7 Metabolism of the Fish Gill

Tea polyphenols protect against Flavobacterium columnare-induced gill injury via suppression of oxidative stress, inflammation, and apoptosis in grass carp

Flavobacterium columnare (F. columnare) is one of the deadliest fish pathogens causing bacterial gill rot disease in various freshwater fish species globally. Tea polyphenols (TPs) are an inexpensive product extracted from tea that have received much attention as a feed additive in aquaculture. The current study was designed to investigate the underlying mechanisms and protective effects of dietary TPs against F. columnare-induced gill injury via suppression of oxidative stress, apoptosis, and inflammation in grass carp. TPs were not supplemented to the diet (control) and were supplemented at 40, 80, 120, 160 or 200 mg/kg diet. The feeding experiment was carried out for 60 days, followed by a 3-Day F. columnare challenge test. The results showed that 120 mg/kg TPs in the diet exerted the following five protective effects in fish gill: (1) control gill-rot disease and improved histopathology, (2) inhibit excessive apoptosis, (3) enhance the activity of antioxidant enzymes and upregulate related gene expression via the Nrf2/Keap1 pathway, (4) increase the activity of immune enzymes, And (5) mediate inflammatory cytokine gene expression via the JAK/STAT3 pathway. Taken together, dietary supplementation with TPs is a compelling approach to protect the gill function of fish against F. columnare.

Salinity-induced transcriptome profiles in marine and freshwater threespine stickleback after an abrupt 6-hour exposure

Saltwater and freshwater environments have opposing physiological challenges, yet, there are fish species that are able to enter both habitats during short time spans, and as individuals they must therefore adjust quickly to osmoregulatory contrasts. In this study, we conducted an experiment to test for plastic responses to abrupt salinity changes in two populations of threespine stickleback, Gasterosteus aculeatus, representing two ecotypes (freshwater and ancestral saltwater). We exposed both ecotypes to abrupt native (control treatment) and non-native salinities (0‰ and 30‰) and sampled gill tissue for transcriptomic analyses after 6 h of exposure. To investigate genomic responses to salinity, we analyzed four different comparisons; one for each ecotype (in their control and exposure salinity; (1) and (2), one between ecotypes in their control salinity (3), and the fourth comparison included all transcripts identified in (3) that did not show any expressional changes within ecotype in either the control or the exposed salinity (4)). Abrupt salinity transfer affected the expression of 10 and 1530 transcripts for the saltwater and freshwater ecotype, respectively, and 1314 were differentially expressed between the controls, including 502 that were not affected by salinity within ecotype (fixed expression). In total, these results indicate that factors other than genomic expressional plasticity are important for osmoregulation in stickleback, due to the need for opposite physiological pathways to survive the abrupt change in salinity.

Differential Expression of miRNAs and Their Predicted Target Genes Indicates That Gene Expression in Atlantic Salmon Gill Is Post-Transcriptionally Regulated by miRNAs in the Parr-Smolt Transformation and Adaptation to Sea Water

Smoltification (parr-smolt transformation) is a complex developmental process consisting of developmental changes that lead to remodeling of the Atlantic salmon gill. Here, the expression changes of miRNAs and mRNAs were studied by small-RNA sequencing and microarray analysis, respectively, to identify miRNAs and their predicted targets associated with smoltification and subsequent sea water adaptation (SWA). In total, 18 guide miRNAs were identified as differentially expressed (gDE miRNAs). Hierarchical clustering analysis of expression changes divided these into one cluster of 13 gDE miRNAs with decreasing expression during smoltification and SWA that included the miRNA-146, miRNA-30 and miRNA-7132 families. Another smaller cluster that showed increasing expression consisted of miR-101a-3p, miR-193b-5p, miR-499a-5p, miR-727a-3p and miR-8159-5p. The gDE miRNAs were predicted to target 747 of the genes (DE mRNAs), showing expression changes in the microarray analysis. The predicted targets included genes encoding NKA-subunits, aquaporin-subunits, cystic fibrosis transmembrane conductance regulator and the solute carrier family. Furthermore, the predicted target genes were enriched in biological processes associated with smoltification and SWA (e.g., immune system, reactive oxygen species, stress response and extracellular matrix organization). Collectively, the results indicate that remodeling of the gill involves the post-transcriptional regulation of gene expression by the characterized gDE miRNAs.

