Limberg, Jacqueline K.

Background Individuals with sleep apnea experience repeated intermittent hypoxic exposures during sleep. Previous work in sleep apnea showed patients have an increased heart rate (HR), corrected QT interval (QTc), QTc dispersion and P‐wave maximum duration and dispersion (max‐min P‐wave duration). Such changes have been linked to increased risk of cardiovascular disease and sudden cardiac death. Unfortunately, the mechanisms contributing to these changes are not completely understood. Given that individuals with sleep apnea experience repeated intermittent hypoxic exposures during sleep, we tested the hypothesis that exposure to intermittent hypoxia (IH) in young healthy adults would result in similar ECG changes to those seen in patients with sleep apnea. Methods To test this hypothesis, we completed ECG analysis of 7 healthy humans (6M/1F, Age=29±5 years, BMI=24±4kg/m 2 ) without sleep apnea before and after 3‐hours of exposure to IH. Subjects were exposed to hypoxic gas (0.05 FiO2) for 25‐seconds separated by 2‐minutes of room air breathing. This resulted in 25 hypoxic events per hour in which oxygen saturation was significantly reduced (Normoxia: 97%, Hypoxia: 89%, p<0.01). ECG analysis was completed in Lab Chart 7 software and consisted of measurement of intervals (RR, PR, QT), durations (P, QRS), and amplitudes (P, Q, R, S, T) during 10 minutes of normoxic quiet rest before and after IH exposure. QTc was calculated using Bazett's formula (QT duration/√HR). Dispersion of QTc and P waves were calculated as max duration ‐min duration. Results HR was significantly increased following IH (60±8 vs. 66±8 beats/min, baseline vs. post‐IH, respectively, p<0.05). There was no significant change in P‐wave maximum duration or dispersion following IH. QT interval was significantly decreased and QTc was significantly increased following IH (QT interval: 377±13 vs. 368±8ms, QTc: 377±22 vs 384±22ms, baseline vs. post‐IH, p<0.05 for both). QTc dispersion was not significantly different from baseline following IH. Conclusions We found that 3 hours of experimentally‐induced IH in young healthy individuals resulted in increased HR, decreased QT interval, and increased QTc. These data suggest that IH exposure experienced by patients with sleep apnea may contribute to ECG changes commonly observed in this population and subsequent increases in cardiovascular morbidity and mortality. Support or Funding Information Funding : DK90541 (MJJ), HL120570 (JKL)

Background We have previously shown that the carotid body chemoreceptors play an important role in glucoregulation. When the carotid bodies are experimentally desensitized in humans, counterregulatory responses to hypoglycemia are attenuated. Whether carotid body activation conversely increases circulating glucose levels was previously unexamined. We hypothesize plasma glucose levels would be increased in young healthy adults after carotid body activation via acute intermittent hypoxia. Methods Seven (6M/1F, 21–34 years) healthy, non‐obese, non‐diabetic, subjects without sleep apnea completed 2 visits randomized to 3‐hours of intermittent hypoxia or continuous normoxia. Oxygen saturation and ventilation (turbine ventilation flow transducer) were measured continuously. Plasma glucose levels and carotid body chemosensitivity (hypoxic ventilatory response, HVR) were measured before and after the 3‐hour protocol. Intermittent hypoxia consisted of 25‐s exposures to hypoxic gas (0.05 FiO 2 ) separated by 2‐min of room air breathing, resulting in 25‐events per hour in which oxygen saturation was significantly reduced (Normoxia: 97%, Hypoxia: 89%, p<0.01). Results No changes in plasma glucose (93 vs 93 mg/dL, p=0.92) or HVR (−0.35 vs. −0.39 L/min/%SpO 2, p=0.56) were observed after 3‐hours of continuous normoxia. In contrast, circulating glucose levels and chemosensitivity increased after intermittent hypoxia when compared to baseline (92 vs 97 mg/dL, p<0.01) and (−0.39 vs −0.60 L/min/%SpO 2 , p=0.04), respectively. Additionally, the rise in glucose was significantly correlated with the rise in carotid body chemosensitivity (R=0.38, p=0.05). Conclusion Circulating glucose levels increased after 3‐hours of intermittent hypoxia. The significant relationship between HVR and circulating glucose suggests that the carotid bodies may play a role in altered glucose handling. These findings may have important implications for the high prevalence of diabetes in patients with sleep apnea. Support or Funding Information DK90541, HL120570

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.

