Limberg, Jacqueline K.

Background Maintaining baroreflex sensitivity is integral to ensuring blood pressure maintenance and end‐organ perfusion during hemorrhage. This is especially true under conditions of hypoxemia, where decreased tolerance to central hypovolemia is commonly observed. Hypoxia activates peripheral chemoreceptors and reduces baroreceptor‐vascular resistance reflex sensitivity. However, a direct role for the peripheral chemoreceptors in the hemodynamic response to hemorrhage in hypoxia remains unclear. Objective We tested the hypothesis that attenuation of peripheral chemoreceptor activity (low‐dose dopamine; 1–2 μg/kg/min) during acute hypoxia would promote maintenance of mean arterial pressure, pulse pressure, and other hemodynamic markers [heart rate (HR), stroke volume (SV), cardiac output (CO), total peripheral resistance (TPR)] during simulated hemorrhage. Methods Ten healthy men (35±3 years, BMI 25±1 kg/m 2 ) participated in a randomized, double‐blind, placebo controlled study. Participants completed two experimental visits (saline and low‐dose dopamine) at which lower body negative pressure (LBNP) was conducted during hypoxia (SpO 2 ~85%; PaO2 ~52 mmHg) delivered via a non‐rebreather mask. Ventilation [V̇ (turbine transducer)], arterial blood pressure (brachial artery catheter), HR (ECG), and central venous pressure (peripherally inserted central catheter) were measured continuously and SV, CO, and TPR were calculated. Following a 5‐min rest, participants underwent LBNP for 5‐min at 3 intervals (−15, −30, and −45 mmHg). Outcome values were analyzed and compared using a mixed linear model (SAS PROC MIXED) with a direct product AR(1) covariance structure using indicator variables for saline vs. dopamine. Data from the mixed linear model are reported as an estimated change per 15 mmHg of LBNP. Results Dopamine had no effect on hypoxia P a O 2 , P a CO 2 , V̇ or respiratory rate (all P >0.05) and this relationship persisted throughout all LBNP stages. The mixed linear model showed no effect of dopamine vs. saline on changes in: central venous pressure (−1.1±0.4 vs −1.3±0.3 mmHg, P =0.15), mean arterial pressure (−4.7±2.3 vs −5.3±1.4 mmHg, P =0.25), pulse pressure (−6.7±1.5 vs −8.3±1.4 mmHg, P =0.50), HR (7.8±1.5 vs 6.1±1.5 bpm, P =0.41), SV (−10.9±1.9 vs −11.6±3.3 mL, P =0.41), CO (−0.3±0.2 vs −0.4±0.4 L · min −1 , P =0.31) or TPR (0.2±0.4 vs 0.7±0.6 mmHg · L · min −1 P =0.27). Discussion Intravenous infusion of low‐dose dopamine designed to attenuate peripheral chemoreflex activity during acute hypoxia did not alter the integrated hemodynamic response to simulated hemorrhage. Our data suggests that peripheral chemoreceptors have a limited role during hypoxic central hypovolemia. More research is needed to understand the sensitivity and cross‐talk between chemoreceptors and baroreceptors during hemorrhage. Support or Funding Information Funding: W81XWH‐13‐2‐0038 (DOD), DK90541 (NIH), UL1 TR000135 (NIH).

