| Bone development is seized of the most important status during the growth of children and youth because the bone development directly effects on other organs and systems. So far, the mechanism of the bone development, especially effect of exercise on the skeletal development remains uncertain, thus, the purpose of this paper is to investigate the mechanism of changes of growing bone using different intensive exercise and mechanical loading. The systematic research includes bone mass, bone histomorphometry, bone biomechanics, bone biochemical marker and morphology of growth plate, local regulation factor-PTHrP, as well as analysis of their interaction.120 Sprague-Dawley rats, 4 weeks of age, were assigned to five groups: a baseline control (BC), exercise, sedentary (CON), tail-suspension (TS) and weighting (W). The exercise group was divided respectively into low loading exercise (LE) and high loading exercise (HE) subgroups and 3, 6 and 9weeks exercise subgroups. The rats of LE and HE groups were treadmill-trained respectively for 3, 6, or 9weeks (5day/week, 1hr/day). TS means tail-suspended and W was weighted (40% body weight) for 3 weeks. The main results are as follows:(1) The body weight of each group rats increased with age. There were no significant differences between LE and CON, but HE, TS and W were remarkable lower than CON. That indicates the increase of body weight of HE, TS and W rats are inhibited and the low loading exercise is contributive to the growth of body weight in our experiment.(2) The longitudinal data of each index in growing rats showed there were no consistent changes among various groups, but the characteristic of changes with increasing age was more obvious in LE than in HE.(3) With regard to the effect of exercise on the skeletal development, there were significant differences among various intensive training groups. LE could improve comprehensively the bone information, bone mass, bone structure and mechanical properties, meanwhile, it could also increase the length of bone and the height of growth pate, and then promoted longitudinal growth of bone. Nevertheless, all of above mention were weaken in HE. Thus, to the subjects of our experiment, the low loading exercise was an appropriate loading that can improve long bone development.(4) There were also remarkable differences among various mechanical loading groups. TS could decrease the bone information, bone mass, bone structure and mechanical properties, and it could reduce the height of growth plate but it didn't affect the length of bone. While W may promote comprehensively the qualities of the growing bone, at the same time, it could inhibit the increasing of bone length, width and thickness, as well as the height of growth plate. It is suggested when the methods of intervene were chosen for children and youth, the positive role that W can improve bone qualities was considered, as well as the amount of loading used during weighting was also thought carefully to avoid retarding the growth of bone length and even body height.(5) The mechanism of the effect of exercise on the long bone development in growing rats is to promote the bone formation by activating the bone modeling, but it isn't improved absolutely with increasing intensity of training. For the low loading exercise, the mechanism of the effect on the bone growth is that the bone formation and absorption, especially the bone formation are enhanced, and then the properties against fracture and deformation are improved by increasing the bone mass and changing the architecture of trabecular bone. The mechanism of weighting is similar to the low loading exercise. But the mechanism of tail-suspension is obvious distinct, it activates the bone remodeling and inhibits the bone formation.(6) This study found first that exercise could change the expression of PTHrP in the growth plate during treadmill training of 6 and 9 weeks, and its expression is more significant in LE than in HE. However, for the expression of PTHrP mRNA, there were no...
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