[Comparisons of effects of vitamin D3 metabolites on mineralization: utilization of incisor dentin in parathyroidectomized rats].

The rat incisor is continuously growing throughout the animal's life recording the state of mineralization within its dentin at all times. Therefore, it was assumed that the incisor dentin was suitable material for studying the effects of various drugs on such mineralization. The present study set out to examine the effects of vitamin D3 metabolites on the remineralization of dentin and the serum calcium level in parathyroidectomized (PTX) rats. Male Wistar rats weighing about 200-250 g were parathyroidectomized. Each rat was given daily 12 g of a synthetic diet containing 0.1% Ca and 0.4% P (normal P) or 0.06% P (P deficient) without vitamin D. Exp. I: Three groups of rats given a normal P diet were orally administered vehicle, 25 (OH) D3 (50 IU/rat) or 1,25 (OH)2D3 (5 IU/rat) for 5 days from the 28th day after PTX, respectively. Exp. II: Two groups of rats given a normal P diet or a P deficient diet were further divided into 4 sub-groups and intramuscularly administered 0, 5,000, 10,000 or 20,000 IU of vitamin D3 on the 9th day after PTX, respectively. The active vitamin D3, 1,25 (OH)2D3 restored the mineralization of the dentin which had been suppressed following PTX, and elevated the serum calcium level to about 9 mg/dl. However, the 25 (OH) D3 neither restored the mineralization of the dentin nor elevated the serum calcium level in the PTX rats. At doses below 1,000 IU, vitamin D3 could not restore the mineralization of the dentin. However, at massive doses above 5,000 IU, the mineralization was restored in proportion to the dose. The serum calcium levels also increased in proportion to the dose. It is assumed that the increase of the serum calcium level was mainly due to the enhancement of calcium absorption from the intestine by vitamin D3. Both remineralization of the incisor dentin and increase of the serum calcium level were more marked in groups of rats given a P deficient diet than in those given a normal P diet. In all cases in the present experiments, the degree of mineralization of the dentin seemed to correspond with the degree of elevation of the serum calcium levels. These results suggest that the remineralization of the dentin in the PTX rats was attributable to the effect of the vitamin D3 metabolites on serum calcium regulation. Relative biological activities of 1,25 (OH)2D3, 25 (OH) D3 and vitamin D3 on the remineralization of dentin were estimated to be 1, less than 0.02 and less than 0.001, respectively.