Changes of ammonia, urea contents and transaminase activity in the body during aerial exposure and ammonia loading in Chinese loach Paramisgurnus dabryanus

Competitive Bio-Accumulation Between Ammonia and Nitrite Results in Their Antagonistic Toxicity to Hypophthalmichthys molitrix: Antioxidant and Immune Responses and Metabolic Detoxification Evidence

Ammonia and nitrite, as major aquatic pollutants, exhibit significant toxicity toward aquatic organisms. However, their interactive effects on fish are unclear. Aiming to determine their interactive effects, silver carp (Hypophthalmichthys molitrix) were exposed to ammonia, nitrite or ammonia + nitrite for 72 h. Silver carp exhibited pathological damage in the liver and spleen and significant increases in MDA, SOD and CAT in the liver and plasma after ammonia or nitrite exposure. Thus, ammonia and nitrite caused significant histology damage through inducing oxidative stress, and the antioxidative response of SOD-CAT was initiated by silver carp to defend them. A transcriptomic analysis suggested that disruptions in immune responses and metabolism were the main toxic effects caused by ammonia and nitrite. Specifically, nitrite decreased splenic TNF-α and IL-1β but increased splenic C4. Ammonia decreased splenic TNF-α and C4 but increased splenic IL-1β. We noted significant interactions between ammonia and nitrite, and the pathological changes and IBR in the co-exposure groups were less severe than those in the single-factor exposure groups, indicating that ammonia and nitrite have an antagonistic effect. Significant decreases in plasmatic ammonia and NO2-+NO3- were induced by nitrite and ammonia, respectively. Moreover, the plasmatic glutamine, urea-N, and glutamine synthetase and glutamate dehydrogenase activities increased significantly under ammonia and nitrite exposure, while T-NOS decreased significantly. These results suggest an antagonistic interaction between ammonia and nitrite in silver carp, possibly resulting from competitive bioaccumulation. Consequently, the simultaneous monitoring and control of both ammonia and nitrite concentrations are essential to mitigate their compounded toxic effects, which might be exacerbated under isolated exposure conditions.

Chromosome-level genome assembly and annotation of the gynogenetic large-scale loach (Paramisgurnus dabryanus)

The large-scale loach (Paramisgurnus dabryanus; Cypriniformes: Cobitidae) is primarily distributed in East Asia. It is an important economic fish species characterized by fast growth, temperature-dependent sex determination and the ability to breathe air. Currently, molecular mechanism studies related to some aspects such as sex determination, toxicology, feed nutrition, growth and genetic evolution have been conducted. However, the lack of a high-quality reference genome has hindered further research. In this study, we performed PacBio HiFi and Hi-C sequencing on a female (ZW) specimen and assembled the first high-quality chromosome-level genome of the large-scale loach. The assembled genome comprises 24 chromosomes, with a total length of 1.04 Gb, a scaffold N50 of 41.7 Mb, and a BUSCO completeness of 95.8%, including 28,311 protein-coding genes. These findings not only provide new insights into the genome structure of the large-scale loach but also establish a crucial reference point for omics studies and serve as an essential genomic resource for breeding programs in this species.

Transcriptome Analysis Reveals Immune and Antioxidant Defense Mechanisms in the Eriocheir japonica sinensis after Exposure to Ammonia

As a key species in freshwater aquaculture, Eriocheir japonica sinensis was subjected to ammonia stress to assess its impact on the hepatopancreas. A total of 4007 differentially expressed genes (DEGs) were identified between control and treatment groups, comprising 1838 upregulated and 2169 downregulated genes. Following exposure to 300 mg/L of ammonia, the oxidative phosphorylation pathway was activated, while the lysosomal pathway was suppressed, thereby influencing immune functions. Thirteen DEGs from these pathways were further validated via qRT-PCR, revealing gene expression changes of one- to two-fold. Both acid phosphatase (ACP) and alkaline phosphatase (AKP) levels in the hepatopancreas and hemolymph initially increased and then decreased, indicating a disruption in immune functionality. Additionally, alanine transaminase (ALT) and triglyceride (TG) levels were measured, alongside catalase (CAT) activity, total antioxidant capacity (T-AOC), and malondialdehyde (MDA) content, all of which showed an upward trend, signifying oxidative stress and tissue damage. These results offer critical insights into the antioxidant and immune mechanisms of E. j. sinensis in ammonia-enriched environments.

