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

Area of Science:

• Biochemistry
• Bone Biology
• Mineral Metabolism

Background:

• Neonatal mouse calvariae buffer protons (H+) when cultured in low pH media, releasing calcium (Ca).
• The observed stoichiometry (1 neq Ca released for 16-21 neq H+ buffered) suggests a mechanism beyond simple bone mineral dissolution.

Purpose of the Study:

• To investigate if proton for sodium (H+/Na+) exchange explains the observed lack of stoichiometric relationship between H+ buffering and Ca release.
• To analyze the surface elemental composition of neonatal mouse calvariae under varying pH conditions.

Main Methods:

• Culturing neonatal mouse calvariae for 3 hours in control (pH 7.40) or reduced-pH (pH 7.21) media.
• Utilizing a high-resolution scanning ion microprobe to examine surface and cross-section elemental composition (Na, K, Ca).

Main Results:

• The calvarial surface exhibits a high ratio of sodium (Na) and potassium (K) to calcium (Ca) (Na/Ca: 52±17; K/Ca: 61±17).
• In contrast, the calvarial cross-section shows significantly lower Na/Ca (2±1) and K/Ca (1±1) ratios (P < 0.01).
• Culture in reduced-pH medium decreased surface Na/Ca (5±1) and K/Ca (7±1) ratios (P < 0.025), though still higher than cross-section values (P < 0.05).

Conclusions:

• Neonatal mouse calvariae possess a surface layer enriched in Na and K relative to Ca.
• Reduced extracellular pH leads to a depletion of surface Na and K with respect to Ca in cultured calvariae.
• These findings support a model involving ion exchange, specifically H+/Na+ exchange, in the buffering of protons by bone.