Adaptation of mechanical, morphological, and biochemical properties of the rat growth plate to dose-dependent voluntary exercise

Abstract

Mechanical loading has been shown to modulate longitudinal bone growth and cellular activity of the growth plate. Nevertheless, limited and controversial results exist regarding the effect of exercise on a physiological level on the growth plate. The present study investigated whether dose-dependent voluntary exercise has an influence on morphological, biochemical, and mechanical factors of the distal femoral growth plate of immature rats. Female growing Sprague–Dawley rats were randomly assigned to a unlimited exercise group (UE, n = 10), a limited exercise group (LE, n = 10), and a sedentary control group (CON, n = 10). The exercise groups were trained voluntarily in a running wheel for 8 weeks. The UE group could use the running wheel everytime, whereas the LE group had timely restricted (50%) access. After sacrifice, the right femur was prepared for histomorphometric analysis and immunohistochemical staining of the distal growth plate. Mechanical testing was carried out on the distal growth plate of the left femur in shear direction. At the end of the study, the UE group had a significantly lower body mass than the CON group. There was no significant difference in overall femoral length between the groups. The height of the growth plate and the proliferation zone was significantly greater in the CON group than in both exercise groups. Only the LE group had a significantly lower hypertrophic zone and matrilin-3 staining pattern than the CON group. Osteonectin was located in the matrix of the upper hypertrophic zone in the UE group, whereas the LE and CON group showed more chondrocytes in the hypertrophic and lower proliferation zones stained for osteonectin, suggesting a higher level of mineralization in the growth plate of the UE group. No variations of mechanical properties of the distal femoral growth plate were detected. These results clearly demonstrate adaptations of morphology and biochemical parameters to the dose of running exercise, which do not result in significant differences in mechanical properties or bone length between the UE, LE, and CON groups.