Critical role of bicarbonate in calcium release from bone

Calcium release from cultured bone is pH dependent; net calcium flux (JCa) from bone increases with decreasing pH. At a similar decrement in pH there is greater JCa when acidosis is produced by a low medium bicarbonate concentration ([HCO3-]), a model of metabolic acidosis (Met), compared with an increased medium PCO2, a model of respiratory acidosis (Resp). To separate the role of [HCO3-] from that of pH in inducing JCa we cultured calvariae for 3 h under three different neutral (pH approximately 7.4) isohydric environments [control (Ctl), fully compensated Met (C-Met), or fully compensated Resp (C-Resp)] and two different acid (pH approximately 7.1) isohydric environments (Met or Resp). During neutral pH (Ctl, C-Met, and C-Resp) there was JCa from bone during C-Met (decreased [HCO3-]), no net flux during Ctl (normal [HCO3-]), and JCa into bone during C-Resp (increased [HCO3-]); and JCa was correlated inversely with [HCO3-] (r = -0.824, n = 36, P less than 0.001). During acid pH there was greater JCa from bone during Met (decreased [HCO3-]) than during Resp (normal [HCO3-]); and JCa was again correlated inversely with [HCO3-] (r = -0.848, n = 22, P less than 0.001). JCa from bone during Met and Resp was greater than C-Met and C-Resp, respectively. The addition of the osteoclastic inhibitor salmon calcitonin did not alter the relative JCa results. Thus at a constant pH the magnitude of JCa from cultured neonatal mouse calvariae appears dependent on the [HCO3-]; the lower the [HCO3-], the greater the calcium efflux.(ABSTRACT TRUNCATED AT 250 WORDS)