Comparison of the effects of amino-terminal synthetic parathyroid hormone-related peptide (PTHrP) of malignancy and parathyroid hormone on resorption of cultured fetal rat long bones.

We have compared the effects of of various synthetic amino-terminal forms of human parathyroid hormone-related peptide (PTHrP) of malignancy with synthetic parathyroid hormone (PTH) on the resorptive responses of fetal rat long bones in organ culture. PTH and PTHrP increased 45Ca release at concentrations of 0.1-25 nM. PTHrP (1-40) and bovine PTH (1-34) were more potent than human PTH (1-34) and PTHrP (1-34). However, the slopes of the dose-response curves and the maximal resorptive effects were similar. There was a marked decrease in the potency of amino-terminal PTHrP peptides as the length was decreased. PTHrP (1-29) and PTHrP (1-25) were inactive at 120 nM. Further comparison of bPTH (1-34) and PTHrP (1-34) showed that both could induce bone resorption after a brief (6 hours) exposure and that the response to PTHrP (1-34) was qualitatively similar to that of bPTH (1-34) with respect to enhancement by ACTH and inhibition by calcitonin and glucocorticoids. Hydroxyurea and indomethacin did not block the resorptive response to either agonist. Cyclic AMP production in response to PTHrP (1-34) and (1-40) was similar to that for bPTH (1-34) in ROS 17/2.8 cells. The cyclic AMP (cAMP) response was much smaller in fetal rat long bones and calvariae, and bPTH was more potent than PTHrP. These studies confirm that PTHrP is quantitatively similar in its effects on bone resorption to PTH and are consistent with the two agents acting on the same receptor.