EFFECTS OF MEDIUM ACIDIFICATION BY ALTERATION OF CARBON-DIOXIDE OR BICARBONATE CONCENTRATIONS ON THE RESORPTIVE ACTIVITY OF RAT OSTEOCLASTS

Little is known about the extracellular conditions or factors that sti mulate mature osteoclasts to resorb mineralized tissues. Isolated mamm alian osteoclasts are strongly stimulated by protons in HERES-buffered culture media in the absence of CO2 and HCO3-, but it has been report ed that cell-mediated Ca2+ release from bone organ cultures is increas ed only when media are acidified by reduction of HCO3- concentrations, and not by increasing PCO2 (considered models of metabolic and respir atory acidosis, respectively). We investigated this question using dis aggregated rat osteoclasts cultured on dentin slices for 24 h. The num ber of pits resorbed per osteoclast was stimulated in media acidified by manipulation of either HCO3- or CO2 concentrations. In experiments in which incubator CO2 was varied, resorption was almost abolished in the presence of 2.5% CO2 at pH 7.61 but increased in a stepwise manner up to 1.3 pits per osteoclast when dentin slices were cultured with 1 0% CO2 at pH 6.97. The depths and widths of pits, measured using a con focal laser reflection microscope, also tended to increase with increa sing CO2 and decreasing pH. However, in experiments where pH was lower ed by reducing medium HCO3-, pit size decreased, partially offsetting the increased number of pits resorbed per osteoclast. These findings s uggest that rat osteoclasts may be more sensitive to stimulation by CO 2 acidosis than by HCO3- acidosis, at least in the short term, and may possibly reflect local regulatory processes in bone.