Air-breathing magur catfish (Clarias magur) regularly face the problem of exposure to high environmental ammonia (HEA) as one of the major pollutants in their natural habitats that causes considerable toxic effects at the cellular level, including that of oxidative stress. The major objective of the present study was to demonstrate the antioxidant activity of endogenously produced nitric oxide (NO) to defend against ammonia-induced oxidative stress in primary hepatocytes of magur catfish during exposure to HEA. Exposure to NH 4 Cl (5 mmol l −1) led to a significant increase in intracellular ammonia concentration with a sharp rise of hydrogen peroxide (H 2 O 2) and malondialdehyde (MDA) concentrations within 3 h in primary hepatocytes, which decreased gradually at later stages of treatment. This phenomenon was accompanied by a significant increase in superoxide dismutase (SOD) and catalase (CAT) activity as a consequence of induction of corresponding genes. HEA exposure also led to the stimulation of NO production due to induction of inducible nitric oxide synthase (iNOS) activity, as a consequence of up-regulation of the nos2 gene. Most interestingly, when NO production by hepatocytes under ammonia stress was blocked by adding certain inhibitors [aminoguanidine and 3-(4methylphenylsulfonyl)-2-propenenitrile] to the culture medium, there was a further rise of H 2 O 2 and MDA concentrations in hepatocytes. These were accompanied by the lowering of SOD and CAT activity with less expression of corresponding genes. Thus, it can be contemplated that magur catfish use the strategy of stimulation of NO production, which ultimately induces the SOD-CAT enzyme system to defend against ammonia-induced oxidative stress.

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