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

Derangement of open-loop static and dynamic characteristics of the carotid sinus baroreflex in streptozotocin-induced type 1 diabetic rats

Although diabetes mellitus (DM) is a major risk factor for cardiovascular diseases, changes in open-loop static and dynamic characteristics of the arterial baroreflex in the early phase of DM remain to be clarified. We performed an open-loop systems analysis of the carotid sinus baroreflex in type 1 DM rats 4 to 5 wk after intraperitoneal streptozotocin injection ( n = 9) and we compared the results with control rats ( n = 9). The operating-point baroreflex gain was maintained in the DM rats compared with the control rats (2.07 ± 0.67 vs. 2.66 ± 0.22 mmHg/mmHg, P = 0.666). However, the range of arterial pressure (AP) control was narrower in the DM than in the control group (48.0 ± 5.0 vs. 77.1 ± 4.5 mmHg, P = 0.001), suggesting that the reserve for AP buffering is lost in DM. Although baroreflex dynamic characteristics were relatively preserved, coherences were lower in the DM than in the control group. The decreased coherence in the neural arc may be related to the narrowed quasi-linear range in the static relationship between carotid sinus pressure and sympathetic nerve activity in the DM group. Although the reason for the decreased coherences in the peripheral arc and the total reflex arc was inconclusive, the finding may indicate a loss of integrity of the baroreflex-mediated sympathetic AP control in the DM group. The derangement of the baroreflex dynamic characteristics is progressing occultly in this early stage of type 1 DM in a manner where dynamic gains are relatively preserved around the normal operating point.

Desipramine increases cardiac parasympathetic activity via α2-adrenergic mechanism in rats

Desipramine (DMI) is a blocker of neuronal norepinephrine (NE) uptake transporter. Although intravenous DMI has been shown to cause centrally-mediated sympathoinhibition and peripheral NE accumulation, its parasympathetic effect remains to be elucidated. We hypothesized that intravenous DMI activates the cardiac vagal nerve via an α₂-adrenergic mechanism. Using a cardiac microdialysis technique, changes in myocardial interstitial acetylcholine (ACh) levels in the left ventricular free wall in response to intravenous DMI (1mg·kg^-1) were examined in anesthetized rats. In rats with intact vagi (n=7), intravenous DMI increased ACh from 1.67±0.43 to 2.48±0.66nM (P<0.01). In rats with vagotomy (n=5), DMI did not significantly change ACh (from 0.92±0.16 to 0.85±0.23nM). In rats with intact vagi pretreated with intravenous yohimbine (2mg·kg^-1), DMI did not significantly change ACh (from 1.25±0.23 to 1.13±0.15nM). In conclusion, while DMI is generally considered to be an agent that predominantly affects sympathetic neurotransmission, it can activate the cardiac vagal nerve via α₂-adrenergic stimulation in experimental settings in vivo.

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.

Cardiac Response Rate Variability in Physically Abused Women of Childbearing Age

Physical abuse directly affects maternal and fetal/infant health, with documented reports of higher rates of pregnancy termination, neonatal death, and lower birth weights. Although the Centers for Disease Control and Prevention recommend repeated interviews of women of childbearing age to screen for abuse, the paper-and-pencil instruments available for such screening are adversely affected by the hesitancy of women to disclose physical abuse. Biophysical measures of physiological stress adaptations may hold potential for identifying physically abused childbearing women. This pilot investigation used a Latin square design to assess the effects of physically abusive trauma on the cardiac rate response of three clinical groups and one control group of childbearing-age women. Participants were screened using the Child-bearing Health Questionnaire. Cardiac response rates were measured during a standardized orthostatic challenge using a Tanito cardiac rate response monitor. Forty participants participated with an average age of 27. Multiple analyses of variance revealed that there were significant differences between cardiac rate responses at the 5-min interval. Post hoc testing using Dunnett’s t indicated that only the abused pregnant women had significantly higher cardiac responses to orthostatic challenges; differences were apparent at the 5-min testing period. The findings suggest that physical abuse may alter the vasovagal response beyond the attenuation associated with pregnancy. These findings support further testing with larger samples to identify vasovagal changes in abused pregnant women.

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.

