Normal bone particles are preferentially resorbed in the presence of osteocalcin-deficient bone particles in vivo.

In an in vivo model of osteoclastic bone resorption, we previously showed that osteocalcin-deficient bone particles (BPs), derived from warfarin-treated rats, were resorbed 50% as well as normal BPs and that they recruited fewer osteoclastic cells with decreased tartrate-resistant acid phosphatase (TRAP) activity. In order to determine the specificity of the resorption response, we evaluated the fate of implanted mixtures of normal and osteocalcin-deficient BPs. Normal and warfarin-treated donor rats were prelabeled in vivo with oxytetracycline to permit identification of BPs from either source. Normal, osteocalcin-deficient, and 50:50 mixtures of BPs (either labeled or unlabeled) were implanted into normal rats and recovered 12 days later for enzymatic (TRAP) and nondecalcified histomorphometric analyses. The incorporated oxytetracycline had no significant effect on resorption of bone particles. The recovered osteocalcin-deficient BPs were surrounded by fewer osteoclastic cells, were resorbed less, and contained less extractable TRAP activity than normal BPs. In mixed BP implants with normal and osteocalcin-deficient BPs, each type of bone particle elicited the same tissue response as when implanted separately. Remarkably, the different particles evoked dissimilar osteoclastic responses and were resorbed to different extents, even when adjacent within the same implant. These data suggest that osteocalcin may act as a substrate signal for resorption and that osteocalcin in the normal BPs does not influence the cellular response to adjacent osteocalcin-deficient BPs.