Cyprinus carpio is a stenohaline species but can tolerate some ranges of changes in environmental salinities, so histomorphological methods and Na+-K+ ATPase and Aquaporin 3 immunohistochemistry were performed on common carp kidney as an osmoregulatory organ in experimental groups and control in order to investigate their possible roles during salinity challenge. Five groups of fish (n=25) with salinities ranging from 3, 6, 9 and 12 g/l marine salt and a control group (tap water) were used. The experiment was continued for two weeks. Kidney samples from control and experimental groups were fixed in 4% paraformaldehyde and were embedded in paraffin. The Na+-K+ ATPase and Aquaporin 3 intensity of the immunostaining and the renal tubules dilation had direct relation with environmental salinities, and showed the involvement of these proteins in physiological responses to environmental salinity. Furthermore, in the salinities 9 and 12 g/l epithelium of the renal tubules, profound histomorphological alteration was present.
