Labile or Surface Pools of Magnesium, Sodium, and Potassium in Developing Porcine Enamel Mineral

Abstract

The present study was undertaken to assess the labile or surface pools of Mg, Na, and K ions in porcine enamel tissues at various developmental stages. The enamel samples, corresponding to the outer and the inner secretory, the early maturing, and the mature hard enamel, were dissected from the labial sides of permanent incisors of 6- to 8-month-old piglets. Each enamel sample was extracted successively with solutions of de-ionized water and 50 mmol/L Tris-4 mol/L guanidine buffer (for removal of organic matrix proteins, mainly amelogenins). The labile (free or organically bound) pools of Mg, Na, and K were assessed by the total amounts of these ionic species extracted by the water and Tris-guanidine buffer. The surface (adsorbed onto enamel mineral) pool of Mg was assessed directly by determination of the adsorption of Mg onto enamel mineral at various developmental stages. The results showed that: (i) 30-40% of the Mg in the secretory and early maturation enamel was in the surface pool (adsorbed onto the enamel mineral); (ii) 25 to 40% of the total sodium in the enamel samples was in labile forms; and (iii) most (around 70-80%) of the total potassium was readily extracted in water and appeared to originate from the enamel fluid; only marginal portions remained in the solids. The present adsorption studies also indicated that the maximum uptake of magnesium in the early maturation enamel was due mostly to an increase of the occupancy by Mg ions of adsorption sites on the crystal surfaces, which become accessible with a massive removal of enamel matrix proteins. The observation that the surface pool of Mg (and the total Mg content) decreased markedly with the advancement of mineralization is explained by the displacement of the adsorbed Mg from the crystal surfaces by Ca2+ having a higher adsorption affinity for the same adsorption sites and by changes in the properties of the mineral surfaces. The overall results support the contention that incorporation of Mg into developing enamel upon resorption of the organic matrix may play a key role in regulating enamel crystal growth.