A direct action of human calcitonin gene-related peptide on isolated osteoclasts

The calcitonin gene encodes a small family of peptides: calcitonin, calcitonin gene-related peptide (CGRP) and katacalcin. Whereas calcitonin is concerned with skeletal maintenance, the function, if any, of katacalcin is still unknown. In the present study we have assessed resorption of human cortical bone substrate by isolated rat osteoclasts and have shown that CGRP acts directly on the osteoclast to inhibit bone resorption. The three CGRP peptides (rat, human(alpha) and human(beta) caused an almost equivalent decrease in osteoclastic bone resorption and were approximately 1000-fold less potent than human calcitonin in this respect. The responses of human calcitonin and human CGRP(alpha) were additive. Furthermore, prior treatment with trypsin to destroy receptors abolished the responsiveness of osteoclasts to CGRP and calcitonin. The carboxyl- and amino-terminal fragments of CGRP were found not to inhibit bone resorption, suggesting that the whole molecule of CGRP is necessary for biological activity. We have therefore suggested that the calcitonin-like effects of CGRP, seen both in vivo in the rat bioassay and in vitro in organ cultures, are due to the direct action of CGRP on the osteoclast, probably mediated through the calcitonin receptor. Though it is unlikely that CGRP is involved in the regulation of plasma calcium, the peptide may be an important local regulator of bone cell function.