Acute effects of nitrite on ion regulation in two neotropical fish species

To broaden the understanding of physiological responses of tropical fish to environmental stressors, the effects of nitrite on haematological parameters and plasma and red blood cell ion regulation were studied in two neotropical fish species, Astyanax altiparanae and Prochilodus lineatus. Both fish species were exposed to NaNO2 (30 mg l(-1)) over a 96-h period and blood samples were taken for ion and haematological analyses. The results revealed that nitrite leads to a decrease in P. lineatus blood haematocrit and haemoglobin content and an increase in blood methaemoglobin. A. altiparanae did not exhibit any significant difference in these haematological parameters. During the exposure to NO2- both fish species had significantly reduced plasma Na+ concentration and red blood cell (RBC) K+ concentration, but only P. lineatus showed an increase in extracellular K+ concentration. When RBC volume was analyzed in vitro, after 2 min of exposure to NaNO2, a 36% shrinkage was observed in P. lineatus cells, while only a 10% shrinkage was observed in A. altiparanae cells. These results suggest that for P. lineatus, nitrite entrance into the cell leads to methaemoglobin formation and K+ efflux, causing red cell shrinkage and increased plasma K+. However, A. altiparanae proved to be a species more resistant to nitrite, exhibiting fewer responses to this compound.