Dynamic characteristics of baroreflex neural and peripheral arcs are preserved in spontaneously hypertensive rats

Altered Autonomic Function in Metabolic Syndrome: Interactive Effects of Multiple Components

Metabolic syndrome (MetS) describes a set of disorders that collectively influence cardiovascular health, and includes hypertension, obesity, insulin resistance, diabetes, and dyslipidemia. All these components (hypertension, obesity, dyslipidemia, and prediabetes/diabetes) have been shown to modify autonomic function. The major autonomic dysfunction that has been documented with each of these components is in the control of sympathetic outflow to the heart and periphery at rest and during exercise through modulation of the arterial baroreflex and the muscle metaboreflex. Many studies have described MetS components in singularity or in combination with the other major components of metabolic syndrome. However, many studies lack the capability to study all the factors of metabolic syndrome in one model or have not focused on studying the effects of how each component as it arises influences overall autonomic function. The goal of this review is to describe the current understanding of major aspects of metabolic syndrome that most likely contribute to the consequent/associated autonomic alterations during exercise and discuss their effects, as well as bring light to alternative mechanisms of study.

Exaggerated renal sympathetic nerve and pressor responses during spontaneously occurring motor activity in hypertensive rats

Stimulation of the mesencephalic locomotor region elicits exaggerated sympathetic nerve and pressor responses in spontaneously hypertensive rats (SHR) as compared with normotensive Wistar-Kyoto rats (WKY). This suggests that central command or its influence on vasomotor centers is augmented in hypertension. The decerebrate animal model possesses an ability to evoke intermittent bouts of spontaneously occurring motor activity (SpMA) and generates cardiovascular responses associated with the SpMA. It remains unknown whether the changes in sympathetic nerve activity and hemodynamics during SpMA are altered by hypertension. To test the hypothesis that the responses in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) during SpMA are exaggerated with hypertension, this study aimed to compare the responses in decerebrate, paralyzed SHR, WKY, and normotensive Sprague-Dawley (SD) rats. In all strains, an abrupt increase in RSNA occurred in synchronization with tibial motor discharge (an index of motor activity) and was followed by rises in MAP and heart rate. The centrally evoked increase in RSNA and MAP during SpMA was much greater (306 ± 110%) in SHR than WKY (187 ± 146%) and SD (165 ± 44%). Although resting baroreflex-mediated changes in RSNA were not different across strains, mechanically or pharmacologically induced elevations in MAP attenuated or abolished the RSNA increase during SpMA in WKY and SD but had no effect in SHR. It is likely that the exaggerated sympathetic nerve and pressor responses during SpMA in SHR are induced along a central command pathway independent of the arterial baroreflex and/or result from central command-induced inhibition of the baroreflex.

Very short-term beat-by-beat blood pressure variability in the supine position at rest correlates well with the nocturnal blood pressure variability assessed by ambulatory blood pressure monitoring

Blood pressure variability (BPV) is an important indicator in risk stratification for hypertension. Among the daily BPVs assessed using a 24-h ambulatory blood pressure (BP) monitor nocturnal systolic BPV has been suggested to predict cardiovascular risks. We hypothesized that very short-term BPV at rest would correlate with nocturnal BPV because of the shared autonomic BP regulatory system under no daily exertion. Thirty untreated normotensive and hypertensive adults underwent 30-min continuous beat-by-beat BP recordings in the supine position, followed by 24-h ambulatory blood pressure monitoring (ABPM). The relationship between very short-term BPV (standard deviation (SD), coefficient of variation (CV)) and daytime and nocturnal BPV (SD, CV, average real variability (ARV), and standardized ARV (CV-ARV)) was assessed with Pearson's correlation coefficients. Very short-term BPV correlated significantly with nocturnal BPV (ARV, r = 0.604, p < 0.001) but not with daytime BPV. These trends were more pronounced with the increasing data length of continuous beat-by-beat BP recording. Using a data segment from the last 10 min of a 30-min continuous beat-by-beat BP recording resulted in a stronger correlation between very short-term BPV and nocturnal BPV than using earlier segments. The findings of this study suggest that very short-term BPV in the supine position at rest may predict nocturnal BPV. Since the burden of ABPM for patients has hindered clinical dissemination, very short-term BPV has the potential to develop a novel index of BPV.

