In this study, we evaluated the rat cortical bone changes after a two‐cycle, 60‐day each (ON/OFF/ON/OFF) treatment with either prostaglandin E2 (OVX/c‐PGE2) alone or in combination with risedronate (OVX/c‐PGE2+Ris), in comparison with daily treatment with PGE2 for 240 days (OVX/PGE2−240d) in ovariectomized (OVX) rats. At the end of the study, we found that: (1) the overall effectiveness of the treatment on bone mass in the tibial shaft indicates the following ranking: OVX/PGE2−240d = OVX/c‐PGE2+Ris > OVX/c‐PGE2 > OVX/c‐Ris ≥ OVX = aging; (2) the same bone mass and architecture were produced in the OVX/PGE2−240d and the OVX/c‐PGE2+Ris groups, but the histomorphometric profiles differed in that the former exhibited a higher bone turnover and index of resorption; (3) OVX/c‐PGE2+Ris treatment prevented endocortical bone loss and minimized trabecular bone loss during the OFF periods; and (4) the OVX/c‐PGE2 alone treatment resulted in the accumulation of less total bone than OVX/PGE2−240d and OVX/c‐PGE2+Ris because it could not maintain most of the new subendocortical and marrow trabecular bone generated earlier. In summary, both continuous daily PGE2 and two cycles ON/OFF combined PGE2 and Ris treatments result in more bone mass than two cycles ON/OFF PGE2 alone and Ris alone in estrogen‐deficient rats. This study showed that the anabolic effects of PGE2 can be induced and maintained either by continuous administration or by cyclical PGE2+Ris.

Our laboratory has previously demonstrated that the T-lymphocyte is critical in the development of cyclosporin A-induced osteopenia in the rat model, A similar state of osteopenia is induced by estrogen depletion in the ovariectomized (OVX) rat, which is the animal model of postmenopausal bone loss, However, the role of the immune system, and particularly the T-lymphocyte, in estrogen deplete osteopenia has not been elucidated, We used the Rowett athymic nude rat as our model of T-lymphocyte deficiency, In this study, the experimental rats were divided into four groups as follows: (1) sham-operated Rowett heterozygous (rnu/+) euthymic rats (control group); (2) OVX Rowett heterozygous (rnu/+) euthymic rats; (3) sham-operated Rowett homozygous (rnu/rnu) athymic nude rats, which are T-lymphocyte deficient; and (4) ovariectomized Rowett homozygous (rnu/rnu) rats, Rats were weighed, and venous blood was taken in weeks 2, 4, and 6 for determination of serum osteocalcin, Serum 1,25-dihydroxyvitamin D (1,25(OH)(2)D) was determined on the day of sacrifice, Following sacrifice, histomorphometry was performed on double-labeled proximal tibial metaphyses, Flow cytometric analysis of splenic mononuclear cell isolates stained for OX19-positive (CD5) T-lymphocytes was performed, T-lymphocyte analysis revealed significant reductions in both athymic nude groups, while OVX euthymic rats demonstrated a diminished number of T-cells relative to their sham-operated counterparts, Histomorphometric data indicated that both OVX groups exhibited a significant loss of trabecular volume, with associated increases in indices for bone formation and resorption, with resorption likely outstripping formation, resulting in osteopenia. Serum osteocalcin was significantly elevated in the ovariectomized euthymic group throughout the experimental period compared with the control group (p < 0.01); it was elevated in the ovariectomized athymic group on week 4 only (p < 0.01 vs. control), It appears that the T-lymphocyte may not be an essential component in the pathogenesis of estrogen deficiency osteopenia, The contribution of circulating T-lymphocytes as well as other T-lymphocyte-rich organs needs to be explored further.

This study is designed to test how intermittent application of prostaglandin E(2) (PGE(2)) and risedronate (Ris) alone or in combination acts on the cancellous bone mass in estrogen-deficient rats. Sprague-Dawley rats were ovariectomized (ovx) or sham-ovx'd at 6 months of age. PGE(2) (6mg/kg/d), Ris (5 mu g/kg/twice a week) or PGE(2) plus Ris were given for 60 days to ovx rats immediately after operation and followed by 60 days without treatment. The drugs were then reapplied for another 60 days. Static histomorphometry was performed on the secondary spongiosa of proximal tibial metaphysis (PTM). Sixty days of ovx lost trabecular bone and number, Ris prevented ovx-induced bone loss. PGE(2) added 48% extra cancellous bone, but the new bone was completely lost after 60 days of withdrawal. Another 60 days of PGE(2) treatment only partially restored the trabecular bone, the bone mass was still -42% lower than that of sham-ovx controls. Co-treatment of PGE(2) with Ris added the same amount of bone as PGE(2) alone after the first 60 days treatment period, but differed from PGE(2) alone in that the new bone lost less during the 60 days withdrawal period. Re-application of co-treatment for another 60 days added more extra bone. We concluded that intermittent co-treatment with anabolic and anti-resorptive agents is more effective than anabolic agent alone in long-term therapy of cancellous bone in estrogen-deficient rats.

Effects of risedronate and prostaglandin E2 (PGE2) alone or in combination on the distal tibia, a non-growing bone site with closed epiphysis at 3 months of age, were studied in ovariectomized (ovx) rats. Six-month-old Sprague-Dawley female rats were either ovx or sham-ovx. Rats were treated immediately after operation either with risedronate (5 μg/kg/2x/wk), PGE2 (6 mg/kg/d), or risedronate+ PGE2 for 60 days (on-groups) and followed by 60 days without treatment (off-groups). Trabecular area, width and numbers were determined in metaphyseal cancellous bone of the distal tibia. No significant bone loss or structural changes were observed in the distal tibial metaphysis after 120 days of ovx. Risedronate alone did not produce any affect on bone mass during the treatment and the withdrawal periods. PGE2 alone increased the trabecular bone mass associated with thickened trabeculae and increased trabecular numbers. However, some of the newly formed bone was lost at the end of 60 days withdrawal. Combination of risedronate and PGE2 treatment added the same amount of bone mass as PGE2 alone, and the added new bone was maintained during the 60 days withdrawal. These results indicate that treatment with risedronate and PGE2 can preserve the anabolic effect of PGE2 on bone mass for at least 60 days after treatment.

Effects of intermittent administration of prostaglandin E2 (PGE2), risedronate (Ris) and their combination on bone mass were studied on the cortical bone of tibial shafts of ovariectomized (ovx) rats. Six month old ovx rats were treated immediately after operation with subcutaneous injections of 6 mg PGE2 /kg/d, 5 μg Ris /kg/2x/wk or their combination for 60 days each of an on/off/on cycle. PGE2 alone and in combination with Ris added the same amount of new bone in the first 60 days on period. During the 60 days off period, newly added endocortical and marrow trabecular bone disappeared in PGE2 alone treatment groups. In co-treatment group, the marrow trabeculae were only partly lost. There was no difference in total bone area between co-treatment and PGE2 alone groups after another 60 days on treatment. Our findings indicate that co-treatment was better in the maintenance of newly added endocortical and marrow trabecular bone during the off period; however, it formed less bone than PGE2 alone during the second treatment period, and ended up with the same amount of bone with both treatments.