Nicholson, Wayne T.

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 The risk of hypertension increases as women reach menopause. Our group has recently reported that autonomic support of blood pressure (BP) is increased in post‐menopausal women. Given regular exercise is known to decrease the risk of developing hypertension, we investigated the role of aerobic fitness on autonomic support of BP in women. We hypothesized that trained premenopausal, but not postmenopausal, women would have less autonomic support of BP than age‐matched untrained women. Methods Twelve healthy, young premenopausal (27±1yrs) and 10 older postmenopausal (56±1yrs) women completed VO 2max tests on cycle ergometers. The participants were then stratified by fitness level based on VO 2max percentiles from the American College of Sports Medicine's Guidelines (≤50 th percentile = untrained; >50 th percentile = trained; premenopausal: 7 untrained, 5 trained; postmenopausal: 5 untrained, 5 trained). On a separate day muscle sympathetic nerve activity (MSNA, microneurography, peroneal nerve), heart rate (electrocardiography), and blood pressure (brachial arterial catheter) were recorded before and during ganglionic blockade using trimethaphan camsylate (dose:1.5–7mg/min). Stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were calculated from the blood pressure waveform (WinCPRS). Results Muscle sympathetic nerve activity was higher in trained young women than in untrained young women (burst incidence: 35±11 vs. 12±2 bursts/100 heart beats, p=0.03). Young trained women had a smaller reduction in mean arterial pressure (MAP) with trimethaphan than young untrained women (ΔMAP:−5±3 vs. −15±4mmHg, p=0.01), despite a larger reduction in total peripheral resistance (ΔTPR: −6±1 vs. −2±1mmHg*min/L, p=0.02). The trained young women were able to maintain their MAP due to a greater increase in cardiac output (ΔCO: 1.4±0.4 vs. −0.3±0.3L/min, p= 0.01). In older postmenopausal women, MSNA and the fall in BP, CO, and TPR with trimethaphan were not different between trained and untrained individuals (p= 0.81, 0.16, 0.52 and 0.09, respectively). Conclusion Baseline MSNA was higher in trained premenopausal women when compared to untrained premenopausal women; however, aerobic fitness did not influence baseline MSNA in postmenopausal women. In young trained women, the fall in MAP following ganglionic blockade was only one‐third of the response seen in young untrained women. Baseline MSNA and the change in MAP, CO, and TPR with ganglionic blockade were not different between trained and untrained postmenopausal women. These data suggest aerobic fitness reduces autonomic support of blood pressure in young trained women, but not in trained postmenopausal women. Support or Funding Information Support: HL83947 (MJJ); HL131151 (SEB), DK 7352‐37 (SEB), HL118154 (JNB)

