THE EFFECTS OF BRANCHIAL CHLORIDE CELL PROLIFERATION ON RESPIRATORY FUNCTION IN THE RAINBOW TROUT ONCORHYNCHUS MYKISS

Detection of Diclofenac-Induced Alterations in Rainbow Trout (Oncorhynchus mykiss) Using Quantitative Stereological Methods

In 2013, the nonsteroidal anti-inflammatory drug diclofenac (DCF) was included in the watch list for emerging pollutants under the European Union Water Framework Directive. Frequently, monitoring data revealed DCF concentrations in surface waters exceeding the proposed environmental quality standards of 0.04 µg L^-1 and 0.126 µg L^-1 . In recent literature, the possible effects of DCF on fish are discussed controversially. To contribute to a realistic risk assessment of DCF, a 28-day exposure experiment was carried on rainbow trout (Oncorhynchus mykiss). To warrant reliability of data, experiments were conducted considering the Criteria for Reporting and Evaluating Ecotoxicity Data. The test concentrations of DCF used (0.1, 0.5, 1, 5, 25, and 100 µg L^-1 ) also included environmentally relevant concentrations. The lowest-observed-effect concentration (LOEC) for a significant decrease in the plasma concentrations of the DCF biomarker prostaglandin E2 was 0.5 µg L^-1 (male fish). For objective evaluation of relevant histomorphological parameters of gills and trunk kidneys, unbiased quantitative stereological methods were applied. In the gills, significant increases in the thickness of the secondary lamella and in the true harmonic mean of barrier thickness in secondary lamellae were present at DCF concentrations of 25 µg L^-1 and 100 µg L^-1 . In the trunk kidneys, the absolute and relative volumes of nephrons were significantly decreased, paralleled by a significant increase of the volume of the interstitial renal tissue. With regard to quantitative histomorphological alterations in the trunk kidney, the observed LOEC was 0.5 µg L^-1 . The quantitative histomorphological analyses that were conducted allow identification and objective quantification of even subtle but significant morphological effects and thus provide an important contribution for the comparability of study results for the determination of no-observed-effect concentrations (NOEC). Environ Toxicol Chem 2023;42:859-872. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Cortisol regulation of Na+, K+-ATPase β1 subunit transcription via the pre-receptor 11β-hydroxysteroid dehydrogenase 1-like (11β-Hsd1L) in gills of hypothermal freshwater milkfish, Chanos chanos

Hypothermal stress changes the balance of osmoregulation by affecting Na⁺, K⁺-ATPase (Na-K-ATPase) activity or inducing modulation to epithelium permeability in fish. Meanwhile, cellular concentrations of cortisol can be modulated by the pre-receptor enzymes 11β-hydroxysteroid dehydrogenase 1 and 2 (11β-Hsd1 and 2). In fish, increasing levels of exogenous cortisol stimulate Na⁺ uptake via specific interaction with cortisol. This study investigated cortisol effects on expression of Na-K-ATPase subunit proteins and activity in gills of milkfish under hypothermal stress and revealed that the plasma cortisol contents as well as gill 11β-hsd1l and na-k-atpase β1 mRNA abundance were decreased in fresh water (FW) milkfish. Meanwhile, in the seawater (SW) milkfish, the plasma cortisol contents and gill 11β-hsd1l and na-k-atpase β1 mRNA abundance was increased under hypothermal stress. On the other hand, the abundance of 11β-hsd2 mRNA increased in both FW and SW. In addition, 11β-hsd1l expression increased in FW milkfish but decreased in SW milkfish after cortisol injection. Accordingly, the results that gill Na-K-ATPase activity of FW milkfish was affected by environmental temperatures as well as cortisol-dependent Na-K-ATPase β1-subunit levels might be due to increased expression of 11β-hsd1l that elevated intracellular cortisol contents. In hypothermal SW milkfish, decreasing abundance of Na-K-ATPase β1 protein due to reduced expression of 11β-hsd1l was found after cortisol injection. Thus, under hypothermal stress, 11β-HSD1L in FW milkfish gills was used to modulate cortisol and the following effects on increasing the transcription of Na-K-ATPase β1 protein.

Rates of hypoxia induction alter mechanisms of O2 uptake and the critical O2 tension of goldfish

The rate of hypoxia induction (RHI) is an important but overlooked dimension of environmental hypoxia that may affect an organism's survival. We hypothesized that, compared with rapid RHI, gradual RHI will afford an organism more time to alter plastic phenotypes associated with O₂ uptake and subsequently reduce the critical O₂ tension (P_crit) of the rate of O₂ uptake (Ṁ_O2 ). We investigated this by determining P_crit values for goldfish exposed to short (∼24 min), typical (∼84 min) and long (∼480 min) duration P_crit trials to represent different RHIs. Consistent with our predictions, long duration P_crit trials yielded significantly lower P_crit values (1.0-1.4 kPa) than short and typical duration trials, which did not differ (2.6±0.3 and 2.5±0.2 kPa, respectively). Parallel experiments revealed these time-related shifts in P_crit were associated with changes to aspects of the O₂ transport cascade that took place over the hypoxia exposures: gill surface areas and haemoglobin-O₂ binding affinities were significantly higher in fish exposed to gradual RHIs over 480 min than fish exposed to rapid RHIs over 60 min. Our results also revealed that the choice of respirometric technique (i.e. closed versus intermittent) does not affect P_crit or routine Ṁ_O2 , despite the significantly reduced water pH and elevated CO₂ and ammonia levels measured following closed-circuit P_crit trials of ∼90 min. Together, our results demonstrate that gradual RHIs result in alterations to physiological parameters that enhance O₂ uptake in hypoxic environments. An organism's innate P_crit is therefore most accurately determined using rapid RHIs (<90 min) so as to avoid the confounding effects of hypoxic acclimation.

