly differs from the complex RNA pattern reported for the higher vertebrates PRL-R and provides an additional argument for the existence of a single PRL-R for both prolactin isoforms in this fish species.

Hypercortisolemia does not affect the branchial osmoregulatory responses of the marine teleost Sparus sarba

The effect of cortisol treatment on branchial Na +-K +-ATPase subunit mRNA abundance, enzyme activity, chloride cell number/morphometrics and serum electrolyte levels were investigated for the marine teleost Sparus sarba. Groups of fish received intraperitoneal injections of cortisol at a concentration of 4 μg/g body weight, daily, over a seven-day period. This dose of cortisol was sufficiently high enough to maintain a condition of hypercortisolemia as serum cortisol levels in treated fish were eleven fold higher than controls at time of sacrifice. By using branchial Na +-K +-ATPase α- and β-subunit cDNA clones we were able to demonstrate that cortisol administration to S. sarba caused a significant elevation in the abundance of α-mRNA whereas the levels of β-mRNA were unchanged. In addition Na +-K +-ATPase activity remained unaltered by cortisol administration. Branchial chloride cell number, exposure, apical area as well as serum Na + and Cl − levels remained unchanged after cortisol administration. The results of this study suggest that elevated cortisol level may not necessarily translate into modulated branchial Na +-K +-ATPase activity and chloride cell function in hypo-osmoregulating marine fish.

Na(+)/K(+)-ATPase immunoreactivity in branchial chloride cells of Oreochromis mossambicus exposed to copper.

Chloride cells were identified by Na(+)/K(+)-ATPase immunocytochemistry at the light and electron microscope levels in gills of freshwater tilapia Oreochromis mossambicus. Turnover of chloride cells was enhanced by exposing the fish to waterborne copper (3.2 micromol l(-)(1)) for 14 days, as indicated by a 38 % increase in cells expressing proliferating cell nuclear antigen (PCNA) relative to controls. The expression of PCNA was most marked in the central area of the filamental epithelium, from where the chloride cells are thought to originate and migrate. In control fish, chloride cells were associated exclusively with the filamental epithelium. In both controls and copper-exposed fish, two chloride cell populations were seen after Na(+)/K(+)-ATPase immunostaining. These probably represent subpopulations of newly emerged chloride cells: (1) strongly stained cells (mature chloride cells) in the filamental and lamellar epithelium and (2) weakly stained cells, identified by electron microscopy as apoptotic and necrotic chloride cells, mainly in the filamental epithelium. Absolute numbers of mature chloride cells fell, while necrotic and apoptotic chloride cell numbers increased, in copper-exposed fish. A strong correlation could be established for gill Na(+)/K(+)-ATPase specific activity and the number of strongly stained chloride cells in controls and copper-exposed fish and for Na(+)/K(+)-ATPase specific activity and total numbers of immunoreactive cells in copper-exposed fish owing to an increased incidence of weakly staining cells.

The response of sea bream following abrupt hyposmotic exposure

The response of a marine teleost, silver sea bream Sparus sarba, to abrupt hyposmotic exposure was investigated over a 120‐h period following direct exposure from sea water (SW, 33‰) to a hyposmotic environment of 6‰. Aspects of serum chemistry, stored metabolites and gill morphology were used to gain further insight into the biochemical, physiological and morphological alterations that take place following low salinity exposure of a marine fish. Rapid (&lt;1 h) reductions in serum [Cl−] occurred while serum [Na+] exhibited only transient perturbations during initial exposure. Serum total [Ca] declined 24 h after exposure and returned to pre‐exposure levels by 120 h. Despite a tendency for muscle moisture to increase during the early stages of low salinity exposure to significant change could be detected. The response of the branchial chloride cell (CC) was rapid, with apical and fractional exposure area increasing after 6 h. The number of CCs exposed at the branchial surface reduced after 6 h but subsequently increased to elevated levels. Serum cortisol levels had increased three fold 1 h after hyposmotic exposure and stabilised at pre‐exposure levels within 12–24 h. Serum triiodothyronine (T3) levels exhibited a biphasic response, significantly elevating and decreasing after 3 and 6 h respectively. Significant post‐hyposmotic exposure elevations in serum glucose and protein occurred after 1 h, peaking at 3 h and returning to lower levels after 6 h. Total free ninhydrin reactive substances were significantly elevated 3 h post‐hyposmotic exposure, a phenomenon attributable to elevated levels of ammonia, alanine, arginine, glycine, isoleucine, lysine, methionine, phenylalanine, serine, taurine, threonine and valine. Of these substances, glycine, lysine, serine and taurine remained elevated for up to 12 h or longer. Serum urea levels elevated 1 h after exposure to 6%‰ and returned to SW levels 3 h post‐exposure. The relevance of these results is discussed within the context of current knowledge on the effects of hyposomotic adaptation on marine fish.

