Inhibition of bone resorption and lysosomal enzyme release from calvarial bones cultured for 24 hours: synergism between cyclic AMP analogues and phosphodiesterase inhibitors

The effect of N6-monobutyryl adenosine 3′,5′-cyclic monophosphate (mbcAMP), N6,O2′-dibutyryl adenosine 3′,5′-cyclic monophosphate (dbcAMP), 8-bromo adenosine 3′,5′-cyclic monophosphate (8-brcAMP), adenosine 3′,5′-cyclic monophosphate (cAMP) and two phosphodiesterase (PDE) inhibitors, theophylline and 3-isobutyl-methylxanthine (IBMX) on bone resorption was studied in an organ culture system for 24 h using half calvaria from 6–7 day old mice. The parameters studied were: the release of calcium (Ca2+), inorganic phosphate (Pi), β-glucuronidase, β-galactosidase, lactate dehydrogenase (LDH), and 45Ca from the bones to the medium. With dbcAMP, in concentrations between 5 × 10−5m and 2.5 × 10−4m, and with 8-brcAMP, in concentrations between 10−5m and 5 × 10−5m, a dose-dependent inhibitory effect on the spontaneous release of 45Ca from the explants was found. IBMX and theophylline in doses of 10−3m and 2.5 × 10−3m, respectively, inhibited the spontaneous mobilization of 45Ca, while hypoxanthine, which lacks PDE inhibitory capacity, did not affect the release of 45Ca. When cAMP or its analogues were combined with IBMX, a potentiated inhibitory effect on mineral mobilization and lysosomal enzyme release was seen. In contrast, adenosine 5′-monophosphate, 8-bromo adenosine 5′-monophosphate and sodium butyrate did not reduce the release of 45Ca when applied alone or combined with IBMX. PDE inhibitors combined with parathyroid hormone (PTH) resulted in a reduction of the PTH-stimulated bone resorption. The results provide further evidence that cAMP is not a mediator of the early stage of PTH-induced bone resorption, but on the contrary inhibits mineral mobilization and lysosomal enzyme release from cultured bones.