Combined Effects of Exercise and Propranolol on Bone Tissue in Ovariectomized Rats

The bone response to physical exercise may be under control of the SNS. Using a running session in rats, we confirmed that exercise improved trabecular and cortical properties. SNS blockade by propranolol did not affect this response on cortical bone but surprisingly inhibited the trabecular response. This suggests that the SNS is involved in the trabecular response to exercise but not in the cortical response. Introduction: Animal studies have suggested that bone remodeling is underadrenergic control through the sympathetic nervous system (SNS). However, the SNS contribution to bone response under mechanical loading remains unclear. The purpose of this study was to examine the preventive effect of exercise coupled with propranolol on cancellous and cortical bone compartments in ovariectomized rats. Materials and Methods: Six-month-old female Wistar rats were ovariectomized (OVX, n 44) or sham- operated (n 24). OVX rats received subcutaneous injections of propranolol 0.1 mg/kg/day or vehicle and were submitted or not submitted to treadmill exercise (13 m/minute, 60 minutes/day, 5 days/week) for 10 weeks. Tibial and femoral BMD was analyzed longitudinally by DXA. At death, the left tibial metaphysis and L4 vertebrae were removed, and CT was performed to study trabecular and cortical bone structure. Histo- morphometric analysis was performed on the right proximal tibia. Results: After 10 weeks, BMD and trabecular strength decreased in OVX rats, whereas bone turnover rate and cortical porosity increased compared with the Sham group (p < 0.001). Either propranolol or exercise allowed preservation of bone architecture by increasing trabecular number (+50.35% versus OVX; p < 0.001) and thickness (+16.8% versus OVX; p < 0.001). An additive effect of propranolol and exercise was observed on cortical porosity but not on trabecular microarchitecture or cortical width. Biomechanical properties indicated a higher ultimate force in the OVX-propranolol-exercise group compared with the OVX group (+9.9%; p < 0.05), whereas propranolol and exercise alone did not have any significant effect on bone strength. Conclusions: Our data confirm a contribution of the SNS to the determinants of bone mass and quality and show a antagonistic effect of exercise and a -antagonist on trabecular bone structure. J Bone Miner Res 2007;22:578-588. Published online on January 22, 2007; doi: 10.1359/JBMR.070117