Characteristics of renal sympathetic nerve activity in experimental congestive heart failure in the rat

The role of the renal afferent and efferent nerve fibers in heart failure

Renal nerves contain afferent, sensory and efferent, sympathetic nerve fibers. In heart failure (HF) there is an increase in renal sympathetic nerve activity (RSNA), which can lead to renal vasoconstriction, increased renin release and sodium retention. These changes are thought to contribute to renal dysfunction, which is predictive of poor outcome in patients with HF. In contrast, the role of the renal afferent nerves remains largely unexplored in HF. This is somewhat surprising as there are multiple triggers in HF that have the potential to increase afferent nerve activity, including increased venous pressure and reduced kidney perfusion. Some of the few studies investigating renal afferents in HF have suggested that at least the sympatho-inhibitory reno-renal reflex is blunted. In experimentally induced HF, renal denervation, both surgical and catheter-based, has been associated with some improvements in renal and cardiac function. It remains unknown whether the effects are due to removal of the efferent renal nerve fibers or afferent renal nerve fibers, or a combination of both. Here, we review the effects of HF on renal efferent and afferent nerve function and critically assess the latest evidence supporting renal denervation as a potential treatment in HF.

Autonomic nervous system and immune system interactions

The present review assesses the current state of literature defining integrative autonomic-immune physiological processing, focusing on studies that have employed electrophysiological, pharmacological, molecular biological, and central nervous system experimental approaches. Central autonomic neural networks are informed of peripheral immune status via numerous communicating pathways, including neural and non-neural. Cytokines and other immune factors affect the level of activity and responsivity of discharges in sympathetic and parasympathetic nerves innervating diverse targets. Multiple levels of the neuraxis contribute to cytokine-induced changes in efferent parasympathetic and sympathetic nerve outflows, leading to modulation of peripheral immune responses. The functionality of local sympathoimmune interactions depends on the microenvironment created by diverse signaling mechanisms involving integration between sympathetic nervous system neurotransmitters and neuromodulators; specific adrenergic receptors; and the presence or absence of immune cells, cytokines, and bacteria. Functional mechanisms contributing to the cholinergic anti-inflammatory pathway likely involve novel cholinergic-adrenergic interactions at peripheral sites, including autonomic ganglion and lymphoid targets. Immune cells express adrenergic and nicotinic receptors. Neurotransmitters released by sympathetic and parasympathetic nerve endings bind to their respective receptors located on the surface of immune cells and initiate immune-modulatory responses. Both sympathetic and parasympathetic arms of the autonomic nervous system are instrumental in orchestrating neuroimmune processes, although additional studies are required to understand dynamic and complex adrenergic-cholinergic interactions. Further understanding of regulatory mechanisms linking the sympathetic nervous, parasympathetic nervous, and immune systems is critical for understanding relationships between chronic disease development and immune-associated changes in autonomic nervous system function.

Electroacupuncture improves cardiac function and remodeling by inhibition of sympathoexcitation in chronic heart failure rats

Chronic heart failure (CHF) is responsible for significant morbidity and mortality worldwide, mainly as a result of neurohumoral activation. Acupuncture has been used to treat a wide range of diseases and conditions. In this study, we investigated the effects of electroacupuncture (EA) on the sympathetic nerve activity, heart function, and remodeling in CHF rats after ligation of the left anterior descending coronary artery. CHF rats were randomly selected to EA and control groups for acute and chronic experiments. In the acute experiment, both the renal sympathetic nerve activity and cardiac sympathetic afferent reflex elicited by epicardial application of capsaicin were recorded. In the chronic experiment, we performed EA for 30 min once a day for 1 wk to test the long-term EA effects on heart function, remodeling, as well as infarct size in CHF rats. The results show EA significantly decreased the renal sympathetic nerve activity effectively, inhibited cardiac sympathetic afferent reflex, and lowered the blood pressure of CHF rats. Treating CHF rats with EA for 1 wk dramatically increased left ventricular ejection fraction and left ventricular fraction shortening, reversed the enlargement of left ventricular end-systolic dimension and left ventricular end-diastolic dimension, and shrunk the infarct size. In this experiment, we demonstrated EA attenuates sympathetic overactivity. Additionally, long-term EA improves cardiac function and remodeling and reduces infarct size in CHF rats. EA is a novel and potentially useful therapy for treating CHF.