Ammonia-induced oxidative stress triggered apoptosis in the razor clam (Sinonovacula constricta)

As one of the most significant contaminants and stressors in aquaculture systems, ammonia adversely jeopardizes the health of aquatic animals. Ammonia exposure affects the development, metabolism, and survival of shellfish. However, the responses of the innate immune and antioxidant systems and apoptosis in shellfish under ammonia stress have rarely been reported. In this study, razor clams (Sinonovacula constricta) were exposed to different concentrations of non-ion ammonia (0.25 mg/L, 2.5 mg/L) for 72 h and then placed in ammonia-free seawater for 72 h for recovery. The immune responses induced by ammonia stress on razor clams were investigated by antioxidant enzyme activities and degree of apoptosis in digestive gland and gill tissues at different time points. The results showed that exposure to a high concentration of ammonia greatly disrupted the antioxidant system of the razor clam by exacerbating the accumulation of reactive oxygen species ( O 2 - , H2O2) and disordering the activities of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), and the level of activity remained at a significantly high level after recovering for 72 h (P < 0.05). In addition, there were significant differences (P < 0.05) in the expression of key genes (Caspase 7, Cyt-c, Bcl-2, and Bax) in the mitochondrial apoptotic pathway in the digestive glands and gills of razor clams as a result of ammonia stress and were unable to return to normal levels after 72 h of recovery. TUNEL staining indicated that apoptosis was more pronounced in gills, showing a dose and time-dependent pattern. As to the results, ammonia exposure leads to the activation of innate immunity in razor clams, disrupts the antioxidant system, and activates the mitochondrial pathway of apoptosis. This is important for comprehending the mechanism underlying the aquatic toxicity resulting from ammonia in shellfish.

α-lipoic acid ameliorates hepatotoxicity induced by chronic ammonia toxicity in crucian carp (Carassius auratus gibelio) by alleviating oxidative stress, inflammation and inhibiting ERS pathway

High environment ammonia (HEA) poses a deadly threat to aquatic animals and indirectly impacts human healthy life, while nutritional regulation can alleviate chronic ammonia toxicity. α-lipoic acid exhibits antioxidative effects in both aqueous and lipid environments, mitigating cellular and tissue damage caused by oxidative stress by aiding in the neutralization of free radicals (reactive oxygen species). Hence, investigating its potential as an effective antioxidant and its protective mechanisms against chronic ammonia stress in crucian carp is highly valuable. Experimental fish (initial weight 20.47 ± 1.68 g) were fed diets supplemented with or without 0.1% α-lipoic acid followed by a chronic ammonia exposure (10 mg/L) for 42 days. The results revealed that chronic ammonia stress affected growth (weight gain rate, specific growth rate, and feed conversion rate), leading to oxidative stress (decreased the activities of antioxidant enzymes catalase, superoxide dismutase, glutathione peroxidase; decreased total antioxidant capacity), increased lipid peroxidation (accumulation of malondialdehyde), immune suppression (decreased contents of nonspecific immune enzymes AKP and ACP, 50% hemolytic complement, and decrease of immunoglobulin M), impaired ammonia metabolism (reduced contents of Glu, GS, GSH, and Gln), imbalance of expression of induced antioxidant-related genes (downregulation of Cu/Zu SOD, CAT, Nrf2, and HO-1; upregulation of GST and Keap1), induction of pro-apoptotic molecules (transcription of BAX, Caspase3, and Caspase9), downregulation of anti-apoptotic gene Bcl-2 expression, and induction of endoplasmic reticulum stress (upregulation of IRE1, PERK, and ATF6 expression). The results suggested that the supplementation of α-lipoic acid could effectively induce humoral immunity, alleviate oxidative stress injury and endoplasmic reticulum stress, and ultimately alleviate liver injury induced by ammonia poisoning (50-60% reduction). This provides theoretical basis for revealing the toxicity of long-term ammonia stress and provides new insights into the anti-ammonia toxicity mechanism of α-lipoic acid.

