The effect of the chelator 1,10-phenanthroline on the DNA-binding of 1,25-dihydroxyvitamin D receptor was demonstrated. Increasing concentrations of the chelator, up to 3 mM, decreased receptor binding to DNA-cellulose. This effect was specific for the chelating function because the nonchelating analog, 1,7-phenanthroline, did not inhibit DNA-cellulose binding of the receptor. The cations, Cd[superscript]2+ and Zn[superscript]2+, were able to block chelator-mediated inhibition. These cations were unable to reverse the inhibition unless 1,10-phenanthroline was removed by dialysis;The rat osteosarcoma 17/2.8 cells were shown to express both the cellular retinoic acid binding protein and the nuclear retinoic acid receptor. These two proteins exhibit saturable binding of retinoic acid with estimated equilibrium dissociation constants of 51 nM and 0.95 nM, respectively. The binding of various analogs to the nuclear receptor correlates with their ability to up-regulate the 1,25-dihydroxyvitamin D receptor. Northern blot analysis confirmed that these cells express mRNA for retinoic acid receptors [alpha] and [beta];The concentration of retinoic acid receptor [beta] mRNA was essentially unaffected by retinoic acid treatment. A rapid (2 h) increase in retinoic acid receptor [alpha] mRNA concentration was seen with retinoic acid treatment. This increase reached as high as 5-fold by 24 h. 1,25-Dihydroxyvitamin D[subscript]3 did not affect retinoic acid receptor mRNA concentration. Both 1,25-dihydroxyvitamin D[subscript]3 and parathyroid hormone decreased retinoic acid receptor concentration by 50% after 20 h of treatment. Thyroid hormone slightly increased 1,25-dihydroxyvitamin D receptor concentration, whereas retinoic acid substantially increased this receptor concentration in rat osteosarcoma 17/2.8 cells. The retinoic acid increase in 1,25-Dihydroxyvitamin D receptor concentration was attenuated in the presence of thyroid hormone. This effect of thyroid hormone was specific for retinoic acid because thyroid hormone did not alter the 1,25-Dihydroxyvitamin D[subscript]3-mediated regulation of either the 1,25-Dihydroxyvitamin D or the retinoic acid receptors.