Individual and combined effects of calciotropic hormones and growth factors on mineral metabolism in embryonic chick tibiae.

We have investigated single and combined effects of calciotropic hormones and growth factors on the regulation of alkaline phosphatase (ALP) activity and calcium metabolism in an optimized serum-free bone organ culture system of embryonic chick tibiae. Parathyroid hormone PTH(1-34) alone mobilized calcium from bone tissue time- and dose-dependently and inhibited ALP activity. Both the bisphosphonate (BM 21.0955) and to a lesser extent salmon calcitonin alone slightly increased calcium uptake and inhibited the stimulation of bone resorption by PTH(1-34). 1,25(OH)2D3 mobilized calcium and inhibited ALP activity in contrast to 24,25(OH)2D3 which inhibited ALP activity but had no significant effect on calcium metabolism. Interestingly the combination of PTH(1-34) with 1,25(OH)2D3 but not 24,25(OH)2D3 reduced calcium mobilization. The combination of the midregional fragment PTH(28-48), which by itself has no effect on calcium metabolism, with 1,25(OH)2D3 reduced calcium mobilization more efficiently. Several PTH-regulated mediators have been assayed in this system. Of the tested growth factors, IGF-I at high concentrations caused bone resorption with no effect on ALP activity. TGF-beta 1 (transforming growth factor beta) and BMP-2 had no significant effect on calcium metabolism; however, ALP activity was inhibited by TGF-beta 1 and induced dose dependently by BMP-2. Of the other factors known to be present in bone, platelet-derived growth factor (PDGFA/B) and epidermal growth factor (EGF) had a small effect on calcium mobilization but had no effect on ALP activity. bFGF reduced ALP activity slightly without an effect on calcium metabolism. Our results show that this in vitro system can mimic some interactions of calciotropic hormones in vivo and allows the assaying of mediators in terms of regulation of ALP activity and of calcium metabolism.