Prostaglandin E2 initially inhibits and then stimulates bone resorption in isolated rabbit osteoclast cultures

Osteoclasts were isolated from the long bones of neonatal rabbits and cultured on devitalized bovine bone slices for 8, 24, 48 and 72 h with and without prostaglandin E2 (PGE2) (10(-6) M). The number of osteoclasts present at the end of the culture periods was counted after staining the cells for tartrate resistant acid phosphatase (TRAP). After removal of the cells, the resorption lacunae excavated by the osteoclasts were observed by scanning electron microscopy (SEM) and their size and depth calculated by computer-assisted morphometric and stereomorphometric techniques. PGE2 had no effect on the number of TRAP positive multinucleated osteoclasts, but decreased the number of TRAP positive mononuclear cells. The total area of the excavated pits and the area excavated per osteoclast in PGE2-treated cultures were decreased by 62 and 58% respectively after 8 h in culture. After 24 h in culture, the total excavated area and the excavated area per osteoclast were still 44 and 38% lower in the PGE2-treated cultures than in the corresponding control cultures. However, after 48 h of culture, resorptive activity in PGE2-treated cultures was consistently greater than in control cultures. In the course of a 48 h culture period, the PGE2 concentration decreased from 1.0 x 10(-6) to 0.3 x 10(-6) M. Thus, despite the continuous presence of PGE2, the resorptive activity of osteoclasts not only recovered from the transient inhibitory effect of PGE2, but was actually greater than in the control cultures. This confirms that the effects of PGE2 in isolated osteoclast preparations are inhibitory in short term cultures, but shows that the effects of PGE2 in such preparations are stimulatory in longer term cultures. Proliferating stromal cells with osteoblast-like characteristics comprised approximately 45% of the 'osteoclast' cultures at the start of the cultures, but their number increased to 93% of the total cell population at 48 h and to 98% at 72 h. Our results suggest that the PGE2-induced stimulation of osteoclastic activity represents an indirect effect mediated by stromal cells derived from bone marrow. Our results also indicate that the increased resorptive activity in PGE2 treated cultures can be accounted for by an increase in the size of the resorption lacunae and is not caused by an increase in osteoclast number.