Closed-Loop Identification of Baroreflex Properties in the Frequency Domain

The arterial baroreflex system plays a key role in maintaining the homeostasis of arterial pressure (AP). Changes in AP affect autonomic nervous activities through the baroreflex neural arc, whereas changes in the autonomic nervous activities, in turn, alter AP through the baroreflex peripheral arc. This closed-loop negative feedback operation makes it difficult to identify open-loop dynamic characteristics of the neural and peripheral arcs. Regarding sympathetic AP controls, we examined the applicability of a nonparametric frequency-domain closed-loop identification method to the carotid sinus baroreflex system in anesthetized rabbits. This article compares the results of an open-loop analysis applied to open-loop data, an open-loop analysis erroneously applied to closed-loop data, and a closed-loop analysis applied to closed-loop data. To facilitate the understanding of the analytical method, sample data files and sample analytical codes were provided. In the closed-loop identification, properties of the unknown central noise that modulated the sympathetic nerve activity and the unknown peripheral noise that fluctuated AP affected the accuracy of the estimation results. A priori knowledge about the open-loop dynamic characteristics of the arterial baroreflex system may be used to advance the assessment of baroreflex function under closed-loop conditions in the future.

Power Spectral Analysis of Short-Term Blood Pressure Recordings for Assessing Daily Variations of Blood Pressure in Human

Although daily variations of blood pressure (BP) predict cardiovascular event risk, their assessment requires ambulatory BP monitoring which hinders the clinical application of this approach. Since the baroreflex is a major determinant of BP variations, especially in the frequency range of 0.01-0.1 Hz (baro-frequency), we hypothesized that the power spectral density (PSD) of short-term BP recordings in the baro-frequency range may predict daily variations of BP. In nine-week-old Wister-Kyoto male rats (N =5) with or without baroreflex dysfunction, we telemetrically recorded continuous BP for 24 hours and estimated PSD using Welch's periodogram for the recordings during the 12-hour light period. We compared the reference PSD of 12-hour recording with the PSDs obtained from shorter data lengths ranging from 5 to 240 minutes. The 30-minute BP recordings reproduced PSD of 12-hour recordingswell, and PSD in the baro-frequency range paralleled the standard deviation of 12-hour BP. Thus, the PSD of 30-minute BP reflects the daily BP variability in rats. In human subjects, we estimated PSD from 30-minute noninvasive continuous BP recordings. The rat and human PSDs shared remarkably similar characteristics. Furthermore, comparison of PSD between elderly and young subjects suggested that the baro-frequency range in humans overlapped with that in rats. In conclusion, PSD derived from 30-minute BP recordings is capable of predicting daily BP variations. Our proposed method may serve as a simple, noninvasive and practical tool for predicting cardiovascular events in the clinical setting.

Suppressed baroreflex peripheral arc overwhelms augmented neural arc and incapacitates baroreflex function in rats with pulmonary arterial hypertension

NEW FINDINGS

What is the central question of this study? The impact of pulmonary arterial hypertension on open-loop baroreflex function, which determines how powerfully and rapidly the baroreflex operates to regulate arterial pressure, remains poorly understood. What is the main finding and its importance? The gain of the baroreflex total arc, indicating the baroreflex pressure-stabilizing function, is markedly attenuated in rats with monocrotaline-induced pulmonary arterial hypertension. This is caused by a rightward shift of the baroreflex neural arc and a downward shift of the peripheral arc. These findings contribute greatly to our understanding of arterial pressure regulation by the sympathetic nervous system in pulmonary arterial hypertension.

ABSTRACT

Sympathoexcitation has been documented in patients with established pulmonary arterial hypertension (PAH). Although the arterial baroreflex is the main negative feedback regulator of sympathetic nerve activity (SNA), the way in which PAH impacts baroreflex function remains poorly understood. In this study, we conducted baroreflex open-loop analysis in a rat model of PAH. Sprague-Dawley rats were injected with monocrotaline (MCT) s.c. to induce PAH (60 mg kg^-1 ; n = 11) or saline as a control group (CTL; n = 8). At 3.5 weeks after MCT injection, bilateral carotid sinuses were isolated, and intrasinus pressure (CSP) was controlled while SNA at the coeliac ganglia and arterial pressure (AP) were recorded. To examine the static baroreflex function, CSP was increased stepwise while steady-state AP (total arc) and SNA (neural arc) responses to CSP and the AP response to SNA (peripheral arc) were measured. Monocrotaline significantly decreased the static gain of the baroreflex total arc at the operating AP compared with CTL (-0.80 ± 0.31 versus -0.22 ± 0.22, P < 0.05). Given that MCT markedly increased plasma noradrenaline, an index of SNA, by approximately 3.6-fold compared with CTL, calibrating SNA by plasma noradrenaline revealed that MCT shifted the neural arc to a higher SNA level and shifted the peripheral arc downwards. Monocrotaline also decreased the dynamic gain of the baroreflex total arc (-0.79 ± 0.16 versus -0.35 ± 0.17, P < 0.05), while the corner frequencies that reflect the speed of the baroreflex remained unchanged (0.06 ± 0.02 versus 0.08 ± 0.02 Hz, n.s.). In rats with MCT-induced PAH, the suppressed baroreflex peripheral arc overwhelms the augmented neural arc and, in turn, attenuates the gain of the total arc, which determines the pressure-stabilizing capacity of the baroreflex.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

