Cancellous bone behavior in hindlimb immobilized rats during and after naproxen treatment.

Temporary immobilization creates bone loss. The purpose of this investigation was to use an agent to protect the skeleton from bone loss bone during temporary immobilization. Eighty-nine 6-month-old retired breeder Sprague-Dawley female rats were used. Animals were randomly divided into six groups of equal numbers. Four groups were given drinking water from day 0, containing naproxen (100 or 200 mg/l). At day 7, half the animals in all groups had their right hindlimb immobilized. At day 49, half the immobilized rats and non-immobilized controls were sacrificed. The remaining rats were remobilized and the drug was stopped. At day 91, all remaining rats were sacrificed. Gastrocnemius and soleus muscle weights were determined. Right tibiae were analyzed for cancellous bone mass, bone structural and bone dynamic variables. At the close of immobilization, bone mass was lower in the right (immobilized) hindlimb of the immobilized group than in the non-immobilized group. Immobilized rats drinking 100 mg/l naproxen water had significantly higher bone mass in their immobilized limbs than did untreated immobilized rats, but all rats drinking 200 mg/l naproxen water had lower bone mass than controls. After 6 weeks of recovery, bone mass in the immobilized limb of untreated formerly immobilized rats improved, but remained below untreated never-immobilized rats. Formerly immobilized rats that had been treated with 100 mg/l naproxen water had normal bone mass after 6 weeks of recovery. Naproxen, an agent that mildly depresses activation frequency, prevents some of the transient bone mass and structural deterioration during temporary immobilization. Such treatment facilitates a more rapid return to normal bone mass, though not to normal structure. The more rapid recovery occurs because the difference from normal is less, not because of more rapid formation in recovering animals.