The fasted and post-prandial physiological responses of the Patagonian blennie Eleginops maclovinus

Eleginops maclovinus is a native species with potential for Chilean aquaculture. Understanding the variations between the post-prandial and fasted metabolic responses can contribute to improving the aquaculture of this species. This study aimed to characterize variations in intermediate metabolism during the course of the day in the liver, serum, and gills of fed and unfed fish. For this, 72 fish were assigned to two experimental groups, "fed" and "fasted". The first group was fed "ad libitum" at 8.30, while the fasted group was not fed for 24 h. Samples were taken from both groups at 9:00, and every 2 h: 11:00, 13:00, 15:00, 17:00, and 19:00. In the fed group, food spent a long time in the gastrointestinal tract, with a large increase in stomach size and without evidence of complete emptying of the stomach at 19:00 (10.5 h post-feeding). In serum, the levels of amino acids, glucose, and triglycerides presented significant differences with peak levels at different times of day in the fed group. The cortisol in the fasted group presented a diurnal pattern with high levels during the morning and very low levels after 13:00, while in the fed group, the high cortisol variability did not allow a clear pattern to be established. In the liver, the effect of time on the enzymatic activity of the intermediary metabolism was greater compared to the effect of feeding. In the liver, enzyme activity decreased at later hours of the day, while glycogen levels increased at later hours of the day in both groups: but its levels were higher in the fed group. In gills, as well as in the liver, time had a greater effect than feeding on intermediate metabolism, since feeding only had a significant effect on the levels of hexokinase, lactate, and amino acids, suggesting an effect on carbohydrate metabolism. Meanwhile, time significantly affected the levels of Na⁺, K⁺-ATPase, glutamate dehydrogenase, aspartate aminotransferase, amino acids, and proteins, suggesting an effect on amino acid metabolism. In conclusion, the intermediate metabolism of E. maclovinus presents variations according to the time of day, with an increased metabolism during the morning and decreased metabolism as the day progresses, especially at the hepatic level. The gill tissue, despite not being a metabolic organ, presents feeding-dependent variations in its metabolism. Additional studies will be required to corroborate if coordinating a feeding strategy during the first hours of the day when metabolism is greater would improve the growth of E. maclovinus.

Environmental Salinity Modifies Mucus Exudation and Energy Use in European Sea Bass Juveniles

The European sea bass (Dicentrarchus labrax) is a euryhaline marine teleost that can often be found in brackish and freshwater or even in hypersaline environments. Here, we exposed sea bass juveniles to sustained salinity challenges for 15 days, simulating one hypoosmotic (3‰), one isosmotic (12‰) and one hyperosmotic (50‰) environment, in addition to control (35‰). We analyzed parameters of skin mucus exudation and mucus biomarkers, as a minimally invasive tool, and plasma biomarkers. Additionally, Na⁺/K⁺-ATPase activity was measured, as well as the gill mucous cell distribution, type and shape. The volume of exuded mucus increased significantly under all the salinity challenges, increasing by 130% at 50‰ condition. Significantly greater amounts of soluble protein (3.9 ± 0.6 mg at 50‰ vs. 1.1 ± 0.2 mg at 35‰, p < 0.05) and lactate (4.0 ± 1.0 µg at 50‰ vs. 1.2 ± 0.3 µg at 35‰, p < 0.05) were released, with clear energy expenditure. Gill ATPase activity was significantly higher at the extreme salinities, and the gill mucous cell distribution was rearranged, with more acid and neutral mucin mucous cells at 50‰. Skin mucus osmolality suggested an osmoregulatory function as an ion-trap layer in hypoosmotic conditions, retaining osmosis-related ions. Overall, when sea bass cope with different salinities, the hyperosmotic condition (50‰) demanded more energy than the extreme hypoosmotic condition.

