Inhibition of parathyroid hormone-stimulated bone resorption by monovalent cation ionophores.

Abstract

The actions of divalent cation ionophores on bone resorption in vitro are complex; both enhancement of resorption and inhibition of stimulated resorption have been observed with the ionophore A23187. We have found in neonatal mouse calvaria, in which divalent ionophores were only inhibitory, that monovalent cation ionophores were even more potent inhibitors of stimulated bone resorption. Nigericin, monensin, and X206 each inhibited the release of calcium (Ca) from calvaria that were stimulated to resorb by 0.1 U/ml parathyroid hormone (PTH). Actions of the three ionophores were dose dependent and were maximal at 10−7M, 3×10−7M, and 1.2×10−7M, respectively, compared to A23187, which was maximally inhibitory at 2×10−6M. After pretreatment with nigericin alone or together with PTH for 24 h, inhibition of stimulated resorption was partially reversible. Prolonged (48 h) treatment, either with ionophore alone or together with PTH, caused irreversible inhibition of stimulated Ca release. However, the ionophore was only partially inhibitory if it was added to calvaria stimulated by pretreatment with PTH alone for 24 or 48 h. Resorption stimulated by prostaglandin E2, 1,25-dihydroxyvitamin D3, and epidermal growth factor was also inhibited by monovalent ionophores, indicating that the inhibition was not at the level of the PTH receptor. In addition, ionophores did not lower basal or PTH-stimulated production of cyclic AMP by calvaria. Submaximal doses of nigericin were synergistic with calcitonin or ouabain in inhibiting PTH-stimulated resorption. These results are consistent with the hypothesis that stimulated release of Ca from bone occurs by a Na/Ca exchange mechanism. Thus monovalent cation ionophores would increase intracellular Na+, thereby decreasing the Na gradient across bone cell membranes, leading to conditions unfavorable for Ca efflux coupled to further Na influx.