Dual mechanisms of angiotensin-induced activation of mouse sympathetic neurones.

Ang II directly activates neurones in sympathetic ganglia. Our goal was to define the electrophysiological basis of this activation. Neurones from mouse aortic-renal and coeliac ganglia were identified as either 'tonic' or 'phasic'. With injections of depolarizing currents, action potentials (APs) were abundant and sustained in tonic neurones (TNs) and scarce or absent in phasic neurones (PNs). Resting membrane potentials were equivalent in TNs (-48 +/- 2 mV, n = 18) and PNs (-48 +/- 1 mV, n = 23) while membrane resistance was significantly higher in TNs. Ang II depolarized and increased membrane resistance equally in both TNs (n = 8) and PNs (n = 8) but it induced APs only in TNs, and enhanced current-evoked APs much more markedly in TNs (P < 0.05). The AT1 receptor antagonist losartan (2 microm, n = 6) abolished all responses to Ang II, whereas the AT2 receptor blocker PD123,319 had no effect. The transient K+ current (IA), which was more than twice as large in TNs as in PNs, was significantly inhibited by Ang II in TNs only whereas the delayed sustained K+ current (IK), which was comparable in both TNs and PNs, was not inhibited. M currents were more prominent in PNs and were inhibited by Ang II. The IA channel blocker 4-aminopyridine triggered AP generation in TNs and prevented the Ang II-induced APs but not the depolarization. Blockade of M currents by oxotremorine M or linopirdine prevented the depolarizing action of Ang II. The protein kinase C (PKC) inhibitor H7 (10 microm, n = 9) also prevented the Ang II-induced inhibition of IA and the generation APs but not the depolarization nor the inhibition of M currents. Conversely, the PKC agonist phorbol 12-myristate 13-acetate mimicked the Ang II effects by triggering APs. The results indicate that Ang II may increase AP generation in sympathetic neurones by inducing a PKC-dependent inhibition of IA currents, and a PKC-independent depolarization through inhibition of M currents. The differential expression of various K+ channels and their sensitivity to phosphorylation by PKC may determine the degree of activation of sympathetic neurones and hence may influence the severity of the hypertensive response.

Slow inactivation of sodium currents in the rat nodose neurons.

Nodose neurons express sodium currents that can be differentiated based on their sensitivity to tetrodotoxin. Several studies have demonstrated significant differences in voltage-dependence and kinetics of activation and inactivation between tetrodotoxin-sensitive and tetrodotoxin-resistant currents. However, little is known about the slow inactivation. Using whole cell patch-clamp technique fast and slow inactivation of sodium currents were studied in cultured rat nodose neurons. Tetrodotoxin-resistant currents recovered much more rapidly after a 15-ms depolarization than tetrodotoxin-sensitive currents. However, repeated 5-ms depolarizations at 10 Hz induced a cumulative inhibition that was more prolonged in tetrodotoxin-resistant compared to tetrodotoxin-sensitive currents. Consistent with these findings, slow inactivation proceeded more rapidly and was more complete for the tetrodotoxin-resistant than for tetrodotoxin-sensitive currents. While the voltage-dependence of fast inactivation differed significantly between the pharmacologically distinct currents, the voltage-dependence of slow inactivation was similar for both sodium currents. We conclude that slow inactivation of sodium currents can be triggered by trains of brief depolarizations. The resulting prolonged decrease in membrane excitability may contribute to the different patterns of action potential generation observed in primary afferent neurons.

Mechanisms determining sensitivity of baroreceptor afferents in health and disease.

Baroreceptors sense and signal the central nervous system of changes in arterial pressure through a series of sensory processes. An increase in arterial pressure causes vascular distension and baroreceptor deformation, the magnitude of which depends on the mechanical viscoelastic properties of the vessel wall. Classic methods (e.g., isolated carotid sinus preparation) and new approaches, including studies of isolated baroreceptor neurons in culture, gene transfer using viral vectors, and genetically modified mice have been used to define the cellular and molecular mechanisms that determine baroreceptor sensitivity. Deformation depolarizes the nerve endings by opening a new class of mechanosensitive Ion channel. This depolarization triggers action potential discharge through opening of voltage-dependent sodium (Na+) and potassium (K+) channels at the "spike initiating zone" (SIZ) near the sensory terminals. The resulting baroreceptor activity and its sensitivity to changes in pressure are modulated through a variety of mechanisms that influence these sensory processes. Modulation of voltage-dependent Na+ and K+ channels and the Na+ pump at the SIZ by membrance potential, action potential discharge, and chemical autocrine and paracrine factors are important mechanisms contributing to changes in baroreceptor sensitivity during sustained increases in arterial pressure and in pathological states associated with endothelial dysfunction, oxidative stress, and platelet activation.