Transcriptional analysis reveals physiological response to acute acidification stress of barramundi Lates calcarifer (Bloch) in coastal areas

To understand the physiological response of estuarine fish to acidification, barramundi (Lates calcarifer) juveniles were exposed to acidified seawater in experimental conditions. The molecular response of barramundi to acidification stress was assessed by RNA-seq analysis. A total of 2188 genes were identified as differential expression genes. The gene ontology classification system and Kyoto Encyclopedia of Genes and Genomes database analysis showed that acidification caused differential expressions of genes and pathways in the gills of barramundi. Acidification had a great influence on the signal transduction pathway in cell process. Furthermore, we detected that numerous unigenes involved in the pathways associated with lipid metabolism, carbohydrate metabolism, amino acid metabolism, glycan biosynthesis and metabolism specific and non-specific immunity were changed. This study indicates that the physiological responses in barramundi especially the immune system and energy allocation correspond to the variation of environmental pH. This study reveals the necessity for assessment of the potential of estuarine fishes to cope with acidification of the environment and the need to develop strategies for fish conservation in coastal areas.

Proteome changes in muscles, ganglia, and gills in Corbicula fluminea clams exposed to crude oil: Relationship with behavioural disturbances

The use of online remote control for 24/7 behavioural monitoring can play a key role in estimating the environmental status of aquatic ecosystems. Recording the valve activity of bivalve molluscs is a relevant approach in this context. However, a clear understanding of the underlying disturbances associated with behaviour is a key step. In this work, we studied freshwater Asian clams after exposure to crude oil (measured concentration, 167 ± 28 μg·L^-1) for three days in a semi-natural environment using outdoor artificial streams. Three complementary approaches to assess and explore disturbances were used: behaviour by high frequency non-invasive (HFNI) valvometry, tissue contamination with polycyclic aromatic hydrocarbons (PAH), and proteomic analysis. Two tissues were targeted: the pool adductor muscles - retractor pedal muscle - cerebral and visceral ganglia, which is the effector of any valve movement and the gills, which are on the frontline during contamination. The behavioural response was marked by an increase in valve closure-duration, a decrease in valve opening-amplitude and an increase in valve agitation index during opening periods. There was no significant PAH accumulation in the muscle plus nervous ganglia pool, contrary to the situation in the gills, although the latter remained in the low range of data available in literature. Major proteomic changes included (i) a slowdown in metabolic and/or cellular processes in muscles plus ganglia pool associated with minor toxicological effect and (ii) an increase of metabolic and/or cellular processes in gills associated with a greater toxicological effect. The nature of the proteomic changes is discussed in terms of unequal PAH distribution and allows to propose a set of explanatory mechanisms to associate behaviour to underlying physiological changes following oil exposure. First, the first tissues facing contaminated water are the inhalant siphon, the mantle edge and the gills. The routine nervous activity in the visceral ganglia should be modified by nervous information originating from these tissues. Second, the nervous activity in the visceral ganglia could be modified by its own specific contamination. Third, a decrease in nervous activity of the cerebral ganglia close to the mouth, including some kind of narcosis, could contribute to a decrease in visceral ganglia activity via a decrease or blockage of the downward neuromodulation by the cerebro-visceral connective. This whole set of events can explain the decrease of metabolic activity in the adductor muscles, contribute to initiate the catch mechanism and then deeply modify the valve behaviour.

Haematological, blood biochemical and immunological responses to gradual acclimation to low-salinity water in Walton’s mudskipper Periophthalmus waltoni Koumans, 1941 (Perciformes: Gobiidae)

The present study investigates and reports the effects of gradual acclimation to low salinity water on some haematological, biochemical and immunological responses in Walton’s mudskipper, Periophthalmus waltoni. For this purpose, mudskippers caught from Persian Gulf coastal area (Bandar Khamir, Hormozgan Province, Iran) were maintained in laboratory aquaria with half seawater (50% SW, 17 ppt) and fed daily with frozen blood worms (Chironomus spp.) for one month prior to the start of experiments. After acclimation, groups of 18 individuals were either directly transferred to 50% SW (control), or acclimated to low salinity water during two sub-periods. In the first sub-period, fish were exposed to low salinity water namely to a gradual water salinity decrease of 1 ppt per day (during 17 days) until the final salinity of 0.4 ppt was reached. Afterwards, fish continued to maintain in this point of salinity (0.4 ppt), for further 15 days until day 32 (second sub-period). Fish were sampled on day 0, 17 and 32. Statistical analysis showed a significant influence of reduced salinity on erythrocytes, haemoglobin, haematocrit, leukocytes, lymphocytes, neutrophils, monocytes and on all biochemical and immunological parameters tested on day 17. However, these indices returned to the control level on day 32. Based on results, the extremely euryhaline p. waltoni can be acclimated to freshwater medium without showing any health disturbance if a gradual decrease in salinity is carried out for a long period of time.

