Extracellular calcium modulates vitamin d-dependent calbindin-D_28K gene expression in chick kidney cells

The effect of extracellular calcium ion (Ca²⁺) concentration on 1, 25-dihydroxyvitamin D3 (l, 25-(OH)2D3)-induction of vitamin D-dependent calcium-binding protein (calbindin-D28K) and its mRNA levels was examined in primary chick kidney cells in vitro. When exposed to normal medium Ca²⁺ (1.0 mM), 1, 25-(OH)2D3 increased calbindin-D28K mRNA, as measured by Northern analysis, by 4-10 fold over basal levels by 12 to 24 h after addition of hormone. In the presence of 0.5 mM Ca²⁺, 1, 25-(OH)2D3 induced calbindin-D28K mRNA by only 2 fold, whereas, when cells were exposed to 2 mM Ca²⁺, the induction was 10-15 fold. This calcium modulation of 1, 25-(OH)2D3 induction was also observed at the level of calbindin-D28K protein concentrations as measured by radioimmunoassay. the alterations in medium Ca²⁺ were not associated with any change in the rate of total RNA or protein synthesis. These studies suggest that both Ca²⁺ and 1, 25-(OH)2D3 participate in the regulation of calbindin-D2gK gene expression in the kidney.