Greater unidirectional calcium efflux from bone during metabolic, compared with respiratory, acidosis

Abstract

There is a smaller net calcium efflux from bone in vitro during respiratory (increased PCO2) than metabolic (decreased [HCO3-] acidosis. This could be due to the elevated PCO2, which would lessen the driving force for mineral dissolution and increase the driving force for mineralization with respect to carbonated apatite in the bone mineral. To test this hypothesis, we injected neonatal mice with 45Ca and dissected the radiolabeled calvariae 24 h later. The live calvariae were then cultured for 24 h under conditions simulating respiratory acidosis (Resp, pH = 7.225 +/- 0.003, PCO2 = 87.5 +/- 0.1 mmHg), severe respiratory acidosis (SResp, pH = 7.072 +/- 0.004, PCO2 = 103.0 +/- 0.5 mmHg), metabolic acidosis (Met, pH = 7.212 +/- 0.003, HCO3- = 15.5 +/- 0.1 meq/l), or normal acid-base status (Ctl, pH = 7.452 +/- 0.003, PCO2 = 40.0 +/- 0.2 mmHg, HCO3- = 27.8 +/- 0.2 meq/l) and bidirectional net calcium flux (JCa) and unidirectional 45Ca release were determined. There was greater JCa from bone during Met than Resp, and JCa was not different from Met during SResp despite the latter having a significantly lower pH. There was greater unidirectional 45Ca release from bone during Met than Resp, SResp, or Ctl. There was a similar direct correlation between JCa and 45Ca efflux in the respiratory and metabolic groups. However, when calvarial osteoclast activity was inhibited with calcitonin,although there was again greater JCa and 45Ca release with a metabolic compared with respiratory acidosis, there was a greater proportion of 45Ca release than JCa from bone.(ABSTRACT TRUNCATED AT 250 WORDS)