Responses of cardiac sympathetic nerve activity to changes in circulating volume differ in normal and heart failure sheep

Sympathetic transduction of cardiac sympathetic nerve activity in healthy, conscious sheep

Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Recently, the transduction of resting bursts of muscle SNA (MSNA) has been investigated and shown to have a role in the maintenance of blood pressure through changes in vascular tone in humans. In the present study, we investigate whether directly recorded resting cardiac SNA (CSNA) regulates heart rate (HR), coronary blood flow (CoBF), coronary vascular conductance (CVC), cardiac output (CO) and mean arterial pressure. Instrumentation was undertaken to record CSNA and relevant vascular variables in conscious sheep. Recordings were performed at baseline, as well as after the infusion of a β-adrenoceptor blocker (propranolol) to determine the role of β-adrenergic signalling in sympathetic transduction in the heart. The results show that after every burst of CSNA, there was a significant effect of time on HR (n = 10, ∆: +2.1 ± 1.4 beats min-1 , P = 0.002) and CO (n = 8, ∆: +100 ± 150 mL min-1 , P = 0.002) was elevated, followed by an increase in CoBF (n = 9, ∆: +0.76 mL min-1 , P = 0.001) and CVC (n = 8, ∆: +0.0038 mL min-1  mmHg-1 , P = 0.0028). The changes in HR were graded depending on the size and pattern of CSNA bursts. The HR response was significantly attenuated after the infusion of propranolol. Our study is the first to explore resting sympathetic transduction in the heart, suggesting that CSNA can dynamically change HR mediated by an action on β-adrenoceptors. KEY POINTS: Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Previous studies have examined sympathetic transduction primarily in the skeletal muscle and shown that bursts of muscle SNA alter blood flow to skeletal muscle and mean arterial pressure, although this has not been examined in the heart. We investigated sympathetic transduction in the heart and show that, in the conscious condition, the size of bursts of SNA to the heart can result in incremental increases in heart rate and coronary blood flow mediated by β-adrenoceptors. The pattern of bursts of SNA to the heart also resulted in incremental increases in heart rate mediated by β-adrenoceptors. This is the first study to explore the transduction of bursts of SNA to the heart.

Renal, Cardiac, and Autonomic Effects of Catheter-Based Renal Denervation in Ovine Heart Failure

[Figure: see text].

Regulation of Coronary Blood Flow by the Carotid Body Chemoreceptors in Ovine Heart Failure

Carotid bodies (CBs) are peripheral chemoreceptors, which are primary sensors of systemic hypoxia and their activation produces respiratory, autonomic, and cardiovascular adjustments critical for body homeostasis. We have previously shown that carotid chemoreceptor stimulation increases directly recorded cardiac sympathetic nerve activity (cardiac SNA) which increases coronary blood flow (CoBF) in conscious normal sheep. Previous studies have shown that chemoreflex sensitivity is augmented in heart failure (HF). We hypothesized that carotid chemoreceptor stimulation would increase CoBF to a greater extent in HF than control sheep. Experiments were conducted in conscious HF sheep and control sheep (n = 6/group) implanted with electrodes to record diaphragmatic electromyography (dEMG), flow probes to record CoBF as well as arterial pressure. There was a significant increase in mean arterial pressure (MAP), CoBF and coronary vascular conductance (CVC) in response to potassium cyanide (KCN) in both groups of sheep. To eliminate the effects of metabolic vasodilation, the KCN was repeated while the heart was paced at a constant level. In this paradigm, the increase in CoBF and CVC was augmented in the HF group compared to the control group. Pre-treatment with propranolol did not alter the CoBF or the CVC increase in the HF group indicating this was not mediated by an increase in cardiac sympathetic drive. The pressor response to CB activation was abolished by pre-treatment with intravenous atropine in both groups, but there was no change in the CoBF and vascular conductance responses. Our data suggest that in an ovine model of HF, carotid body (CB) mediated increases in CoBF and CVC are augmented compared to control animals. This increase in CoBF is mediated by an increase in cardiac SNA in the control group but not the HF group.

