Heart rate deceleration capacity as a marker of perioperative risk: identifying relevant patient phenotypes surgical procedures.

Loss of regulation of the autonomic nervous system is found in many diseases from the age of 50 to 60 yr and even more so in older patients. The imbalance is usually manifested by an increase in sympathetic tone, long considered to be the most deleterious element in terms of cardiac rhythmic risk, but also by a reduction in the effectiveness of short-term regulation of the baroreflex arc (partial loss of parasympathetic control). Techniques for analysing this autonomic disorder by analysing heart rate regulation are widely available in outpatient clinics and provide interesting indicators of cardiovascular and cerebrovascular risk. Deceleration capacity of cardiac autonomic control has been identified for its prognostic role in high-risk patients and in the general population. Further research is indicated to assess the value of this marker in anaesthetic risk management by targeting procedures with greater risk of intraoperative and postoperative autonomic dysfunction.

Rhythmic firing of neurons in the medulla of conscious freely behaving rats: rhythmic coupling with baroreceptor input.

Recent investigations emphasized the importance of neural control of cardiovascular adjustments in complex behaviors, including stress, exercise, arousal, sleep-wake states, and different tasks. Baroreceptor feedback is an essential component of this system acting on different time scales from maintaining stable levels of cardiovascular parameters on the long-term to rapid alterations according to behavior. The baroreceptor input is essentially rhythmic, reflecting periodic fluctuations in arterial blood pressure. Cardiac rhythm is a prominent feature of the autonomic control system, present on different levels, including neuron activity in central circuits. The mechanism of rhythmic entrainment of neuron firing by the baroreceptor input was studied in great detail under anesthesia, but recordings of sympathetic-related neuron firing in freely moving animals remain extremely scarce. In this study, we recorded multiple single neuron activity in the reticular formation of the medulla in freely moving rats during natural behavior. Neurons firing in synchrony with the cardiac rhythm were detected in each experiment (n = 4). In agreement with prior observations in anesthetized cats, we found that neurons in this area exhibited high neuron-to-neuron variability and temporal flexibility in their coupling to cardiac rhythm in freely moving rats, as well. This included firing in bursts at multiples of cardiac cycles, but not directly coupled to the heartbeat, supporting the concept of baroreceptor input entraining intrinsic neural oscillations rather than imposing a rhythm of solely external origin on these networks. It may also point to a mechanism of maintaining the basic characteristics of sympathetic neuron activity, i.e., burst discharge and cardiac-related rhythmicity, on the background of behavior-related adjustments in their firing rate.

Effect of maturation on parasympathetic modulation during exercise and recovery.

OBJECTIVES: This study examined the effect of maturation on parasympathetic nervous system (PNS) response from rest to light- to moderate-intensity exercise and recovery from maximal exercise in pre- (n = 10; maturity offset = -3.0 ± 1.2 years; age = 10.1 ± 1.9 years), mid- (n = 9; maturity offset = -0.1 ± 0.6 years; age = 13.7 ± 1.0 years), and postpubertal (n = 10; maturity offset = 1.9 ± 0.6 years; age = 15.6 ± 1.2 years) boys and men (n = 10; age = 24.1 ± 2.0 years). DESIGN: Participants completed seated rest, light-intensity exercise (50% HRmax), and moderate-intensity exercise (65% HRmax). Following moderate-intensity exercise, intensity was ramped to elicit maximal HR and followed by 25 min of seated recovery. Log transformed values for root mean square of successive differences (lnRMSSD), high-frequency power (lnHF) and normalized HF power (lnHFnu) assessed PNS modulation during 3 min of rest, light-intensity exercise, moderate-intensity exercise, and 3-min epochs throughout recovery. RESULTS: During light-intensity exercise, lnRMSSD and lnHF were greater in prepubertal (lnRMSSD = 3.4 ± 0.3 ms; lnHF = 5.4 ± 0.7 ms2) compared to men (lnRMSSD = 2.8 ± 0.5 ms; lnHF = 4.0 ± 0.9 ms2). During moderate-intensity exercise, lnHF differed between prepubertal and men (2.8 ± 1.0 vs. 1.4 ± 1.0 ms2). During recovery, HRV variables were greater in prepubertal compared to postpubertal and men. CONCLUSIONS: Prepubertal boys have reduced PNS withdrawal during light-intensity exercise and greater PNS reactivation following exercise.