Background: Hypertension prevalence increases in females following menopause. Poor endothelial microvascular function increases risk of hypertension. Endothelial-mediated vasodilation is dependent on nitric oxide (NO). Estrogen enhances NO production; therefore, reduced estrogen levels in postmenopausal females may result in a reduced contribution of NO to endothelial-mediated vasodilation, although studies are limited. Herein, we aimed to investigate the contribution of NO to endothelial-mediated vasodilation in premenopausal and postmenopausal females. We hypothesized that postmenopausal females would have attenuated endothelium-mediated vasodilation and reduced contribution of NO compared to premenopausal females. Methods: Ten premenopausal (27±3 yrs) and eleven postmenopausal (59±4 yrs) normotensive females were studied. Resting brachial blood pressures were measured using a standard arm cuff. Brachial arterial catheters were placed for continuous blood pressure measurement and pharmacologic infusions. Forearm blood flow (FBF) was measured using venous occlusion plethysmography at baseline (saline) and during infusion of an endothelium-dependent vasodilator (acetylcholine; 1.0, 2.0, 4.0, and 8.0 μg/100mL tissue/min) with and without a NO synthase inhibitor (L-NG-monomethylarginine [L-NMMA]; 1.0 mg/min) co-infusion. Forearm vascular conductance (FVC) was calculated as FBF/mean arterial pressure✕100. Data were analyzed using a two-way repeated-measures ANOVA. Results: Resting systolic blood pressure (SBP) but not diastolic blood pressure (DBP) was greater in postmenopausal compared to premenopausal females (SBP: 121±15 vs. 103±9 mmHg, respectively; P<0.01; DBP: 72±11 vs. 65±5 mmHg, respectively; P=0.07). FBF responses to acetylcholine did not differ between premenopausal and postmenopausal females (Group Main Effect: P=0.11). When correcting for blood pressure, premenopausal females exhibited greater FVC responses to acetylcholine compared to postmenopausal females (Group Main Effect: P=0.04). In premenopausal females, FBF and FVC responses to acetylcholine were reduced with co-infusion of L-NMMA compared to acetylcholine infusion only (L-NMMA Main Effect: P=0.02 and P=0.01, respectively). In postmenopausal females, the FBF and FVC responses to acetylcholine were unaffected by L-NMMA (L-NMMA Main Effect: P=0.79 and P=0.76, respectively). Conclusion: Compared to premenopausal females, postmenopausal females have impaired endothelial microvascular function. Attenuated NO contribution during endothelium-mediated vasodilation in postmenopausal women may partially contribute to the increased hypertension risk in this population. AHA 14PRE18040000, AHA 898649, NIH HL083947, NIH HL148144, NCATS UL1 TR000135 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.

Insulin acts centrally to stimulate sympathetic vasoconstrictor outflow to skeletal muscle and peripherally to promote vasodilation. Given these divergent actions, the “net effect” of insulin on the transduction of muscle sympathetic nerve activity (MSNA) into vasoconstriction and thus, blood pressure (BP) remains unclear. We hypothesized that sympathetic transduction to BP would be attenuated during hyperinsulinemia compared with baseline. In 22 young healthy adults, MSNA (microneurography), and beat-to-beat BP (Finometer or arterial catheter) were continuously recorded, and signal-averaging was performed to quantify the mean arterial pressure (MAP) and total vascular conductance (TVC; Modelflow) responses following spontaneous bursts of MSNA at baseline and during a euglycemic-hyperinsulinemic clamp. Hyperinsulinemia significantly increased MSNA burst frequency and mean burst amplitude (baseline: 46 ± 6 au; insulin: 65 ± 16 au, P < 0.001) but did not alter MAP. The peak MAP (baseline: 3.2 ± 1.5 mmHg; insulin: 3.0 ± 1.9 mmHg, P = 0.67) and nadir TVC ( P = 0.45) responses following all MSNA bursts were not different between conditions indicating preserved sympathetic transduction. However, when MSNA bursts were segregated into quartiles based on their amplitudes at baseline and compared with similar amplitude bursts during hyperinsulinemia, the peak MAP and TVC responses were blunted (e.g., largest burst quartile: MAP, baseline: Δ4.4 ± 1.7 mmHg; hyperinsulinemia: Δ3.0 ± 0.8 mmHg, P = 0.02). Notably, ∼15% of bursts during hyperinsulinemia exceeded the size of any burst at baseline, yet the MAP/TVC responses to these larger bursts (MAP, Δ4.9 ± 1.4 mmHg) did not differ from the largest baseline bursts ( P = 0.47). These findings indicate that increases in MSNA burst amplitude contribute to the overall maintenance of sympathetic transduction during hyperinsulinemia.