Background Sympathetic nerve activity and its effect on blood pressure are altered by female sex hormones; however, this effect differs between exogenous and endogenous hormones. With oral contraceptive (OC) use, cross‐sectional data has demonstrated higher blood pressure (BP) with no corresponding change in muscle sympathetic nerve activity (MSNA). This results in a higher BP/MSNA ratio with OC use although contributing mechanisms are unclear. The higher BP/MSNA ratio in women on OC suggests OC use may limit the ability of sympathetic baroreflex (sBRS) to suppress MSNA. Alternatively, a “burst” of MSNA could result in a greater change in BP (augmented neurovascular transduction). Objective The goal of this study was to determine if OCs alter blood pressure control through autonomic mechanisms in young women. Hypotheses We hypothesized (1) sympathetic baroreflex sensitivity would be decreased and (2) neurovascular transduction would be augmented in women on OCs compared to naturally cycling women. Methods We conducted a retrospective analysis of data from healthy young women taking OCs (n=30, 25±4yrs) and with natural menstrual cycles (n=23, 27±4yrs). Women were studied during the placebo phase of OC use or early follicular phase of the menstrual cycle. Heart rate (HR, 3‐lead ECG), BP (arterial line or photoplethysmography for both groups), and MSNA (peroneal nerve microneurography) were measured over 5 min of quiet rest and data were analyzed for measures of sBRS and neurovascular transduction (Ensemble, Elucimed Inc., Dunedin, New Zealand). Results Groups did not differ in mean BP (Control: 94±9, OC:92±8mmHg, p=0.60), or HR (Control: 61±8, OC: 62±9 bpm, p=0.57), but women on OCs had lower MSNA than controls (Control: 16±6, OC: 12±5 bursts/min, p=0.006). There were no significant differences in TPR (Control: 0.022±0.005, OC: 0.020±0.005 mmHg min/L, p=0.12), however there was a trend toward increased cardiac output in women on OCs (Control: 4.45±0.86, OC: 4.94±1.02 L/min, p=.068). There were no significant group differences in sBRS (Control: −4.3±2.1, OC: −4.0±2.6 bursts ·100 heartbeats −1 ·mmHg −1 , p=0.79), or neurovascular transduction (Control: 0.093±0.042, OC: 0.091±0.058 ΔmmHg/% burst area, p=0.87). Furthermore, there were no significant correlations between BP and BRS, MSNA, or transduction in either group. Conclusions Our data strengthen previous findings which have shown women taking OCs have higher BP for a given MSNA. The analysis we performed build on these results and demonstrate that the increase in BP for a given MSNA in women taking OCs is not due to attenuated sBRS or augmented neurovascular transduction. This work has advanced our understanding of the alteration of blood pressure with OC use in young women. Support or Funding Information NIH HL083947

Background Repetitive hypoxic apneas, similar to what is observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP) in humans. This resetting of the baroreflex is associated with increased BP in preclinical models of sleep apnea (chronic intermittent hypoxia, CIH); however, the majority of our understanding comes from men. There are data to suggest female rats exposed to CIH do not develop high BP. With this, we sought to examine potential sex‐related differences in the effect of intermittent hypoxia (IH) on the sympathetic baroreflex. We hypothesized young men would exhibit resetting of the baroreflex to higher BP following IH, with no change in young women. Methods BP and muscle sympathetic nerve activity (MSNA) were assessed in 17 men (30±1 yrs) and 10 women (28±1 yrs) during quiet normoxic rest prior to and following 30‐min of experimental IH [30‐s hypoxic exposures (0.05 F i O 2 ) alternating with 90‐s room air breathing] resulting in 15 hypoxic events (91.7±0.5 % SpO2). Sympathetic baroreflex sensitivity was evaluated using the modified Oxford technique before and after 30‐min of IH. Results Diastolic BP increased in men following IH (71±2 to 74±2 mmHg; p<0.01) with no change in women (75±3 to 75±3 mmHg; p=0.97). Sympathetic baroreflex sensitivity did not change following IH in either group (Men: −1.8±0.2 to −1.9±0.3; Women: −1.6±0.3 to −2.1± 0.5 bursts/100 heart beats/mmHg; Main effect of IH, p=0.14; Interaction of group and IH, p=0.28). Conclusion Acute IH resets the sympathetic baroreflex to higher BP in young men, with no change in young women. Baroreflex sensitivity slopes were unchanged in either group following IH. Our results support sex differences in the effect of IH on sympathetic control of BP and show that young women are protected from the BP‐raising effects of IH. These data enhance our understanding of sex‐specific mechanisms that may contribute to the development of hypertension in sleep apnea. Support or Funding Information NIH HL130339, Mayo Clinic Center for Biomedical Discovery