Associations between 24-h heart rate variability and aerobic fitness in high-level female soccer players.

OBJECTIVE: The aim of this study was to investigate the changes in 24-h heart rate variability and aerobic fitness, and their associations, in female soccer players during the preseason period. METHODS: Sixteen players were assessed (24-h HRV and Yo-Yo Intermittent Recovery Test, level 1 [YYIR1]) before and after 4 weeks of preseason. The relationship between R-R24h length and high-frequency oscillations (HF24h) was analyzed by a quadratic regression model (revealing or not saturation of vagal activity) assessed 48-h before (PRE-preseason) and 48-h after (POST-preseason) the preseason period. Additionally, the mean HF24h was calculated from the linear portion of the R-R interval versus the HF24h regression curve (HF index). The average of the corresponding R-R24h values was defined as the R-R index. RESULTS: In PRE-preseason, seven players had a saturated HF24h, while in POST-preseason, five new cases of saturated HF24h were observed. The mean R-R24h, HF24h, R-R index, and HF index lengths significantly increased after preseason (p < 0.001). Significant differences were found in YYIR1 PRE- compared with POST-preseason (930 ± 286 m [individual range: 400-1240 m] versus 1265 ± 252 m [640-1640 m], respectively; p < 0.001). Additionally, the relative changes in HF24h and HF index were largely correlated with improvements in the distance covered during the YYIR1 (r = 0.68 and r = 0.56; respectively). CONCLUSION: Enhanced vagal activity after 4-week preseason period of soccer training increased the occurrence of vagal saturation in high-level female soccer players. Additionally, the increases in HF24h and HF index were significantly correlated with aerobic fitness change.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour.

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Promotion of vascular integrity in sepsis through modulation of bioactive adrenomedullin and dipeptidyl peptidase 3.

Sepsis represents one of the major medical challenges of the 21st century. Despite substantial improvements in the knowledge on pathophysiological mechanisms, this has so far not translated into novel adjuvant treatment strategies for sepsis. In sepsis, both vascular tone and vascular integrity are compromised, and contribute to the development of shock, which is strongly related to the development of organ dysfunction and mortality. In this review, we focus on dipeptidyl peptidase 3 (DPP3) and adrenomedullin (ADM), two molecules that act on the vasculature and are involved in the pathophysiology of sepsis and septic shock. DPP3 is an ubiquitous cytosolic enzyme involved in the degradation of several important signalling molecules essential for regulation of vascular tone, including angiotensin II. ADM is a key hormone involved in the regulation of vascular tone and endothelial barrier function. Previous studies have shown that circulating concentrations of both DPP3 and ADM are independently associated with the development of organ failure and adverse outcome in sepsis. We now discuss new evidence illustrating that these molecules indeed represent two distinct pathways involved in the development of septic shock. Recently, both ADM-enhancing therapies aimed at improving endothelial barrier function and vascular tone and DPP3-blocking therapies aimed at restoring systemic angiotensin responses have been shown to improve outcome in various preclinical sepsis models. Given the current lack of effective adjuvant therapies in sepsis, additional research on the therapeutic application of these peptides in humans is highly warranted.

Combined mental task and metaboreflex impair cerebral oxygenation in patients with type 2 diabetes mellitus.

