Limberg, Jacqueline K.

Objective Inadequate sleep is associated with an increased risk of hypertension. Exaggerated blood pressure (BP) reactivity to psychological stress is predictive of the development of hypertension. Following 24‐hours of total sleep deprivation, individuals exhibit an exaggerated BP response to mental stress (MS). The mechanisms contributing to this response and the generalizability to more prolonged exposure to restricted sleep are not clear. Thus we sought to examine the early hemodynamic response to MS in individuals before and during prolonged sleep restriction. We hypothesized the early rise in BP during MS would occur more slowly and would achieve a higher peak following 9 nights of restricted sleep when compared to control. We further hypothesized the contribution of total peripheral resistance (TPR) to the rise in BP would be greater following restricted sleep when compared to control. Methods Individuals completed two visits randomized to control sleep or experimental sleep restriction. Ten young (23±1 yrs) healthy, non‐obese (24±1 kg/m 2 ) men and women (5M/5F) completed the restriction visit and seven of the same individuals (23±1 yrs; 5M/2F; 24±1 kg/m 2 ) completed the control visit. Each visit consisted of 4 days of acclimation (9 hours of sleep per night) followed by 9 days of control (9 hours/night) or restricted (4 hours/night) sleep. For the purpose of this investigation, individuals were studied on days 2 and 13. Beat‐by‐beat heart rate (HR, electrocardiography) and BP (finger plethysmography) were measured continuously during 2 min of quiet rest followed by 5 min of mental arithmetic. Stroke volume (SV) was estimated from the pressure waveform and cardiac output (CO) and TPR were calculated. Peak change, time to peak, and rate of rise in BP were assessed during the initiation of mental stress (first 30‐s). Results BP increased during MS ( P <0.05) and the early peak BP did not differ between conditions ( P =0.35–0.84). However, following prolonged experimental sleep restriction, individuals achieved peak systolic BP earlier (Time to peak: 19±4 vs 9±3 s, P =0.04) and the rate of rise in diastolic BP at the onset of MS was significantly greater (0.7± 0.3 vs 2.3±0.7 mmHg/s, P =0.01) when compared to the acclimation period. No differences in time to peak BP or rate of rise at the onset of MS were observed on the control visit ( P =0.16–0.97). The peak change in SV (0.5±1.4 vs 6.3±2.1, P =0.03) and CO (1.1±0.2 vs 1.7±0.2 L/min, p=0.01) at the onset of MS were greater following prolonged sleep restriction when compared to the acclimation period. No differences in the change in SV or CO were observed on the control visit ( P =0.39–0.41) and no differences in the change in TPR were observed on either visit ( P =0.36–0.86). Conclusions Prolonged (9‐day) experimental sleep restriction (4 hours/night) results in a faster increase in BP in response to mental arithmetic. This rapid increase in BP at the onset of MS is achieved primarily via a greater increase in SV and CO when compared to the acclimation period. These data highlight an important role for CO in the BP response to stress in sleep restricted individuals and provide important insight into potential mechanisms which may contribute to the development of hypertension following inadequate sleep. Support or Funding Information NIH HL130339, NIH HL114676, NIH HL114024, NIH HL083947, APS UGSRF This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Objective Patients with sleep apnea exhibit impairments in reflex responses to decreasing, but not increasing, arterial blood pressures (BP). Attenuated BP control may contribute to the increased risk of developing hypertension observed in patients with sleep apnea; however the mechanisms involved are not completely understood. Therefore, we aimed to examine the differential effect of acute intermittent hypoxia (IH) on baroreflex sensitivity during decreasing compared to increasing BP. We hypothesized acute IH would