Increase In Sympathetic Nerve Activity Research Articles

Protein kinase C epsilon contributes to chronic mechanoreflex sensitization in rats with heart failure.

We investigated second-messenger signalling components linked to the stimulation of Gq protein-coupled receptors (e.g. thromboxane A2 and bradykinin B2 receptors) on the sensory endings of thin fibre muscle afferents in the chronic mechanoreflex sensitization in rats with myocardial infarction-induced heart failure with reduced ejection fraction (HF-rEF). We hypothesized that injection of either the inositol 1,4,5-trisphosphate (IP3) receptor antagonist xestospongin C (5µg) or the PKCε translocation inhibitor PKCe141 (45µg) into the arterial supply of the hindlimb would reduce the increase in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) evoked during 30s of 1Hz dynamic hindlimb muscle stretch in decerebrate, unanaesthetized HF-rEF rats but not sham-operated controls (SHAM). Ejection fraction was significantly reduced in HF-rEF (45 (19)%) compared to SHAM (80 (9)%; P<0.001) rats. In HF-rEF rats (n=3M/2F), IP3 receptor blockade had no effect on the peak ΔRSNA (pre: 99 (74)%; post: 133 (79)%; P=0.974) or peak ΔMAP response to stretch (peak ΔMAP: pre: 32 (14) mmHg; post: 36 (21) mmHg; P=0.719). Conversely, in another group of HF-rEF rats (n=4M/3F), the PKCε translocation inhibitor reduced the peak ΔRSNA (pre: 110 (77)%; post: 62 (58)%; P=0.029) and peak ΔMAP response to stretch (pre: 30 (20) mmHg; post: 17 (16) mmHg; P=0.048). In SHAM counterparts, neither drug affected the mechanoreflex responses. Our findings highlight PKCε, but not IP3 receptors, as a significant second-messenger in the chronic mechanoreflex sensitization in HF-rEF which may play a crucial role in the exaggerated sympathetic response to exercise in this patient population. KEY POINTS: Skeletal muscle contraction results in an exaggerated reflex increase in sympathetic nerve activity in heart failure patients with reduced ejection fraction (HF-rEF) compared to healthy individuals, contributing to increased cardiovascular risk and impaired tolerance for mild exercise. The exaggerated reflex sympathetic responses in HF-rEF may be attributed to a chronic sensitization of mechanically sensitive thin fibre muscle afferents mediated, at least in part, by stimulation of Gq protein-coupled thromboxane A2 and bradykinin B2 receptors on muscle afferent sensory endings. The specific Gq protein-linked signalling mechanisms that produce the chronic mechanoreflex sensitization in HF-rEF have not been investigated but may involve inositol 1,4,5-trisphosphate (IP3) receptors and/or protein kinase C epsilon (PKCε). Here we demonstrate that PKCε, but not IP3 receptors, within the sensory endings of thin fibre muscle afferents plays a role in the sensitization of mechanically sensitive thin fibre muscle afferents in rats with HF-rEF.

Palmitic Acid Induces Oxidative Stress and Senescence in Human Brainstem Astrocytes, Downregulating Glutamate Reuptake Transporters-Implications for Obesity-Related Sympathoexcitation.

Obesity has been associated with a chronic increase in sympathetic nerve activity, which can lead to hypertension and other cardiovascular diseases. Preliminary studies from our lab found that oxidative stress and neuroinflammation in the brainstem contribute to sympathetic overactivity in high-fat-diet-induced obese mice. However, with glial cells emerging as significant contributors to various physiological processes, their role in causing these changes in obesity remains unknown. In this study, we wanted to determine the role of palmitic acid, a major form of saturated fatty acid in the high-fat diet, in regulating sympathetic outflow. Human brainstem astrocytes (HBAs) were used as a cell culture model since astrocytes are the most abundant glial cells and are more closely associated with the regulation of neurons and, hence, sympathetic nerve activity. In the current study, we hypothesized that palmitic acid-mediated oxidative stress induces senescence and downregulates glutamate reuptake transporters in HBAs. HBAs were treated with palmitic acid (25 μM for 24 h) in three separate experiments. After the treatment period, the cells were collected for gene expression and protein analysis. Our results showed that palmitic acid treatment led to a significant increase in the mRNA expression of oxidative stress markers (NQO1, SOD2, and CAT), cellular senescence markers (p21 and p53), SASP factors (TNFα, IL-6, MCP-1, and CXCL10), and a downregulation in the expression of glutamate reuptake transporters (EAAT1 and EAAT2) in the HBAs. Protein levels of Gamma H2AX, p16, and p21 were also significantly upregulated in the treatment group compared to the control. Our results showed that palmitic acid increased oxidative stress, DNA damage, cellular senescence, and SASP factors, and downregulated the expression of glutamate reuptake transporters in HBAs. These findings suggest the possibility of excitotoxicity in the neurons of the brainstem, sympathoexcitation, and increased risk for cardiovascular diseases in obesity.

Functional-Molecular Mechanisms of Sympathetic-Parasympathetic Dysfunction in PVC-Induced Cardiomyopathy Revealed by Dual Stressor PVC-Exercise Challenge.

