PLATELET-ACTIVATING-FACTOR INDUCES PSEUDOPOD FORMATION IN CALCITONIN-TREATED RABBIT OSTEOCLASTS

We demonstrated previously that platelet-activating factor (PAP), a po tent inflammatory mediator, acts on osteoclasts to elevate cytosolic [ Ca2+] and stimulate resorption. However, it is not clear whether the e ffects of PAF on resorptive activity are direct or indirect. In the pr esent study, we investigated the effects of PAF on osteoclast motility . Osteoclasts were isolated from the long bones of neonatal rabbits, a nd cell motility and morphology were monitored using time-lapse video microscopy. Calcitonin, a hormone known to induce retraction of pseudo pods and inhibit resorptive activity, was used to render osteoclasts q uiescent. Within 10 minutes of calcitonin treatment (100 ng/ml, final) , pronounced retraction of pseudopods was observed in 68 of 112 cells tested. When PAF (200 nM, final) was added 10 minutes after calcitonin treatment, pseudopods were evident 1 h later in 15 of 37 calcitonin-r esponsive cells tested. In contrast, pseudopods were evident in only 4 of 31 calcitonin-responsive cells treated with control solutions (PAF -vehicle or S-PAF, the biologically inactive stereoisomer of PAF). Pse udopod formation was quantified by measuring the planar area of pseudo pods with a computer-based video analysis system. When assessed 60 min utes following PAF treatment, the pseudopod area was significantly gre ater in PAF-treated cells than in control cells. In some calcitonin-tr eated osteoclasts, PAF induced pseudopod formation when applied focall y using an extracellular micropipette, consistent with a direct action of PAF. We conclude that PAF directly induces pseudopod formation in calcitonin-inhibited osteoclasts, a morphologic response indicative of osteoclast activation.