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Alpha‐adrenergic regulation of muscle, adipose tissue and bone blood flow at rest and during exercise in humans

Positron emission tomography allows for determination of muscle and tissue‐specific blood flow in humans. Since hamstrings remain inactive during one leg exercise performed by knee extensors, the technique and model provide an opportunity to elucidate the role of nervous control of blood flow in the exercising limb in humans. Here we measured muscle and tissue‐specific blood flow in nine healthy young men under the femoral arterial infusions of norepinephrine (NE) and phentolamine. At rest, NE reduced blood flow by ~40% similarly in all muscles and in bone and adipose tissue. During exercise, NE reduced exercising muscle blood flow by 16%, but did not affect inactive muscle blood flow. Bone blood flow was not affected by NE, but blood flow in adipose tissue was reduced similarly as at rest. Phentolamine elevated blood flow similarly in different muscles and tissues of the limb at rest (by 3.5–6‐fold), and during exercise, the ratio of active‐to‐inactive muscle blood flow reduced from 18.3 ± 10.7 at control to 5.3 ± 4.0 during phentolamine, since blood flow in inactive muscles increased 3.4‐fold, but blood flow in exercising muscle remained essentially comparable to control exercise. In conclusion, the major novel finding here is that the release of á‐adrenergic tone markedly disturbs the distribution of blood flow in exercising human limb by elevating flow especially around inactive muscle fibers.