Untargeted metabolic profiling reveals distinct patterns of thermal sensitivity in two related notothenioids

Antarctic marine ectothermal animals may be affected more than temperate species by rising temperatures due to ongoing climate change. Their specialisation on stable cold temperatures makes them vulnerable to even small degrees of warming. Thus, addressing the impacts of warming on Antarctic organisms and identifying their potentially limited capacities to respond is of interest. The objective of the study was to determine changes in metabolite profiles related to temperature acclimation. In a long-term experiment adult fish of two Antarctic sister species Notothenia rossii and Notothenia coriiceps were acclimated to 0°C and 5°C for three months. Impacts and indicators of acclimation at the cellular level were determined from metabolite profiles quantified in gill tissue extracts using nuclear magnetic resonance (NMR) spectroscopy. Furthermore, the metabolite profiles of the two con-generic species were compared. NMR spectroscopy identified 37 metabolites that were present in each sample, but varied in their absolute concentration between species and between treatments. A decrease in amino acid levels indicated an increased amino acid catabolism after incubation to 5°C. In addition, long term warming initiated shifts in organic osmolyte concentrations and modified membrane structure observed by altered levels of phospholipid compounds. Differences in the metabolite profile between the two notothenioid species can be related to their divergent lifestyles, especially their different rates of motor activity. Increased levels of the Krebs cycle intermediate succinate and a higher reduction of amino acid concentrations in warm-acclimated N. rossii showed that N. rossii is more affected by warming than N. coriiceps.

Evaluation of potential candidate genes involved in salinity tolerance in striped catfish (Pangasianodon hypophthalmus) using an RNA-Seq approach

Increasing salinity levels in freshwater and coastal environments caused by sea level rise linked to climate change is now recognized to be a major factor that can impact fish growth negatively, especially for freshwater teleost species. Striped catfish (Pangasianodon hypophthalmus) is an important freshwater teleost that is now widely farmed across the Mekong River Delta in Vietnam. Understanding the basis for tolerance and adaptation to raised environmental salinity conditions can assist the regional culture industry to mitigate predicted impacts of climate change across this region. Attempt of next generation sequencing using the ion proton platform results in more than 174 million raw reads from three tissue libraries (gill, kidney and intestine). Reads were filtered and de novo assembled using a variety of assemblers and then clustered together to generate a combined reference transcriptome. Downstream analysis resulted in a final reference transcriptome that contained 60,585 transcripts with an N50 of 683 bp. This resource was further annotated using a variety of bioinformatics databases, followed by differential gene expression analysis that resulted in 3062 transcripts that were differentially expressed in catfish samples raised under two experimental conditions (0 and 15 ppt). A number of transcripts with a potential role in salinity tolerance were then classified into six different functional gene categories based on their gene ontology assignments. These included; energy metabolism, ion transportation, detoxification, signal transduction, structural organization and detoxification. Finally, we combined the data on functional salinity tolerance genes into a hypothetical schematic model that attempted to describe potential relationships and interactions among target genes to explain the molecular pathways that control adaptive salinity responses in P. hypophthalmus. Our results indicate that P. hypophthalmus exhibit predictable plastic regulatory responses to elevated salinity by means of characteristic gene expression patterns, providing numerous candidate genes for future investigations.

Vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata): an osmoregulatory story

To investigate the physiological roles of arginine vasotocin (AVT) and isotocin (IT) in osmoregulatory process in gilthead sea bream (Sparus aurata), a time course study (0, 12h, and 1, 3, 7 and 14 days) has been performed in specimens submitted to hypoosmotic (from 40‰ salinity to 5‰ salinity) or hyperosmotic (from 40‰ salinity to 55‰ salinity) challenges. Plasma and liver osmoregulatory and metabolic parameters, as well as AVT and IT pituitary contents were determined concomitantly with hypothalamic pro-vasotocin (pro-VT) and pro-isotocin (pro-IT) mRNA expression levels. Previously, sequences coding for pro-VT and pro-IT cDNAs were cloned. Two osmoregulatory periods related to plasma osmolality and metabolic parameter variations could be distinguished: i) an adaptative period, from 12h to 3 days after transfer, and ii) a chronic regulatory period, starting at day 3 after transfer. Higher values in hypothalamic pro-VT and pro-IT mRNA expression as well as in pituitary AVT and IT storage levels in both hypo- and/or hyper-osmotic transfers have been distinguished. These increase correlated with changes in plasma cortisol levels, suggesting an interaction between this hormone and pro-VT expression. Furthermore, pro-IT expression enhancement also suggests a role of the isotocinergic system as a modulator in the acute stress response induced by hyper-osmotic challenge in S. aurata.