Acute effects of arterial baroreflex on sympathetic nerve activity and plasma norepinephrine concentration

Arterial pressure (AP) elevates as a logarithmic function of exogenously administered dose of norepinephrine (NE). In contrast, AP is nearly linearly correlated with efferent sympathetic nerve activity (SNA) during acute baroreflex intervention. The present study aimed at quantifying the relationship between SNA and plasma NE concentration during acute baroreflex intervention. Carotid sinus regions were isolated from systemic circulation in five Wistar Kyoto rats, and carotid sinus pressure was changed among 60, 100, 120, 140, and 180 mm Hg every 2 min. Arterial blood (0.2 ml) was obtained at each pressure level for plasma NE measurement. Maximum AP and minimum AP were 153.34 ± 6.28 and 67.31 ± 4.92 mm Hg, respectively, in response to pressure perturbation. Plasma NE correlated linearly with SNA for individual animal data (slope: 0.957 ± 0.090 pg · ml(-1) · %(-1), intercept: 46.57 ± 7.22 pg/ml, r(2): ranged from 0.923 to 0.992) and also for group averaged data (NE = 0.956 × SNA + 47.97, r(2 )= 0.982). Blockade of neuronal NE uptake by intravenous desipramine (1 mg/kg) administration increased the slope (2.966 ± 0.686 pg · ml(-1) · %(-1), P < 0.05) and the intercept (168.73 ± 28.53 pg/ml, P < 0.01) of the plasma NE-SNA relationship. These results indicate that the relationship between SNA and plasma NE concentration was nearly linear within the normal physiological range of acute baroreflex control of AP. While plasma NE concentration can reflect changes in SNA, it may also overestimate the sympathetic outflow from the central nervous system when neuronal NE uptake is impaired systemically.

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

Although baroreceptors are known to reset to operate in a higher pressure range in spontaneously hypertensive rats (SHR), the total profile of dynamic arterial pressure (AP) regulation remains to be clarified. We estimated open-loop transfer functions of the carotid sinus baroreflex in SHR and Wistar Kyoto (WKY) rats. Mean input pressures were set at 120 (WKY₁₂₀ and SHR₁₂₀) and 160 mmHg (SHR₁₆₀). The neural arc transfer function from carotid sinus pressure to efferent splanchnic sympathetic nerve activity (SNA) revealed derivative characteristics in both WKY and SHR. The slope of dynamic gain (in decibels per decade) between 0.1 and 1 Hz was not different between WKY₁₂₀ (10.1 ± 1.0) and SHR₁₂₀ (10.4 ± 1.1) but was significantly greater in SHR₁₆₀ (13.2 ± 0.8, P < 0.05 with Bonferroni correction) than in SHR₁₂₀. The peripheral arc transfer function from SNA to AP showed low-pass characteristics. The slope of dynamic gain (in decibels per decade) did not differ between WKY₁₂₀ (-34.0 ± 1.2) and SHR₁₂₀ (-31.4 ± 1.0) or between SHR₁₂₀ and SHR₁₆₀ (-32.8 ± 1.3). The total baroreflex showed low-pass characteristics and the dynamic gain at 0.01 Hz did not differ between WKY₁₂₀ (0.91 ± 0.08) and SHR₁₂₀ (0.84 ± 0.13) or between SHR₁₂₀ and SHR₁₆₀ (0.83 ± 0.11). In both WKY and SHR, the declining slope of dynamic gain was significantly gentler for the total baroreflex than for the peripheral arc, suggesting improved dynamic AP response in the total baroreflex. In conclusion, the dynamic characteristics of AP regulation by the carotid sinus baroreflex were well preserved in SHR despite significantly higher mean AP.

In vivo direct monitoring of interstitial norepinephrine levels at the sinoatrial node

We assessed in vivo interstitial norepinephrine (NE) levels at the sinoatrial node in rabbits, using microdialysis technique. A dialysis probe was implanted adjacent to the sinoatrial node of an anesthetized rabbit and dialysate was sampled during sympathetic nerve stimulation. Atrial dialysate NE concentration correlated well with heart rate. Desipramine significantly increased dialysate NE concentrations both before and during sympathetic nerve stimulation compared with the absence of desipramine. However, desipramine did not affect the relation between heart rate and dialysate NE concentration. These results suggest that atrial dialysate NE level reflects the relative change of NE concentration in the synaptic cleft. Microdialysis is a powerful tool to assess in vivo interstitial NE levels at the sinoatrial node.