Inhibition of glial glutamate transporter GLT1 in the nucleus of the solitary tract attenuates baroreflex control of sympathetic nerve activity and heart rate

The astrocytic glutamate transporter (GLT1) plays an important role in the maintenance of extracellular glutamate concentration below neurotoxic levels in brain. However, the functional role of GLT1 within the nucleus of the solitary tract (NTS) in the regulation of cardiovascular function remains unclear. We examined the effect of inhibiting GLT1 in the subpostremal NTS on mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA) and heart rate (HR) in anesthetized, artificially ventilated rats. It was found that dihydrokainate (DHK; inhibitor of GLT1, 5 mmol/L, 100 nL) injections into the NTS (n = 6) decreased MAP (50 ± 10 mmHg, mean ± SD), RSNA (89 ± 14%) and HR (37 ± 6 bpm). Pretreatment with kynurenate (KYN; glutamate receptor antagonist, 5 mmol/L, 30 μL) topically applied to the dorsal surface of the brainstem (n = 4) attenuated the responses to NTS injections of DHK (P < 0.01). The effect of DHK on arterial baroreflex function was examined using i.v. infusions of phenylephrine and nitroprusside. DHK reduced baroreflex response range (maximum-minimum) of RSNA by 91 ± 2% and HR by 83 ± 5% (n = 6, P < 0.001). These results indicate that inhibition of GLT1 within the NTS decreases MAP, RSNA, and HR by the activation of ionotropic glutamate receptors. As a result, baroreflex control of RSNA and HR was dramatically attenuated. The astrocytic glutamate transporter in the NTS plays an important role in the maintenance and regulation of cardiovascular function.

Tonic aortic depressor nerve stimulation does not impede baroreflex dynamic characteristics concomitantly mediated by the stimulated nerve

Although electrical activation of the carotid sinus baroreflex (baroreflex activation therapy) is being explored as a device therapy for resistant hypertension, possible effects on baroreflex dynamic characteristics of interaction between electrical stimulation and pressure inputs are not fully elucidated. To examine whether the electrical stimulation of the baroreceptor afferent nerve impedes normal short-term arterial pressure (AP) regulation mediated by the stimulated nerve, we electrically stimulated the right aortic depressor nerve (ADN) while estimating the baroreflex dynamic characteristics by imposing pressure inputs to the isolated baroreceptor region of the right ADN in nine anesthetized rats. A Gaussian white noise signal with a mean of 120 mmHg and standard deviation of 20 mmHg was used for the pressure perturbation. A tonic ADN stimulation (2 or 5 Hz, 10 V, 0.1-ms pulse width) decreased mean sympathetic nerve activity (367.0 ± 70.9 vs. 247.3 ± 47.2 arbitrary units, P < 0.01) and mean AP (98.4 ± 7.8 vs. 89.2 ± 4.5 mmHg, P < 0.01) during dynamic pressure perturbation. The ADN stimulation did not affect the slope of dynamic gain in the neural arc transfer function from pressure perturbation to sympathetic nerve activity (16.9 ± 1.0 vs. 14.7 ± 1.6 dB/decade, not significant). These results indicate that electrical stimulation of the baroreceptor afferent nerve does not significantly impede the dynamic characteristics of the arterial baroreflex concomitantly mediated by the stimulated nerve. Short-term AP regulation by the arterial baroreflex may be preserved during the baroreflex activation therapy.

