Induction of nitric oxide synthesis: A strategy to defend against the high environmental ammonia-induced oxidative stress in primary hepatocytes of air-breathing catfish, Clarias magur.

The 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 the ammonia-induced oxidative stress in primary hepatocytes of magur catfish during exposure to HEA. Exposure to NH4Cl (5 mM) led to a significant increase of intracellular ammonia concentration with a sharp rise of hydrogen peroxide (H2O2) 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 of superoxide dismutase (SOD) and catalase (CAT) activities 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 nos2 gene. Most interestingly, when NO production by hepatocytes under ammonia stress was blocked by adding certain inhibitors (aminoguanidine and BAY) in the culture media, there was a further rise of H2O2 and MDA concentrations in hepatocytes. These were accompanied by the lowering of SOD and CAT activities with less expression of corresponding genes. Thus, it can be contemplated that magur catfish uses the strategy of stimulation of NO production, which ultimately induces the SOD/CAT enzyme system to defend against the ammonia-induced oxidative stress.