Limberg, Jacqueline K.

Sex-related differences in respiratory modulation of sympathetic activity have been observed in rodent models of sleep apnea [intermittent hypoxia (IH)]. In light of sex disparities in the respiratory response to acute IH in humans as well as changes in respiratory modulation of muscle sympathetic nerve activity (MSNA) in clinical sleep apnea, we examined sex-related differences in respiratory modulation of MSNA following acute IH. We hypothesized that respiratory modulation of MSNA would be altered in both male and female participants after IH; however, the respiratory patterning of MSNA following IH would be sex specific. Heart rate, MSNA, and respiration were evaluated in healthy male ( n = 21, 30 ± 5 yr) and female ( n = 10, 28 ± 5 yr) participants during normoxic rest before and after 30 min of IH. Respiratory modulation of MSNA was assessed by fitting polynomials to cross-correlation histograms constructed between sympathetic spikes and respiration. MSNA was elevated after IH in male (20 ± 6 to 24 ± 8 bursts/min) and female (19 ± 8 to 22 ± 10 bursts/min) participants ( P < 0.01). Both male and female participants exhibited respiratory modulation of MSNA ( P < 0.01); however, the pattern differed by sex. After IH, modulation of MSNA within the breath was reduced in male participants ( P = 0.03) but increased in female participants ( P = 0.02). Both male and female adults exhibit changes in respiratory patterning of MSNA after acute IH; however, this pattern differs by sex. These data support sex disparities in respiratory modulation of MSNA and may have implications for conditions such as sleep apnea.

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 .

PURPOSE Sympathetic nervous system responses to voluntary apnea are increased by hypoxia and attenuated by hyperoxia. However, whether neural firing and recruitment strategies in response to apnea can be modified by hypoxia and/or hyperoxia were previously unknown. We examined the influence of chemoreceptor stimuli on sympathetic neural firing and recruitment strategies during voluntary end‐expiratory apnea. We hypothesized increases in the firing frequency and probability of low‐threshold axons during apnea would be exaggerated during hypoxia and attenuated during hyperoxia. METHODS Multi‐unit muscle sympathetic nerve activity (MSNA, microneurography) was measured in 10 young, healthy men (31±2 yrs, 25±1 kg/m 2 ). Data were collected at baseline and during maximal voluntary end‐expiratory apnea under normoxic (S p O 2 : 98±1%), hypoxic (F i O 2 : 0.17±0.01; S p O 2 : 82±1%), and hyperoxic (F i O 2 : 0.92±0.03; S p O 2 : 100±0%) conditions. Condition order was randomized and trials were separated by a minimum of 15‐min quiet normoxic rest. Action potential (AP) patterns were studied from the filtered raw signal using wavelet‐based methodology. Data are reported from the last 2‐min of baseline and during the second half of apnea, reflecting the time of highest MSNA. RESULTS There was an increase in multi‐unit MSNA (24±3 to 35±3 bursts/min, p=0.04) during normoxic apnea, which was due to an increase in the frequency and incidence of AP spikes (243±75 to 519±134 AP/min, p=0.048; 412±133 to 773±185 AP/100 heart beats, p=0.04). An increase in the probability of AP firing more than once per burst was also observed (p=0.01). There was an increase in multi‐unit MSNA (20±4 to 46±7 bursts/min, p<0.01) during hypoxic apnea that was due to an increase in the frequency and incidence of AP spikes (192±59 to 952±266 AP/min, p<0.01; 326±89 to 1212±327 AP/100 heart beats, p<0.01). Hypoxic apnea also resulted in an increase in the probability of AP firing more than once per burst (p<0.01). Hyperoxia attenuated any increase in MSNA with apnea, such that no changes in multi‐unit MSNA (24±4 to 28±5 bursts/min, p=0.14) nor frequency or incidence of AP spikes (155±35 to 253±71 AP/min, p=0.12; 249±61 to 502±188 AP/100 heart beats, p=0.28) were observed. Hyperoxia also attenuated any increase in the probability of AP firing more than once per burst (p=0.47). CONCLUSION These data are the first to show neural firing and recruitment strategies in response to voluntary end‐expiratory apnea are modified by hypoxia and hyperoxia. Our results demonstrate that hypoxic apnea increases the frequency and incidence of action potential spikes, as well as the probability of multiple within‐burst firing; whereas this response was suppressed when individuals were hyperoxic. These data may have important implications for neural control of the circulation in recreational activities (i.e. diving) and/or clinical conditions prone to apnea (i.e. sleep apnea). Support or Funding Information FUNDING: AHA #15SDG25080095, NIH HL130339, Natural Sciences and Engineering Research Council of Canada Discovery Grant program. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