Neurohumoral interactions contributing to renal vasoconstriction and decreased renal blood flow in heart failure

In heart failure (HF), increases in renal sympathetic nerve activity (RSNA), renal norepinephrine spillover, and renin release cause renal vasoconstriction, which may contribute to the cardiorenal syndrome. To increase our understanding of the mechanisms causing renal vasoconstriction in HF, we investigated the interactions between the increased activity of the renal nerves and the renal release of norepinephrine and renin in an ovine pacing-induced model of HF compared with healthy sheep. In addition, we determined the level of renal angiotensin type-1 receptors and the renal vascular responsiveness to stimulation of the renal nerves and α1-adrenoceptors. In conscious sheep with mild HF (ejection fraction 35%–40%), renal blood flow (276 ± 13 to 185 ± 18 mL/min) and renal vascular conductance (3.8 ± 0.2 to 3.1 ± 0.2 mL·min−1·mmHg−1) were decreased compared with healthy sheep. There were increases in the burst frequency of RSNA (27%), renal norepinephrine spillover (377%), and plasma renin activity (141%), whereas the density of renal medullary angiotensin type-1 receptors decreased. In anesthetized sheep with HF, the renal vasoconstrictor responses to electrical stimulation of the renal nerves or to phenylephrine were attenuated. Irbesartan improved the responses to nerve stimulation, but not to phenylephrine, in HF and reduced the responses in normal sheep. In summary, in HF, the increases in renal norepinephrine spillover and plasma renin activity are augmented compared with the increase in RSNA. The vasoconstrictor effect of the increased renal norepinephrine and angiotensin II is offset by reduced levels of renal angiotensin type-1 receptors and reduced renal vasoconstrictor responsiveness to α1-adrenoceptor stimulation.

Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress

The differences in the degree of sympathetic nerve activity (SNA) over cutaneous blood vessels, although known to be more prominent in the periphery than the core vasculature, has not been thoroughly investigated quantitatively. Hence, two studies were carried out to investigate the differences in SNA between the periphery and the core during the cold pressor test (CPT) (right-hand immersion in ice water) and cold exposure (whole body exposed to cold air) using photoplethysmography (PPG). Two methods utilizing PPG, namely differential multi-site PTT measurements and low-frequency spectral analysis were explored for quantitative determination of SNA. Each study involved 12 healthy volunteers, and PPG signals were acquired from the right index finger (RIF), left index finger (LIF) (periphery) and the ear canal (core). During CPT, Pulse Transit Time (PTT) was measured to the respective locations and the mean percentage change in PTT during ice immersion at each location was used as an indicator for the extent of SNA. During cold exposure, the low-frequency spectral analysis was performed on the acquired raw PPGs to extract the power of the sympathetic [low-frequency (LF): 0.04–0.15 Hz] and parasympathetic components [high-frequency (HF): 0.15–0.4 Hz]. The ratio of LF/HF components was then used to quantify the differences in the influence of SNA on the peripheral and core circulation. PTT measured from the EC, and the LIF has dropped by 5 and 7%, respectively during ice immersion. The RIF PTT, on the other hand, has dropped significantly (P < 0.05) by 12%. During the cold exposure, the LF/HF power ratio at the finger has increased to 86.4 during the cold exposure from 19.2 at the baseline (statistically significant P = 0.002). While the ear canal LF/HF ratio has decreased to 1.38 during the cold exposure from 1.62 at baseline (P = 0.781). From these observations, it is evident that differential PTT measurements or low-frequency analysis can be used to quantify SNA. The results also demonstrate the effectiveness of the central auto-regulation during both short and long-term stress stimulus as compared to the periphery.

Baroreflex Sensitivity and Blood Pressure Variability can Help in Understanding the Different Response to Therapy During Acute Phase of Septic Shock

BACKGROUND

Mean values of hemodynamic variables are poorly effective in evaluating an actual recovery of the short-term autonomic mechanisms for blood pressure (BP) and heart rate (HR) regulation. The aim of this work is to analyze the response to therapy in the early phase of septic shock to verify possible associations between BP recovery and BP autonomic control.

METHODS

This is an ancillary study from the multicenter prospective observational trial Shockomics (NCT02141607). A total of 21 septic shock patients were studied at two time points during the acute phase of shock and were classified according to changes in SOFA score. Time series of BP components and HR were analyzed in time and frequency domain. Baroreflex sensitivity (BRS) was assessed, and a mathematical model for the decomposition of diastolic arterial pressure (DAP) oscillations was used to understand the different contributions of BRS and HR on peripheral vascular resistance control.