Dietary Supplementation with R-(+)-Limonene Improves Growth, Metabolism, Stress, and Antioxidant Responses of Silver Catfish Uninfected and Infected with Aeromonas hydrophila

R-(+)-limonene is a monoterpene from plants of the genus Citrus with diverse biological properties. This research evaluated the effects of dietary supplementation with R-(+)-limonene on growth, metabolic parameters in plasma and liver, and the antioxidant and stress responses in silver catfish, Rhamdia quelen, challenged or not with Aeromonas hydrophila. Fish were fed for 67 days with different doses of R-(+)-limonene in the diet (control 0.0, L0.5, L1.0, and L2.0 mL/kg of diet). On the 60th day, a challenge with A. hydrophila was performed. R-(+)-limonene in the diet potentiated the productive performance of the fish. The metabolic and antioxidant responses indicate that R-(+)-limonene did not harm the health of the animals and made them more resistant to the bacterial challenge. Histological findings showed the hepatoprotective effect of dietary R-(+)-limonene against A. hydrophila. Igf1 mRNA levels were upregulated in the liver of fish fed with an L2.0 diet but downregulated with bacterial challenge. The expression levels of crh mRNA were higher in the brains of fish fed with the L2.0 diet. However, the L2.0 diet downregulated crh and hspa12a mRNA expression in the brains of infected fish. In conclusion, the results indicated that R-(+)-limonene can be considered a good dietary supplement for silver catfish.

Toxicological effect of endocrine disrupting insecticide (deltamethrin) on enzymatical, haematological and histopathological changes in the freshwater iridescent shark, Pangasius hypothalamus

This study investigated the deltamethrin (DMN) induced harmful effects on Pangasius hypophthalmus using enzymatic activity, haematological, and histopathological changes. LC50 value was 0.021 mg/L at 96 h, and sublethal toxicity was tested for 45 days at two `concentrations (i.e., 1/5th and 1/10th of LC50). Haematological parameters and enzymatic activities significantly changed between DMN-exposed and control groups (p < 0.05). Histopathologically, both DMN doses induced liver hyperemia, hepatic cell rupture, necrosis, hypertrepheoid bile duct, shifting nuclei, vascular haemorrhage, and hepatocyte degeneration, while in gill, secondary lamellae destruction, a fusion of adjacent gill lamellae, hypertrophy, hyperplasia, adhesion, and fusion were noticed. Kidney developed melanomacrophages, increased periglomerular and peritubular space, vacuolation, decreased glomerulus, hyaline droplets in tubular cells, loss of tubular epithelium, distal convoluted segment hypertrophy, and granular layer in brain pyramid and Purkinje cell nucleus. But, limiting pesticide impacts on freshwater fish and their habitat requires a holistic, cradle-to-grave approach and toxicological studies.

Inhibition of the NF-κB signaling pathway affects gonadal differentiation and leads to male bias in Paramisgurnus dabryanus

In recent years, sex-controlled breeding has emerged as an effective strategy to enhance the yields of economic animals with different growth characteristics, while increasing the economic benefits of aquaculture. It is known that the NF-κB pathway participates in gonadal differentiation and reproduction. Therefore, we used the large-scale loach as a research model for the present study and selected an effective inhibitor of the NF-κB signaling pathway (QNZ). This, to investigates the impacts of the NF-κB signaling pathway on gonadal differentiation during a critical period of gonad development and after maturation. Simultaneously, the sex ratio bias and the reproductive capacities of adult fish were analyzed. Our results indicated that the inhibition of the NF-κB signaling pathway influenced the expression of genes related to gonad development, regulated the gene expression related to the brain-gonad-liver axis of juvenile loaches, and finally impacted the gonadal differentiation of the large-scale loach and promoted a male-biased sex ratio. Meanwhile, high QNZ concentrations affected the reproductive abilities of adult loaches and inhibited the growth performance of offspring. Thus, our results deepened the exploration of sex control in fish and provided a certain research basis for the sustainable development of the aquaculture industry.

