Evidence for in vivo upregulation of the intestinal vitamin D receptor during dietary calcium restriction in the rat

1,25-Dihydroxyvitamin D3 [1,25(OH)2D31 increases intestinal calcium absorption through events that include binding of 1,25(OH)2D3 to the intracellular vitamin D receptor. In vitro studies using mammalian cell cultures reveal an increase in vitamin D receptor content after exposure to 1,25(OH)2D3. To test the hypothesis that 1,25(OH)2D3 increases enterocyte vi-tamin D receptor content in vivo, male rats were fed either a normal calcium diet (NCD, 1.2 % Ca) or low calcium diet (LCD, 0.002 % Ca). After 21 d LCD increased serum 1,25(011)213 levels (27±3 vs. 181±17 pg/ml, P < 0.001) and increased transepithelial mucosal to serosal calcium fluxes (J.m) across duodenum (65±21 vs. 204±47 nmol/cm2 h, NCD vs. LCD, P < 0.01) and jejunum (23±3 vs. 46±4, P < 0.007). No change in serosal to mucosal calcium fluxes (1..) were observed. LCID increased 1,25(OH)2D3 receptor number threefold in duode-num (32.9±6.7 vs. 98.7±13.7 fmol 1,25(OH)2D3/mg protein) and jejunum (34.1±9.5 vs. 84.9±7.7) without a change in the receptor affinity for 1,25(OH)2D3 (Kd is 0.17±0.06 vs. 0.21±0.02 nM for NCD and LCD duodenum, respectively). Duodenal polyadenylated vitamin D receptor mRNA deter-mined by Northern blot analysis did not increase appreciably during LCD, suggesting that upregulation in vivo may not be due primarily to increased receptor synthesis. The results of this study indicate that under physiologic conditions as during chronic dietary calcium restriction, in-creased intestinal vitamin D receptor content accompanies in-creased calcium active transport. Upregulation of the vitamin D receptor by 1,25(OH)2D3 may result primarily from post-translational processes that decrease degradation of the recep-tor with increased receptor synthesis responsible for a negligi-ble portion of the accumulation