Vascular myogenic control of tibial bone blood flow in young healthy individuals
Authors: Draghici Adina E. , Ely Matthew R. , Taylor J. A. 
Published: May 2022

Bone health is critically dependent on adequate blood flow supplied by its extensive vascular network. Without adequate perfusion to provide oxygen and essential nutrients critical for bone metabolism, nearly all skeletal functions are compromised. However, despite the importance of blood flow to bone, regulation of bone blood flow is poorly understood in humans. The broad hypothesis of the current work is that the primary regulators of blood flow to bone are not different from other vascular beds. We have recently shown that sympathetic innervation of the bone vasculature has an active role in controlling bone blood flow in young healthy individuals. This work furthers our previous findings by investigating additional key bone blood flow regulators ‐ myogenic vasodilation and vasoconstriction. While vascular myogenic control has been investigated in soft tissue, to our knowledge there has been no research in the bone vasculature in humans, with only a handful of animal studies.

In young healthy individuals (N=8), we characterized myogenic control of tibial blood flow via novel custom‐made near infrared spectroscopy (NIRS). Tibial blood flow via NIRS was measured as oxy‐, deoxy‐, and total hemoglobin (Hb). We employed reactive hyperemia and leg dependency to assess myogenic vasodilation and vasoconstriction in the whole calf and in the tibia. The magnitude of maximum myogenic vasodilation was defined by the peak reactive blood flow response to sustained ischemia. Arterial flow to the lower leg was occluded by rapidly inflating a pneumatic thigh cuff above the knee to 220 mmHg in supine subjects. Tibial blood flow was assessed via NIRS and whole leg blood flow velocity in the popliteal artery of the same leg was measured via Doppler ultrasound during 1 min of rest, 10 min of cuff inflation, and 5 min of recovery. Myogenic vasoconstriction was assessed via leg dependency, i.e., during two levels of increased transmural pressure achieved by lowering the leg below heart level. Only tibial blood flow was assessed during 5 min in supine position, 5 min in seated upright position, and 5 min in seated position with legs lowered at 90deg knee flexion.

During cuff occlusion, there was a time dependent decrease in bone oxyHb accompanied by an increase in deoxyHb indicating lack of flow with continued metabolism. Tibial total Hb (sum of oxy‐ and deoxy‐Hb) did not change significantly during cuff inflation, but increased rapidly and markedly with cuff release, far surpassing baseline values when flow was restored. A similar overshoot response was seen in whole leg blood flow with cuff release. Thus, the response of tibial flow to reactive hyperemia follows the same behavior as the whole limb, indicating an active myogenic vasodilator response in bone similar to other vascular beds. During leg dependency, despite increases in arterial perfusion pressure at both levels (ΔP, 25.8 + 3.67, and 40.0 + 3.02 mmHg), there was no change in tibial flow (total Hb, ‐0.39 + 7.40, and ‐0.29 + 16.2 μM). Hence, unchanged tibial flow despite increased pressure indicates a compensatory vasoconstrictor response in the tibia. Our results indicate that similar to other vascular beds, myogenic control plays an active role in regulating the bone vasculature in young healthy individuals. Characterizing blood flow regulation in bone constitutes the basis of defining its role in overall bone health and its contribution to numerous bone loss conditions.