Effects of pH on bone calcium and proton fluxes in vitro

Bone mineral is thought to decompose during acute and chronic metabolic acidosis and thereby contribute to buffering of the acid load. We cultured neonatal mouse calvariae for 3 h and found calcium efflux from bone when the medium pH was below 7.40, calcium influx into bone when the pH was above 7.40, and no net flux at pH 7.40. The calcium flux varied to the same extent when medium pH was altered by a primary change in the medium bicarbonate concentration or in the partial pressure of carbon dioxide. Calcium and proton fluxes were inversely correlated (r = -0.713, P less than 0.001), and the slope of the linear regression indicated that between 16 and 21 neq of proton entered the calvariae in exchange for each neq of calcium that left. In 24-h cultures, acid medium also caused net calcium efflux from bone, and alkaline medium caused net influx. PTH increased calcium efflux at acid but not at alkaline medium pH. Sodium azide resulted in net influx of calcium into bone at all values of medium pH. Calcium release by cultured calvariae in response to low medium pH is associated with proton buffering; over 3 h the stoichiometry indicates that little buffering is due to the dissolution of calcium-containing crystals. Effects of medium pH on calcium release are amplified by PTH, and calcium efflux can be prevented by the metabolic inhibitor sodium azide.