Cardiovascular regulation is altered by type 2 diabetes mellitus (DM2), producing an abnormal response to muscle metaboreflex. During physical exercise, cerebral blood flow is impaired in patients with DM2, and this phenomenon may reduce cerebral oxygenation (COX). We hypothesized that the simultaneous execution of a mental task (MT) and metaboreflex activation would reduce COX in patients with DM2. Thirteen individuals suffering from DM2 (6 women) and 13 normal age-matched controls (CTL, 6 women) participated in this study. They underwent five different tests, each lasting 12 min: postexercise muscle ischemia (PEMI) to activate the metaboreflex, control exercise recovery (CER), PEMI + MT, CER + MT, and MT alone. COX was evaluated using near-infrared spectroscopy with sensors applied to the forehead. Central hemodynamics was assessed using impedance cardiography. We found that when MT was superimposed on the PEMI-induced metaboreflex, patients with DM2 could not increase COX to the same extent reached by the CTL group (101.13% ± 1.08% vs. 104.23% ± 2.51%, P < 0.05). Moreover, patients with DM2 had higher mean blood pressure and systemic vascular resistance as well as lower stroke volume and cardiac output levels compared with the CTL group, throughout our experiments. It was concluded that patients with DM2 had reduced capacity to enhance COX when undertaking an MT during metaboreflex. Results also confirm that patients with DM2 had dysregulated hemodynamics during metaboreflex, with exaggerated blood pressure response and vasoconstriction. This may have implications for these patients' lack of inclination to exercise.

ACE2, a multifunctional protein - from cardiovascular regulation to COVID-19.

This Editorial, written by Guest Editors Professor Michael Bader, Professor Anthony J. Turner and Dr Natalia Alenina, proudly introduces the Clinical Science-themed collection on angiotensin-converting enzyme 2 (ACE2), a multifunctional protein - from cardiovascular regulation to coronavirus disease 2019 (COVID-19).

Endogenous AT1 receptor-protein kinase C activity in the hypothalamus augments glutamatergic input and sympathetic outflow in hypertension.

KEY POINTS: The angiotensin AT1 receptor expression and protein kinase C (PKC)-mediated NMDA receptor phosphorylation levels in the hypothalamus are increased in a rat genetic model of hypertension. Blocking AT1 receptors or PKC activity normalizes the increased pre- and postsynaptic NMDA receptor activity of hypothalamic presympathetic neurons in hypertensive animals. Inhibition of AT1 receptor-PKC activity in the hypothalamus reduces arterial blood pressure and sympathetic nerve discharges in hypertensive animals. AT1 receptors in the hypothalamus are endogenously activated to sustain NMDA receptor hyperactivity and elevated sympathetic outflow via PKC in hypertension. ABSTRACT: Increased synaptic N-methyl-d-aspartate receptor (NMDAR) activity in the hypothalamic paraventricular nucleus (PVN) plays a major role in elevated sympathetic output in hypertension. Although exogenous angiotensin II (AngII) can increase NMDAR activity in the PVN, whether endogenous AT1 receptor-protein kinase C (PKC) activity mediates the augmented NMDAR activity of PVN presympathetic neurons in hypertension is unclear. Here we show that blocking AT1 receptors with losartan or inhibiting PKC with chelerythrine significantly decreased the frequency of NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and the amplitude of puff NMDA currents of retrogradely labelled spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs). Also, treatment with chelerythrine abrogated the potentiating effect of AngII on mEPSCs and puff NMDA currents of labelled PVN neurons in SHRs. In contrast, neither losartan nor chelerythrine had any effect on mEPSCs or puff NMDA currents in labelled PVN neurons in Wistar-Kyoto (WKY) rats. Furthermore, levels of AT1 receptor mRNA and PKC-mediated NMDAR phosphorylation in the PVN were significantly higher in SHRs than in WKY rats. In addition, microinjection of losartan or chelerythrine into the PVN substantially reduced blood pressure and renal sympathetic nerve discharges in SHRs but not in WKY rats. Chelerythrine blocked sympathoexcitatory responses to AngII microinjected into the PVN. Our findings suggest that endogenous AT1 receptor-PKC activity is essential for presynaptic and postsynaptic NMDAR hyperactivity of PVN presympathetic neurons and for the augmented sympathetic outflow in hypertension. This information advances our mechanistic understanding of the interplay between angiotensinergic and glutamatergic excitatory inputs in hypertension.

The impact of 2 years of high-intensity exercise training on a model of integrated cardiovascular regulation.

