title: Extreme nitrite tolerance in the clown knifefish Chitala ornata is linked to up-regulation of methaemoglobin reductase activity.
abstract: The clown knifefish is a facultative air breather, which is widely farmed in freshwater ponds in Vietnam. Here we report a very high nitrite tolerance (96h LC50of 7.82mM) in this species and examine the effects of 1mM (LC5) and 2.5mM (LC10) ambient nitrite on haemoglobin (Hb) derivatives, electrolyte levels, acid-base status, and total body water content during 7days of exposure. Furthermore, we tested the hypothesis that erythrocyte methaemoglobin (metHb) reductase activity is upregulated by nitrite exposure. Plasma nitrite levels increased for 2-3days but stayed below environmental levels and fell towards control values during the last half of the exposure period. Plasma nitrate, in contrast, rose continuously, reflecting detoxification of nitrite to nitrate. MetHb generated from the reaction between nitrite and erythrocyte Hb reached 38% at day 2, but then decreased to 17% by the end of experiment. The first order rate constant for metHb reduction by erythrocyte metHb reductase increased from 0.01 in controls to 0.046min-1after 6days of nitrite exposure, showing up-regulation of this enzyme. While such upregulation has been suggested in nitrite-exposed fish species, this study provides the first experimental evidence.
authors.affiliation: College of Aquaculture and Fisheries, Can Tho Uiniversity, Can Tho City, Viet Nam.
authors.forename: Le Thi Hong
authors.initials: LTH
authors.lastname: Gam
authors.affiliation: Department of Biology, University of Southern Denmark, Odense, Denmark.
authors.forename: Frank Bo
authors.initials: FB
authors.lastname: Jensen
authors.affiliation: Zoophysiology, Department of Bioscience, Aarhus University, Aarhus, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Damsgaard
authors.affiliation: College of Aquaculture and Fisheries, Can Tho Uiniversity, Can Tho City, Viet Nam.
authors.forename: Do Thi Thanh
authors.initials: DTT
authors.lastname: Huong
authors.affiliation: College of Aquaculture and Fisheries, Can Tho Uiniversity, Can Tho City, Viet Nam.
authors.forename: Nguyen Thanh
authors.initials: NT
authors.lastname: Phuong
authors.affiliation: Zoophysiology, Department of Bioscience, Aarhus University, Aarhus, Denmark. Electronic address: mark.bayley@bios.au.dk.
authors.forename: Mark
authors.initials: M
authors.lastname: Bayley
mesh_terms: D000818:Animals; D042966:Cytochrome-B(5) Reductase / Q000378:metabolism*; D004361:Drug Tolerance; D004789:Enzyme Activation / Q000187:drug effects; D004912:Erythrocytes / Q000187:drug effects* / Q000201:enzymology; D005399:Fishes / Q000097:blood* / Q000378:metabolism; D005618:Fresh Water / Q000737:chemistry; D007928:Lethal Dose 50; D008706:Methemoglobin / Q000378:metabolism*; D009573:Nitrites / Q000097:blood / Q000633:toxicity; D015854:Up-Regulation; D014874:Water Pollutants, Chemical / Q000097:blood / Q000633:toxicity
chemical_list: D009573:Nitrites; D014874:Water Pollutants, Chemical; D008706:Methemoglobin; D042966:Cytochrome-B(5) Reductase
keywords: CYB5R3; Osteoglossiformes; clown featherback; cytochrome b5 reductase; nitrite detoxification; oxygen carrying capacity