RESULTS

Only those patients, who significantly improved organ function (responders, R), showed an increase of mean value and low frequency (LF) power in BP time series. Fluid accumulation was higher in the non-responders (NR). BRS increased in NR and the model of DAP variability showed that the contribution of HR was highly reduced in NR.

CONCLUSIONS

Although patients reached the mean BP target of 65 mmHg, our analyses highlighted important differences in terms of autonomic nervous system control. BP variability, HR variability and baroreflex trends can add information to individual vital sign measure such as mean BP, and can help in understanding the responsiveness to the combination of symphatomimetic drugs and fluid therapy.

Mechanisms underlying the increased cardiac norepinephrine spillover in heart failure

Patients with heart failure (HF) have increased levels of cardiac norepinephrine (NE) spillover, which is an independent predictor of mortality. We hypothesized that this increase in NE spillover in HF depends not only on increases in sympathetic nerve activity (SNA) but also on changes in the mechanisms controlling NE release and reuptake. Such changes would lead to differences between the increases in directly recorded SNA and NE spillover to the heart in HF. Experiments were conducted in conscious sheep implanted with electrodes to record cardiac SNA (CSNA). In addition, arterial pressure and cardiac NE spillover were determined. In HF, the levels of both CSNA (102 ± 8 vs. 45 ± 8 bursts/min, P < 0.05) and cardiac NE spillover (21.6 ± 3.8 vs. 3.9 ± 0.8 pmol/min, P < 0.05) were significantly higher than in normal control animals. In HF, baroreflex control of cardiac NE spillover was impaired, and when CSNA was abolished by increasing arterial pressure, there was no reduction in cardiac NE spillover. A decrease in cardiac filling pressures in the HF group led to a significant increase in CSNA, but it significantly decreased cardiac NE spillover. In HF, the levels of cardiac NE spillover were enhanced above those expected from the high level of SNA, suggesting that changes in mechanisms controlling NE release and reuptake further increase the high level of NE at the heart, which will act to enhance the deleterious effects of increased CSNA in HF. NEW & NOTEWORTHY This is the first study, to our knowledge, to compare direct recordings of cardiac sympathetic nerve activity with simultaneously measured cardiac norepinephrine (NE) spillover. Our results indicate that in heart failure, increased cardiac sympathetic nerve activity is a major contributor to the increased NE spillover. In addition, there is enhanced NE spillover for the levels of synaptic nerve activity.

The need for and the challenges of measuring renal sympathetic nerve activity

Renal denervation (RDN) was primarily developed to treat hypertension and is potentially a new method for treating arrhythmias. Because of the lack of a standardized protocol to measure renal sympathetic nerve activity, RDN is administered in a blind manner. This inability to assess efficacy at the time of treatment delivery may be a large contributor to the ambiguity of RDN outcomes reported in the hypertension literature. The advancement of RDN as a treatment of hypertension or arrhythmias will be hampered by the lack of delivery assessment, a deficiency that the cardiovascular electrophysiology community, with its expertise in recording and mapping, may have a role in addressing and overcoming. The development of endovascular recording of renal nerve action potentials may provide a useful accessory tool for RDN. Innovation in this area will be crucial as we as a community reconsider the therapeutic value of RDN.

Radionuclide imaging of cardiac sympathetic innervation in heart failure: unlocking untapped potential

Heart failure (HF) is associated with sympathetic overactivity, which contributes to disease progression and arrhythmia development. Cardiac sympathetic innervation imaging can be performed using radiotracers that are taken up in the presynaptic nerve terminal of sympathetic nerves. The commonly used radiotracers are (123)I-metaiodobenzylguanidine ((123)I-mIBG) for planar and single-photon emission computed tomography imaging, and (11)C-hydroxyephedrine for positron emission tomography imaging. Sympathetic innervation imaging has been used in assessing prognosis, response to treatment, risk of ventricular arrhythmias and sudden death and prediction of response to cardiac resynchronization therapy in patients with HF. Other potential applications of these techniques are in patients with chemotherapy-induced cardiomyopathy, predicting myocardial recovery in patients with left ventricular assist devices, and assessing reinnervation following cardiac transplantation. There is a lack of standardization with respect to technique of (123)I-mIBG imaging that needs to be overcome for the imaging modality to gain popularity in clinical practice.