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

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

Hyperosmotic shock adaptation by cortisol involves upregulation of branchial osmotic stress transcription factor 1 gene expression in Mozambique Tilapia

The Mozambique tilapia (Oreochromis mossambicus) is a euryhaline species that does not survive direct seawater exposure. Cortisol is involved in re-establishing electrolyte homeostasis in seawater and is thought to play a role in allowing tilapia to cope with abrupt seawater exposure, but the mechanism(s) are far from clear. Recently, osmotic stress transcription factor 1 (OSTF1) was identified as a key signaling molecule involved in hyperosmotic stress adaptation in tilapia. Consequently, we tested the hypothesis that upregulation of OSTF1 expression by cortisol is a key response for hyperosmotic stress adaptation in tilapia. Fish were exposed to different salinities over a 24h period, while a major electrolyte disturbance and mortality was observed only with full-strength seawater exposure. Therefore, we administered cocoa butter implants of cortisol (50mg/kg) intraperitoneally to tilapia maintained in fresh water and after three days exposed these fish to full-strength seawater. There was 50% mortality in the control fish upon seawater exposure, but this was abolished by cortisol treatment. Abrupt seawater exposure did not affect plasma cortisol levels, while, as expected, exogenous administration of this steroid elevated plasma cortisol levels both in fresh water and seawater. Cortisol treatment significantly induced OSTF1 gene expression in fresh water tilapia, and also enhanced further the seawater-induced OSTF1 mRNA abundance. Plasma osmolality decreased, while gill Na(+)/K(+)-ATPase activity was suppressed in the cortisol group in seawater compared to the sham group. This corresponded with a significant reduction in gill ionocyte size and Na(+)/K(+)-ATPase activity and protein expression after seawater exposure. Cortisol did not modify liver metabolism, but significantly suppressed gill metabolic capacity in seawater. Overall, cortisol adapts tilapia to a hyperosmotic shock associated with abrupt seawater exposure. This involves upregulation of OSTF1 gene expression and a concomitant suppression of branchial metabolism in tilapia.

Some insights into energy metabolism for osmoregulation in fish

A sufficient and timely energy supply is a prerequisite for the operation of iono- and osmoregulatory mechanisms in fish. Measurements of whole-fish or isolated-gill (or other organs) oxygen consumption have demonstrated regulation of the energy supply during acclimation to different osmotic environments, and such regulation is dependent on species, the situation of acclimation or acclimatization, and life habits. Carbohydrate metabolism appears to play a major role in the energy supply for iono- and osmoregulation, and the liver is the major source supplying carbohydrate metabolites to osmoregulatory organs. Compared with carbohydrates, the roles of lipids and proteins remain largely unclear. Energy metabolite translocation was recently found to occur between fish gill ionocytes and neighboring glycogen-rich (GR) cells, indicating the physiological significance of a local energy supply for gill ion regulatory mechanisms. Spatial and temporal relationships between the liver and other osmoregulatory and non-osmoregulatory organs in partitioning the energy supply for ion regulatory mechanisms during salinity challenges were also proposed. A novel glucose transporter was found to specifically be expressed and function in gill ionocytes, providing the first cue for investigating energy translocation among gill cells. Advanced molecular physiological approaches can be used to examine energy metabolism relevant to a particular cell type (e.g., gill ionocytes), and functional genomics may also provide another powerful approach to explore new metabolic pathways related to fish ion regulation.

