Impaired homologous upregulation of vitamin D receptor in rats with chronic renal failure.

We studied the homologous regulation of the vitamin D receptor (VDR) in the duodenum of rats with chronic renal failure. Mean basal nuclear 3H-labeled 1 alpha,25-dihydroxyvitamin D3 ([3H]1,25(OH)2D3) binding capacity was 48 and 43 fmol/mg protein for sham-operated and uremic rats with similar dissociation constants (Kd), respectively. These results coincided with those of immunoblot analysis, which found that VDR protein level of uremic rats was 87.6% that of sham-operated rats. In uremic rats, 1,25(OH)2D3, 2.0 micrograms/kg, failed to upregulate VDR protein levels until 24 h, in contrast to the significant increases produced in sham-operated rats at both 12 (1.55-fold) and 24 h (1.75-fold). Baseline level of VDR mRNA in uremic rats, determined by Northern blot analysis, was comparable to that in sham-operated rats. Treatment with 1,25(OH)2D3 slightly decreased VDR mRNA at 6-24 h in the sham-operated rats, in contrast to the increase seen at 6 h in uremic rats. These results suggest that the homologous upregulation of VDR was attenuated in rats with chronic renal failure because of an impairment at a translational and/or posttranslational step.