Effects of different input pressure waveforms on the carotid sinus baroreflex-mediated sympathetic arterial pressure response in rats

Although the pulsatility of an input pressure is an important factor that determines the arterial baroreflex responses, whether the difference in the input waveforms can meaningfully affect the baroreflex function remains unknown. This study aimed to compare baroreflex responses between two distinct pressure waveforms: a forward saw wave (FSW) and a backward saw wave (BSW). In seven anesthetized rats, carotid sinus pressure was exposed to the FSW or the BSW with a mean of 120 mmHg, pulse pressure of 40 mmHg, and pulse frequency of 1 Hz. Changes in efferent sympathetic nerve activity (SNA) and arterial pressure (AP) during six consecutive saw wave trials (FSW1, BSW1, FSW2, BSW2, FSW3, and BSW3) were examined. The steady-state SNA value during FSW1 was 91.1 ± 1.9%, which was unchanged during FSW2 and FSW3 but significantly increased during BSW1 (106.6 ± 3.4%, P < 0.01), BSW2 (110.6 ± 2.5%, P < 0.01), and BSW3 (111.6 ± 2.3%, P < 0.01). The steady-state AP value during FSW1 was 98.2 ± 8.1 mmHg, which was unchanged during FSW2 and FSW3 but significantly increased during BSW1 (106.7 ± 7.4 mmHg, P < 0.01), BSW2 (105.6 ± 7.8 mmHg, P < 0.01), and BSW3 (103.8 ± 7.2 mmHg, P < 0.05). In conclusion, the FSW was more effective than the BSW in reducing mean SNA and AP. The finding could be applied to designing an artificial pulsatile pressure such as that generated by left ventricular assist devices.

NEW & NOTEWORTHY This study examined whether the waveforms of an input pressure alone can affect the baroreflex function by using a forward saw wave and a backward saw wave with the same mean pressure, pulse pressure, and pulse frequency. The forward saw wave was more effective than the backward saw wave in reducing sympathetic nerve activity and arterial pressure. The finding could be applied to designing an artificial pulsatile pressure such as that generated by left ventricular assist devices.

Aortic depressor nerve stimulation does not impede the dynamic characteristics of the carotid sinus baroreflex in normotensive or spontaneously hypertensive rats

Recent clinical trials in patients with drug-resistant hypertension indicate that electrical activation of the carotid sinus baroreflex can reduce arterial pressure (AP) for more than a year. To examine whether the electrical stimulation from one baroreflex system impedes normal short-term AP regulation via another unstimulated baroreflex system, we electrically stimulated the left aortic depressor nerve (ADN) while estimating the dynamic characteristics of the carotid sinus baroreflex in anesthetized normotensive Wistar-Kyoto (WKY; n = 8) rats and spontaneously hypertensive rats (SHR; n = 7). Isolated carotid sinus regions were perturbed for 20 min using a Gaussian white noise signal with a mean of 120 mmHg for WKY and 160 mmHg for SHR. Tonic ADN stimulation (2 Hz, 10 V, and 0.1-ms pulse width) decreased mean sympathetic nerve activity (73.4 ± 14.0 vs. 51.6 ± 11.3 arbitrary units in WKY, P = 0.012; and 248.7 ± 33.9 vs. 181.1 ± 16.6 arbitrary units in SHR, P = 0.018) and mean AP (90.8 ± 6.6 vs. 81.2 ± 5.4 mmHg in WKY, P = 0.004; and 128.6 ± 9.8 vs. 114.7 ± 10.3 mmHg in SHR, P = 0.009). The slope of dynamic gain in the neural arc transfer function from carotid sinus pressure to sympathetic nerve activity was not different between trials with and without the ADN stimulation (12.55 ± 0.93 vs. 13.03 ± 1.28 dB/decade in WKY, P = 0.542; and 17.37 ± 1.01 vs. 17.47 ± 1.64 dB/decade in SHR, P = 0.946). These results indicate that the tonic ADN stimulation does not significantly modify the dynamic characteristics of the carotid sinus baroreflex.

