PARATHYROID-HORMONE INDUCES SUPEROXIDE ANION BURST IN THE OSTEOCLAST - EVIDENCE FOR THE DIRECT INSTANTANEOUS ACTIVATION OF THE OSTEOCLAST BY THE HORMONE

We have shown that superoxide anion (O-2(-)) production by the osteocl ast can be used as an index of the osteoclast activity since the agent s that inhibit and stimulate the osteoclast also diminish and stimulat e O-2(-) production respectively. Therefore, we have investigated the mechanism of parathyroid hormone (PTH)-mediated stimulation of osteocl ast function in terms of its effect on O-2(-) generation. The determin ation of O-2(-) generation was carried out by employing cytochrome c i mmobilised on a surface-modified gold electrode. The basal level of fr ee radical production by the osteoblast-like cells (ROS 17/2.8) was 10 (4)-fold lower than by osteoclasts cultured on bone. PTH had no acute effect on free radical production by the osteoblasts. The exposure of the osteoclasts cultured on bone to PTH led to a dramatic and immediat e stimulation of O-2(-) generation which was unaffected by the presenc e of ROS 17/2.8 cells. The osteoclasts cocultured with ROS 17/2.8 cell s and exposed to PTH for 3 h were also found to produce greater stimul ation of O-2(-) than the osteoclasts exposed to PTH alone. A competiti ve leukotriene D-4 antagonist REV 5901, which also inhibits 5-lipoxyge nase, did not block O-2(-) generation by osteoclasts cultured alone or in the presence of osteoblasts. Therefore, we conclude that PTH direc tly stimulates osteoclasts to produce O-2(-); this may be the main mod e of activation of the osteoclasts, although an osteoblast-mediated ef fect of the hormone cannot be ruled out.