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.

Keywords: cardiopulmonary baroreceptors; dynamic leg exercise; sympathetic activity.

Figure 1.

Changes in HR (A), SV (B), and CO (C) during the 60EX and 80EX trials. *P < 0.05 vs. the last minute of rest in the 60EX trial. ^†P < 0.05 vs. the last minute rest in the 80EX trial. ^§P < 0.05 when comparing the 60EX and 80EX trials.

Figure 2.

Changes in thoracic impedance ((Z values) from rest to exercise in the 60EX and 80EX trials. ^§P < 0.05 when comparing the 60EX and 80EX trials.

Figure 3.

Representative ECG and MSNA recordings made during the 60EX and 80 EX trials.

Figure 4.

Changes in MSNA BF in the 60EX and 80 EX trials. *P < 0.05 vs. the last minute of rest in the 60EX trial. ^†P < 0.05 vs. the last minute of rest in the 80EX trial. ^§P < 0.05 when comparing the 60EX and 80EX trials.