Gill structural change in response to turbidity has no effect on the oxygen uptake of a juvenile sparid fish

Turbidity as a result of increased suspended sediments in coastal waters is an environmental stress of worldwide concern. Recent research on fish suggests that detrimental changes to gill structure can occur in turbid waters, with speculation that these alterations diminish fitness variables, such as growth and development, by negatively impacting the O₂ uptake capacity (respiration) of fish. Specifically to address this unknown, the impact of turbid water on the gill structure, somatic growth rate and O₂ uptake rates of a juvenile sparid species (Pagrus auratus) was addressed following exposure to five different turbidity treatments (<10, 20, 40, 60 or 80 nephelometric turbidity units) for 30 days. Significant gill structural change was apparent with a progressive increase in turbidity and was quantified as a reduction in lamellar density, as well as an increase in basal hyperplasia, epithelial lifting and increased oxygen diffusion distance across the lamellae. The weight of control fish did not change throughout the experiment, but all fish exposed to turbid waters lost weight, and weight loss increased with nephelometric turbidity units, confirming that long-term turbidity exposure is detrimental to growth productivity. The growth of fish could be impacted in a variety of ways, but the specific hypothesis that structural alteration of the gills impairs O₂ uptake across the gills and limits growth fitness was not supported because there was no measurable difference in the standard metabolic rate, maximal metabolic rate, aerobic metabolic scope or critical oxygen saturation limit of fish measured in clear water after 30 days of exposure. Although impaired O₂ uptake as a result of structurally adjusted gills is unlikely to be the cause of poor fish growth, the exact mechanism by which growth productivity is affected in turbid conditions remains unclear and warrants further investigation.

Effects of hypoxia-induced gill remodelling on the innervation and distribution of ionocytes in the gill of goldfish, Carassius auratus

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish acclimated to 7°C leads to preferential distribution of branchial ionocytes to the distal edges of the ILCM, where they are likely to remain in contact with the water and hence remain functional. Upon exposure to hypoxia, the ILCM retracts, and the ionocytes become localized to the lamellar surfaces and on the filament epithelium, owing to their migration and the differentiation of new ionocytes from progenitor cells. Here we demonstrate that the majority of the ionocytes receive neuronal innervation, which led us to assess the consequences of ionocyte migration and differentiation during hypoxic gill remodelling on the pattern and extent of ionocyte neuronal innervation. Normoxic 7°C goldfish (ILCM present) possessed significantly greater numbers of ionocytes/mm² (951.2 ± 94.3) than their 25°C conspecifics (ILCM absent; 363.1 ± 49.6) but a statistically lower percentage of innervated ionocytes (83.1% ± 1.0% compared with 87.8% ± 1.3%). After 1 week of exposure of goldfish to hypoxia, the pool of branchial ionocytes was composed largely of pre-existing migrating cells (555.6 ± 38.1/mm²) and to a lesser extent newly formed ionocytes (226.7 ± 15.1/mm²). The percentage of new (relative to pre-existing) ionocytes remained relatively constant (at ∼30%) after 1 or 2 weeks of normoxic recovery. After hypoxia, pre-existing ionocytes expressed a greater percentage of innervation than newly formed ionocytes in all treatment groups; however, their percentage innervation steadily decreased over 2 weeks of normoxic recovery.

Physiological effects of gasoline on the freshwater fish Prochilodus lineatus(Characiformes: Prochilodontidae)

The purpose of this work was to evaluate the effects of the water-soluble fraction of gasoline (WSFG) on the Neotropical freshwater fish Prochilodus lineatus. The WSFG was prepared by mixing gasoline in water (1:4) and animals were exposed for 6, 24 and 96h to 5% diluted WSFG or only to water. After exposure, blood was collected from the caudal vein and the gills were removed. The following parameters were analyzed: hematological (hemoglobin, hematocrit, number of red blood cells), osmo-ionic (plasma Na+, Cl- and K+ and plasma osmolarity), metabolic (total plasma proteins and glucose), endocrine (cortisol), density and distribution of chloride cells [CC] in the gills (immunohistochemistry), and branchial Na+/K+-ATPase (NKA) activity. Hemolysis was found to occur after 96h exposure to WSFG, as indicated by the decrease in the hematological parameters analyzed, followed by an increase in plasma K+. Secondary stress response was revealed by the occurrence of hyperglycemia in the three periods of exposure, despite the absence of significant increases in the plasma cortisol. The exposure to WSFG also caused an increase in the quantity of CC and in plasma Na+, after 24h, as well as in the enzymatic activity of NKA and plasma osmolarity, after 24h and 96h. These results indicate that fish exposed to the WSFG showed physiological adjusts to maintain their osmotic balance. However, the increase in the quantity of CC in the lamellae may interfere in the gas exchange impairing respiration.

Implications for osmorespiratory compromise by anatomical remodeling in the gills of Arapaima gigas

The gill structure of the Amazonian fish Arapaima gigas, an obligatory air breather, was investigated during its transition from water breathing to the obligatory air breathing modes of respiration. The gill structure of A. gigas larvae is similar to that of most teleost fish; however, the morphology of the gills changes as the fish grow. The main morphological changes in the gill structure of a growing fish include the following: (1) intense cell proliferation in the filaments and lamellae, resulting in increasing epithelial thickness and decreasing interlamellar distance; (2) pillar cell system atrophy, which reduces the blood circulation through the lamellae; (3) the generation of long cytoplasmic processes from the epithelial cells into the intercellular space, resulting in continuous and sinuous paracellular channels between the epithelial cells of the filament and lamella that may be involved in gas, ion, and nutrient transport to epithelial cells; and (4) intense mitochondria-rich cell (MRC) proliferation in the lamellar epithelium. All of these morphological changes in the gills contribute to a low increase of the respiratory surface area for gas exchange and an increase in the water-blood diffusion distance increasing their dependence on air-breathing as fish developed. The increased proliferation of MRCs may contribute to increased ion uptake, which favors the regulation of ion content and pH equilibrium.