ENaC subunits are molecular components of the arterial baroreceptor complex.

Mechanosensation is essential to the perception of our environment. It is required for hearing, touch, balance, proprioception, and blood pressure homeostasis. Yet little is known about the identity of ion-channel complexes that transduce mechanical stimuli into neuronal responses. Genetic studies in Caenorhabditis elegans suggest that members of the DEG/ENaC family may be mechanosensors. Therefore we tested the hypothesis that mammalian epithelial Na(+)-channel (ENaC) subunits contribute to the mechanosensor in baroreceptor neurons. The data presented here show that ENaC transcripts and proteins are expressed in mechanosensory neurons and at the putative sites of mechanotransduction in baroreceptor sensory-nerve terminals. Additionally, known ENaC inhibitors, amiloride and benzamil, disrupt mechanotransduction in arterial baroreceptor neurons. These data are consistent with the hypothesis that DEG/ENaC proteins are components of mechanosensitive ion-channel complexes.

A novel effect of angiotensin on renal sympathetic nerve activity in mice.

OBJECTIVE: The goals of this study were to characterize the effects of angiotensin II (Ang II) on renal sympathetic nerve activity (RSNA) and to define mechanisms of its actions in mice. DESIGN: The experiments were performed in sodium pentobarbital anesthetized C57BL/6J mice to investigate the effects of intravenous administration of Ang II on RSNA recorded from renal sympathetic post-ganglionic nerve fibers. RESULTS: Intravenous (i.v.) administration of Ang II (4 ng/g) increased arterial pressure and evoked a biphasic change in RSNA: inhibition of high-amplitude phasic bursts of RSNA secondary to the initial rise of arterial pressure followed by activation of low-amplitude continuously discharging RSNA that exceeded baseline activity (255 +/- 72% baseline, n = 8). The peak change of mean arterial pressure (MAP) was +60 +/- 4 mmHg (n = 8). In the same group of animals, norepinephrine (40 ng/g) caused an equivalent increase in MAP (+57 +/- 5 mmHg) and essentially abolished RSNA. The Ang II-induced activation of RSNA was dose-dependent (0.5-4 ng/g, n = 7) and was abolished by the Ang II type 1 (AT1) receptor blocker, losartan (10 microg/g, i.v.) (301 +/- 61 versus 117 +/- 22% baseline, before versus after losartan, n = 5). The ganglionic blocker, hexamethonium (30 microg/g, i.v.), eliminated baseline high-amplitude bursts of RSNA but did not blunt the Ang II-induced RSNA (n = 6). In baroreceptor denervated and vagotomized mice, Ang II failed to inhibit high-amplitude bursts of RSNA but continued to trigger low-amplitude continuous RSNA. CONCLUSION: We conclude that Ang II activates renal sympathetic nerves that discharge in a continuous pattern, distinctly different than the normal baseline high-amplitude bursts of RSNA. The mechanism may involve direct activation of post-ganglionic sympathetic neurons mediated through AT1 receptors.

Angiotensin selectively activates a subpopulation of postganglionic sympathetic neurons in mice.

