Objective  To establish a hypobaric hypoxia rat model in a real high-altitude environment, to investigate the effects of the real high-altitude environment on rat bone mass and bone microstructure using multiple methods such as Micro CT, blood biochemistry, and pathology, and to explore the potential mechanisms involved.

Methods Sprague Dawley (SD) rats were transported to the Yushu Plateau Laboratory (at 4250 m above sea level) in Qinghai Province and kept there for 4, or 8, or 18 months. These groups were designated as H-4, H-8, and H-18, respectively. Upon completion of the high-altitude exposure, these animals were transported to the Molecular Imaging Laboratory, West China Hospital, Sichuan University (at 500 m above sea level) in Chengdu for relevant testing and comparison with the control animals raised in a low-altitude environment for the same durations (designated L-4, L-8, and L-18). The tests performed included blood biochemistry, Micro CT imaging, and pathological assessments such as ELISA, Western blot, and HE and TRAP staining.

Results Compared with that of the control group, the body mass of rats in the H-4 and H-18 groups decreased significantly (H-4 group vs. L-4 group: 513.75±35.10 g vs. 649.18±60.03 g, P<0.01; H-18 group vs. L-18 group: 535.58±66.65 g vs. 670.86±44.96 g, P<0.01). The serum Ca²⁺ concentration was higher in the H-8 group and H-18 group compared to that in the control group (H-8 group vs. L-8 group: 2.48±0.09 mmol/L vs. 2.38±0.07 mmol/L, P<0.05; H-18 group vs. L-18 group: 2.55±0.11 mmol/L vs. 2.13±0.27 mmol/L, P<0.05). No statistically significant difference was observed in the concentration of P³⁺. Bone metabolism indicator cross-linked carboxy-terminal telopeptide of type Ⅰ collagen (CTX-Ⅰ) was significantly increased in all high-altitude groups compared to the low-altitude groups (H-4 group vs. L-4 group: 1.44±0.08 ng/mL vs. 0.70±0.13 ng/mL, P<0.01; H-8 group vs. L-8 group: 1.52±0.10 ng/mL vs. 0.75±0.10 ng/mL, P<0.01; H-18 group vs. L-18 group: 2.70±0.13 ng/mL vs. 1.94±0.15 ng/mL, P<0.01). In addition, CT results showed a decrease in bone volume fraction of trabecular bone in the three high-altitude groups (H-4 group vs. L-4 group: 7.48±2.35% vs. 10.40±2.93%, P<0.05; H-8 group vs. L-8 group: 7.17±2.68% vs. 10.09±2.95%, P<0.05; H-18 group vs. L-18 group: 2.90±2.91% vs. 8.68±4.11%, P<0.01), and increased trabecular separation in the three high-altitude groups (H-4 group vs. L-4 group: 0.70±0.12 mm vs. 0.60±0.06 mm, P<0.05; H-8 group vs. L-8 group: 0.68±0.07 mm vs. 0.59±0.05 mm, P<0.01; H-18 group vs. L-18 group: 0.80±0.09 mm vs. 0.70±0.09 mm, P<0.05). TRAP staining showed an increase in osteoclasts in the H-4 and H-18 groups. Western blot results indicated an increase in the expression of receptor activator of nuclear factor-κB ligand (RANKL) and hypoxia inducible factor-1α (HIF-1α) in high-altitude environment, while the expression of osteoprotegerin (OPG) was inhibited.

Conclusion The impact of high-altitude environment on rat femurs is characterized primarily by a reduction in trabecular bone mass and damage to bone microstructure.