Intestinal Na+/K+-ATPase activity in salmonids

The sodium-potassium dependent adenosine triphosphatase (Na+/K+-ATPase) is assumed to be a key enzyme in adaptive processes in euryhaline fish. The present study demonstrates activity of Na+/K+-ATPase in intestinal preparations from rainbow trout (Oncorhynchus mykiss Walbaum) and Atlantic salmon (Salmo salar L.), examines the ion and substrate requirements and effects of incubation temperature, pH, and ouabain. Maximal enzyme activity was obtained at 13 to 16 mM ATP; 18 to 24 mM MgCl2; Na:K-ratio of 4:1; total ionic strength of Na+ and K+ at 185 mM, and pH at 6.5 to 6.6. The presence of an intestinal Ca2+-ATPase was demonstrated in both species. Na+/K+-ATPase activity in intestinal preparations required a higher concentration of substrate than the corresponding enzyme activity in gill preparations. Compared to gill preparations the optimal incubation temperature for intestinal preparations is 20 degrees C instead of 37 degrees C. Moreover, the incubation time may be prolonged to 60 min if enzyme activity is low. In conclusion, this work indicates that salmonid intestinal Na+/K+-ATPase exhibits properties that are not substantially different from those of the corresponding enzyme in the gills, but intestinal preparations seem to require a higher concentration of substrate and a lower incubation temperature.