Regulation of Rhesus glycoprotein-related genes in large-scale loach Paramisgurnus dabryanus during ammonia loading

Waterborne ammonia is one of the crucial issues that limited production and animal health in aquaculture. Ammonia-tolerant varieties are highly desired in intensive fish farming. Screening for the key regulatory genes of ammonia tolerance is essential for variety breeding. According to the previous hypothesis, Rh glycoproteins play an important role in ammonia excretion in teleosts. However, the ammonia defensive mechanisms are not well described at present for large-scale loach (Paramisgurnus dabryanus), a typical air-breathing and commercially important fish in East Asia. Here we show that the transcription of Rh glycoprotein-related genes was significantly affected by ammonia exposure in this species. Probit analysis showed that 96 h-LC₅₀ of NH₄Cl at 23 ℃ and pH 7.2 was 92.64 mmol/L. A significant increase of Rhcg expression in gills was observed after 48 h of 60 mmol/L and 36 h of 80 mmol/L NH₄Cl exposure, suggesting that Rhcg present on the apical side of the branchial epithelium facilitates NH₃ excretion out of gills. A high concentration of acute ammonia exposure induced elevated Rhbg transcript in the gills of large-scale loaches, while a slight change in Rhbg expression was observed in response to lower ammonia, suggesting that transcriptions of Rhbg genes are activated by a considerably high level of ambient ammonia to eliminate excessive endogenous nitrogen. The Rhag mRNA level in gills of large-scale loaches increased markedly with the prolonging of exposure time from 0 to 36 h of ammonia loading, suggesting Rhag localized in gills may be primarily associated with ammonia handling. During 7-21 days of ammonia exposure, the expression of most Rh glycoproteins-related genes in the gills decreased, indicating that the functional role of Rh glycoproteins is not primarily associated with ammonia defense over a long period (more than 7 days). Although a significant transcript of Rhbg was found in the skin of a large-scale loach, the lack of Rhcg and down-regulation of Rhag may indicate that the skin is not an essential location of ammonia excretion, at least when submerged to high levels of ammonia in the environment. In conclusion, Rh glycoproteins localized in gills as ammonia transporters play a momentous role in ammonia detoxification in this species during acute ammonia loading. However, it does not show a positive function during long-term ammonia exposure. Furthermore, the physiological function of Rh glycoproteins localized in the skin is still unclear and deserves further study.

PacBio Full-Length and Illumina Transcriptomes of the Gill Reveal the Molecular Response of Corbicula fluminea under Aerial Exposure

Air exposure is a common stress for Corbicula fluminea, an economically important freshwater shellfish consumed in China, during aquaculture and transportation. However, little is known about its molecular responses to air exposure. Therefore, this study used a combination of PacBio full-length and Illumina transcriptomes to investigate its molecular responses to air exposure. A total of 36,772 transcripts were obtained using PacBio sequencing. Structural analysis identified 32,069 coding sequences, 1906 transcription factors, 8873 simple sequence repeats, and 17,815 long non-coding RNAs. Subcellular localization analysis showed that most transcripts were located in the cytoplasm and nucleus. After 96-h of air exposure, 210 differentially expressed genes (DEGs) in the gill were obtained via Illumina sequencing. Among these DEGs, most of the genes related to glycolysis, tricarboxylic acid cycle, lipid metabolism, and amino acid metabolism were upregulated. Additionally, many DEGs associated with immunity, cytoskeleton reorganization, autophagy, and ferroptosis were identified. These findings indicated that metabolic strategy change, immune response, cytoskeleton reconstruction, autophagy, and ferroptosis might be the important mechanisms that C. fluminea use to cope with air exposure. This study will enrich the gene resources of C. fluminea and provide valuable data for studying the molecular mechanisms coping with air exposure in C. fluminea and other freshwater mollusks.

