Nitrite and carbon dioxide are common environmental contaminants in the intensive aquaculture ponds used to farm clown knifefish (Chitala ornata) in the Mekong delta, Vietnam. Here we tested the hypothesis that hypercapnia reduces nitrite uptake across the gills, because pH regulation will reduce chloride uptake and hence nitrite uptake as the two ions compete for the same transport route via the branchial HCO₃⁻/Cl⁻ exchanger. Fish fitted with arterial catheters were exposed to normocapnic/normoxic water (control), nitrite (1 mM), hypercapnia (21 mmHg CO₂), or combined hypercapnia (acclimated hypercapnia) and nitrite for 96 h. Blood was sampled to measure acid-base status, haemoglobin derivatives and plasma ions. Plasma nitrite increased for 48 h, but levels stayed below the exposure concentration, and subsequently decreased as a result of nitrite detoxification to nitrate. The total uptake of nitrite (evaluated as [NO₂⁻] + [NO₃⁻]) was significantly decreased in hypercapnia, in accordance with the hypothesis. Methemoglobin and nitrosylhemoglobin levels were similarly lower during hypercapnic compared to normocapnic nitrite exposure. The respiratory acidosis induced by hypercapnia was half-compensated by bicarbonate accumulation in 96 h, which was mainly chloride-mediated (i.e. reduced Cl⁻ influx via the branchial HCO₃⁻/Cl⁻ exchanger). Plasma osmolality and main ions (Na⁺, Cl⁻) were significantly decreased by hypercapnia and by nitrite exposure, consistent with inhibition of active transport. We conclude that hypercapnia induces a long-lasting, and mainly chloride-mediated acid-base regulation that reduces the uptake of nitrite across the gills.

亚硝酸盐和二氧化碳是在越南湄公河三角洲养殖小丑刀鱼（Chitala ornata）的集约化养殖池塘中常见的环境污染物。在这里，我们测试了高碳酸血症减少了穿过鳃亚硝酸盐摄取的假说，因为调节pH值将减少氯离子摄取，并因此亚硝酸盐摄取作为两种离子经由鳃HCO竞争相同的传送路径₃ ^- /氯^-热交换器。将装有动脉导管的鱼暴露于高碳酸/高氧水（对照），亚硝酸盐（1 mM），高碳酸血症（21 mmHg CO ₂），或合并高碳酸血症（适应性高碳酸血症）和亚硝酸盐96小时。采样血液以测量酸碱状态，血红蛋白衍生物和血浆离子。血浆亚硝酸盐增加48小时，但水平保持在暴露浓度以下，随后由于亚硝酸盐解毒为硝酸盐而降低。亚硝酸盐的总摄取量（评价为[NO ₂ ^- ] + [NO ₃ ^- ]）是高碳酸血症显著下降，按照假设。与正常人亚硝酸盐暴露相比，高碳酸血症期间的高铁血红蛋白和亚硝酰基血红蛋白水平同样较低。高碳酸血症引起的呼吸性酸中毒在96小时内被碳酸氢盐的积累补偿了一半，碳酸氢盐的积累主要由氯介导（即Cl-降低）^。通过鳃HCO涌入₃ ^- /氯^-交换器）。血浆渗透压和主离子（Na ⁺，氯^- ）由高碳酸血症和亚硝酸盐曝光，具有抑制活性的运输一致均显著降低。我们得出的结论是，高碳酸血症会导致持久的，主要是氯离子介导的酸碱调节，从而减少整个g的亚硝酸盐吸收。