The blood: bone disequilibrium. II. Evidence against the active accumulation of calcium or phosphate into the bone extracellular fluid.

An attempt was made to quantitate in vitro the unidirectional fluxes of Ca2+ and Pi across periosteal and endosteal membranes of frontal bones from calvaria of 20 day chick embryos. The influx of Ca2+, as previously reported, seemed to be passive [12]. The uncoupler, CCCP, had no effect on the influx of Ca2+, but increased both the endosteal and periosteal effluxes. La2+ decreased the influx of Ca2+. The influx of phosphate was also passive and proportional to the concentration of phosphate. The influx was unaffected by either iodoacetate or CCCP, but increased when the periosteal and endosteal membranes were removed; the efflux of phosphate was greater from the endosteal side and was unaffected by CCCP, while iodoacetate increased both the periosteal and endosteal effluxes. The evidence presented refutes the generally accepted hypothesis that Ca2+ (or phosphate) fluxes into and out of bone are governed by a pumpleak system. However, the increase in rate of influx when the cellular layers are removed indicates that the periosteum and endosteum constitute an important structural conponent in the maintenance of bone mineral and its equilibration with the circulation.