Bone resorption induced by A23187 is abolished by indomethacin: implications for second messenger utilised by parathyroid hormone

Abstract

Parathyroid hormone acts on the osteoblast to induce osteoclastic bone resorption. Parathyroid hormone utilises cyclic AMP as a second messenger in osteoblasts, but may also cause an increase in cytoplasmatic free calcium ions ([Ca2+]i) in the same cell. To investigate the role of osteoblastic [Ca2+]i in the induction of bone resorption, we have compared the effects of parathyroid hormone and the Ca2+-ionophore, A23187, as well as the adenylate cyclase stimulating agent, forskolin, and the phorbol ester, phorbole 12,13 dibutyrate (PDB), on bone resorption in neonatal mouse calvarial bones. Parathyroid hormone (0.1 and 1 nM) dose dependently stimulated the release of prelabelled 45Ca2+ in 72 h culture. Parathyroid hormone-induced bone resorption was not affected by the addition of 1 microM indomethacin to the incubation media, and was therefore, not mediated by local prostaglandin formation. A23187 stimulated the release of 45Ca2+ at 1-10 nM. Above 100 nM, A23187 inhibited bone resorption. The A23187 (3 and 10 nM)-induced bone resorption was abolished by the cyclooxygenase inhibitor, indomethacin (1 microM), indicating that the stimulatory effect was mediated via prostaglandin formation. The adenylate cyclase stimulating agent, forskolin, dose dependently stimulated bone resorption at and above 1 microM. There was no additive or synergistic effect of forskolin and A23187 on 45Ca2+ release. Forskolin-induced bone resorption was, as with parathyroid hormone but in contrast to ionophore-induced bone resorption, not abolished by indomethacin (1 microM). The protein kinase C activator, PDB, at 10 and 1000 nM stimulated the release of prelabelled 45Ca2+. The stimulatory effect of the protein kinase C stimulating phorbol ester, PDB, on bone resorption was abolished by the addition of indomethacin. In summary, bone resorption induced by a Ca2+-ionophore is abolished by indomethacin. This indicates that bone resorbing agents known to increase [Ca2+]i subsequently enhance local prostaglandin formation. Bone resorption induced by the protein kinase C activator, PDB, was also abolished by indomethacin, whereas, forskolin and parathyroid hormone-induced bone resorption was unaffected. These data indicate that cyclic AMP, but not [Ca2+]i, is involved as a second messenger in parathyroid-induced bone resorption.