Sub chronic exposure to crude oil, dispersed oil and dispersant induces histopathological alterations in the gills of the juvenile rabbit fish (Siganus canaliculatus)

There is little existing information on the sub-lethal effects of experimental exposure of Arabian Gulf fish to oil pollution. This study investigated the potential sub-lethal effects of the water accommodated fraction (WAF) of light Arabian crude oil, dispersed oil and dispersant (Maxi Clean 2) on the gills of the juvenile rabbit fish (Siganus canaliculatus), observing several histopathological biomarkers at different time points and different doses. These laboratory exposures simulated a range of possible oil pollution events. Significant alterations in four health categories (circulatory, proliferative, degenerative and inflammatory) were identified and form the basis for understanding the short-term response of fish to oil. Evaluations of histopathological lesions in gill tissue were carried out following 3, 6, 9, 12, 15, 18 and 21 days of exposure. The main lesions observed and quantified were lamellar capillary aneurysms, vasodilatation of lamellae, hemorrhage, edema, lifting of lamellar and filamentary epithelium and epithelium necrosis, epithelial and chloride cell hypertrophy and hyperplasia, fusion of adjacent lamellae, epitheliocystis and inflammatory infiltration. Exposure of juvenile fish to WAF, dispersant oil and dispersant caused significant changes in the gill lesions and reaction patterns. Dispersed oil caused the most significant effect followed by WAF and then dispersant. The present study is one of the first which explores the relationship between oil pollution and epitheliocystis and reports that exposure to crude oil and dispersed oil increases the prevalence of epitheliocystis formation under controlled laboratory conditions.

Organochlorines and metals induce changes in the mitochondria-rich cells of fish gills: an integrative field study involving chemical, biochemical and morphological analyses

Through integrating chemical, biochemical and morphological analyses, this study investigated the effects of multiple pollutants on the gill mitochondria-rich cells (MRCs) in two fish species, Astyanax fasciatus and Pimelodus maculatus, collected from five sites (FU10, FU20, FU30, FU40 and FU50) in the Furnas Hydroelectric Power Station reservoir. Water analyses revealed aluminum, iron and zinc as well as organochlorine (aldrin/dieldrin, endosulfan, heptachlor/heptachlor epoxide and metolachlor) contamination at all of the sites, with the exception of FU10. Copper, chrome, iron and zinc were detected in the gills of both species, and aldrin/dieldrin, endosulfan and heptachlor/heptachlor epoxide were detected in the gills of fish from all of the sites, with the exception of FU10. Fish collected at FU20, FU30 and FU50 exhibited numerous alterations in the surface architecture of their pavement cells and MRCs. The surface MRC density and MRC fractional area were lower in fish from FU20, FU30, FU40 and FU50 than in those from the reference site (FU10) in the winter, and some variability between the sites was observed in the summer. The organochlorine contamination at FU20 and FU50 was associated with variable changes in the MRCs and inhibition of Na(+)/K(+)-ATPase (NKA) activity, especially in P. maculatus. At FU30, the alterations in the MRCs were associated with the contaminants present, especially metals. A multivariate analysis demonstrated a positive association between the biological responses of both species and environmental contamination, indicating that under realistic conditions, a mixture of organochlorines and metals affected the MRCs by inhibiting NKA activity and inducing morphological changes, which may cause an ionic imbalance.

Cortisol regulates Na+ uptake in zebrafish, Danio rerio, larvae via the glucocorticoid receptor

Unlike other freshwater fish previously examined, zebrafish are capable of increasing their rate of Na(+) uptake during chronic exposure to acidic water (pH 4). In the present study, the potential role of cortisol in the induction of Na(+) uptake during acid-exposure was investigated. When zebrafish larvae (4 days post-fertilization) were treated with waterborne cortisol, the rate of Na(+) uptake was significantly increased; this effect was blocked by co-incubating larvae with RU-486, an antagonist selective for the glucocorticoid receptor (GR). A similar induction in Na(+) uptake, which was also blocked by RU-486, was observed when larvae were treated with dexamethasone, a selective GR agonist. Conversely, treating larvae with aldosterone, a selective agonist for the mineralocorticoid receptor (MR) had no effect on Na(+) uptake. Acid-exposure increased whole body cortisol levels and translational knockdown of GR using antisense morpholinos prevented the full induction of Na(+) uptake during exposure to acidic water, further confirming the role of cortisol and GR in Na(+) uptake stimulation. Using immunohistochemistry, GR was localized to ionocytes known to be responsible for Na(+) uptake (HR-cells). Knockdown of Rhcg1, an apical membrane ammonia channel or Na(+)/H(+) exchanger 3b (NHE3b), proteins known to play an important role in facilitating Na(+) uptake in acidic water, prevented the stimulatory effects of cortisol treatment on Na(+) uptake, suggesting that cortisol regulates Na(+) uptake by stimulating an Rhcg1-NHE3b "functional metabolon".

Mechanisms and regulation of Na(+) uptake by freshwater fish

Mechanisms of ion uptake by freshwater (FW) fish have received considerable attention over the past 80 years. Through an assortment of techniques incorporating whole animal physiology, electrophysiology and molecular biological approaches, three models have been proposed to account for Na(+) uptake. (1) Direct exchange of Na(+) and H(+) via one or more types of Na(+)/H(+) exchanger (slc9), (2) uptake of Na(+) through epithelial Na(+) channels energized by an electrical gradient created by H(+)-ATPase and (3) Na(+)/Cl(-) co-transport (slc12). While each mechanism is supported at least in part by theoretical or experimental data, there are several outstanding questions that have not yet been fully resolved. Furthermore, there are few details concerning how these Na(+) uptake mechanisms are fine tuned in response to the fluctuating FW environments. In this review, we summarize the current understanding of these three Na(+) uptake mechanisms and discuss their regulation by endocrine (cortisol and prolactin) and neurohumoral (catecholamines) factors.

