The Effect of Progressive Resistance Exercise on Bone Quality in Hindlimb Suspended Growing Rats and Its Potential Mediator FGF-2

Abstract

ABSTRACT

Introduction: Bone is an organ that interacts with surrounding environments and has adaptability to change its microstructure in the presence of mechanical loading. Such a mechanism is well demonstrated in the case of space flight crew or patients in bed resting for long-terms as they experience significant loss of bone quality. The mechanical loading produced from resistance training may bring positive effects on bone quality. However, the effect of progressive resistance ladder climbing exercise on bone quality following a period of hindlimb suspension has not been thoroughly demonstrated. Furthermore, fibroblast growth factor-2 (FGF-2) is a myokine that has been demonstrated to induce an osteogenic effect on bone and its potential role is needed to be investigated. Therefore, this study has been conducted to investigate the effect of hindlimb suspension and progressive resistance exercise on the restoration of bone quality of debilitated bone from a period of hindlimb suspension treatment. Furthermore, in order to investigate its potential mediator between muscle and bone, the expression of fibroblast growth factor-2 (FGF-2 or bFGF) was performed and analyzed.

Methods: Fifty 9-week-old growing male Sprague-Dawley rats were randomly assigned to each of the following groups: HS, hindlimb suspension for 2 weeks (n=10)

CO, control group against HS (n=10)

HEX, hindlimb suspension intervened for 2 weeks followed by 4 weeks of resistance ladder climbing exercise (n=10)

HCO, control group against HEX, hindlimb suspension intervened for 2 weeks followed by 4 weeks of weight bearing spontaneous recovery (n=10)

CON, control group against HCO. Following 1 week of adaptation, the tail of rats in group HS, HEX, HCO were taped for hindlimb suspension. The rats were hindlimb suspended for 2 weeks and then distributed to the regular cages. Following the HS intervention, the ladder climbing resistance exercise was performed for 4 weeks in a progressive manner by progressively increasing the weight loads. For bone quality measurement, Na18F PET/CT, micro-CT and DXA were scanned and serum bone turnover markers were analyzed. For muscle FGF-2 mRNA and protein level analysis, Real-Time PCR and Western Blot analysis were performed in soleus and extensor digitorum longus muscles.

Results: Following 2 weeks of hindlimb suspension intervention, there were significant losses in bone quality indices of whole body BMC, BMD, bone density, BV/TV and aBMD of the right tibia. Also, in blood serum bone formation marker, P1NP, there was a significant reduction found. 4 weeks of progressive ladder climbing resistance exercise demonstrated significant increases in bone quality in bone density, BV/TV and aBMD of the right tibia, even surpassing those of the age-matched control. From PET/CT analysis, abnormal bone turnover rate was found in HCO group, whereas that of HEX group was down-regulated back to that of CON group from the effect of training. There was no significant result found in the protein and mRNA expression of FGF-2 in soleus and extensor digitorum longus muscles among all groups, but in decreased mRNA level following exercise in EDL muscle.

Conclusion: 2 weeks of hindlimb suspension clearly demonstrated debilitating effect on bone quality in growing rats and 4 weeks of progressive ladder climbing resistance exercise exerted beneficial effects on the recovery of the tibia of the growing rats. The further study is needed to demonstrate the potential role of FGF-2 between muscle and bone.