Scruggs, Zachariah M.

Background Nitric oxide (NO) contributes ~30% to the rate of vasodilation during dynamic exercise in healthy humans. This NO‐mediated dilation occurs via increases in cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase‐5 (PDE‐5). Therefore, we sought to determine the nature of NO‐cGMP signalling to the exercise vasodilator response in healthy humans. We hypothesized the time‐course of exercise vasodilation would be: 1) accelerated by sildenafil citrate (SDF, PDE‐5 inhibitor) 2) slowed by the NO synthase inhibitor L‐N G ‐monomethyl arginine (L‐NMMA) 3) similarly slowed by the combination of SDF+L‐NMMA. Methods Data from 29 healthy individuals were included in the present study. All subjects completed 5‐min of dynamic forearm exercise at 15% of maximal voluntary contraction under control conditions (CON, arterial saline infusion), followed by an identical exercise protocol during: 1) PDE‐5 inhibition via SDF (100mg oral dose, n = 8), 2) NOS inhibition via intra‐arterial infusion of L‐NMMA (50 mg loading dose and 1 mg/min maintenance dose, n = 15), or 3) combined SDF+L‐NMMA (n = 6). Forearm blood flow (FBF; Doppler ultrasound) and mean brachial artery blood pressure (MAP, transducer) were measured continuously. To determine time to steady‐state vasodilation, forearm vascular conductance (FVC = FBF/100mmHg/dL FAV) data were curve fit with a monoexponential model and mean response time (MRT, time to achieve 63% of steady state response) was calculated. In addition, the increase in relative forearm vascular conductance from rest to steady state exercise (average of the last 30s of exercise, ΔFVC = FVC exercise − FVC rest ) was used to assess the total vasodilatory response. Results Data are reported as mean ± SE (FVC = mL/dL/100mmHg/min, MRT = s) . In contrast to our hypothesis, SDF alone had no effect on exercise vasodilation (Δ FVC CON = 20±3 vs. SDF = 21±3, p = 0.50) or time to steady‐state vasodilation (MRT: CON = 32± 7 vs SDF= 33±4, p = 0.90) in young healthy adults. NOS inhibition with L‐NMMA reduced ΔFVC (CON = 16±2 vs. L‐NNMA = 14±1, p = 0.049) and increased the time to reach steady‐state vasodilation (MRT: CON = 22±5 vs. L‐NMMA = 40±6, p = 0.01). Surprisingly, when subjects were given L‐NMMA combined with SDF, reductions in Δ FVC or increases in MRT were no longer observed (Δ FVC: CON = 23±4 vs. SDF+L‐NMMA = 23±6, p = 0.90; MRT: CON = 45±8 vs. SDF+L‐NMMA = 49±11, p = 0.80). Conclusion These data indicate inhibiting PDE‐5 during moderate exercise does not improve the magnitude or time‐course of vasodilation. Whereas, NOS inhibition limits the magnitude and time‐course of the vasodilatory response to dynamic handgrip exercise in healthy adults, combined inhibition of PDE‐5 restores the response to control levels. These data suggest PDE‐5 inhibition does not potentiate the NO‐cGMP pathway during moderate exercise. However, PDE‐5 inhibition may have therapeutic use in pathologies where limited NO bioavailability may contribute to impaired blood flow responses to exercise. Support or Funding Information American Heart Association (15POST23100020) ‐ JMKNational Institutes of Health (HL46493) ‐ MJJ National Institutes of Health (HL078019) ‐ MJJ National Institutes of Health (HL105820) ‐ WGS National Institutes of Health (RR17520) ‐ N/A National Institutes of Health (UL1 TR000135) ‐ N/A