The interactive effects of hypoxemia, hyperoxia, and temperature on the gill morphology of goldfish (Carassius auratus)

Acclimation of crucian carp and goldfish to temperatures below 15°C causes covering of the gill lamellae by a mass of cells termed the interlamellar cell mass (ILCM). Here we explore the cues underlying gill remodeling (removal or growth of an ILCM) and specifically test the hypotheses that 1) depletion of internal O2 stores in the absence of any change in external O2 status can trigger the removal of the ILCM in goldfish acclimated to 7°C, 2) exposing fish acclimated to 25°C to an abundance of O2 (hyperoxia) can reverse the gill remodeling (i.e., cause the covering of lamellae by an expansion of the ILCM), and 3) neuroepithelial cells (NECs) are involved in signaling the shedding of the ILCM. Hypoxemia induced by phenylhydrazine (anemia) or 5% CO caused a decrease in the ILCM from 80% to 23% and 35%, respectively. Hyperoxia exposure at 25°C caused an increase to 67% of total ILCM and a smaller decrease in the size of the ILCM when fish were transferred from 7 to 25°C. Daily sodium cyanide injections were used to stimulate NECs; this treatment led to a significant decrease in the ILCM. Thus, the three major conclusions of this study are 1) that gill remodeling can occur during periods of internal hypoxemia, 2) that O2 supply and demand may be a significant driving force shaping gill remodeling in goldfish, and 3) the NECs may play a role in triggering the shedding of the ILCM during hypoxia.

Respiratory responses to hypoxia or hypercapnia in goldfish (Carassius auratus) experiencing gill remodelling

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish significantly decreases the functional lamellar surface area and increases the diffusion distance for gas transfer and thus may impose a serious challenge for the transfer of respiratory gases (O₂ and CO₂). Here we tested the hypothesis that the presence of the ILCM in goldfish acclimated to 7°C impedes the uptake of O2 and excretion of CO₂. While Pa(O₂) remained unaltered, the baseline values of Pa(CO)₂ were significantly higher in goldfish at 7°C with ILCM present (5.55 ± 0.54 mmHg; mean ± SEM) than in goldfish at 25°C without the ILCM (3.98 ± 0.18 mmHg). Carbonic anhydrase (CA) injections relieved the apparent diffusion limitation imposed by the presence of the ILCM on CO₂ excretion (Pw(CO₂) levels dropped to 3.07 ± 0.32 mmHg). Interestingly, the exposure of fish to acute hypoxia evoked similar changes in Pa(O₂) at the two acclimation temperatures. Ethanol (EtOH) exposure was also used as a tool to further investigate the potential effects of the ILCM on branchial solute transfer. The results showed that the ILCM does not impede EtOH uptake in 7°C goldfish. Overall, the results of this study demonstrate that the remodelling of the goldfish gill associated with acclimation to 7°C water, while increasing Pw(CO₂) , has minimal impact on branchial O2 transfer.

The phylogeny of central chemoreception

Respiratory chemoreceptors responsive to changes in CO(2)/H(+) appear to be present in all vertebrates from fish to birds and mammals. They appear to have arisen first in the periphery sensitive to the external environment. Thus, in most fish CO(2)/H(+) chemoreceptors reside primarily in the gills and respond to changes in aquatic rather than arterial P(CO)₂ . In the air-breathing tetrapods (amphibians, mammals, reptiles and birds), the branchial arches regress developmentally and the derivatives of the branchial arteries are now exclusively internal. The receptors associated with these arteries now sense only arterial (not environmental) P(CO)₂/pH . Central CO(2)/H(+) chemoreception also appears to have arisen with the advent of air breathing, presumably as a second line of defense. These receptors may have arisen multiple times in association with several (but not all) of the independent origins of air breathing in fishes. There is strong evidence for multiple central sites of CO(2)/H(+) sensing, at least in amphibians and mammals, suggesting that it may not only have originated multiple times in different species but also multiple times within a single species.

Morphology and changes of chloride cell of Rutilus rutilus Caspicus (Cyprinidea, teleost) in Caspian Sea

An ultrastructural study was performed on chloride cells of euryhaline R.r.Caspicus of south of Caspian Sea. The chloride cells are distributed in the interlamellar region of filaments. They are oval to elongated form with an apical positioned nucleus, expanded tubular system and heteromorphic mitochondria. These cells are surrounded by pavement cell and accessory cell. A small and depressed surface formed by pavement cells is in contact with the aquatic milieu. There is also channel system in accessory cells. One of the typical features was the important changes in microtubules and mitochondria of chloride cells in some fishes. Swelling and rupture of cristae and degeneration of microtubules were from these changes.

Histochemical technique for the detection of chloride cells in fish

Chloride cells are responsible for ionic exchanges in the fish gill. These cells have been widely studied, considering its importance in vital functions of the gill, and because they proliferate when exposed to unfavorable environments. One of the main characteristic of these cells is an acidic cytoplasm, which has been used for identification through histochemical techniques with dyes such as Toluidine Blue and Hematoxylin and Eosin. However, these techniques can be problematic, since epithelial cells can, in certain situations, acquire acidic characteristics similar to those of chloride cells, thus staining in a similar way. Among other functions, chloride cells play a role in calcium uptake from the environment, and therefore have a high concentration of this element. Based on this information, this study aims at developing a specific protocol for the identification of chloride cells. With this purpose, the Von Kossa method specific for calcium was used combined with Hematoxylin counterstaining. Chloride cells had cytoplasm slightly stained with Hematoxylin and the presence of dark stained granules dispersed in the cytoplasm resulted from the Von Kossa reaction due to the calcium present in these cells. This was not found in any other gill cell. Thus, the technique used in this study was specific and efficient to identify chloride cells in fish gills.