The forgotten role of central volume in low frequency oscillations of heart rate variability

The hypothesis that central volume plays a key role in the source of low frequency (LF) oscillations of heart rate variability (HRV) was tested in a population of end stage renal disease patients undergoing conventional hemodialysis (HD) treatment, and thus subject to large fluid shifts and sympathetic activation. Fluid overload (FO) in 58 chronic HD patients was assessed by whole body bioimpedance measurements before the midweek HD session. Heart Rate Variability (HRV) was measured using 24-hour Holter electrocardiogram recordings starting before the same HD treatment. Time domain and frequency domain analyses were performed on HRV signals. Patients were retrospectively classified in three groups according to tertiles of FO normalized to the extracellular water (FO/ECW%). These groups were also compared after stratification by diabetes mellitus. Patients with the low to medium hydration status before the treatment (i.e. 1st and 2nd FO/ECW% tertiles) showed a significant increase in LF power during last 30 min of HD compared to dialysis begin, while no significant change in LF power was seen in the third group (i.e. those with high pre-treatment hydration values). In conclusion, several mechanisms can generate LF oscillations in the cardiovascular system, including baroreflex feedback loops and central oscillators. However, the current results emphasize the role played by the central volume in determining the power of LF oscillations.

Stimulation of the cardiopulmonary baroreflex enhances ventricular contractility in awake dogs: a mathematical analysis study

The cardiopulmonary baroreflex responds to an increase in central venous pressure (CVP) by decreasing total peripheral resistance and increasing heart rate (HR) in dogs. However, the direction of ventricular contractility change is not well understood. The aim was to elucidate the cardiopulmonary baroreflex control of ventricular contractility during normal physiological conditions via a mathematical analysis. Spontaneous beat-to-beat fluctuations in maximal ventricular elastance (Emax), which is perhaps the best available index of ventricular contractility, CVP, arterial blood pressure (ABP), and HR were measured from awake dogs at rest before and after β-adrenergic receptor blockade. An autoregressive exogenous input model was employed to jointly identify the three causal transfer functions relating beat-to-beat fluctuations in CVP to Emax (CVP → Emax), which characterizes the cardiopulmonary baroreflex control of ventricular contractility, ABP to Emax, which characterizes the arterial baroreflex control of ventricular contractility, and HR to Emax, which characterizes the force-frequency relation. The CVP → Emax transfer function showed a static gain of 0.037 ± 0.010 ml(-1) (different from zero; P < 0.05) and an overall time constant of 3.2 ± 1.2 s. Hence, Emax would increase and reach steady state in ∼16 s in response to a step increase in CVP, without any change to ABP or HR, due to the cardiopulmonary baroreflex. Following β-adrenergic receptor blockade, the CVP → Emax transfer function showed a static gain of 0.0007 ± 0.0113 ml(-1) (different from control; P < 0.10). Hence, Emax would change little in steady state in response to a step increase in CVP. Stimulation of the cardiopulmonary baroreflex increases ventricular contractility through β-adrenergic receptor system mediation.

Central exogenous nitric oxide decreases cardiac sympathetic drive and improves baroreflex control of heart rate in ovine heart failure

Heart failure (HF) is associated with increased cardiac and renal sympathetic drive, which are both independent predictors of poor prognosis. A candidate mechanism for the centrally mediated sympathoexcitation in HF is reduced synthesis of the inhibitory neuromodulator nitric oxide (NO), resulting from downregulation of neuronal NO synthase (nNOS). Therefore, we investigated the effects of increasing the levels of NO in the brain, or selectively in the paraventricular nucleus of the hypothalamus (PVN), on cardiac sympathetic nerve activity (CSNA) and baroreflex control of CSNA and heart rate in ovine pacing-induced HF. The resting level of CSNA was significantly higher in the HF than in the normal group, but the resting level of RSNA was unchanged. Intracerebroventricular infusion of the NO donor sodium nitroprusside (SNP; 500 μg · ml(-1)· h(-1)) in conscious normal sheep and sheep in HF inhibited CSNA and restored baroreflex control of heart rate, but there was no change in RSNA. Microinjection of SNP into the PVN did not cause a similar cardiac sympathoinhibition in either group, although the number of nNOS-positive cells was decreased in the PVN of sheep in HF. Reduction of endogenous NO with intracerebroventricular infusion of N(ω)-nitro-l-arginine methyl ester decreased CSNA in normal but not in HF sheep and caused no change in RSNA in either group. These findings indicate that endogenous NO in the brain provides tonic excitatory drive to increase resting CSNA in the normal state, but not in HF. In contrast, exogenously administered NO inhibited CSNA in both the normal and HF groups via an action on sites other than the PVN.