Neurohumoral stimulation

The temporal relationship between the development of heart failure and activation of the neurohumoral systems involved in chronic heart failure (CHF) has not been precisely defined. When a compensatory mechanism switches to a deleterious contributing factor in the progression of the disease is unclear. This article addresses these issues through evaluating the contribution of various cardiovascular reflexes and cellular mechanisms to the sympathoexcitation in CHF. It also sheds light on some of the important central mechanisms that contribute to the increase in sympathetic nerve activity in CHF.

Enhanced activation of RVLM-projecting PVN neurons in rats with chronic heart failure

Previous studies have indicated that there is increased activation of the paraventricular nucleus (PVN) in rats with chronic heart failure (CHF); however, it is not clear if the preautonomic neurons within the PVN are specifically overactive. Also, it is not known if these neurons have altered responses to baroreceptor or osmotic challenges. Experiments were conducted in rats with CHF (6-8 wk after coronary artery ligation). Spontaneously active neurons were recorded in the PVN, of which 36% were antidromically activated from the rostral ventrolateral medulla (RVLM). The baseline discharge rate in RVLM-projecting PVN (PVN-RVLM) neurons from CHF rats was significantly greater than in sham-operated (sham) rats (6.0 ± 0.6 vs. 2.6 ± 0.3 spikes/s, P < 0.05). Picoinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid significantly decreased the basal discharge of PVN-RVLM neurons by 80% in CHF rats compared with 37% in sham rats. Fifty-two percent of spontaneously active PVN-RVLM neurons responded to changes in the mean arterial pressure (MAP). The changes in discharge rate in PVN-RVLM neurons after a reduction in MAP (+52 ± 7% vs. +184 ± 61%) or an increase in MAP (-42 ± 8% vs. -71 ± 6%) were significantly attenuated in rats with CHF compared with sham rats. Most PVN-RVLM neurons (63%), including all barosensitive PVN-RVLM neurons, were excited by an internal carotid artery injection of hypertonic NaCl (2.1 osmol/l), whereas a smaller number (7%) were inhibited. The increase in discharge rate in PVN-RVLM neurons to hypertonic stimulation was significantly enhanced in rats with CHF compared with sham rats (134 ± 15% vs. 92 ± 13%). Taken together, these data suggest that PVN-RVLM neurons are more active under basal conditions and this overactivation is mediated by an enhanced glutamatergic tone in rats with CHF. Furthermore, this enhanced activation of PVN-RVLM neurons may contribute to the altered responses to baroreceptor and osmotic challenges observed during CHF.

Central neural control of sympathetic nerve activity in heart failure following exercise training

Typical characteristics of chronic congestive heart failure (HF) are increased sympathetic drive, altered autonomic reflexes, and altered body fluid regulation. These abnormalities lead to an increased risk of mortality, particularly in the late stage of chronic HF. Recent evidence suggests that central nervous system (CNS) mechanisms may be important in these abnormalities during HF. Exercise training (ExT) has emerged as a nonpharmacological therapeutic strategy substitute with significant benefit to patients with HF. Regular ExT improves functional capacity as well as quality of life and perhaps prognosis in chronic HF patients. The mechanism(s) by which ExT improves the clinical status of HF patients is not fully known. Recent studies have provided convincing evidence that ExT significantly alleviates the increased sympathetic drive, altered autonomic reflexes, and altered body fluid regulation in HF. This review describes and highlights the studies that examine various central pathways involved in autonomic outflow that are altered in HF and are improved following ExT. The increased sympathoexcitation is due to an imbalance between inhibitory and excitatory mechanisms within specific areas in the CNS such as the paraventricular nucleus (PVN) of the hypothalamus. Studies summarized here have revealed that ExT improves the altered inhibitory pathway utilizing nitric oxide and GABA mechanisms within the PVN in HF. ExT alleviates elevated sympathetic outflow in HF through normalization of excitatory glutamatergic and angiotensinergic mechanisms within the PVN. ExT also improves volume reflex function and thus fluid balance in HF. Preliminary observations also suggest that ExT induces structural neuroplasticity in the brain of rats with HF. We conclude that improvement of the enhanced CNS-mediated increase in sympathetic outflow, specifically to the kidneys related to fluid balance, contributes to the beneficial effects of ExT in HF.

