EXTRA CANCELLOUS BONE INDUCED BY COMBINED PROSTAGLANDIN E(2) AND RISEDRONATE ADMINISTRATION IS MAINTAINED AFTER THEIR WITHDRAWAL IN OLDER FEMALE RATS

Prostaglandin E(2) (PGE(2)) has been recognized for its marked anaboli c effect on bone, but the bone gain is lost after the cessation of PGE (2) treatment, In previous studies, we were successful in maintaining the new bone by administering a bisphosphonate after the withdrawal of PGE, treatment, The objective of this study was to determine the fate of the extra bone induced by a combination with PGE, and risedronate after discontinuing treatment, Ninety-six 9-month-old virgin female Sp rague-Dawley rats were treated with 1 or 5 mu g of risedronate/kg/twic e weekly, 6 mg of PGE(2)/kg/day alone or 6 mg of PGE(2)/kg/day plus 1 or 5 mu g of risedronate/kg/twice weekly for 60 days (day 0) and follo wed by 60 days without treatment (day 60), We have reported the result s from the groups treated for 60 days previously, This report is restr icted to the histomorphometric findings on the secondary spongiosa of the proximal tibial metaphysis in the groups after withdrawal for 60 d ays, We found that the only group that maintained the PGE, induced new bone after withdrawal was the group treated with 6 mg of PGE(2)/kg/da y plus 5 mu g of risedronate/kg/twice a week withdrawal of this combin ed treatment depressed bone turnover (bone-based bone formation rate, activation frequency) and bone resorption (percent eroded perimeter). The tissue mechanisms responsible for the protection drew from the pre viously deposited risedronate.