Environmental salinity regulates the in vitro production of [ 3H]-1,25-dihydroxyvitamin D 3 and [ 3H]-24,25 dihydroxyvitamin D 3 in rainbow trout ( Oncorhynchus mykiss)

Abstract

Previous studies have shown that specific binding of 1,25-dihydroxyvitamin D 3 (1,25(OH) 2D 3) and 24,25-dihydroxyvitamin D 3 (24,25(OH) 2D 3) to enterocyte basolateral membranes (BLM), as well as circulating concentrations, is affected in response to changes in environmental salinity. It is not known if the production of 1,25(OH) 2D 3 and 24,25(OH) 2D 3 is affected by environmental salinity. The aim of the present study was to measure the in vitro production of [ 3H]-1,25(OH) 2D 3 and [ 3H]-24,25(OH) 2D 3 in fresh water (FW) and after 1, 2, 3, and 7 days after transfer to seawater (SW). Pooled sub-cellular fractions (mitochondria and microsomes) from liver or kidney was incubated with [ 3H]-25(OH)D 3 and the produced metabolites were separated using HPLC. Hepatic production of [ 3H]-1,25(OH) 2D 3 was decreased after 24 h in SW. This was followed by an up-regulation after 48 h and a second, slower decrease in production rate which leveled out after 7 days in SW. The production rate in SW was lower than the original rate in FW-adapted fish. For hepatic [ 3H]-24,25(OH) 2D 3 production the pattern was reversed. Renal production of [ 3H]-24,25(OH) 2D 3 increased significantly during the period of SW acclimation. These results suggest that environmental salinity regulates the production rate of the two antagonizing calcium regulatory hormones; 1,25(OH) 2D 3 and 24,25(OH) 2D 3. This gives further evidence to the hypothesis that there is a physiological regulation and a differentiated importance of 1,25(OH) 2D 3 and 24,25(OH) 2D 3 in relation to environmental calcium concentrations.