The role of environmental salinity on Na-dependent intestinal amino acid uptake in rainbow trout (Oncorhynchus mykiss).

Na/K-ATPases (NKA) in the basolateral membrane of the intestinal enterocytes create a Na-gradient that drives both ion-coupled fluid uptake and nutrient transport. Being dependent on the same gradient as well as on the environmental salinity, these processes have the potential to affect each other. In salmonids, L-lysine absorption has been shown to be higher in freshwater (FW) than in seawater (SW) acclimated fish. Using electrophysiology (Ussing chamber technique), the aim was to explore if the decrease in L-lysine transport was due to allocation of the Na-gradient towards ion-driven fluid uptake in SW, at the cost of amino acid transport. Intestinal NKA activity was higher in SW compared to FW fish. Exposure to ouabain, an inhibitor of NKA, decreased L-lysine transport. However, exposure to bumetanide and hydrochlorothiazide, inhibitors of Na, K, 2Cl-co-transporter (NKCC) and Na, Cl-co-transporter (NCC) respectively, did not affect the rate of intestinal L-lysine transport. In conclusion, L-lysine transport is Na-dependent in rainbow trout and the NKA activity and thus the available Na-gradient increases after SW acclimation. This increased Na-gradient is most likely directed towards osmoregulation, as amino acid transport is not compromised in SW acclimated fish.