Evaluation of Ammonia Nitrogen Exposure in Immune Defenses Present on Spleen and Head-Kidney of Wuchang Bream (Megalobrama amblycephala)

Ammonia is one of the most important environmental factors in aquatic ecosystems. However, there are limited studies on the effects of chronic or long-term ammonia stress and its potential molecular mechanism in fish. This study aimed to investigate the immune response and molecular mechanisms in the spleen and head-kidney of fish following chronic ammonia exposure. Megalobrama amblycephala (9.98 ± 0.48 g) were exposed to different concentrations of total ammonia nitrogen (0-30 mg/L) for 30 days. Ammonia exposure caused significant increases in cortisol levels and decreases in lysozyme and complement 3/4 concentrations in the serum, indicating inhibitory effects of ammonia stress on innate immune responses. Ammonia exposure also induced concentration-dependent increases in ammonia concentrations in tissue, pathological damage and indexes of spleen and head-kidney. Additionally, the contents of immunoglobulin M (IgM), interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) as well as mRNA levels of toll-like receptors (TLRs)/Myeloid differentiation factor 88 (MyD88)-independent signaling molecules in the spleen and head-kidney were significantly downregulated after ammonia exposure. Our findings suggested that chronic ammonia exposure caused the suppression of innate and adaptive immune responses through downregulating TLR/MyD88-independent signaling. Adverse influences of chronic ammonia stress were more severe in the spleen than in the head-kidney.

Transcriptomic analyses of the acute aerial and ammonia stress response in the gill and liver of large-scale loach (Paramisgurnus dabryanus)

The large-scale loach (Paramisgurnus dabryanus) is one of the most commercially important cultured species. Ammonia nitrogen accumulation is one of the key issue which limited production and animal health in aquaculture, but few of information is available on the molecular mechanisms of ammonia detoxification. We performed transcriptomic analyses of the gill and liver of large-scale loach subjected to 48 h of aerial and ammonia exposure. We obtained 47,473,424 to 56,791,496 clean reads from the aerial exposure, ammonia exposure and control groups, assembled and clustered a total of 92,658 unigenes with an average length of 909 bp and N50 of 1787 bp. Totals of 489/145 and 424/140 differentially expressed genes (DEGs) were detected in gill/liver of large-scale loach after aerial and ammonia exposure through comparative transcriptome analyses, respectively. In addition, totals of 43 gene ontology (GO) terms and 266 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified. After aerial and ammonia exposure, amino acid metabolism pathways in liver of large-scale loach were significantly enriched, suggesting that large-scale loach responded to high exogenous and endogenous ammonia stress by enhancing amino acid metabolism. Besides, the expression of several ammonia transporters (i.e., Rhesus glycoproteins and Aquaporins) in gill of large-scale loach were markedly changed after 48 h of aerial exposure, suggesting that large-scale loach responded to high endogenous ammonia stress by regulating the expression of Rh glycoproteins and Aqps related genes in gill. The results provide valuable information on the molecular mechanism of ammonia detoxification of large-scale loach to endogenous and environmental ammonia loading, will facilitate the molecular assisted breeding of ammonia resistant varieties, and will offer beneficial efforts for establishing an environmental-friendly and sustainable aquaculture industry.

