An open-source application for the standardized burst identification from the integrated muscle sympathetic neurogram

Reflex sympathetic activation to inspiratory muscle loading is attenuated in females relative to males

Intense inspiratory muscle work can evoke a metabolite-stimulated pressor reflex, commonly referred to as the respiratory muscle metaboreflex. When completing similar relative and absolute levels of inspiratory work, females have an attenuated blood pressure response. We sought to test the hypothesis that the lower blood pressure response to the respiratory muscle metaboreflex in females is associated with a reduced sympathetic response. Healthy young (26 ± 4 yr) males (n = 9) and females (n = 7) completed two experimental days. On day 1, participants completed pulmonary function testing and became familiarized with an inspiratory pressure-threshold loading (PTL) task. On the second day, balloon-tipped catheters were placed in the esophagus and stomach to measure pleural and gastric pressures, and transdiaphragmatic pressure was calculated. A microelectrode was inserted into the fibular nerve to quantify muscle sympathetic nerve activity (MSNA), and participants then completed isocapnic PTL to task failure. There was a significant sex-by-time interaction in the mean arterial pressure (MAP, P = 0.015) and burst frequency (P = 0.039) response to PTL. Males had a greater rise in MAP (Δ21 ± 9 mmHg) than females (Δ13 ± 5 mmHg, P = 0.026). Males also demonstrated a greater rise in MSNA burst frequency (Δ18 ± 7 bursts/min) than females (Δ10 ± 5 bursts/min, P = 0.015). The effect of sex was observed despite females and males completing the same magnitude of diaphragm work throughout the task (P = 0.755). Our findings provide novel evidence that the lower blood pressure response to similar relative and absolute inspiratory muscle work in females is associated with lower sympathetic activation.NEW & NOTEWORTHY The blood pressure response to high levels of inspiratory muscle work is lower in females and occurs alongside a reduced sympathetic response. The reduced blood pressure and sympathetic response occur despite males and females performing similar levels of absolute inspiratory work. Our findings provide evidence that sex differences in the respiratory muscle metaboreflex are, in part, sympathetically mediated.

Influence of sex and sedentary conditions on sympathetic burst characteristics in prepubertal, postpubertal, and young adult rats

Recent evidence indicates that sex-based differences in cardiovascular disease (CVD) begin early in life, particularly when associated with risk factors such as a sedentary lifestyle. CVD is associated with elevated sympathetic nerve activity (SNA), quantified as increased SNA burst activity in humans. Whether burst characteristics are influenced by sex or sedentary conditions at younger ages is unknown. The purpose of our study is to compare SNA bursts in active and sedentary female and male rats at ages including prepuberty and young adulthood. We hypothesized that burst characteristics and blood pressure are higher under sedentary conditions and lower in female rats compared with males. We analyzed splanchnic SNA (SpSNA) recordings from Inactin-anesthetized male and female rats at 4-, 8-, and 16-wk of age. Physically active and sedentary rats were each housed in separate, environmentally controlled chambers where physically active rats had free access to an in-cage running wheel. Sympathetic bursts were obtained by rectifying and integrating the raw SpSNA signal. Burst frequency, burst height, and burst width were calculated using the Peak Parameters extension in LabChart. Our results showed that sedentary conditions produced a greater burst width in 8- and 16-wk-old rats compared with 4-wk-old rats in both males and females (P < 0.001 for both). Burst frequency and incidence were both higher in 16-wk-old males compared with 16-wk-old females (P < 0.001 for both). Our results suggest that there are sedentary lifestyle- and sex-related mechanisms that impact sympathetic regulation of blood pressure at ages that range from prepuberty into young adulthood.NEW & NOTEWORTHY The mechanisms of decreased incidence of cardiovascular disease (CVD) in reproductive-age women compared with age-matched men are unknown. The strong association between elevated sympathetic activity and CVD led us to characterize splanchnic sympathetic bursts in female and male rats. Prepubescent males and females exhibited narrower sympathetic bursts, whereas young adult males had higher resting burst frequency compared with age-matched females. Sex-based regulation of sympathetic activity suggests a need for sex-dependent therapeutic strategies to combat CVD.

Acute hypoxia elicits lasting reductions in the sympathetic action potential transduction of arterial blood pressure in males

