Inhibition of osteoclast formation and function by bicarbonate: Role of soluble adenylyl cyclase

High [HCO-₃^-] inhibits and low [CO₃ ^- stimulates bone resorption, which mediates part of the effect of chronic acidosis or acid feeding on bone. Soluble adenylyl cyclase (sAC) is a bicarbonate sensor that can potentially mediate the effect of bicarbonate on osteoclasts. Osteoclasts were incubated in 0, 12, and 24mMHCO₃ ^-at pH 7.4 for 7-8 days and assayed for tartrate-resistant acid phosphatase (TRAP) and vacuolar-ATPase expression, and H⁺ccumulation. Total number and area of TRAP (+)ultinucleated osteoclasts was decreased by HCO₃^-in a dose-dependent manner. V-ATPase expression and H⁺accumulation normalized to cell cross-sectional area or protein were not significantly changed. The HCO₃^--induced inhibition of osteoclast growth and differentiation was blocked by either 2-hydroxyestradiol, an inhibitor of sAC or sAC knockdown by sAC specific siRNA. The model of CO₃^-inhibiting osteoclast via sAC was further supported by the fact that the HCO₃^-dose-response on osteoclasts is flat when cells were saturated with 8-bromo-cAMP, a permeant cAMP analog downstream from sAC thus simulating sAC activation. To confirm our in vitro findings in intact bone, we developed a 1-week mouse calvaria culture system where osteoclasts were shown to be viable. Bone volume density (BV/TV) determined by micro-computed tomography (μmCT), was higher in 24 mM HCO ₃^-compared to 12 mM HCO₃^-treated calvaria. This HCO₃^-effect on BV/TV was blocked by 2-hydroxyestradiol. In summary, sAC mediates the inhibition of osteoclast function byHCO₃^-, by acting as aHCO₃ ^- sensor.