Chronic AT1 receptor blockade normalizes NMDA-mediated changes in renal sympathetic nerve activity and NR1 expression within the PVN in rats with heart failure

Exercise training normalizes enhanced glutamatergic mechanisms within the paraventricular nucleus (PVN) concomitant with the normalization of increased plasma ANG II levels in rats with heart failure (HF). We tested whether ANG II type 1 (AT(1)) receptors are involved in the normalization of PVN glutamatergic mechanisms using chronic AT(1) receptor blockade with losartan (Los; 50 mg.kg(-1).day(-1) in drinking water for 3 wk). Left ventricular end-diastolic pressure was increased in both HF + vehicle (Veh) and HF + Los groups compared with sham-operated animals (Sham group), although it was significantly attenuated in the HF + Los group compared with the HF + Veh group. The effect of Los on cardiac function was similar to exercise training. At the highest dose of N-methyl-d-aspartate (NMDA; 200 pmol) injected into the PVN, the increase in renal sympathetic nerve activity was 93 +/- 13% in the HF + Veh group, which was significantly higher (P < 0.05) than the increase in the Sham + Veh (45 +/- 2%) and HF + Los (47 +/- 2%) groups. Relative NMDA receptor subunit NR(1) mRNA expression within the PVN was increased 120% in the HF + Veh group compared with the Sham + Veh group (P < 0.05) but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). NR(1) protein expression increased 87% in the HF + Veh group compared with the Sham + Veh group but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). Furthermore, in in vitro experiments using neuronal NG-108 cells, we found that ANG II treatment stimulated NR(1) protein expression and that Los significantly ameliorated the NR(1) expression induced by ANG II. These data are consistent with our hypothesis that chronic AT(1) receptor blockade normalizes glutamatergic mechanisms within the PVN in rats with HF.

Exercise training normalizes enhanced glutamate-mediated sympathetic activation from the PVN in heart failure

Exercise training (ExT) normalizes the increased sympathetic outflow in heart failure (HF), but the mechanisms are not known. We hypothesized that ExT would normalize the augmented glutamatergic mechanisms mediated by N-methyl-d-aspartic acid (NMDA) receptors within the paraventricular nucleus (PVN) that occur with HF. Four groups of rats were used: 1) sham-operated (Sham) sedentary (Sed), 2) Sham ExT, 3) HF Sed, and 4) HF ExT. HF was induced by left coronary artery ligation, and ExT consisted of 3 wk of treadmill running. In alpha-chloralose-urethane-anesthetized rats, the increase in renal sympathetic nerve activity in response to the highest dose of NMDA (200 pmol) injected into the PVN in the HF Sed group was approximately twice that of the Sham Sed group. In the HF ExT group the response was not different from the Sham Sed and Sham ExT groups. Relative NMDA NR1 receptor subunit mRNA expression was 63% higher in the HF Sed group compared with the Sham Sed group but in the HF ExT group was not different from the Sham Sed and Sham ExT groups. NR1 receptor subunit protein expression was increased 87% in the HF Sed group compared with the Sham Sed group but in the HF ExT group was not significantly different from the Sham Sed and Sham ExT groups. Thus one mechanism by which ExT alleviates elevated sympathetic outflow in HF may be through normalization of glutamatergic mechanisms within the PVN.

Decreased renal sympathetic activity in response to cardiac unloading with nitroglycerin in patients with heart failure*

AIMS

To examine changes in renal sympathetic outflow in response to cardiac unloading with nitroglycerin (GTN) in patients with chronic heart failure (CHF) and healthy subjects (HS).