Intermediary metabolism of Arctic char Salvelinus alpinus during short-term salinity exposure

The migration of Arctic char Salvelinus alpinus from freshwater to seawater requires a substantial reorganization of the osmoregulatory tissues to regulate plasma ion levels. These modifications have an inherent metabolic cost, which must be met through the upregulation of intermediary metabolism. Arctic char intermediary metabolism was monitored during the initial 96 h of seawater acclimation through measurement of key enzymes in gill, liver, red and white muscle as well as tissue and blood free amino acid (FAA) levels, and plasma glucose and non-esterified fatty acid content. In general, seawater exposure stimulated large changes in amino acid metabolism, but no change in lipid or carbohydrate metabolism. White muscle FAA content increased significantly following seawater exposure, with levels of essential FAAs doubling after 96 h. Similar increases were seen in the plasma, suggesting a rapid mobilization of FAAs to the circulation. These changes were accompanied by significant increases in the activities of enzymes involved in amino acid metabolism in the gill, liver, red and white muscle, suggesting seawater-acclimated fish have an enhanced capacity for energy production from amino acids. Increased energy requirements were evident in the gill of seawater-acclimated char, as citrate synthase activity increased significantly. The results of this study suggest a rapid upregulation of amino acid metabolism may be critical for the successful acclimation of Arctic char to seawater.

Daily changes in parameters of energy metabolism in liver, white muscle, and gills of rainbow trout: Dependence on feeding

We assessed the daily patterns of parameters involved in energy metabolism in liver, white muscle, and gills of rainbow trout. Where daily rhythms were found, we analyzed the potential influence of feeding. Immature rainbow trout were randomly distributed in 3 groups: fish fed for 7 days, fish fasted for 7 days, and fish fasted for 7 days and refed for 4 days. On sampling day, fish of fed and refed groups were fed at 11.00 h, and all fish were sampled from each treatment group using the following time schedule: 14.00, 18.00, 21.00, 00.00, 04.00, 07.00, 10.00 and 14.00 h. The results obtained from metabolic parameters can be grouped into four different categories, such as i) those displaying no daily changes in any group assessed in liver (acetoacetate and lactate levels), white muscle (protein levels, and low Km (glucose) hexokinase (HK) and HK-IV activities) and gills (protein levels), ii) those displaying no 24 h changes in fed fish but in refed or fasted fish in liver (glucose, glycogen, amino acid and protein levels, and HK-IV activity), white muscle (glycogen and amino acid levels) and gills (glucose levels), iii) those displaying 24 h changes that were apparently dependent on feeding since they disappear in fasted fish in liver (Low Km (glucose) HK, lactate dehydrogenase (LDH-O), glucose 6-phosphatase (G6Pase), fructose 1,6-bisphosphatase (FBPase) , alpha-glycerophosphate dehydrogenase (G3PDH), glutamate dehydrogenase (GDH) and aspartate aminotransferase (Asp-AT) activities), white muscle (glucose levels, and pyruvate kinase (PK), LDH-O, G3PDH and Asp-AT activities) and gills (glycogen and lactate levels, and Low Km (glucose) HK, HK-IV, LDH-O and Asp-AT activities), and iv) those parameters displaying 24 h changes apparently not dependent on feeding in liver (lactate levels and PK activity) and gills (amino acid levels, and PK and GDH activities). In general, most 24 h changes observed were dependent on feeding and can be also related to daily changes in activity.

Tissue-specific modulation of glucocorticoid receptor expression in response to salinity acclimation in rainbow trout

While studies clearly point to a role for cortisol signaling in seawater adaptation, very little is known about salinity impact on glucocorticoid receptor (GR) expression in fish. To this end, we investigated the temporal GR expression in the gill and liver of rainbow trout (Oncorhynchus mykiss) to salinity exposure. Trout were subjected to gradual salinity increases (11 ppt for 1 d, 17 ppt for 2 d and 23 ppt for 2 d) over a five day period. Gill Na(+), K(+)-ATPase alpha-subunit mRNA showed a transient elevation with salinity exposure, while gill cystic fibrosis transmembrane conductance regulator mRNA was not significantly affected by salinity. Liver PEPCK transcript levels showed a transient increase at day 1, but not at day 3 or day 5 of salinity exposure, while the activity of this enzyme was significantly depressed at all time points. Liver glycogen content was also significantly reduced by salinity exposure compared to the freshwater group. Gill GR transcript levels were 3-fold greater upon salinity exposure and this level was maintained over the 5 day period, while gill GR protein content remained unchanged except for a significant drop at day 1 of salinity exposure. Liver GR transcript levels showed no significant change with salinity exposure, while GR protein content was transiently elevated at day 3, but not at day 1 or day 5 of salinity exposure. The tissue-specific GR transcript response in the gill leads us to hypothesize a role for osmosensory signal transduction pathway in the regulation of GR expression in fish. Collectively, salinity exposure modulates GR expression and glucocorticoid signaling in rainbow trout.

