Effects of Aging and Exercise Training on Bone Vascular Function and Bone Structural Adaptations

Purpose: Adequate skeletal perfusion is necessary to maintain bone remodeling processes and bone health. Natural aging leads to progressive bone loss, which may be associated with impaired bone vasomotor function and impaired perfusion. Although these age-related changes can be ameliorated by exercise training, the specific mechanisms underlying exercise-induced alterations of bone vasomotor function in advanced age remain unknown. The purpose of this study was to elucidate whether the exercise-induced vasomotor effects on the femoral principal nutrient artery (PNA) as well as bone blood flow are associated to bone structural adaptions in old age. Methods: Young (4–6 mo old) and old (20–22 mo old) male Fischer-344 rats either did treadmill endurance exercise training (15 m/min, 15° incline, 60 min/day, 5 days/week) for 10–12 weeks or remained sedentary (SED) in cages. Multiple vasomotor functions were evaluated in femoral PNAs: 1) Endothelium-dependent vasodilation to acetylcholine (ACh) alone, or in the presence of eNOS inhibitors (L-NAME). 2) Endothelium-independent vasodilation was tested using the nitric oxide (NO) donor, Dea-NONOate. 3) Receptor-dependent vasoconstrictor responses was tested using the α-adrenergic agonist phenylephrine (PE). 4) Receptor-independent responses were examined using potassium chloride (KCl). 5) Finally, the pressure-induced myogenic responses (0 up to 100 mmHg) were also examined. Protein expression of endothelial NO synthase (eNOS) and antioxidant enzyme superoxide dismutase (SOD1) were examined in femoral PNAs. Regional blood flow to the femur was also assessed at rest and during treadmill exercise. Lastly, trabecular bone structures of the distal femoral metaphysis were determined by micro-computed tomography. Results: NO-mediated, endothelium-dependent vasodilation (ACh) and endothelium-independent relaxation (Dea-NONOate) were impaired by advanced age, but exercise training enhanced the response to ACh in femoral PNAs from both young and old rats. Age did not alter α-adrenergic receptor-dependent (PE) or independent (KCl) vasoconstrictor responses, whereas exercise training showed enhanced vasoconstrictor response to PE in old rats. In addition, advanced age impaired the pressure-sensitive myogenic response, while there was no alteration following exercise training in both young and old groups. Exercise training restored the age-induced decline of eNOS expression in femoral PNA by 55% from old rats, whereas age and exercise training had no effect on SOD1 expression (p<0.05). Regional femur blood flow to the proximal and distal metaphysis and epiphysis, as well as diaphyseal marrow, were lower in old rats than in young rats at rest (-38%, -39%, and -39%, respectively), while exercise training led to increased blood flow (42%) to distal metaphysis and epiphysis in old rats. Moreover, exercise-trained old rats showed significantly increased regional blood flow during exercise compared to old SED rats (proximal distal metaphysis and epiphysis and diaphyseal marrow: 114%, 135% and 119%, respectively). This increased blood flow to the proximal and distal metaphysis and epiphysis was even higher in older exercise-trained rats than in young exercise-trained rats. Trabecular bone structures, including BV/TV, trabecular connectivity density, number, and separation (-30%, -66%, -35% and 68%, respectively) (p<0.001) were deteriorated in the distal femoral metaphysis in old rats compared to young rats, while these bone properties were not altered with exercise training. Conclusion: Taken together, the findings of this study demonstrated that vasomotor responses are impaired in the femoral PNAs of old rats, which are closely associated with age-induced alterations in bone structures. However, exercise training in old rats reversed the diminished vasomotor regulation, including vasodilator responses and enhanced regional bone and marrow blood flow. However, these exercise training-induced improvements in bone and marrow perfusion and vasodilator responsiveness do not appear to enhance the structural properties of trabecular bone in aged rats.