Angiotensin II (Ang II) increases renal sympathetic nerve activity in anesthetized mice before and after ganglionic blockade, suggesting that Ang II may directly activate postganglionic sympathetic neurons. The present study directly tested this hypothesis in vitro. Neurons were dissociated from aortic-renal and celiac ganglia of C57BL/6J mice. Cytosolic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratio imaging using fura 2. Ang II increased [Ca(2+)](i) in a subpopulation of sympathetic neurons. At a concentration of 200 nmol/L, 14 (67%) of 21 neurons responded with a rise in [Ca(2+)](i). The Ang II type 1 (AT(1)) receptor blocker (losartan, 2 micromol/L) but not the Ang II type 2 (AT(2)) receptor blocker (PD123,319, 4 micromol/L) blocked this effect. The Ang II-induced [Ca(2+)](i) increase was abolished by removal of extracellular Ca(2+) but not altered by depletion of intracellular Ca(2+) stores with thapsigargin. Ang II no longer elicited a [Ca(2+)](i) increase in the presence of lanthanum (25 micromol/L). The specific N-type and L-type Ca(2+) channel blockers, omega-conotoxin GVIA and nifedipine, respectively, significantly inhibited the Ang II-induced [Ca(2+)](i) increase. The protein kinase C inhibitor H7 but not the protein kinase A inhibitor H89 blocked the response to Ang II. These results demonstrate that Ang II selectively activates a subpopulation of postganglionic sympathetic neurons in aortic-renal and celiac ganglia, triggering Ca(2+) influx through voltage-gated Ca(2+) channels. This effect is mediated through AT(1) receptors and requires the activation of protein kinase C. The activation of a subgroup of sympathetic neurons by Ang II may exert unique effects on kidney function in pathological states associated with elevated Ang II.

Localization of beta and gamma subunits of ENaC in sensory nerve endings in the rat foot pad.

The molecular mechanisms underlying mechanoelectrical transduction and the receptors that detect light touch remain uncertain. Studies in Caenorhabditis elegans suggest that members of the DEG/ENaC cation channel family may be mechanoreceptors. Therefore, we tested the hypothesis that subunits of the mammalian epithelial Na(+) channel (ENaC) family are expressed in touch receptors in rat hairless skin. We detected betaENaC and gammaENaC, but not alphaENaC transcripts in cervical and lumbar dorsal root ganglia (DRG). Using immunofluorescence, we found betaENaC and gammaENaC expressed in medium to large lumbar DRG neurons. Moreover, we detected these two subunits in Merkel cell-neurite complexes, Meissner-like corpuscles, and small lamellated corpuscles, specialized mechanosensory structures of the skin. Within these structures, betaENaC and gammaENaC were localized in the nerve fibers believed to contain the sensors responsive to mechanical stress. Thus beta and gammaENaC subunits are good candidates as components of the molecular sensor that detects touch.

Nitric oxide enhances slow inactivation of voltage-dependent sodium currents in rat nodose neurons.

Nitric oxide (NO) can alter neuronal excitability by decreasing the current through voltage-sensitive sodium channels. We hypothesized that NO inhibits sodium currents in part by promoting slow inactivation. We performed whole-cell voltage clamp experiments on sensory neurons from the nodose ganglion. The voltage-dependence of inactivation was determined after stepping the neurons to various potentials between -100 and 30 mV for 200 ms (fast inactivation) and 3 min (slow inactivation) prior to depolarization to 10 mV. NO shifted the voltage of half-inactivation for fast and slow inactivation to more hyperpolarized potentials by 7 and 12 mV, respectively. Sodium currents exhibited a more profound closed state and slow inactivation after exposure to NO. These results demonstrate for the fist time that the slow inactivation of sodium currents is subject to modulation. Due to its effects on fast and slow inactivation, NO may cause a prolonged decrease in neuronal excitability.

Central vagotonic effects of atropine modulate spectral oscillations of sympathetic nerve activity.

BACKGROUND: Low-dose atropine causes bradycardia either by acting on the sinoatrial node or by its effects on central muscarinic receptors increasing vagal activity. Any central muscarinic effects of high-dose atropine on RR interval are masked by peripheral muscarinic blockade at the sinoatrial node, which causes tachycardia. Effects of central parasympathetic activation on sympathetic activity are not known. METHODS AND RESULTS: Using power spectral analysis of RR interval, intra-arterial blood pressure, respiration, and muscle sympathetic nerve activity (MSNA), we examined the effects of both low (2 microgram/kg IV) and high (15 microgram/kg IV) doses of atropine. After low-dose atropine, RR increased by 9+/-1% (P<0.0001), the low-frequency (LF) component (in normalized units, NU) of RR variability decreased by -32+/-8%, and the high-frequency (HF)NU component increased (+74+/-19%); hence, LF/HF of RR variability fell by 52+/-10% (all P<0.01). Although overall MSNA did not change, LFNU of MSNA decreased (-15+/-5%), HFNU of MSNA increased (+31+/-3%), and LF/HF of MSNA fell (-41+/-8%) (all P<0.01). After high-dose atropine, LFNU of MSNA decreased (-17+/-12%), HFNU of MSNA increased (+22+/-3%), and LF/HF of MSNA fell (-51+/-21%) (all P<0.02). CONCLUSIONS: Increasing central parasympathetic activity with low-dose atropine is associated with an increase in the HF and a decrease in the LF oscillations of both RR interval and MSNA variability. High-dose atropine similarly induces an increase in the HF and a decrease in the LF components of MSNA variability. Thus, central parasympathetic activation is able to modulate the oscillatory characteristics of sympathetic nerve traffic to peripheral blood vessels.