The significance of autonomic dysfunction in premature ventricular contraction-induced cardiomyopathy (PVC-CM) remain unknown. Utilizing a novel "dual stressor" provocative challenge combining exercise with premature ventricular contraction (PVCs), the authors characterized the functional and molecular mechanisms of cardiac autonomic (cardiac autonomic nervous system) remodeling in a PVC-CM animal model. In 15 canines (8 experimental, 7 sham), we implanted pacemakers and neurotelemetry devices and subjected animals to 12weeks of bigeminal PVCs to induce PVC-CM. Sympathetic nerve activity (SNA), vagal nerve activity (VNA), and heart rate were continuously recorded before, during, and after treadmill exercise challenge with and without PVCs, at baseline and after development of PVC-CM. Western blot and enzyme-linked immunosorbent assay were used to evaluate molecular markers of neural remodeling. Exercise triggered an increase in both SNA and VNA followed by late VNA withdrawal. With PVCs, the degree of exercise-induced SNA augmentation was magnified, whereas late VNA withdrawal became blunted. After PVC-CM development, SNA was increased at rest but failed to adequately augment during exercise, especially with PVCs, coupled with impaired VNA and heart rate recovery after exercise. In the remodeled cardiac autonomic nervous system, there was widespread sympathetic hyperinnervation and elevated transcardiac norepinephrine levels but unchanged parasympathetic innervation, indicating sympathetic overload. However, cardiac nerve growth factor was paradoxically downregulated, suggesting an antineurotrophic counteradaptive response to PVC-triggered sympathetic overload. Sympathetic overload, sympathetic dysfunction, and parasympathetic dysfunction in PVC-CM are unmasked by combined exercise and PVC challenge. Reduced cardiac neurotrophic factor might underlie the mechanisms of this dysfunction. Neuromodulation therapies to restore autonomic function could constitute a novel therapeutic approach for PVC-CM.

Mechanisms mediating muscle metaboreflex control of cardiac output during exercise: Impaired regulation in heart failure.

The ability to increase cardiac output during dynamic exercise is paramount for the ability to maintain workload performance. Reflex control of the cardiovascular system during exercise is complex and multifaceted involving multiple feedforward and feedback systems. One major reflex thought to mediate the autonomic adjustments to exercise is termed the muscle metaboreflex and is activated via afferent neurons within active skeletal muscle which respond to the accumulation of interstitial metabolites during exercise when blood flow and O2 delivery are insufficient to meet metabolic demands. This is one of the most powerful cardiovascular reflexes capable of eliciting profound increases in sympathetic nerve activity, arterial blood pressure, central blood volume mobilization, heart rate and cardiac output. This review summarizes the mechanisms meditating muscle metaboreflex-induced increases in cardiac output. Although much has been learned from studies using anaesthetized and/or decerebrate animals, we focus on studies in conscious animals and humans performing volitional exercise. We discuss the separate and interrelated roles of heart rate, ventricular contractility, ventricular preload and ventricular-vascular coupling as well as the interaction with other cardiovascular reflexes which modify muscle metaboreflex control of cardiac output. We discuss how these mechanisms may be altered in subjects with heart failure with reduced ejection fraction and offer suggestions for future studies.

The effects of dark chocolate on cognitive performance during cognitively demanding tasks: A randomized, single-blinded, crossover, dose-comparison study

Dark chocolate, rich in polyphenols, increases cerebral blood flow and improves cognitive function. This study aimed to determine whether the consumption of chocolate with a high concentration of polyphenols helps to maintain cognitive performance during cognitively demanding tasks. In this randomized, single-blinded, crossover, dose-comparison study, 18 middle-aged adults consumed two types of chocolate (25 g each), one with a high concentration (635.0 mg) and the other with a low concentration (211.7 mg) of cacao polyphenols, and performed a cognitive task requiring response inhibition and selective attention over two time periods (15–30 min and 40–55 min after consumption, respectively). Autonomic nerve function and subjective feelings, such as fatigue and concentration, were measured before food intake and after the second task to assess the participant's state. The results showed that the average reaction time between the first and second sessions was not significantly different for either high- or low-concentration chocolate consumption. However, the percentage of correct responses was similar in the first (96.7 %) and second (96.8 %) sessions for high-concentration chocolate consumption and significantly lower for low-concentration chocolate consumption in the second (96.4 %) session than in the first session (97.3 %). Autonomic nerve function showed a significant increase in sympathetic nerve activity after the second task with high-concentration chocolate consumption, while subjective feelings showed an increase in mental fatigue for both chocolate types but a significant decrease in concentration only after the second task with low-concentration chocolate consumption. These findings suggest that dark chocolate consumption contributes to the maintenance of performance and concentration in continuous and demanding cognitive tasks.

Selective Nrf2 Overexpression in the RVLM ameliorates Obesity-induced Sympathoexcitation

