Regulation of 1,25(OH)2 vitamin D3 receptor content in cultured LLC-PK1 kidney cells limits hormonal responsiveness

Receptor content in cultured kidney (LLC-PK1) cells was found to be modulated following the introduction of a culture medium change, declining to 40% of control values at 18 h. Scatchard analysis indicated that the reduced 1,25(OH)2-[3H]D3 nuclear binding we detected was due to decreased abundance of receptors (3811 vs 1619 sites/cell) with no change in the Kd (0.4-0.5 nM). Cells with reduced receptors exhibited diminished ability to respond to 1,25(OH)2D3 as measured by induction of 25(OH)vitamin D-24-hydroxylase activity. There was a close coupling between decreased receptor levels and diminished hormone responsiveness. The data suggest the absence of "spare" receptors and that receptor abundance is a limiting factor in cell responsiveness to 1,25(OH)2D3.