Mechanosensitive ion channels in putative aortic baroreceptor neurons.

Cell-attached patch-clamp experiments were performed on dissociated neurons from nodose ganglia of adult rats. Putative aortic baroreceptor neurons were identified by labeling nerve endings in the adventitia of the aortic arch with the carbocyanine dye DiI. Whereas previous experiments demonstrated the presence of mechanosensitive (MS) whole cell currents, these experiments studied single MS ion channels and examined the influence of culture conditions on their expression. Single MS channels were activated by applying negative pressure through the recording pipette. Channel openings became more frequent as the negative pressure was increased, with open probability increasing significantly above 30 mmHg. MS channels had a slope conductance of 114 pS and a reversal potential of approximately 0 mV, consistent with a nonspecific cation conductance. Channels were not affected by antagonists of voltage-gated conductances but were blocked by 20 microM gadolinium, a known blocker of MS ion channels. When nodose neurons were cocultured with aortic endothelial cells, but not aortic smooth muscle cells, the percentage of patches exhibiting MS ion channels increased significantly, suggesting that aortic endothelial cells secrete a diffusible factor that increases channel expression.

Nitric oxide as an autocrine regulator of sodium currents in baroreceptor neurons.

Arterial baroreceptors are mechanosensitive nerve endings in the aortic arch and carotid sinus that play a critical role in acute regulation of arterial blood pressure. A previous study has shown that nitric oxide (NO) or NO-related species suppress action potential discharge of baroreceptors. In the present study, we investigated the effects of NO on Na+ currents of isolated baroreceptor neurons in culture. Exogenous NO donors inhibited both tetrodotoxin (TTX) -sensitive and -insensitive Na+ currents. The inhibition was not mediated by cGMP but by NO interaction with channel thiols. Acute inhibition of NO synthase increased the Na+ currents. NO scavengers (hemoglobin and ferrous diethyldithiocarbamate) increased Na+ currents before but not after inhibition of NO synthase. Furthermore, NO production in the neuronal cultures was detected by chemiluminescence and immunoreactivity to the neuronal isoform of NO synthase was identified in fluorescently identified baroreceptor neurons. These results indicate that NO/NO-related species function as autocrine regulators of Na+ currents in baroreceptor neurons. Modulation of Na+ channels may represent a novel response to NO.

A molecular component of the arterial baroreceptor mechanotransducer.

Baroreceptor nerve endings detect acute fluctuations in arterial pressure. We tested the hypothesis that members of the DEG/ENaC family of cation channels, which are responsible for touch sensation in Caenorhabditis elegans, may be components of the baroreceptor mechanosensor. We found the gamma subunit of ENaC localized to the site of mechanotransduction in baroreceptor nerve terminals innervating the aortic arch and carotid sinus. A functional role for DEG/ENaC members was suggested by blockade of baroreceptor nerve activity and baroreflex control of blood pressure by an amiloride analog that inhibits DEG/ENaC channels. These data suggest that ENaC subunits may be components of the baroreceptor mechanotransducer and pave the way to a better definition of mechanisms responsible for blood pressure regulation and hypertension.

The prostacyclin analogue carbacyclin inhibits Ca(2+)-activated K+ current in aortic baroreceptor neurones of rats.

