Osmoregulatory and metabolic changes in the gilthead sea bream Sparus auratus after arginine vasotocin (AVT) treatment.

The influence of arginine vasotocin (AVT) on osmoregulation and metabolism in gilthead sea bream Sparus auratus was evaluated by two experimental approaches. In the first, seawater (SW, 36 ppt)-acclimatized fish were injected intraperitoneally with vehicle (vegetable oil) or two doses of AVT (0.5 and 1 microg/g body weight). Twenty-four hours later, eight fish from each group were sampled; the remaining fish were transferred to low saline water (LSW, 6 ppt, hypoosmotic test), SW (transfer control), and hypersaline water (HSW, 55 ppt, hyperosmotic test). After another 24h (48-h post-injection), fish were sampled. The only significant effect observed was the increase of sodium levels in AVT-treated fish transferred to HSW. In the second experiment, fish were injected intraperitoneally with slow-release vegetable oil implants (mixture 1:1 of coconut oil and seeds oil) alone or containing AVT (1 microg/g body weight). After 3 days, eight fish from each group were sampled; the remaining fish were transferred to LSW, SW, and HSW as above, and sampled 3 days later (i.e. 6 days post-injection). In the AVT-treated group transferred from SW to SW, a significant increase vs. control was observed in gill Na(+),K(+)-ATPase activity. Kidney Na(+),K(+)-ATPase activity decreased in the AVT-treated group transferred to LSW and no changes were observed in the other groups. These osmoregulatory changes suggest a role for AVT during hyperosmotic acclimation based on changes displayed by gill Na(+),K(+)-ATPase activity. AVT treatment increased plasma cortisol levels in fish transferred to LSW and HSW. In addition, AVT treatment affected parameters of carbohydrate, lipid, amino acid, and lactate metabolism in plasma and tissues (gills, kidney, liver, and brain). The most relevant effects were the increased potential of liver for glycogen mobilization and glucose release resulting in increased plasma levels of glucose in AVT-treated fish transferred to LSW and HSW. These changes may be related to the energy repartitioning process occurring during osmotic adaptation of S. auratus to extreme environmental salinities and could be mediated by increased levels of cortisol in plasma.