The rostral part of the ventrolateral medulla (RVLM) in the brainstem is an important region responsible for both the resting levels and the reflex control of sympathetic nerve activity (SNA). Oxidative stress-induced hyperactivation of the glutamatergic neurons in the RVLM has been associated with increases in SNA. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a major transcription factor ubiquitously expressed in all cell types and regulates the expression of several antioxidant and anti-inflammatory genes. Previous studies from our lab have demonstrated that obesity-induced sympathoexcitation was associated with Nrf2 dysfunction in the RVLM. Based on this, we hypothesized that Nrf2 overexpression at the level of the RVLM would reverse obesity-induced sympathoexcitation. To address this hypothesis, we selectively overexpressed Nrf2 at the level of the RVLM using a Nrf2 lentiviral vector (HIV-CamKIIa-GFP-Nrf2). This vector drives Nrf2 gene overexpression under the control of CamKIIa promoter that specifically targets the excitatory glutamatergic neurons. A control GFP vector (CamKIIa-GFP-virus) was used in the sham surgeries. For this experiment, two-month-old C57BL/6J mice (n=34, both sexes) were fed a high-fat diet (HFD; 60kcal% fat) or a chow diet (Control; 10kcal% fat) for 4-6 months. The animals were then subdivided into four subgroups (each group included an equal number of animals of both sexes): Chow + Control GFP lentiviral vector, HFD + Control GFP lentiviral vector, Chow + Nrf2 lentiviral vector, HFD + Nrf2 lentiviral vector. After 16 weeks of dietary treatment, the animals were microinjected with either control GFP or Nrf2 lentiviral vector in the RVLM. Two weeks after the microinjection surgery, radiotelemeters were implanted in the mice, and hemodynamic parameters were measured. Hexamethonium, a ganglionic blocker was injected into the animals to measure the drop in MAP, which is an indirect measurement of the whole-body SNA. Major findings from this study include a) As expected, HFD treatment increased systolic blood pressure (SBP), and mean arterial pressure (MAP). Nrf2 overexpression in the RVLM reversed SBP but not MAP in HFD mice when compared to control GFP vector group. b) Consistent with our earlier findings, animals fed with HFD diet showed a significant drop in blood pressure after hexamethonium injection compared to the control animals indicating HFD-induced increase in SNA. Nrf2 overexpression was also able to reduce the drop in MAP in response to hexamethonium in the HFD animals compared to control GFP vector group, suggesting Nrf2 overexpression attenuated obesity-mediated sympathoexcitation. These results indicate that Nrf2 upregulation in the RVLM can be a potential therapeutic option for obesity-induced increase in blood pressure and sympathoexcitation. Supported by: National Heart, Blood and Lung Institute grant R15HL148844 and American Heart Association Institutional Research Enhancement Award - 959725 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.

Mediobasal hypothalamic neurons contribute to the control of brown adipose tissue sympathetic nerve activity and cutaneous vasoconstriction

The mediobasal hypothalamus (MBH) contains heterogeneous neuronal populations that regulate food intake and energy expenditure. However, the role of MBH neurons in the neural control of thermoeffector activity for thermoregulation is not known. This study sought to determine the effects of modulating the activity of MBH neurons on the sympathetic outflow to brown adipose tissue (BAT), BAT thermogenesis, and cutaneous vasomotion. Pharmacological inhibition of MBH neurons by local administration of muscimol, a GABAA receptor agonist, reduced skin cooling-evoked BAT thermogenesis, expired CO2, body temperature, heart rate, and mean arterial pressure, while blockade of GABAA receptors by nanoinjection of bicuculline in the MBH induced large increases in BAT sympathetic nerve activity (SNA), BAT temperature, body temperature, expired CO2, heart rate, and cutaneous vasoconstriction. Neurons in the MBH send projections to neurons in the dorsal hypothalamic area and dorsomedial hypothalamus (DMH), which excite sympathetic premotor neurons in the rostral raphe pallidus area (rRPa) that control sympathetic outflow to BAT. The increases in BAT SNA, BAT temperature, and expired CO2 elicited by blockade of GABAA receptors in the MBH were reversed by blocking excitatory amino acid receptors in the DMH or in the rRPa. Together, our data show that MBH neurons provide a modest contribution to BAT thermogenesis for cold defense, while GABAergic disinhibition of these neurons produces large increases in the sympathetic outflow to BAT, and cutaneous vasoconstriction. Activation of glutamate receptors on BAT thermogenesis-promoting neurons of the DMH and rRPa is necessary for the increased sympathetic outflow to BAT evoked by disinhibition of MBH neurons. These data demonstrate neural mechanisms that contribute to the control of thermoeffector activity, and may have important implications for regulating body temperature and energy expenditure.

Rapid onset vasodilation in response to forearm muscle contraction in Hispanic American and Caucasian American Adults

Hispanic Americans (HA) are the fastest growing ethnic populations in the U.S., estimated to double in the next four decades. According to NHANES 2013-2016 data, prevalence of type 2 diabetes (T2D) is 15.1% in HA men and 14.1% in HA women, compared to 9.4 and 7.3% in non-Hispanic Caucasian American (CA) men and women, respectively. Even HA children have a 5-fold higher rate of T2D compared to their age-matched CA counterparts. Impaired vascular function is a hallmark of T2D however, limited number of studies have investigated the effect of ethnicity on vascular function at rest and during exercise. To our knowledge, only one study has reported a higher flow-mediated dilation at rest in healthy HA compared to CA adults. Hence, the purpose of this study was to investigate the rapid onset vasodilation in response to a single muscle contraction of the forearm at progressively increasing intensity (20, 40, and 60% of maximum voluntary contraction (MVC)) in young, healthy HA compared to age- and weight-matched CA individuals. A single muscle contraction model was chosen to isolate the robust local vasodilatory responsiveness of the vasculature without increases in sympathetic nerve activity or large hemodynamic changes. We hypothesized that HA would demonstrate an attenuated forearm vascular conductance (FVC) and forearm blood flow (FBF) in response to skeletal muscle contractions, compared to CA group. Young, healthy HA (n=4) and CA (n=4) adults performed skeletal muscle contractions at 20%, 40 % and 60% of their maximum voluntary contraction (MVC). To isolate the effect of mechanical compression during muscle contractions, FBF and FVC responses to a brachial cuff inflation were also measured. Heart rate (ECG), mean arterial pressure (MAP; finger photoplethysmography) and forearm blood flow (FBF; duplex Doppler ultrasound) were measured. FVC was calculated as FBF/MAP. Baseline FBF (CA: 115.21 ± 48.9 and HA: 68.89 ± 21.9 ml/min; p=0.27), MAP (CA: 82 ± 7.48 and HA: 81 ± 2.87 mmHg; p=0.77), FVC (p=0.15), and MVCs (CA: 67 ± 26.8 and HA: 70 ± 7.4 kg; p=0.82) were similar between the groups. Following single muscle contractions, both groups exhibited an intensity dependent increase in FVC and FBF, however, there was no difference between HA and CA groups. For instance, following muscle contraction at 40% MVC, peak FVC (%Δ) was 251% ± 46.35 in HA and 189% ± 34.98 in CA (race effect p = 0.23, time effect = 0.01, interaction p = 0.029). Similarly, peak %Δ FBF or %Δ FVC following 60% contractions were similar between the groups (FVC: 327% ± 84.36 in HA vs. 286% ± 74.45 in CA (race effect p=0.62, time effect p&lt;0.01, interaction effect p=0.01)). Both groups exhibited a similar response to brachial cuff inflation for peak FVC (p=0.21) and peak FBF (p=0.30). Our preliminary findings indicate that HA have a preserved rapid onset vasodilation in response to a single forearm contraction when compared to CA adults. Supported by Kinesiology and Health Education UT Austin Start-up Account 19-2635-91 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.

