Periodontal inflammation triggers a site‐specific and wide radius of calcium metabolic effects on alveolar bone

Abstract Background and Objective There is a close relationship between inflammation and bone remodeling in the periodontium. However, previous studies have not delineated the alterations in calcium (Ca) metabolism during periodontitis progression. The aim of this current investigation was to examine Ca dynamics in alveolar bone of rats during progression of ligature‐induced periodontal inflammation by using 45 Ca, which is an index of hard tissue neogenesis. Material and Methods To induce periodontitis, the maxillary right first molar (M1) of 8‐week‐old male rats was ligated with a silk suture for 1, 3, 7, and 28 days. The left M1 was not ligated as a control. To evaluate resultant changes in bone neogenesis, 45 CaCl 2 was injected intraperitoneally 24 hours before euthanasia. The left‐and‐right palatal mucosa, molar teeth (M1 and M2), and alveolar bone were harvested for evaluation of 45 Ca radioactivity using a liquid scintillation counter. The distribution of 45 Ca in maxillary tissues was evaluated using autoradiography (ARG). In addition, we analyzed the bone volume fraction (BV/TV) and bone mineral density (BMD) of the alveolar bone by micro‐computed tomography. To investigate the number of osteoclasts and osteoblasts, tartrate‐resistant acid phosphatase (TRAP) and bone‐specific alkaline phosphatase (BAP) were measured by an enzymatic assay and immunohistochemistry, respectively. Results 45 Ca radioactivity in the alveolar bone of the ligature side decreased by 8% compared to the unligated control‐side on day 1, whereas on day 7, it markedly increased by 33%. The 45 Ca levels in the gingival tissue and molar teeth were slightly but significantly lower than the control‐side on day 1 and higher from day 3 to 28. The variation in 45 Ca levels for the alveolar bone was greater and specific compared with other tissues. Furthermore, on day 7, ARG data revealed that 45 Ca on the control side was primarily localized to the periodontal ligament (PDL) space and alveolar bone crest and barely detected in the gingival tissues and deeper parts of the alveolar bone. On the ligature side, 45 Ca disappeared from the PDL and alveolar crest, but instead was broadly and significantly increased within the deeper zones of the alveolar bone and furcation areas and distant from the site of ligature placement and periodontal inflammation. In the shallow zone of the alveolar bone, these changes in 45 Ca levels on day 7 were consistent with decreases in the bone structural parameters (BV/TV and BMD), enhanced osteoclast presence, and suppressed levels of BAP expression in osteoblasts. In contrast, the deep zone and furcation area showed that TRAP‐positive cells increased, but BAP expression was maintained in the resorption lacunae of the alveolar bone. Conclusion During periodontitis progression in rats, 45 Ca levels in the alveolar bone exhibited biphasic alterations, namely decreases and increases. These data indicate that periodontitis induces a wide range of site‐specific Ca metabolism alterations within the alveolar bone.