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.
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