attenuate baroreflex sensitivity to decreasing, but not increasing, BP. Methods Heart rate (HR, electrocardiography), BP (brachial arterial catheter), and muscle sympathetic nerve activity (MSNA, microneurography of the peroneal nerve) were assessed in 14 healthy adults (8M/6F; 32±1 yrs, 26±1 kg/m 2 ). Baroreflex sensitivity was evaluated during decreasing and increasing BP using the modified Oxford technique before and after 30‐min of IH [25‐s hypoxic (0.10 F i O 2 ) exposures alternating with 120‐s room air breathing]. Data were analyzed from the depressor (nitroprusside) and pressor (phenylephrine) phases of the modified Oxford for measures of cardiovagal and sympathetic baroreflex sensitivity, as well as the T50 (diastolic BP with a 50% likelihood of an MSNA burst). Results IH resulted in 15 events where oxygen saturation was reduced (S p O 2 : 98±1% vs 92±2%, p<0.01). Resting normoxic ventilation, carotid chemosensitivity to hypoxia, and MSNA were increased following IH (all p<0.05). HR, systolic BP, diastolic BP, and the T50 were not altered following 30‐min of IH (p‐value range 0.08 to 0.90). Diastolic BP was greater than T50 at baseline (Decreasing BP: p=0.046); however, after IH, diastolic BP was no longer significantly different than the T50 (p=0.52). Cardiovagal baroreflex sensitivity to decreasing and increasing BP differed at baseline (14.0±1.7 vs 22.0±3.3 ms/mmHg, p=0.02) and following IH (11.3±1.1 vs 19.1±2.0 ms/mmHg, p<0.01). Cardiovagal baroreflex sensitivity to decreasing BP tended to be lower following IH (p=0.09). Sympathetic baroreflex sensitivity to decreasing and increasing BP did not differ at baseline (−2.1±0.3 vs −2.5±0.3 %/mmHg; p=0.28) or following IH (−2.6±0.6 vs −3.0±0.4 %/mmHg, p=0.55) and was unaffected by IH (p‐value range 0.20–0.40). Conclusions Acute IH has a differential effect on baroreflex sensitivity. Specifically, cardiovagal baroreflex sensitivity to decreasing, but not increasing, BP tends to be lower following IH. In contrast, sympathetic baroreflex sensitivity is unaffected by IH. Prior to IH, diastolic BP is greater than the T50, thus promoting lower MSNA. Despite no change in baroreflex sensitivity after IH, diastolic BP is no longer significantly different than the T50 – consistent with higher MSNA following IH. Support or Funding Information NIH HL130339, Mayo Clinic Center for Biomedical Discovery This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Objective We examined the contribution of the arterial baroreflex to sympathetic discharge patterns in humans by systematically manipulating the reflex using intravenous nitroprusside and phenylephrine. We hypothesized nitroprusside would increase firing of low‐threshold axons, with no effect on recruitment of latent axons and/or synaptic delays. We further hypothesized phenylephrine would decrease firing of low‐threshold axons, with no effect on synaptic delays. Methods Heart rate (HR, ECG), arterial blood pressure (BP, brachial arterial catheter), and multi‐unit muscle sympathetic nerve activity (MSNA, microneurography of the peroneal nerve) were measured in 13 male subjects (30±2 yrs, 25±1 kg/m 2 ) at baseline and during systemic intravenous infusion of nitroprusside (0.4–1.7 μg/kg/min) and phenylephrine (0.2–1.0 μg/kg/min), administered separately. Action potential (AP) patterns were studied from the filtered raw signal using wavelet‐based methodology. Results Compared to baseline, BP decreased and HR and multi‐unit MSNA (41±5 to 53±4 bursts/100 heart beats) increased with nitroprusside (all P <0.01). Nitroprusside increased AP incidence (326±66 to 579±129 AP/100 heart beats) and AP content per integrated burst (8±1 to 11±2 AP/burst) (all P <0.01). The firing probability of low‐threshold axons increased with nitroprusside, and recruitment of previously latent, high‐threshold axons was observed (22±3 to 24±3 max clusters, 9±1 to 11±1 clusters/burst; all P <0.05). Compared to baseline, BP increased and HR and multi‐unit MSNA (41±3 to 21±3 bursts/100 