The teleost pseudobranch is a gill-like structure often fused to the anterior of the opercular cavity Pseudobranch cells are mitochondria rich and have high levels of Na+, K+-ATPase activity. In this study, pseudobranch Na+, K+-ATPase activity in juvenile chinook salmon (Oncorhynchus tshawytscha) was compared to gill Na+, K+-ATPase activity, a known marker of parr-smolt transformation, in three experiments. In two stocks of New Zealand chinook salmon, pseudobranch Na+, K+-ATPase activity was found to significantly increase during development. At these times gill Na+, K+-ATPase activity was also elevated. Pseudobranch Na+, K+-ATPase activity did not increase 10 days after transfer from fresh water to 34 ppt seawater, a treatment that resulted in a twofold increase in gill Na+, K+-ATPase activity. Cortisol (50 mug/g) and ovine growth hormone (5 mug/g) implants had no effect on pseudobranch Na+, K+-ATPase activity in underyearling chinook salmon, while gill Na+, K+-ATPase activity was stimulated by each hormone. In yearling chinook salmon, only cortisol stimulated pseudobranch Na+, K+-ATPase activity 14 days post-implantation. It was concluded that the pseudobranch differs from the gill in terms of the regulation of Na+, K+-ATPase activity and a role during adaptation to seawater is likely to be limited. (C) 2003 Elsevier Science Inc. All rights reserved.