Time course of osmoregulatory and metabolic changes during osmotic acclimation in Sparus auratus

Changes in different osmoregulatory and metabolic parameters over time were assessed in gills, kidney, liver and brain of gilthead sea bream Sparus auratus transferred either from seawater (SW, 38 p.p.t.) to hypersaline water (HSW, 55 p.p.t.) or from SW to low salinity water (LSW, 6 p.p.t.) for 14 days. Changes displayed by osmoregulatory parameters revealed two stages during hyperosmotic and hypo-osmotic acclimation: (i) an adaptive period during the first days of acclimation (1-3 days), with important changes in these parameters, and (ii) a chronic regulatory period (after 3 days of transfer) where osmotic parameters reached homeostasis. From a metabolic point of view, two clear phases can also be distinguished during acclimation to hyperosmotic or hypo-osmotic conditions. The first one coincides with the adaptive period and is characterized by enhanced levels of plasma metabolites (glucose, lactate, triglycerides and protein), and use of these metabolites by different tissues in processes directly or indirectly involved in osmoregulatory work. The second stage coincides with the chronic regulatory period observed for the osmoregulatory parameters and is metabolically characterized in HSW-transferred fish by lower energy expenditure and a readjustment of metabolic parameters to levels returning to normality, indicative of reduced osmoregulatory work in this stage. In LSW-transferred fish, major changes in the second stage include: (i) decreased glycolytic potential, capacity for exporting glucose and potential for amino acid catabolism in liver; (ii) enhanced use of exogenous glucose through glycolysis, pentose phosphate and glycogenesis in gills; (iii) increased glycolytic potential in kidney; and (iv) increased glycogenolytic potential and capacity for use of exogenous glucose in brain.

Copper impact on heat shock protein 70 expression and apoptosis in rainbow trout hepatocytes

The mechanism underlying copper hepatotoxicity was investigated in primary cultures of rainbow trout hepatocytes maintained in Leibovitz-15 media. CuSO4 treatment (0, 25, 50, 100 and 200 microM) resulted in a dose-dependent elevation in heat shock protein 70 (hsp70) expression at 24 and 48 h post-exposure. There was no effect of copper (200 microM CuSO4) on hepatotoxicity at 24 h, whereas longer exposures (48 h) resulted in increased lactate dehydrogenase (LDH) leakage and apoptosis, demonstrated by fluorescence nuclear staining and DNA fragmentation. Vitamin C (1 mM), a free radical scavenger, inhibited this copper-induced apoptosis implying a role for reactive oxygen species in copper toxicity. However, no parallel inhibition of either LDH leakage or hsp70 protein expression was observed with vitamin C suggesting that at least two independent mechanisms are involved in the cellular response to copper. Also, copper exposed (24 h) cells were unable to mount an hsp70 response to a standardized heat shock (+15 degrees C for 1 h), even in the presence of vitamin C. Together, these results suggest that hepatotoxicity of copper includes impairment of hsp70 response to subsequent stressors and/or signals, which is crucial for protecting cells from proteotoxicity.

Energy cost of NaCl transport in isolated gills of cutthroat trout

Few studies have made direct estimates of the energy required for ion transport in gills of freshwater (FW) and seawater (SW) fish. Oxygen consumption was measured in excised gill tissue of FW-adapted cutthroat trout (Oncorhynchus clarki clarki) to estimate the energy cost of NaCl transport in that osmoregulatory organ. Ouabain (0.5 mM) and bafilomycin A1 (1 microM) were used to inhibit the Na+-K+ and H+ pumps, respectively. Both inhibitors significantly decreased gill tissue oxygen consumption, accounting for 37% of total tissue respiration. On a whole mass basis, the cost of NaCl uptake in the FW trout gill was estimated to be 1.8% of whole animal oxygen uptake. An isolated, saline-perfused gill arch preparation was also used to compare gill energetics in FW- and SW-adapted trout. The oxygen consumption of FW gills was significantly (33%) higher than SW gills. On a whole animal basis, total gill oxygen consumption in FW and SW trout accounted for 3.9 and 2.4% of resting metabolic rate, respectively. The results of both experiments suggest that the energy cost of NaCl transport in FW and SW trout gills represents a relatively small (<4%) portion of the animal's total energy budget.

