Effects of environmental hypercapnia on hemato-immunological parameters, carbonic anhydrase, and Na+, K+-ATPase enzyme activities in rainbow trout Oncorhynchus mykiss) tissues

In this study, the effects of environmental hypercapnia on hemato-immunological parameters and the activities of respiratory enzymes such as carbonic anhydrase (CA) and Na+, K+-ATPase were investigated in rainbow trout (Oncorhynchus mykiss) tissues (gill, liver and kidney). Batches of 12 fish were exposed to 4.5mg L-1 (control) and 14mg L-1 CO2. No mortalities occurred during the 14days of the experimental period. Red blood cell (RBC), hemoglobin (Hb), and hematocrit (Ht) levels, and innate immune parameters such as nitro blue tetrazolium (NBT), lysozyme, and myeloperoxidase activities, and the melano-macrophage frequency were negatively affected by elevated CO2 levels. Patterns of change in CA activity differed among the gill, liver, and kidney. Compared with the activities of CA in the control group, the CA enzyme was significantly stimulated at day 7 in the gill tissue, whereas it was stimulated at day 14 of the experiment in the liver tissue of fish exposed to 14mg L-1 CO2 (P < 0.05). In contrast to the pattern of CA enzyme activities, the Na+, K+-ATPase enzymes were stimulated significantly in the liver after day 7 but inhibited in the kidney and gill (P < 0.05). These results suggest that a subchronic exposure to hypercapnia of rainbow trout tissues may lead to adaptive changes in the respiratory enzymes and negatively affects hemato-immunological parameters.