The effects of extracellular pH on bone resorption by avian osteoclasts in vitro

It has previously been reported that low extracellular pH stimulates the excavation of resorption lacunae by rodent osteoclasts in vitro. Using avian bone cells in a similar in vitro assay we have demonstrated that osteoclast activity is optimal at pH 7.20-7.40 and is inhibited at extremes of pH (less than 7.10 and greater than 7.60). Over the first 24 h of incubation at low pH there may be an increase in osteoclastic resorption but to a lesser extent than that reported for rodent cells. However, after 24-30 h in culture there is little or no further increase in bone resorption, presumably due to a cytotoxic effect of low pH acting either on the osteoclast directly or via nonosteoclastic bone cells. In contrast to a previous report, in which preincubation of wafers for 24 h had no effect on bone resorption, we found that preincubation of bone substrates at pH 6.50 for longer periods enhances subsequent resorption at pH 7.20.