1,25-Dihydroxyvitamin D3 up-regulates the 1,25-dihydroxyvitamin D3 receptor in vivo.

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Abstract
The level of mRNA encoding the 1,25-dihydroxyvitamin D3 receptor in the intestine of vitamin D-deficient rats given 1,25-dihydroxyvitamin D3 was determined by Northern blot analysis using a 32P-labeled cDNA probe to the 1,25-dihydroxyvitamin D3 receptor. mRNA levels increased 10-fold above deficiency levels at 6 and 12 hr after an intravenous dose of 1,25-dihydroxyvitamin D3, returning to predosing levels at 24 hr. Total receptor protein level determined by an immunoradiometric assay was increased 2-fold at 12 hr. No change in unoccupied receptor levels determined by ligand-binding assay was observed during this period. These results suggest that 1,25-dihydroxyvitamin D3 increases receptor mRNA and total receptor level to maintain constant levels of unoccupied receptor.