Responsiveness of parathyroid hormone to bone resorption in 13-day-old embryonic chick calvaria cultures

To clarify what kind of process participates in bone resorption, time course of indices of bone resorption was investigated using 13-day-old embryonic chick calvaria. When calvariae were cultured with parathyroid hormone (PTH) at 0.01 U/ml for 8 days, hydroxyproline (Hyp) release was already stimulated by PTH in cultures by 1 to 2 days but stimulation of⁴⁵Ca release was not observed even in cultures by 6 to 8 days. Furthermore, stimulation of collagenase release by PTH was observed prior to that of Hyp release. These results indicate that collagenase release precedes Hyp release, which is followed by⁴⁵Ca release. The release of tartrate-resistant acid phosphatase (TrACP), a marker enzyme of osteoclast, was stimulated by PTH at 0.1 U/ml and above to a greater extent in cultures by 5 to 8 days (phase 11) than in cultures by 0 to 4 days (phase 1). E-64, an inhibitor of cysteine proteinase, inhibited PTH-stimulated⁴⁵Ca release strongly in phase 11 but showed a slight decrease in Hyp release in the same phase. These results suggest that first PTH stimulates collagenase production from osteoblasts, secondly collagenase degrades uncalcified collagen and lastly osteoclasts resorb mineralized bones.