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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Salinity acclimation affects the somatotropic axis in rainbow trout

¹National Center for Cool and Cold Water Aquaculture/Agricultural Research Service/United States Department of Agriculture, Kearneysville, West Virginia; ²Department of Biology, University of Kentucky, Lexington, Kentucky; ³Department of Biology, Wilfrid Laurier University, Waterloo, Ontario, Canada; and ⁴School of Optometry and ⁵Department of Biology, University of Waterloo, Waterloo, Ontario, Canada

Submitted 5 July 2004 ; accepted in final form 9 December 2004

In this study, we set out to examine the role of the somatotropic axis in the ion-regulation process in rainbow trout. Specifically, our objective was to examine whether plasma insulin-like growth factor-binding proteins (IGFBPs) are modulated by gradual salinity exposure. To this end, freshwater (FW)-adapted rainbow trout were subjected to gradual salinity increases, up to 66% seawater, over a period of 5 days. During this acclimation process, minimal elevations in plasma Ca²⁺ and Cl^– were seen in the salinity-acclimated groups compared with FW controls. There were no changes in plasma Na⁺ levels, and only a minor transient change in plasma cortisol levels was seen with salinity exposure. The salinity challenged animals responded with elevations in plasma growth hormone (GH) and IGF-I levels and gill Na⁺-K⁺-ATPase activity. We identified IGFBPs of 21, 32, 42, and 50 kDa in size in the plasma of these animals, and they were consistently higher with salinity. Despite the overall increase in IGFBPs with salinity, transient changes in individual BPs over the 5-day period were noted in the FW and salinity-exposed fish. Specifically, the transient changes in plasma levels of the 21-, 42-, and 50-kDa IGFBPs were different between the FW and salinity groups, while the 32-kDa IGFBP showed a similar trend (increases with sampling time) in both groups. Considered together, the elevated plasma IGFBPs suggest a key role for these binding proteins in the regulation of IGF-I during salinity acclimation in salmonids.

insulin-like growth factor binding-proteins; insulin-like growth factor-I; growth hormone; rainbow trout; salinity

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