Background Psychological stress and insufficient sleep are both associated with an increased risk of developing hypertension. While the underlying mechanisms are not well defined, it has been postulated that this may be facilitated by modulation of baroreflex control of sympathetic activity. Short periods (<5 min) of mental stress attenuate sympathetic baroreflex sensitivity (sBRS), but it is currently unclear whether this holds true for longer bouts of stress. As such, we sought to examine the effect of 10 min of mental stress on sBRS. Additionally, we evaluated the impact of experimental sleep restriction on baroreflex control of blood pressure during mental stress. We hypothesized that mental stress would attenuate sBRS, and that prolonged exposure to restricted sleep would result in a greater reduction in sBRS during mental stress. Methods Fourteen healthy adults (10M/4F, 25±1 yrs) underwent 10 min of mental stress consisting of a 5‐min mental arithmetic task, followed immediately by a 5‐min Stroop color‐word conflict test. Heart rate (electrocardiography), blood pressure (finger photoplethysmography), and muscle sympathetic nerve activity (microneurography of the common peroneal nerve) were continuously measured to determine sBRS at baseline and during mental stress. A subset of participants (n=9) completed the mental stress protocol under both experimental sleep restriction (4 hr/night) and control sleep conditions (8–9 hr/night) while inpatient at a hospital‐based clinical research unit. Subjects underwent a 3‐night acclimation period, followed by 9 or 14 nights of either sleep restriction or control sleep. Data from Day 2/1 or Day 13/15 (during control sleep only) were used to assess the effect of mental stress alone, and data from Day 13/15 (under both conditions) were used to assess the effect of sleep restriction. Results Acute mental stress resulted in a marked blunting of sBRS from baseline (−2.7±0.5 to −1.5±0.4 bursts·100hb −1 ·mmHg −1 , p=0.04). Examining the mental stress interventions individually revealed a significant decrease in sBRS during the initial 5 min of stress (mental arithmetic, p=0.03), but not during the subsequent 5 min of testing (Stroop test, p=0.75). Mental stress also reduced baroreflex function following sleep restriction (main effect of condition, p<0.01); however, the reduction in sBRS was not significantly different in restricted sleep compared to control (main effect of visit, p=0.78). There was no significant interaction effect between sleep restriction and mental stress (p=0.81). Conclusions These data provide further support that, in young healthy individuals, sBRS is attenuated early in mental stress but quickly returns to baseline values. In contrast, prolonged sleep restriction does not appear to alter sympathetic control of blood pressure. Support or Funding Information NIH HL114024 (VKS), NIH HL114676 (VKS), NIH HL083947 (MJJ), FAER Medical Student Anesthesia Research Fellowship (NMP)

We examined the effect of acute intermittent hypoxia (IH) on sympathetic neural firing patterns and the role of the carotid chemoreceptors. We hypothesized exposure to acute IH would increase muscle sympathetic nerve activity (MSNA) via an increase in action potential (AP) discharge rates and within-burst firing. We further hypothesized any change in discharge patterns would be attenuated during acute chemoreceptor deactivation (hyperoxia). MSNA (microneurography) was assessed in 17 healthy adults (11 male/6 female; 31 ± 1 yr) during normoxic rest before and after 30 min of experimental IH. Prior to and following IH, participants were exposed to 2 min of 100% oxygen (hyperoxia). AP patterns were studied from the filtered raw MSNA signal using wavelet-based methodology. Compared with baseline, multiunit MSNA burst incidence ( P < 0.01), AP incidence ( P = 0.01), and AP content per burst ( P = 0.01) were increased following IH. There was an increase in the probability of a particular AP cluster firing once ( P < 0.01) and more than once ( P = 0.03) per burst following IH. There was no effect of hyperoxia on multiunit MSNA at baseline or following IH ( P > 0.05); however, hyperoxia following IH attenuated the probability of particular AP clusters firing more than once per burst ( P < 0.01). Acute IH increases MSNA by increasing AP discharge rates and within-burst firing. A portion of the increase in within-burst firing following IH can be attributed to the carotid chemoreceptors. These data advance the mechanistic understanding of sympathetic activation following acute IH in humans.

New Findings What is the central question of this study? What is the role of β2‐adrenergic receptor (β2AR) vasodilatation in older postmenopausal women as compared to premenopausal women and the role of nitric oxide (NO) in β2AR‐mediated vasodilatation in both groups of women? What is the main finding and its importance? β2AR responsiveness is blunted in postmenopausal women compared to young premenopausal women. Additionally, NO may contribute to β2AR‐mediated vasodilatation in young premenopausal women. Abstractβ2‐Adrenergic receptor (β2AR)‐mediated vasodilatation, which is partially dependent on nitric oxide (NO) formation, is blunted in men at risk for developing hypertension. However, the role of β2AR vasodilatation in hypertension pathophysiology in ageing postmenopausal women is unclear. Therefore, the goals of this study were to determine if forearm vasodilatation to the selective β2AR agonist terbutaline is blunted in older postmenopausal women (59 ± 4 years) compared to young premenopausal women (27 ± 3 years) and to assess NO contribution to β2AR‐mediated vasodilatation in both groups of women. Forearm blood flow (FBF) and forearm vascular conductance (FVC) were measured using venous occlusion plethysmography at baseline and during intra‐arterial infusions of terbutaline at 0.1–2.0 µg (100 ml tissue)−1 min−1 with and without the NO synthase inhibitor l‐NG‐monomethylarginine (l‐NMMA). Mean arterial pressure was significantly greater in postmenopausal women than in young women at baseline (P = 0.01). Baseline FBF and FVC did not differ between young and postmenopausal women (P > 0.05) and rose significantly within each group during terbutaline infusion (P < 0.05). There were significant group × dose interactions for FBF (P = 0.01) and FVC (P = 0.001), indicating vasodilator responses were lower in postmenopausal women. In young women, FVC response to the highest dose of terbutaline tended to be lower with l‐NMMA co‐infusion vs. without l‐NMMA (P = 0.05). There were no significant decreases in FBF or FVC responses to terbutaline in postmenopausal women with l‐NMMA co‐infusion (P > 0.05 for all). These data suggest that β2AR responsiveness is blunted in postmenopausal women compared to young premenopausal women, and that NO may contribute to β2AR‐mediated vasodilatation in young premenopausal women.

