Effects of aluminum on bone surface ion composition

Abstract

Aluminum induces net calcium efflux from cultured bone. To determine whether aluminum alters the bone surface ion composition in a manner consistent with predominantly cell-mediated resorption, a combination of cell-mediated resorption and physicochemical dissolution or physicochemical dissolution alone, we utilized an analytic high-resolution scanning ion microprobe with secondary ion mass spectroscopy to determine the effects of aluminum on bone surface ion composition. We cultured neonatal mouse calvariae with or without aluminum (10⁻⁷ M) for 24 h and determined the relative ion concentrations of ²³Na, ²⁷AI, ³⁹K, and ⁴⁰Ca on the bone surface and eroded subsurface. Control calvariae have a surface (depth ∼6 nm) that is rich in Na and K compared with Ca (Na/Ca = 24.4 + 1.4, mean + 95% confidence limit of counts per second of detected secondary ions, K/Ca = 13.2 + 0.9). Aluminum is incorporated into the bone and causes a depletion of surface Na and K relative to Ca (Na/Ca = 9.6 + 0.7, K/Ca = 4.9 + 0.4; each p < 0.001 versus control). After erosion (depth ∼50 nm), control calvariae have more Na and K than Ca (Na/Ca = 16.0 + 0.1, K/Ca = 7.5 + 0.1); aluminum again depleted Na and K relative to Ca (Na/Ca = 4.1 + 0.1, K/Ca = 1.9 + 0.1; each p < 0.001 versus control). Aluminum produced a greater net efflux of Ca (362 ± 53, mean ± SE, nmol/bone/24 h) than control (60 ± 30, p < 0.001). With aluminum, the fall in the ratios of both Na/Ca and K/Ca coupled with net Ca release from bone indicates that aluminum induces a greater efflux of Na and K than Ca from the bone surface and is consistent with an aluminum-induced removal of the bone surface. This alteration in surface ion concentration and calcium efflux is consistent with that observed when calcium is lost from bone through a combination of cell-mediated resorption and physicochemical dissolution.