Influence of environmental ammonia on the production of nitric oxide and expression of inducible nitric oxide synthase in the freshwater air-breathing catfish (Heteropneustes fossilis)

Nitric oxide (NO) is a highly versatile and unique ubiquitous signaling molecule, and is known to play diverse physiological functions in mammals including those of adaptation to various stresses. The present study reports on the influence of exposure to high external ammonia (HEA) on the production of nitric oxide (NO) and the expression of inducible nitric oxide synthase (iNOS), that produces NO from l-arginine in the freshwater air-breathing catfish (Heteropneustes fossilis), which is reported to tolerate a very HEA. Some levels of NO were found to be present in all the tissues and also in plasma of control fish, which further enhanced significantly in fishes treated with high concentrations of environmental ammonia (25 and 50mM ammonium chloride) for 7 days, accompanied by more efflux of NO from the perfused liver. This was accomplished by the induction of iNOS activity in different tissues of fish exposed to HEA, which otherwise was not detectable in control fish. Exposure to 25mM ammonium chloride also led to a significant expression of iNOS protein in different tissues, followed by further increase at 50mM ammonium chloride. Further, there was an increase in the expression of iNOS mRNA in ammonia-treated fish, thus suggesting that the expression of iNOS gene under hyper-ammonia stress was probably regulated at the transcriptional level. Immunocytochemical analysis indicated that the expression of iNOS in different tissues was zonal specific and not expressed uniformly throughout the organ. Hyper-ammonia stress also led to activation and nuclear translocation of nuclear factor κB (NFκB) in hepatic cells. These results suggest that the activation of iNOS gene under hyper-ammonia stress was probably mediated through the activation of one of the major transcription factors, the NFκB. This is the first report of ammonia-induced expression of iNOS gene, iNOS protein expression leading to more generation of NO under hyper-ammonia stress in any teleosts.