METHODS AND RESULTS

Renal (RNAsp) and total body (TBNAsp) noradrenaline (NA) spillover were measured with radiotracer methods in 16 patients with CHF (50+/-3 years, LVEF 20+/-1%) and nine HS (57+/-2 years) during right heart and renal vein catheterisation. Low dose GTN decreased mean pulmonary artery pressure (PAm: CHF -7+/-2 mm Hg, HS -4+/-1 mm Hg, p<0.05 vs. baseline) but not mean arterial pressure (MAP: CHF -2+/-1 mm Hg, HS -2+/-1 mm Hg) and did not affect RNAsp in any of the study groups. High dose GTN lowered MAP (CHF -12+/-1 mm Hg, HS -12+/-2 mm Hg, p<0.05 vs. baseline) and PAm (CHF -13+/-2 mm Hg, HS -5+/-1 mm Hg, p<0.05 vs. baseline) and was accompanied by a significant reduction in RNAsp only in CHF (1.3+/-0.1 nmol/min baseline to 0.9+/-0.2 nmol/min, p<0.05), whereas RNAsp in HS remained unchanged.

CONCLUSIONS

In spite of a reduction in both arterial pressure and cardiac filling pressures, renal sympathetic activity decreased in CHF and did not increase in HS. These findings suggest that the altered loading conditions resulting from high-dose GTN infusion have renal sympathoinhibitory effects.

Role of paraventricular nucleus in mediating sympathetic outflow in heart failure

A number of neurohumoral processes are activated in heart failure, including an increase in the plasma concentration of norepinephrine. Few studies have been performed to examine the role of the central nervous system in the activation of sympathetic outflow during heart failure (HF). In this paper I review these limited studies, with particular emphasis on examining the role of the paraventricular nucleus (PVN) in the exaggerated sympathetic outflow commonly observed in heart failure. The conclusion is that heart failure is associated with changes in specific areas in the brain and that alterations in the activation of neurons in the PVN are likely related to abnormalities in vasopressin production, blood volume regulation, and sympathoexcitation observed in the heart failure state. Furthermore, neuronal nitric oxide within the PVN that is involved in mediating sympathetic outflow via a GABA mechanism from the PVN may be deficient in inhibiting overall sympathetic outflow leading to the exaggerated sympathetic outflow commonly observed in heart failure.

Central angiotensin II-enhanced splenic cytokine gene expression is mediated by the sympathetic nervous system

We tested the hypothesis that central angiotensin II (ANG II) administration would activate splenic sympathetic nerve discharge (SND), which in turn would alter splenic cytokine gene expression. Experiments were completed in sinoaortic nerve-lesioned, urethane-chloralose-anesthetized, splenic nerve-intact (splenic-intact) and splenic nerve-lesioned (splenic-denervated) Sprague-Dawley rats. Splenic cytokine gene expression was determined using gene-array and real-time RT-PCR analyses. Splenic SND was significantly increased after intracerebroventricular administration of ANG II (150 ng/kg, 10 microl), but not artificial cerebrospinal fluid (aCSF). Splenic mRNA expression of IL-1beta, IL-6, IL-2, and IL-16 genes was increased in ANG II-treated splenic-intact rats compared with aCSF-treated splenic-intact rats. Splenic IL-1beta, IL-2, and IL-6 gene expression responses to ANG II were significantly reduced in splenic-denervated compared with splenic-intact rats. Splenic gene expression responses did not differ significantly in ANG II-treated splenic-denervated and aCSF-treated splenic-intact rats. Splenic blood flow responses to intracerebroventricular ANG II administration did not differ between splenic-intact and splenic-denervated rats. These results provide experimental support for the hypothesis that ANG II modulates the immune system through activation of splenic SND, suggesting a novel relation between ANG II, efferent sympathetic nerve outflow, and splenic cytokine gene expression.

Brain angiotensin-converting enzyme activity and autonomic regulation in heart failure

