Neuronal uptake affects dynamic characteristics of heart rate response to sympathetic stimulation

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.

Disentangling cardiovascular control mechanisms during head-down tilt via joint transfer entropy and self-entropy decompositions

A full decomposition of the predictive entropy (PE) of the spontaneous variations of the heart period (HP) given systolic arterial pressure (SAP) and respiration (R) is proposed. The PE of HP is decomposed into the joint transfer entropy (JTE) from SAP and R to HP and self-entropy (SE) of HP. The SE is the sum of three terms quantifying the synergistic/redundant contributions of HP and SAP, when taken individually and jointly, to SE and one term conditioned on HP and SAP denoted as the conditional SE (CSE) of HP given SAP and R. The JTE from SAP and R to HP is the sum of two terms attributable to SAP or R plus an extra term describing the redundant/synergistic contribution to the JTE. All quantities were computed during cardiopulmonary loading induced by −25° head-down tilt (HDT) via a multivariate linear regression approach. We found that: (i) the PE of HP decreases during HDT; (ii) the decrease of PE is attributable to a lessening of SE of HP, while the JTE from SAP and R to HP remains constant; (iii) the SE of HP is dominant over the JTE from SAP and R to HP and the CSE of HP given SAP and R is prevailing over the SE of HP due to SAP and R both in supine position and during HDT; (iv) all terms of the decompositions of JTE from SAP and R to HP and SE of HP due to SAP and R were not affected by HDT; (v) the decrease of the SE of HP during HDT was attributed to the reduction of the CSE of HP given SAP and R; (vi) redundancy of SAP and R is prevailing over synergy in the information transferred into HP both in supine position and during HDT, while in the HP information storage synergy and redundancy are more balanced. The approach suggests that the larger complexity of the cardiac control during HDT is unrelated to the baroreflex control and cardiopulmonary reflexes and may be related to central commands and/or modifications of the dynamical properties of the sinus node.

Spectral Peak Frequency in Low-Frequency Band in Cross Spectra of Blood Pressure and Heart Rate Fluctuations in Young Type 1 Diabetic Patients

In this study we tested whether joint evaluation of the frequency (fcs) at which maxima of power in the cross-spectra between the variability in systolic blood pressure and inter-beat intervals in the range of 0.06-0.12 Hz occur together with the quantification of baroreflex sensitivity (BRS) may improve early detection of autonomic dysfunction in type 1 diabetes mellitus (T1DM). We measured 14 T1DM patients (age 20.3-24.2 years, DM duration 10.4-14.2 years, without any signs of autonomic neuropathy) and 14 age-matched controls (Co). Finger arterial blood pressure was continuously recorded by Finapres for one hour. BRS and fcs were determined by the spectral method. Receiver-operating curves (ROC) were calculated for fcs, BRS, and a combination of both factors determined as F(z)=1/(1+exp(-z)), z=3.09–0.013*BRS–0.027*fcs. T1DM had significantly lower fcs than Co (T1DM: 88.8±6.7 vs. Co: 93.7±3.8 mHz; p<0.05), and a tendency towards lower BRS compared to Co (T1DM: 10.3±4.4 vs. Co: 14.6±7.1 ms/mm Hg; p=0.06). The ROC for Fz showed the highest sensitivity and specificity (71.4 % and 71.4 %) in comparison with BRS (64.3 % and 71.4 %) or fcs (64.3 % and 64.3 %). The presented method of evaluation of BRS and fcs forming an integrated factor Fz could provide further improvement in the risk stratification of diabetic patients.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

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.

Contrasting effects of presynaptic α2-adrenergic autoinhibition and pharmacologic augmentation of presynaptic inhibition on sympathetic heart rate control

Presynaptic α2-adrenergic receptors are known to exert feedback inhibition on norepinephrine release from the sympathetic nerve terminals. To elucidate the dynamic characteristics of the inhibition, we stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring heart rate (HR) in anesthetized rabbits ( n = 6). We estimated the transfer function from cardiac sympathetic nerve stimulation to HR and the corresponding step response of HR, with and without the blockade of presynaptic inhibition by yohimbine (1 mg/kg followed by 0.1 mg·kg−1·h−1 iv). We also examined the effect of the α2-adrenergic receptor agonist clonidine (0.3 and 1.5 mg·kg−1·h−1 iv) in different rabbits ( n = 5). Yohimbine increased the maximum step response (from 7.2 ± 0.8 to 12.2 ± 1.7 beats/min, means ± SE, P < 0.05) without significantly affecting the initial slope (0.93 ± 0.23 vs. 0.94 ± 0.22 beats·min−1·s−1). Higher dose but not lower dose clonidine significantly decreased the maximum step response (from 6.3 ± 0.8 to 6.8 ± 1.0 and 2.8 ± 0.5 beats/min, P < 0.05) and also reduced the initial slope (from 0.56 ± 0.07 to 0.51 ± 0.04 and 0.22 ± 0.06 beats·min−1·s−1, P < 0.05). Our findings indicate that presynaptic α2-adrenergic autoinhibition limits the maximum response without significantly compromising the rapidity of effector response. In contrast, pharmacologic augmentation of the presynaptic inhibition not only attenuates the maximum response but also results in a sluggish effector response.