Background Patients with sleep apnea experience repeated, intermittent hypoxic exposures during sleep and are at increased risk of developing hypertension. The mechanisms that contribute to high blood pressure in these patients are not completely understood. In this context, we sought to examine the effect of intermittent hypoxic exposures on baroreflex sensitivity and blood pressure. We hypothesized 3‐hours of intermittent hypoxia would result in a decrease in baroreflex sensitivity in healthy adults. We further hypothesized any decrease baroreflex sensitivity would be related to an increase in blood pressure following intermittent hypoxia. Methods Six (5M/1F) young healthy, non‐obese, subjects without sleep apnea completed 3‐hours of intermittent hypoxia. Intermittent hypoxia consisted of 25‐second exposures to hypoxic gas (0.05 F i O 2 ) separated by 2 minutes of room air breathing, resulting in 25‐hypoxic events per hour in which oxygen saturation (S p O 2 ) was significantly reduced (Normoxia: 97%, Hypoxia: 89%, p<0.01). Beat‐to‐beat heart rate (electrocardiogram) and blood pressure (finger photoplethysmography) were measured continuously. Spontaneous cardiac baroreflex sensitivity was determined using the sequence method (WinCPRS) and data are reported from 10‐minutes of quiet, normoxic rest before and after the intermittent hypoxia protocol. Results We observed a significant reduction in spontaneous cardiac baroreflex sensitivity to falling pressures (“down‐down”) following intermittent hypoxia (Control=22±2, IH=15±2 ms/mmHg; p=0.02). No change in baroreflex sensitivity to rising pressures (“up‐up”) was observed (Control=25±2, IH=20±6 ms/mmHg; p=0.41). A relationship between baroreflex sensitivity and mean arterial blood pressure was observed, such that a reduction in baroreflex sensitivity tended to be related to an increase in blood pressure (“down‐down”: R=−0.45, p=0.14; “up‐up”: R=−0.54, p=0.07). Conclusions Intermittent hypoxia results in a decrease in baroreflex sensitivity, which is related to an increase in blood pressure. These data may have important implications for mechanisms contributing to the risk of hypertension in individuals with sleep apnea. Support or Funding Information DK90541 (MJJ), HL120570 (JKL)

Repeat exposures to low oxygen (intermittent hypoxia, IH) elicit increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in men. Endothelin (ET) receptor antagonists can attenuate the sympathetic and BP response to IH in rodents; whether these data translate to humans were unclear. We show acute IH results in an increase in ET-1, MSNA, and BP in healthy young men; however, the effect of IH on MSNA and BP does not occur through activation of ET-receptors in healthy young men.

Background Muscle sympathetic nerve activity (MSNA) is augmented in patients with sleep apnea. This increase in activity is due to increases in firing probability and mean firing rates of individual vasoconstrictor neurons, in addition to multiple within‐burst firing; however, contributing mechanisms and the impact of such firing patterns on peripheral vascular tone are less clear. We examined the effect of acute intermittent hypoxia (IH) on sympathetic neural firing and recruitment strategies and the relationship between changes in MSNA and neurovascular transduction following acute IH. We hypothesized exposure to acute IH would increase MSNA via an increase in action potential (AP) discharge rates and within‐burst firing. We further hypothesized any change in discharge patterns following acute IH would be positively related to a change in neurovascular transduction. Methods Blood pressure (brachial arterial catheter) and MSNA (microneurography of the peroneal nerve) were assessed in 17 healthy adults (11M/6F; 31 ± 1 yrs) during quiet normoxic rest prior to and following 30‐min of experimental IH [30‐s hypoxic (0.05 F i O 2 ) exposures alternating with 90‐s room air breathing]. AP patterns were studied from the filtered raw MSNA signal using wavelet‐based methodology. The slope of the relationship between diastolic blood pressure (DBP) and the preceding MSNA burst area at a fixed cardiac cycle lag was used to quantify neurovascular transduction. Results IH resulted in 15 events where oxygen saturation was reduced (S p O 2 : 99 ± 1% vs 92 ± 1%, P <0.01). Compared to baseline, multi‐unit MSNA burst incidence (33 ± 4 to 43 ± 5 bursts/100 heart beats), AP incidence (272 ± 50 to 501 ± 112 AP/100 heart beats) and AP content per integrated burst (7 ± 1 to 10 ± 1 AP/burst; 4 ± 1 to 5 ± 1 clusters/burst) were increased following IH (all P< 0.05). Neurovascular transduction was not altered following IH ( P =0.34); however, there was a positive relationship between the change (Δ) in neurovascular transduction (mmHg/%/s) and changes in AP discharge rate (AP/100 heart beats; R=0.52, P= 0.03) and within‐burst firing (AP/burst; R=0.38, P= 0.14) following IH. Conclusions Acute IH increases sympathetic neuronal discharge by increasing within‐burst firing of low‐threshold axons. A positive relationship exists between increases in the neural discharge rates and neurovascular transduction following acute IH, such that those individuals with the greatest increase in AP firing exhibited the greatest increase in transduction of sympathetic activity into blood pressure. These data may have important implications for mechanistic understanding of sympathetic activation in sleep apnea and its down‐stream effects, including the development of hypertension and cardiovascular disease. Support or Funding Information Funding: NIH HL130339, Mayo Clinic Center for Biomedical Discovery, Mizzou HES PURE, APS UGSRF. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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 .

