Plasma growth hormone‐binding protein levels in Atlantic salmon Salmo salar during smoltification and seawater transfer

Specific growth hormone ( GH )‐binding protein (Ghbp) was purified from Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss plasma with immunoprecipitation and characterized in cross‐linking studies using autoradiography and western blots. The size of the Ghbp was estimated to be c. 53 kDa . A radioimmunoassay was established to measure Ghbp in salmonids, using antibodies specific against the extracellular segment of the S. salar growth hormone receptor 1 (grh1 ; GenBank AY462105 ). Plasma Ghbp levels were measured in S. salar smolts in fresh water and after transfer to seawater ( SW ; experiments 1 and 2), and in post‐smolts kept at different salinities (0, 12, 22 and 34) for 3 months (experiment 3). A transient increase in plasma Ghbp, which lasted for 1 month or less, was noted in smolts after transfer to SW . Concomitantly, plasma GH and gill Na + –K + – ATPase activity increased during smoltification (in experiment 2). No difference in plasma Ghbp was evident between post‐smolts kept at different salinities, although the fish kept at salinity 34 had higher plasma GH than the group kept at salinity 22 and higher hepatic ghr1 expression than post‐smolts kept at salinity 12. This suggests that plasma Ghbp regulation may respond to salinity changes in the short term. The lack of correlation between Ghbp, plasma GH and hepatic ghr1 expression in the long‐term post‐smolt experiment indicates that Ghbp levels may be regulated independently of other components of the endocrine GH system in salmonids.