pH dependence of bone resorption: mouse calvarial osteoclasts are activated by acidosis

We examined the effects of HCO3- and CO2 acidosis on osteoclast-mediated Ca 2+ release from 3-day cultures of neonatal mouse calvaria. Ca2+ release was minimal above pH 7.2 in control cultures but was stimulated strongly by th e addition of small amounts of H+ to culture medium (HCO3- acidosis). For e xample, addition of 4 meq/l H+ reduced pH from 7.12 to 7.03 and increased C a2+ release 3.8-fold. The largest stimulatory effects (8- to 11-fold), obse rved with 15-16 meq/l added H+, were comparable to the maximal Ca2+ release elicited by 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3; 10 nM], parathyroid hormone (10 nM), or prostaglandin E-2 (1 muM); the action of these osteoly tic agents was attenuated strongly when ambient pH was increased from simil ar to7.1 to similar to7.3. CO2 acidosis was a less effective stimulator of Ca2+ release than HCO3- acidosis over a similar pH range. Ca2+ release stim ulated by HCO3- acidosis was almost completely blocked by salmon calcitonin (20 ng/ml), implying osteoclast involvement. In whole mount preparations o f control half-calvaria, similar to 400 inactive osteoclast-like multinucle ate cells were present; in calvaria exposed to HCO3- acidosis and to the ot her osteolytic agents studied, extensive osteoclastic resorption, with perf oration of bones, was visible. HCO3- acidosis, however, reduced numbers of osteoclast-like cells by similar to 50%, whereas 1,25(OH)(2)D-3 treatment c aused increases of similar to 75%. The results suggest that HCO3- acidosis stimulates resorption by activating mature osteoclasts already present in c alvarial bones, rather than by inducing formation of new osteoclasts, and p rovide further support for the critical role of acid-base balance in contro lling osteoclast function.