Spontaneous bursts of muscle sympathetic nerve activity decrease leg vascular conductance in resting humans

Abstract

Previous studies in humans attempting to assess sympathetic vascular transduction have related large reflex-mediated increases in muscle sympathetic nerve activity (MSNA) to associated changes in limb vascular resistance. However, such procedures do not provide insight into the ability of MSNA to dynamically control vascular tone on a beat-by-beat basis. Thus we examined the influence of spontaneous MSNA bursts on leg vascular conductance (LVC) and how variations in MSNA burst pattern (single vs. multiple bursts) and burst size may affect the magnitude of the LVC response. In 11 young men, arterial blood pressure, common femoral artery blood flow, and MSNA were continuously recorded during 20 min of supine rest. Signal averaging was used to characterize percent changes in LVC for 15 cardiac cycles following heartbeats associated with and without MSNA bursts. LVC significantly decreased following MSNA bursts, reaching a nadir during the 6th cardiac cycle (single bursts, -2.9 ± 1.1%; and multiple bursts, -11.0 ± 1.4%; both, P < 0.001). Individual MSNA burst amplitudes and the total amplitude of consecutive bursts were related to the magnitude of peak decreases in LVC. In contrast, cardiac cycles without MSNA bursts were associated with a significant increase in LVC (+3.1 ± 0.5%; P < 0.001). Total vascular conductance decreased in parallel with LVC also reaching a nadir around the peak rise in arterial blood pressure following an MSNA burst. Collectively, these data are the first to assess beat-by-beat sympathetic vascular transduction in resting humans, demonstrating robust and dynamic decreases in LVC following MSNA bursts, an effect that was absent for cardiac cycles without MSNA bursts.

Fig. 1.

Summary data showing beat-by-beat percent changes in leg vascular conductance (LVC) following spontaneous muscle sympathetic nerve activity (MSNA) bursts. A: changes in LVC following all bursts, nonbursts, and white noise. B: changes following single and multiple MSNA bursts. Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Fig. 2.

Summary data showing beat-by-beat percent changes in LVC following MSNA bursts of varying height grouped into quartiles from smallest to largest (Q1–Q4). Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Fig. 3.

Summary data showing beat-by-beat percent changes in LVC following MSNA burst clusters grouped into singlets (A), couplets (B), triplets (C), and quadruplets (D). Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Fig. 4.

Summary data relating the nadir percent decrease in LVC and the total amplitude of each MSNA burst cluster. Color denotes the cluster grouping (single, red; couplet, green; triplet, yellow; and quadruplet, blue), whereas symbols differentiate the cluster quartile (Q1–Q4) of each cluster group. Brackets denote significant difference from white noise percent changes. Values are means ± SE. AU, arbitrary unit.

Fig. 5.

Summary data showing beat-by-beat percent changes in cardiac output (top), total vascular conductance (middle), and mean arterial pressure (bottom) following all MSNA bursts, nonbursts, and white noise. Asterisks and brackets denote significant difference from white noise percent changes. Values are means ± SE.