Nonlinear identification of the total baroreflex arc: higher-order nonlinearity

The total baroreflex arc is the open-loop system relating carotid sinus pressure (CSP) to arterial pressure (AP). The nonlinear dynamics of this system were recently characterized. First, Gaussian white noise CSP stimulation was employed in open-loop conditions in normotensive and hypertensive rats with sectioned vagal and aortic depressor nerves. Nonparametric system identification was then applied to measured CSP and AP to establish a second-order nonlinear Uryson model. The aim in this study was to assess the importance of higher-order nonlinear dynamics via development and evaluation of a third-order nonlinear model of the total arc using the same experimental data. Third-order Volterra and Uryson models were developed by employing nonparametric and parametric identification methods. The R² values between the AP predicted by the best third-order Volterra model and measured AP in response to Gaussian white noise CSP not utilized in developing the model were 0.69 ± 0.03 and 0.70 ± 0.03 for normotensive and hypertensive rats, respectively. The analogous R² values for the best third-order Uryson model were 0.71 ± 0.03 and 0.73 ± 0.03. These R² values were not statistically different from the corresponding values for the previously established second-order Uryson model, which were both 0.71 ± 0.03 (P > 0.1). Furthermore, none of the third-order models predicted well-known nonlinear behaviors including thresholding and saturation better than the second-order Uryson model. Additional experiments suggested that the unexplained AP variance was partly due to higher brain center activity. In conclusion, the second-order Uryson model sufficed to represent the sympathetically mediated total arc under the employed experimental conditions.

Nonlinear identification of the total baroreflex arc: chronic hypertension model

The total baroreflex arc is the open-loop system relating carotid sinus pressure (CSP) to arterial pressure (AP). Its linear dynamic functioning has been shown to be preserved in spontaneously hypertensive rats (SHR). However, the system is known to exhibit nonlinear dynamic behaviors. The aim of this study was to establish nonlinear dynamic models of the total arc (and its subsystems) in hypertensive rats and to compare these models with previously published models for normotensive rats. Hypertensive rats were studied under anesthesia. The vagal and aortic depressor nerves were sectioned. The carotid sinus regions were isolated and attached to a servo-controlled piston pump. AP and sympathetic nerve activity were measured while CSP was controlled via the pump using Gaussian white noise stimulation. Second-order, nonlinear dynamics models were developed by application of nonparametric system identification to a portion of the measurements. The models of the total arc predicted AP 21-43% better (P < 0.005) than conventional linear dynamic models in response to a new portion of the CSP measurement. The linear and nonlinear terms of these validated models were compared with the corresponding terms of an analogous model for normotensive rats. The nonlinear gains for the hypertensive rats were significantly larger than those for the normotensive rats [-0.38 ± 0.04 (unitless) vs. -0.22 ± 0.03, P < 0.01], whereas the linear gains were similar. Hence, nonlinear dynamic functioning of the sympathetically mediated total arc may enhance baroreflex buffering of AP increases more in SHR than normotensive rats.

Open-loop static and dynamic characteristics of the arterial baroreflex system in rabbits and rats

The arterial baroreflex system is the most important negative feedback system for stabilizing arterial pressure (AP). This system serves as a key link between the autonomic nervous system and the cardiovascular system, and is thus essential for understanding the pathophysiology of cardiovascular diseases and accompanying autonomic abnormalities. This article focuses on an open-loop systems analysis using a baroreceptor isolation preparation to identify the characteristics of two principal subsystems of the arterial baroreflex system, namely, the neural arc from pressure input to efferent sympathetic nerve activity (SNA) and the peripheral arc from SNA to AP. Studies on the static and dynamic characteristics of the two arcs under normal physiological conditions and also under various interventions including diseased conditions are to be reviewed. Quantitative understanding of the arterial baroreflex function under diseased conditions would help develop new treatment strategies such as electrical activation of the carotid sinus baroreflex for drug-resistant hypertension.

Nonlinear identification of the total baroreflex arc

The total baroreflex arc [the open-loop system relating carotid sinus pressure (CSP) to arterial pressure (AP)] is known to exhibit nonlinear behaviors. However, few studies have quantitatively characterized its nonlinear dynamics. The aim of this study was to develop a nonlinear model of the sympathetically mediated total arc without assuming any model form. Normal rats were studied under anesthesia. The vagal and aortic depressor nerves were sectioned, the carotid sinus regions were isolated and attached to a servo-controlled piston pump, and the AP and sympathetic nerve activity (SNA) were measured. CSP was perturbed using a Gaussian white noise signal. A second-order Volterra model was developed by applying nonparametric identification to the measurements. The second-order kernel was mainly diagonal, but the diagonal differed in shape from the first-order kernel. Hence, a reduced second-order model was similarly developed comprising a linear dynamic system in parallel with a squaring system in cascade with a slower linear dynamic system. This "Uryson" model predicted AP changes 12% better (P < 0.01) than a linear model in response to new Gaussian white noise CSP. The model also predicted nonlinear behaviors, including thresholding and mean responses to CSP changes about the mean. Models of the neural arc (the system relating CSP to SNA) and peripheral arc (the system relating SNA to AP) were likewise developed and tested. However, these models of subsystems of the total arc showed approximately linear behaviors. In conclusion, the validated nonlinear model of the total arc revealed that the system takes on an Uryson structure.

