Net proton influx into bone during metabolic, but not respiratory, acidosis

During acute metabolic acidosis there is a net influx of protons into bone, decreasing the elevated proton concentration. Whether there is an influx of protons into bone during acute respiratory acidosis is not known. To determine the effect of respiratory acidosis on net proton flux (JH) relative to bone, we compared JH from neonatal mouse calvariae incubated for 3 h in medium acidified by an increase in PCO2 (respiratory acidosis) with that from calvariae incubated in medium acidified to the same extent by a decrease in bicarbonate concentration (metabolic acidosis). The initial medium pH with respiratory acidosis was not different from that with metabolic acidosis (7.108 +/- 0.005 vs. 7.091 +/- 0.007, respectively, P = NS). During respiratory acidosis there was no JH from bone relative to the medium (JH = 236 +/- 93 neq.bone-1.3h-1, P = NS vs. 0); however, during metabolic acidosis there was net proton influx from the medium into bone (JH = -703 +/- 108, P less than 0.05 vs. 0, P less than 0.001 vs. respiratory acidosis). There was less calcium efflux from bone during respiratory than during metabolic acidosis (JCa = 68 +/- 6 nmol.bone-1.3 h-1 vs. 100 +/- 9, respectively, P less than 0.001). There is a net influx of protons into bone in vitro during acute metabolic, but not during acute respiratory, acidosis. The smaller calcium efflux during respiratory acidosis may indicate less net bone mineral dissolution and thus less buffer release into the medium.