PTH increases renal 25(OH)D₃-1α-hydroxylase (CYP1α) mRNA but not renal 1,25(OH)₂D₃production in adult rats

The capacity of parathyroid hormone (PTH) to stimulate renal 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] production declines with age in the rat. The purpose of these studies was to determine whether this decline is due to a decreased capacity of PTH to increase the mRNA levels of CYP1alpha, the cytochrome P-450 component of the 25(OH)D(3)-1alpha-hydroxylase. Young (2 mo) and adult (12 mo) male Fischer 344 rats were parathyroidectomized (PTX). After 72 h, PTX rats were injected with PTH or vehicle at 24, 6, and 3 h before death, and renal CYP1alpha mRNA levels were measured by ribonuclease protection assay. In young rats, PTH markedly increased plasma 1,25(OH)(2)D(3) and renal 1,25(OH)(2)D(3) production. However, in adult rats, the response to PTH was less than 30% of that seen in young rats. Renal CYP1alpha mRNA levels, on the other hand, were increased over fivefold by PTH in both young and adult rats. In in vitro studies, PTH/forskolin increased CYP1alpha mRNA levels over twofold in renal slices from both young and adult PTX rats. These studies demonstrate that the decreased capacity of PTH to increase 1,25(OH)(2)D(3) production in adult rats is not due to decreased induction of CYP1alpha mRNA.