Sympathetic Nerves Inhibit Conducted Vasodilatation Along Feed Arteries during Passive Stretch of Hamster Skeletal Muscle

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Abstract
Ascending vasodilatation is integral to blood flow control in exercising skeletal muscle and is attributable to conduction from intramuscular arterioles into proximal feed arteries. Passive stretch of skeletal muscle can impair muscle blood flow but the mechanism is not well understood. We hypothesized that the conduction of vasodilatation along feed arteries can be modulated by changes in muscle length. In anaesthetized hamsters, acetylcholine (ACh) microiontophoresis triggered conducted vasodilatation along feed arteries (diameter, 50-70 μm) of the retractor muscle secured at 100 % resting length or stretched by 30 %. At 100 % length, ACh evoked local dilatation (> 30 μm) and this response conducted rapidly along the feed artery (14 ± 1 μm dilatation at 1600 μm upstream). During muscle stretch, feed arteries constricted ≈10 μm (P < 0.05) and local vasodilatation to ACh was maintained while conducted vasodilatation was reduced by half (P < 0.01). Resting diameter and conduction recovered upon restoring 100 % length. Sympathetic nerve stimulation (4-8 Hz) produced vasoconstriction and attenuated conduction in the manner observed during muscle stretch, as did noradrenaline or phenylephrine (10 nm). Inhibiting nitric oxide production (Nω-nitro-L-arginine, 50 μm) produced similar vasoconstriction yet had no effect on conduction. Phentolamine, prazosin, or tetrodotoxin (1 μm) during muscle stretch abolished vasoconstriction and restored conduction. Inactivation of sensory nerves with capsaicin had no effect on vasomotor responses. Thus, muscle stretch can attenuate conducted vasodilatation by activating α-adrenoreceptors on feed arteries through noradrenaline released from perivascular sympathetic nerves. This autonomic feedback mechanism can restrict muscle blood flow during passive stretch.