Effects of exercise on the bone mineral density and bone cells after unloading in rats

Introduction

Osteocytes are the cells embedded in extracellular matrices generated by osteoblasts, and communicate each other via gap junctions. It is reported that gap junctions are broken by the absence of load. However, the effects of exercise on osteocytes as receptors of mechanical stress after disuse bone atrophy have not been clarified. In this study, we evaluated the effects of exercise after bone loss on the bone mineral density (BMD) and osteocytes.

Methods

The experimental animals were 22 male 9-week-old Sprague-Dawley rats, and experiments were performed under the following conditions: 1) Plaster fixation of the left hindlimb was performed in the position of knee joint flexion and foot joint dorsiflexion during the period of 10-13 weeks of age (non-loading), 2) After removal of the fixation, the animals were divided into a non-exercise group (n=7) and an exercise group (n=7), and 3) The animals in the exercise group were allowed to perform free running in a rotatory cage during the period of 14-17 weeks of age, while in the non-exercise group, the animals were bred in individual cages by the routine method (re-loading).
After removal of the plaster fixation and completion of the experiments, fixation was performed by perfusion with Karnovsky’s solution under anesthesia with Nembutal, and the femur was excised. BMD was analyzed in the metaphysis of the proximal femur (L1), the diaphysis (L2-L3) and the metaphysis of the distal femur (L4) by the DXA method (QDR-2000). The bone matrix was dissolved by the EDTA-KOH method, and the morphology of osteocytes and osteoblasts was observed by scanning electron microscopy (SEM).

Results

BMD of L4 in the left femur, which had not been loaded by plaster fixation, was reduced (p<0.01). BMD of L4 was more increased by re-loading than that of L1 and L2-L3 in both of the non-exercise and exercise groups, indicating large changes in BMD of the metaphysis of the distal femur. SEM demonstrated that osteocytes were spherical with a diameter of about 10 μm during the non-loading period, while in the non-exercise and exercise groups, spherical and spindle-shaped osteocytes with a diameter of 15 μm were observed. The osteocytes and osteoblasts of the bone surface and cell processes of osteocytes had complex structures in the exercise group.

Discussion/Conclusions

L4 is positioned in the knee joint for support of and action on load and weight caused by exercise, and BMD of L4 may have been high due to strong effects of mechanical stress. In the process of the increase in BMD, osteocytes enhance ossification, and are closely involved in transmission of mechanical stress, suggesting that they are functionally activated.