PTH/PTHrP receptor expression on osteoblasts and osteocytes but not resorbing bone surfaces in growing rats

Using in situ hybridization, we correlated the expression of mRNA for the parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) receptor with bone formation and resorption in undecalcified serial sections of bones from growing rats. In addition we investigated the presence of biologically active receptors in the same locations using an in vivo autoradiographic technique. In the ulnae of growing rats, there are well defined zones of cortical bone formation and resorption. These contribute to the modeling drifts by which the bone achieves its adult shape. Forming surfaces incorporate fluorochrome labels, are lined with osteoid, and have a layer of cuboidal osteoblasts that have a high alkaline phosphatase activity. Resorbing surfaces have no fluorochrome incorporation, no osteoid, and are lined with resorbing cells with high tartrate-resistant acid phosphatase (TRAP) activity. PTH/PTHrP receptor mRNA was expressed predominantly on forming but not on resorbing bone surfaces and colocalized with sites of binding of radiolabeled PTH after intravenous injection. PTH/PTHrP mRNA expression on osteocytes was inconclusive but radiolabeled PTH bound to a proportion of osteocytes in all regions of the cortex although binding was not specifically related to areas of bone formation or resorption. These results suggest that in growing animals the actions of PTH or PTHrP are connected more with bone formation than resorption. Such a role may be linked to the ability of PTH to induce bone formation in adults but does not explain the actions of the hormone in regulating resorption. Binding of PTH to osteocytes increases the evidence for a physiological role for these cells.