Development of a vitamin D-responsive organ culture system for adult and old rat intestine.

The purpose of this study was to develop an organ culture system for adult and old rat small intestine that is responsive to vitamin D. Explants from F344 rats were cultured on Millipore tissue culture inserts placed in 6-well dishes at a temperature of 28 degrees C and in the presence of 95% oxygen. Explants from young (2 months old), adult (12 months old), and old (22 months old) rats were viable for up to 12 hr as determined by constant rates of DNA and protein synthesis. Hormonal responsiveness was characterized by measuring the capacity of 1,25-dihydroxyvitamin D [1,25(OH)2D], the hormonal form of vitamin D, to increase mRNA levels of the intestinal 24-hydroxylase cytochrome P-450 (CYP24). Jejunal explants from young rats increased CYP24 mRNA levels in a linear fashion with an EC50 of 3 nM in response to 1,25(OH)2D. There was no change with age in the magnitude of the jejunal response with regard to time (0-12 hr) or dose (0.1-100 nM). However, in the duodenum, 1,25(OH)2D increased CYP24 mRNA to significantly higher levels in the adult compared to the young. Since the 24-hydroxylase is the first step in the degradative pathway for 1,25(OH)2D in the intestine, increased duodenal expression of the 24-hydroxylase may contribute to the decreased action of 1,25(OH)2D on the adult duodenum.