Cellular contribution to pH-mediated calcium flux in neonatal mouse calvariae

Net calcium flux from cultured neonatal mouse calvariae into the culture medium is pH dependent, and acidified culture medium causes egress of calcium from bone. To determine whether calcium flux is mediated by pH effects on bone cell function, we cultured calvariae for 24 h with sodium azide, acetazolamide, parathyroid hormone (PTH), 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or after three successive freeze-thaw cycles, treatments that would be expected to alter bone cell function. We recultured bones for 3 h with the respective treatment and measured calcium flux. Sodium azide and freeze-thaw cycles produced a net influx of calcium (JCa = -22 +/- 7 and -23 +/- 6 nmol X bone-1 X 3 h-1, respectively) compared with net efflux of control bones (JCa = 35 +/- 6) at a similar initial medium pH. Acetazolamide reduced net flux to 0 (JCa = 7 +/- 6). PTH and 1,25(OH)2D3 increased net calcium efflux from bone (JCa = 78 +/- 7 and 74 +/- 10, respectively). Despite changing net flux, the slope dependence of net flux on medium pH was the same in the control group and all five treated groups of bones. The similarity of slopes indicates that the pH dependence of net flux is not a result of pH acting on bone cells but probably an effect of altered mineral equilibria. The difference in net flux at similar pH indicates that calcium efflux is partially inhibited by acetazolamide and stimulated by both PTH and 1,25(OH)2D3.