Insignificant effects of plasma catecholamines on dynamic heart rate regulation by the cardiac sympathetic nerve

Although plasma catecholamines such as norepinephrine (NE) and epinephrine (Epi) increase during severe exercise, the effects of high levels of plasma catecholamines on dynamic heart rate (HR) regulation by the cardiac sympathetic nerve remains unknown. The aim of the present study was to examine the effects of plasma catecholamines on the transfer function from sympathetic nerve stimulation to HR. In anesthetized rabbits, we randomly stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring HR. The effects of intravenous NE administration at 1 and 10 mugmiddotkg^-1middoth^-1 were examined in 6 rabbits. The effects of intravenous Epi administration at 1 and 10 mugmiddotkg^-1middoth^-1 were examined in different 6 rabbits. Although plasma NE increased 10 times as high as the baseline level during the NE administration at mugmiddotkg^-1middoth^-1 , dynamic gain of the transfer function was not changed significantly (7.1plusmn1.2, 6.9plusmn1.1, and 7.7plusmn1.1 beatsmiddotmin^-1middotHz^-1). Similarly, although plasma Epi increased 10 times as high as the baseline level during the Epi administration at 10 mugmiddotkg^-1middoth^-1, dynamic gain of the transfer function was not changed significantly (7.5plusmn0.8, 7.9plusmn0.8, and 7.6plusmn1.2 beatsmiddotmin^-1middotHz^-1). In conclusion, plasma catecholamines of physiologically-relevant high concentrations did not interfere with the dynamic HR regulation by the cardiac sympathetic nerve.

Sympathetic Neural Regulation of Heart Rate Is Robust against High Plasma Catecholamines

The sympathetic regulation of heart rate (HR) may be attained by neural and humoral factors. With respect to the humoral factor, plasma noradrenaline (NA) and adrenaline (Adr) can reportedly increase to levels approximately 10 times higher than resting level during severe exercise. Whether such high plasma NA or Adr interfered with the sympathetic neural regulation of HR remained unknown. We estimated the transfer function from cardiac sympathetic nerve stimulation (SNS) to HR in anesthetized and vagotomized rabbits. An intravenous administration of NA (n = 6) at 1 and 10 microg.kg(-1).h(-1) increased plasma NA concentration (pg/ml) from a baseline level of 438 +/- 117 (mean +/- SE) to 974 +/- 106 and 6,830 +/- 917 (P < 0.01), respectively. The dynamic gain (bpm/Hz) of the transfer function did not change significantly (from 7.6 +/- 1.2 to 7.5 +/- 1.1 and 8.1 +/- 1.1), whereas mean HR (in bpm) during SNS slightly increased from 280 +/- 24 to 289 +/- 22 (P < 0.01) and 288 +/- 22 (P < 0.01). The intravenous administration of Adr (n = 6) at 1 and 10 microg.kg(-1).h(-1) increased plasma Adr concentration (pg/ml) from a baseline level of 257 +/- 86 to 659 +/- 172 and 2,760 +/- 590 (P < 0.01), respectively. Neither the dynamic gain (from 8.0 +/- 0.6 to 8.4 +/- 0.8 and 8.2 +/- 1.0) nor the mean HR during SNS (from 274 +/- 13 to 275 +/- 13 and 274 +/- 13) changed significantly. In contrast, the intravenous administration of isoproterenol (n = 6) at 10 microg.kg(-1).h(-1) significantly increased mean HR during SNS (from 278 +/- 11 to 293 +/- 9, P < 0.01) and blunted the transfer gain value at 0.0078 Hz (from 5.9 +/- 1.0 to 1.0 +/- 0.4, P < 0.01). In conclusion, high plasma NA or Adr hardly affected the dynamic sympathetic neural regulation of HR.

Effects of neuronal norepinephrine uptake blockade on baroreflex neural and peripheral arc transfer characteristics

