HOMOLOGOUS UP-REGULATION OF VITAMIN-D RECEPTORS IS TISSUE-SPECIFIC INTHE RAT

1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) receptors (VDR) are express ed 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 nor mal vitamin D-snfficient (+D) diet showed elevated VDR levels in kidne y (391 +/- 53 vs. 913 +/- 76 fmol/g of tissue; p < 0.05), but not in t estis, 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 se en in intestine, testis, or lung. Because 1,25(OH)(2)D-3-induced hyper calcemia may independently affect VDR regulation, 1,25(OH)(2)D-3 was i nfused into -D rats, and normocalcemia was maintained by reduced dieta ry 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 nat ure of the homologous VDR regulation. Assay of VDR levels by L-1-tosyl amido-2-phenylethyl chloromethyl ketone-H-3-1,25(OH)(2)D-3 exchange as say ruled out differences in endogenous 1,25(OH)(2)D-3 occupancy as th e basis for the observed differences in VDR regulation, Finally, coide ntity of the VDR-like sites in kidney versus testis was confirmed by c ompetitive 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 s pecific monoclonal anti-rat VDR antibody. Thus, under a wide variety o f experimental conditions, homologous up-regulation of the VDR occurs in the wt kidney in vivo, but not in several other target tissues whic h do not regulate plasma calcium homeostasis. Moreover, this different ial VDR regulation did not result from secondary changes in plasma cal cium, from differential 1,25(OH)(2)D-3 responsiveness in the various t issues, nor from differences in endogenous 1,25(OH)(2)D-3 occupancy of the VDR. These studies thus establish that, in contrast to observatio ns in vitro, the widely described phenomenon of homologous VDR np-regu lation in kidney and intestine is not a universal property of 1,25(OH) (2)D-3 target tissues in vivo in the rat.