Hypoxia attenuates cardiopulmonary reflex control of sympathetic nerve activity during mild dynamic leg exercise

Abstract

New Findings r What is the central question of this study?The cardiopulmonary baroreflex inhibits adjustment of sympathetic vasomotor outflow during mild-intensity dynamic exercise. However, it is unclear how suppression of sympathetic vasomotor outflow by the cardiopulmonary baroreflex is modulated by a powerful sympatho-excitatory drive from the exercise pressor reflex, central command and/or the arterial chemoreflex. r What is the main finding and its importance?Hypoxia-induced heightened sympathetic nerve activity during dynamic exercise attenuated cardiopulmonary baroreflex control of sympathetic vasomotor outflow. This could facilitate the redistribution of blood flow to the active muscles by sympathetically mediated vasoconstriction of inactive muscles.Muscle sympathetic nerve activity (MSNA) does not increase during mild-intensity dynamic leg exercise in normoxic conditions, despite activation of central command and the exercise pressor reflex. Suppression of MSNA could be caused by muscle pump-induced loading of cardiopulmonary baroreceptors. In contrast, MSNA increases during mild dynamic leg exercise in hypoxic conditions. We hypothesized that hypoxic exercise, which induces a powerful sympatho-excitatory drive from the exercise pressor reflex, central command and/or arterial chemoreflex, attenuates cardiopulmonary reflex control of sympathetic vasomotor outflow. To test this hypothesis, MSNA was recorded during leg cycling in hypoxic conditions and with increased central blood volume by increasing the pedalling frequency to change the cardiopulmonary baroreflex. Subjects performed two leg cycle exercises at different pedal cadences of 60 and 80 r.p.m. (60EX and 80EX trials, respectively) in two (haemodynamic and MSNA) measurement conditions while breathing a hypoxic gas mixture (inspired oxygen fraction = 0.12). Thoracic impedance, stroke volume and cardiac output were measured non-invasively using impedance cardiography. During the MSNA test, MSNA was recorded via microneurography at the right median nerve at the elbow. Changes in thoracic impedance, stroke volume and cardiac output during the 80EX trial were greater than those during the 60EX trial. The MSNA burst frequency during hypoxic exercise in the 80EX trial (39 ± 4 bursts min −1 ) did not differ from that during the 60EX trial (39 ± 3 bursts min −1 ). These results suggest that the cardiopulmonary baroreflex of sympathetic vasomotor outflow during dynamic exercise is modulated by heightened hypoxia-induced sympathetic nerve activity.

Work of breathing influences muscle sympathetic nerve activity during semi‐recumbent cycle exercise

“…Others investigating resting MSNA have found considerable between subject variability . Similarly, the current study and others utilizing dynamic exercise found variability in MSNA at a given external work rate (Watts) or Wb. For example, in Figure Panel A, at any intensity and for a given relative Wb, the MSNA BF varies ~15% of maximal levels.…”

“…The recording of MSNA from the upper limb during leg cycling is technically difficult. Furthermore, most investigations have recording MSNA during low intensity exercise (<50% ). As such, a secondary purpose of our study was to provide a quantitative assessment of MSNA during heavy exercise.…”

Elevated sympathetic vasomotor outflow in response to increased inspiratory muscle activity during exercise is less in young women compared with men

“…We used 40% exercise intensity for several reasons, as in our previous study, because the rate of successful MSNA recordings is relatively high when the exercise intensity is light (Katayama et al., , , , ). Movements of the arm and the body become greater during leg cycling with inspiratory resistance.…”

“…The mean voltage neurogram was continuously digitized through an analog-to-digital converter with a sampling frequency of 200 Hz. The MSNA bursts were identified from the mean voltage neurogram using a customized computer program-assisted inspection (Katayama et al, 2015;Katayama, Ishida, Saito, Koike, & Ogoh, 2016), which accounted for the latency from the ECG R wave to the sympathetic burst (Fagius & Wallin, 1980). The MSNA was quantified as burst frequency (BF; in bursts per minute) and burst incidence (BI; in bursts per 100 heart beats).…”

Role of the Hypoxia-Inducible Factor in Periodontal Inflammation

“…This is made possible by a cellular HIF system. Cellular hypoxia, or a lower than "normal" concentration of O 2 in cells, occurs commonly and could induce signiicant changes, immediate or delayed, on cellular processes, including cell growth and apoptosis, cell proliferation and survival, pH regulation and energy metabolism, cell migration, matrix and barrier function, angiogenesis, and vasomotor regulation [8][9][10][11][12][13]. These biological processes involve active responses by the body to secure an additional oxygen supply via circulation.…”