Transfer function characteristics of the neural and peripheral arterial baroreflex arcs at rest and during postexercise muscle ischemia in humans

Previous studies have demonstrated an increase in the arterial baroreflex (ABR) control of muscle sympathetic nerve activity (MSNA) during isolated activation of the muscle metaboreflex with postexercise muscle ischemia (PEMI). However, the increased ABR-MSNA control does not appear to manifest in an enhancement in the ABR control of arterial blood pressure (BP), suggesting alterations in the transduction of MSNA into a peripheral vascular response and a subsequent ABR-mediated change in BP. Thus we examined the operating gains of the neural and peripheral arcs of the ABR and their interactive relationship at rest and during muscle metaboreflex activation. In nine healthy subjects, graded isolation of the muscle metaboreflex was achieved by PEMI following isometric handgrip performed at 15% and 30% maximal voluntary contraction (MVC). To obtain the sensitivities of the ABR neural and peripheral arcs, the transfer function gain from BP to MSNA and MSNA to femoral vascular conductance, respectively, was analyzed. No changes from rest were observed in the ABR neural or peripheral arcs during PEMI after 15% MVC handgrip. However, PEMI following 30% MVC handgrip increased the low frequency (LF) transfer function gain between BP and MSNA (ABR neural arc; +58 +/- 28%, P = 0.036), whereas the LF gain between MSNA and femoral vascular conductance (ABR peripheral arc) was decreased from rest (-36 +/- 8%, P = 0.017). These findings suggest that during high-intensity muscle metaboreflex activation an increased ABR gain of the neural arc appears to offset an attenuation of the peripheral arc gain to help maintain the overall ABR control of systemic BP.

Yohimbine Attenuates Baroreflex-Mediated Bradycardia in Humans

α-2 Adrenoreceptor stimulation profoundly augments baroreflex-mediated bradycardia presumably through parasympathetic activation. We tested the hypothesis that endogenous α-2 adrenergic tone mediates a similar response. In 10 healthy men (age: 33±3 years; body mass index: 24±1.3 kg/m 2 ), we determined baroreflex control of heart rate and sympathetic traffic after ingestion of the selective α-2 adrenoceptor antagonist yohimbine (20 mg) or placebo. Testing was conducted in a randomized, double-blind, crossover fashion. We measured heart rate, brachial and finger blood pressure, and muscle sympathetic nerve activity. Sympathetic and parasympathetic baroreflex curves were determined using incremental phenylephrine and nitroprusside infusions (0.3, 0.6, 0.9, 1.2, and 1.5 μg/kg per minute). Plasma norepinephrine increased with yohimbine (50±38 ng/L; P <0.05) and was unchanged with placebo (2.2±7.6 ng/L). Blood pressure increased 13±4/8±1 mm Hg with yohimbine and 6±2/3±1 mm Hg with placebo ( P <0.01). HR increased 5±1 bpm with yohimbine but did not change with placebo ( P <0.01). Ninety minutes after drug ingestion, resting muscle sympathetic nerve activity was similar with yohimbine and with placebo. Baroreflex control of heart rate was decreased with yohimbine (6 ms/mm Hg versus 10 ms/mm Hg; P <0.01) and reset to higher blood pressure and heart rate values. In contrast, yohimbine did not alter the sympathetic baroreflex curve. Yohimbine selectively attenuates baroreflex heart rate control in normotensive young men possibly through parasympathetic mechanisms.

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.

Contribution of baroreflex sensitivity and vascular reactivity to variable haemodynamic responses to cocaine in conscious rats

1. Baroreflex function is critical for short-term arterial pressure regulation and decreased baroreflex responsivity may predict a predisposition to hypertension and sudden cardiac death. In the present study, we assessed whether baroreflex sensitivity (BRS) and/or vascular reactivity covary with haemodynamic responsiveness to cocaine in vascular and mixed responders. 2. We assessed the heart rate index of BRS in resting animals. We examined dose-response relationships to pressor and depressor agents to determine cardiovascular reactivity. Subsequently, rats were given cocaine (5 mg/kg, i.v.) to classify them as vascular or mixed responders. Vascular responders (n=16) were defined as those rats with a substantial (>8%) decrease in cardiac output in response to cocaine owing to a larger increase in systemic vascular resistance. The remaining rats (n=8) were mixed responders because they had smaller increases in vascular resistance and little change or an increase in cardiac output. 3. The BRS determined with angiotensin (Ang) II, but not with phenylephrine, was impaired in mixed responders compared with vascular responders. At equipressor doses, there were significantly greater reductions in cardiac output in vascular responders compared with mixed responders in response to phenylephrine or AngII. Methacholine produced greater decreases in heart rate in vascular responders, suggesting greater muscarinic responsivity. 4. We conclude that differences in vascular reactivity to AngII may contribute to differences in haemodynamic response profiles to cocaine in individual rats. More importantly, the differences in vascular responsivity and BRS do not appear to be primary determinants of haemodynamic response variability.