Modulation of the control of muscle sympathetic nerve activity during incremental leg cycling

Abstract

We tested the hypotheses that arterial baroreflex (ABR) control over muscle sympathetic nerve activity (MSNA) in humans does not remain constant throughout a bout of leg cycling ranging in intensity from very mild to exhausting. ABR control over MSNA (burst incidence, burst strength and total MSNA) was evaluated by analysing the relationship between beat-to-beat spontaneous variations in diastolic arterial pressure (DAP) and MSNA in 15 healthy subjects at rest and during leg cycling in a seated position at five workloads: very mild (10 W), mild (82 ± 5.0 W), moderate (126 ± 10.2 W), heavy (156 ± 14.3 W), and exhausting (190 ± 21.2 W). The workload was incremented every 6 min. The linear relationships between DAP and MSNA variables were significantly shifted downward during very mild exercise, but then shifted progressively upward as exercise intensity increased. During heavy and exhausting exercise, moreover, the DAP-MSNA relationships were also significantly shifted rightward from the resting relationship. The sensitivity of ABR control over burst incidence and total MSNA was significantly lower during very mild exercise than during rest, and the sensitivity of the burst incidence control remained lower than the resting level at all higher exercise intensities. By contrast, the sensitivity of the total MSNA control recovered to the resting level during mild and moderate exercise, and was significantly increased during heavy and exhausting exercise (versus rest). We conclude that, in humans, ABR control over MSNA is not uniform throughout a leg cycling exercise protocol in which intensity was varied from very mild to exhausting. We suggest that this non-uniformity of ABR function is one of the mechanisms by which sympathetic and cardiovascular responses are matched to the exercise intensity.

Autonomic Adjustments to Exercise in Humans

“…Indeed, for the most part, studies using direct muscle SNA recordings, plasma norepinephrine as well as norepinephrine spillover have all suggested that sympathetic activation becomes progressively greater as exercise intensity and duration is increased (24,103,146,186). Galbo et al (103) reported a progressive increase in venous norepinephrine concentration during graded treadmill exercise at workloads above 50% maximal oxygen uptake (VO 2 max).…”

A cholinergic contribution to the circulatory responses evoked at the onset of handgrip exercise in humans

“…In the present study, although there was a tendency for a blunted fall in SVR and BP at the onset of leg cycling exercise with glycopyrrolate, the lack of a strong effect perhaps signifies an effect of exercise modality on the mechanisms involved in control of circulatory responses. For example, a reduction in sympathetic vasoconstrictor neural outflow is documented at the onset of seated isometric quadriceps exercise (37) and low-intensity leg cycling (20) that may be secondary to activation of the cardiopulmonary baroreceptors resulting from an increase in venous return and right atrial pressure by the skeletal muscle pump (46,48). In contrast, sympathetic vasoconstrictor activity is typically unchanged at the onset of moderate intensity isometric handgrip and in- creases at higher handgrip intensities (26).…”

Hemodynamic response to exercise in supine and standing attitudes: an integrated model

“…Despite that, the blood pressure fall in the early stage of one-leg extension (red arrow in Fig.3 (A)), and the prolonged decrease of the blood pressure during the one-leg cycling (between the red and green arrow in Fig.3 (B)) were observed in the experiment conducted by Radegran, et al, (1998), andSprangers, et al, (1991), respectively. In addition, the workloads in both cases are mild, which was defined in the previous study (Ichinose, et al (2008)). The auto-regulation mechanisms is responsible for the recovery of arterial pressure after the initial fall in arterial pressure during one-leg extension, which was also apparent in experimental data (Radegran, et al, (1998)).…”

“…The CVS is influenced dynamically by the muscle pump and the metabovasodilation in terms of blood pressure, while the ANS is affected by a combination of the resetting of the operating pressure threshold of arterial baroreflex, central command model, and summary of mechanoreflex, metaboreflex and baroreflex. Here the oxygen intake (workload) is determined based on the previous studies (Ichinose, et al 2008;Melvin Keller, et al 2004;Papelier, et al 1994).…”