KEY POINTS: Heart rate variability, a common and easily measured index of cardiovascular dynamics, is the output variable of complicated cardiovascular and respiratory control systems. Both neural and non-neural control mechanisms may contribute to changes in heart rate variability. We previously developed an innovative method using transfer function analysis to assess the effect of prolonged exercise training on integrated cardiovascular regulation. In the present study, we modified and applied this to investigate the effect of 2 years of high-intensity training on circulatory components to tease out the primary effects of training. Our method incorporated the dynamic Starling mechanism, dynamic arterial elastance and arterial-cardiac baroreflex function. The dynamic Starling mechanism gain and arterial-cardiac baroreflex gain were significantly increased in the exercise group. These parameters remained unchanged in the controls. Conversely, neither group experienced a change in dynamic arterial elastance. The integrated cardiovascular regulation gain in the exercise group was 1.34-fold larger than that in the control group after the intervention. In these previously sedentary, otherwise healthy, middle-aged adults, 2 years of high-intensity exercise training improved integrated cardiovascular regulation by enhancing the dynamic Starling mechanism and arterial-cardiac baroreflex sensitivity. ABSTRACT: Assessing the effects of exercise training on cardiovascular variability is challenging because of the complexity of multiple mechanisms. In a prospective, parallel-group, randomized controlled study, we examined the effect of 2 years of high-intensity exercise training on integrated cardiovascular function, which incorporates the dynamic Starling mechanism, dynamic arterial elastance and arterial-cardiac baroreflex function. Sixty-one healthy participants (48% male, aged 53 years, range 52-54 years) were randomized to either 2 years of exercise training (exercise group: n = 34) or control/yoga group (controls: n = 27). Before and after 2 years, subjects underwent a 6 min recording of beat-by-beat pulmonary artery diastolic pressure (PAD), stroke volume index (SV index), systolic blood pressure (sBP) and RR interval measurements with controlled respiration at 0.2 Hz. The dynamic Starling mechanism, dynamic arterial elastance and arterial-cardiac baroreflex function were calculated by transfer function gain between PAD and SV index; SV index and sBP; and sBP and RR interval, respectively. Fifty-three participants (controls: n = 25; exercise group: n = 28) completed the intervention. After 2 years, the dynamic Starling mechanism gain (Group × Time interaction: P = 0.008) and the arterial-cardiac baroreflex gain (P = 0.005) were significantly increased in the exercise group but remained unchanged in the controls. There was no change in dynamic arterial elastance in either of the two groups. The integrated cardiovascular function gain in the exercise group increased 1.34-fold, whereas there was no change in the controls (P = 0.02). In these previously sedentary, otherwise healthy middle-aged adults, a 2 year programme of high-intensity exercise training improved integrated cardiovascular regulation by enhancing the dynamic Starling mechanism and arterial-cardiac baroreflex sensitivity, without changing dynamic arterial elastance.

Cardiovascular regulation in response to multiple hemorrhages: analysis and parameter estimation.

Mathematical models can provide useful insights explaining behavior observed in experimental data; however, rigorous analysis is needed to select a subset of model parameters that can be informed by available data. Here we present a method to estimate an identifiable set of parameters based on baseline left ventricular pressure and volume time series data. From this identifiable subset, we then select, based on current understanding of cardiovascular control, parameters that vary in time in response to blood withdrawal, and estimate these parameters over a series of blood withdrawals. These time-varying parameters are first estimated using piecewise linear splines minimizing the mean squared error between measured and computed left ventricular pressure and volume data over four consecutive blood withdrawals. As a final step, the trends in these splines are fit with empirical functional expressions selected to describe cardiovascular regulation during blood withdrawal. Our analysis at baseline found parameters representing timing of cardiac contraction, systemic vascular resistance, and cardiac contractility to be identifiable. Of these parameters, vascular resistance and cardiac contractility were varied in time. Data used for this study were measured in a control Sprague-Dawley rat. To our knowledge, this is the first study to analyze the response to multiple blood withdrawals both experimentally and theoretically, as most previous studies focus on analyzing the response to one large blood withdrawal. Results show that during each blood withdrawal both systemic vascular resistance and contractility decrease acutely and partially recover, and they decrease chronically across the series of blood withdrawals.

