Accumulation of cAMP augments dynamic vagal control of heart rate

Dynamic accentuated antagonism of heart rate control during different levels of vagal nerve stimulation intensity in rats

Accentuated antagonism refers to a phenomenon in which the vagal effect on heart rate (HR) is augmented by background sympathetic tone. The dynamic aspect of accentuated antagonism remains to be elucidated during different levels of vagal nerve stimulation (VNS) intensity. We performed VNS on anesthetized rats (n = 8) according to a binary white noise signal with a switching interval of 500 ms at three different stimulation rates (low-intensity: 0-10 Hz, moderate-intensity: 0-20 Hz, and high-intensity: 0-40 Hz). The transfer function from VNS to HR was estimated with and without concomitant tonic sympathetic nerve stimulation (SNS) at 5 Hz. The asymptotic low-frequency (LF) gain (in beats/min/Hz) of the transfer function increased with SNS regardless of the VNS rate [low-intensity: 3.93 ± 0.70 vs. 5.82 ± 0.65 (P = 0.021), moderate-intensity: 3.87 ± 0.62 vs. 5.36 ± 0.53 (P = 0.018), high-intensity: 4.77 ± 0.85 vs. 7.39 ± 1.36 (P = 0.011)]. Moreover, SNS slightly increased the ratio of high-frequency (HF) gain to the LF gain. These effects of SNS were canceled by the pretreatment of ivabradine, an inhibitor of hyperpolarization-activated cyclic nucleotide-gated channels, in another group of rats (n = 6). Although background sympathetic tone antagonizes the vagal effect on mean HR, it enables finer HR control by increasing the dynamic gain of the vagal HR transfer function regardless of VNS intensity. When interpreting the HF component of HR variability, the augmenting effect from background sympathetic tone needs to be considered.

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.

Exercise training improves hypertension-induced autonomic dysfunction without influencing properties of peripheral cardiac vagus nerve

We examined the vagal transfer function of autonomic heart rate (HR) control in anesthetized sedentary and exercise-trained Spontaneously Hypertensive Rats (SHR). To this end, male SHR and Wystar-Kyoto (WKY) rats with 48-50weeks of age-old were divided into 4 groups: sedentary (SHR_S, n=12) and trained (SHR_T, n=14) hypertensive rats, sedentary (WKY_S, n=13) and trained (WKY_T, n=13) normotensive rats. The trained groups were submitted to swimming protocol for 9weeks. Blood pressure (BP), HR, HR variability (HRV), BP variability (BPV), baroreflex sensitivity and cardiac tonus were recorded in baseline conditions. Following, electric stimulation of peripheral vagus nerve was performed in anesthetized conditions. Resting bradycardia was observed in SHR_T and WKY_T when compared to their respective sedentary groups (p<0.001). The BP was lower in SHR_T than in SHR_S (p<0.001). The SHR_T and WKY_T rats showed higher baroreflex-mediated tachycardia values when compared to their respective sedentary counterparts (p<0.001). Baroreflex bradycardic response in SHR_T was higher than in SHR_S (p<0.005). The SHR_T and WKY_T rats showed a decreased sympathetic activity in comparison to their respective sedentary groups (p<0.05). The cardiac vagal tonus was higher in SHR_T than in SHR_S (p<0.05). Regarding the dynamic transducer properties of peripheral vagus nerve to the heart no difference was observed among the groups. In conclusion, our results demonstrate that exercise training decreased BP in SHR and improved cardiovascular autonomic balance to the heart without changes in transduction properties of peripheral cardiac vagus nerve.

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.

