Homologous up-regulation of vitamin D receptors is tissue specific in the rat

1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) receptors (VDR) are expressed in multiple tissues within the body. VDR levels are increased by 1,25(OH)(2)D-3 in intestine and kidney and in numerous cell models. The ability of 1,25(OH)(2)D-3 to affect VDR levels in other target tissues in vivo was studied by assessing VDR levels by the 3(H)-1,25(OH)(2)D-3 binding assay under varied physiological conditions in the rat. When compared with vitamin D-deficient (-D) controls, rats raised on a normal vitamin D-snfficient (+D) diet showed elevated VDR levels in kidney (391 +/- 53 vs. 913 +/- 76 fmol/g of tissue; p < 0.05), but not in testis, heart, or lung Up-regulation of the VDR also occurred in kidney of +D rats 1 day after a single 100-ng dose of 1,25(OH)(2)D-3 (454 +/- 43 vs. 746 +/- 113 fmol/mg of DNA; p < 0.05), but no changes were seen in intestine, testis, or lung. Because 1,25(OH)(2)D-3-induced hypercalcemia may independently affect VDR regulation, 1,25(OH)(2)D-3 was infused into -D rats, and normocalcemia was maintained by reduced dietary calcium intake. In this model, the renal VDR was again up-regulated (446 +/- 115 vs, 775 +/- 58 fmol/mg of DNA; p < 0.05), but VDR levels in testis and lung were unaffected. Scatchard analysis and tests of 1,25(OH)(2)D-3 dose (1-100 ng/day for 7 days) and temporal (100 ng/day for 1-7 days) responsiveness further supported the tissue-specific nature of the homologous VDR regulation. Assay of VDR levels by L-1-tosylamido-2-phenylethyl chloromethyl ketone-H-3-1,25(OH)(2)D-3 exchange assay ruled out differences in endogenous 1,25(OH)(2)D-3 occupancy as the basis for the observed differences in VDR regulation, Finally, coidentity of the VDR-like sites in kidney versus testis was confirmed by competitive binding analysis comparing their relative affinities for 25(OH)D-3 versus 1,25(OH)(2)D-3 (30.5 +/- 6.4 vs, 35.6 +/- 3.6 in kidney and testis, respectively) and by immunoblot analysis using a highly specific monoclonal anti-rat VDR antibody. Thus, under a wide variety of experimental conditions, homologous up-regulation of the VDR occurs in the wt kidney in vivo, but not in several other target tissues which do not regulate plasma calcium homeostasis. Moreover, this differential VDR regulation did not result from secondary changes in plasma calcium, from differential 1,25(OH)(2)D-3 responsiveness in the various tissues, nor from differences in endogenous 1,25(OH)(2)D-3 occupancy of the VDR. These studies thus establish that, in contrast to observations in vitro, the widely described phenomenon of homologous VDR np-regulation in kidney and intestine is not a universal property of 1,25(OH)(2)D-3 target tissues in vivo in the rat.