Central chemoreflex activation induces sympatho-excitation without altering static or dynamic baroreflex function in normal rats

Central chemoreflex activation induces sympatho-excitation. However, how central chemoreflex interacts with baroreflex function remains unknown. This study aimed to examine the impact of central chemoreflex on the dynamic as well as static baroreflex functions under open-loop conditions. In 15 anesthetized, vagotomized Sprague-Dawley rats, we isolated bilateral carotid sinuses and controlled intra-sinus pressure (CSP). We then recorded sympathetic nerve activity (SNA) at the celiac ganglia, and activated central chemoreflex by a gas mixture containing various concentrations of CO₂ Under the baroreflex open-loop condition (CSP = 100 mmHg), central chemoreflex activation linearly increased SNA and arterial pressure (AP). To examine the static baroreflex function, we increased CSP stepwise from 60 to 170 mmHg and measured steady-state SNA responses to CSP (mechanoneural arc), and AP responses to SNA (neuromechanical arc). Central chemoreflex activation by inhaling 3% CO₂ significantly increased SNA irrespective of CSP, indicating resetting of the mechanoneural arc, but did not change the neuromechanical arc. As a result, central chemoreflex activation did not change baroreflex maximum total loop gain significantly (-1.29 ± 0.27 vs. -1.68 ± 0.74, N.S.). To examine the dynamic baroreflex function, we randomly perturbed CSP and estimated transfer functions from 0.01 to 1.0 Hz. The transfer function of the mechanoneural arc approximated a high-pass filter, while those of the neuromechanical arc and total (CSP-AP relationship) arcs approximated a low-pass filter. In conclusion, central chemoreflex activation did not alter the transfer function of the mechanoneural, neuromechanical, or total arcs. Central chemoreflex modifies hemodynamics via sympatho-excitation without compromising dynamic or static baroreflex AP buffering function.

Acute Effects of Vagotomy on Baroreflex Equilibrium Diagram in Rats with Chronic Heart Failure

The arterial baroreflex system can be divided into the neural arc, from pressure input to efferent sympathetic nerve activity (SNA), and the peripheral arc, from SNA to arterial pressure (AP). Plotting the neural and peripheral arcs on a pressure–SNA plane yields a baroreflex equilibrium diagram. We examined the effects of vagotomy on the open-loop static characteristics of the carotid sinus baroreflex in normal control rats (NC, n = 10) and rats with heart failure after myocardial infarction (MI, n = 10). In the NC group, vagotomy shifted the neural arc toward higher SNA and decreased the slope of the peripheral arc. Consequently, the operating-point SNA increased without a significant change in the operating-point AP on the baroreflex equilibrium diagram. These vagotomy-induced effects were not observed in the MI group, suggesting a loss of vagal modulation of the carotid sinus baroreflex function in heart failure.

Acute arterial baroreflex-mediated changes in plasma catecholamine concentrations in a chronic rat model of myocardial infarction

While it may be predictable that plasma norepinephrine (NE) concentration changes with efferent sympathetic nerve activity (SNA) in response to baroreceptor pressure inputs, an exact relationship between SNA and plasma NE concentration remains to be quantified in heart failure. We examined acute baroreflex-mediated changes in plasma NE and epinephrine (Epi) concentrations in normal control (NC) rats and rats with myocardial infarction (MI) (n = 6 each). Plasma NE concentration correlated linearly with SNA in the NC group (slope: 2.17 ± 0.26 pg mL(-1) %(-1), intercept: 20.0 ± 18.2 pg mL(-1)) and also in the MI group (slope: 19.20 ± 6.45 pg mL(-1) %(-1), intercept: -239.6 ± 200.0 pg mL(-1)). The slope was approximately nine times higher in the MI than in the NC group (P < 0.01). Plasma Epi concentration positively correlated with SNA in the NC group (slope: 1.65 ± 0.79 pg mL(-1) %(-1), intercept: 115.0 ± 69.5 pg mL(-1)) and also in the MI group (slope: 7.74 ± 2.20 pg mL(-1) %(-1), intercept: 24.7 ± 120.1 pg mL(-1)). The slope was approximately 4.5 times higher in the MI than in the NC group (P < 0.05). Intravenous administration of desipramine (1 mg kg(-1)) significantly increased plasma NE concentration but decreased plasma Epi concentration in both groups, suggesting that neuronal NE uptake had contributed to the reduction in plasma NE concentration. These results indicate that high levels of plasma catecholamine in MI rats were still under the influence of baroreflex-mediated changes in SNA, and may provide additional rationale for applying baroreflex activation therapy in patients with chronic heart failure.

