Comparison of parathyroid hormone and calcium ionophore A23187 effects on bone resorption and nucleic acid synthesis in cultured fetal rat bone.

It has recently been demonstrated that calcium ionophore A23187 mimics certain of the effects of parathyroid hormone (PTH) on bone in vitro, including stimulation of 45Ca release and cAMP formation. To further examine the relative effects of these two agents on bone cell metabolism, we compared the effects of synthetic PTH 1-34 (50 ng/ml) and calcium ionophore A23187 (0.5 micrograms/ml) on 45Ca release, DNA concentration, and nucleic acid synthesis in fetal rat forelimb rudiments cultured for periods up to 120 h. Both agents stimulated 45Ca release; however, the effects of PTH were apparent after a shorter period of exposure. Bone DNA concentration (expressed as microgram DNA/mg bone) was not affected by PTH but was significantly increased relative to control values by exposure to A23187 for 8-120 h of incubation. PTH increased the incorporation of 3H-thymidine into DNA at 30 and 48 h, and increased the incorporation of 14C-uridine into RNA at 48 h, time points which corresponded to a period of accelerated PTH stimulation of 45Ca release. In contrast, 3H-thymidine and 14C-uridine incorporation were both uniformly suppressed by A23187 at all time points examined. Thus the increased DNA concentration observed in A23187-treated rudiments appeared to be the result of a decreased rate of bone maturation and mineralization. The markedly different patterns of nucleic acid synthesis in response to PTH and A23187 suggest that these agents differ significantly in their mechanisms of action on bone cell metabolism.

Ouabain effects on hormonally-stimulated bone resorption and cyclic AMP content in cultured fetal rat bones.

Ouabain in concentrations from 20-100 micromoles produced a dose-related inhibition of in vitro stimulation of bone resorption by parathyroid hormone, 1,25-dihydroxyvitamin D3 and calcium ionophore A23187, as measured by 45Ca and [3H]-hydroxyproline release in 5-day cultures of fetal rat forelimb rudiments. The inhibitory effect on 45Ca release was completely reversed by subsequent incubation in ouabain-free medium. At a concentration of 100 micromoles ouabain virtually abolished active bone resorption; however, basal and stimulated bone cyclic AMP (cAMP) content were significantly increased above levels observed in the absence of ouabain. The increased cAMP content did not appear to be the result of phosphodiesterase inhibition. It is concluded that intact Na/K ATPase function is required for hormonally-stimulated bone resorptive processes and that the inhibitory effect of ouabain on bone resorption is produced at a point subsequent to cyclic AMP generation.