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 microm spatial resolution) were performed on metatarsal bones of weanling mice after drilling a 100-mum 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 (>or=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.
Marenzana
M
M
Bone Metabolic Unit, Sci. Inst. H. San Raffaele, Via Olgettina 60, 20132 Milan, Italy.
Shipley
A M
AM
Squitiero
P
P
Kunkel
J G
JG
Rubinacci
A
A
D000818:Animals; D001862:Bone Resorption; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D017136:Ion Transport; D051379:Mice; D008807:Mice, Inbred BALB C
D002118:Calcium