Chronic acidosis-induced alteration in bone bicarbonate and phosphate

Chronic metabolic acidosis increases urinary calcium excretion without altering intestinal calcium absorption, suggesting that bone mineral is the source of the additional urinary calcium. In vivo and in vitro studies have shown that metabolic acidosis causes a loss of mineral calcium while buffering the additional hydrogen ions. Previously, we studied changes in femoral, midcortical ion concentrations after 7 days of in vivo metabolic acidosis induced by oral ammonium chloride. We found that, compared with mice drinking only distilled water, ammonium chloride induced a loss of bone sodium and potassium and a depletion of mineral HCO(3)(-) and phosphate. There is more phosphate than carbonate in neonatal mouse bone. In the present in vitro study, we utilized a high-resolution scanning ion microprobe with secondary ion mass spectroscopy to test the hypothesis that chronic acidosis would decrease bulk (cross-sectional) bone phosphate to a greater extent than HCO(3)(-) by localizing and comparing changes in bone HCO(3)(-) and phosphate after chronic incubation of neonatal mouse calvariae in acidic medium. Calvariae were cultured for a total of 51 h in medium acidified by a reduction in HCO(3)(-) concentration ([HCO(3)(-)]; pH similar to7.14, [HCO(3)(-)] similar to13) or in control medium ( pH similar to7.45, HCO(3)(-) similar to26). Compared with incubation in control medium, incubation in acidic medium caused no change in surface total phosphate but a significant fall in cross-sectional phosphate, with respect to the carbon-carbon bond (C(2)) and the carbon-nitrogen bond (CN). Compared with incubation in control medium, incubation in acidic medium caused no change in surface HCO(3)(-) but a significant fall in cross-sectional HCO(3)(-) with respect to C2 and CN. The fall in cross-sectional phosphate was significantly greater than the fall in cross-sectional HCO(3)(-). The fall in phosphate indicates release of mineral phosphates, and the fall in HCO(3)(-) indicates release of mineral HCO(3)(-), both of which would be expected to buffer the additional protons and help restore the pH toward normal. Thus a model of chronic acidosis depletes bulk bone proton buffers, with phosphate depletion exceeding that of HCO(3)(-).