CANCELLOUS BONE BEHAVIOR IN HINDLIMB IMMOBILIZED RATS DURING AND AFTER NAPROXEN TREATMENT

Temporary immobilization creates bone loss. The purpose of this invest igation was to use an agent to protect the skeleton from bone loss bon e during temporary immobilization. Eighty-nine 6-month-old retired bre eder Sprague-Dawley female rats were used. Animals were randomly divid ed into six groups of equal numbers. Four groups were given drinking w ater 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 da y 49, half the immobilized rats and nonimmobilized controls were sacri ficed. The remaining rats were remobilized and the drug was stopped. A t day 91, all remaining rats were sacrificed. Gastrocnemius and soleus muscle weights were determined. Right tibiae were analyzed for cancel lous bone mass, bone structural and bone dynamic variables. At the clo se of immobilization, bone mass was lower in the right (immobilized) h indlimb of the immobilized group than in the non-immobilized group. Im mobilized rats drinking 100 mg/l naproxen water had significantly high er bone mass in their immobilized limbs than did untreated immobilized rats, but all rats drinking 200 mg/l naproxen water had lower bone ma ss than controls. After 6 weeks of recovery, bone mass in the immobili zed limb of untreated formerly immobilized rats improved, but remained below untreated never-immobilized rats. Formerly immobilized rats tha t had been treated with 100 mg/l naproxen water had normal bone mass a fter 6 weeks of recovery. Naproxen, an agent that mildly depresses act ivation frequency, prevents some of the transient bone mass and struct ural deterioration during temporary immobilization. Such treatment fac ilitates 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 ani mals.