A rehabilitation exercise program induces severe bone mineral deficits in estrogen-deficient rats after extended disuse.
OBJECTIVE
Both estrogen and mechanical loading regulate bone maintenance. However, mechanical overload seems less effective in enhancing bone mineral density (BMD) in estrogen-deficient women. The aim of this study was to determine whether estradiol (E2) influences early-phase bone adaptations to reambulation (REAMB) and/or rehabilitation exercises after hindlimb unloading (HLU) of ovariectomized rats.


METHODS
Eighty-one 5-month-old female Sprague-Dawley rats were randomized into the following groups: (1) intact controls, (2) ovariectomy (OVX), (3) OVX + E2, (4) OVX + 4 weeks of HLU, (5) OVX + E2 + HLU, (6) OVX + HLU + 2 weeks of quadrupedal REAMB, (7) OVX + E2 + HLU + REAMB, (8) OVX + HLU + REAMB + supplemental climbing, jumping, and balance exercises (EX), or (9) OVX + E2 + HLU + REAMB + EX. Serial dual-energy x-ray absorptiometry scans were performed to track total body bone characteristics throughout the study, and peripheral quantitative computerized tomography was used to determine distal femoral metaphyseal bone mineral characteristics.


RESULTS
Total body BMD increased by 4% to 8% in all animals receiving supplemental E2, whereas BMD did not change in animals without E2. OVX reduced trabecular BMD at the femoral metaphysis, and HLU exacerbated this loss while also reducing cortical BMD. E2 protected against OVX + HLU-induced bone loss at the femoral metaphysis. Conversely, REAMB did not alter BMD, regardless of estrogen status. In the absence of E2, REAMB + EX resulted in severe bone loss after OVX + HLU, with trabecular BMD and cortical BMD measurements that were 91% and 7% below those of controls, respectively (P ≤ 0.001). However, in the presence of E2, REAMB + EX did not negatively influence bone mineral characteristics.


CONCLUSIONS
E2 protects against bone loss resulting from combined OVX + HLU of rodents. In the absence of estrogen, exercise induces disadvantageous early-phase bone adaptations after extended disuse.
1VA Medical Center, Gainesville, FL 32607, USA. jfyarrow@ufl.edu
Joshua F
JF
Yarrow
Sean C
SC
McCoy
J Andries
JA
Ferreira
Jennifer E
JE
Pingel
Bryan P
BP
Conrad
Thomas J
TJ
Wronski
Alyssa A
AA
Williams
Stephen E
SE
Borst
Marybeth
M
Brown
D015502:Absorptiometry, Photon; D000818:Animals; D015519:Bone Density; D004958:Estradiol / Q000008:administration & dosage*; D004967:Estrogens / Q000172:deficiency*; D005081:Exercise Therapy; D005260:Female; D019417:Hindlimb Suspension; D008903:Minerals; D010024:Osteoporosis / Q000209:etiology / Q000534:rehabilitation / Q000628:therapy; D010052:Ovariectomy; D051381:Rats; D017207:Rats, Sprague-Dawley
D004967:Estrogens; D008903:Minerals; D004958:Estradiol