A Narrative Review of Commercial Platforms Offering Tracking of Heart Rate Variability in Corporate Employees to Detect and Manage Stress.

The COVID-19 pandemic has resulted in employees being at risk of significant stress. There is increased interest by employers to offer employees stress monitoring via third party commercial sensor-based devices. These devices assess physiological parameters such as heart rate variability and are marketed as an indirect measure of the cardiac autonomic nervous system. Stress is correlated with an increase in sympathetic nervous activity that may be associated with an acute or chronic stress response. Interestingly, recent studies have shown that individuals affected with COVID will have some residual autonomic dysfunction that will likely render it difficult to track both stress and stress reduction using heart rate variability. The aims of the present study are to explore web and blog information using five operational commercial technology solution platforms that offer heart rate variability for stress detection. Across five platforms we found a number that combined HRV with other biometrics to assess stress. The type of stress being measured was not defined. Importantly, no company considered cardiac autonomic dysfunction because of post-COVID infection and only one other company mentioned other factors affecting the cardiac autonomic nervous system and how this may impact HRV accuracy. All companies suggested they could only assess associations with stress and were careful not to claim HRV could diagnosis stress. We recommend that managers think carefully about whether HRV is accurate enough for their employees to manage their stress during COVID.

Leptin Increases: Physiological Roles in the Control of Sympathetic Nerve Activity, Energy Balance, and the Hypothalamic–Pituitary–Thyroid Axis

It is well established that decreases in plasma leptin levels, as with fasting, signal starvation and elicit appropriate physiological responses, such as increasing the drive to eat and decreasing energy expenditure. These responses are mediated largely by suppression of the actions of leptin in the hypothalamus, most notably on arcuate nucleus (ArcN) orexigenic neuropeptide Y neurons and anorexic pro-opiomelanocortin neurons. However, the question addressed in this review is whether the effects of increased leptin levels are also significant on the long-term control of energy balance, despite conventional wisdom to the contrary. We focus on leptin's actions (in both lean and obese individuals) to decrease food intake, increase sympathetic nerve activity, and support the hypothalamic-pituitary-thyroid axis, with particular attention to sex differences. We also elaborate on obesity-induced inflammation and its role in the altered actions of leptin during obesity.

Sleep Deprivation and Heart Rate Variability in Healthy Volunteers: Effects of REM and SWS Sleep Deprivation.

Using PSG-guided acute selective REM/SWS sleep deprivation in volunteers, this study examined the effects of sleep deprivation on the cardiovascular and autonomic nervous systems, as well as the relationship between cardiac neuromodulation homeostasis and cardiovascular disease. An experiment was conducted using 30 healthy volunteers (male : female = 1 : 1, aged 26.33 ± 4.5 years) divided into groups for sleep deprivation of SWS and REM sleep, and then, each group was crossed over for normal sleep (2 days) and repeated sleep deprivation (1 day, 3 times). During the study period, PSG and ELECTRO ECG monitoring were conducted, and five-minute frequency domain parameters and blood pressure values were measured before and after sleep deprivation. Changes in VLF, LFnu, LF/HF, HF, and HFnu after SWS sleep deprivation were statistically significant (P < 0.05), but not LF (P = 0.063). Changes in VLF, LF, HF, LF/HF, LFnu, and HFnu after REM sleep deprivation were not statistically significant (P > 0.05). An increase in sympathetic nerve activity results from sleep deprivation and sudden awakening from SWS sleep is associated with a greater risk of cardiovascular disease.

PS-BPB04-5: SELECTIVE ACTIVATION OF GLUTAMATERGIC NEURONS WITHIN THE SFO EVOKES A PRESSOR RESPONSE THAT IS DEPENDENT ON THE PVN

