Ion microprobe determination of bone surface elements: effects of reduced medium pH.

When neonatal mouse calvariae are cultured in a medium having a low pH they release calcium (Ca) while buffering protons (H). However, for 1 neq Ca released, 16-21 neq H enter the calvariae, arguing strongly against simple dissolution of bone mineral as the mechanism of H buffering. To determine if H for sodium (Na) exchange could explain the lack of stoichiometry between H and Ca, we cultured calvariae for 3 h in control (pH = 7.40 +/- 0.01) or reduced-pH (pH = 7.21 +/- 0.01) medium and then examined the surface Na, K, and Ca using a high-resolution scanning ion microprobe. The calvarial surface was rich in Na and K relative to Ca (Na/Ca, 52 +/- 17; K/Ca, 61 +/- 17; all values are the ratios of counts per second of detected secondary ions, means +/- SE). Compared with the surface the calvarial cross section contained far less Na and K relative to Ca (Na/Ca, 2 +/- 1; K/Ca, 1 +/- 1; both P less than 0.01 vs. surface). Compared with the control surface, culture in a reduced-pH medium reduced the surface Na and K relative to Ca (Na/Ca, 5 +/- 1; K/Ca, 7 +/- 1; both P less than 0.025 vs. surface) to values still greater than the cross section (P less than 0.05 for both). Neonatal mouse calvariae have a surface that is rich in Na and K relative to Ca. Reduced medium pH depletes surface Na and K of cultured calvariae with respect to Ca.