Numerous resorptive stimuli have been shown to enhance osteoclast differentiation, increasing osteoclast numbers and accelerating bone resorption. Currently, there is much less understanding of regulation of mature osteoclast activity. Indeed, there is presently only minimal evidence of changes in gene expression as a mechanism for altering bone resorption. We investigate here, in the mature osteoclast, regulation of 2 genes-carbonic anhydrase II (CAII) and calcitonin receptor (CTR) in response to acidosis, which is known to increase bone resorption. We studied the effect of acid pH on CAII and CTR mRNA expression in mature osteoclasts raised in coculture of ST-2 and primary marrow cells. On day 6 of culture, stromal cells were removed with collagenase, the remaining osteoclasts were incubated overnight, and then exposed to varying pH. RT-PCR was performed on total RNA using primers for CAII, CTR, or glyceraldehyde dehydrogenase phosphate (GAP). Expression of CTR mRNA was increased 2.14 +/- 0.41 and 2.56 +/- 0.45 (P < 0.05)-fold by a 4-hour exposure to pH 6.75 and 6.5, respectively. CAII mRNA was similarly increased 2.18 +/- 0.42 and 2.63 +/- 0.48 (P < 0.05)-fold by pH 6.75 and 6.5, respectively. Increased expression of CAII and CTR mRNA was seen by 2 hours and maximally by 4 hours. Increased expression of CTR and CAII mRNA was not explained by increases in osteoclast numbers: pH 7.4-100 +/- 3.7, 6.75-133 +/- 8.3, 6.5-124 +/- 7.8. These results demonstrate upregulation of two osteoclast genes in response to acidosis, illustrating the ability of the mature osteoclast to respond to resorptive signals with increased functional gene expression.