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Muscle metaboreflex control of ventricular contractility during dynamic exercise

Departments of ¹Physiology and ²Surgery, Wayne State University School of Medicine, Detroit, Michigan

Submitted 12 August 2005 ; accepted in final form 19 September 2005

When oxygen delivery to active skeletal muscle is insufficient for the metabolic demands, afferent nerves within muscles are activated, which elicit reflex increases in heart rate (HR), cardiac output (CO), and arterial pressure (AP), termed the muscle metaboreflex (MMR). To what extent the increases in CO are the result of increased ventricular contractility is unclear. A widely accepted index of contractility is maximal left ventricular elastance (E_max), the slope of the end-systolic pressure-volume relationship, such as during rapidly imposed reductions in preload. The objective of the present study was to determine whether MMR activation elicits increases in E_max. Experiments were performed using conscious dogs chronically instrumented to measure left ventricular pressure and volume at rest and during mild or moderate treadmill exercise with and without partial hindlimb ischemia to elicit MMR responses. At both workloads, MMR activation significantly increased CO, HR, AP, and maximum rate of change of left ventricular pressure. During both mild and moderate exercise, MMR activation increased E_max to 159.6 ± 8.83 and 155.8 ± 6.32% of the exercise value under free-flow conditions, respectively. We conclude that the increase of ventricular elastance associated with MMR activation indicates that a substantial increase in ventricular contractility contributes to the rise in CO during dynamic exercise.

elastance; pressor response; cardiac function

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