The influence of the sympathetic nervous system on individual vessels of the microcirculation of skeletal muscle of the rat

Abstract

1. Direct observations have been made on the responses of individual vessels of the microcirculation of rat spinotrapezius muscle to stimulation of the sympathetic paravascular nerve fibres and to topically applied catecholamines. 2. All arteries and arterioles were constricted by sympathetic stimulation, the maximum response occurring at a stimulus frequency of 8‐10 Hz. Primary and secondary arterioles (13‐50 μm internal diameter) showed the greatest percentage change in diameter and remained constricted throughout the 1 min stimulation period whilst terminal arterioles (7‐13 μm internal diameter) constricted initially but then returned towards their control diameter before the stimulus ceased. 3. By contrast the venules and veins showed no response to sympathetic stimulation. 4. In accord with these observations, fluorescence histochemical studies revealed a network of noradrenergic nerve fibres on all arterial vessels but showed no innervation of any venous vessels. 5. Topically applied noradrenaline or adrenaline (10−10‐10−8 g/ml.) dilated the majority of arteries and arterioles while higher concentrations of either agent produced dose‐dependent constrictor responses. In addition, many venules dilated in response to adrenaline (10−9 g/ml.) while others constricted, but concentrations of either noradrenaline or adrenaline greater than 10−8 g/ml. produced dose‐dependent constriction of all venules and veins. 6. The behaviour of the more proximal and more distal arterioles during sympathetic stimulation is in accord respectively with the changes in muscle vascular resistance and in capillary surface area recorded in previous studies during sympathetic stimulation. 7. The observation that venous vessels are not influenced by sympathetic nerve fibres contrasts with the established view that venous vessels of skeletal muscle are strongly constricted during sympathetic stimulation. However, reappraisal of the evidence used to support this view indicates that such results may have been misinterpreted, while other available evidence supports the proposal that the present findings may be representative of skeletal muscle vasculature in general.

Neural control of muscle blood flow during exercise

“…In resting muscles, sympathetic nerve stimulation constricts all resistance vessels. Increasing the stimulus frequency of SNA from 0.5 Hz through 16 Hz produces a full range of vasoconstriction mediated primarily through the release of norepinephrine (5,21,50,60,95). However, there is a rank order of responsiveness to SNA that varies with location in the resistance network.…”

“…In contrast, vasoconstriction is sustained in proximal arterioles and feed arteries (50,95). Such regional differences in the ability to escape from sympathetic vasoconstriction are even more dramatic during exercise and are discussed later in this review.…”

“…In the hamster retractor muscle, the absolute decreases in vessel diameter during SNA are largest in feed arteries and progressively smaller in first-through thirdorder arterioles. However, when these responses to SNA are normalized to the respective resting diameters, the magnitude of vasoconstriction is largest in the distal branches and smallest in the proximal branches (50,95).…”

“…A rich adventitial plexus of sympathetic nerve fibers surrounds feed arteries and extends throughout the arteriolar network, including its terminal branches (26,31,50). However, neither capillaries nor venules in skeletal muscle are directly innervated (26).…”

The roles of catecholamines in responses evoked in arterioles and venules of rat skeletal muscle by systemic hypoxia.

“…Topical application of noradrenaline (10-6 g ml-') to the spinotrapezius muscle evoked a decrease in diameter of between 5 and 15 % from control diameter in each type of arterial and venous vessel although in many of the terminal, secondary and primary arterioles there was a secondary relaxation towards the control diameter (Marshall, 1982). When the noradrenaline had been washed off and the vessels had regained their control diameter, topical application of phentolamine caused a transient increase in the diameter of the majority of arterioles and some venules.…”