The interrelationship of cortisol, Gill (Na + K) ATPase, and homeostasis during the Parr-Smolt transformation of atlantic salmon ( Salmo salar L.)

Abstract

Serum cortisol concentrations were measured in juvenile Atlantic salmon ( Salmo salar L.) undergoing the parr-smolt transformation in fresh water, at either 1 year (S1 population) or 2 years (S2 population) after hatching. Serum cortisol levels were generally low (<10 ng ml −1), but during smoltification became significantly elevated in both populations. In addition, the S2 population showed a small cortisol peak in the autumn prior to smoltification. Simultaneous measurement of gill (Na + K) ATPase activity and serum cortisol concentrations in S2 salmon juveniles revealed that both features rose during smoltification in fresh water. The rise in gill (Na + K) ATPase activity was independent of cortisol levels, and preceded the rise in cortisol titer by approximately 1 month. After seawater transfer, gill enzyme levels remained high while cortisol titers fell sharply. Serum cortisol levels, but not gill (Na + K) ATPase activities, were progressively reduced by acclimation of smolts to increasing salinities. Linear regression studies indicated that, at any one level of gill (Na + K) ATPase, cortisol titer increased with increasing surface area: volume ratio. Extra-cellular fluid volume (sodium space) was found to decline with increasing gill (Na + K) ATPase activity, and to increase with serum cortisol titers. These results indicate that high serum cortisol levels represent a secondary response caused by the development of hypoosmoregulatory ability while still resident in fresh water. Cortisol does not appear to directly stimulate gill (Na + K) ATPase activity in Atlantic salmon smolts.