Avian osteoclast cells are stimulated to resorb calcified matrices by and possess receptors for leukotriene B-4

Leukotriene B-4 (LTB4) is elevated in inflammatory conditions and appears t o be a potential mediator of inflammation. We have recently shown that this 5-lipoxygenase metabolite of arachidonic acid stimulates bone resorption i n vitro and in vivo. In order to determine the mechanism whereby LTB4 cause s bone resorption, avian osteoclasts were examined for the effects of LTB4 and for the presence of LTB4 receptors. Isolated avian osteoclast mononucle ar precursor cells, which fuse in culture to form multinucleated cells, wer e chosen for receptor binding studies because this population is a morpholo gically similar source of osteoclasts, and large numbers of these cells can be obtained from egg-laying hens. Binding of LTB4 and activation would sup port the hypothesis of a direct effect of this compound on osteoclasts. LTB 4 stimulated isolated avian osteoclasts to form resorption lacunae on calci fied matrices and to increase their content of tartrate-resistant acid phos phatase (TRAP), a marker of activated osteoclasts. Receptor binding studies were performed at day 1, when the cells were mononuclear, at day 4, when m ononuclear pre cursors were actively fusing, and at day 7, when fusion has slowed. Scatchard analysis of saturation binding data showed two classes of binding sites, a high- and low-affinity binding site with dissociation con stants (K-D) of 0.2-0.4 nM and 5.6-24 nM. Association studies showed rapid binding of LTB4 to the cells within 10 minutes. These data show that LTB4 a ccelerates fusion and activates highly enriched populations of avian osteoc lasts and that LTB4 receptors are present in this cell population.