Neuronal uptake is the most important mechanism by which norepinephrine (NE) is removed from the synaptic clefts at sympathetic nerve terminals. We examined the effects of neuronal NE uptake blockade on the dynamic sympathetic regulation of the arterial baroreflex because dynamic characteristics are important for understanding the system behavior in response to exogenous disturbance. We perturbed intracarotid sinus pressure (CSP) according to a binary white noise sequence in anesthetized rabbits, while recording cardiac sympathetic nerve activity (SNA), arterial pressure (AP), and heart rate (HR). Intravenous administration of desipramine (1 mg/kg) decreased the normalized gain of the neural arc transfer function from CSP to SNA relative to untreated control (1.03 ± 0.09 vs. 0.60 ± 0.08 AU/mmHg, mean ± SE, P < 0.01) but did not affect that of the peripheral arc transfer function from SNA to AP (1.10 ± 0.05 vs. 1.08 ± 0.10 mmHg/AU). The normalized gain of the transfer function from SNA to HR was unaffected (1.01 ± 0.04 vs. 1.09 ± 0.12 beats·min−1·AU−1). Desipramine decreased the natural frequency of the transfer function from SNA to AP by 28.7 ± 7.0% (0.046 ± 0.007 vs. 0.031 ± 0.002 Hz, P < 0.05) and that of the transfer function from SNA to HR by 64.4 ± 2.2% (0.071 ± 0.003 vs. 0.025 ± 0.002 Hz, P < 0.01). In conclusion, neuronal NE uptake blockade by intravenous desipramine administration reduced the total buffering capacity of the arterial baroreflex mainly through its action on the neural arc. The differential effects of neuronal NE uptake blockade on the dynamic AP and HR responses to SNA may provide clues for understanding the complex pathophysiology of cardiovascular diseases associated with neuronal NE uptake deficiency.

Non-invasive model-based estimation of the sinus node dynamic properties from spontaneous cardiovascular variability series

A non-invasive model-based approach to the estimation of sinus node dynamic properties is proposed. The model exploits the spontaneous beat-to-beat variability of heart period and systolic arterial pressure and the sampled respiration, thus surrogating the information from direct measures of neural activity. The residual heart period variability not related to baroreflex, to direct effects of respiration and to low frequency influences independent of baroreflex, is interpreted as the effect of the dynamic properties of the sinus node and modelled as a regression of the RR interval over its previous value. Therefore the sinus node transfer function is modelled by means of a filter with a real pole z = mu (and a zero in the origin). It was found that: first, in young healthy subjects the nodal tissue responded as a low-pass filter with mu = 0.76 +/- 0.12 (mean +/- SD); secondly, ageing did not significantly modify either its shape or gain at 0 Hz; thirdly, in heart transplant recipients, the dynamic transduction properties were lost (all-pass filter, p = 0.06 +/- 0.16, p < 0.001); fourthly, low-dose atropine left the sinus node dynamic properties unmodified; fifthly, high-dose atropine affected the dynamic transduction properties by increasing the gain at 0 Hz and rendering steeper its roll-off (the percent increase of mu with respect to baseline was 18.3 +/- 22.3, p < 0.05).

Neural influences on cardiovascular variability: possibilities and pitfalls

Altered variability in the cardiovascular system is associated with a range of cardiovascular diseases and increased mortality. Because blood pressure and heart rate show distinct low-frequency oscillations that appear to be affected by either vagal or sympathetic activity, it has been hoped that measurement of the strength of these oscillations could be used as an index of autonomic tone and thus form the basis of a diagnostic test. This review focuses on recent research that has examined the fundamental origin of variability associated with respiration and a slow oscillation at 0.1 Hz in the human. A new hypothesis is proposed to account for the slow oscillation in heart rate and blood pressure that incorporates components of the central nervous system, other reflex pathways regulating sympathetic activity, and resonance in the baroreflex control of blood pressure. Whereas it is clear that sympathetic activity and arterial baroreflexes are critical elements in producing cardiovascular variability, there is also evidence that other factors, including the ability of the vasculature to respond to sympathetic activity, appear to play a role in determining the strength of oscillations. Given the potential impact of other nonbaroreflex or nonautonomic pathways in affecting cardiovascular variability, it is proposed that one must use care in relating changes in the strength of an oscillation in blood pressure and heart rate as definitively due to a change in autonomic control.

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.

Norepinephrine reuptake, baroreflex dynamics, and arterial pressure variability in rats

This study examined the effect of norepinephrine reuptake blockade with desipramine (DMI) on the spontaneous variability of the simultaneously recorded arterial pressure (AP) and renal sympathetic nerve activity (SNA) in conscious rats. Acute DMI administration (2 mg/kg iv) depressed AP Mayer waves ( approximately 0.4 Hz) and increased low-frequency (<0.2 Hz) components of AP variability. DMI decreased renal SNA variability, especially due to the abolition of oscillations related to Mayer waves. To examine whether DMI-induced changes in AP and renal SNA variabilities could be explained by alterations in the dynamic characteristics of the baroreceptor reflex loop, the frequency responses of mean AP to aortic depressor nerve stimulation were studied in urethan-anesthetized rats. DMI accentuated the low-pass filter properties of the transfer function without significantly altering the fixed time delay. The frequency responses of iliac vascular conductance to stimulation of the lumbar sympathetic chain were studied in an additional group of anesthetized rats. DMI did not markedly alter the low-pass filter properties of the transfer function and slightly increased the fixed time delay. These results suggest that the DMI-induced decrease in the dynamic gain of the baroreceptor reflex is responsible for the decreased spontaneous renal SNA variability and the accompanying increased AP variability. The "slowing down" of baroreflex responses cannot be attributed to an effect of DMI at the vascular neuroeffector junction.