Localisation of prostaglandin endoperoxide H synthase (PGHS)-1 and PGHS-2 in bone following mechanical loading in vivo

Recent data suggests that induction of prostaglandin endoperoxide a synthas e-2 (PGHS-2) is critical for the anabolic response of lamellar bone elicite d by mechanical strain in vivo. The aim of the present study was to localis e PGHS-1 and PGHS-2 in rat tibiae following four-point bending in vivo. Rig ht tibiae of 19 adult female rats were subjected to 300 cycles of bending o r sham loading at 2.0 Hz with an applied load of 65 N. At 0, 6, and 24 hr p ostloading, rats were anaesthetised and perfused with Bouin's fixative. Lef t and right tibiae were dissected, postfixed for 4 hr at 4 degrees C, decal cified in EDTA, and embedded in paraffin. Serial 5 mu M sections were stain ed for PGHS-1 and PGHS-2 using standard immunoperoxidase procedures. For th e first time, immunoreactivity for both PGHS-1 and PGHS-2 was localised in bone cells in situ, in the rat tibia. PGHS-1 was distributed widely in all tibiae, while PGHS-2 showed sparse localisation. At the endocortical surfac es (EcS), osteoblasts, lining cells, and osteocytes close to the surface re acted strongly for PG HS-I, as did intracortical osteocytes. At the periost eal surface (PsS), osteoblasts and cells of the osteogenic region were immu nopositive. Immediately after loading, the numerical density (n.mm(-2)) of osteocytes labeled with PGHS-1 was significantly greater in loaded tibiae c ompared to controls. This increase was not; seen after sham loading. At 6 a nd 24 hr postloading, this difference was no longer evident. Staining for P GHS-2 was sparse compared to PGHS-1. Light to moderate reactivity was obser ved in osteocytes and canaliculae, but the numerical density of labeled cel ls was significantly less than that for PGHS-1. Moderate staining was seen in Lining cells and osteoblasts at the EcS and PsS of some tibiae. Osteocla sts at the PsS reacted strongly for both PGHS-1 and PGHS-2. There was a sim ilar load-related increase in the density of PGHS-2-labeled osteocytes 0 hr postloading. The labeled osteocyte density had decreased at 6 hr, but rema ined significantly greater in loaded bones. These results show that both fo rms of PGHS can be localised in bone cells, with PGHS-1 expressed to a grea ter extent than PGHS-2. The data also suggest that both PGHS-1 and PGHS-2 m ay play important roles in the early response of bone to mechanical loading in vivo. Anal. Rec. 252:580-586, 1998. (C) 1998 Wiley-Liss, Inc.