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

In freshwater-adapted rainbow trout, intestinal cells (enterocytes) possess receptors for 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] in the basolateral membrane, and respond to treatment with 1,25(OH)(2)D(3) with increased intracellular calcium concentrations. No receptors are found for the antagonizing hormone 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)] 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)(2)D(3) are down-regulated and specific binding for 24,25(OH)(2)D(3) 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)(2)D(3) and a decreased sensitivity to 1,25(OH)(2)D(3). This results in a partial inhibition of intracellular calcium uptake, which would be beneficial when inhabiting a calcium-rich environment like seawater.