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.

We examined the effect of intermittent hypoxia (IH, a hallmark feature of sleep apnea) on adipose tissue lipolysis and the role of endothelin-1 (ET-1) in this response. We hypothesized that IH can increase ET-1 secretion and plasma free fatty acid (FFA) concentrations. We further hypothesized that inhibition of ET-1 receptor activation with bosentan could prevent any IH-mediated increase in FFA. To test this hypothesis, 16 healthy male participants (32 ± 5 yr, 26 ± 2 kg/m2) were exposed to 30 min of IH in the absence (control) and presence of bosentan (62.5 mg oral twice daily for 3 days prior). Arterial blood samples for ET-1, epinephrine, and FFA concentrations, as well as abdominal subcutaneous adipose tissue biopsies (to assess transcription of cellular receptors/proteins involved in lipolysis), were collected. Additional proof-of-concept studies were conducted in vitro using primary differentiated human white preadipocytes (HWPs). We show that IH increased circulating ET-1, epinephrine, and FFA ( P < 0.05). Bosentan treatment reduced plasma epinephrine concentrations and blunted IH-mediated increases in FFA ( P < 0.01). In adipose tissue, bosentan had no effect on cellular receptors and proteins involved in lipolysis ( P > 0.05). ET-1 treatment did not directly induce lipolysis in differentiated HWP. In conclusion, IH increases plasma ET-1 and FFA concentrations. Inhibition of ET-1 receptors with bosentan attenuates the FFA increase in response to IH. Based on a lack of a direct effect of ET-1 in HWP, we speculate the effect of bosentan on circulating FFA in vivo may be secondary to its ability to reduce sympathoadrenal tone.

Objective Hyperinsulinemia elicits an increase sympathetic nervous system activity directed toward skeletal muscle (muscle sympathetic nerve activity, MSNA). The insulin‐mediated increase in MSNA is primarily attributed to its effect within the central nervous system. In addition to direct central effects, insulin also elicits peripheral vasodilation, which may further increase MSNA via arterial baroreflex‐mediated mechanisms in order to maintain blood pressure (BP). Herein we examined the contribution of insulin‐induced peripheral vasodilation and role for the arterial baroreflex in the MSNA response to hyperinsulinemia. We hypothesized that rescuing peripheral resistance with co‐infusion of the vasoconstrictor phenylephrine would attenuate the MSNA response to hyperinsulinemia. We further hypothesized the insulin‐mediated increase in MSNA would be recapitulated with another vasodilator (sodium nitroprusside, SNP). Methods In 33 healthy adults (28M/5F, 28±1 yrs, BMI 25.0±0.5 kg/m 2 ) MSNA (microneurography) and BP (Finometer/brachial catheter) were measured, and total peripheral resistance (TPR, ModelFlow) was calculated at rest and during intravenous insulin (n=20) or SNP (n=13) infusion. A subset of participants receiving insulin (n=7) were co‐infused with phenylephrine. Spontaneous arterial baroreflex sensitivity was determined over 5‐min at baseline and during pharmacological intervention(s). Results Insulin infusion increased MSNA (p=0.05) and decreased TPR (p<0.05), with no effects on arterial baroreflex sensitivity or BP (p>0.05). Co‐infusion with phenylephrine returned MSNA and TPR to baseline, with no effect on baroreflex sensitivity (p>0.05). Similar to insulin, SNP increased MSNA (p<0.01) and decreased TPR (p<0.05), with no effect on baroreflex sensitivity (p>0.05). Conclusions Present findings reveal that acute hyperinsulinemia increases MSNA without a change in arterial baroreflex sensitivity in healthy men and women. Insulin‐mediated increases in MSNA are lost with co‐infusion of phenylephrine and can be recapitulated with infusion of SNP. Results support a dynamic interplay between central and peripheral mechanisms during hyperinsulinemia to increase MSNA and maintain BP.

We hypothesized that elevation in muscle sympathetic nervous system activity (MSNA) during hyperinsulinemia is mediated by its peripheral vasodilator effect on the arterial baroreflex. Using three separate protocols in humans, we observed increases in both MSNA and cardiac output during hyperinsulinemia, which we attributed to the baroreflex response to peripheral vasodilation induced by insulin. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.