Intestinal transport mechanisms and plasma cortisol levels during normal and out-of-season parr-smolt transformation of Atlantic salmon, Salmo salar

The intestine is one of the major osmoregulatory organs in fish. During the salmon parr-smolt transformation, the intestine must change its functions from the freshwater (FW) role of preventing water inflow, to the seawater (SW) role of actively absorbing ions and water. This development can be assessed as an increased intestinal fluid transport (Jv) during the parr-smolt transformation. The developmental changes taking place during parr-smolt transformation are governed by a number of endocrine systems, of which cortisol is the main stimulator of Jv. In order to further elucidate the mechanisms behind the elevation of Jv during parr-smolt transformation, juvenile Atlantic salmon were followed during natural (1+age) as well as photoperiod-induced (0+age) smoltification. Plasma cortisol levels, gill and intestinal Na+,K+-ATPase activity, Jv (only during natural smoltification) and intestinal paracellular permeability were measured. In natural smolting as well as in photoperiod-induced smolting, normal patterns of plasma cortisol levels and gill Na+,K+-ATPase activity, with clearly defined, transient peaks were obtained. When fish were transferred to SW, a second elevation in plasma cortisol levels and gill Na+,K+-ATPase activity occurred, whereas Jv remained at similar levels as in FW fish. As to the mechanisms behind the increased Jv during parr-smolt transformation, the intestinal Na+,K+-ATPase activity increases in the anterior intestine and the paracellular permeability, as judged by transepithelial resistance (TER), appears to decrease in the posterior intestine. These events correspond with the increase in Jv seen during this developmental stage. Furthermore, the increase in the physiological parameters follows the changes in plasma cortisol levels, shifted by a couple of weeks. When the fish were transferred to SW, a further increase in Na+,K+-ATPase activity was apparent in both anterior and posterior intestine and the paracellular permeability decreases. To summarize, the increased Jv seen during the parr-smolt transformation of Atlantic salmon may be due to an increase in the paracellular water flow of the posterior intestine. When the fish enter SW, the water flow appears to be directed from the paracellular pathway towards a more transcellular route with increased intestinal Na+,K+-ATPase activity as the main driving force.