The specific localization and the characterization of the parathyroid hormone (PTH) receptor in bone have been studied using 18-d embryonic chick calvariae and biologically active, electrolytically labeled [1251] bovine PTH(I-34). Binding was initiated by adding [12Sl]-bPTH(I-34) to bisected calvariae at 30°C. Steady state binding was achieved at 90 min at which time 10 mg dry wt of calvaria specifically bound 17% of the added [12Sl]bPTH (I-34). Nonspecific binding in the presence of 244 nM unlabeled bPTH(1-34) was <2%. Insulin, glucagon, and calcitonin (I/~g/ml) did not compete for PTH binding sites. Half-maximal inhibition of binding was achieved at concentrations of unlabeled bPTH(1-34) or bPTH(I-84) of about 10 nM. The range of concentration (2-100 nM) over which bPTH(1-34) and bPTH(I-84) stimulated cyclic 3'5'adenosine monophosphate (cAMP) production was similar to that which inhibited the binding of [12Sl]bPTH(I-34). Light microscope autoradiograms showed that grains were concentrated over cells (osteoblasts and progenitor cells) at the external surface of the calvariae and in trabeculae. In the presence of excess unlabeled PTH, labeling of control autoradiograms was reduced to near background levels. No labeling of osteocytes or osteoclasts was observed. At the electron microscopic level, grains were localized primarily over cell membranes. A quantitative analysis of grain distribution suggested that cellular internalization of PTH occurred.Parathyroid hormone (PTH) acts directly on bone acutely to increase bone resorption and decrease bone formation (1, 2) and chronically to increase both bone resorption and bone formation (3). These actions are mediated by the ceUular elements of bone by mechanisms involving, at least in part, the stimulation of cyclic Y,5' adenosine monophosphate (cAMP) production (4, 5). The evidence available indicates that PTH directly or indirectly influences all bone cells (osteogenic precursors, osteoblasts, osteocytes, and osteoclasts) (6-9); however, recent reports (10, 11) which describe the effect of PTH on cAMP production in osteoblastlike or osteoclastlike cells in monolayer culture implicate the osteoblast as the major osseous target cell of PTH.During the past five years, major advances in the production of biologically active, high specific-activity, radioiodinated preparations of PTH have made it possible to investigate the previously elusive PTH receptor in kidney (12). It is only recently that similar progress has been made in the identification and characterization of PTH receptors in bone: using electrolytically [L2SI]-labeled, receptor-purified, synthetic bo-
