Extreme nitrite tolerance in the clown knifefish Chitala ornata is linked to up-regulation of methaemoglobin reductase activity

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 (96 h LC50 of 7.82 mM) in this species and examine the effects of 1 mM (LC5) and 2.5 mM (LC10) ambient nitrite on haemoglobin (Hb) derivatives, electrolyte levels, acid-base status, and total body water content during 7 days of exposure. Furthermore, we tested the hypothesis that erythrocyte methaemoglobin (metHb) reductase activity is upregulated by nitrite exposure. Plasma nitrite levels increased for 2-3 days 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 0.046 min−1 after 6 days 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.