Post-hypoxia sympathoexcitation does not elicit corresponding changes in vascular tone, suggesting diminished sympathetic signalling. Blunted sympathetic transduction following acute hypoxia, however, has not been confirmed and the effects of hypoxia on the sympathetic transduction of mean arterial pressure (MAP) as a function of action potential (AP) activity is unknown. We hypothesized that MAP changes would be blunted during acute hypoxia but restored in recovery and asynchronous APs would elicit smaller MAP changes than synchronous APs. Seven healthy males (age: 24 (3) years; BMI: 25 (3) kg/m² ) underwent 20 min isocapnic hypoxia (P_ET O₂ : 47 (2) mmHg) and 30 min recovery. Multi-unit microneurography (muscle sympathetic nerve activity; MSNA) and continuous wavelet transform with matched mother wavelet was used to detect sympathetic APs during baseline, hypoxia, early (first 7 min) and late (last 7 min) recovery. AP groups were classified as synchronous APs, asynchronous APs (occurring outside an MSNA burst) and no AP activity. Sympathetic transduction of MAP was quantified using signal-averaging, with ΔMAP tracked following AP group cardiac cycles. Following synchronous APs, ΔMAP was reduced in hypoxia (+1.8 (0.9) mmHg) and early recovery (+1.5 (0.7) mmHg) compared with baseline (+3.1 (2.2) mmHg). AP group-by-condition interactions show that at rest asynchronous APs attenuate MAP reductions compared with no AP activity (-0.4 (1.1) vs. -2.2 (1.2) mmHg, respectively), with no difference between AP groups in hypoxia, early or late recovery. Sympathetic transduction of MAP is blunted in hypoxia and early recovery. At rest, asynchronous sympathetic APs contribute to neural regulation of MAP by attenuating nadir pressure responses. KEY POINTS: Acute isocapnic hypoxia elicits lasting sympathoexcitation that does not correspond to parallel changes in vascular tone, suggesting blunted sympathetic transduction. Signal-averaging techniques track the magnitude and temporal cardiovascular responses following integrated muscle sympathetic nerve activity (MSNA) burst and non-burst cardiac cycles. However, this does not fully characterize the effects of sympathetic action potential (AP) activity on blood pressure control. We show that hypoxia blunts the sympathetic transduction of mean arterial pressure (MAP) following synchronous APs that form integrated MSNA bursts and that sympathetic transduction of MAP remains attenuated into early recovery. At rest, asynchronous APs attenuate the reduction in MAP compared with cardiac cycles following no AP activity, thus asynchronous sympathetic APs appear to contribute to the neural regulation of blood pressure. The results advance our understanding of sympathetic transduction of arterial pressure during and following exposure to acute isocapnic hypoxia in humans.

Action potential amplitude and baroreflex resetting of action potential clusters mediate hypoxia-induced sympathetic long-term facilitation

KEY POINTS

Acute isocapnic hypoxia resets the arterial baroreflex and permits long-lasting sympathoexcitation called sympathetic long-term facilitation. Our understanding of sympathetic long-term facilitation following hypoxia in humans is based on multiunit muscle sympathetic nerve activity and does not fully characterize the underlying baroreflex control of sympathetic neuronal subpopulations or their discharge/recruitment strategies. We show that sympathetic long-term facilitation is mediated by baroreflex resetting of sympathetic action potential clusters to higher arterial pressure operating points, a reduction in the percentage of action potentials firing asynchronously, and a shift toward larger amplitude action potential activity. The results advance our fundamental understanding of how the sympathetic nervous system mediates sympathetic long-term facilitation following exposure to acute isocapnic hypoxia in humans.

ABSTRACT

Baroreflex resetting permits sympathetic long-term facilitation (sLTF) following hypoxia; however, baroreflex control of action potential (AP) clusters and AP recruitment patterns facilitating sLTF is unknown. We hypothesized that baroreflex resetting of arterial pressure operating points (OPs) of AP clusters and recruitment of large-amplitude APs would mediate sLTF following hypoxia. Eight men (age: 24 (3) yrs; BMI: 24 (3) kg/m² ) underwent 20-min isocapnic hypoxia (P_ET O₂ : 47 (2) mmHg) and 30-min recovery. Multi-unit microneurography (muscle sympathetic nerve activity; MSNA) and a continuous wavelet transform with matched mother wavelet was used to detect sympathetic APs during baseline, hypoxia, early (first 5-min), and late recovery (last 5-min). AP amplitude (normalized to largest baseline AP amplitude), percent APs occurring outside a MSNA burst (% asynchronous APs), and proportion of APs firing in small (1-3), medium (4-6), and large (7-10) normalized cluster sizes was calculated. Normalized clusters were used to assess baroreflex OPs and sensitivity. Hypoxia increased total MSNA activity, which remained elevated during recovery (P<0.0001). Baroreflex OPs were shifted rightward for all clusters in recovery, with no effect on slope. Compared to baseline, AP amplitude was elevated by 3 (2) % and 4 (2) % while asynchronous APs were reduced by 9 (5) % and 7 (6) % in early and late recovery, respectively. In early recovery, the proportion of APs firing in large clusters was increased compared to baseline. Hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. Abstract figure legend Eight healthy men underwent 20-min isocapnic hypoxia and 30-min recovery. The study tested the hypothesis that baroreflex resetting of arterial pressure operating points (OPs) of action potential (AP) clusters and recruitment of large-amplitude APs would mediate sympathetic long-term facilitation (sLTF) following acute hypoxic exposure. Hypoxia increased multi-unit muscle sympathetic nerve activity (MSNA; measured via microneurography), which remained elevated throughout recovery. Sympathetic APs were detected in the filtered MSNA neurogram using a continuous wavelet transform with matched mother wavelet. An effect of condition revealed that compared to baseline, AP amplitude was elevated while asynchronous APs were reduced in early and late recovery, respectively. Our findings show that AP amplitude distributions are shifting towards larger AP amplitudes in all subjects following hypoxia. Our findings indicate that hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. This article is protected by copyright. All rights reserved.