[Dependence of incisor dentin mineralization on plasma calcium levels in parathyroidectomized rats]

This experiment was performed to clarify whether mineralization of incisor dentin depends upon plasma calcium (Ca) levels. Male Wistar rats (7 weeks old) were parathyroidectomized (PTXed) and given vitamin D-deficient diets containing various amounts (0.3-2%) of Ca for 2 weeks in order to maintain the plasma Ca at various levels and to examine their relationship with the mineralization of incisor dentin. Plasma Ca levels, which were lowered to about 6 mg/dl by PTX, ranged from 5.1 to 10.1 mg/dl after feeding on diets containing various amounts of Ca; the plasma Ca level in the group given 2% Ca reached the same level as that in the sham-operated group. On the other hand, plasma phosphorus (P) levels, which were increased by PTX, reduced markedly with increases in dietary Ca, leading to hypophosphatemia in the group given 2.0% Ca. In this series of experiments, the plasma Ca levels in PTXed rats seemed to be kept constant by the dietary Ca without any involvement of hormones regulating the Ca metabolism because a high correlation was found between the plasma Ca and the dietary Ca (r = 0.987). Both dry and ash weights of the upper incisor also increased with the plasma Ca levels, but with the plasma P levels it was just the opposite. The content of ash in the incisor also increased with the plasma Ca levels. Transverse sections of the lower incisors stained with hematoxylin showed a zone of remineralization of the dentin on the lingual aspect in rats with plasma Ca levels of about 6.5-7.0 mg/dl; the width of the zone was greater in groups of rats with higher plasma Ca levels. Appositional rates of dentin (y, micron/3 days) as determined by a time marking method showed a high positive correlation with the plasma Ca levels (x, mg/dl). The regression equations were as follows: y = 4.8 + 5.7x (r = 0.915, p less than 0.001) on the lingual aspect and y = 6.0 + 5.8x (r = 0.959, p less than 0.001) on the labial aspect. There were a high positive correlation and a negative correlation between appositional rates of dentin and plasma Ca: P ratios (r = 0.928) and between appositional rates of dentin and plasma P levels (r = -0.927) or the products of plasma Ca x P (r = -0.834). These results suggest that the mineralization of incisor dentin depends upon the plasma Ca levels in PTXed rats.