Antagonism of fluoride toxicity by high levels of calcium but not of inorganic phosphate during secretory amelogenesis in the hamster tooth germ in vitro

Abstract

Whether the interference by fluoride (F −) with secretory amelogenesis in vitro could be modulated by altering the levels of calcium (Ca) and inorganic phosphate (P) in the medium was investigated. Hamster first upper molar tooth germs in the secretory phase of amelogenesis were exposed to 10 μM–1.31 mM (0.2–25 parts/10 6) of F − in vitro for 2 days in the presence of either low (1.2 mM), moderate (2.1 mM) or high (4.1 mM) levels of Ca, or moderate (1.6 mM) and high (3.6 mM) levels of P. The biosynthesis and secretion of enamel matrix proteins under each of the experimental conditions were examined by labelling with [ 3H]-proline during the last 24 h of culture, and mineralization by labelling with 45Ca and [ 32P]- orthophosphate. With moderate levels of Ca and P (control medium), F − increased the uptake of 45Ca and 32P in a dose-dependent manner; F − did not inhibit the synthesis of matrix proteins but to a moderate extent impaired their secretion. In explants grown in the presence of 52 μM of F − the superficial layers of enamel matrix deposited in vitro (fluorotic matrix) failed to mineralize. Increasing P levels in the medium had no clear histological effect, whereas lowering Ca levels sometimes seemed to aggravate the F − effect. Raising Ca levels improved the histological pattern: in spite of the presence of F −, high Ca levels allowed a limited mineralization of the superficial layer of fluorotic matrix along with a strong rise in mineralization of the deeper layers of pre-exposure enamel. High Ca levels also considerably reduced the cellular changes in secretory ameloblasts induced by 52 μM of F − and slightly counteracted the inhibition of matrix secretion, as measured biochemically. Some of the effects of F − on secretory amelogenesis in vitro can thus be reversed by raising Ca levels in the medium. Therefore, the effect of F − on secretory amelogenesis in vitro seems to be primarily interference with the enamel mineralization process per se and, secondarily, an impairment of matrix secretion.