The key vascular signals regulating muscle blood flow during exercise are unclear. We tested the hypothesis that in young adults, leg blood flow (LBF) during exercise would be reduced independently by pharmacological inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX). In 13 young adults, we measured heart rate (ECG), blood pressure (femoral artery and vein), blood gases, and venous oxygen saturation (indwelling oximeter) during steady‐state dynamic (60/min) knee extension exercise. LBF was determined by Doppler ultrasound of the femoral artery. Exercise (20W) increased leg vascular conductance (LVC) from 4± 1 to 33± 3 ml/min/100mmHg. The NOS inhibitor L‐NAME reduced LVC by ∼10% to 30± 1. Infusion of the COX inhibitor, ketoralac (n=9), increased LVC from 30± 1 to 34± 2. L‐NAME infusion reduced venous oxygen saturation from 48 to 38%, suggesting greater extraction to maintain oxygen consumption. Control intra‐arterial infusions of acetylcholine (ACH) and nitroprusside (NTP) tested inhibitor efficacy. The change in LVC to two doses of ACh was blunted 53 and 15%, by L‐NAME+ketorolac infusion. In contrast, the LVC responses to NTP were unaltered. These data implicate a modest role for NO‐mediated leg exercise hyperemia in adults and a potentially modest LBF restraint by vasoconstrictor prostaglandins. Funded by the NIH HL‐46493 and the Minnesota Obesity Center.

Patients with sleep apnea exhibit ventilatory instability and increased carotid chemoreceptor activity. Whether ventilatory instability is directly related to altered chemoreceptor activity has not been examined in humans. Therefore we tested the hypothesis that acute activation of the carotid chemoreflex with hypoxia (10% O 2 ) would increase breathing variability (assessed by coefficient of variation of tidal volume {%CV}) and blunting the peripheral chemoreflex with IV dopamine would attenuate the response. To test this hypothesis, nine healthy subjects (3 M/6 F) were studied on two separate study days: 1) saline or 2) low‐dose dopamine infusion. Individualized dopamine dose (1‐4 mg/kg/min), that maximally blunted chemoreflex sensitivity, was identified during a separate screening visit. Respiratory rate (RR), tidal volume (TV), end tidal CO 2 (ETCO 2 )and O 2 (ETO 2 ) were measured. Baseline measures were similar between days. Saline infusion did not affect RR, TV, end tidal gases, or %CV. However, dopamine infusion significantly increased %CV (25 ± 5 to 32 ± 6 %; p<0.05) and ETCO 2 (0.3 ± 0.1 %; p<0.0001) without affecting RR, or TV. Hypoxia increased %CV during saline infusion (18 ± 3 to 29 ± 3%; p<0.05), and this effect was blunted during dopamine infusion (31 ± 6 to 38 ± 6%; p>0.05). These data indicate hypoxia increases respiratory variability and this effect is mediated by the carotid chemoreceptors. Furthermore, the effect of increased chemoreceptor drive on ventilatory instability, such as that seen in sleep apnea, may be blunted by low‐dose dopamine in humans. These data provide a potential therapeutic target for improving ventilatory instability in human disease. Funding:NIH DK090541

