Femoral Neck Response to Exercise and Subsequent Deconditioning in Young and Adult Rats

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Abstract
Aged bones have been considered to have reduced capacity to respond to changes in incident loading. By subjecting young and adult rats to increased loading and subsequent deconditioning, we observed quantitatively similar adaptive responses of bone in these two groups, but young skeletons adapted primarily through geometric changes and adult bones through increased volumetric density. Loss of the exercise‐induced bone benefits did not depend on age. Introduction: Aging has been shown to decrease the sensitivity of the mechanosensory cells of bones to loading‐induced stimuli, presumably resulting in not only reduced capacity but also different adaptive mechanism of the aged skeleton to altered loading, as well as poorer capacity to preserve exercise‐induced bone benefits. Materials and Methods: Fifty young (5‐week‐old) and 50 adult (33‐week‐old) male rats were randomized into control and exercise (+deconditioning) groups. After a 14‐week progressively intensified running program, one‐half of the exercised rats (both young and adult) were killed, and the remaining rats underwent subsequent 14‐week period of deconditioning (free cage activity). A comprehensive analysis of the femoral neck was performed using peripheral quantitative computed tomography and mechanical testing. Results: In comparison with the controls, both young and adult exercised rats had significant increases in almost all measured parameters: +25% (p < 0.001) and +10% (not significant [NS]) in the cross‐sectional area; +28% (p < 0.001) and +18% (p < 0.001) in bone mineral content; +11% (p < 0.05) and +23% (p < 0.001) in bone mineral density; and +30% (p < 0.01) and +28% (p < 0.01) in the breaking load, respectively. The skeletal responses were not statistically different between the young and adult rats. After the 14‐week period of deconditioning, the corresponding exercised‐to‐controls differences were +17% (p < 0.05) and +10% (NS), +18% (p < 0.05) and +13% (p < 0.05), +2% (NS) and +2% (NS), and +11% (NS) and +6% (NS), respectively. Again, the response differences were not significant between the age groups. Conclusion: Quantitatively, the capacity of the young and adult skeleton to adapt to increased loading was similar, but the adaptive mechanisms appeared different: growing bones seemed to primarily display geometric changes (increase in bone size), whereas the adult skeleton responded mainly through an increase in density. Despite this apparent difference in the adaptive mechanism, aging did not modulate the ability of the skeleton to preserve the exercise‐induced bone gain, because the bone loss was similar in the young and adult rats after cessation of training.