The vitamin D receptor and its ligand 1 α,25-dihydroxyvitamin D3 in Atlantic salmon (Salmo salar)

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 (similar to 1.25mM 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 (sVDRO) 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)D-3 and 24,25(OH)(2)D-3 did not change significantly. Plasma calcitriol concentrations were lowest in FW part, 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.