Background Hypoglycemia results in a reduction in spontaneous cardiac baroreflex sensitivity (scBRS) and a shift in the baroreflex working range to higher heart rates; this effect may be mediated by the carotid chemoreceptors. We hypothesized changes in baroreflex control of heart rate with hypoglycemia would be blunted in carotid body resected patients when compared with healthy controls. Methods Five patients (3M/2F) with bilateral carotid body resection for glomus tumors and ten healthy controls (7M/3F) completed a 180‐min hyperinsulinemic, hypoglycemic (~3.3 µmol/mL) clamp. Changes in heart rate, blood pressure, and scBRS were assessed. Results scBRS was reduced from baseline during hypoglycemia (p<0.01) and was lower in carotid body resected patients when compared with controls (p=0.01). Hypoglycemia‐mediated reductions in systolic and mean blood pressure were significantly greater in patients when compared with controls (p<0.05). Despite lower blood pressures, increases in heart rate with hypoglycemia were blunted in patients (p<0.01). Conclusion Reductions in scBRS with hypoglycemia cannot be exclusively attributed to the carotid chemoreceptors; however, the chemoreceptors play a role in resetting the baroreflex working range to higher heart rates during hypoglycemia such that a rise in heart rate is not observed in carotid body resected patients. These data support a contribution of the carotid chemoreceptors to baroreflex control of blood pressure during hypoglycemia and highlight potential widespread effects of carotid body resection in humans. Funding DK090541, HL120570

Activation or inhibition of the carotid body chemoreceptors (CBCs) may impact cardiac baroreflex sensitivity (cBRS). The purpose of this study was to test the hypothesis that inhibition of the CBCs will elicit a change in cBRS when compared to normal resting conditions. Young, healthy subjects (n=16, 31±2, 6 women) were given IV saline and a range of IV dopamine (DA) (1‐4 µg/kg/min) for 15 minutes prior to and throughout five hypoxic ventilatory response (HVR) tests. Subjects were initially infused with IV saline. After each HVR, the DA infusion rate was increased by 1 µg/kg/min. A 3‐lead EKG and beat‐by‐beat blood pressure (NexFin) were used to determine cBRS. Data were analyzed from the 5 minutes prior to the HVR associated with the DA dose that elicited the greatest attenuation of the HVR when compared to saline. Heart rate was not significantly different during saline vs. DA (62±3 vs. 65±3, p=0.25). Diastolic blood pressure (77±2 vs. 72±1 mmHg), mean arterial pressure (98±2 vs. 92±2 mmHg), and cBRS “up” sequence (38±7 vs. 21±3 ms/mmHg) were significantly lower during saline vs. DA (p<0.05 for all). cBRS values from the RRI spectrum were not statistically distinguishable between saline and DA (26±4 vs. 21.1±3, p=0.06). In summary, inhibiting the CBCs via DA infusion in young, healthy subjects results in a fall in blood pressure and a reduction in cBRS in response to a rise in blood pressure. These data suggest the CBCs contribute to cBRS and play an important role in modulating parasympathetic control of blood pressure. Funding: NIH R01DK090541

There are known interactions between the chemoreflex and baroreflex. The purpose of the present investigation was to examine the contribution of the carotid body chemoreceptors to changes in baroreflex control of blood pressure with exposure to hypoxia. Low‐dose dopamine blunts carotid body afferent activity and can thus be used to examine the contribution of the carotid chemoreceptors to reflex responses during hypoxia. Hypothesis Spontaneous cardiac baroreflex sensitivity (scBRS) will be reduced with hypoxia and this effect will be blunted when the carotid chemoreceptors are desensitized with low‐dose dopamine. Methods Seven subjects (4M/3F) completed 2 visits randomized to IV dopamine (individualized dose shown to result in a nadir hypoxic ventilatory response, 1‐4 μg/kg/min) or placebo (saline). Subjects were exposed to 5‐min normoxia (99±0% SpO2), followed by 5‐min hypoxia (83±2%SpO2). Blood pressure (intra‐arterial catheter) and heart rate (ECG) were measured continuously and sCBRS was assessed by spectrum analysis. The relative (%) change in sCBRS with hypoxia [(Hypoxia‐Normoxia)/Normoxia x 100)] was compared between dopamine and placebo. Results scBRS was reduced with hypoxia (saline p<0.01; dopamine p<0.01). The fall in scBRS with hypoxia was blunted with dopamine (saline: ‐58±7 %, dopamine: ‐42±9 %, p=0.02). Conclusion These data are consistent with a reduction in scBRS with chemoreceptor activation. Furthermore, we show this effect is partially mediated by the carotid body chemoreceptors, given the fall in scBRS is attenuated when the chemoreceptors are desensitized with low‐dose dopamine. Funding NIH DK090541 (Joyner)

