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 [mb] cAMP, N6,O2''-dibutyryl (db) cAMP, 8-bromo (8-br) cAMP, cAMP and 2 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 Ca2+, Pi, .beta.-glucuronidase, .beta.-galactosidase, lactate dehydrogenase (LDH) and 45Ca from the bones to the medium. With dbcAMP, in concentrations between 5 .times. 10-5 and 2.5 .times. 10-4 M, and with 8-brcAMP, in concentrations between 10-5 and 5 .times. 10-5 M, a dose-dependent inhibitory effect on the spontaneous release of 45Ca from the explants was found. IBMX and theophylline in doses of 10-3 and 2.5 .times. 10-3 M, respectively, inhibited the spontaneous mobilization of 45Ca; hypoxanthine, which lacks PDE inhibitory capacity, did not affect the release of 45Ca. When cAMP or its analogs were combined with IBMX, a potentiated inhibitory effect on mineral mobilization and lysosomal enzyme release was seen. cAMP, 8-brcAMP 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. Evidently 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.