Different roles for calcium and cyclic AMP in the action of PTH: Studies in bone explants and isolated bone cells

In fetal mouse calvaria forskolin (0.1–100 μM), like PTH, stimulated cyclic AMP production in a dose-dependent way. The dose-response curve for forskolin-induced bone mineral release (24 hrs), however, demonstrated a biphasic character, showing stimulation at 0.1 μM and inhibition at 5 and 10 μM. In addition, forskolin-stimulated bone resorption reached a plateau after 48 hrs of incubation, a phenomenon which did not occur with PTH. Forskolin (0.1 μM) strongly stimulated PTH-induced cyclic AMP production in fetal mouse calvaria. However, PTH-stimulated bone resorption and PTH-induced increase in cytosolic free Ca 2+ in bone fetal rat cells were not stimulated by forskolin (0.1 μM). 9-(Tetrahydro-2-furyl) adenine (100 μM) completely blunted PTH-stimulated cyclic AMP response in fetal mouse calvaria. PTH-stimulated bone resorption was also completely inhibited, but only after 6 hrs and not after 24 hrs of incubation. With nifedipine and varabamil PTH-stimulated bone resorption was significantly inhibited after 24 hrs of incubation and not significantly after 6 hrs of incubation. A23187 (1 gmM) significantly stimulated PTH-stimulated cyclic AMP level and increased basal cytosolic free Ca 2+ concentration in cultured rat bone cells. In calvaria, however, it had no effect on either basal and PTH-stimulated cyclic AMP production or on basal and PTH-stimulated bone resorption (6 and 24 hrs). From these observations it follows that in calvaria manipulation of intracellular cyclic AMP only (partially) affects bone resorption. This observation points to a role for an additional second messenger in establishing full blown bone resorption. Some of the results are published in short elsewhere.