Differences in the dynamic baroreflex characteristics of unmyelinated and myelinated central pathways are less evident in spontaneously hypertensive rats

The aim of the study was to identify the contribution of myelinated (A-fiber) and unmyelinated (C-fiber) baroreceptor central pathways to the baroreflex control of sympathetic nerve activity (SNA) and arterial pressure (AP) in anesthetized Wistar-Kyoto (WKY; n = 8) and spontaneously hypertensive rats (SHR; n = 8). The left aortic depressor nerve (ADN) was electrically stimulated with two types of binary white noise signals designed to preferentially activate A-fibers (A-BRx protocol) or C-fibers (C-BRx protocol). In WKY, the central arc transfer function from ADN stimulation to SNA estimated by A-BRx showed strong derivative characteristics with the slope of dynamic gain between 0.1 and 1 Hz ( Gslope) of 14.63 ± 0.89 dB/decade. In contrast, the central arc transfer function estimated by C-BRx exhibited nonderivative characteristics with Gslope of 0.64 ± 1.13 dB/decade. This indicates that A-fibers are important for rapid baroreflex regulation, whereas C-fibers are likely important for more sustained regulation of SNA and AP. In SHR, the central arc transfer function estimated by A-BRx showed higher Gslope (18.46 ± 0.75 dB/decade, P < 0.01) and that estimated by C-BRx showed higher Gslope (8.62 ± 0.64 dB/decade, P < 0.001) with significantly lower dynamic gain at 0.01 Hz (6.29 ± 0.48 vs. 2.80 ± 0.36%/Hz, P < 0.001) compared with WKY. In conclusion, the dynamic characteristics of the A-fiber central pathway are enhanced in the high-modulation frequency range (0.1–1 Hz) and those of the C-fiber central pathway are attenuated in the low-modulation frequency range (0.01–0.1 Hz) in SHR.

Effects of tempol on baroreflex neural arc versus peripheral arc in normotensive and spontaneously hypertensive rats

Although oxidative redox signaling affects arterial pressure (AP) regulation via modulation of vascular tone and sympathetic nerve activity (SNA), it remains unknown which effect plays a dominant role in the determination of AP in vivo. Open-loop systems analysis of the carotid sinus baroreflex was conducted to separately quantify characteristics of the neural arc from baroreceptor pressure input to SNA and the peripheral arc from SNA to AP in normotensive Wistar-Kyoto (WKY; n = 8) and spontaneously hypertensive rats (SHR; n = 8). Responses in SNA and AP to a staircase-wise increase in carotid sinus pressure were examined before and during intravenous administration of the membrane-permeable superoxide dismutase mimetic tempol (30 mg/kg bolus followed by 30 mg·kg−1·h−1). Two-way ANOVA indicated that tempol significantly decreased the response range of SNA (from 89.1 ± 2.4% to 60.7 ± 2.5% in WKY and from 77.5 ± 3.2% to 56.9 ± 7.3% in SHR, P < 0.001) without affecting the lower plateau of SNA (from 12.5 ± 2.4% to 9.5 ± 2.5% in WKY, and from 28.8 ± 2.8% to 30.4 ± 5.7% in SHR, P = 0.800) in the neural arc. While tempol did not affect the peripheral arc characteristics in WKY, it yielded a downward change in the regression line of AP vs. SNA in SHR. In conclusion, oxidative redox signaling plays an important role, not only in the pathological AP elevation, but also in the baroreflex-mediated physiological AP regulation. The effect of modulating oxidative redox signaling on the peripheral arc contributed to the determination of AP in SHR but not in WKY.

