MODULATION OF THE RESORPTIVE ACTIVITY OF RAT OSTEOCLASTS BY SMALL CHANGES IN EXTRACELLULAR PH NEAR THE PHYSIOLOGICAL RANGE

We investigated the effect of small shifts in extracellular pH on the resorptive activity of rat osteoclasts in vitro. Osteoclast-containing mixed bone cell populations disaggregated from neonatal rat long bone s were cultured for 26 h at low density on bovine hone disks in HCO3-/ CO2-buffered medium modified by the addition of small amounts of proto ns as HCl (2.5-15.0 mEq/L) or hydroxyl ions as NaOH(2.5-10.0 mEq/L). L ittle or no bone resorption occurred when ambient pH was above about 7 .30; at pH 7.00, osteoclasts exhibited near-maximal activity. The grea test changes were associated with a pH difference of as little as 0.10 unit, between PH 7.25 and pH 7.15. Over this narrow range, the averag e number of resorption pits formed on each bone wafer increased sixfol d. The steep sigmoidal pH response curve indicates that very slight al terations in ambient hydrogen ion concentration can effectively ''swit ch on'' or ''switch off'' rat osteoclasts in vitro.