Enhanced muscle pump during mild dynamic leg exercise inhibits sympathetic vasomotor outflow

Abstract

Muscle sympathetic nerve activity (MSNA) is not increased during leg cycling at light and mild intensities, despite activation of central command and the exercise pressor reflex. We determined whether increasing central blood volume and loading the cardiopulmonary baroreceptors modulate sympathetic vasomotor outflow during leg cycling. To this end, we changed the pedaling frequency to enhance skeletal muscle pump. Subjects performed two leg cycle exercises at differential pedal rates of 60 and 80 rpm (60EX and 80EX trials) for two conditions (with and without MSNA measurement). In each trial, subjects completed leg cycling with a differential workload to maintain constant oxygen consumption (VO2). MSNA was recorded via microneurography at the right median nerve of the elbow. Without MSNA measurement, thoracic impedance, stroke volume (SV), and cardiac output (CO) were measured non‐invasively using impedance cardiography. Heart rate and VO2 during exercise did not differ between the 60EX and 80EX trials. Changes in thoracic impedance, SV, and CO during the 80EX trial were greater than during the 60EX trial. MSNA during the 60EX trial was unchanged compared with that at rest (25.8 ± 3.1 [rest] to 28.3 ± 3.4 [exercise] bursts/min), whereas a significant decrease in MSNA was observed during the 80EX trial (25.8 ± 2.8 [rest] to 19.7 ± 2.0 [exercise] bursts/min). These results suggest that a muscle pump‐induced increase in central blood volume, and thereby loading of cardiopulmonary baroreceptors, could inhibit sympathetic vasomotor outflow during mild dynamic leg exercise, despite activation of central command and the exercise pressor reflex.

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

“…), and the change in thoracic impedance (Δ Z value) is presented as a negative number by convention (Katayama et al . ). The ECG and impedance signals were sampled at a frequency of 200 Hz using an analog‐to‐digital converter (CBI‐3133B; Interface, Hiroshima, Japan) and stored in a computer (CF‐F8; Panasonic, Osaka, Japan).…”

“…Tidal volume ( V T ), breathing frequency ( f ), and carbon dioxide output () were determined using an online system with a mixing chamber, as we have described previously (Katayama et al . , ). The subjects breathed through a nasal mask (5719; Hans Rudolph, Kansas City, MO, USA), and the expired gas volume was measured using a Fleisch pneumotachometer (PN‐230; Arco System, Chiba, Japan).…”

“…The recording system was identical to that used in our previous studies (Katayama et al . , , ). A tungsten microelectrode with a shaft diameter of 0.1 mm (impedance 1–5 MΩ) was inserted manually by the experimenter into the right median nerve at the cubital fossa.…”

“…The MSNA bursts were identified from the mean‐voltage neurogram using customized computer program‐assisted inspection (Katayama et al . , b , ), which accounted for the latency from the ECG R wave to the sympathetic burst (Fagius & Wallin, ). The MSNA was quantified as burst frequency (BF, in bursts per minute) and incidence (BI, in bursts per 100 heart beats; Saito et al .…”

“…These results suggest that the cardiopulmonary baroreflex during dynamic exercise could be modulated by a powerful sympatho‐excitatory drive from the exercise pressor reflex, central command and/or the arterial chemoreflex (Fadel & Raven, ; Katayama et al . ; Fisher et al . ).…”