Objective: An overactive glutamatergic projection from the subfornical organ (SFO) to the paraventricular nucleus (PVN) is associated with the development of hypertension. Despite this, the functional relationship of this pathway under normotensive conditions remains poorly defined within the literature. Without this knowledge, it remains unknown if a dysfunction of this pathway is the underlying cause of hypertension or rather simply a consequence of the underlying hypertensive disease pathology. As such, the aim of this study is to identify the short-term function of the SFO and PVN pathway under normotensive conditions. Design and Methods: An optogenetic viral vector (pAAV-CaMKIIa-hChR2 (H134R)-EYFP) was microinjected into the SFO of Lewis rats (n = 5). Three weeks later, rats were anaesthetised and instrumented to record blood pressure and sympathetic nerve activity, administer intravenous drugs and to photo-stimulate glutamatergic SFO neurons. Results: Photostimulation of glutamatergic SFO neurons produced a pressor response that was abolished by inhibition of neuronal transmission within the PVN (9 ± 1mmHg vs 3 ± 1mmHg peak change from baseline, baseline vs PVN inhibition, n = 2 rats, 3 replicates per rat). In a separate cohort (n = 3 rats, 5 replicates per rat), we determined if the pressor response elicited was dependant on the sympathetic nervous system and/or the release of the vasoactive hormone vasopressin. Blockade of sympathetic vasomotor outflow attenuated the pressor response by 76%, whereas combined inhibition of sympathetic outflow and vasopressin 1a receptors attenuated the pressor response by 88%. Finally, photostimulation of glutamatergic SFO neurons led to a peak increase in sympathetic nerve activity (n = 2) that was abolished by phenylephrine administration, suggesting that the neuronal circuitry underpinning this response is barosensitive. Conclusions: This study demonstrates that the pressor response elicited by glutamatergic SFO neurons is dependent on the PVN and is largely mediated by the sympathetic nervous system via barosensitive neuronal circuitry. Chronic activation of this pathway will be undertaken to determine if an overactivation of this neuronal pathway induces hypertension in normotensive animals.

Mechanisms Of Mechanoreflex Activation In Rats With Heart Failure: Possible Mechanosensory Role Of ASIC1a

PURPOSE: Skeletal muscle contraction stimulates mechanically activated channels on sensory endings of muscle afferents contributing importantly to the reflex increases in sympathetic nerve activity (SNA). This reflex is termed the mechanoreflex and its activation in healthy populations plays a key role in meeting the metabolic demands associated with exercise. However, mechanoreflex activation in patients with heart failure (HF) results in exaggerated increases in SNA contributing to a reduced exercise capacity. A member of the family of acid sensing ion channels (ASICs) present on these sensory endings, namely ASIC1a, has demonstrated the surprising and largely under-appreciated ability to respond to mechanical stimulation. However, whether ASIC1a contributes to the activation of the exaggerated mechanoreflex in HF is unknown. Thus, we investigated the role played by ASIC1a in evoking the exaggerated mechanoreflex in HF rats with reduced ejection fraction (HF-rEF). We hypothesized that injection of the ASIC1a antagonist psalmotoxin-1 (PcTx-1; 100 ng, 0.1 mL saline) into the arterial supply of the hindlimb would reduce the reflex increase in renal SNA (RSNA, stainless steel electrodes) evoked during 30s static hindlimb muscle stretch (a model of mechanoreflex activation) in HF-rEF rats, but not healthy controls. METHODS: Experiments were performed on male Sprague-Dawley rats. We used the coronary artery ligation-induced myocardial infarction model of HF-rEF. Healthy control rats were subjected to a sham coronary artery ligation. At least six weeks after the initial surgery, in decerebrate, unanesthetized rats (SHAM, n = 6; ejection fraction 80 ± 3%; HF-rEF, n = 7; ejection fraction 52 ± 5%; P < 0.01) we compared the mechanoreflex before and after ASIC1a blockade. RESULTS: In SHAM rats, we found that ASIC1a blockade had no effect on the RSNA response evoked during hindlimb muscle stretch (∫ΔRSNA pre: 298 ± 90; post: 235 ± 64 a.u.; p = 0.58). In HF-rEF rats, ASIC1a blockade reduced the RSNA response evoked during hindlimb muscle stretch (∫ΔRSNA pre: 688 ± 220; post: 304 ± 140 a.u.; p = 0.03). CONCLUSIONS: In HF-rEF rats, ASIC1a demonstrates mechanosensory properties that are not present in healthy rats and, therefore, and may contribute to the exaggerated mechanoreflex present in HF-rEF.

Peripheral chemoreflex modulation of renal hemodynamics and renal tissue PO2 in chronic heart failure with reduced ejection fraction.

Aberrant carotid body chemoreceptor (CBC) function contributes to increased sympathetic nerve activity (SNA) and reduced renal blood flow (RBF) in chronic heart failure (CHF). Intermittent asphyxia (IA) mimicking sleep apnea is associated with additional increases in SNA and may worsen reductions in RBF and renal PO2 (RPO2) in CHF. The combined effects of decreased RBF and RPO2 may contribute to biochemical changes precipitating renal injury. This study sought to determine the role of CBC activity on glomerular filtration rate (GFR), RBF and RPO2 in CHF, and to assess the additive effects of IA. Furthermore, we sought to identify changes in gene expression that might contribute to renal injury. We hypothesized that GFR, RBF, and RPO2 would be reduced in CHF, that decreases in RBF and RPO2 would be worsened by IA, and that these changes would be ameliorated by CBC ablation (CBD). Finally, we hypothesized that CHF would be associated with pro-oxidative pro-fibrotic changes in renal gene expression that would be ameliorated by CBD. CHF was induced in adult male Sprague Dawley rats using coronary artery ligation (CAL). Carotid body denervation was performed by cryogenic ablation. GFR was assessed in conscious animals at the beginning and end of the experimental period. At 8-weeks post-CAL, cardiac function was assessed via echocardiography, and GFR, baseline and IA RBF and RPO2 were measured. Renal gene expression was measured using qRT-PCR. GFR was lower in CHF compared to sham (p < 0.05) but CBD had no salutary effect. RBF and RPO2 were decreased in CHF compared to sham (p < 0.05), and this effect was attenuated by CBD (p < 0.05). RBF and RPO2 were reduced to a greater extent in CHF vs. sham during exposure to IA (p < 0.05), and this effect was attenuated by CBD for RBF (p < 0.05). Downregulation of antioxidant defense and fibrosis-suppressing genes was observed in CHF vs. sham however CBD had no salutary effect. These results suggest that aberrant CBC function in CHF has a clear modulatory effect on RBF during normoxia and during IA simulating central sleep apnea.

