The effect of environmental hypercapnia and size on nitrite toxicity in the striped catfish (Pangasianodon hypophthalmus).

Striped catfish (Pangasianodon hypophthalmus) are farmed intensively at high stocking densities in Vietnam where they are likely to encounter environmental hypercapnia as well as occasional high levels of aquatic nitrite. Nitrite competes with Cl(-) for uptake at the branchial HCO3(-)/Cl(-) exchanger, causing a drastic reduction in the blood oxygen carrying capacity through the formation of methaemoglobin and nitrosylhaemoglobin. Environmental hypercapnia induces a respiratory acidosis where the branchial HCO3(-)/Cl(-) exchange activity is reduced in order to retain HCO3(-) for pH recovery, which should lead to a reduced nitrite uptake. To assess the effect of hypercapnia on nitrite uptake, fish were cannulated in the dorsal aorta, allowing repeated blood sampling for measurements of haemoglobin derivatives, plasma ions and acid-base status during exposure to 0.9mM nitrite alone and in combination with acute and 48h acclimated hypercapnia over a period of 72h. Nitrite uptake was initially reduced during the hypercapnia-induced acidosis, but after pH recovery the situation was reversed, resulting in higher plasma nitrite concentrations and lower functional haemoglobin levels that eventually caused mortality. This suggests that branchial HCO3(-)/Cl(-) exchange activity is reduced only during the initial acid-base compensation, but subsequently increases with the greater availability of internal HCO3(-) counter-ions as pH is compensated. The data further suggest that branchial Na(+)/H(+) exchange plays a significant role in the initial phase of acid-base compensation. Overall, longer term environmental hypercapnia does not protect against nitrite uptake in P. hypophthalmus, but instead enhances it. In addition, we observed a significant size effect in nitrite accumulation, where large fish attained plasma [nitrite] above the ambient concentration, while small fish did not. Small P. hypophthalmus instead had significantly higher plasma [nitrate], and haemoglobin concentrations, revealing greater capacity for detoxifying nitrite by oxidising it to nitrate.