Dietary arginine attenuates hypoxia- induced HIF expression, metabolic responses and oxidative stress in Indian Major Carp, Cirrhinus mrigala.

Hypoxia is a common stressor in aquaculture systems, which causes severe physiological disturbances, ultimately leading to mortality or reduced productivity. Arginine, as a precursor of NO, has a role in enhancing oxygen delivery. Thus, an experiment was conducted to evaluate the effect of dietary arginine (Arg) in Cirrhinus mrigala exposed to hypoxia. The fish were fed with different levels of arginine for 60 days and exposed for 72 h to a sublethal level of hypoxia (0.50 ± 0.16 mg/L dissolved oxygen [DO]). The six treatment groups with three replicates were N0 (0% Arg + Normoxia), H0 (0% Arg + Hypoxia), N0.7 (0.70% Arg + Normoxia), H0.7 (0.70% Arg + Hypoxia), N1.4 (1.40% Arg + Normoxia), H1.4 (1.40% Arg + Hypoxia). Eighteen experimental units with twelve animals (5.8 ± 0.18 g) each were used for the trial.The results indicated that supplementation of arginine at 0.7 and 1.4% enhanced the hypoxia tolerance time, although the high dose (1.4%) did not yield any further increments. The exposure to hypoxia up-regulated Hypoxia Inducible Factor (HIF)-1α mRNA expression and supplementation of arginine significantly decreased hypoxia induced up-regulation of HIF at 1.4%. Arginine supplementation partially or completely normalised the hypoxia induced changes in the metabolic enzymes of C. mrigala. The fish exposed to hypoxic conditions exhibited significantly higher (P < 0.05) lipid peroxidation levels than those maintained under normoxic conditions, while arginine feeding significant in reducing lipid peroxidation. Antioxidant enzyme activities were significantly higher (P < 0.05) in hypoxia-exposed carp, indicating increased oxidative stress during the hypoxic exposure, that was improved in Arg-supplemented groups. However, arginine did not modulate erythrocyte countsalthough itreduced the erythrocyte fragility. We conclude arginine supplementation is effective in ameliorating hypoxia induced metabolic alterations and improving antioxidant defences in fish.

Dietary GABA enhances hypoxia tolerance of a bottom-dwelling carp, Cirrhinus mrigala by modulating HIF-1α, thyroid hormones and metabolic responses.

The Indian major carp, mrigal (Cirrhinus mrigala), is a bottom-dwelling fish that can survive hypoxic episodes in its natural environment. We hypothesise that it can better survive hypoxic conditions by altering metabolic responses through GABA (Gamma-aminobutyric acid) supplementation. In the first experiment, the hypoxia tolerance time of the fishes was evaluated under extreme anoxic conditions after feeding with GABA, which showed that GABA had improved survival time under hypoxia. To study the response of dietary GABA in hypoxia-exposed fish, the branchial HIF-1α expression levels, serum thyroid hormone levels and hepatic metabolic responses were assessed in the subsequent experiment. The treatment groups were fed for 60 days with experimental diets containing 4 levels of GABA (0.00% G, 0.50% G, 0.75% G and 1.0%G) and were subjected to 72-h hypoxia exposure (0.5 ± 0.02 mg L-1 dissolved oxygen (DO)) whereas a control group was maintained under normoxic conditions (6.0 ± 0.21 mg L-1 DO). The five treatment groups with three replicates were C0 (0% G + normoxia), H0 (0% G + hypoxia), H0.5 (0.50% G + hypoxia), H0.75 (0.75% G + hypoxia) and H1.0 (1.00% G + hypoxia). The results indicated that GABA supplementation triggered downregulation of HIF 1 alpha expression. When compared with the control group, decreased thyroxine (T4) and triiodothyronine (T3) levels were observed in the GABA-fed hypoxic groups. However, TSH (thyroid stimulating hormone) level remained unchanged in all the treatments. The LDH (lactate dehydrogenase) level in hypoxia-exposed groups was decreased by GABA supplementation. Our study demonstrated that GABA supplementation restores acute hypoxia-induced HIF-1α expression, thyroid hormone levels and LDH activities. On the other hand, it enhanced the citrate synthase (CS) activities at 0.5-1.00%, which showed a sharp decline in hypoxia. Hypoxia caused increase in the serum metabolites such as glucose, lactate, cholesterol and triglycerides. However, GABA supplementation was partially effective in reducing glucose and lactate level while triglycerides and cholesterol values remained unchanged. Overall, our results suggested a potential role of GABA in suppressing metabolism during hypoxia exposure, which can increase the chances of survival of the species Cirrhinus mrigala during hypoxia.