Several recent studies suggest an important role for the brain renin-angiotensin system in the pathogenesis of heart failure. Angiotensin-converting enzyme (ACE) activity and binding of angiotensin type 1 (AT1) receptors, which mediate the central effects of ANG II, are increased in heart failure. The present study examined the relationship between brain ACE activity and the autonomic dysregulation characteristic of rats with congestive heart failure. Rats with heart failure (HF) induced by coronary artery ligation and sham-operated control (SHAM) rats were treated with chronic (28 days) third cerebral ventricle [intracerebroventricular (ICV)] or intraperitoneal (IP) infusion of a low dose of the ACE inhibitor enalaprilat (ENL) or vehicle (VEH). VEH-treated HF rats had increased sodium consumption, reduced urine sodium and urine volume, and increased sympathetic nerve activity with impaired baroreflex regulation. These responses were minimized or prevented by ICV ENL started 24 h after coronary ligation. IP ENL at the low dose used in these studies had no beneficial effects on HF rats. Neither IP nor ICV ENL had any substantial effect on the SHAM rats. The findings confirm a critically important contribution of the brain renin-angiotensin system to the pathophysiology of congestive heart failure.

[Sympathetic overactivity and the kidney]

Hypertension is present in the majority of patients with chronic renal failure and constitutes a major risk factor for the very high cardiovascular morbidity and mortality in this patient population. Furthermore hypertension is known to be a substantial progression factor in renal disease. In the past, it had been presumed that hypertension in chronic renal failure is due to enhanced sodium retention, chronic hypervolemia and increased activity of the renin-angiotensin-aldosterone-system. Recent studies now provide evidence that sympathetic overactivity plays an additional important role and also promotes progression of renal failure. The treatment goal in renal patients is to delay or even prevent progression of renal failure and to reduce the cardiovascular risk. Recent studies have investigated the respective impact of sympatholytic drugs, e.g. inhibitors of the renin-angiotensin-aldosterone-system, beta-blockers or I1-Imidazolin-receptor-agonists in fulfilling these aims. The present report will review experimental and clinical studies on the role of sympathetic overactivity in hypertension and chronic renal failure and possible new therapeutic options.

Effect of water immersion on renal sympathetic nerve activity and arterial baroreflex in conscious rabbits with heart failure

Several studies have indicated an interaction between cardiopulmonary mechanoreflex and arterial baroreflex. However, the contribution of cardiopulmonary mechanoreflex to an abnormal arterial baroreflex in chronic heart failure (CHF) has not been fully investigated. We examined the effect of the activation of cardiopulmonary mechanoreceptors induced by head-out water immersion (WI) on the arterial baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in conscious rabbits with CHF induced by myocardial infarction. The arterial baroreflex sensitivity (BRS) of both HR and RSNA were decreased in CHF. WI induced a small decrease in RSNA in CHF compared to a sham-operated group (Sham), despite a similar increase in central venous pressure. WI did not affect BRS of HR or RSNA in either Sham or CHF. By averaging rectified RSNA recordings, we found that miniature RSNA in the control in CHF was higher than that in Sham. WI decreased the synchronized RSNA without changing miniature RSNA in either group. These results suggest that cardiopulmonary mechanoreflex control of RSNA is reduced in CHF, and that cardiopulmonary mechanoreflex has little effect on arterial baroreflex. An increase in miniature RSNA may reflect sympatho-excitation in CHF.

Synchrony analysis between blood pressure and sympathetic nerve signal inhibited by atrial receptor stimulation in Wistar rats

Recently attempts have been made to analyse blood pressure (BP) and renal sympathetic nerve activity (RSNA) to determine how patterns contained within them might reflect control by the autonomic nervous system. To date, these studies have primarily used coherence analysis of BP and RSNA in the frequency domain. However, this analysis is unable to assess the non-linear properties of the underlying cardiovascular control system. In this study we employed not only coherence analysis but also cross-entropy analysis using balloon inflation ('balloon-on') to assess the influence of right atrial receptors on the relationship between BP and RSNA under two conditions in anaesthetised Wistar rats. Balloon-on stimulation alone inhibited RSNA by 28 +/- 4% in eight rats without changing BP. This effect on integrated nerve activity was not present when atrial stimulation was applied during stimulation of a site in the paraventricular nucleus (PVN) of the hypothalamus which increased RSNA by 158.7 +/- 58% and increased BP by 17.1 +/- 2.3 mmHg. However, the cross-entropy measurement was significantly decreased (P < 0.05) during balloon-on stimulation in both the conditions revealing that there is greater synchrony between the oscillating signals contained within the BP and RSNA time series. Thus during the enhanced RSNA elicited by stimulation of the PVN, the inhibitory influence of atrial receptors, although apparently blocked, was still effective in that it resulted in the energy of the RSNA spectrum becoming more evenly distributed over a range of frequencies. The data show that cross-entropy calculations are able to characterize the non-linearities of underlying cardiovascular control.