Induction to apoptosis and necrosis in the branchial chloride cells of fish exposed to copper in water

Induction to apoptosis and necrosis in the branchial chloride cells of fish exposed to copper in water

Changes in salinity tolerance and branchial chloride cells of newborn guppy during freshwater and seawater adaptation

To clarify an osmoregulatory role of branchial chloride cells in successful seawater adaptation of newborn guppies Poecilia reticulata, the present study examined the functional and morphological changes in branchial chloride cells during freshwater and seawater adaptation using immunohistochemical localization of Na+,K+-ATPase. In the newborn guppies of the freshwater-adapted strain, chloride cells were located on the primary lamellae and showed strong immunoreactivity for Na+,K+-ATPase. During adaptation of the newborn guppies to freshwater, their salinity tolerance decreased within five days after birth. Although the total number of chloride cells did not change, the immunoreactive chloride cells decreased in number and size and the immunonegative chloride cells appeared on the primary lamellae. In the seawater-adapted strain, on the other hand, the newborn guppies had larger chloride cells and higher salinity tolerance than those of the freshwater-adapted strain. Acclimation of the mothers to freshwater caused reductions in the chloride cell size and the salinity tolerance of their newborn guppies, indicating that the mother's environmental salinity enhanced the chloride cell size and the salinity tolerance of the newborn guppies in the seawater-adapted strain. During adaptation of the newborn guppies to seawater, their salinity tolerance increased within five days after birth. All of the chloride cells showed a strong immunoreactivity and the cell size increased with no changes in cell number. During the freshwater and seawater adaptation, the salinity tolerance significantly correlated with the sectional area of the immunoreactive chloride cells in the gills. These results indicate that the osmoregulatory activity of the chloride cells, expressed as the immunoreactivity for Na+,K+-ATPase and the cell number and size, is important for the successful seawater adaptation of newborn guppies. J. Exp. Zool. 284:137–146, 1999. © 1999 Wiley-Liss, Inc.

Changes in salinity tolerance and branchial chloride cells of newborn guppy during freshwater and seawater adaptation

Changes in salinity tolerance and branchial chloride cells of newborn guppy during freshwater and seawater adaptation

Ultrastructure and distribution dynamics of chloride cells in tilapia larvae in fresh water and sea water.

Integumental and branchial chloride cells of tilapia larvae (Oreochromis mossambicus) were studied at the light-microscopical and ultrastructural level. Total numbers and distribution of chloride cells were quantified after immunostaining of cross sections of the entire larvae with an antibody against the alpha-subunit of Na+/K+-ATPase. The majority (66%) of Na+/K+-ATPase-immunoreactive (ir) cells, i.e. chloride cells, of freshwater tilapia larvae were located extrabranchially up to 48 h after hatching. Five days after hatching, the majority (80%) of chloride cells were found in the buccal cavity. Transfer of 24-h-old larvae to 20% sea water speeded up this process; 24 h after transfer (i.e. 48 h after hatching), the majority (59%) of chloride cells were located in the buccal cavity. The branchial chloride cell population of 24-h- and 120-h-old larvae consisted of immature, mature, apoptotic and necrotic chloride cells. However, relatively more immature chloride cells were observed in freshwater larvae (42-63%) than in (previously studied) freshwater adults (21%), illustrating the developmental state of the gills. After transfer to sea water, the incidence of degenerative chloride cells did not change. Furthermore, the incidence of immature cells had decreased and a new subtype of chloride cells, the "mitochondria-poor" cells, appeared more frequently. These mitochondria-poor chloride cells were characterised by an abundant tubular system and relatively few mitochondria, which were aligned at the border or concentrated in one part of the cytoplasm. Most of these cells did not contact the water. The function of their enhanced appearance after seawater transfer is unknown.