High concentrations of environmental ammonia induced changes in large-scale loach (Paramisgurnus dabryanus) immunity

High concentrations of environmental ammonia can cause reduced immunity and death in fish, causing enormous economic losses. Air-breathing fish usually have a high ammonia tolerance and are very suitable for high-density fish farming. However, research on the effects of environmental ammonia on air-breathing fish immunity is lacking. Therefore, this study investigated the effects of environmental ammonia on the immunity of large-scale loach (Paramisgurnus dabryanus) by exposing fish to 30 mmol/L NH4Cl solution and subsequently analyzing the changes in serum and liver immune indicators, including total protein, albumin, globulin, immunoglobulin (Ig) M, lysozyme, complement component (C) 3 and C4, heat shock protein (HSP) 70, HSP90, tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-12. Results revealed that ammonia exposure significantly affected the total protein, albumin, globulin, IgM, complement C3 and C4, HSP70, HSP90, and inflammatory cytokine contents in the body, indicating that ammonia exposure induced a significant immune response and lowered bodily immunity. However, most of the immune indicators significantly decreased in the later stages of the experiment, suggesting a weakened immune response, which may be due to the species-specific ammonia detoxification ability of large-scale loach that reduces ammonia toxicity in the body.

Assessing the risk of insecticides to Actinopterygii in the combination of ecological planting and rearing

In order to study the co-existing environment of pests and economic animals, the toxicity of 15 insecticides to Plutella xylostella, Monopterus albus, and Paramisgurnus dabryanus was tested. Combined with the recommended maximum doses in the field and bioassay, the results showed that for the three insecticides that were of relatively low toxicity to M. albus and P. dabryanus, spinetoram showed the best control effect on P. xylostella, followed by chlorfenapyr and chlorantraniliprole. However, P. xylostella showed a relatively high resistance to chlorfenapyr. Therefore, the best insecticide suitable for the fields with the cauliflower-finless eel or cauliflower-loach planting and rearing combination was spinetoram, followed by chlorantraniliprole and chlorfenapyr. Other insecticides such as emamectin benzoate, Bacillus thuringiensis (Bt), matrine, and so on were effective against the diamondback moth, but they were not suitable for use because of their high toxicity to the finless eel and loach.

Effects of Acute Ammonia Stress on Antioxidant Responses, Histopathology and Ammonia Detoxification Metabolism in Triangle Sail Mussels (Hyriopsis cumingii)

Ammonia is one of the major pollutants in the aquatic ecosystem. Hyriopsis cumingii has great potential for the restoration of eutrophic water. However, there is no study investigating the effect of ammonia exposure in H. cumingii. The median lethal concentration (96 h LC50) of unionized ammonium was 12.86 mg/L in H. cumingii. In the study, H. cumingii were exposed to 6.43 mg L−1 unionized ammonium (1/2 96 h LC50) for 0, 6, 12, 24, 48, 72, and 96 h. High environment ammonia induced antioxidant response to protect the body from oxidative damage. After exposure to ammonia, there was a same trend of induction followed by inhibition of the activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione-S-transferases (GST) in the hepatopancreas and gills of H. cumingii. However, the antioxidant response could not completely counteract the oxidation effect during the exposure period, resulting in lipid peroxidation (LPO) and tissue injury in the hepatopancreas and gills of H. cumingii eventually. Moreover, this study indicated that glutamine synthetase (GS), glutamate dehydrogenase (GDH), alanine aminotransaminase (ALT), and aspartate aminotransaminase (AST) in the hepatopancreas and gills may play an important role in ammonia detoxification of H. cumingii. Our results will be helpful to understand the mechanism of aquatic toxicology induced by ammonia in shellfish.

Plasma energetic substrates and hepatic enzymes in the four-eyed fish Anableps anableps (Teleostei: Cyprinodontiformes) during the dry and rainy seasons in the Amazonian Island of Maracá, extreme north of Brazil

ABSTRACT Anableps anableps is a viviparous teleost typical from Amazon Delta estuaries. It is representative of this biome in Maracá, which offers a potential for biomonitoring. The aim of this study is to apply different biomarkers to males and females of this species and verify possible seasonal influences on their physiology. To collect fish, three expeditions were carried out from the rainy season of April 2018 to the rainy season of February 2019. Biometric parameters and gonadosomatic (GSI), hepatosomatic (HSI), and viscerosomatic (VSI) indexes were calculated, and blood samples were taken to measure triglycerides, total proteins, glucose, and activity of the enzymes aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP). The GSI of males is higher in the rainy season and of females in the dry season. This is probably related to the embryogenesis process. Males show an increase in biomass during the dry season, a metabolic homogeneity, and females show an increase in plasma glucose, triglycerides, and ALT activity. The tested biomarkers are potential for biomonitoring, preliminarily suggesting that there is a seasonal asynchronism between males and females of A. anableps as for the allocation of energy resources at different times of their life cycle.