Background In models of congestive heart failure, carotid body denervation decreases ventilatory oscillations, thereby increasing ventilatory stability. Low‐dose dopamine can acutely blunt carotid body chemosensitivity assessed by a reduction of the hypoxic ventilatory response (HVR), but whether it affects ventilatory stability was previously unknown. We hypothesized ventilatory oscillations [as assessed by coefficient of variation of tidal volume (%CV V t ) and breathing frequency (%CV F b )] would be reduced when the carotid chemoreceptors were desensitized by low‐dose dopamine. Methods Healthy adults (n=20, 6F/14M, 31±2 years) rested quietly for 5‐min and breath‐by‐breath ventilation was measured during normoxia under 2 IV infusion conditions: 1) saline, 2) an individualized dose of dopamine (1‐4 ug/kg/min) shown to result in a nadir HVR. Results Dopamine infusion increased breathing frequency (13±1 to 14±1 breaths/min, p=0.03) with no change in mean tidal volume (740±65 to 700±68 mL, p=0.18). Infusion of dopamine in young healthy adults increased ventilatory oscillations (%CV V t : saline = 21±2, dopamine = 30±4, p=0.01; %CV F b : saline = 17±1, dopamine = 21±2, p=0.07). There was no relationship between the decrease in HVR and the increase in ventilatory oscillations (V t : r=0.15, p=0.54; F b : r=0.33, p=0.16). Conclusion Contrary to our hypothesis, IV dopamine (at an individualized dose that attenuates HVR) significantly increases ventilatory oscillations in healthy subjects. These data suggest the effect of low‐dose dopamine on ventilatory oscillations in healthy adults is in contrast to that seen with carotid body denervation in disease. Funding : NIH DK090541

Objective Blood pressure (BP) reactivity to stress is predictive of the development of cardiovascular disease. Cardiovascular reactivity is attenuated in individuals after bariatric surgery; however, the underlying mechanisms are not completely understood. We hypothesized the BP response at the onset of isometric handgrip exercise would occur earlier and to a lesser degree in individuals who underwent bariatric surgery when compared to obese adults and the reliance on total peripheral resistance (TPR) would be attenuated. Methods Twenty‐six individuals [7 non‐obese (28±3 yrs, 23±1 kg/m 2 ), 11 obese (33±2 yrs, 39±1 kg/m 2 ), 8 post‐bariatric surgery (35±2 yrs, 29±1 kg/m 2 )] completed isometric handgrip exercise (40% maximum voluntary contraction) to exhaustion. Heart rate (HR, ECG) and arterial BP (brachial catheter) were measured continuously. Stroke volume 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 first 30‐s of exercise. Results Compared to non‐obese controls, obese adults exhibited a slower initial rise in BP (9.3± 3.9 vs 21.7±2.5 s) and higher peak BP change (Δ6.7±2.1 vs Δ11.9±2.1 mmHg) at the onset of isometric handgrip exercise (p<0.05). Peak BP and the rate of rise were not different between individuals who underwent bariatric surgery (14.0±4.1 s, Δ5.6±1.3 mmHg) and non‐obese controls (p>0.05). Non‐obese controls exhibited an exercise‐mediated increase in CO whereas obese adults increased TPR (p<0.05). The increases in CO and TPR were less apparent in individuals who underwent bariatric surgery (p>0.05). Low cardiac baroreflex sensitivity (BRS) and heart rate variability (HRV: SDNN, standard deviation of normal to normal R‐R intervals) at rest were related to smaller increases in CO (BRS: R=0.47, p<0.01; SDNN: R=0.57, p<0.01) and greater increases in TPR (BRS: R=−0.49, p=0.01; SDNN: R=−0.60, p<0.01) during exercise. Conclusions In contrast to obese adults, individuals who underwent bariatric surgery exhibit a rapid rise in BP at the onset of isometric handgrip exercise. This rapid increase in BP is associated with a fall in TPR and results in lower peak BP at the onset of isometric exercise. These data suggest bariatric surgery attenuates BP reactivity to physical stress via improvements in the time‐course of hemodynamic responses and reflex autonomic adjustments. Support or Funding Information NIH HL130339, NIH HL083947, NIH DK82424, NIH UL1 RR024150, Mayo Clinic Department of Anesthesiology, Mayo Foundation for Medical Education and Research This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .