Effects of Carbonic Anhydrase Inhibitor Acetazolamide (AZ) on Osteoclasts and Bone Structure

We examined the changes of osteoclasts and bone structure in vivo and in vitro under the condition of low activity of bone resorption induced by acetazolamide (AZ). AZ is a specific inhibitor of carbonic anhydrase II (CA II), which is the enzyme indispensable for substantial proton generation by osteoclasts. In vivo study, we investigated histologically and histomorphometrically the changes of trabecular bone and osteoclasts at the cartilage-bone junction of tibiae in rats injected subcutaneously with AZ for 21 days. The area of trabecular bone of AZ treated group increased compared with control group. Moreover, AZ treatment decreased about 20% the number of osteoclasts at the cartilage-bone junction. In vitro study, we examined the effects of AZ on osteoclasts using culture system of rat bone marrow. The number of osteoclasts on the bone slice was suppressed by AZ of a concentration from 10-5 to 10-4 M in a doze-dependent manner. The number of osteoclasts cultured with 10-4 M AZ for 24 hours and with or without parathyroid hormone (PTH) on the bone slice and on the glass showed a significant decrease compared with the control group. Some osteoclasts treated with AZ showed morphological changes of apoptosis-like cell death, including shrinking of cytoplasm and fragmentation of nuclei. These results show that high-doze of AZ decreases bone resorption not only by inhibiting CA II but also by decreasing the number of osteoclasts via induction of cell death. This suggests that CA II plays an important role not only in proton formation but also in osteoclast survival.