Abstract
High ${\rm [HCO}_3^ - ]$ inhibits and low ${\rm [HCO}_3^ - ]$ 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 24 mM ${\rm HCO}_3^ -$ at pH 7.4 for 7–8 days and assayed for tartrate‐resistant acid phosphatase (TRAP) and vacuolar‐ATPase expression, and H^+^ accumulation. Total number and area of TRAP (+) multinucleated osteoclasts was decreased by ${\rm HCO}_3^ -$ in a dose‐dependent manner. V‐ATPase expression and H^+^ accumulation normalized to cell cross‐sectional area or protein were not significantly changed. The ${\rm HCO}_3^ -$‐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 ${\rm HCO}_3^ -$ inhibiting osteoclast via sAC was further supported by the fact that the ${\rm HCO}_3^ -$ 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 (µCT), was higher in 24 mM ${\rm HCO}_3^ -$ compared to 12 mM ${\rm HCO}_3^ -$ treated calvaria. This ${\rm HCO}_3^ -$ effect on BV/TV was blocked by 2‐hydroxyestradiol. In summary, sAC mediates the inhibition of osteoclast function by ${\rm HCO}_3^ -$, by acting as a ${\rm HCO}_3^ -$ sensor. J. Cell. Physiol. 220: 332–340, 2009. © 2009 Wiley‐Liss, Inc.