heart beats) decreased with phenylephrine (all P <0.01). Phenylephrine decreased AP discharge rate (406±128 to 166±42 AP/100 heart beats) and resulted in fewer unique clusters (15±2 to 9±1 max clusters, all P <0.05); however, components of an integrated burst were not altered (9±2 vs 8±1 AP/burst; 5±1 vs 4±1 clusters/burst, P >0.05). AP cluster latency decreased with cluster size under both conditions ( P <0.01), but this relationship was not altered with nitroprusside ( P =0.42) or phenylephrine ( P =0.39). Conclusions Baroreflex unloading with systemic nitroprusside increases sympathetic neuronal discharge rate by increasing the firing probability of low‐threshold axons, in addition to recruitment of previously latent, high‐threshold axons. In contrast, baroreflex loading with systemic phenylephrine decreases neuronal discharge rate, including decreased firing probability of high‐threshold axons; however, the components of an integrated burst (AP and clusters per burst) and synaptic delays are unaffected. These data support a hierarchical pattern of sympathetic neural recruitment/de‐recruitment and suggest acute modification of synaptic delays are not influenced by the baroreflex. Support or Funding Information NIH HL083947, NIH HL130339 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Action potentials (APs) of varying sizes and firing probabilities, that are synchronized into bursts of efferent traffic, characterize recordings of muscle sympathetic nerve activity (MSNA). Recent evidence points to ordered recruitment of these APs but the sites of this control remain unknown. This study investigated the role of the paravertebral ganglia in sympathetic AP emission patterns using pharmacological blockade of ganglionic N N ‐nicotinic receptors via trimethaphan camsylate (TM; n = 7 females, 37 ±5 years, 112 ±2/70 ±3 mmHg, 22 ±1 kg/m 2 ). Heart rate (HR; ECG), blood pressure (BP; arterial catheter) and MSNA (microneurography) were recorded during baseline, TM infusion (1–7 mg/min) and Valsalva's manoeuvre. MSNA APs were detected from the filtered raw signal using a wavelet deconstruction approach and were binned in clusters (Scott's rule) based on peak‐to‐peak amplitude (K‐means). In the final minute of MSNA burst detection during TM, HR increased (57 ±11 to 66 ±12 bpm) and mean BP decreased (97 ±7 to 90 ±7 mmHg) (both P < 0.01 compared to baseline). Relative to baseline, TM elicited a progressive reduction in total sympathetic outflow (833 ±458 to 157 ±120 AU/min), characterized by fewer integrated bursts (30 ±14 to 9 ±8 bursts/100 beats) with decaying amplitude (49 ±3 to 31 ±3 AU) (all P < 0.01). Underlying TM‐mediated attenuations in integrated MSNA were reductions in AP incidence (186 ±101 to 29 ±31 AP/100beats) and AP content per integrated burst (7 ±2 to 3 ±1 APs/burst) (all P < 0.01). Compared to baseline, the total number of active AP clusters was reduced with TM (14 ±3 to 9 ±3 AP clusters), resulting in fewer unique clusters per burst (4 ±1 to 3 ±1 AP clusters/burst) (both P < 0.01). Relative to baseline, most participants demonstrated a reduction in the within‐burst firing probability of all AP clusters, with the largest two to six AP clusters ceasing to fire in the last minute before integrated bursting activity was abolished. Following cessation of integrated MSNA bursts, the smallest 6 ±2 sympathetic AP clusters (range: 4 – 8 AP clusters) persisted to fire in an asynchronous pattern (49 ±41 AP/100beats) in all participants. Valsalva's Maneuver did not increase the incidence of these persistent small APs (60 ±42 AP/100beats, P = 0.52), or instigate any larger APs (previously active at baseline) in six of seven participants (6 ±1 total AP clusters, P = 0.30). These data suggest: 1) the paravertebral ganglia may contribute to the heterogeneity in the firing probabilities of varying sized postganglionic sympathetic APs active under baseline conditions, and 2) the presence of non‐synchronized sympathetic APs that express resistance to blockade with trimethaphan. Support or Funding Information Supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, Discovery Grant #217916‐2013. