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

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 mum internal diameter) showed the greatest percentage change in diameter and remained constricted throughout the 1 min stimulation period whilst terminal arterioles (7-13 mum 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.