Intestinal metabolism and portal venous transport of 1,25(OH)2D3, 25(OH)D3, and vitamin D3 in the rat.

We compared the intestinal absorption of three vitamin D3 sterols and tested the hypothesis that the intestine hydroxylates absorbed vitamin D and transports polar metabolites in portal venous blood. Micellar solutions containing 50 nmol of a radiolabeled vitamin D3 sterol (1,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3, or vitamin D3) were placed in closed jejunal segments of rats prepared with lymphatic and mesenteric venous fistulas. Venous blood loss was replaced by infusion of donor rat blood into the saphenous vein. After 1-2 h the rats were killed, and intestinal homogenates, mesenteric blood, and lymph were analyzed. The average rate of absorption of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] was two- and fivefold higher than that of 25-hydroxyvitamin D3 [25(OH)D3] and vitamin D3 (D3), respectively. Transport of hydroxylated vitamin D sterols was primarily via the venous route; the average rate of venous transport of 1,25(OH)2D3 was 18.3 X 10(2) nmol X min-1 X g-1 compared with 8.8 X 10(2) for 25(OH)D3 and 0.13 X 10(2) for D3. High-performance liquid chromatography of intestinal and plasma extracts revealed that there was 25-hydroxylation of absorbed D3, 24- and putative 1-hydroxylation of absorbed 25(OH)D3, and prompt portal venous transport of all hydroxylated metabolites. When 1,25(OH)2D3 was infused into the lumen, the composition of radiolabeled sterols found in intestinal homogenates and mesenteric venous plasma was virtually identical to that of the infusate. These studies provide in vivo evidence for the intestinal metabolism of pharmacological quantities of absorbed vitamin D3 sterols and the prompt portal venous transport of more polar metabolites.