Optimizing the parameters of vagus nerve stimulation by uniform design in rats with acute myocardial infarction

Vagus nerve stimulation (VNS) has been shown to improve left ventricular function and survival in rats with acute myocardial infarction (AMI), and this maneuver has also been adopted clinically for the treatment of patients with chronic heart failure (CHF). Recent in vitro and in vivo studies have suggested that VNS can modulate the level of pro-inflammatory factors. Despite the beneficial effects of VNS, the stimulation parameters for obtaining favorable outcomes appear highly variable. To optimize VNS parameters, we set up different stimulation protocols with different pulse width (1-2 ms), frequency (1-6 Hz), voltage (1-6 V) and duration (40-240 min) of VNS by uniform design (UD). Rats were divided into seven groups with (Group1-Group6) or without VNS (MI group). Our results demonstrate that (1) the parameter sets in Group1, Group2 and Group3 yield the best post-MI protection by VNS, while the protective role were not observed in Group4, Group5 and Group6; (2) baroreflex sensitivity and the α7 nicotinic acetylcholine receptor level were also increased in Group1, Group2 and Group3. (3) the parameter set in Group1 (G1:1 ms, 2 Hz, 3 V, 240 min) is judged the most optimal parameter in this study as rats in this group not only showed a reduced myocardial injury with better-preserved cardiac function compared with other groups, more important, but also exhibited minimal heart rate (HR) reduction. (4) the duration of VNS plays an important role in determining the protection effect of VNS. In conclusion, VNS displays a beneficial role in Group1, Group2 and Group3. Of note, the parameter set in Group1 provides the most optimal cardioprotective effect. These results may provide insight into development of novel treatment for ischemic heart diseases.

Protection against ischemia-induced oxidative stress conferred by vagal stimulation in the rat heart: involvement of the AMPK-PKC pathway

Reactive oxygen species (ROS) production is an important mechanism in myocardial ischemia and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is one of major sources of ROS in the heart. Previous studies showed that vagus nerve stimulation (VNS) is beneficial in treating ischemic heart diseases. However, the effect of VNS on ROS production remains elusive. In this study, we investigated the role of VNS onischemia-induced ROS production. Our results demonstrated that VNS alleviated the myocardial injury, attenuated the cardiac dysfunction, reserved the antioxidant enzyme activity and inhibited the formation of ROS as evidenced by the decreased NADPH oxidase (Nox) activity and superoxide fluorescence intensity as well as the expression of p67phox, Rac1 and nitrotyrosine. Furthermore, VNS resulted in the phosphorylation and activation of adenosine monophosphate activated protein kinase (AMPK), which in turn led to an inactivation of Nox by protein kinase C (PKC); however, the phenomena were repressed by the administration of a muscarinic antagonist atropine. Taken together, these data indicate that VNS decreases ROS via AMPK-PKC-Nox pathway; this may have potential importance for the treatment of ischemic heart diseases.

Angiotensin II disproportionally attenuates dynamic vagal and sympathetic heart rate controls

To better understand the pathophysiological role of angiotensin II (ANG II) in the dynamic autonomic regulation of heart rate (HR), we examined the effects of intravenous administration of ANG II (10 μg·kg−1·h−1) on the transfer function from vagal or sympathetic nerve stimulation to HR in anesthetized rabbits with sinoaortic denervation and vagotomy. In the vagal stimulation group ( n = 7), we stimulated the right vagal nerve for 10 min using binary white noise (0–10 Hz). The transfer function from vagal stimulation to HR approximated a first-order low-pass filter with pure delay. ANG II attenuated the dynamic gain from 7.6 ± 0.9 to 5.8 ± 0.9 beats·min−1·Hz−1 (means ± SD; P < 0.01) without affecting the corner frequency or pure delay. In the sympathetic stimulation group ( n = 7), we stimulated the right postganglionic cardiac sympathetic nerve for 20 min using binary white noise (0–5 Hz). The transfer function from sympathetic stimulation to HR approximated a second-order low-pass filter with pure delay. ANG II slightly attenuated the dynamic gain from 10.8 ± 2.6 to 10.2 ± 3.1 beats·min−1·Hz−1 ( P = 0.049) without affecting the natural frequency, damping ratio, or pure delay. The disproportional suppression of the dynamic vagal and sympathetic regulation of HR would result in a relative sympathetic predominance in the presence of ANG II. The reduced high-frequency component of HR variability in patients with cardiovascular diseases, such as myocardial infarction and heart failure, may be explained in part by the peripheral effects of ANG II on the dynamic autonomic regulation of HR.

