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 (P-i) uptake by the small intestine in mammals and birds, we determined the effects of dietary cholecalciferol (vitamin D-3) on P-i uptake by the small intestine, P-i concentrations in the plasma, P-i 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 D-3/kg diet over a 7- to 8-day feeding period. Plasma P-i 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 lU/kg. There were no significant, diet-related differences in plasma levels of 25-hydroxycholecalciferol [25(OH)D-3] and 1,25 dihydroxycholecalciferol [1,25(OH)(2)D-3]. Increasing levels of dietary cholecalciferol also did not enhance in vitro P-i uptakes by the intestine (range of means: 0.22-0.29 nmol/mg tissue min) and P-i concentrations in the intestinal lumen (8.5-13.5 mmol/L). P-i uptake did not differ among tissues incubated in vitamin D-3, 25(OH)D-3, or 1,25(OH)(2)D-3. These results demonstrate that when fish are fed P-sufficient diets, dietary cholecalciferol increases plasma P-i concentrations but decreases liver weights, alterations which are not accompanied by changes in intestinal weight, P-i uptake by the intestine, P-i concentration in the intestinal lumen, and circulating metabolites of cholecalciferol. (C) 1999 Academic Press.