Aquatic Toxicology 104 (2011) 86–93 Contents lists available at ScienceDirect Aquatic Toxicology journal homepage: www.elsevier.com/locate/aquatox Effects of nitrite exposure on functional haemoglobin levels, bimodal respiration, and swimming performance in the facultative air-breathing fish Pangasianodon hypophthalmus Sjannie Lefevre a,∗ , Frank B. Jensen b , Do.T.T. Huong c , Tobias Wang a , Nguyen T. Phuong c , Mark Bayley a a Zoophysiology, Department of Biological Sciences, Aarhus University, Aarhus, Denmark b Department of Biology, University of Southern Denmark, Odense, Denmark c College of Aquaculture and Fisheries, Can Tho University, Can Tho City, Viet Nam article info Article history: Received 25 February 2011 Received in revised form 21 March 2011 Accepted 26 March 2011 Keywords: Methaemoglobin Striped catfish Blood oxygen carrying capacity Aerobic scope Swimming performance Bimodal respirometry LC50 abstract In this study we investigated nitrite (NO 2 - ) effects in striped catfish, a facultative air-breather. Fish were exposed to 0, 0.4, and 0.9 mM nitrite for 0, 1, 2, 4, and 7 days, and levels of functional haemoglobin, methaemoglobin (metHb) and nitrosyl haemoglobin (HbNO) were assessed using spectral deconvolu- tion. Plasma concentrations of nitrite, nitrate, chloride, potassium, and sodium were also measured. Partitioning of oxygen consumption was determined to reveal whether elevated metHb (causing func- tional hypoxia) induced air-breathing. The effects of nitrite on maximum oxygen uptake (MO 2max ) and critical swimming speed (U crit ) were also assessed. Striped catfish was highly tolerant to nitrite exposure, as reflected by a 96 h LC 50 of 1.65 mM and a moderate nitrite uptake into the blood. Plasma levels of nitrite reached a maximum after 1 day of exposure, and then decreased, never exceeding ambient levels. MetHb, HbNO and nitrate (a nitrite detoxification product) also peaked after 1 day and then decreased. Only high levels of nitrite and metHb caused reductions in MO 2max and U crit . The response of striped catfish contrasts with that seen in most other fish species and discloses efficient mechanisms of com- bating nitrite threats. Furthermore, even though striped catfish is an efficient air-breather, this species has the ability to sustain aerobic scope and swimming performance without air-breathing, even when faced with nitrite-induced reductions in blood oxygen carrying capacity. Our study is the first to confirm that high levels of nitrite and metHb reduce MO 2max and thereby aerobic scope, while more moderate elevations fail to do so. Further studies are needed to elucidate the mechanisms underlying the low nitrite accumulation in striped catfish. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Water-borne nitrite (NO 2 - ) enters the blood of freshwater fish via the gill chloride/bicarbonate exchanger that partakes in the active branchial chloride uptake mechanism, and this nitrite entry can lead to a number of physiological disturbances (Eddy and Williams, 1987; Jensen, 2003). The rate of nitrite uptake depends on the rate of active ion uptake and the relative affinity of nitrite and chloride for the exchanger (Williams and Eddy, 1986). Fish with high rates of branchial Cl - uptake typically accumulate plasma nitrite well above ambient values (Williams and Eddy, 1986; Jensen, 2003). Since nitrite and chloride compete for the same uptake route, chloride uptake will be reduced, and plasma chloride concentra- ∗ Corresponding author at: Zoophysiology, Biological Sciences, Aarhus University, C.F. Møllers allé, Building 1131, 8000 Aarhus C, Denmark. Tel.: +45 89 42 26 95. E-mail address: sjannie.lefevre@biology.au.dk (S. Lefevre). tion may decrease (Jensen et al., 1987). Once in the blood, nitrite enters the red blood cells, where it reacts with the haemoglobin (Hb) molecule. The reaction between nitrite and oxygenated Hb leads to oxi- dation of haem iron (from the ferrous to the ferric state) to form methaemoglobin (metHb) and oxidation of nitrite to nitrate, while three haem bound O 2 become reduced (Eq. (1)). The reaction of nitrite with deoxygenated Hb produces metHb and nitric oxide (NO) (Eq. (2)), with NO being trapped by ferrous haem to form nitrosyl-Hb (HbNO; Eq. (3))(Kosaka and Tyuma, 1987; Jensen, 2009): 4Hb(Fe 2+ )O 2 + 4NO 2 - + 4H + → 4Hb(Fe 3+ ) + 4NO 3 - + O 2 + 2H 2 O (1) Hb(Fe 2+ ) + NO 2 - + H + → Hb(Fe 3+ ) + NO + OH - (2) Hb(Fe 2+ ) + NO → Hb(Fe 2+ )NO (3) 0166-445X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2011.03.019