Interrenal Response in Climbing Perch (Anabas testudineus Bloch) to Nitrate Exposure: Hydromineral and Metabolic Considerations

The physiological response of climbing perch to water-borne nitrate, an important component of the effluents of coconut husk retting, was examined to identify the mechanism of nitrate tolerance in fish. Indices of interrenal function, and metabolic and osmoregulatory homeostasis were analyzed in fish treated with potassium nitrate. Nitrate loading in water for 48 h produced a significant increase in the plasma cortisol by a low dose (247 μM), whereas a higher dose (494 μM) had little effect. A remarkable cortisol surge was found in the nitrate-treated fish kept for recovery in clean water for 96 h, which correlated with the rise in the plasma Na⁺. Glucose, lactate and Na⁺ concentrations in the plasma showed reduction in the nitrate-exposed fish, whereas plasma urea increased. Nitrate exposure had little influence on the gill and kidney Na⁺, K⁺-ATPase activities but had a stimulatory effect on liver Na⁺, K⁺-ATPase activity, indicating a major role of liver in nitrate tolerance. Overall, the present data indicate that nitrate exposure induces an integrated stress response in climbing perch as a result of an activated interrenal axis and disturbed metabolic and hydromineral regulations. This suggests a protective role of cortisol in the regulation of nitrate tolerance in this fish.