Ambient salinity modulates the response of the tilapia, Oreochromis mossambicus (Peters), to net confinement

The stress response of the euryhaline tilapia Oreochromis mossambicus adapted to either fresh water (FW) or salt water (Instant Ocean™, 950 mosm; (SW)) was investigated to establish the influences of ambient salinity in this species. The fish were considered to be adapted to the salinities as pre-confinement plasma cortisol and glucose levels were typical for unstressed fish. Two hour net confinement increased plasma cortisol and glucose to a similar extent in both FW and SW. Individual plasma sodium and chloride levels were unaffected by confinement, although plasma Na:Cl ratio increased in FW. Confinement increased intestinal Na +/K +-ATPase activity in FW, but not in SW. In contrast, kidney Na +/K +-ATPase activity increased in SW only. Branchial Na +/K +-ATPase activity decreased with confinement in SW, but not in FW. In SW, confinement reduced the numbers of opercular chloride cells. Increased aging of the branchial chloride cell (CC) population of SW-confined fish was indicated by large numbers of apoptotic CCs in the interlamellar areas. This effect on the CC population was absent in FW-confined fish. Overall, confinement in SW-adapted fish had a more profound impact than confinement in FW-adapted fish. This is likely to have associated energetic consequences in terms of branchial oxygen and ATP consumption. Therefore, results suggest the possibility of different effects of confinement on subsequent growth in FW and SW.

Alterations in Na +–K +–ATPase activity and gill chloride cell morphometrics of juvenile black sea bream ( Mylio macrocephalus) in response to salinity and ration size

The effect of salinity (6, 12 and 33‰) and ration size (5 and 10% body ww day −1) on branchial chloride cell surface ultrastructural morphometrics and osmoregulatory response of juvenile black sea bream ( Mylio macrocephalus) was assessed. The osmoregulatory performance of fish held in sea water (33‰) or acclimated to an isosmotic environment of 12‰ was unimpaired by a reduction in ration size. Fish acclimated to a hyposmotic environment of 6‰ exhibited competent hyperosmoregulation when fed a 10% dietary ration size. Under such conditions chloride cell apical exposure was significantly elevated, however, branchial and renal Na +–K +–ATPase activity were unaltered. A reduction in ration size (5% body ww day −1) did not alter renal Na +–K +–ATPase activity, but did result in an elevation in branchial Na +–K +–ATPase activity of both 33- and 6‰-acclimated fish and a reduction in the chloride cell exposure of 6‰-acclimated fish. A dietary related decrease in chloride cell exposure of 6‰-acclimated fish fed a 5% ration size appeared to result in a hypochloraemic response while serum Na + levels were not significantly affected. Renal Na +–K +–ATPase activity was lower in 12‰-acclimated fish regardless of ration size. The weight of fish fed a 10% ration size after 84 days culture in 12‰ was significantly greater than that of fish held in 33 and 6‰. Fish fed a 5% ration size and acclimated to 12‰ exhibited a greater body mass than those in 33‰ but did not differ significantly from fish acclimated to 6‰. Regardless of ration size, the final body weights of fish in 33‰ and 6‰ were not significantly different. Data suggest that M. macrocephalus juveniles may grow better under isosmotic conditions while fish acclimated to a hyposmotic environment of 6‰ and fed low ration sizes experience a degree of osmoregulatory disequilibrium, the source of which is likely the reduction in chloride cell exposure.

