Bone as an ion exchange organ: Evidence for instantaneous cell-dependent calcium efflux from bone not due to resorption

The current study tests the hypothesis that basal level and minute-by-minute correction of plasma Ca²⁺ by outward and inward Ca²⁺ fluxes from and into an exchangeable ionic pool in bone is controlled by an active partition system without contributions from the bone remodeling system. Direct real-time measurements of Ca²⁺ fluxes were made using the scanning ion-selective electrode technique (SIET) on living bones maintained ex vivo in physiological conditions. SIET three-dimensional measurements of the local Ca²⁺ concentration gradient (10 μm spatial resolution) were performed on metatarsal bones of weanling mice after drilling a 100-μm hole through the cortex to expose the internal bone extracellular fluid (BECF) to the bathing solution, whose composition mimicked the extracellular fluid (ECF). Influxes of Ca²⁺ towards the center of the cortical hole (15.1 ± 4.2 pmol cm^-2 s^-1) were found in the ECF and were reversed to effluxes (7.4 ± 2.9 pmol cm ^-2 s^-1) when calcium was depleted from the ECF, mimicking a plasma demand. The reversal from influx to efflux and vice versa was immediate and fluxes in both directions were steady throughout the experimental time (≥2 h, n = 14). Only the efflux was nullified within 10 min by the addition of 10 mM/L Na-Cyanide (n = 7), demonstrating its cell dependence. The timeframes of the exchanges and the stability of the Ca²⁺ fluxes over time suggest the existence of an exchangeable calcium pool in bone. The calcium efflux dependency on viable cells suggests that an active partition system might play a central role in the short-term error correction of plasma calcium without the contribution of bone remodeling.