The arcuate nucleus: A site of synergism between Angiotensin II and leptin to increase sympathetic nerve activity and blood pressure in rats

The action of leptin in brain to increase sympathetic nerve activity (SNA) and blood pressure depends upon functional Angiotensin II (AngII) type 1a receptors (AT1aR); however, the sites and mechanism of interaction are unknown. Here we identify one site, the hypothalamic arcuate nucleus (ArcN), since prior local blockade of AT1aR in the ArcN with losartan or candesartan in anesthetized male rats essentially eliminated the sympathoexcitatory and pressor responses to ArcN leptin nanoinjections. Unlike mice, in male and female rats, AT1aR and LepR rarely co-localized, suggesting that this interdependence occurs indirectly, via a local interneuron or network of neurons. ArcN leptin increases SNA by activating pro-opiomelanocortin (POMC) inputs to the PVN, but this activation requires simultaneous suppression of tonic PVN Neuropeptide Y (NPY) sympathoinhibition. Because AngII-AT1aR inhibits ArcN NPY neurons, we propose that loss of AT1aR suppression of NPY blocks leptin-induced increases in SNA; in other words, ArcN-AngII-AT1aR is a gatekeeper for leptin-induced sympathoexcitation. With obesity, both leptin and AngII increase; therefore, the increased AT1aR activation could open the gate, allowing leptin (and insulin) to drive sympathoexcitation unabated, leading to hypertension.

Protein Kinase C Epsilon Contributes to the Exaggerated Mechanoreflex in Rats with Heart Failure

BackgroundDuring exercise, skeletal muscle contraction stimulates mechanically sensitive channels on the sensory endings of thin fiber muscle afferents. Stimulation of these mechanically sensitive channels initiates a reflex, termed the mechanoreflex, which contributes importantly to reflex increases in sympathetic nerve activity (SNA) and mean arterial blood pressure (MAP). In patients and animals with heart failure with reduced ejection fraction (HF‐rEF), mechanoreflex activation contributes to exaggerated increases in SNA and is linked to decreased exercise tolerance and increased mortality. The exaggerated mechanoreflex in HF‐rEF is partially attributed to an increased responsiveness of thin fiber skeletal muscle afferents to mechanical stimulation. Several lines of evidence demonstrate that signaling within sensory neurons mediated by the non‐traditional protein kinase c epsilon (PKCε) isoform augments mechanically activated channel function and produces mechanical hyperalgesia. Whether PKCε‐mediated signaling within sensory neurons contributes the exaggerated mechanoreflex in HF‐rEF is unknown.Purpose &amp; HypothesisWe determined the role played by PKCε within sensory endings of thin fiber muscle afferents in evoking the mechanoreflex in rats with HF‐rEF and healthy control rats. We tested the hypothesis that injection of the selective PKCε inhibitor PKCε141 (44μg) into the arterial supply of the hindlimb would reduce the reflex increase in renal SNA (RSNA) and MAP evoked during 30 seconds of 1 Hz dynamic hindlimb muscle stretch (a model of mechanoreflex activation) in HF‐rEF rats, but not in healthy control rats.MethodsExperiments were performed on Sprague‐Dawley rats subjected to coronary artery ligation surgery to produce myocardial infarction and HF‐rEF or a sham ligation. At least six weeks after the initial surgery, in decerebrate, unanesthetized rats we compared (Student’s t‐tests) the mechanoreflex evoked by hindlimb skeletal muscle stretch before and after injection of PKCε141 into the arterial supply of the hindlimb.ResultsEjection fraction (determined via echocardiography) was significantly reduced in HF‐rEF rats (48±1%) compared to sham rats (80±2%, P&lt;0.01). Intra‐arterial PKCε141 injection reduced the pressor and RSNA response to hindlimb muscle stretch in HF‐rEF rats (n=6, 4M/2F, peak ΔMAP pre: 34±8; post: 19±7 mmHg; P=0.04; peak ΔRSNA pre: 121±32; post: 70±24%; P=0.04), but not SHAM rats (n=4, 3M/1F, peak ΔMAP pre: 14±4; post: 14±3 mmHg; P=0.87; peak ΔRSNA pre: 62±32; post: 58±26%; P=0.80) rats. In control experiments (n=3 HF‐rEF rats, 2M/1F), injection of PKCε141 into the jugular vein had no effect on the pressor or RSNA response to muscle stretch demonstrating a lack of central nervous system effects.ConclusionsThese data suggest that that PKCε signaling within the sensory neuron endings contributes to the exaggerated mechanoreflex in HF‐rEF and enhance the understanding of the sympathetic and cardiovascular adjustments to exercise in the ~6 million Americans with HF‐rEF.

Elimination of Senescent Cells Attenuates Obesity‐Induced Increase in Sympathetic Nerve Activity