Predominant role of neural arc in sympathetic baroreflex resetting of spontaneously hypertensive rats

BACKGROUND

There is ongoing controversy over whether neural or peripheral factors are the predominant cause of hypertension. The closed-loop negative feedback operation of the arterial baroreflex hampers understanding of how arterial pressure (AP) is determined through the interaction between neural and peripheral factors. METHODS AND RESULTS: A novel analysis of an isolated open-loop baroreceptor preparation to examine sympathetic nervous activity (SNA) and AP responses to changes in carotid sinus pressure (CSP) in adult spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) was conducted. In the neural arc (CSP-SNA relationship), the midpoint pressure (128.9±3.8 vs. 157.9±8.1 mmHg, P<0.001) and the response range of SNA to CSP (90.5±3.7 vs. 115.4±7.6%/mmHg, P=0.011) were higher in SHR. In the peripheral arc (SNA-AP relationship), slope and intercept did not differ. A baroreflex equilibrium diagram was obtained by depicting neural and peripheral arcs in a pressure-SNA plane with rescaled SNA (% in WKY). The operating-point AP (111.3±4.4 vs. 145.9±5.2 mmHg, P<0.001) and SNA (90.8±3.2 vs. 125.1±6.9% in WKY, P<0.001) were shifted towards a higher level in SHR.

CONCLUSIONS

The shift of the neural arc towards a higher SNA range indicated a predominant contribution to baroreflex resetting in SHR. Notwithstanding the resetting, the carotid sinus baroreflex in SHR preserved an ability to reduce AP if activated with a high enough pressure.

Resetting of the sympathetic baroreflex is associated with the onset of hypertension during chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) is a model of arterial hypoxemia that accompanies sleep apnea and increases resting arterial pressure (AP). We examined the effects of 7 days of exposure to CIH on arterial baroreflex control of renal sympathetic nerve activity (RSNA) and heart rate (HR) in rats. Sprague-Dawley rats (15±2 weeks old) were exposed to CIH (9% oxygen for 3 min every 10 min, 8 h per day) for 7 days (n=16) while control rats (n=18) were maintained in normoxia. Baroreflex regulation of RSNA and HR were estimated in Inactin anesthetized and artificially ventilated rats during infusions of phenylephrine and nitroprusside to manipulate AP. After exposure to CIH, resting mean AP was higher in CIH than that in control group (115±7 vs. 105±7, P<0.001). Resting HR did not differ between the two groups. Exposure to CIH shifted the AP-RSNA relationship rightward (approximately 10 mm Hg, P<0.01). CIH did not alter maximum gain of the baroreflex control of RSNA (-2.6±0.6 vs. -2.5±0.6 arbitrary units (a.u.)/mm Hg) and HR (-1.8±0.6 vs. -1.8±0.7 bpm/mm Hg, CIH vs. control). In addition, cardiac spontaneous baroreflex sensitivity in conscious rats (n=8) also did not change during exposure to CIH. These results indicate that resetting of the sympathetic baroreflex control, rather than an impairment of its sensitivity, is associated with an onset of hypertension induced by CIH.

Time course of the hemodynamic responses to aortic depressor nerve stimulation in conscious spontaneously hypertensive rats

The time to reach the maximum response of arterial pressure, heart rate and vascular resistance (hindquarter and mesenteric) was measured in conscious male spontaneously hypertensive (SHR) and normotensive control rats (NCR; Wistar; 18-22 weeks) subjected to electrical stimulation of the aortic depressor nerve (ADN). The parameters of stimulation were 1 mA intensity and 2 ms pulse length applied for 5 s, using frequencies of 10, 30, and 90 Hz. The time to reach the hemodynamic responses at different frequencies of ADN stimulation was similar for SHR (N = 15) and NCR (N = 14); hypotension = NCR (4194 ± 336 to 3695 ± 463 ms) vs SHR (3475 ± 354 to 4494 ± 300 ms); bradycardia = NCR (1618 ± 152 to 1358 ± 185 ms) vs SHR (1911 ± 323 to 1852 ± 431 ms), and the fall in hindquarter vascular resistance = NCR (6054 ± 486 to 6550 ± 847 ms) vs SHR (4849 ± 918 to 4926 ± 646 ms); mesenteric = NCR (5574 ± 790 to 5752 ± 539 ms) vs SHR (5638 ± 648 to 6777 ± 624 ms). In addition, ADN stimulation produced baroreflex responses characterized by a faster cardiac effect followed by a vascular effect, which together contributed to the decrease in arterial pressure. Therefore, the results indicate that there is no alteration in the conduction of the electrical impulse after the site of baroreceptor mechanical transduction in the baroreflex pathway (central and/or efferent) in conscious SHR compared to NCR.