OSTEOCLAST RECRUITMENT IN MICE IS STIMULATED BY (3-AMINO-1-HYDROXYPROPYLIDENE)-1,1-BISPHOSPHONATE

Though some evidence suggests that bisphosphonates (BPs) act directly on osteoclasts to inhibit bone resorption, other evidence suggests that they inhibit the development of the osteoclast. We found an increase in osteoclast recruitment in 2-day-old mice given (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD). A threefold increase in 5-bromo-2'-deoxyuridine (BrdU)-labeled osteoclast nuclei was observed on mouse parietal bones 3 days after APD injection. This suggests that inhibition of osteoclast development is not an action of APD in mice of this age. The mechanism of the increased recruitment was investigated. As osteoclast progenitors were not detected on parietal bones in vitro, we looked for an increase in circulating monocytes to account for the recruitment. No such increase was found, but when Cr-51-labeled bone marrow was injected intraperitoneally into mice given APD there was an increase in accumulation of Cr-51 in calvaria and in femur and tibia over controls. This increase did not occur when Cr-51-labeled erythrocytes or free Cr-51 was injected. We conclude that APD causes increased recruitment of osteoclast precursors by increasing the avidity of bone for hematopoietically derived cells.