Muscarinic potassium channels augment dynamic and static heart rate responses to vagal stimulation

Vagal control of heart rate (HR) is mediated by direct and indirect actions of ACh. Direct action of ACh activates the muscarinic K+ (KACh) channels, whereas indirect action inhibits adenylyl cyclase. The role of the KACh channels in the overall picture of vagal HR control remains to be elucidated. We examined the role of the KACh channels in the transfer characteristics of the HR response to vagal stimulation. In nine anesthetized sinoaortic-denerved and vagotomized rabbits, the vagal nerve was stimulated with a binary white-noise signal (0–10 Hz) for examination of the dynamic characteristic and in a step-wise manner (5, 10, 15, and 20 Hz/min) for examination of the static characteristic. The dynamic transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with a lag time. Tertiapin, a selective KACh channel blocker (30 nmol/kg iv), significantly decreased the dynamic gain from 5.0 ± 1.2 to 2.0 ± 0.6 (mean ± SD) beats·min−1·Hz−1 ( P < 0.01) and the corner frequency from 0.25 ± 0.03 to 0.06 ± 0.01 Hz ( P < 0.01) without changing the lag time (0.37 ± 0.04 vs. 0.39 ± 0.05 s). Moreover, tertiapin significantly attenuated the vagal stimulation-induced HR decrease by 46 ± 21, 58 ± 18, 65 ± 15, and 68 ± 11% at stimulus frequencies of 5, 10, 15, and 20 Hz, respectively. We conclude that KACh channels contribute to a rapid HR change and to a larger decrease in the steady-state HR in response to more potent tonic vagal stimulation.

Cardiac sympathetic nerve stimulation does not attenuate dynamic vagal control of heart rate via α-adrenergic mechanism

Complex sympathovagal interactions govern heart rate (HR). Activation of the postjunctional beta-adrenergic receptors on the sinus nodal cells augments the HR response to vagal stimulation, whereas exogenous activation of the presynaptic alpha-adrenergic receptors on the vagal nerve terminals attenuates vagal control of HR. Whether the alpha-adrenergic mechanism associated with cardiac postganglionic sympathetic nerve activation plays a significant role in modulation of the dynamic vagal control of HR remains unknown. The right vagal nerve was stimulated in seven anesthetized rabbits that had undergone sinoaortic denervation and vagotomy according to a binary white-noise signal (0-10 Hz) for 10 min; subsequently, the transfer function from vagal stimulation to HR was estimated. The effects of beta-adrenergic blockade with propranolol (1 mg/kg i.v.) and the combined effects of beta-adrenergic blockade and tonic cardiac sympathetic nerve stimulation at 5 Hz were examined. The transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with pure delay. beta-Adrenergic blockade decreased the dynamic gain from 6.0 +/- 0.4 to 3.7 +/- 0.6 beats x min(-1) x Hz(-1) (P < 0.01) with no alteration of the corner frequency or pure delay. Under beta-adrenergic blockade conditions, tonic sympathetic stimulation did not further change the dynamic gain (3.8 +/- 0.5 beats x min(-1) x Hz(-1)). In conclusion, cardiac postganglionic sympathetic nerve stimulation did not affect the dynamic HR response to vagal stimulation via the alpha-adrenergic mechanism.