Cardiovascular regulation: associations between exercise and head-up tilt.

It was hypothesized that faster cardiorespiratory kinetics during exercise are associated with higher orthostatic tolerance. Cardiorespiratory kinetics of 14 healthy male subjects (30 ± 4 years, 179 ± 8 cm, 79 ± 8 kg) were tested on a cycle ergometer during exercise with changing work rates of 30 and 80 W. Pulmonary oxygen uptake ( ) was measured breath-by-breath and heart rate (HR), mean arterial blood pressure (MAP), and total peripheral resistance (TPR) were measured beat-to-beat. Muscular oxygen uptake ( ) was estimated from HR and . Kinetic parameters were determined by time-series analysis, using cross-correlation functions (CCFmax(x)) between the parameter and the work rate. Cardiovascular regulations of MAP, HR, and TPR during orthostatic stress were measured beat-to-beat on a tilt seat. Changes between the minima and maxima during the 6° head-down tilt and the 90° head-up tilt positions were calculated for each parameter (Δtilt-up). correlated significantly with ΔTPRtilt-up (r = 0.790, p ≤ 0.001). CCFmax(HR) was significantly correlated with ΔHRtilt-up (r = -0.705, p = 0.002) and the amplitude in HR from 30 to 80 W (rSP = -0.574, p = 0.016). The observed correlations between cardiorespiratory regulation in response to exercise and orthostatic stress during rest might allow for a more differential analysis of the underlying mechanisms of orthostatic intolerance in, for example, patient groups.

Exposure to antenatal corticosteroids and reduced respiratory sinus arrhythmia in adult survivors of extremely low birth weight.

Purpose/aim: Antenatal corticosteroid (ACS) therapy has dramatically increased survival rates among extremely low birth weight (ELBW) infants. However, the long-term effects of ACS on autonomic nervous system function have not been explored. Using the world's oldest longitudinally followed cohort of ELBW infants we compared respiratory sinus arrhythmia (RSA) among ELBW survivors whose mothers received ACS (ELBW-S), those who did not (ELBW-NS) and normal birth weight (NBW) controls in their 20 and 30 s. Methods: Resting electrocardiogram (ECG) was recorded from ELBW-S (n = 28), ELBW-NS (n = 36), and matched NBW controls (n = 79) at 22-26 and 29-36 years. Resting RSA was compared across groups via analyses of covariance (ANCOVA), adjusting for sex, medication use, postnatal steroid exposure and the presence of chronic health conditions. RSA was also compared across assessments for each group. Results: At 29-36 years, resting RSA in ELBW-S was significantly lower than in NBW controls. RSA in the ELBW-NS group was intermediate between ELBW-S and NBW groups. Although the ELBW-S group also showed nominally reduced RSA compared to NBW controls at the 22-26-year visit, this difference was not statistically significant. Conclusions: ELBW survivors exposed to ACS had lower RSA than NBW controls during their 30 s, suggestive of a decline in parasympathetic input to heart. ELBW survivors who received ACS may be particularly vulnerable to cardiovascular problems in later life.

Gabapentin Reduces Blood Pressure and Heart Rate through the Nucleus Tractus Solitarii.

BACKGROUND: Oral and intravenous gabapentin can markedly attenuate blood pressure (BP) in hypertensive rats. The nucleus tractus solitarii (NTS) is the primary integrative center for cardiovascular control and other autonomic functions in the central nervous system. However, the signaling mechanisms involved in gabapentin-mediated cardiovascular effects in the NTS remain unclear. We investigated whether the nitric oxide synthase (NOS) signaling pathway was involved in gabapentin-mediated BP regulation in the NTS of spontaneously hypertensive (SHR) rats. METHODS: SHR rats were anesthetized with urethane at age 10-12 weeks. Arterial pressure and heart rate (HR) were monitored through a femoral artery catheter. For stereotaxic intra-NTS microinjection, the dorsal surface of the medulla was exposed by limited craniotomy. We observed that unilateral microinjection of gabapentin into the NTS whether to change dose-related BP and HR. Then, unilateral microinjection of gabapentin into the NTS before and after N(ω)-nitro-L-arginine methyl ester (L-NAME) treatment whether to change blood pressure and heart rate. RESULTS: Unilateral microinjection of gabapentin into the NTS produced prominent dose-related depressor and bradycardic effects in SHR rats. The cardiovascular effects of gabapentin were attenuated by the prior administration of the NOS inhibitor, L-NAME. CONCLUSIONS: Gabapentin modulated central BP and HR control in the NTS of SHR rats in this study through NOS signaling.

