Metabolic modulation of sympathetic vasoconstriction in exercising skeletal muscle

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Abstract
The tight coupling of oxygen supply and utilization in exercising skeletal muscle is the result of complex interactions between local mechanisms that control muscle blood flow and substrate utilization and systemic mechanisms that control cardiac output and arterial pressure. The role of the sympathetic nervous system in the integration of these responses, specifically the interaction between sympathetic activation and local vasodilator mechanisms in exercising muscle, has been an active area of research for many years yet remains incompletely understood. The functional consequence of sympathetic activation in exercising skeletal muscle has been the subject of considerable debate. Previous studies in animals and humans have suggested that sympathetic vasoconstricton in active muscle is (a) well maintained and serves to limit active hyperaemia, thereby preventing muscle blood flow from outstripping cardiac output in order to preserve blood pressure and vital organ perfusion or (b) greatly attenuated in order to optimize muscle perfusion, a concept that has been termed ‘functional sympatholysis’. Studies performed over the past 70 years have provided conflicting evidence regarding the relative importance of sympathetic vasoconstriction vs. functional sympatholysis in exercising skeletal muscle. The focus of this review is mainly on recent studies in anaesthetized animal preparations and in conscious humans that have provided evidence for the metabolic modulation of sympathetic vasoconstriction in contracting skeletal muscle and have identified a number of key underlying mechanisms that extend the initial concept of sympatholysis.