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¹ National Institute of Nutrition and Seafood Research, PO Box 2029 Nordnes, N-5817, Bergen, Norway
² Department of Animal Physiology, Faculty of Science, Radboud University Nijmegen, Toernooiveld 1 NL-6525ED, Nijmegen, The Netherlands
³ Division of Pediatrics, Department of Clinical and Molecular Medicine, University of Bergen, Haukeland University Hospital, N-5021, Bergen, Norway

(Requests for offprints should be addressed to G Flik; Email: g.flik{at}science.ru.nl)

Seaward migration of Salmo salar is preceded by preparatory physiological adaptations (parr–smolt transformation) to allow for a switch from freshwater (FW) to seawater (SW), which also means a switch in ambient calcium from hypocalcic (<1 mM Ca²⁺) to the plasma (~1.25 mM Ca²⁺) and to strongly hypercalcic (8–12 mM Ca²⁺). Uptake, storage (skeleton, scales) and excretion of calcium need careful regulation. In fish, the vitamin D endocrine system plays a rather enigmatic role in calcium physiology. Here, we give direct evidence for calcitriol involvement in SW migration. We report the full sequence of the nuclear vitamin D receptor (sVDR0) and two alternatively spliced variants resulting from intron retention (sVDR1 and sVDR2). In FW parr, SW adapting smolts, and in SW adults, plasma concentrations of 25(OH)D₃ and 24,25(OH)₂D₃ did not change significantly. Plasma calcitriol concentrations were lowest in FW parr, doubled during smoltification and remained elevated in SW adults. Increased calcitriol coincided with a twofold decrease in sVDR mRNA levels in gill, intestine, and kidney of FW smolts and SW adults, when compared with parr. Clearly, there was a negative feedback and dynamic response of the vitamin D endocrine system during parr–smolt transformation. The onset of these dynamic changes in FW parr warrants a further search for the endocrines that initiate these changes. We speculate that the vitamin D system plays a crucial role in calcium and phosphorus handling in Atlantic salmon.