Cardiovascular reflex contributions to sympathetic inhibition during low intensity dynamic leg exercise in healthy middle-age.

Aging augments resting muscle sympathetic nerve activity (MSNA) and sympatho-inhibition during mild dynamic 1-leg exercise. To elucidate which reflexes elicit exercise-induced inhibition, we recruited 19 (9 men) healthy volunteers (mean age 56 ± 9 SD years), assessed their peak oxygen uptake (VO 2peak ), and, on another day, measured heart rate (HR), blood pressure (BP) and MSNA (microneurography) at rest and during 1-leg cycling (2 min each at 0 load and 30%-40% VO 2peak ), 3 times: (1) seated +2 min of postexercise circulatory occlusion (PECO) (elicit muscle metaboreflex); (2) supine (stimulate cardiopulmonary baroreflexes);and (3) seated, breathing 32% oxygen (suppress peripheral chemoreceptor reflex). While seated, MSNA decreased similarly during mild and moderate exercise (p < 0.001) with no increase during PECO (p = 0.44). Supine posture lowered resting MSNA (main effect p = 0.01) BP and HR. MSNA fell further (p = 0.04) along with diastolic BP and HR during mild, not moderate, supine cycling. Hyperoxia attenuated resting (main effect p = 0.01), but not exercise MSNA. In healthy middle-age, the cardiopulmonary baroreflex and arterial chemoreflex modulate resting MSNA, but contrary to previous observations in young subjects, without counter-regulatory offset by the sympatho-excitatory metaboreflex, resulting in an augmented sympatho-inhibitory response to mild dynamic leg exercise.