High plasma norepinephrine attenuates the dynamic heart rate response to vagal stimulation

To better understand the pathophysiological significance of high plasma norepinephrine (NE) concentration in regulating heart rate (HR), we examined the interactions between high plasma NE and dynamic vagal control of HR. In anesthetized rabbits with sinoaortic denervation and vagotomy, using a binary white noise sequence (0-10 Hz) for 10 min, we stimulated the right vagus and estimated the transfer function from vagal stimulation to HR response. The transfer function approximated a first-order low-pass filter with pure delay. Infusion of NE (100 microg. kg(-1) x h(-1) iv) attenuated the dynamic gain from 6.2 +/- 0.8 to 3.9 +/- 1.2 beats x min(-1) x Hz(-1) (n = 7, P < 0.05) without affecting the corner frequency or pure delay. Simultaneous intravenous administration of phentolamine (1 mg x kg(-1) x h(-1)) and NE (100 microg x kg(-1) x h(-1)) abolished the inhibitory effect of NE on the dynamic gain (6.3 +/- 0.8 vs. 6.4 +/- 1.3 beats x min(-1) x Hz(-1), not significant, n = 7). The inhibitory effect of NE at infusion rates of 10, 50, and 100 microg x kg(-1) x h(-1) on dynamic vagal control of HR was dose-dependent (n = 5). In conclusion, high plasma NE attenuated the dynamic HR response to vagal stimulation, probably via activation of alpha-adrenergic receptors on the preganglionic and/or postganglionic cardiac vagal nerve terminals.

Dynamic sympathetic control of atrioventricular conduction time and heart period

Although power spectra of R-R and P-R intervals in response to random respiration show similar frequency distributions, the way in which dynamic sympathetic regulation contributes to such similarity remains unknown. We estimated the transfer function from sympathetic stimulation to the atrioventricular interval (AV conduction time; T(AV)) with and without constant atrial pacing in seven anesthetized cats. The transfer function from sympathetic stimulation to T(AV), except for absolute gain values, approximated a low-pass filter similar to that from sympathetic stimulation to the A-A interval (heart period; T(AA)). The 90%-rise times did not differ between the T(AA) and T(AV) step responses (32.3 +/- 1.8 vs. 29.6 +/- 3.2 s). Constant pacing augmented the T(AV) step response (-0.58 +/- 0.10 vs. -0.86 +/- 0.12 ms/Hz, P < 0.05) without affecting the 90%-rise time. These findings suggest that the dynamic characteristics of sympathetic control are similar between T(AA) and T(AV) despite the different electrophysiological mechanisms determining T(AA) and T(AV). A numerical simulation indicated that if the dynamic characteristics of the sympathetic control do not match between T(AA) and T(AV), a critical condition for initiation of reentrant tachycardia would be encountered.

Dynamic nonlinear vago-sympathetic interaction in regulating heart rate

Although the characteristics of the static interactions between the sympathetic and parasympathetic nervous systems in regulating heart rate have been well established, how the dynamic interaction modulates the heart rate response remains unknown. Thus, we investigated the dynamic interaction by estimating the transfer function from nerve stimulation to heart rate, using band-limited Gaussian white noise, in anesthetized rabbits. Concomitant tonic vagal stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic sympathetic stimulation to heart rate by 55.0%+/-40.1% and 80.7%+/-50.5%, respectively (P < 0.05). Concomitant tonic sympathetic stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic vagal stimulation to heart rate by 18.2%+/-17.9% and 24.1%+/-18.0%, respectively (P < 0.05). Such bidirectional augmentation was also observed during simultaneous dynamic stimulation of the sympathetic and vagal nerves independent of their stimulation patterns. Because of these characteristics, changes in sympathetic or vagal tone alone can alter the dynamic heart rate response to stimulation of the other nerve. We explained this phenomenon by assuming a sigmoidal static relationship between autonomic nerve activity and heart rate. To confirm this assumption, we identified the static and dynamic characteristics of heart rate regulation by a neural network analysis, using large-amplitude Gaussian white noise input. To examine the mechanism involved in the bidirectional augmentation, we increased cytosolic adenosine 3',5'-cyclic monophosphate (cAMP) at the postjunctional effector site by applying pharmacological interventions. The cAMP accumulation increased the gain of the transfer function relating dynamic vagal stimulation to heart rate. Thus, accumulation of cAMP contributes, at least in part, to the sympathetic augmentation of the dynamic vagal control of heart rate.