Lack of significant effect of carboxyl-terminal parathyroid hormone-related peptide fragments on isolated rat and chick osteoclasts.

Parathyroid hormone-related protein (PTHrP) fragments 107-111, 107-138, and 107-139 were all found to be without consistent significant inhibitory effect on the resorptive activity of isolated rat and chick osteoclasts over the dose range 10(-13) M-10(-9) M. In the rat, these results contrasted with a strong and significant inhibition (100%) by calcitonin. Our results differ from that previously reported for C-terminal fragments of PTHrP by Fenton et al. [1-3], who noted up to a 70% inhibition of resorptive activity of isolated rat or chick osteoclasts at femtomolar doses and greater. It is possible that the lack of response observed in our osteoclast assays is due to unknown variables in the bone slice assay that influence the responsiveness of isolated osteoclasts to these fragments.