Distributional Changes in Branchial Chloride Cells during Freshwater Adaptation in Japanese Sea Bass Lateolabrax japonicus

Abstract Distributional changes in branchial chloride cells were examined in Japanese sea bass (Lateolabrax japonicus) juveniles transferred from seawater (SW) to fresh water (FW) during their migration season toward low salinity habitat in nature. Chloride cells were identified by immunocytochemistry with a specific antiserum for Na+,K+-ATPase. In fish reared in SW as controls, branchial chloride cells were localized exclusively in the filaments and absent in the lamellae. When sea bass were transferred from SW to FW, chloride cells emerged in gill lamellae, starting at the proximal part of the lamellae and thereafter spread over the lamellar epithelium. On 7th and 15th days after FW transfer, chloride cells were mostly found on the lamellae, whereas the number of filament chloride cells was decreased. These results suggest that, in Japanese sea bass juveniles, chloride cells in the gill lamellae are important in FW adaptation, and that lamellar chloride cells originated from the filaments and migrated t...

Haloplasticity of black seabream (Mylio macrocephalus): Hypersaline to freshwater acclimation

Black seabream (Mylio macrocephalus) were acclimated to various salinities (50, 33, 12 and 6‰) for eight months. Acclimation of fish to 6‰ for eight months allowed successful adaptation to freshwater (0‰) for a further 21 days without mortality. This is the first report of freshwater acclimation of a “true” marine fish for an acceptable experimental duration. Osmoregulatory and metabolic strategies were characterized via alterations in branchial chloride cell (CC) numbers and surface ultrastructural morphometrics along with changes in serum chemistry, muscle moisture, liver glycogen and branchial, renal, hepatic and intestinal enzyme activities. Branchial CC numbers were elevated in 50 and 6‰ environments; however, freshwater acclimation resulted in return to low numbers. Branchial Na+-K+-ATPase activity was generally higher in 50 and 33‰ environments and exhibited a declining trend in 12 and 6‰ environments. Freshwater acclimation resulted in a marked elevation in branchial Na+-K+-ATPase activity. Elevated CC exposure areas were typically found at salinity extremes. Serum Na+, Cl– and muscle moisture content did not vary between groups acclimated from 50 to 6‰. Freshwater acclimation resulted in significant hyponatremia, hypochloremia and muscle hydration. Branchial ICDH activity was lowest in a 12‰ environment and highest at salinity extremes. Renal Na+-K+-ATPase exhibited lower activity in 12 and 6‰ and was markedly elevated in 0‰. Enzyme activities of both liver and kidney indicated elevated gluconeogenic activity in freshwater-adapted fish. Total intestinal Na+-K+-ATPase activity tended to decline in lower salinities; however, lowest activity was found in fish adapted to 12‰. Na+-K+-ATPase activities in different segments of the intestine may reflect the osmoregulatory role of this organ in varying salinities. The data indicated efficient homeostatic control in Mylio macrocephalus acclimated from hypersaline to freshwater environments and clearly demonstrates the haloplasticity of this marine fish species. J. Exp. Zool. 283:226–241, 1999. © 1999 Wiley-Liss, Inc.

Effects of Prolactin and Growth Hormone on Strategies of Hypoosmotic Adaptation in a Marine Teleost,Sparus sarba

Silver seabream (Sparus sarba) held in seawater (33‰) or acclimated to a hypoosmotic environment of 6‰ were given intraperitoneal injections of saline (0.8% NaCl), recombinant bream growth hormone (rbGH, 1 μg/g), or ovine prolactin (oPRL, 6μg/g) for 7 consecutive days. Serum Na+levels were unaffected by hypoosmotic acclimation and rbGH and oPRL treatment. Treatment of seawater fish with oPRL resulted in hyperchloremia. In 6‰, saline-treated fish exhibited elevated branchial chloride cell (CC) numbers and exposure indices, all of which were markedly reduced by oPRL. CC numbers and morphometrics were unaffected by oPRL in seawater fish. In contrast, rbGH treatment of seawater fish resulted in elevated CC numbers, apical area, and fractional area and, in 6‰ fish, elevated CC fractional area and exposure numbers. Branchial Na+-K+-ATPase activity reduced in saline-treated fish adapted to 6% but was unaffected by rbGH regardless of salinity. oPRL reduced activity in both seawater and 6‰-adapted fish. Neither hypoosmotic adaptation nor oPRL had any effect on renal Na+-K+-ATPase activity whereas rbGH reduced activity in both 33 and 6‰. Saline-treated fish adapted to 6‰ exhibited reduced Na+-K+-ATPase activity in most regions of the intestine. Treatment with rbGH did not change intestinal Na+-K+-ATPase activity of seawater fish but elevated activity in the anterior regions (esophagus and stomach) of 6‰-adapted fish. Treatment with oPRL elevated Na+-K+-ATPase activity throughout the gastrointestinal tract of seawater fish and in the anterior reaches of 6‰-adapted fish. The data indicated that the as yet uncharacterized osmoregulatory roles of PRL and GH in seabream may warrant further attention as the present study connoted differing responses to that of other teleosts studied.