1. Previous studies indicate that prostacyclin (PGI2) increases the activity of baroreceptor afferent fibres. The purpose of this study was to test the hypothesis that PGI2 inhibits Ca(2+)-activated K+ current (IK(Ca))in isolated baroreceptor neurones in culture. 2. Rat aortic baroreceptor neurones in the nodose ganglia were labelled in vivo by applying a fluorescent dye (DiI) to the aortic arch 1-2 weeks before dissociation of the neurones. Outward K+ currents in baroreceptor neurones evoked by depolarizing voltage steps from a holding potential of -40 mV were recorded using the whole-cell patch-clamp technique. 3. Exposure of baroreceptor neurones to the stable PGI2 analogue carbacyclin significantly inhibited the steady-state K+ current in a dose-dependent and reversible manner. The inhibition of K+ current was not caused indirectly by changes in cytosolic Ca2+ concentration. The Ca(2+)-activated K+ channel blocker charybdotoxin (ChTX, 10(-7) M) also inhibited the K+ current. In the presence of ChTX or in the absence of Ca2+, carbacyclin failed to inhibit the residual K+ current. Furthermore, in the presence of high concentrations of carbacyclin, ChTX did not cause further reduction of K+ current. 4. Carbacyclin-induced inhibition of IK(Ca) was mimicked by 8-bromo-cAMP and by activation of G-protein with GTP gamma S. The inhibitory effect of carbacyclin on IK(Ca) was abolished by GDP beta S, which blocks G-protein activation, and by a selective inhibitor of cAMP-dependent protein kinase, PKI5-24. 5. The results demonstrate that carbacyclin inhibits ChTX-sensitive IK(Ca) in isolated aortic baroreceptor neurones by a G-protein-coupled activation of cAMP-dependent protein kinase. This mechanism may contribute to the PGI2-induced increase in baroreceptor activity demonstrated previously.

Reactive oxygen species and calcium homeostasis in cultured human intestinal smooth muscle cells.

Reactive oxygen species (ROS) significantly alter cell function. We examined the effects of hydrogen peroxide (H2O2) and xanthine/xanthine oxidase (X/XO) on isolated intestinal muscle cells. We assessed cell viability with the exclusion dye trypan blue and assayed the effects of H2O2 and X/XO on the intracellular redox state with the fluorescent probe 2',7'-dichlorofluorescein. Intracellular calcium concentration was measured in cells loaded with fura 2-acetoxymethyl ester, and we recorded whole membrane currents with conventional patch-clamp methods. Cells remained viable after a 5-min exposure to H2O2 and X/XO. H2O2 and X/XO led to a significant rise of the intracellular concentration of ROS. H2O2 (270 microM to 2.7 mM) as well as X/XO (0.25-16 mU; 0.5 mM xanthine) significantly increased intracellular calcium concentrations. Depletion of intracellular calcium with ryanodine or thapsigargin did not abolish the effect of ROS on the intracellular calcium concentration. In the absence of external calcium or in the presence of the calcium channel blocker nifedipine, H2O2 and X/XO still increased the intracellular calcium level. Thus calcium influx and calcium release from internal stores contributed to this rise in cytosolic calcium. Catalase and superoxide dismutase blunted or completely abolished the changes in calcium concentration elicited by H2O2 and X/XO. Exposure to ROS resulted in a rapid decline of the membrane resistance without significant changes in voltage-sensitive ion currents. We conclude that ROS disrupt the calcium homeostasis of cells at concentrations that do not lead to immediate cell death. The resulting elevation in cytosolic free calcium will activate a variety of biochemical reactions and may thus contribute to the cytotoxicity of reactive oxygen molecules.

Non-voltage-gated Ca2+ influx through mechanosensitive ion channels in aortic baroreceptor neurons.

