In VivoDistribution of Parathyroid Hormone Receptors in Bone: Evidence that a Predominant Osseous Target Cell Is Not the Mature Osteoblast*

Previous studies in vitro and in vivo have demonstrated the presence of receptor sites for PTH on cells of the osteoblast phenotype. Nevertheless, it is unclear whether the diverse functions of this hormone in bone can all be attributed to its interaction with a single cell type. In this study, we have used a radioautographic method to examine the competitive binding of 125I-labeled rat PTH-(1-34) to the long bones of rats in vivo. Our studies confirm the presence of competitive binding to mature osteoblasts and the absence of significant competitive binding to multinucleated osteoclasts. However, by light and electron microscopic radioautographic analysis, the majority of specific competitive PTH binding was present over a cell in the intertrabecular space of the metaphyseal region, which was distinct from the mature osteoblast. This large mononuclear cell with multiple cytoplasmic extensions appeared to interface with both the bone matrix and the microvascular osseous circulation and may provide an additional target to mediate hormonal effects on the skeleton.