Sympathoexcitation constrains vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia.

We used diffuse correlation spectroscopy to investigate sympathetic vasoconstriction, local vasodilation, and integration of these two responses in the skeletal muscle microvasculature of 20 healthy volunteers. Diffuse correlation spectroscopy probes were placed on the flexor carpi radialis muscle or vastus lateralis muscle, and a blood flow index was derived continuously. We measured hemodynamic responses during sympathoexcitation induced by forehead cooling, after which the effects of the increased sympathetic tone on vasodilatory responses during postocclusive reactive hyperemia (PORH) were examined. PORH was induced by releasing arterial occlusion (3 min) in an arm or leg. To increase sympathetic tone during PORH, forehead cooling was begun 60 s before the occlusion release and ended 60 s after the release. During forehead cooling, mean arterial pressure rose significantly and was sustained at an elevated level. Significant vasoconstriction and decreases in blood flow index followed by gradual blunting of the vasoconstriction also occurred. The time course of these responses is in good agreement with previous observations in animals. The acute sympathoexcitation diminished the peak vasodilation during PORH only in the vastus lateralis muscle, but it hastened the decline in vasodilation after the peak in both the flexor carpi radialis muscle and vastus lateralis muscle. Consequently, the total vasodilatory response assessed as the area of the vascular conductance during the first minute of PORH was significantly diminished in both regions. We conclude that, in humans, the integrated effects of sympathetic vasoconstriction and local vasodilation have an important role in vascular regulation and control of perfusion in the skeletal muscle microcirculation. NEW & NOTEWORTHY We used diffuse correlation spectroscopy to demonstrate that acute sympathoexcitation constrains local vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia. This finding indicates that integration of sympathetic vasoconstriction and local vasodilation is importantly involved in vascular regulation and the control of perfusion of the skeletal muscle microcirculation in humans.

Effects of vagotomy on cardiovascular and heart rate variability alterations following chronic normobaric hypoxia in adult rabbits.

BACKGROUND: chronic hypoxia increases basal ventilation and pulmonary vascular resistance, with variable changes in arterial blood pressure and heart rate, but it's impact on heart rate variability and autonomic regulation have been less well examined. We studied changes in arterial blood pressure, heart rate and heart rate variability (HRV) in rabbits subjected to chronic normobaric hypoxia (CNH; PB ~ 719 mmHg; FIO2 ~ 9.2%) for 14 days and assess the effect of autonomic control by acute bilateral vagal denervation. RESULTS: exposure to CNH stalled animal weight gain and increased the hematocrit, without affecting heart rate or arterial blood pressure. Nevertheless, Poincaré plots of the electrocardiographic R-R intervals showed a reduced distribution parallel to the line of identity, which interpreted as reduced long-term HRV. In the frequency domain, CNH reduced the very-low- (< 0.2 Hz) and high-frequency components (> 0.8 Hz) of the R-R spectrograms and produced a prominent component in the low-frequency component (0.2-0.5 Hz) of the power spectrum. In control and CNH exposed rabbits, bilateral vagotomy had no apparent effect on the short- and long-term HRV in the Poincaré plots. However, bilateral vagotomy differentially affected higher-frequency components (> 0.8 Hz); reducing it in control animals without modifying it in CNH-exposed rabbits. CONCLUSIONS: These results suggest that CNH exposure shifts the autonomic balance of heart rate towards a sympathetic predominance without modifying resting heart rate or arterial blood pressure.