Gill protein turnover: costs of adaptation

Measurements of gill protein synthesis, and hence turnover, were greatly facilitated over the last decade by the application of "flooding dose" methodology to non-mammalian species. Numerous studies show that in fish and aquatic invertebrates, gills are among the most active tissues with respect to protein turnover, this being true under a variety of environmental and nutritional conditions. The main components being turned over in fish gills are probably collagen, primarily in the gill arches, and epithelial cell proteins in the filaments, both arches and filaments having similar protein synthesis rates. Intriguingly, differences are apparent between protein synthesis rates of adjacent holobranchs, the first (most anterior) being significantly more active than the second or third, perhaps hinting at functional differences between holobranchs. Experimental estimates of energetic costs for protein synthesis, derived from cycloheximide treatment of isolated perfused gills, give a maximum value of 14 mmol O2/g protein synthesized, which is about double theoretical costs. Environmental stressors, such as heavy metals or acid/aluminum, have variable effects on branchial protein turnover. Limited data suggest that zinc or acid exposure depresses protein synthesis, whereas acid/aluminum increases it quite markedly. Calculations indicate that whereas effects within the gills may be substantial, in terms of whole animal energetics, the costs of branchial adaptation are likely to be small.

Regulation of 3,3',4,4'-tetrachlorobiphenyl induced cytochrome P450 metabolism by thiols in tissues of rainbow trout

We observed that glutathione (GSH) status regulates the Ah receptor inducible cytochrome P4501A (CYP1A) gene expression and catalytic activity in 3,3',4,4'-tetrachlorobiphenyl (TCB) exposed rainbow trout. Tissue GSH status of TCB (1 mg/kg body weight, in corn oil) injected fish was manipulated by a) injecting (i.p.) GSH (0.25 g/kg), b) arresting GSH synthesis by L-buthionine-[S,R]-sulfoximine (BSO; 6 mmol/kg) injection for 3 and 6 days. Our attempt to manipulate GSH levels by lipoate supplementation (16 mg/kg) was not productive. Both BSO- and lipoate-supplemented fish maintained a low tissue redox (GSSG/GSH) ratio. Activities of glutathione peroxidase and glutathione reductase were elevated following 3 days of GSH supplementation in GSH rich tissues. Low activities of these enzymes were observed in BSO treated GSH deficient tissues. TCB injection markedly induced hepatic and renal CYP1A catalytic (ethoxyresorufin O-deethylase [EROD]) activities. This effect was further potentiated (3-fold) in GSH-supplemented fish tissues. In contrast, EROD induction by TCB was markedly suppressed in GSH deficient (BSO-treated) and lipoate-supplemented fish. The suppression of CYP1A catalytic activities in GSH deficient and lipoate-supplemented fish was consistently associated with a suppression of TCB induced CYP1A mRNA and protein expressions in these groups. In glutathione-supplemented fish, TCB induced CYP1A protein expression was markedly higher following 3 days of GSH supplementation. Results of our study suggest that tissue thiol status modulates cytochrome P450 CYP1A gene expression and catalytic activity.

Evolution and regulation of urea synthesis and ureotely in (batrachoidid) fishes

Selected teleostean (bony) fish species of the family Batrachoididae (toadfishes and midshipmen) possess high titers of all enzymes of the ornithine-urea cycle in their livers. These species have proven valuable in understanding the short-term regulation of urea synthesis, urea permeability, and transport across epithelial tissues, and how urea synthesis and excretion have evolved among vertebrates. One species in particular, the gulf toadfish (Opsanus beta), has been shown to rapidly switch from ammonia excretion to urea synthesis and excretion during a variety of stress conditions (including confinement). The transition is accompanied by an upregulation of hepatic glutamine synthetase activity, and a switch to pulsatile urea excretion from the anterior end of the fish. In fact, a single day's excretion can be voided in a period of < 3 h. Hypotheses on the environmental significance of these patterns of urea synthesis and excretion are discussed.