Bone resorption by isolated chick osteoclasts in culture is stimulated by murine spleen cell supernatant fluids (osteoclast-activating factor) and inhibited by calcitonin and prostaglandin E2.

The question of whether any of the agents known to activate bone resorption in vivo or in organ cultures acts directly on the osteoclast or via intermediate target cells that secondarily secrete locally paracrine factors is important for our understanding of bone remodeling. In an attempt to clarify this issue for some of the agents, we have taken advantage of the recent progress in obtaining and culturing relatively pure populations of osteoclasts. We performed an in vitro bone-resorbing assay in which isolated and partially purified chick osteoclasts were cultured on devitalized, paired and standardized bone disks prepared from rat calvaria prelabeled with both 45Ca and 3H-proline. Some of the isolated osteoclasts attached to the devitalized bone matrix, formed a ruffled border, and acidified the bone-resorbing compartment that they established with the matrix, thereby indicating that they resorbed bone in a physiologic manner. Salmon calcitonin added to these cultures (0.3 U/ml = 60 ng/ml) and prostaglandin E2 (PGE2) (10(-6) M) inhibited both basal and stimulated 45Ca and 3H-proline release. Neither parathyroid hormone (PTH) 1-34 (1 U/ml), 1,25-(OH)2-D3 (10(-8) and 10(-9) M), nor interleukin 1 (IL-1) (purified from P388D1 macrophage culture supernatant fluids or recombinant murine IL-1-alpha) (100 ng/ml) stimulated bone resorption in these cultures. In contrast, supernatant fluids from concanavalin A (Con-A)-activated murine spleen cell cultures (murine osteoclast-activating factor; OAF) consistently and significantly induced a 3- to 5-fold stimulation of bone resorption in this system.