The central neural mechanisms affecting sympathetic neuronal discharge strategies supporting homeostatic adjustments in human muscle sympathetic nerve activity (MSNA) remain unclear. This study investigated the hypothesis that central alpha-2 adrenergic mechanisms affect sympathetic action potential (AP) discharge, recruitment, and latency patterns within human MSNA (peroneal nerve microneurography and continuous wavelet transform). An alpha-2 adrenergic agonist (dexmedetomidine) was infused in eight healthy male and female participants (5 females; 28 ± 7years). Data were recorded during a 5-minute resting control condition, a 10-minute intravenous dexmedetomidine loading dose (0.225 μg/kg), and a dexmedetomidine maintenance dose (~0.1 μg/kg/hr). Data are reported as mean (SD) for the 5-minute control condition and 2-minutes of the dexmedetomidine infusion when MSNA bursting activity was demonstrably reduced. Two-tailed paired t-tests were performed. Dexmedetomidine reduced mean pressure (92 ± 7 to 80 ± 8 mmHg; P < 0.01) but did not affect heart rate (61 ± 13 to 60 ± 14 bpm; P = 0.75). Dexmedetomidine reduced integrated MSNA burst frequency (14 ± 6 to 4 ± 3 bursts/min; P < 0.01), sympathetic AP frequency (75 ± 46 to 16 ± 14 AP/min; P < 0.01), APs per burst (5 ± 1 to 4 ± 1 AP/burst; P = 0.01), AP clusters per burst (3.5 ± 0.7 to 3.0 ± 0.7 clusters/burst; P = 0.03), and de-recruited the largest AP clusters (12 ± 3 to 7 ± 2 AP clusters; P < 0.01). Despite de-recruiting large AP clusters with short conduction times, dexmedetomidine elicited shorter sympathetic AP conduction times (1.18 ± 0.12 to 1.14 ± 0.13 s; P = 0.03). Six participants performed a Valsalva Maneuver (20 s, 40 mmHg mouth pressure) during the control condition and during the dexmedetomidine maintenance dose. Data are reported as mean (SD) change (Δ) from the 2-minute period preceding the Valsalva Maneuver to the Valsalva Maneuver for each condition. During the control condition, Valsalva Maneuver increased AP frequency (Δ 311 ± 317 AP/min; P = 0.048), recruited larger previously silent AP clusters (Δ 2 ± 1 AP clusters; P < 0.01), and elicited shorter AP conduction times (Δ ‑0.09 ± 0.07 s; P = 0.02). Compared to the control condition, Valsalva Maneuver performed during the dexmedetomidine maintenance dose condition elicited lesser increases in AP frequency (Δ 116 ± 189 AP/min; P vs. control = 0.02) and lesser recruitment of larger previously silent AP clusters (Δ 0 ± 1 AP clusters; P vs. control = 0.02). Dexmedetomidine did not affect the latency reduction elicited by Valsalva Maneuver (Δ ‑0.07 ± 0.14 s; P vs. control = 0.61). These data suggest that human central alpha-2 adrenergic mechanisms affect: i) sympathetic postganglionic neuronal discharge and recruitment patterns, and ii) synaptic delays within the sympathetic neurocircuitry influencing AP latency. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Institutes of Health (NIH). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The ganglionic blocker trimethaphan (TMP) is no longer produced; alternatives are needed to inhibit baroreflex control of the circulation during systemic hemodynamic studies. We tested the hypothesis that combination anti‐cholinergic glycopyrrolate (Gly) and α2 receptor agonist dexmedetomidine (Dex) would inhibit baroreflex buffering in response to vasodilation to a similar degree as ganglionic blockade with TMP. In 2 separate trials, 5 males received an I.V. bolus and infusion of Gly‐Dex, and 11 males received TMP infusion. To offset the fall in blood pressure with TMP, phenylephrine (PE) was used to raise pressure to pre‐TMP baseline. Terbutaline was then infused for 15 minutes at 2 doses (33 and 67 ng/kg/min). Heart rate (HR), blood pressure (BP, oscillometric cuff for Gly‐Dex, brachial arterial line for TMP+PE), and cardiac output (CO, acetylene breathing) were recorded. Resting HR was higher with Gly‐Dex (mean ± SEM: 93±7mmHg) than TMP+PE (74±3mmHg, p<0.001) but resting BP, CO, stroke volume (SV), and total peripheral resistance (TPR) were similar between Gly‐Dex and TMP+PE. The cardiovascular responses to terbutaline were similar between Gly‐Dex and TMP+PE, except for the increase in CO which was significantly greater in the TMP + PE group. We conclude that Gly‐Dex is a viable pharmacologic alternative to TMP in physiologic measures of BP and TPR during vasodilator infusions.