Renal sympathetic nerve regulation to heating is altered in rats with heart failure

Heart failure (HF) alters the regulation of basal sympathetic nerve discharge (SND); however, the effect of HF on SND responses to acute stress is not well established. In the present study, renal SND responses to hyperthermia were determined in chloralose-anesthetized HF rats and in sham controls. Whole body heating (colonic temperature increased from 38 to 41 degrees C) was used as an acute stressor because increased internal body temperature provides a potent stimulus to the sympathetic nervous system. Left ventricular end-diastolic pressure and the right ventricular wt-to-body wt ratio were increased (P < 0.05) in HF compared with sham rats. The following observations were made: 1) renal sympathoexcitatory responses to heating were significantly reduced in HF compared with sham rats, 2) renal blood flow remained unchanged from control levels during heating in HF rats but was significantly reduced in sham rats, and 3) renal SND responses to heating were significantly higher in HF rats with bilateral lesions of the hypothalamic paraventricular nucleus (PVN) compared with sham PVN-lesioned HF rats. These results demonstrate a marked attenuation in the responsiveness of renal SND to heating in HF rats and suggest that HF alters the organization of neural pathways mediating SND responses to heating.

Endogenous angiotensin II in the NTS contributes to sympathetic activation in rats with aortocaval shunt

Recent studies have suggested that the central nervous system is responsible for activation of sympathetic nerve activity (SNA) and the renin-angiotensin system in heart failure (HF). The aim of this study was to determine whether activation of the renin-angiotensin system within the nucleus of the solitary tract (NTS) plays a role in enhanced SNA in HF. High-output HF was induced by an aortocaval (A-V) shunt with some modifications in the rat. These rats exhibited a left ventricular dilatation and hemodynamic signs of high-output HF. Urinary catecholamine excretion and maximal renal SNA (RSNA) were greater in the A-V shunted rats than in the control rats. Microinjection of an angiotensin II type 1-receptor antagonist, CV11974, into the NTS was performed. The arterial pressure and RSNA were reduced by CV11974 to a greater degree in the A-V shunted rats than in the control rats. The expression of angiotensin-converting enzyme mRNA in the medulla was greater in the A-V shunted rats than in the control rats. These results suggest that activation of the renin-angiotensin system within the NTS contributes to an enhanced SNA in this model.

Neural control of the kidney: functionally specific renal sympathetic nerve fibers

The sympathetic nervous system provides differentiated regulation of the functions of various organs. This differentiated regulation occurs via mechanisms that operate at multiple sites within the classic reflex arc: peripherally at the level of afferent input stimuli to various reflex pathways, centrally at the level of interconnections between various central neuron pools, and peripherally at the level of efferent fibers targeted to various effectors within the organ. In the kidney, increased renal sympathetic nerve activity regulates the functions of the intrarenal effectors: the tubules, the blood vessels, and the juxtaglomerular granular cells. This enables a physiologically appropriate coordination between the circulatory, filtration, reabsorptive, excretory, and renin secretory contributions to overall renal function. Anatomically, each of these effectors has a dual pattern of innervation consisting of a specific and selective innervation by unmyelinated slowly conducting C-type renal sympathetic nerve fibers in addition to an innervation that is shared among all the effectors. This arrangement permits the maximum flexibility in the coordination of physiologically appropriate responses of the tubules, the blood vessels, and the juxtaglomerular granular cells to a variety of homeostatic requirements.