Effects of soft-water acclimation on the physiology, swimming performance, and cardiac parameters of the rainbow trout, Oncorhynchus mykiss

Rainbow trout acclimated to soft water were submitted to an incremental velocity trial, and exhibited a 14% decrease in critical swimming speed (U(crit) * 1.37 +/- 0.055 vs. 1.54 +/- 0.044 m s(-1)) compared to fish kept in hard water. After a standardized swimming protocol, soft-water-acclimated fish had higher blood lactate concentrations (6.5 +/- 0.66 and 6.0 +/- 0.64 mmol L(-1) (soft water) vs. 5.0 +/- 0.46 and 3.9 +/- 0.32 mmol L(-1) (hard water)), revealing a greater use of anaerobic metabolism for the same exercise. Cardiovascular parameters were investigated while fish were swimming at increasing water velocities, revealing that soft-water-acclimated fish had lower increases in heart rate (105% vs. 118% of pre-exercise values), due to higher heart rates observed during acclimation and during the first 10 min of the swimming trial. This was also reflected by the plateau in heart rate and stroke volume observed during the swimming protocol, which can be attributed to increased cardiovascular function in response to soft-water acclimation. These results are in accord with previously reported increases in blood-to-water diffusion distance, due to proliferation of chloride cells at the gills in response to soft-water conditions, and underscore the costs and limitations of soft-water acclimation.

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models

Carbon dioxide (CO(2)) excretion and acid-base regulation in fish are linked, as in other animals, though the reversible reactions of CO(2) and the acid-base equivalents H(+) and HCO(3)(-): CO(2)+H(2)O<-->H(+)+HCO(3)(-). These relationships offer two potential routes through which acid-base disturbances may be regulated. Respiratory compensation involves manipulation of ventilation so as to retain CO(2) or enhance CO(2) loss, with the concomitant readjustment of the CO(2) reaction equilibrium and the resultant changes in H(+) levels. In metabolic compensation, rates of direct H(+) and HCO(3)(-) exchange with the environment are manipulated to achieve the required regulation of pH; in this case, hydration of CO(2) yields the necessary H(+) and HCO(3)(-) for exchange. Because ventilation in fish is keyed primarily to the demands of extracting O(2) from a medium of low O(2) content, the capacity to utilize respiratory compensation of acid-base disturbances is limited and metabolic compensation across the gill is the primary mechanism for re-establishing pH balance. The contribution of branchial acid-base exchanges to pH compensation is widely recognized, but the molecular mechanisms underlying these exchanges remain unclear. The relatively recent application of molecular approaches to this question is generating data, sometimes conflicting, from which models of branchial acid-base exchange are gradually emerging. The critical importance of the gill in acid-base compensation in fish, however, has made it easy to overlook other potential contributors. Recently, attention has been focused on the role of the kidney and particularly the molecular mechanisms responsible for HCO(3)(-) reabsorption. It is becoming apparent that, at least in freshwater fish, the responses of the kidney are both flexible and essential to complement the role of the gill in metabolic compensation. Finally, while respiratory compensation in fish is usually discounted, the few studies that have thoroughly characterized ventilatory responses during acid-base disturbances in fish suggest that breathing may, in fact, be adjusted in response to pH imbalances. How this is accomplished and the role it plays in re-establishing acid-base balance are questions that remain to be answered.

Differential freshwater adaptation in juvenile sea-bassDicentrarchus labrax: involvement of gills and urinary system

The effects of long-term freshwater acclimatization were investigated in juvenile sea-bass Dicentrarchus labrax to determine whether all sea-bass juveniles are able to live in freshwater and to investigate the physiological basis of a successful adaptation to freshwater. This study particularly focused on the ability of sea-bass to maintain their hydromineral balance in freshwater and on their ion (re)absorbing abilities through the gills and kidneys. Two different responses were recorded after a long-term freshwater acclimatization. (1) Successfully adapted sea-bass displayed standard behavior; their blood osmolality was maintained almost constant after the freshwater challenge, attesting to their efficient hyperosmoregulation. Their branchial and renal Na+/K+-ATPase abundance and activity were high compared to seawater fish due to a high number of branchial ionocytes and to the involvement of the urinary system in active ion reabsorption, producing hypotonic urine. (2) Sea-bass that had not successfully adapted to freshwater were recognized by abnormal schooling behavior. Their blood osmolality was low (30% lower than in the successfully adapted sea-bass), which is a sign of acute osmoregulatory failure. High branchial Na+/K+-ATPase abundance and activity compared to successfully adapted fish were coupled to a proliferation of gill chloride cells, whose ultrastructure did not display pathological signs. The large surface used by the gill chloride cells might negatively interfere with respiratory gas exchanges. In their urinary system, enzyme abundance and activity were low, in accordance with the observed lower density of the kidney tubules. Urine was isotonic to blood in unsuccessfully adapted fish, ruling out any participation of the kidney in hyperosmoregulation. The kidney failure seems to generate a compensatory ion absorption through increased gill activity, but net ion loss through urine seems higher than ion absorption by the gills, leading to lower hyper-osmoregulatory performance and to death.

