Expression of gill Vacuolar-type H⁺-ATPase B subunit, and Na⁺, K⁺-ATPase α₁ and β₁ subunit messenger RNAs in smolting Salmo salar

Changes in gill vacuolar-type H ⁺-ATPase B subunit, and Na ⁺,K ⁺-ATPase α and β subunit mRNA expression were examined during the course of smoltification 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 α ₁, subunit, and iii) Na ⁺,K ⁺-ATPase β ₁ subunit, and used these as Northern blotting probes. During smoltification, the salmon showed a typical increase in gill Na ⁺,K ⁺-ATPase activity and improved hypo-osmoregulatory ability as judged by their ability to regulate plasma [Cl ^-] in a 24-hr seawater challenge test (35 ppt). Gill Na ⁺,K ⁺-ATPase α ₁ and β ₁ subunit mRNA levels were regulated at a constant ratio during smoltification. Both transcripts were elevated during the build-up of gill Na ⁺,K ⁺-ATPase activity, underlining the importance of increased mRNA levels for increased enzyme activity. Gill V-H ⁺-ATPase B subunit mRNA levels were high during the early phase of smoltification. These results support our hypothesis that gill V-H ⁺-ATPase expression may be elevated during the early stages of smoltification in order to counter the effects of increased ionic efflux when in FW. The peak smolt 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-osmoregulatory organ and hence the development of optimal SW-tolerance of the smolt.