Competitive Effects of Sympathetic Control and Tissue Metabolites on Resistance and Capacitance Vessels and Capillary Filtration in Skeletal Muscle

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Abstract
Reduction in regional blood flow to skeletal muscle impairs and eventually abolishes the responses of the resistance and capacitance vessels to both sympathetic vasoconstrictor nerve fibre activation and close intraarterial infusion of l‐noradrenaline. The resistance response declines more rapidly and is abolished earlier than the capacitance response. The more severe the degree of flow reduction the more rapid is the decline and the shorter is the time to abolition of both responses. Increased frequency of nerve stimulation and increased dosage of noradrenaline produce greater responses and can ‘protect’ reactivity, temporarily, against the effects of flow reduction. This protection is greater for the capacitance response than for the resistance response. Flow reduction also impairs precapillary resistance to a greater degree and more rapidly than postcapillary resistance. This interferes with and eventually abolishes the normal ability of the nerves to decrease mean capillary hydrostatic pressure and, thereby, to cause a net inward movement of extravascular fluid. Retention of postcapillary resistance response beyond that of precapillary results eventually in an outward movement of fluid on nerve stimulation. The data indicate that precapillary responses (precapillary resistance vessels, precapillary sphincters) are more under the influence of local dilating metabolic factors than are postcapillary responses (postcapillary resistance vessels, major capacitance vessels) which, in turn, are more dominated by extrinsic nervous factors. These observations provide a more precise understanding of the regulation of the local circulation and may aid in explaining the nature of the circulatory derangement in shock.