Chloride cell responses to ion challenge in two tropical freshwater fish, the erythrinids Hoplias malabaricus and Hoplerythrinus unitaeniatus

Chloride cell (CC) responses to ion challenge and plasma ion concentration were evaluated in two ecologically distinct erythrinids, Hoplias malabaricus, an exclusively water-breathing species, and Hoplerythrinus unitaeniatus, a facultative air-breathing fish, at one, two, seven, and 15 days of exposure to deionized water and to ion-rich water. H. malabaricus displayed high CC proliferation on filament and lamellar epithelium during exposure to deionized water and significant CC proliferation in the filament epithelium on the first day of exposure to water rich in NaCl and Ca2+ and in the lamellar epithelium on the first, second, and seventh day of exposure to such water. CC proliferation in H. unitaeniatus occurred only in the lamellar epithelium of fish exposed to deionized water. CC proliferation on both species was not accompanied by significant increase of CC density in contact with the external medium. The increase in the CC fractional area (CCFA) resulted from the increase of individual CC apical surface area on the first and second days of exposure to deionized water in H. malabaricus and only on the first day in H. unitaeniatus. Plasma ions in both erythrinid species showed transitory changes and, on the fifteenth day of exposure to the two types of experimental water, the plasma ion concentration was similar to the control fish. The CC responses of these erythrinid fish showed that CC proliferation depends on previous CC density in the gill and is not related solely to exposure to ion-poor water. Furthermore, CC proliferation in gill epithelium did not always involve an increase of CC density in contact with the external medium.

Stimulation of Cl- uptake and morphological changes in gill mitochondria-rich cells in freshwater tilapia (Oreochromis mossambicus)

The purpose of the present article is to examine the relationships between ion uptakes and morphologies of gill mitochondria-rich (MR) cells in freshwater tilapia. Tilapia were acclimated to three different artificial freshwaters (high Na [10 mM], high Cl [7.5 mM]; high Na, low Cl [0.02-0.07 mM], and low Na [0.5 mM], low Cl) for 1 wk, and then morphological measurements of gill MR cells were made and ion influxes were determined. The number and the apical size of wavy-convex MR cells positively associated with the level of Cl(-) influx. Conversely, Na(+) influx showed no positive correlation with the morphologies of MR cells. The dominant MR cell type in tilapia gills changed from deep-hole to wavy-convex within 6 h after acute transfer from a high-Cl(-) to a low-Cl(-) environment. Deep-hole MR cells became dominant 24-96 h after acute transfer from a low-Cl(-) to a high-Cl(-) environment. We conclude that wavy-convex MR cells associate with Cl(-) uptake but not Na(+) uptake, and the rapid formation of wavy-convex MR cells reflects the timely stimulation of Cl(-) uptake to recover the homeostasis of internal Cl(-) levels on acute challenge with low environmental Cl(-).

The distribution of mitochondria-rich cells in the gills of air-breathing fishes

Respiration and ion regulation are the two principal functions of teleostean gills. Mainly found in the gill filaments of fish, mitochondria-rich cells (MRCs) proliferate to increase the ionoregulatory capacity of the gill in response to osmotic challenges. Gill lamellae consist mostly of pavement cells, which are the major site of gas exchange. Although lamellar MRCs have been reported in some fish species, there has been little discussion of which fish species are likely to have lamellar MRCs. In this study, we first compared the number of filament and lamellar MRCs in air-breathing and non-air-breathing fish species acclimated to freshwater and 5 g NaCl L(-1) conditions. An increase in filament MRCs was found in both air-breathing and non-air-breathing fish acclimated to freshwater. Lamellar MRCs were found only in air-breathing species, but the number of lamellar MRCs did not change significantly with water conditions, except in Periophthalmus cantonensis. Next, we surveyed the distribution of MRCs in the gills of 66 fish species (including 29 species from the previous literature) from 12 orders, 28 families, and 56 genera. Our hypothesis that lamellar MRCs are more likely to be found in air-breathing fishes was supported by a significant association between the presence of lamellar MRCs and the mode of breathing at three levels of systematic categories (species, genus, and family). Based on this integrative view of the multiple functions of fish gills, we should reexamine the role of MRCs in freshwater fish.

Sensing and transfer of respiratory gases at the fish gill

The gill is both a site of gas transfer and an important location of chemoreception or gas sensing in fish. While often considered separately, these two processes are clearly intricately related because the gases that are transferred between the ventilatory water and blood at the gill are simultaneously sensed by chemoreceptors on, and within, the gill. Modulation of chemoreceptor discharge in response to changes in O(2) and CO(2) levels, in turn, is believed to initiate a series of coordinated cardiorespiratory reflexes aimed at optimising branchial gas transfer. The past decade has yielded numerous advances in terms of our understanding of gas transfer and gas sensing at the fish gill, particularly concerning the transfer and sensing of carbon dioxide. In addition, recent research has moved from striving to construct a single model that covers all fish species, to recognition of the considerable inter-specific variation that exists with respect to the mechanics of gas transfer and the cardiorespiratory responses of fish to changes in O(2) and CO(2) levels. The following review attempts to integrate gas transfer and gas sensing at the fish gill by exploring recent advances in these areas.

Sensitivity of CO2 excretion to blood flow changes in trout is determined by carbonic anhydrase availability

The blood transit time through the gills of rainbow trout (Oncorhynchus mykiss) was modified by manipulation of cardiac output (Vb). The experiments tested the hypothesis that efficiency of CO2 excretion is sensitive to changes in blood flow owing to chemical equilibrium limitations. An extracorporeal blood shunt was used to continuously monitor blood gases in fish in which Vb was elevated (by 13.3 +/- 2.4 ml x min(-1) x kg(-1)) by intravascular saline injection or reduced (by 10.8 +/- 1.8 ml x min(-1) x kg(-1)) by removal of plasma. The arterial partial pressure of CO2 (Pa(CO2); an index of CO2 excretion efficiency) was increased with elevated Vb and was decreased with reduced Vb such that the changes in Pa(CO2) exhibited a significant positive sigmoidal relationship with the changes in Vb (r2 =0.75; P < 0.05). In contrast, there was no significant relationship between changes in the arterial partial pressure of O2 (Pa(O2); an index of O2 uptake efficiency) and changes in Vb (r2 = 0.07; P > 0.05). The intravenous administration of carbonic anhydrase (CA; 10 mg/kg) before vascular volume loading eliminated the increase in Pa(CO2) with increased Vb that was observed in control fish.

