Further studies on the role of carbon dioxide in one resorption

An adequate concentration of carbon dioxide was found necessary for optimal effects of parathyroid hormone (PTH) on fetal rat bones cultured for 72 hr in vitro. A submaximal dose of PTH effective in a medium gassed with 5% CO₂ in air was found to have little or no effect in a medium gassed with 0.6% CO₂ in air and maintained at the same pH (7.4). The effect of PTH in 0.6% CO₂ could be significantly increased by addition of sodium oxaloacetate (OAA), 10^-3M. PTH effect in an air gassed medium buffered with amine buffers was also increased by the addition of oxaloacetate. These data suggest that oxaloacetate production may have been inadequate in CO₂ deficient media. The requirement for CO₂ was further investigated by the use of a carbonic anhydrase inhibitor, acetazolamide. At 4 x 10^-4M, acetazolamide completely inhibited the effect of a maximal concentration of PTH in 5% CO₂ gassed medium. At 10^-5M, it had no effect in the 5% CO₂, but completely inhibited the effect of this dose of PTH in 0.6% CO₂ or air. These latter results confirm the importance of the CO₂ bicarbonate system in bone resorption.