Optogenetic identification of hypothalamic orexin neuron projections to paraventricular spinally projecting neurons.

Orexin neurons, and activation of orexin receptors, are generally thought to be sympathoexcitatory; however, the functional connectivity between orexin neurons and a likely sympathetic target, the hypothalamic spinally projecting neurons (SPNs) in the paraventricular nucleus of the hypothalamus (PVN) has not been established. To test the hypothesis that orexin neurons project directly to SPNs in the PVN, channelrhodopsin-2 (ChR2) was selectively expressed in orexin neurons to enable photoactivation of ChR2-expressing fibers while examining evoked postsynaptic currents in SPNs in rat hypothalamic slices. Selective photoactivation of orexin fibers elicited short-latency postsynaptic currents in all SPNs tested (n = 34). These light-triggered responses were heterogeneous, with a majority being excitatory glutamatergic responses (59%) and a minority of inhibitory GABAergic (35%) and mixed glutamatergic and GABAergic currents (6%). Both glutamatergic and GABAergic responses were present in the presence of tetrodotoxin and 4-aminopyridine, suggesting a monosynaptic connection between orexin neurons and SPNs. In addition to generating postsynaptic responses, photostimulation facilitated action potential firing in SPNs (current clamp configuration). Glutamatergic, but not GABAergic, postsynaptic currents were diminished by application of the orexin receptor antagonist almorexant, indicating orexin release facilitates glutamatergic neurotransmission in this pathway. This work identifies a neuronal circuit by which orexin neurons likely exert sympathoexcitatory control of cardiovascular function.NEW & NOTEWORTHY This is the first study to establish, using innovative optogenetic approaches in a transgenic rat model, that there are robust heterogeneous projections from orexin neurons to paraventricular spinally projecting neurons, including excitatory glutamatergic and inhibitory GABAergic neurotransmission. Endogenous orexin release modulates glutamatergic, but not GABAergic, neurotransmission in these pathways.

The carotid baroreflex modifies the pressor threshold of the muscle metaboreflex in humans.

The purpose of the present study was to test our hypothesis that unloading the carotid baroreceptors alters the threshold and gain of the muscle metaboreflex in humans. Ten healthy subjects performed a static handgrip exercise at 50% of maximum voluntary contraction. Contraction was sustained for 15, 30, 45, and 60 s and was followed by 3 min of forearm circulatory arrest, during which forearm muscular pH is known to decrease linearly with increasing contraction time. The carotid baroreceptors were unloaded by applying 0.1-Hz sinusoidal neck pressure (oscillating from +15 to +50 mmHg) during ischemia. We estimated the threshold and gain of the muscle metaboreflex by analyzing the relationship between the cardiovascular responses during ischemia and the amount of work done during the exercise. In the condition with unloading of the carotid baroreceptors, the muscle metaboreflex thresholds for mean arterial blood pressure (MAP) and total vascular resistance (TVR) corresponded to significantly lower work levels than the control condition (threshold for MAP: 795 ± 102 vs. 662 ± 208 mmHg and threshold for TVR: 818 ± 213 vs. 572 ± 292 kg·s, P < 0.05), but the gains did not differ between the two conditions (gain for MAP: 4.9 ± 1.7 vs. 4.4 ± 1.6 mmHg·kg·s-1·100 and gain for TVR: 1.3 ± 0.8 vs. 1.3 ± 0.7 mmHg·l-1·min-1·kg·s-1·100). We conclude that the carotid baroreflex modifies the muscle metaboreflex threshold in humans. Our results suggest the carotid baroreflex brakes the muscle metaboreflex, thereby inhibiting muscle metaboreflex-mediated pressor and vasoconstriction responses.NEW & NOTEWORTHY We found that unloading the carotid baroreceptors shifts the pressor threshold of the muscle metaboreflex toward lower metabolic stimulation levels in humans. This finding indicates that, in the normal loading state, the carotid baroreflex inhibits the muscle metaboreflex pressor response by shifting the reflex threshold to higher metabolic stimulation levels.