The CO2/pH ventilatory drive in fish

That ventilation in fish is driven by O2 has long been accepted. The O2 ventilatory drive reflects the much lower capacitance of water for O2 than for CO2, and is mediated by O2 receptors that are distributed throughout the gill arches and that monitor both internal and external O2 levels. In recent years, however, evidence has amassed in support of the existence of a ventilatory drive in fish that is keyed to CO2 and/or pH. While ventilatory responses to CO2/pH may be mediated in part by the O2 drive through CO2/pH-induced changes in blood O2 status, CO2/pH also appear to stimulate ventilation directly. The receptors involved in this pathway are as yet unknown, but the experimental evidence available to date supports the involvement of branchial CO2-sensitive chemoreceptors with an external orientation. Internally-oriented CO2-sensitive chemoreceptors may also be involved, although evidence on this point remains equivocal. In the present paper, the evidence for a CO2/pH-keyed ventilatory drive in fish will be reviewed.

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.

The effects of sediment-associated triorganotin compounds on the gills of the European flounder, Platichthys flesus (L.)

The effects of exposure to sediment-associated tri-n-butyltin chloride (TBT) and triphenyltin chloride (TPhT) were examined in the euryhaline European flounder, Platichthys flesus (L.). The effects were quantified by measuring the changes in sodium efflux, Na(+)/K(+)-ATPase activity and the numbers, areas and distribution of chloride cells in the gills of freshwater-adapted fish, following a rapid transfer to seawater. After transfer, the Na(+)/K(+)-ATPase activity and the sodium efflux significantly increased in both the TPhT and control groups but not in the TBT group. However, Na(+)/K(+)-ATPase activity and the sodium efflux in the TPhT group had returned to pre-salinity transfer levels by day 15 after the initial exposure to TPhT. Morphological changes in the numbers and areas of chloride cells, known to be associated with seawater adaptation, took place in the control group, i.e. there was a significant reduction in the number of lamellar chloride cells accompanied by an increase in the number of interlamellar chloride cells. There was a reduction in the numbers of lamellar chloride cells in the TBT-exposed group following transfer to seawater but the mean number was significantly higher than the control group by the end of the experiment. In the TPhT-exposed group, the reduction was not significantly different to that seen in the control group. By the end of the experiment, both organotin-exposed groups had significantly lower mean numbers of interlamellar chloride cells than the control group. Before transfer to seawater, the mean areas of lamellar and interlamellar chloride cells of all three groups were not significantly different. On transfer, the mean areas of lamellar chloride cells in the control group became significantly smaller than the mean areas of the organotin groups. There was no significant difference in the mean areas of interlamellar chloride cells in the control and TBT groups between the start and finish of the experiment but there was a significant increase in the mean area of TPhT-treated animals at the end of the experiment when compared to the control group. The results presented in this study lead to the conclusion that tri-n-butyltin chloride and triphenyltin chloride in sediments are capable of significantly disrupting both the physiological as well as morphological components of ionic regulatory functions of an estuarine fish, at concentrations currently found in estuarine sediments.

Distinct seawater and freshwater types of chloride cells in killifish, Fundulus heteroclitus

Physiological and morphological differences between killifish adapted to seawater (SW) and fresh water (FW) were examined with special reference to chloride cells. There was no difference in plasma osmolality between SW- and FW-adapted fish, reflecting their euryhalinity. A rich population of chloride cells was detected in whole-mount preparations of the gills and opercular membrane from SW- and FW-adapted fish. There was no difference between SW- and FW-adapted fish in gill Na+,K+-ATPase activity or oxygen-consumption rates. The gill chloride cells were located mostly in a flat region of the afferent-vascular edge of the filaments. In both tissues, the cells were larger in FW- than in SW-adapted fish. The apical membrane of chloride cells was invaginated to form a pit in SW-adapted fish, whereas it was flat or showed projections and was equipped with microvilli in FW-adapted fish. Chloride cells often interdigitated with neighboring accessory cells in SW-adapted fish, forming multicellular complexes. In FW-adapted fish, on the other hand, a pair of chloride cells that were similar in size was occasionally associated to form "twin cells." Thus, distinct SW and FW types of chloride cells were defined. Our findings suggest that SW- and FW-type chloride cells are equally active in the two environments, but exhibit different ion-transporting functions.

Morphology and function of gill mitochondria-rich cells in fish acclimated to different environments

The objective of this study is to test the hypothesis that morphologically different mitochondria-rich (MR) cells may be responsible for the uptake of different ions in freshwater-adapted fish. Tilapia (Oreochromis mossambicus) were acclimated to high-Ca, mid-Ca, low-Ca, and low-NaCl artificial freshwater, respectively, for 2 wk. Cell densities of wavy-convex, shallow-basin, and deep-hole types of gill MR cells as well as whole-body Ca(2+), Na(+), and Cl(-) influxes were measured. Low-Ca fish developed more shallow-basin MR cells in the gills and a higher Ca(2+) influx than those acclimated to other media. However, fish acclimated to low-NaCl artificial freshwater predominantly developed wavy-convex cells, and this was accompanied by the highest Na(+) and Cl(-) influxes. Relative abundance of shallow-basin and wavy-convex MR cells appear to be associated with changes in Ca(2+) and Na(+)/Cl(-) influxes, suggesting that shallow-basin and wavy-convex MR cells are mainly responsible for the uptake of Ca(2+) and Na(+)/Cl(-), respectively.

