KIDNEY ATPASE RESPONSE IN SEAWATER-TRANSFERRED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) - EFFECT OF SALINITY AND FISH SIZE

Two sizes of domesticated rainbow trout (Oncorhynchus mykiss, 40 +/- 0 .67 and 180 +/- 3.9 g) were directly transferred to brackishwater (9 p pt) and seawater (28 ppt). Kidney Na+-K+-ATPase and Mg2+-ATPase activi ties were measured in fresh water, and after long-term seawater adapta tion (up to 21 days). Renal Na+-K+-ATPase activity increased after sal twater loading in small trout, while large trout displayed an unmodifi ed ATPase activity. The smallest trout showed a low but progressive in crease in renal Mg2+-ATPase activity after the transfer to both salini ties. However, ATPase activity remained unchanged or significantly dec reased in large trout after the transfer to seawater or brackishwater, respectively.