Contrasting open-loop dynamic characteristics of sympathetic and vagal systems during baroreflex-mediated heart rate control in rats

Effects of Spinal Cord Injury Site on Cardiac Autonomic Regulation: Insight from Analysis of Cardiovascular Beat by Beat Variability during Sleep and Orthostatic Challenge

The goal of this study on Spinal Cord Injury (SCI) patients with cervical or thoracic lesion was to assess whether disturbances of ANS control, according to location, might differently affect vagal and sympatho-vagal markers during sleep and orthostatic challenge. We analyzed with linear and nonlinear techniques beat-by-beat RR and arterial pressure (and respiration) variability signals, extracted from a polysomnographic study and a rest-tilt test. We considered spontaneous or induced sympathetic excitation, as obtained shifting from non-REM to REM sleep or from rest to passive tilt. We obtained evidence of ANS cardiac (dys)regulation, of greater importance for gradually proximal location (i.e., cervical) SCI, compatible with a progressive loss of modulatory role of sympathetic afferents to the spinal cord. Furthermore, in accordance with the dual, vagal and sympathetic bidirectional innervation, the results suggest that vagally mediated negative feedback baroreflexes were substantially maintained in all cases. Conversely, the LF and HF balance (expressed specifically by normalized units) appeared to be negatively affected by SCI, particularly in the case of cervical lesion (group p = 0.006, interaction p = 0.011). Multivariate analysis of cardiovascular variability may be a convenient technique to assess autonomic responsiveness and alteration of functionality in patients with SCI addressing selectively vagal or sympathetic alterations and injury location. This contention requires confirmatory studies with a larger population.

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.

Ivabradine augments high-frequency dynamic gain of the heart rate response to low- and moderate-intensity vagal nerve stimulation under β-blockade

Our previous study indicated that intravenously administered ivabradine (IVA) augmented the dynamic heart rate (HR) response to moderate-intensity vagal nerve stimulation (VNS). Considering an accentuated antagonism, the results were somewhat paradoxical; i.e., the accentuated antagonism indicates that an activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels via the accumulation of intracellular cyclic adenosine monophosphate (cAMP) augments the HR response to VNS, whereas the inhibition of HCN channels by IVA also augmented the HR response to VNS. To remove the possible influence from the accentuated antagonism, we examined the effects of IVA on the dynamic vagal control of HR under β-blockade. In anesthetized rats (n = 7), the right vagal nerve was stimulated for 10 min according to binary white noise signals between 0 and 10 Hz (V_0-10), between 0 and 20 Hz (V_0-20), and between 0 and 40 Hz (V_0-40). The transfer function from VNS to HR was estimated. Under β-blockade (propranolol, 2 mg/kg iv), IVA (2 mg/kg iv) did not augment the asymptotic low-frequency gain but increased the asymptotic high-frequency gain in V_0-10 (0.53 ± 0.10 vs. 1.74 ± 0.40 beats/min/Hz, P < 0.01) and V_0-20 (0.79 ± 0.14 vs. 2.06 ± 0.47 beats/min/Hz, P < 0.001). These changes, which were observed under a minimal influence from sympathetic background tone, may reflect an increased contribution of the acetylcholine-sensitive potassium channel (I_K,ACh) pathway after IVA, because the HR control via the I_K,ACh pathway is faster and acts in the frequency range higher than the cAMP-mediated pathway.NEW & NOTEWORTHY Since ivabradine (IVA) inhibits hyperpolarization-activated cyclic nucleotide-gated channels, interactions among the sympathetic effect, vagal effect, and IVA can occur in the control of heart rate (HR). To remove the sympathetic effect, we estimated the transfer function from vagal nerve stimulation to HR under β-blockade in anesthetized rats. IVA augmented the high-frequency dynamic gain during low- and moderate-intensity vagal nerve stimulation. Untethering the hyperpolarizing effect of acetylcholine-sensitive potassium channels after IVA may be a possible underlying mechanism.

Contribution of afferent pathway to vagal nerve stimulation-induced myocardial interstitial acetylcholine release in rats

Vagal nerve stimulation (VNS) has been explored as a potential therapy for chronic heart failure. The contribution of the afferent pathway to myocardial interstitial acetylcholine (ACh) release during VNS has yet to be clarified. In seven anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release during right VNS with the following combinations of stimulation frequency (F in Hz) and voltage readout (V in volts): F0V0 (no stimulation), F5V3, F20V3, F5V10, and F20V10. F5V3 did not affect the ACh level. F20V3, F5V10, and F20V10 increased the ACh level to 2.83 ± 0.47 (P < 0.01), 4.31 ± 1.09 (P < 0.001), and 4.33 ± 0.82 (P < 0.001) nM, respectively, compared with F0V0 (1.76 ± 0.22 nM). After right vagal afferent transection (rVAX), F20V3 and F20V10 increased the ACh level to 2.90 ± 0.53 (P < 0.001) and 3.48 ± 0.63 (P < 0.001) nM, respectively, compared with F0V0 (1.61 ± 0.19 nM), but F5V10 did not (2.11 ± 0.24 nM). The ratio of the ACh levels after rVAX relative to before was significantly <100% in F5V10 (59.4 ± 8.7%) but not in F20V3 (102.0 ± 8.7%). These results suggest that high-frequency and low-voltage stimulation (F20V3) evoked the ACh release mainly via direct activation of the vagal efferent pathway. By contrast, low-frequency and high-voltage stimulation (F5V10) evoked the ACh release in a manner dependent on the vagal afferent pathway.

Threshold and saturation pressures of baroreflex-mediated myocardial interstitial acetylcholine release in rats

Cardiac microdialysis allows the assessment of cardiac efferent vagal nerve activity from myocardial interstitial acetylcholine (ACh) levels with minimal influence on the neural control of the heart; however, a total picture of the baroreflex-mediated myocardial interstitial ACh release including the threshold and saturation pressures has yet to be quantified. In eight anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release simultaneously with efferent sympathetic nerve activity (SNA) during a carotid sinus baroreceptor pressure input between 60 and 180 mm Hg. The baroreflex-mediated ACh release approximated a positive sigmoid curve, and its threshold and saturation pressures were not significantly different from those of an inverse sigmoid curve associated with the baroreflex-mediated SNA response (threshold: 94.3 ± 8.6 vs. 99.3 ± 6.0 mm Hg; saturation: 150.0 ± 10.3 vs. 158.8 ± 5.8 mm Hg). The sympathetic and vagal systems have certain levels of activities across most of the normal pressure range.