Radioiodinated rat parathyroid hormone-(1-34) binds to its receptor on rat osteosarcoma cells in a manner consistent with two classes of binding sites

Binding of 125I-labeled rat (r) PTH-(1-34) to ROS 17/2.8 osteoblastic bone cells and to membranes from these cells was examined. Competitive binding inhibition experiments were performed using unlabeled rPTH-(1-34) with particular emphasis on concentrations of peptide below 1 nM. In intact cells, binding of labeled rPTH-(1-34) was highly specific, and inhibition of binding by unlabeled ligand suggested the presence of two classes of binding sites, one with high affinity and low capacity (KD = 40 pM, approximately 20% of total binding sites) and the other with lower affinity and high capacity (KD = 2 nM, approximately 80% of the sites). Membranes prepared from ROS cells also exhibited a pattern of binding from competitive inhibition curves consistent with two distinct binding sites (KD = 30 pM and 6 nM). In intact ROS cells, cellular cAMP levels increased over the range of 10(-11)-10(-9) M rPTH-(1-34) with an ED50 intermediate between the two KD values (0.25 nM). These data suggest that osteoblastic bone cells possess two distinct classes of membrane receptors for PTH. Since the KD of the higher affinity site more closely approximates circulating concentrations of PTH, binding to this site may have physiologic relevance