Resting metabolic rate, critical swimming speed, and routine activity of the euryhaline cyprinodontid, Aphanius dispar, acclimated to a wide range of salinities

Specimens of the euryhaline cyprinodontid fish, Aphanius dispar, collected in salt ponds, were acclimated to salinities of <1 (freshwater), 35 (seawater), 70, 105, and 140 ppt for 4 wk before measurement of oxygen consumption, critical swimming speed, and routine activity level. Oxygen consumption was similar in <1, 35, and 70 ppt (0.18+/-0.07, 0.17+/-0.06, and 0.16+/-0.04 mL h(-1) g(-1), respectively [mean+/-SD]) but decreased in 105 and 140 ppt (0. 12+/-0.02 and 0.09+/-0.2 mL h(-1) g(-1), respectively). Critical swimming speed and routine activity levels showed the same trend. These results suggest a general decrease in physiological function of A. dispar at extreme salinities.

Apparent diffusion limitations for CO(2) excretion in rainbow trout are relieved by injections of carbonic anhydrase

Experiments were performed in vivo to elucidate the underlying mechanism(s) of apparent diffusion limitations for CO(2) excretion in rainbow trout (Oncorhynchus mykiss). Ligation of two gill arches and the associated expected reduction in gill surface area of 30% caused pronounced respiratory acidosis as indicated by elevated arterial blood P(CO(2)) (Pa(CO(2))) and reduced arterial blood pH. Under conditions of normoxia, arterial blood P(O(2)) (Pa(O(2))) was not significantly (statistically) reduced. However, during hypoxia (water P(O(2))=70-80 mmHg), the apparent trend for reduced Pa(O(2)) values became statistically significant in fish with 15% surface area reduction. To determine whether the elevated Pa(CO(2)) in fish with reduced surface area (30%) reflected true diffusion limitations or chemical equilibrium limitations imposed by the relatively slow rate of red blood cell Cl(-)/HCO(3)(-) exchange, fish were injected with carbonic anhydrase (CA) to permit catalysis of HCO(3)(-) dehydration within the plasma. Injection of CA caused a lowering of Pa(CO(2)) by 0.87+/-0.32 mmHg within 120 min and thus essentially eliminated the increase in Pa(CO(2)) (1.04+/-0.33 mmHg) that was caused by the reduction in surface area. These results clearly demonstrate that the elevation in Pa(CO(2)) evoked by gill surface area reduction is a consequence of chemical equilibrium limitations rather than true diffusion limitations, per se.

The effect of highly alkaline water (pH 9.5) on the morphology and morphometry of chloride cells and pavement cells in the gills of the freshwater rainbow trout: relationship to ionic transport and ammonia excretion

Exposure of rainbow trout (Oncorhynchus mykiss) to alkaline water (pH 9.5) impairs ammonia excretion (JAmm) and gill-mediated ion-exchange processes, as characterized by decreased Cl- (JC1in) and Na+ influx (JNain) across the gill. Scanning electron microscopy suggested that the depression of JC1in was concomitant with an early decrease in the population of the most active chloride cells (CCs), partly compensated for by an increasing number of immature CCs. However, within 72 h after the onset of exposure to alkaline water, there was a 2-fold increase in the fractional apical surface area of CCs that paralleled complete recovery of the maximal Cl- influx rate (JC1max). These results suggest that recovery of JC1max was associated with greater CC surface area, resulting in more transport sites on the gill epithelium. Morphometric analysis of the outermost layer of pavement cells on the lamellar epithelium showed a greater density of microvilli during exposure to alkaline water, which may have contributed to partial restoration of the number of Na+ transport sites (JNamax). Finally, the blood-to-water gill-diffusion distance decreased by 27% after 72 h at pH 9.5, and likely contributed to progressive restoration of ammonia excretion in alkaline water.

Ultrastructural changes in fish gills as biomarker to assess small stream pollution

In order to verify the principal suitability of gill ultrastructure as a biomarker, semi-field studies with two endigoneous fish, trout (Salmo trutta f. fario) and loach (Barbatula barbatula), were performed. The fish were exposed in flow-through systems to one heavily polluted (Körsch) and one lightly polluted small stream (Krähenbach) in South-West Germany. Ultrastructural responses in gills were correlated with limnological and chemical data recorded over a 2 year period in each stream. After 8 weeks of exposure to the heavily polluted stream, fish showed ultrastructural changes in the gills, such as cell proliferation, dilation of the endoplasmic reticulum, hyperplasia, hypersecretion, and epithelial lifting in chloride, epithelial, and mucus cells. The results of the study demonstrate that ultrastructural reactions in the gills of fish kept under semi-field conditions are potentially useful biomarkers indicating small stream pollution.

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.

The chloride cell: structure and function in the gills of freshwater fishes

This review focuses on the structure and function of the branchial chloride cell in freshwater fishes. The mitochondria-rich chloride cell is believed to be the principal site of trans-epithelial Ca2+ and Cl- influxes. Though currently debated, there is accruing evidence that the pavement cell is the site of Na+ uptake via channels linked electrically to an apical membrane vacuolar H(+)-ATPase (proton pump). Chloride cells perform an integral role in acid-base regulation. During conditions of alkalosis, the surface area of exposed chloride cells is increased, which serves to enhance base equivalent excretion as the rate of Cl-/HCO3- exchange is increased. Conversely, during acidosis, the chloride cell surface area is diminished by an expansion of the adjacent pavement cells. This response reduces the number of functional Cl-/HCO3- exchangers. Under certain conditions that challenge ion regulation, chloride cells proliferate on the lamellae. This response, while optimizing the Ca2+ and Cl- transport capacity of the gill, causes a thickening of the blood-to-water diffusion barrier and thus impedes respiratory gas transfer.

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.