Forskolin sensitizes parathyroid hormone-induced cyclic AMP response, but not the bone resorptive effect, in mouse calvarial bones

Abstract

The effect of forskolin on parathyroid hormone (PTH) stimulated bone resorption, as assessed in vitro by the release of 45Ca from prelabelled neonatal mouse calvarial bones, and cyclic AMP formation in mouse calvarial bones and osteoblast-like cells was investigated. Much higher concentrations (100–300-times) of PTH were required to stimulate cyclic AMP accumulation than to stimulate mineral mobilization in murine calvarial bones. PTH, in the absence of phosphodiesterase inhibitor, stimulated cyclic AMP formation in mouse calvarial bones at and above concentrations of 3–10 nmol/l with EC 50 at 10–15 nmol/l. In the presence of forskolin ( 1 or 10 μmol/l) the minimal concentration required to obtain a cyclic AMP response to PTH was decreased by a factor of 30–100 and the EC 50 value was decreased to 1–2 nmol/l. Similar results were seen in osteoblast-cnriched cells. In addition, the magnitude of the PTH-induced cyclic AMP response was substantially potentiated by forskolin, both in calvarial bones and in isolated osteoblasts. Forskolin, in the absence of PTH, stimulated cyclic AMP levels in mouse calvaria at and above 1 μmol/l. In the presence of PTH, the response to forskolin was potentiated over the whole dose-response curve with apparent EC 50 value at 1–2 μmol/l of forskolin. Forskolin (1 μmol/l) did not affect the magnitude of the 45Ca release response to PTH in 24 or 48 h cultures. In 96 h cultures, forskolin, in an additive manner, potentiated the effect of PTH on calcium mobilization. These results show that forskolin, in mouse calvarial bones and in isolated osteoblasts, in addition to directly stimulating cyclic AMP, can enhance receptor-mediated activation of adenylate cyclase. The finding that forskolin did not synergistically potentiate PTH-induced bone resorption suggests that there is no simple relationship between PTH-induccd cyclic AMP formation and stimulation of bone resorption.