Differential regulation of vitamin D receptor and intestinal calcium transport occurring during sexual maturation in the fowl (Gallus domesticus).

Quantitative measurements of intestinal vitamin D receptor (VDR) and calcium transport taking place in individual villus-attached enterocytes have been compared during the onset of sexual maturity to test for a direct involvement of VDR in controlling calcium transport across the chick small intestine. Chickens of all ages showed VDR to be present in crypt and villus enterocytes, the amounts of VDR declining only slightly during enterocyte migration from the crypts to the tips of villi. Calcium transport corrected for initial adsorption was lowest in the crypt and highest in villus tip enterocytes. These results are consistent with VDR initiation and possible later maintenance of calcium transport across differentiating enterocytes. The total amount of VDR determined along the crypt-villus axis was significantly less in immature 11-week-old compared with 17-week-old sexually mature non-laying and 25-week-old laying chickens. Calcium transport was significantly greater in 25-week-old compared with 17- and 11-week-old birds. This unexpected up-regulation of VDR in 17-week-old birds was not affected by dietary restriction of calcium. Increased VDR expression in 17-week-old sexually mature birds is probably initiated by estrogen acting indirectly to increase 1,25 (OH)2D3 production in the kidney. Increased calcium transport in 25-week-old laying chickens could involve estrogen interacting with estrogen receptors as well as 1,25 (OH)2D3 interacting with vitamin D receptors to promote gene transcription in the intestine.