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Articles in PresS, published online ahead of print August 21, 2002
Am J Physiol Renal Physiol, 10.1152/ajprenal.00155.2002
Submitted on April 22, 2002
Accepted on July 10, 2002
1 Department of Medicine, University of Rochester School of Medicine, Rochester, NY, USA
2 Department of Physics, University of Chicago, Chicago, IL, USA
* To whom correspondence should be addressed. E-mail: David_Bushinsky{at}URMC.Rochester.edu.
During an acute fall in systemic pH due to a decrease in the concentration of serum bicarbonate, metabolic acidosis, there is an influx of hydrogen ions into the mineral phase of bone buffering the decrement in pH. When bone is cultured in medium modeling acute metabolic acidosis, the influx of hydrogen ions is coupled to an efflux of sodium and potassium and a depletion of mineral carbonate. These ionic fluxes would be expected to neutralize some of the excess hydrogen ions and restore the pH toward normal. Approximately one third of bone carbonate is located on the hydration shell of apatite where it is readily accessible to the systemic circulation while the remainder is located in less assessable areas. We hypothesize that the surface of bone would respond to acidosis in a different manner than the interior of bone, with depletion of carbonate preferentially occurring on the bone surface. We utilized a high resolution scanning ion microprobe with secondary ion mass spectroscopy to localize the changes in bone carbonate, as measured by bicarbonate, and phosphate, and determine their relative contribution to the buffering of hydrogen ions during acute metabolic acidosis. Neonatal mouse calvariae were incubated in control medium (pH ~ 7.44, [HCO3-] ~ 27 mM) or in medium acidified by a reduction in bicarbonate concentration (pH ~ 7.14, [HCO3-] ~ 13). Compared to control, after a 3 hr incubation in acidic medium there is a 5 fold decrease in surface bicarbonate with respect to C2 (carbon-carbon bond) and a 3 fold decrease in surface bicarbonate with respect to CN (carbon-nitrogen bond) with no change in cross-sectional bicarbonate. Compared to control, after a 3 hr incubation in acidic medium there is a 10 fold decrease in cross-sectional phosphate with respect to C2 and a 10 fold decrease in cross-sectional phosphate with respect to CN with no change in surface phosphate. On the bone surface there is a 4 fold depletion of bicarbonate in relation to phosphate and in cross-section a 7 fold depletion of phosphate in relation to bicarbonate. Thus, acute hydrogen ion buffering by bone involves preferential dissolution of surface bicarbonate and of cross-sectional phosphate.
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