Background The baroreflex plays an integral role in blood pressure maintenance by altering heart rate and sympathetic activity in response to changes in arterial pressure. Baroreflex sensitivity (BRS) has been shown to decrease linearly during progressive exposure to central hypovolemia as induced by lower body negative pressure (LBNP), and can be used as an index of decompensation status during hemorrhage. Circulating epinephrine is significantly elevated during high stress situations such as hemorrhage; however, it is unclear whether epinephrine influences BRS during progressive exposure to central hypovolemia. Based on previous work, we hypothesized epinephrine would attenuate any reduction in spontaneous cardiac baroreflex sensitivity during LBNP. Methods Ten young (35±3 yrs), healthy (BMI 24±1 kg/m 2 ) men and women (6M/4F) underwent LBNP protocols on 2 separate days consisting of a 10‐min pre‐infusion baseline, 15‐min infusion period (randomized to saline or epinephrine, 0.1 μg/kg/min), 10‐min post‐infusion baseline, and 5 minutes of LBNP at −15, −30, −45, −60, −70, and −80 mmHg until the onset of presyncope (range= −30 to −80 mmHg). Presyncope was defined as a rapid decrease in systolic blood pressure greater than 15 mmHg, systolic blood pressure less than 80 mmHg, bradycardia, or voluntary termination due to presyncopal symptoms. Heart rate (HR; electrocardiogram) and blood pressure (BP; radial arterial catheter) were recorded continuously. Spontaneous cardiac BRS was determined using the sequence method (WinCPRS). Data are reported from both 10‐min baselines and from 1 minute at 20, 40, 60, 80, and 100 percent of each individual's total LBNP exposure time. Results We observed no significant differences in total exposure time (1475±150 vs. 1415±79s; p=0.69) or cumulative stress index (820±135 vs. 699±69 min*mmHg; p=0.37) between saline and epinephrine visits, respectively. Baroreflex sensitivity did not change during post‐infusion baselines (ΔDown‐Down =−2±1 vs. −2±2 ms/mmHg; saline vs. epinephrine, respectively; p=0.09). Baroreflex sensitivity was significantly reduced with LBNP (p<0.01 main effect of LBNP). There was no difference between the change in baroreflex sensitivity during LBNP with saline or epinephrine infusion (ΔDown‐Down =−14±4 vs. −8±4 ms/mmHg, p=0.17). Conclusions Consistent with previous work, spontaneous cardiac baroreflex sensitivity fell during LBNP. Contrary to our hypothesis, epinephrine infusion did not attenuate the fall in spontaneous cardiac baroreflex sensitivity observed during simulated blood loss. Because epinephrine did not alter the effect of LBNP on BRS, these data suggest that BRS may accurately reflect decompensation status during traumatic hemorrhage independent of elevated circulating epinephrine. Support or Funding Information US Army Medical Research and Materiel Command Contract W81XWH‐13‐2‐0038

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.

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.

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.