[Effects of 1,25-dihydroxyvitamin D3 on mineralization of incisor dentin in parathyroidectomized rats]

Male Wistar rats (8 weeks old) were parathyroidectomized (PTX) and given daily subcutaneous injections of different doses (0, 25, 50 or 100 ng/kg) of 1,25 (OH)2D3 for 13 days. Changes in plasma calcium and phosphorus levels were monitored at 3 day intervals. An appositional rate of dentin was estimated by a time marking method; that is, the distance of two lead lines deposited in a transverse section of the incisor dentin on the 9th and 12th days of the experimental period was measured. The plasma calcium levels that had been lowered by PTX were elevated dose-dependently following injections of the drug, but the plasma phosphorus levels that had been elevated by PTX were lowered. The body weight was not affected by the administration of the drug. However, the values for the dry weight and ash weight, and the calcium and phosphorus contents of the upper incisors increased dose-dependently. The mineralization of the dentin, that had been suppressed by PTX, also recovered dose-dependently. Apposition rates of the dentin in groups of rats given various doses of the drug (25, 50 and 100 ng/kg/day) averaged 46, 61 and 69 microns/3 days respectively. There was a significant correlation between the apposition rate of the dentin and the plasma calcium level. The correlation coefficients were estimated to be 0.956 (y = 17.2 + 4.6 chi, p less than 0.001) on the lingual aspect and 0.941 (y = 11.4 + 5.3 chi, p less than 0.001) on the labial aspect. However, the values for the mineralization of incisor dentin were inversely proportional to the plasma phosphorus levels. These results indicate that the stimulating effect of 1,25 (OH)2D3 on the mineralization of the dentin in PTX rats is primarily dependent upon the increase in plasma calcium levels by the hormone. The direct effect of 1,25 (OH)2D3 on the mineralization of dentin could not be demonstrated in this experiment.