Intracarotid hypertonic sodium chloride differentially modulates sympathetic nerve activity to the heart and kidney

Hypertonic NaCl infused into the carotid arteries increases mean arterial pressure (MAP) and changes sympathetic nerve activity (SNA) via cerebral mechanisms. We hypothesized that elevated sodium levels in the blood supply to the brain would induce differential responses in renal and cardiac SNA via sensors located outside the blood-brain barrier. To investigate this hypothesis, we measured renal and cardiac SNA simultaneously in conscious sheep during intracarotid infusions of NaCl (1.2 M), sorbitol (2.4 M), or urea (2.4 M) at 1 ml/min for 4 min into each carotid. Intracarotid NaCl significantly increased MAP (91 ± 2 to 97 ± 3 mmHg, P < 0.05) without changing heart rate (HR). Intracarotid NaCl was associated with no change in cardiac SNA (11 ± 5.0%), but a significant inhibition of renal SNA (-32.5 ± 6.4%, P < 0.05). Neither intracarotid sorbitol nor urea changed MAP, HR, central venous pressure, cardiac SNA, and renal SNA. The changes in MAP and renal SNA were completely abolished by microinjection of the GABA agonist muscimol (5 mM, 500 nl each side) into the paraventricular nucleus of the hypothalamus (PVN). Infusion of intracarotid NaCl for 20 min stimulated a larger increase in water intake (1,100 ± 75 ml) than intracarotid sorbitol (683 ± 125 ml) or intracarotid urea (0 ml). These results demonstrate that acute increases in blood sodium levels cause a decrease in renal SNA, but no change in cardiac SNA in conscious sheep. These effects are mediated by cerebral sensors located outside the blood-brain barrier that are more responsive to changes in sodium concentration than osmolality. The renal sympathoinhibitory effects of sodium are mediated via a pathway that synapses in the PVN.

The role of the paraventricular nucleus of the hypothalamus in the regulation of cardiac and renal sympathetic nerve activity in conscious normal and heart failure sheep

The paraventricular nucleus of the hypothalamus (PVN) plays a major role in central cardiovascular and volume control, and has been implicated in controlling sympathetic nerve activity (SNA) during volume expansion and in heart failure (HF). The objectives were to determine the role of the PVN on cardiac and renal SNA (CSNA and RSNA) in conscious normal sheep and sheep with pacing-induced heart failure. In normovolaemic sheep in the normal state and in HF, bilateral microinjection of the GABA agonist muscimol (2 mm, 500 nl), had no effects on resting mean arterial pressure (MAP), heart rate (HR), CSNA or RSNA. In addition, neither chemical inhibition of the PVN using the inhibitory amino acid glycine (0.5 m, 500 nl), nor electrolytic lesion of the PVN reduced the elevated level of CSNA in HF. Dysinhibition of the PVN with bilateral microinjection of bicuculline (1 mm, 500 nl) in normal sheep increased MAP, HR and CSNA, but decreased RSNA, whereas in HF bicuculline had no effects on MAP, HR or CSNA, but inhibited RSNA. During volume expansion in normal sheep, muscimol reversed the inhibition of RSNA, but not of CSNA. In summary, removal of endogenous GABAergic inhibition to the PVN indicated that CSNA is normally under inhibitory control. Although this inhibition was absent in HF, the responses to pharmacological inhibition, or lesion of the PVN, indicates that it does not drive the increased CSNA in HF. These findings indicate the PVN has a greater influence on RSNA than CSNA in the resting state in normal and HF sheep, and during volume expansion in normal sheep.

