Dose-response Effects of Exercise Intensity on Bone in Ovariectomized Rats

Abstract

The purpose of this study was to investigate the effects of different intensities of resistance exercise training on established bone loss in ovariectomized (ovx-ed) rats by densitometry and histomorphometry. Thirty Female Wistar rats were ovx-ed or sham-operated (SHM) at 3 months of age and maintained untreated for 5 months after surgery to establish osteopenia. When they reached 8 months, the ovx-ed rats were divided into four groups in accordance with varying weights applied to a squat-training device: The weight classifications were 1) kept sedentary (OVX); 2) lifted 0 g (LOW); 3) 750 g (MID); and 1500 g (HIGH). The rats in the three training groups performed weight-lifting of 10 reps, performing 2 sets per day, 3 days a week for a ten week period. The Femora and tibiae were removed from each rat and were used for analyses. Ovx induced a significant loss of total BMC in all the bones tested. The ovx-induced femoral BMC loss was observed at all locations tested on the bone (proximal, shaft, and distal), and exercise-intensity dependent restoration was found at the proximal and the distal sites, but not at the shaft. In the tibia, ovx-induced significant bone loss occurred only at the proximal metaphyseal site. The training increased the tibial BMC of all sites in an exercise-intensity dependently, irrespective of the degree of ovx effect. At the tibial shaft, the training increased the cortical bone mass significantly above sham level by the bone apposition at the periosteum. At the proximal tibial metaphysis, exercise had no effect on the cancellous bone volume after ovx-induced bone loss. This finding suggests that the exercise induced bone increase in the ovx-ed rats was from cortical bone, not from cancellous bone, at least in the proximal tibia. These findings indicate that the weight-lifting exercise in rats reversed the ovx-induced bone loss in an exercise-intensity dependent and site-specific manner, even in established osteopenic skeleton 5 mon after ovx.