Dexamethasone downregulates vitamin D receptors in rat kidney, unmasking a high affinity binding site

Dexamethasone exerted tissue-specific effects on rat vitamin D receptor levels: upregulation of receptors in intestine, downregulation in kidney, and no effect on receptors in testis, heart, and lung. Scatchard analysis showed selective downregulation of the low affinity receptor site (Kd = 0.9 nM) in kidney, thus unmasking a high affinity binding site (Kd = 0.05 nM). The single low affinity site in intestine (Kd = 0.3-0.7 nM) was upregulated, while the single high affinity site in testis (Kd = 0.07-0.09 nM) was not changed. These results demonstrate the existence of two types of nuclear binding sites in rat kidney and establish that there are similarities between the high affinity binding site in the rat kidney and the single high affinity receptor site in the testis.