Calibration of baroreflex equilibrium diagram based on exogenous pressor agents in chronic heart failure rats

A baroreflex equilibrium diagram describes the relation between input pressure and sympathetic nerve activity (SNA) and that between SNA and arterial pressure (AP). To calibrate the SNA axis (abscissa) of the baroreflex equilibrium diagram, the AP response to intravenous bolus injections of phenylephrine (0.2-50 μg/kg) or norepinephrine (NE, 0.02-5 μg/kg) was examined in normal control rats (NC, n = 9) and rats with chronic heart failure (CHF, n = 6). The maximum slope of the dose-effect curve was significantly smaller in the CHF group than in the NC group (57.3 ± 5.2 vs 80.9 ± 6.3 mmHg/decade for phenylephrine, 60.2 ± 7.8 vs 80.4 ± 5.9 mmHg/decade for NE; P < 0.01). The CHF/NC ratio of the maximum slope was used to calibrate SNA. While the calibrated baroreflex equilibrium diagram showed increased maximum SNA and operating-point SNA in CHF rats compared with NC rats, the magnitude of increase was smaller than that expected from the excess plasma NE concentration in CHF rats. Plasma NE concentration in the CHF group could be disproportionally high relative to SNA.

Afferent vagal nerve stimulation resets baroreflex neural arc and inhibits sympathetic nerve activity

It has been established that vagal nerve stimulation (VNS) benefits patients and/or animals with heart failure. However, the impact of VNS on sympathetic nerve activity (SNA) remains unknown. In this study, we investigated how vagal afferent stimulation (AVNS) impacts baroreflex control of SNA. In 12 anesthetized Sprague–Dawley rats, we controlled the pressure in isolated bilateral carotid sinuses (CSP), and measured splanchnic SNA and arterial pressure (AP). Under a constant CSP, increasing the voltage of AVNS dose dependently decreased SNA and AP. The averaged maximal inhibition of SNA was ‐28.0 ± 10.3%. To evaluate the dynamic impacts of AVNS on SNA, we performed random AVNS using binary white noise sequences, and identified the transfer function from AVNS to SNA and that from SNA to AP. We also identified transfer functions of the native baroreflex from CSP to SNA (neural arc) and from SNA to AP (peripheral arc). The transfer function from AVNS to SNA strikingly resembled the baroreflex neural arc and the transfer functions of SNA to AP were indistinguishable whether we perturbed ANVS or CSP, indicating that they likely share common central and peripheral neural mechanisms. To examine the impact of AVNS on baroreflex, we changed CSP stepwise and measured SNA and AP responses with or without AVNS. AVNS resets the sigmoidal neural arc downward, but did not affect the linear peripheral arc. In conclusion, AVNS resets the baroreflex neural arc and induces sympathoinhibition in the same manner as the control of SNA and AP by the native baroreflex.

Afferent vagal nerve stimulation resets the baroreflex neural arc and inhibits sympathetic nerve activity.

Discharges of aortic and carotid sinus baroreceptors during spontaneous motor activity and pharmacologically evoked pressor interventions

Our laboratory has demonstrated that the cardiomotor component of aortic baroreflex is temporarily inhibited at the onset of spontaneous motor activity in decerebrate cats, without altering carotid sinus baroreflex. A reason for this dissociation may be attributed to a difference in the responses between aortic nerve activity (AoNA) and carotid sinus nerve activity (CsNA) during spontaneous motor activity. The stimulus-response curves of AoNA and CsNA against mean arterial blood pressure (MAP) were compared between the pressor interventions evoked by spontaneous motor activity and by intravenous administration of phenylephrine or norepinephrine, in which the responses in heart rate (HR) were opposite (i.e., tachycardia vs. baroreflex bradycardia), despite the identical increase in MAP of 34-40 mmHg. In parallel to the pressor response, mean AoNA and CsNA increased similarly by 78-81 and by 88 % of the baseline control, respectively, irrespective of whether the pressor response was evoked by spontaneous motor activity or by a pharmacological intervention. The slope of the stimulus-response curve of the mean AoNA became greater (P < 0.05) during spontaneous motor activity as compared to the pharmacological intervention. On the other hand, the stimulus-response curve of the mean CsNA and its slope were equal (P > 0.05) between the two pressor interventions. Furthermore, the slopes of the stimulus-response curves of both diastolic AoNA and CsNA (defined as the minimal value within a beat) exhibited a greater increase during spontaneous motor activity. All differences in the slopes of the stimulus-response curves were abolished by restraining HR at the intrinsic cardiac frequency. In conclusion, mean mass activities of both aortic and carotid sinus baroreceptors are able to encode the beat-by-beat changes in MAP not only at rest but also during spontaneous motor activity and spontaneous motor activity-related reduction of aortic baroreceptor activity is denied accordingly.