Involvement of Endothelial Nitric Oxide Synthase Activation in Midkine-Mediated Central Hypotensive Effects.

The growth factor midkine (MK) has been implicated in various biologic and pathologic events. It has been shown that the peripheral influence of MK on cardiovascular regulation is due to an influence on the renin-angiotensin system (RAS). The nucleus tractus solitarii (NTS) is the primary integrative center for cardiovascular control and other autonomic functions in the central nervous system. However, the signaling mechanisms involved in MK-mediated cardiovascular effects in the NTS remain unclear. In this study, we investigated whether the RAS and/or N-methyl-D-aspartate (NMDA) receptor-calmodulin-endothelial nitric oxide synthase (eNOS) signaling pathways were both involved in MK-mediated blood pressure (BP) regulation in the NTS of Wistar-Kyoto (WKY) rats. Intra-NTS microinjection and immunoblot analysis were used to evaluate the signal pathway. WKY rats were anesthetized with urethane. Unilateral microinjection of MK (600 fmol) into the NTS produced a dose-dependent decrease in BP and heart rate (HR). The depressor effects were observed before and after microinjection of the angiotensin-converting enzyme (ACE) inhibitor lisinopril (2.4 fmol), or the angiotensin receptor blockers (ARB) inhibitor valsartan (7.5 pmol). However, lisinopril and valsartan did not diminish the MK-mediated cardiovascular effects in the NTS. Microinjection of the NMDA receptor antagonist MK801 (1 nmol) or the NOS inhibitor N-nitro l-arginine methyl ester (L-NAME), (33 nmol), into the NTS attenuated the MK-induced hypotensive effects. Pretreatment with an eNOS inhibitor N5-iminoethyl-l-ornithine (L-NIO) (6 nmol) attenuated the MK-induced hypotensive effects. In this study, the data showed that MK might play a role in central cardiovascular regulation in the NTS. These results suggest that MK decreased BP and HR in the NTS probably acting via the NMDA receptor-calmodulin-eNOS signaling pathway.

Sumoylation of IkB attenuates NF-kB-induced nitrosative stress at rostral ventrolateral medulla and cardiovascular depression in experimental brain death.

BACKGROUND: Small ubiquitin-related modifier (SUMO) is a group of proteins that participates in post-translational modifications. One known SUMO target is the transcription factor nuclear factor-kB (NF-kB) that plays a pivotal role in many disease processes; sumoylation inactivates NF-kB by conjugation with inhibitors of NF-kB (IkB). Our laboratory demonstrated previously that transcriptional upregulation of nitric oxide synthase II (NOS II) by NF-kB, leading to nitrosative stress by the formation of peroxynitrite in the rostral ventrolateral medulla (RVLM), underpins the defunct brain stem cardiovascular regulation that precedes brain death. Based on an experimental endotoxemia model, this study evaluated the hypothesis that sumoylation plays a pro-life role in brain death by interacting with the NF-kB/NOS II/peroxynitrite signaling pathway in the RVLM. RESULTS: In Sprague-Dawley rats, intravenous administration of Escherichia coli lipopolysaccharide (LPS; 10 mg kg-1) elicited an augmentation of SUMO-1 and ubiquitin-conjugase 9 (Ubc9) mRNA or protein levels, alongside SUMO-1-conjugated proteins in the RVLM. Immunoneutralization of SUMO-1 or Ubc9 in the RVLM significantly potentiated the already diminished sumoylation of IkBα and intensified NF-kB activation and NOS II/peroxynitrite expression in this brain stem substrate, together with exacerbated fatality, cardiovascular depression and reduction of an experimental index of a life-and-death signal detected from arterial pressure that disappears in comatose patients signifying failure of brain stem cardiovascular regulation before brain death. CONCLUSION: We conclude that sumoylation of IkB in the RVLM ameliorates the defunct brain stem cardiovascular regulation that underpins brain death in our experimental endotoxemia modal by reducing nitrosative stress via inhibition of IkB degradation that diminishes the induction of the NF-kB/NOS II/peroxynitrite signaling cascade.