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Environmental Salinity Regulates Receptor Expression, Cellular Effects, and Circulating Levels of Two Antagonizing Hormones, 1,25-Dihydroxyvitamin D₃ and 24,25-Dihydroxyvitamin D₃, in Rainbow Trout

Fish Endocrinology Laboratory (D.L., K.S.), Department of Zoology/Zoophysiology, Göteborg University, SE-405 30 Göteborg, Sweden; Department of Nutrition and Food Sciences and the Biotechnology Center (D.L., I.N.), Utah State University, Logan, Utah 84322-8700; and Department of Pediatrics (L.A.), University of Bergen, Haukeland Hospital, N-5021 Bergen, Norway

Address all correspondence and requests for reprints to: Dennis Larsson, Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, Göteborg University, Box 463, SE-405 30 Göteborg, Sweden. E-mail: dennis.larsson{at}zool.gu.se.

In freshwater-adapted rainbow trout, intestinal cells (enterocytes) possess receptors for 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] in the basolateral membrane, and respond to treatment with 1,25(OH)₂D₃ with increased intracellular calcium concentrations. No receptors are found for the antagonizing hormone 24,25-dihydroxyvitamin D₃ [24,25(OH)₂D₃] at the enterocyte basolateral membrane, and it has no effect on enterocyte calcium homeostasis. After acclimation to seawater, however, the enterocyte membrane receptors for 1,25(OH)₂D₃ are down-regulated and specific binding for 24,25(OH)₂D₃ appears, which is further up-regulated with time spent in seawater. This shift in receptor expression is concurrent with an increased sensitivity of the enterocytes to 24,25(OH)₂D₃ and a decreased sensitivity to 1,25(OH)₂D₃. This results in a partial inhibition of intracellular calcium uptake, which would be beneficial when inhabiting a calcium-rich environment like seawater.

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