The vitamin D receptor and its ligand 1 ,25-dihydroxyvitamin D3 in Atlantic salmon (Salmo salar)
Journal of Endocrinology
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 Ca 2C ) to the plasma (w1 . 25 mM Ca 2C ) and to strongly hypercalcic (8-12 mM Ca 2C ). 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
... 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)D 3 and 24,25(OH) 2 D 3 did not change significantly. Plasma calcitriol concentrations were lowest in FW parr, doubled during smoltification and remained elevated in SWadults. 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.