Comparative study of the effects of cyclic nucleotide phosphodiesterase inhibitors on bone resorption and cyclic AMP formation in vitro.

The relation between the level of cyclic AMP and bone resorption was studied in a bone organ culture system, using calvaria from newborn mice. Two methylxanthines, iso-butyl-methylxanthine and theophylline and two non-xanthine inhibitors of cyclic AMP phosphodiesterase, Ro 20-1724 and rolipram, stimulated the release of [45Ca] and [3H] from bones prelabelled in vivo with [45Ca]- and [3H]proline, respectively. The release occurred after a delay of more than 24 hr. In 120-hr cultures, theophylline, IBMX, rolipram and Ro 20-1724, all stimulated the release of stable calcium, inorganic phosphate and the lysosomal enzymes, beta-glucuronidase and beta-N-acetylglucosaminidase from mouse calvarial bones. In addition, all four phosphodiesterase inhibitors decreased the amount of hydroxyproline in the bones at the end of the culture period. The release of minerals and the decrease of hydroxyproline was abolished by indomethacin. In short-term cultures (24 hr), rolipram and Ro 20-1724 did not reduce PTH-stimulated mineral mobilization, whereas the two methylxanthines, and dibutyryl cyclic AMP and 8-bromo cyclic AMP, did cause a reduction of PTH-stimulated mineral release during the first 24 hr. All four phosphodiesterase inhibitors increased the accumulation of cyclic AMP in the calvaria and inhibited cyclic AMP hydrolysis in extracts of calvarial bone. There was a correlation between the magnitude of the initial rise in cyclic AMP and the delayed stimulation of bone resorption. However, much lower concentrations of the PDE inhibitors were sufficient to produce a delayed increase in bone resorption than to block phosphodiesterase and significantly raise cyclic AMP levels. It is suggested that the elevation of cyclic AMP in a subset of bone cells results in an acute reduction of bone mobilization and the cAMP elevation in another subset to a delayed rise in bone resorption.