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Chronic acidosis-induced alteration in bone bicarbonate and phosphate

¹Nephrology Unit, Department of Medicine, University of Rochester School of Medicine, Rochester, New York 14642; and ²Department of Physics, Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637

Submitted 1 April 2003 ; accepted in final form 11 May 2003

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 HCO₃^- 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 HCO₃^- by localizing and comparing changes in bone HCO₃^- 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 HCO₃^- concentration ([HCO₃^-]; pH 7.14, [HCO₃^-] 13) or in control medium (pH 7.45, HCO₃^- 26). 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 (C₂) and the carbon-nitrogen bond (CN). Compared with incubation in control medium, incubation in acidic medium caused no change in surface HCO₃^- but a significant fall in cross-sectional HCO₃^- with respect to C₂ and CN. The fall in cross-sectional phosphate was significantly greater than the fall in cross-sectional HCO₃^-. The fall in phosphate indicates release of mineral phosphates, and the fall in HCO₃^- indicates release of mineral HCO₃^-, 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 HCO₃^-.

ion microprobe; calcium; proton; metabolic acidosis

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