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions. Listen to this article’s corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-doppler-ultrasound-and-resistance-vessel-function/ .

We examined the contribution of the carotid chemoreceptors to insulin-mediated increases in muscle sympathetic nerve activity (MSNA) in healthy humans. We hypothesized that reductions in carotid chemoreceptor activity would attenuate the sympathoexcitatory response to hyperinsulinemia. Young, healthy adults (9 male/9 female, 28 ± 1 yr, 24 ± 1 kg/m2) completed a 30-min euglycemic baseline followed by a 90-min hyperinsulinemic (1 mU·kg fat-free mass−1·min−1), euglycemic infusion. MSNA (microneurography of the peroneal nerve) was continuously measured. The role of the carotid chemoreceptors was assessed at baseline and during hyperinsulinemia via 1) acute hyperoxia, 2) low-dose dopamine (1–4 µg·kg−1·min−1), and 3) acute hyperoxia + low-dose dopamine. MSNA burst frequency increased from baseline during hyperinsulinemia ( P < 0.01). Acute hyperoxia had no effect on MSNA burst frequency at rest ( P = 0.74) or during hyperinsulinemia ( P = 0.83). The insulin-mediated increase in MSNA burst frequency ( P = 0.02) was unaffected by low-dose dopamine ( P = 0.60). When combined with low-dose dopamine, acute hyperoxia had no effect on MSNA burst frequency at rest ( P = 0.17) or during hyperinsulinemia ( P = 0.85). Carotid chemoreceptor desensitization in young, healthy men and women does not attenuate the sympathoexcitatory response to hyperinsulinemia. Our data suggest that the carotid chemoreceptors do not contribute to acute insulin-mediated increases in MSNA in young, healthy adults.

We sought to examine the effect of varying chemoreflex stress on sympathetic neural recruitment strategies during end-expiratory apnea. We hypothesized that increases in the firing frequency and probability of low-threshold axons at the asphyxic “break point” would be exaggerated during hypoxia and attenuated during hyperoxia. Multiunit muscle sympathetic nervous system activity (MSNA) (peroneal nerve microneurography) was measured in 10 healthy male subjects (31 ± 2 yr, 25 ± 1 kg/m2). Individuals completed maximal voluntary end-expiratory apnea under normoxic, hypoxic (inspired O2 fraction: 0.17 ± 0.01), and hyperoxic (inspired O2 fraction: 0.92 ± 0.03) conditions. Action potential (AP) patterns were examined from the filtered raw signal with wavelet-based methodology. Multiunit MSNA was increased ( P ≤ 0.05) during normoxic apnea, because of an increase in the frequency and incidence of AP spikes (243 ± 75 to 519 ± 134 APs/min, P = 0.048; 412 ± 133 to 733 ± 185 APs/100 heartbeats, P = 0.02). Multiunit MSNA increased from baseline ( P < 0.01) during hypoxic apnea, which was due to an increase in the frequency and incidence of APs (192 ± 59 to 952 ± 266 APs/min, P < 0.01; 326 ± 89 to 1,212 ± 327 APs/100 heartbeats, P < 0.01). Hypoxic apnea also resulted in an increase in the probability of a particular AP cluster firing more than once per burst ( P < 0.01). Hyperoxia attenuated any increase in MSNA with apnea, such that no changes in multiunit MSNA or frequency or incidence of AP spikes were observed ( P > 0.05). We conclude that increases in frequency and incidence of APs during apnea are potentiated during hypoxia and suppressed when individuals are hyperoxic, highlighting the important impact of chemoreflex stress in AP discharge patterns. The results may have implications for neural control of the circulation in recreational activities and/or clinical conditions prone to apnea. NEW & NOTEWORTHY Our results demonstrate that, compared with normoxic end-expiratory apnea, hypoxic apnea increases the frequency and incidence of action potential spikes as well as the probability of multiple firing. We further show that this response is suppressed when individuals are hyperoxic. These data highlight the potentially important role of chemoreflex stress in neural firing and recruitment and may have implications for neural control of the circulation in recreational and/or clinical conditions prone to apnea.