| Summary: | In preparation for migration from freshwater to marine habitats, Atlantic salmon ( Salmo salar ) undergo smoltification, a springtime transformation that includes the development of salinity tolerance. The growth hormone/insulin‐like growth‐factor (Igf) network is known to mediate the development of branchial ionoregulatory functions underlying salinity tolerance. Igfs interact with a suite of Igf binding proteins (Igfbps) that modulate hormone activity. In Atlantic salmon, igfbp4 , ‐5a , ‐5b1 , ‐5b2 , ‐6b2 and ‐6b1 are highly expressed in branchial tissue. Here, we characterized the dynamics of igfbp expression during smoltification and following acute seawater (SW) challenges. We observed increased branchial Na + , K + ‐ATPase activity and Na + ,K + ,2Cl ‐ ‐cotransporter expression characteristic of smoltification. igfbp5a , ‐5b1 and ‐5b2 expression did not peak in Spring. Nonetheless, igfbp5a , ‐5b1 and ‐5b2 showed diminished expression following SW challenge in March. igfbp6b1 and ‐6b2 expression, however, did increase in Spring, peaking with smoltification. Following a SW challenge in March, igfbp6b1 showed increased expression while igfbp6b2 exhibited diminished expression. These data suggest that dynamic igfbp expression accompanies smoltification and SW challenges. Transcriptional control of igfbps may thus provide a mechanism for the local modulation of Igf activity in gill.
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