The mechanisms underlying mechanotransduction in baroreceptor neurons (BRNs) are undefined. In this study, we specifically identified aortic baroreceptor neurons in primary neuronal cell cultures from nodose ganglia of rats. Aortic baroreceptor neurons were identified by labeling their soma with the fluorescent dye 1,1'-dioleyl-3,3,3',3'-tetramethylin-docarbocyanine (DiI) applied to the aortic arch. Using Ca2+ imaging with fura 2, we examined these BRNs for evidence of Ca2+ influx and determined its mechanosensitivity and voltage dependence. Mechanical stimuli were produced by ejecting buffer from a micropipette onto the cell surface with a pneumatic picopump, producing a shift in the center of mass of the cell that was related to intensity of stimulation. Ninety-three percent of DiI-labeled neurons responded to mechanical stimulation with an increase in [Ca2+]i. The magnitude of the increases in [Ca2+]i was directly related to the intensity of the stimulus and required the presence of external Ca2+. The trivalent cations Gd3+ or La3+ in equimolar concentrations (20 mumol/L) eliminated the K(+)-induced rises in [Ca2+]i, demonstrating that both trivalent cations are equally effective at blocking voltage-gated Ca2+ channels in these baroreceptor neurons. In contrast, the mechanically induced increases in [Ca2+]i were blocked by Gd3+ (20 mumol/L) only and not by La3+ (20 mumol/L). Stretch-activated channels (SACs) have been shown in other preparations to be blocked by Gd3+ specifically. Our data demonstrate that (1) BRNs, specifically identified as projecting to the aortic arch, have ion channels that are sensitive to mechanical stimuli; (2) mechanically induced Ca2+ influx in these cells is mediated by a Gd(3+)-sensitive ion channel and not by voltage-gated Ca2+ channels; (3) the magnitude of the Ca2+ influx is dependent on the intensity of the stimulus and the degree and duration of deformation; and (4) repeated stimuli of the same intensity result in comparable increases in [Ca2+]i. We conclude that mechanical stimulation increases Ca2+ influx into aortic BRNs independent of voltage-gated Ca2+ channels. The results suggest that Gd(3+)-sensitive SACs are the mechanoelectrical transducers in baroreceptors.

Mechanical stimulation of neurites generates an inward current in putative aortic baroreceptor neurons in vitro.

We investigated the responses of putative aortic baroreceptor neurons to mechanical stimulation of their processes. Putative aortic baroreceptor neurons were identified by applying the carbocyanine dye DiI to the adventitia of the aortic arch of anesthetized rats. After at least 1 week, the nodose ganglia were removed and the neurons were cultured. Within 2-3 days, neurite outgrowth was evident on many neurons. The soma was voltage-clamped using whole cell patch clamp techniques while the neurites were deformed with pneumatic ejection of bath solution at 5-15 psi using a glass pipette (7-15 microm) positioned at least 50 microm from the neurite. Mechanical stimulation induced an inward current in 15 out of 17 putative aortic baroreceptor neurons. The magnitude of the current was related to the intensity of stimulation. The current was blocked by 20 microM gadolinium (n = 11), a reported blocker of mechanically sensitive ion channels, or by incubating the cells overnight in 10 microM phalloidin, which binds to actin filaments (n = 5). We conclude that mechanical deformation of neurites of putative baroreceptor neurons activates a mechanosensitive inward current in the soma and that the cytoskeletal actin filaments are involved in the generation of this current.

Relationship between spectral components of cardiovascular variabilities and direct measures of muscle sympathetic nerve activity in humans.

BACKGROUND: Spectral analysis of RR interval and systolic arterial pressure variabilities may provide indirect markers of the balance between sympathetic and vagal cardiovascular control. METHODS AND RESULTS: We examined the relationship between power spectral measurements of variabilities in RR interval, systolic arterial pressure, and muscle sympathetic nerve activity (MSNA) obtained by microneurography over a range of blood pressures. In eight healthy human volunteers, MSNA, RR interval, intra-arterial pressure, and respiration were measured during blood pressure reductions induced by nitroprusside and during blood pressure increases induced by phenylephrine. Both low-frequency (LF; 0.10 +/- 0.01 Hz) and high-frequency (HF; 0.23 +/- 0.01 Hz) components were detected in MSNA variability. Increasing levels of MSNA were associated with a shift of the spectral power toward its LF component. Decreasing levels of MSNA were associated with a shift of MSNA spectral power toward the HF component. Over the range of pressure changes, the LF component of MSNA variability was positively and tightly correlated with LF components of RR interval (in normalized units; P < 10(-6)) and of systolic arterial pressure variability (both in millimeters of mercury squared and normalized units; P < 5 x 10(-5) and P < 5 x 10(-6), respectively). The HF component of MSNA variability was positively and tightly correlated with the HF component (in normalized units) of RR-interval variability (P < 3 x 10(-4)) and of systolic arterial pressure variability (P < .01). CONCLUSIONS: During sympathetic activation in normal humans, there is a predominance in the LF oscillation of blood pressure, RR interval, and sympathetic nerve activity. During sympathetic inhibition, the HF component of cardiovascular variability predominates. This relationship is best seen when power spectral components are normalized for total power. Synchronous changes in the LF and HF rhythms of both RR interval and MSNA during different levels of sympathetic drive are suggestive of common central mechanisms governing both parasympathetic and sympathetic cardiovascular modulation.