The effect of vitamin D restriction and repletion on bone apposition in the rat and its dependence on parathyroid hormone.

The regulatory role of vitamin D in bone formation and its interaction with parathyroid hormone (PTH) were analyzed in rats in vivo. The bone apposition rate was determined by measuring the distances between tetracycline lines deposited at 48-h intervals. Vitamin D restriction was associated with a decrease in the bone apposition rate and was paralleled by the expected decrease in serum 25-hydroxyvitamin D3 levels but not by changes in 1,25-dihydroxyvitamin D3 levels. Vitamin D3 repletion restored the apposition rate to normal in parathyroid-intact animals but had no effect in parathyroidectomized animals. Vitamin D repletion of PTH-repleted parathyroidectomized animals restored the bone apposition rate. These results indicate that vitamin D or vitamin D metabolites are required for the expression of the effect of PTH on bone apposition. Conversely, vitamin D repletion only affects the bone formation rate in the presence of PTH, thus suggesting the requirement for PTH-dependent vitamin D metabolism.