Obesity is a major risk factor for the development of cardiovascular diseases like hypertension. It is estimated that obesity accounts for approximately 65‐78% of hypertension in adults. Accumulating evidence suggests that increase in sympathetic nerve activity (SNA) plays a crucial role in the pathophysiology of obesity‐induced hypertension. Previous studies from our lab have shown that obesity induces cellular senescence at the level of the rostral ventrolateral medulla (RVLM), which is responsible for basal and reflex control of sympathetic activity associated with cardiovascular function. Cellular senescence is a state of permanent, irreversible cell cycle arrest in proliferative cells. Senescent cells acquire senescence‐associated secretory phenotype (SASP) and secrete proinflammatory cytokines, chemokines and proteases. These SASP factors could serve as a potential source of neuroinflammation. Based on the previous studies, our hypothesis is that obesity‐induced senescence in the RVLM contributes to neuroinflammation and increases in SNA leading to the development of hypertension. To address this hypothesis, we eliminated senescent cells using a pharmacological approach (senolytic drug) and investigated its effect on sympathoexcitation and hypertension in obesity. Two‐month‐old male C57BL/6J mice were fed a high‐fat diet (HFD; 60% fat) or a chow diet (Controls; 10% fat) for 4‐6 months. After 4‐6 months of feeding, the two groups were again subdivided into two subgroups: Vehicle and a senolytic drug combination of Dasatinib+Quercetin (D+Q). So, the four different groups are Control + Vehicle, Controls + (D+Q), HFD + Vehicle, and HFD + (D+Q). Dasatinib (5 mg/kg) plus Quercetin (50 mg/kg) was prepared in a diluted solution comprising 10% ethanol, 30% polyethylene glycol 400, and 60% Phosal 50 PG. The intervention (Vehicle or D+Q) was performed for 3 consecutive days and for 2 cycles with a 2‐week interval between cycles. Hemodynamic parameters (Blood pressure &amp; Heart rate) and SNA were analyzed by radiotelemetry. As expected, the low frequency/high frequency heart rate variability (LF/HF HRV) ratio which is an indicator of SNA was significantly higher in HFD animals, which was reversed by senolytic drug treatment with D+Q. No changes were observed in the blood pressure. These results suggest that obesity‐induced cellular senescence causes sympathoexcitation which was reversed by the removal of senescent cells using senolytics. Future studies will investigate the mechanistic role of cellular senescence at the level of RVLM in obesity‐induced sympathoexcitation.

Systemic Interleukin‐1β Elicits Cyclooxygenase‐Independent Fever

Interleukin‐1β (IL‐1β) induces neuroimmune responses that help to fight infections. IL‐1β‐induced fever is not abolished by cyclooxygenase (COX) inhibitors used to treat inflammation and the neural pathways responsible for COX‐independent fever are still unknown. Considering that neurons that reside in the rostral raphe pallidus area (rRPa) and in the dorsomedial hypothalamus (DMH) play a key role in the control of thermoeffectors, we hypothesized that neurons in these areas contribute to COX‐independent fever elicited by intravenous (iv) IL‐1β. Female and male Sprague Dawley rats were anesthetized with urethane and α‐chloralose (ur/ch) or isoflurane (iso) and treated with saline or indomethacin, prior to receiving iv IL‐1β. Systemic IL‐1β increased brown adipose tissue (BAT) sympathetic nerve activity (SNA) (+2852 ± 2020% baseline (BL), p = 0.003), BAT temperature (TBAT; +2.6 ± 1.3°C, p &lt; 0.001), expired CO2 (+1.0 ± 0.4%, p &lt; 0.001), core body temperature (TCORE; +0.6 ± 0.2°C, p &lt; 0.001), heart rate (HR; +102 ± 42 bpm, p &lt; 0.001), mean arterial pressure (MAP; +11 ± 7 mmHg, p = 0.002), and paw temperature (TPAW; ‐6.9 ± 1.2°C, p &lt; 0.001), which is an index of cutaneous vasoconstriction (CVC). Indomethacin did not prevent IL‐1β‐evoked increases in BAT SNA (+2545 ± 756% BL, p &lt; 0.001), TBAT (+2.9 ± 1.3°C, p &lt; 0.001), expired CO2 (+1.2 ± 0.6%, p = 0.003), TCORE (+0.9 ± 0.5°C, p = 0.005), HR (+117 ± 47 bpm, p &lt; 0.001), MAP (+27 ± 20 mmHg, p = 0.01), or TPAW (‐5.9 ± 3.4°C, p = 0.02). Nanoinjection of muscimol in the rRPa prevented IL‐1β‐evoked increases in BAT SNA (p &lt; 0.01), TBAT (p &lt; 0.01), expired CO2 (p &lt; 0.01) and TCORE (p &lt; 0.01), but not in HR (p = 0.08), in comparison to the group that received IL‐1β only. Nanoinjection of muscimol in the rRPa in indomethacin‐pretreated rats prevented IL‐1β‐evoked increases in BAT SNA (p &lt; 0.01), TBAT (p &lt; 0.01), expired CO2 (p &lt; 0.01), TCORE (p &lt; 0.01), and TPAW (p = 0.03), but not in HR (p = 0.12), in comparison to an indomethacin‐pretreated group that received IL‐1β. Nanoinjections of glutamate receptor antagonists in the DMH markedly reduced IL‐1β‐evoked increases in BAT SNA (‐838 ± 542% BL, p = 0.03), TBAT (‐1.1 ± 0.4°C, p = 0.003), expired CO2 (‐0.4 ± 0.1%, p = 0.009) and HR (‐38 ± 20 bpm, p = 0.01), and reduced MAP (‐4 ± ‐2 mmHg, p = 0.02). Plasma PGE2 levels following iv IL‐1β were significantly higher (by ~3 fold) in rats that were not anesthetized when receiving IL‐1b (263.1 ± 136.4 pg/ml) compared to all other groups of rats that were maintained under anesthesia for 2 h (ur/ch anesthetized, saline pretreated: 96.7 ± 30.8 pg/ml, p = 0.007, or indomethacin pretreated: 119.7 ± 51.9 pg/ml, p = 0.02; iso anesthetized, saline pretreated: 82.5 ± 21.2 pg/ml, p = 0.003, or indomethacin‐pretreated: 108.4 ± 39.1 pg/ml, p = 0.01). Therefore, we suggest that the anesthetics used inhibited the systemic production of PGE2. Together, our data suggest that a glutamatergic input to the DMH is necessary for providing the excitatory drive for increasing BAT thermogenesis and tachycardia in response to circulating IL‐1β, likely via rRPa neurons. Additionally, IL‐1β‐evoked CVC requires activation of neurons in the rRPa. These data define critical components of the neural circuit for systemic IL‐1β‐induced fever and provide a foundation for elucidating additional brain mechanisms responsible for COX‐independent fever.

