Young animals adapt to a low calcium diet by increasing renal production of 1,25-dihydroxyvitamin D [1,25(OH)2D], the active metabolite of vitamin D. However, the capacity of adult animals to adapt is markedly diminished. With the recent cloning of the cytochrome P450 component (CYP1a) of the renal 1-hydroxylase enzyme complex, it is now possible to determine directly the effect of dietary calcium and maturation on the expression of renal 1-hydroxylase. Using a ribonuclease protection assay, it was found that feeding a low Ca diet markedly increased renal CYP1a mRNA levels in young rats. However, feeding this diet to adult rats produced an increase in CYP1a mRNA that was only 10% that of the young rats. These studies demonstrate that a low calcium diet increases renal 1,25-dihydroxyvitamin D production in young animals but not in adult animals by increasing CYP1a expression. Since the low calcium diet increased plasma parathyroid hormone levels to similar levels in both age groups, this suggests that in the adult there is a renal refractoriness to parathyroid hormone.
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