Central angiotensin type 1 receptor blockade decreases cardiac but not renal sympathetic nerve activity in heart failure

In heart failure (HF), cardiac sympathetic nerve activity (SNA; CSNA) is increased, which has detrimental effects on the heart and promotes arrhythmias and sudden death. There is evidence that the central renin-angiotensin system plays an important role in stimulating renal SNA in HF. Because SNA to individual organs is differentially controlled, we have investigated whether central angiotensin receptor blockade decreases CSNA in HF. We simultaneously recorded CSNA and renal SNA in conscious normal sheep and in sheep with HF induced by rapid ventricular pacing (ejection fraction: <40%). The effect of blockade of central angiotensin type 1 receptors by intracerebroventricular infusion of losartan (1 mg/h for 5 hours) on resting levels and baroreflex control of CSNA and renal SNA were determined. In addition, the levels of angiotensin receptors in central autonomic nuclei were determined using autoradiography. Sheep in HF had a large increase in CSNA (43±2 to 88±3 bursts per 100 heart beats; P<0.05) and heart rate, with no effect on renal SNA. In HF, central infusion of losartan for 5 hours significantly reduced the baseline levels of CSNA (to 69±5 bursts per 100 heart beats) and heart rate. Losartan had no effect in normal animals. In HF, angiotensin receptor levels were increased in the paraventricular nucleus and supraoptic nucleus but reduced in the area postrema and nucleus tractus solitarius. In summary, infusion of losartan reduced the elevated levels of CNSA in an ovine model of HF, indicating that central angiotensin receptors play a critical role in stimulating the increased sympathetic activity to the heart.

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.

Response of cardiac sympathetic nerve activity to intravenous irbesartan in heart failure

To determine the effect of irbesartan treatment on resting levels and arterial baroreflex control of cardiac sympathetic nerve activity (CSNA) in heart failure (HF), we studied conscious normal sheep and sheep with HF induced by rapid ventricular pacing for 8-10 wk (n = 7 per group). In HF, there is a large increase in CSNA that is detrimental to outcome. The causes of this increase in CSNA and the effect of angiotensin receptor blockers on CSNA in HF are unclear. CSNA, arterial blood pressure, heart rate (HR), and arterial baroreflex curves were recorded during a resting period and after 90 min of irbesartan infusion (12 mg.kg(-1).h(-1) iv). This dose of irbesartan abolished the pressor response to intravenous ANG II infusion but caused only a slight decrease in the pressor response to centrally administered ANG II. In HF, there was a large increase in CSNA (from 44 +/- 3 to 87 +/- 3 bursts/100 heartbeats). Irbesartan reduced arterial pressure in the normal and HF groups, but the usual baroreflex-mediated increases in CSNA and HR were prevented. This resulted from a significant leftward shift in the CSNA and HR baroreflex curves in both groups. Irbesartan also decreased the sensitivity of the arterial baroreflex control of CSNA. Short-term treatment with an angiotensin receptor blocker, at a dose that abolished the response to circulating, but not central, ANG II, prevented the reflex increase in CSNA in response to the drug-induced fall in arterial pressure.

Discharge properties of cardiac and renal sympathetic nerves and their impaired responses to changes in blood volume in heart failure

Sympathetic nerve activity (SNA) consists of discharges that vary in amplitude and frequency, reflecting the level of recruitment of nerve fibers and the rhythmic generation and entrainment of activity by the central nervous system. It is unknown whether selective changes in these amplitude and frequency components account for organ-specific changes in SNA in response to alterations in blood volume or for the impaired SNA responses to volume changes in heart failure (HF). To address these questions, we measured cardiac SNA (CSNA) and renal SNA (RSNA) simultaneously in conscious, normal sheep and sheep in HF induced by rapid ventricular pacing. Volume expansion decreased CSNA (-62 +/- 10%, P < 0.05) and RSNA (-59 +/- 10%, P < 0.05) equally (n = 6). CSNA decreased as a result of a reduction in burst frequency, whereas RSNA fell because of falls in burst frequency and amplitude. Hemorrhage increased CSNA (+74 +/- 9%, P < 0.05) more than RSNA (+21 +/- 5%, P < 0.09), in both cases because of increased burst frequency, whereas burst amplitude decreased. In HF, burst frequency of CSNA (from 26 +/- 3 to 75 +/- 3 bursts/min) increased more than that of RSNA (from 63 +/- 4 to 79 +/- 4 bursts/min). In HF, volume expansion caused no change in CSNA and an attenuated decrease in RSNA, due entirely to decreased burst amplitude. Hemorrhage did not significantly increase SNA in either nerve in HF. These findings support the concept that the number of sympathetic fibers recruited and their firing frequency are controlled independently. Furthermore, afferent stimuli, such as changes in blood volume, cause organ-specific responses in each of these components, which are also selectively altered in HF.