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Objective: Given recent evidence from rodents supporting a novel role for the carotid bodies in insulin‐mediated sympathoexcitation, we sought to determine whether carotid chemoreceptor desensitization would reduce muscle sympathetic nerve activity (MSNA) during hyperinsulinemia in humans. Methods: Young, healthy subjects (n=4, 30±2 yrs) breathed hyperoxic gas (End‐tidal O2 = 100%, PaO2 505±13 mmHg) for 15 min at baseline and during steady‐state hyperinsulinemia (5±1 vs 53±1 µU/mL). Hyperinsulinemia was achieved using a 90‐min hyperinsulinemic (1 mU/kg FFM/min), euglycemic (108±4 mg/dL) clamp. CO2 was maintained at resting levels (PaCO2 41±1 mmHg). Changes in heart rate, blood pressure, minute ventilation, and MSNA (microneurography of the peroneal nerve) were assessed. Results: Hyperinsulinemia increased MSNA [burst frequency (p<0.01), burst incidence (p=0.02), total activity (p=0.02)]. Hyperoxia decreased heart rate (p=0.01) and minute ventilation (p=0.11), and increased mean blood pressure (p=0.04). Consistent with our hypothesis, the effect of hyperoxia on MSNA (Δ) was different between baseline and hyperinsulinemia (burst frequency, p=0.03; burst incidence, p=0.10). Conclusion: These data are the first to suggest the carotid chemoreceptors may contribute to insulin‐mediated sympathoexcitation in humans. Grant Funding Source : Supported by: NIH R01 DK090541 (MJJ), NIH T32 DK07352 (JKL)

Insulin plays an important role in post‐prandial blood pressure (BP) regulation. Within the central nervous system insulin acts to stimulate sympathetic outflow to skeletal muscle. Concurrently, insulin acts locally in the periphery to promote vasodilation and is suggested to also attenuate sympathetically‐mediated vasoconstriction. Given these divergent actions, the ‘net effect’ of insulin on the sympathetic control of BP remains incompletely understood. Therefore, the purpose of this study was to examine the effect of hyperinsulinemia on sympathetic transduction to BP. To accomplish this, in 10 young healthy men, heart rate (ECG), BP (finometer or brachial arterial line) and muscle sympathetic nerve activity (MSNA) were measured at rest and during euglycemic‐hyperinsulinemia. Signal‐averaging was used to characterize the changes in mean arterial pressure (MAP) and total vascular conductance (TVC; derived from ModelFlow) over 10 cardiac cycles following spontaneous bursts of MSNA at rest and during hyperinsulinemia. No significant differences were found in the peak MAP (Rest: ∆2.8 ± 0.3; Insulin: ∆2.4 ± 0.2 mmHg, P=0.15) or TVC (Rest: ∆‐3.0 ± 0.4; Insulin: ∆‐2.7 ± 0.5 mL/min/mmHg, P=0.38) responses following bursts of MSNA between rest and hyperinsulinemia (i.e., ‘net’ sympathetic transduction was unchanged). Notably, the average size of normalized MSNA bursts was increased during hyperinsulinemia (Rest: 45.6 ± 1.8; Insulin 65.3 ± 5.0 Au, P<0.01). To take into account changes in burst height, MSNA bursts were divided into amplitude‐matched bins. When MSNA burst amplitude bins were matched between rest and hyperinsulinemia, sympathetic transduction for a given burst amplitude was blunted during insulin infusion (e.g., largest resting bursts: MAP, Rest: ∆4.4 ± 0.5; Insulin: ∆3.0 ± 0.3 mmHg, P=0.02; TVC, Rest: ∆‐5.2 ± 0.8; Insulin: ∆‐3.1 ± 0.7 mL/min/mmHg, P=0.05). However, it is important to note that 15 ± 5% of MSNA bursts exhibited amplitudes greater than that which existed at rest and overall during hyperinsulinemia ~50% of all MSNA bursts were comparable or greater in amplitude to the largest bursts present at rest. Collectively, these preliminary findings suggest that during hyperinsulinemia, sympathetic transduction for a given burst amplitude is attenuated while ‘net’ sympathetic transduction remains unchanged due to the generation of larger MSNA bursts.