The vascular effects of prostacyclin and contribution of endothelial derived nitric oxide (NO) to prostacyclin mediated dilation are unknown in aging. A pilot study was performed in healthy older subjects (n=6, 61–73 years old) and younger controls (n=6, 19–45 years old) that were matched for BMI. Subjects underwent brachial artery catheterization and received escalating doses of epoprostenol (PGI 2 2.5, 5, 10 ng/100ml forearm tissue/min) followed by sodium nitroprusside (SNP 0.25, 0.5, 1 mcg/100ml forearm tissue/min) after PGI 2 washout. SNP and PGI 2 were repeated after NO synthase inhibition with N G ‐Monomethyl‐L arginine acetate (L‐NMMA 1mg/min after 50mg load). Using venous occlusion plethysmography, forearm vascular conductance (FVC) was calculated using mean arterial pressure (MAP), as FVC= (FBF/MAP X 100). Percent change in FVC from baseline after PGI 2 was lower in the aging individuals (47, 160, 227 % change FVC) compared to younger controls (85, 218, 327 %change FVC) (p=0.039), L‐NMMA reduced baseline FVC similarly, and the dilator response to PGI 2 was no longer significant between groups. SNP effects were not significant between groups and unaffected by L‐NMMA. This data suggests a reduced effect to prostacyclin in the aging individual may be due to a reduction in the contribution of endothelial derived NO. Support provided by HL‐46493 and UL1‐RR24150.

We previously demonstrated a bimodal distribution of vasodilator responsiveness to adenosine (ADO) infusion in human subjects: one subgroup demonstrated robust vasodilator responses to ADO (ADO responders) and the other showed blunted responses (non‐responders). Furthermore, ADO receptor antagonism blunted ADO vasodilator responses only in ADO responders and blunted exercise vasodilation only at high workloads in both groups. To test the hypothesis that ADO transporter antagonism would augment vasodilator responsiveness to ADO and exercise only in ADO responders, we compared forearm vasodilation to intra‐arterial infusion of three doses of ADO to vasodilation during three workloads of forearm handgrip exercise before and after ADO transporter antagonism with dipyridamole (DYP) in 10 subjects. In ADO responders (n=5), the change in forearm vascular conductance above baseline (ml min −1 100mmHg −1 ) for the low, medium, and high doses of ADO, respectively, was 61 ± 15, 117 ± 35, 164 ± 47 before DYP and 160 ± 50, 171 ± 59, and 216 ± 87 after DYP. For non‐responders (n=5), these values were 20 ± 2, 36 ± 6, 57 ± 14 before DYP ( P < 0.05 vs. responders) and were 159 ± 40, 293 ± 53, and 377 ± 82 after DYP ( P > 0.05 vs. responders). ADO transporter blockade had no effect on exercise hyperemia in either group ( P > 0.05 for pre vs. post‐DYP). We conclude that there may be increased ADO transporter activity in non‐responders resulting in increased clearance of ADO from the interstitium and that, in both subgroups, ADO is not obligatory for exercise hyperemia.

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 .

Reduced cerebral blood flow and reactivity may explain age‐related changes in cognitive function. We examined cerebrovascular reactivity, and the potential role of prostaglandins, in young and old healthy humans. Middle cerebral artery velocity (MCAv) responses to stepped hypercapnia were measured in young (26±2y; 6M/5F) and old (66±2y, 3M/4F) adults before (PRE) and 90 min after (POST) oral indomethacin (1.2mg/kg). To account for any changes in blood pressure, cerebrovascular conductance index (CVCi) was calculated as the MCAv divided by mean arterial pressure. Cerebrovascular reactivity slopes were calculated using linear regression between either MCA velocity or CVCi and end‐tidal CO 2 . There were no differences between young and old adults in height, weight, BMI, or brachial blood pressures. Young adults demonstrated greater MCAv (53.7±6.7 vs. 32.0±3.3), MCAv reactivity (1.62±0.21 vs. 0.94±0.10), and CVCi (0.66±0.08 vs. 0.39±0.04) compared with old adults at PRE (P<0.05 for all). In both young and old adults, MCAv, MCAv reactivity, CVCi, and CVCi reactivity decreased between PRE and POST measurements. Furthermore, any age‐related differences in these cerebrovascular variables were abolished by indomethacin. These results suggest that cerebral blood flow and cerebrovascular reactivity is attenuated in aging adults and that this reduction may be mediated by prostaglandins. NIH: AG16574‐11PP2