Expression of gill vacuolar-type H+-ATPase B subunit, and Na+, K+-ATPase alpha(1) and beta(1) subunit messenger RNAs in smolting Salmo salar

Changes in gill vacuolar-type H+-ATPase B subunit, and Na+,K+-ATPase alpha and beta subunit mRNA expression were examined during the course of smoltif ication in Salmo salar. We cloned and sequenced cDNA fragments of S. salar gill i) vacuolar-type H+-ATPase (V-H+-ATPase) B subunit, ii) Na+,K+-ATPase alpha (1) subunit, and iii) Na+,K+-ATPase beta (1) subunit, and used these as Northern blotting probes. During smoltification, the salmon showed a typ ical increase in gill Na+,K+-ATPase activity and improved hypo-osmoregulato ry ability as judged by their ability to regulate plasma [Cl-] in a 24-hr s eawater challenge test (35 ppt). Gill Na+,K+-ATPase alpha (1) and beta (1) subunit mRNA levels were regulated at a constant ratio during smoltificatio n. Both transcripts were elevated during the build-up of gill Na+,K+-ATPase activity, underlining the importance of increased mRNA levels for increase d enzyme activity. Gill V-H+-ATPase B subunit mRNA levels were high during the early phase of smoltification. These results support our hypothesis tha t gill V-H+-ATPase expression may be elevated during the early stages of sm oltification in order to counter the effects of increased ionic efflux when in FW. The peak smelt stage was, however, characterized by simultaneously elevated gill Na+,K+-ATPase expression and low V-H+-ATPase expression, and possibly ensures the complete transformation of the gill into a hypo-osmore gulatory organ and hence the development of optimal SW-tolerance of the sme lt.