Background Sex‐related differences in respiratory‐sympathetic coupling have been described in preclinical models of intermittent hypoxia (IH) and have been implicated in the development of neurogenic hypertension. Based on sex disparities in the respiratory and blood pressure response to acute IH in humans as well as changes in respiratory‐sympathetic coupling in clinical sleep apnea, we examined sex‐related differences in respiratory modulation of muscle sympathetic nerve activity (MSNA) following acute IH in healthy humans. We hypothesized the magnitude of respiratory‐sympathetic coupling would increase in both men and women following IH; however, the respiratory patterning of MSNA following IH would be sex‐specific. Methods Heart rate, MSNA, and respiration were assessed in 21 healthy young men (30±1 yrs) and 10 healthy young women (28±1 yrs) during normoxic rest prior to and following IH. IH was achieved by alternating between a hypercapnic hypoxic (3% carbon dioxide, 5% oxygen) gas cylinder and room air (21% oxygen) to target 15 hypoxic events over 30‐min. Respiratory modulation of MSNA was assessed by fitting polynomials to cross‐correlation histograms constructed between sympathetic spikes and respiration. Results MSNA was elevated following IH in both men (34±2 to 41±3 bursts/100 heart beats) and women (30±4 to 35±5 bursts/100 heart beats) (p<0.01). When examining the normalized temporal profile of MSNA, both men and women exhibited changes in respiratory patterning of MSNA following IH; however, the pattern of MSNA differed by sex. Following IH, women exhibited more sympathetic activity during late expiration and less during post‐inspiration (interaction of IH and time, p<0.01). In contrast, men exhibited greater sympathetic activity during post‐inspiration following IH (interaction of IH and time, p<0.01). Conclusions Both men and women exhibit changes in respiratory patterning of MSNA following acute IH; however, this pattern differs by sex. These data support sex disparities in respiratory‐sympathetic coupling and advance our understanding of sex‐specific changes in sympathetic activation following IH which may have important implications for the development of neurogenic hypertension.

Repetitive hypoxic apneas, similar to those observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP). This baroreflex resetting is associated with hypertension in preclinical models of sleep apnea (intermittent hypoxia, IH); however, the majority of understanding comes from males. There are data to suggest that female rats exposed to IH do not develop high BP. Clinical data further support sex differences in the development of hypertension in sleep apnea, but mechanistic data are lacking. Here we examined sex-related differences in the effect of IH on sympathetic control of BP in humans. We hypothesized that after acute IH we would observe a rise in muscle sympathetic nerve activity (MSNA) and arterial BP in young men ( n = 30) that would be absent in young women ( n = 19). BP and MSNA were measured during normoxic rest before and after 30 min of IH. Baroreflex sensitivity (modified Oxford) was evaluated before and after IH. A rise in mean BP following IH was observed in men (+2.0 ± 0.7 mmHg, P = 0.03), whereas no change was observed in women (−2.7 ± 1.2 mmHg, P = 0.11). The elevation in MSNA following IH was not different between groups (4.7 ± 1.1 vs. 3.8 ± 1.2 bursts/min, P = 0.65). Sympathetic baroreflex sensitivity did not change after IH in either group ( P > 0.05). Our results support sex-related differences in the effect of IH on neurovascular control of BP and show that any BP-raising effects of IH are absent in young women. These data enhance our understanding of sex-specific mechanisms that may contribute to BP changes in sleep apnea.

We examined the role of sympathetic activation in restraining vasodilatory responses to hyperinsulinemia and sustaining blood pressure in healthy adults. Data are reported from two separate experimental protocols in humans and one experimental protocol in isolated arteries from mice. Contrary to our hypothesis, the present findings support the idea that during hyperinsulinemia, a sympathetically mediated increase in cardiac output, rather than restraint of peripheral vasodilation, is the principal contributor to the maintenance of systemic blood pressure.

Aging increases autonomic support of blood pressure; however, the impact of aerobic fitness on autonomic support of blood pressure has not been addressed in women. As such, we hypothesized that aerobic fitness would be related to the change in blood pressure during ganglionic blockade such that women with greater aerobic fitness would have a blunted fall in blood pressure during ganglionic blockade due to increased vagal tone. Thirteen young premenopausal and 13 older postmenopausal women completed a screening visit where aerobic fitness (maximal oxygen consumption, VO 2max ) was measured. On a separate study day, participants were instrumented for assessment of muscle sympathetic nerve activity, heart rate (electrocardiography), and beat by beat blood pressure (arterial catheter and pressure transducer) and underwent pharmacological blockade of the autonomic ganglia using trimethaphan camyslate. Heart rate, blood pressure, and muscle sympathetic nerve activity were analyzed before and during ganglionic blockade. In young women, there was a significant relationship between aerobic fitness and the change in blood pressure during ganglionic blockade ( r =0.761, P =0.003). In older women, there was no relationship between aerobic fitness and the change in blood pressure during ganglionic blockade ( r =−0.106, P =0.73). Measures of heart rate variability were related to fitness in young women, but not older women (root mean square of successive differences between normal heartbeats, r =0.713, P =0.006 versus r =−0.172, P =0.575). Our data suggest that in young women, autonomic support of blood pressure is attenuated in those that are highly fit; however, this relationship is not significant in older women.