Chronic vagal nerve stimulation improves baroreflex neural arc function in heart failure rats

We tested whether 6-wk vagal stimulation (VS) treatment improved open-loop baroreflex function in rats after myocardial infarction (MI). The following three groups of Sprague-Dawley rats were examined: normal control (NC, n = 9), MI with no treatment (MI-NT, n = 8), and MI treated with VS (MI-VS, n = 7). Under anesthesia, a stepwise input ranging from 60 to 180 mmHg was imposed on isolated carotid sinus baroreceptor regions, while the responses in splanchnic sympathetic nerve activity (SNA) and arterial pressure (AP) were measured. The response range of percent SNA was greater in the MI-VS than in the MI-NT group (63.8 ± 4.9% vs. 33.1 ± 3.8%, P < 0.01). The slope of the AP response to percent SNA was not different between the MI-VS and MI-NT groups (0.611 ± 0.076 vs. 0.781 ± 0.057 mmHg/%). The difference in the response range of AP between the MI-VS and MI-NT groups did not reach statistical significance (40.7 ± 6.2 vs. 26.4 ± 3.5 mmHg). In conclusion, the 6-wk VS treatment significantly improved the baroreflex control of SNA, but the effect was limited for the baroreflex total-loop function due to the lack of significant improvement in the AP response to percent SNA.

High levels of circulating angiotensin II shift the open-loop baroreflex control of splanchnic sympathetic nerve activity, heart rate and arterial pressure in anesthetized rats

Although an acute arterial pressure (AP) elevation induced by intravenous angiotensin II (ANG II) does not inhibit sympathetic nerve activity (SNA) compared to an equivalent AP elevation induced by phenylephrine, there are conflicting reports as to how circulating ANG II affects the baroreflex control of SNA. Because most studies have estimated the baroreflex function under closed-loop conditions, differences in the rate of input pressure change and the magnitude of pulsatility may have biased the estimation results. We examined the effects of intravenous ANG II (10 microg kg(-1) h(-1)) on the open-loop system characteristics of the carotid sinus baroreflex in anesthetized and vagotomized rats. Carotid sinus pressure (CSP) was raised from 60 to 180 mmHg in increments of 20 mmHg every minute, and steady-state responses in systemic AP, splanchnic SNA and heart rate (HR) were analyzed using a four-parameter logistic function. ANG II significantly increased the minimum values of AP (67.6 +/- 4.6 vs. 101.4 +/- 10.9 mmHg, P < 0.01), SNA (33.3 +/- 5.4 vs. 56.5 +/- 11.5%, P < 0.05) and HR (391.1 +/- 13.7 vs. 417.4 +/- 11.5 beats/min, P < 0.01). ANG II, however, did not attenuate the response range for AP (56.2 +/- 7.2 vs. 49.7 +/- 6.2 mmHg), SNA (69.6 +/- 5.7 vs. 78.9 +/- 9.1%) or HR (41.7 +/- 5.1 vs. 51.2 +/- 3.8 beats/min). The maximum gain was not affected for AP (1.57 +/- 0.28 vs. 1.20 +/- 0.25), SNA (1.94 +/- 0.34 vs. 2.04 +/- 0.42%/mmHg) or HR (1.11 +/- 0.12 vs. 1.28 +/- 0.19 beats min(-1) mmHg(-1)). It is concluded that high levels of circulating ANG II did not attenuate the response range of open-loop carotid sinus baroreflex control for AP, SNA or HR in anesthetized and vagotomized rats.