Acute Effect of Intermittent Hypoxia on Peripheral Vascular Function in Young Healthy Adults

Continuous exposure to hypoxia elicits a chemoreceptor‐mediated increase in sympathetic nerve activity and a simultaneous peripheral vasodilation due to an increase in nitric oxide production. The aim of the present study was to determine whether short, repeated bouts of hypoxia induce brachial artery vasodilation and acutely improve endothelial function. Ten young healthy adults (4 women, age: 24±3 years, height: 176±9 cm, body weight: 80±13 kg) visited the laboratory on two separate occasions. Endothelium‐dependent vasodilation was assessed by brachial artery flow‐mediated dilation using a semiautomated diagnostic ultrasound system before and after exposure to either intermittent hypoxia (IH) or intermittent normoxia (IN). Intermittent hypoxia consisted of eight 4‐min hypoxic cycles at a targeted arterial oxygen saturation of 80% interspersed with breathing room air to resaturation, and intermittent normoxia consisted of eight 4‐min normoxic cycles separated by one minute of breathing room air. Air was made hypoxic by titrating nitrogen into a breathing circuit. Brachial artery diameter was assessed via ultrasound throughout the protocols. Hemodynamics, arterial oxygen saturation, and pulmonary gas exchange were also continuously assessed during the protocols. By design, intermittent hypoxia resulted in an arterial oxygen saturation of 81±2%, corresponding to oxygen levels of 11.9±2.2%. Exposure to intermittent hypoxia, but not intermittent normoxia, elicited a brachial artery vasodilation (IH: 0.38±0.06 to 0.40±0.06 vs. IN: 0.38±0.06 to 0.38±0.06 cm, p&lt;0.01). However, neither intermittent hypoxia nor intermittent normoxia acutely affected brachial artery flow‐mediated dilation (IH: 5.8±1.9 to 5.3±1.6% vs. IN: 6.1±2.4 to 6.5±1.7%, p=0.96). Intermittent hypoxia increased cardiac output (5.2±0.9 to 5.9±1.4 L·min‐1, p=0.03) mainly due to an increase in heart rate (57±9 to 68±11 bpm, p&lt;0.01). In conclusion, exposure to intermittent hypoxia triggered a brachial artery vasodilation but did not affect endothelium‐dependent vasodilation in young healthy adults. Multiple sessions of intermittent hypoxia may be necessary to improve nitric oxide synthesis and endothelial function.

Novel mechanosensory role for acid sensing ion channel subtype 1a in evoking the exercise pressor reflex in rats with heart failure.

Mechanical and metabolic signals associated with skeletal muscle contraction stimulate the sensory endings of thin fibre muscle afferents, which, in turn, generates reflex increases in sympathetic nerve activity (SNA) and blood pressure (the exercise pressor reflex; EPR). EPR activation in patients and animals with heart failure with reduced ejection fraction (HF-rEF) results in exaggerated increases in SNA and promotes exercise intolerance. In the healthy decerebrate rat, a subtype of acid sensing ion channel (ASIC) on the sensory endings of thin fibre muscle afferents, namely ASIC1a, has been shown to contribute to the metabolically sensitive portion of the EPR (i.e. metaboreflex), but not the mechanically sensitive portion of the EPR (i.e. the mechanoreflex). However, the role played by ASIC1a in evoking the EPR in HF-rEF is unknown. We hypothesized that, in decerebrate, unanaesthetized HF-rEF rats, injection of the ASIC1a antagonist psalmotoxin-1 (PcTx-1; 100ng) into the hindlimb arterial supply would reduce the reflex increase in renal SNA (RSNA) evoked via 30s of electrically induced static hindlimb muscle contraction, but not static hindlimb muscle stretch (model of mechanoreflex activation isolated from contraction-induced metabolite-production). We found that PcTx-1 reduced the reflex increase in RSNA evoked in response to muscle contraction (n=8; mean (SD) ∫ΔRSNA pre: 1343 (588) a.u.; post: 816 (573) a.u.; P=0.026) and muscle stretch (n=6; ∫ΔRSNA pre: 688 (583) a.u.; post: 304 (370) a.u.; P=0.025). Our data suggest that, in HF-rEF rats, ASIC1a contributes to activation of the exercise pressor reflex and that contribution includes a novel role for ASIC1a in mechanosensation that is not present in healthy rats. KEY POINTS: Skeletal muscle contraction results in exaggerated reflex increases in sympathetic nerve activity in heart failure patients compared to healthy counterparts, which likely contributes to increased cardiovascular risk and impaired tolerance for even mild exercise (i.e. activities of daily living) for patients suffering with this condition. Activation of acid sensing ion channel subtype 1a (ASIC1a) on the sensory endings of thin fibre muscle afferents during skeletal muscle contraction contributes to reflex increases in sympathetic nerve activity and blood pressure, at least in healthy subjects. In this study, we demonstrate that ASIC1a on the sensory endings of thin fibre muscle afferents plays a role in both the mechanical and metabolic components of the exercise pressor reflex in male rats with heart failure. The present data identify a novel role for ASIC1a in evoking the exercise pressor reflex in heart failure and may have important clinical implications for heart failure patients.