Effects of water-borne copper on branchial chloride cells and Na +/K +-ATPase activities in Mozambique tilapia ( Oreochromis mossambicus)

Freshwater tilapia ( Oreochromis mossambicus) were exposed for different periods up to 28 days to 3.2 μM of water-borne Cu. Electron microscopical analysis of the gills demonstrated significant changes in the structure and number of chloride cells (CCs) from Cu-exposed fish when compared to controls. These cells, which are the main location of the Na +/K +-ATPase of the gills and which play a crucial role in transepithlial Na + transport, showed a time-related increase of degeneration by apoptosis and necrosis in the Cu-exposed fish. After 28 days of Cu exposure, apoptotic CCs had doubled in number while necrotic CCs had even increased by a factor of ten. The activity of the gill Na +/K +-ATPase and the plasma Na + concentration decreased in time and in parallel. An inverse relationship between the Na +/K +-ATPase specific activity and the branchial Cu content further supports the notion that this enzyme is very sensitive to Cu 2+ inhibition. In contrast to controls, no significant correlation was found in the Cu-exposed fish between the opercular CC number and the gill Na +/K +-ATPase total activities, despite the large increase in number of these cells. This study provides further evidence that not only the number but also the quality of the CCs, may determine to a large extent the branchial capacity of a freshwater fish to absorb Na + from the surrounding water.

Developmental Sequence of Chloride Cells in the Body Skin and Gills of Japanese Flounder (Paralichthys olivaceus) Larvae

The developmental sequence of chloride cells was examined in both the body skin and gills of Japanese flounder (Paralichthys olivaceus) larvae by whole-mount immunocytochemistry using an antiserum specific for Na(+),K(+)-ATPase. In premetamorphic larvae at 0 and 4 days after hatching (days 0 and 4), immunoreactive chloride cells were distributed only in the yolk-sac membrane and body skin. Premetamorphic larvae at days 8-18 possessed both cutaneous and branchial chloride cells. Large chloride cells in the skin of premetamorphic larvae often formed multicellular complexes, suggestive of their ion-secreting function. Cutaneous chloride cells decreased in size and density at the beginning of metamorphosis (days 21 and 24), and disappeared at the metamorphic climax (days 28 and 33). In contrast, branchial chloride cells first appeared at day 8, and increased during metamorphosis. These results indicate that the site for ion secretion in seawater may shift from cutaneous to branchial chloride cells during metamorphosis. The appearance of branchial chloride cells before the differentiation of gill lamellae suggests that the primary function of the gills during the early development is ion regulation rather than gas exchanges.

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) byin situhybridization 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 byin situhybridization with biotin-labeled synthetic oligonucleotide probe, and specific immunostaining with anti-cortisol 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.

Seawater Adaptability, Osmoregulatory Function, and Branchial Chloride Cells in the Strain Selected for High Salinity Tolerance of the Guppy &lt;i&gt;Poecilia reticulata&lt;/i&gt;

Seawater Adaptability, Osmoregulatory Function, and Branchial Chloride Cells in the Strain Selected for High Salinity Tolerance of the Guppy &lt;i&gt;Poecilia reticulata&lt;/i&gt;

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.