Survival strategies of Wuchang bream (Megalobrama amblycephala) juveniles for chronic ammonia exposure: Antioxidant defense and the synthesis of urea and glutamine

This study aimed to explore how Wuchang bream (Megalobrama amblycephala) survive and defend against the toxicity of ambient total ammonia nitrogen (0, 5, 10, 20 and 30 mg/L TA-N) during 30-day exposure. As a result, hepatic malondialdehyde and protein carbonylation as well as histopathological alterations increased with increasing TA-N level, which suggested that chronic ammonia exposure caused oxidative stress and damage in the liver of fish. Meanwhile, the activities of hepatic total superoxide dismutase (T-SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) and glucose 6-phosphate dehydrogenase (G6PD) as well as the mRNA expression of Cu/Zn sod, cat, gpx and g6pd were elevated significantly along with significant reduction of glutathione (GSH) and nicotinamide adenine dinucleotide phosphate (NADPH) (P < 0.05). These results indicated that hepatic antioxidant responses were activated to alleviate oxidative damages induced by ammonia, in which lower-concentration ammonia only initiate SOD-CAT-GR-G6PDH defense and higher ammonia activated the SOD-CAT-GPx-GSH-GR-G6PDH antioxidant response. In addition, significant increases of serum urea and hepatic ammonia, urea, glutamine, arginase as well as glutamine synthetase were detected with the increase of TA-N (P < 0.05), while serum ammonia levels kept stable (P > 0.05). The present findings further revealed that ammonia could be detoxified directly into glutamine and urea in Wuchang bream to cope with ammonia exposure. In conclusion, under chronic ammonia exposure, enhanced hepatic antioxidant responses as well as increased urea and glutamine synthesis worked in combination to allow Megalobrama amblycephala to defend against environmental ammonia toxicity.

Partial Amino Acid Metabolism and Glutamine Synthesis as the Ammonia Defensive Strategies During Aerial Exposure in Chinese Loach Paramisgurnus dabryanus

The Paramisgurnus dabryanus was exposed to air to assess the changes in plasma, liver and muscle free amino acid (FAA) contents. The FAA concentrations in plasma, liver and muscle of P. dabryanus were significantly affected by aerial exposure (P < 0.05). After 12 h of aerial exposure, the plasma glutamate contents increased significantly (P < 0.05) and reached peak value at 24 h of air exposure. With increasing air exposure time, the plasma alanine contents increased significantly and more dramatically than the control values (P < 0.05). From 24 to 48 h of aerial exposure, the liver free glutamate contents increased significantly and reached the peak value at 48 h of air exposure (P < 0.05). The liver free alanine contents in air exposure group were markedly higher than these values in the control group (P < 0.05). After 72 h of air exposure, the muscle free glutamate contents increased markedly (P < 0.05) and were significantly higher than the control values (P < 0.05). The muscle free alanine contents remained at constant values during the first 12 h of aerial exposure (P > 0.05), thereafter, these concentrations increased significantly until the end of experiment (P < 0.05). Our results showed that glutamate and NH₄⁺ could be used to synthesize glutamine via glutamine synthetase to convert internal ammonia into non-toxic glutamine in P. dabryanus during air exposure. Furthermore, the P. dabryanus could catabolize several certain amino acids, leading alanine form to reduce endogenous ammonia production. The decrease in tissue free glutamate, arginine and proline in P. dabryanus indicated that these certain amino acids should be the starting substrate to be converted to alanine and energy.