Bone Mechanical Loading Reduces Heart Rate and Increases Heart Rate Variability via the Autonomic Nervous System in Anesthetized Mice
Authors: Klump Jackson , Vallejo Julian , Gray Mark , Dallas Mark , Johnson Mark , Wacker Michael 
Published: May 2024

Mechanical loading of bone creates interstitial fluid flow and shear stress within the bone lacunocanalicular system, leading to alterations of bone size and mass. Bone plays an important role as an endocrine organ, releasing endocrine signaling molecules that impact systemic physiology. Further, the central and peripheral nervous systems regulate bone mass in response to strain through alterations in autonomic tone. This suggests the potential for interplay between bone and other organs via endocrine signaling and autonomic innervation. In this study, our group investigated this potential interplay by focusing on bone and heart. We hypothesized that mechanical bone loading of mice in vivo would result in altered cardiac function. To test this hypothesis, we performed acute in vivo mechanical loading on right leg tibias of anesthetized TOPGAL and CD-1 mice. Tibias underwent compressive cyclic loading (a sine wave) that modulates between -0.3N and -9.0N for 300 cycles at 2 Hz. Cardiac function was monitored with lead II electrocardiogram (ECG) data, heart rate (HR), and heart rate variability (HRV). Immediate, transient reduction in resting HR (0.93 ± 0.013 fold change from baseline, n=6-7, p<0.01 compared to control) was achieved during tibial loading in 6-month-old male and female mice, with concurrent increase in HRV (1.24 ± 0.11 fold change from baseline, n=6-7, p<0.01). Both HR and HRV returned to baseline levels upon completion of the loading process. ECG measurements, QRS and corrected QT (QTc), were not found to be altered (p>0.05) during loading and 30 minutes following loading. In further studies, 1N-2N did not produce a decrease in HR, while 3N (0.92 ± 0.077 fold change from baseline, n=12, p<0.05) and 9N (0.93 ± 0.061 fold change from baseline, n=13, p<0.05) decreased HR. There was decreased magnitude and responsiveness with aging to 11 months old at 3N and 9N (p>0.05), suggesting the response may weaken with age. The immediate, transient nature of the changes to resting HR and HRV suggest a neural mechanism for this response. To test this mechanism, we inhibited local neuronal afferent activity by injecting lidocaine (2.5mg/kg) near the tibia prior to loading. In doing so, the decrease in HR during loading was significantly diminished (vehicle 0.90 ± 0.060 vs. lidocaine 0.97 ± 0.042 fold change from baseline, n=7-8, p<0.05). To test the efferent arm of the response, mice were injected with the parasympathetic, muscarinic acetylcholine receptor antagonist atropine (2mg/kg), or the sympathetic, β1/β2 receptor antagonist propranolol (10mg/kg). Propranolol significantly inhibited the HR decrease during loading (vehicle 0.88 ± 0.095 vs. propranolol 0.98 ± 0.026 fold change from baseline, n=7-8, p<0.05), while atropine did not (n=8-11, p>0.05). These findings suggest that reductions in sympathetic tone on the heart during bone loading led to the decrease in HR. In conclusion, this study uncovered a reflexive neural connection between bone and heart that is affected by age and mediated by afferent neurons in the tibia and alterations in efferent sympathetic tone to the heart. These findings provide new knowledge that builds on the current science of bone remodeling and its regulation. This study was funded by HRSA Medical Student Education Grant T99HP39202 and NIH P01AG039355. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.