Effects of long-day on gill Na+, K+-ATPase gene expression and the development of seawater tolerance in sockeye salmon

Photoperiod is one of the major environmental cues that initiate smolting, but its mechanisms of action are not clear. We therefore examined possible effects of increased daylength on the development of seawater tolerance and gill Na+, K+-ATPase (NKA) in sockeye salmon (Oncorhynchus nerka). Seawater tolerance and related biological changes were assessed in terms of plasma sodium concentration after 24 h seawater challenge tests (plasma Na), gill NKA activity, expression of two isoform genes encoding gill NKA α subunits (α1a and α1b), and plasma levels of cortisol and growth hormone (GH). Juvenile fish transferred to an artificial long-day (16 h light/8 h dark cycle) in December showed lower plasma Na and higher gill NKA activity during winter than fish with held on a short-day (8 h light/16 h dark cycle). The amount of α1b mRNA in the long-day group was higher than that in the short-day group, whereas the amount of α1a mRNA in the long-day group was lower than that in the short-day group. Furthermore, the plasma levels of GH and cortisol in the long-day group increased earlier than in the short-day group. These results indicate that the increased daylength stimulates seawater tolerance, possibly through induction of the gill NKA α1b gene by elevated GH and cortisol.