BONE-RESORPTION INDUCED BY A23187 IS ABOLISHED BY INDOMETHACIN - IMPLICATIONS FOR 2ND-MESSENGER UTILIZED BY PARATHYROID-HORMONE

Parathyroid hormone acts on the osteoblast to induce osteoclastic bone resorption. Parathyroid hormone utilises cyclic AMP as a second messe nger in osteoblasts, but may also cause an increase in cytoplasmatic f ree calcium ions ([Ca2+](i)) in the same cell. To investigate the role of osteoblastic [Ca2+](i) in the induction of bone resorption, we hav e 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 resor ption in neonatal mouse calvarial bones. Parathyroid hormone (0.1 and 1 nM) dose dependently stimulated the release of prelabelled Ca-45(2+) in 72 h culture. Parathyroid hormone-induced bone resorption was not affected by the addition of 1 mu M indomethacin to the incubation medi a, and was therefore, not mediated by local prostaglandin formation. A 23187 stimulated the release of Ca-45(2+) at 1-10 nM. Above 100 nM, A2 3187 inhibited bone resorption. The A23187 (3 and 10 nM)-induced bone resorption was abolished by the cyclooxygenase inhibitor, indomethacin (1 mu M), indicating that the stimulatory effect was mediated via pros taglandin formation. The adenylate cyclase stimulating agent, forskoli n, dose dependently stimulated bone resorption at and above 1 mu M. Th ere was no additive or synergistic effect of forskolin and A23187 on C a-45(2+) release. Forskolin-induced bone resorption was, as with parat hyroid hormone but in contrast to ionophore-induced bone resorption, n ot abolished by indomethacin (1 mu M). The protein kinase C activator, PDB, at 10 and 1000 nM stimulated the release of prelabelled Ca-45(2). Th, stimulatory effect of the protein kinase C stimulating phorbol ester, PDB, on bone resorption was abolished by the addition of indome thacin. In summary, bone resorption induced by a Ca2+-ionophore is abo lished by indomethacin. This indicates that bone resorbing agents know n to increase [Ca2+](i) subsequently enhance local prostaglandin forma tion. 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 t hat cyclic AMP,but not [Ca2+](i), is involved as a second messenger in parathyroid-induced bone resorption. (C) 1998 Elsevier Science B.V.