Cholecalciferol modulates plasma phosphate but not plasma vitamin D levels and intestinal phosphate absorption in rainbow trout (Oncorhynchus mykiss).

Since the vitamin D endocrine system modulates phosphorus homeostasis and regulates inorganic phosphate (Pi) uptake by the small intestine in mammals and birds, we determined the effects of dietary cholecalciferol (vitamin D3) on Pi uptake by the small intestine, Pi concentrations in the plasma, Pi concentrations in the intestinal lumen, intestinal weights, liver weights, and concentrations of vitamin D metabolites in the plasma of rainbow trout (Oncorhynchus mykiss) fed phosphorus-sufficient (0.6 g/100 g) diets. Five groups of trout initially weighing 55.8 +/- 0.6 g were fed purified diets containing 0, 300, 2,500, 10,000, and 40,000 IU vitamin D3/kg diet over a 7- to 8-day feeding period. Plasma Pi concentration was higher in trout fed 2,500-40,000 IU/kg diet (8.26 +/- 0.27 mmol/L) than in those fed 0 and 300 IU/kg (6.99 +/- 0.30). Liver weights were 30-50% greater in fish fed 0 IU/kg than in those fed 300-40,000 IU/kg. There were no significant, diet-related differences in plasma levels of 25-hydroxycholecalciferol [25(OH)D3] and 1,25 dihydroxycholecalciferol [1,25(OH)2D3]. Increasing levels of dietary cholecalciferol also did not enhance in vitro Pi uptakes by the intestine (range of means: 0.22-0.29 nmol/mg tissue. min) and Pi concentrations in the intestinal lumen (8.5-13.5 mmol/L). Pi uptake did not differ among tissues incubated in vitamin D3, 25(OH)D3, or 1,25(OH)2D3. These results demonstrate that when fish are fed P-sufficient diets, dietary cholecalciferol increases plasma Pi concentrations but decreases liver weights, alterations which are not accompanied by changes in intestinal weight, Pi uptake by the intestine, Pi concentration in the intestinal lumen, and circulating metabolites of cholecalciferol. Copyright 1999 Academic Press.