Norepinephrine turnover in peripheral tissues of rats with heart failure

Experiments were performed to determine if there is regional heterogeneity in sympathetic neural activation of peripheral tissues in rats with chronic heart failure (HF; 6-8 wk after coronary artery ligation). Norepinephrine (NE) turnover, an index of sympathetic activation, was determined on the basis of the decline in tissue NE levels that occurs during the 8-h after tyrosine hydroxylase inhibition (alpha-methyl-DL-p-tyrosine, 300 mg/kg ip at 4-h intervals). Compared with sham-operated rats, NE turnover was increased in the cardiac left ventricle, skeletal muscle, duodenum, and kidney of rats with HF, but was unaltered in liver and spleen. The increased renal NE turnover in HF was largely a reflection of increased turnover in the cortex, with no change evident in the medulla. Blockade of sympathetic ganglionic traffic (hexamethonium, 2 mg/kg sc at 2-h intervals) eliminated the tissue-specific effects of HF on tissue NE levels measured 8-h after tyrosine hydroxylase inhibition. These data support the contention that chronic HF evokes a central nervous system-mediated increase in basal sympathetic tone that exhibits regional heterogeneity (both between and within organs), a phenomenon that likely contributes to the functional consequences of this pathophysiological state.

Decreased responsiveness of vascular postjunctional alpha1-, alpha2-adrenoceptors and neuropeptide Y1 receptors in rats with heart failure

Heart failure is associated with increased sympathetic nerve activity. We hypothesized that chronic sympathetic stimulation in heart failure resulted in decreased vascular sympathetic responsiveness. A pithed rat model was employed to evaluate peripheral vascular alpha-adrenoceptor and neuropeptide Y (NPY) receptor responsiveness. Heart failure was induced in Sprague-Dawley rats by coronary artery ligation. Sham operated rats (Sham) served as controls. Two months after this surgical procedure, both heart failure (n = 30) and Sham (n = 30) rats underwent standard pithing procedure. Pressor responses to preganglionic sympathetic nerve stimulation (PNS) and activation of postjunctional alpha1- and alpha2-adrenoceptors as well as Y1 receptors were studied. In response to PNS, cardiac index was similar between heart failure and sham rats (P = n.s.). Mean arterial pressure (MAP) increased in a frequency-dependent fashion after PNS in heart failure rats as well as in control rats. All the agonists used, i.e. the alpha1-adrenoceptor agonist phenylephrine, the alpha2-adrenoceptor agonists clonidine and BHT933 as well as NPY, induced dose-dependent increases in MAP in heart failure and in sham rats. However, in rats with heart failure, the response to all the agonists studied was significantly decreased and the dose response curves were shifted to the right (P < 0.01). We conclude that in vivo vascular response to postjunctional alpha1- and alpha2-adrenoceptors as well as Y1 receptors are decreased in rats with heart failure.

Cold stress alters characteristics of sympathetic nerve discharge bursts

Frequency-domain analyses were used to determine the effect of cold stress on the relationships between the discharge bursts of sympathetic nerve pairs, sympathetic and aortic depressor nerve pairs, and sympathetic and phrenic nerve pairs in chloralose-anesthetized, baroreceptor-innervated rats. Sympathetic nerve discharge (SND) was recorded from the renal, lumbar, splanchnic, and adrenal nerves during decreases in core body temperature from 38 to 30 degrees C. The following observations were made. 1) Hypothermia produced nonuniform changes in the level of activity in regionally selective sympathetic nerves. Specifically, cold stress increased lumbar and decreased renal SND but did not significantly change the level of activity in splanchnic and adrenal nerves. 2) The cardiac-related pattern of renal, lumbar, and splanchnic SND bursts was transformed to a low-frequency (0-2 Hz) pattern during cooling, despite the presence of pulse-synchronous activity in arterial baroreceptor afferents. 3) Peak coherence values relating the discharges between sympathetic nerve pairs decreased at the cardiac frequency but were unchanged at low frequencies (0-2 Hz), indicating that the sources of low-frequency SND bursts remain prominently coupled during progressive reductions in core body temperature. 4) Coherence of discharge bursts in phrenic and renal sympathetic nerve pairs in the 0- to 2-Hz frequency band increased during mild hypothermia (36 degrees C) but decreased during deep hypothermia (30 degrees C). We conclude that hypothermia profoundly alters the organization of neural circuits involved in regulation of sympathetic nerve outflow to selected regional circulations.

Animal models of heart failure

Animal models of heart failure present homogenous groups of animals all with heart failure produced by a well defined lesion at a particular stage of evolution, in contrast to humans, who present with heart failure of uncertain duration from a wide variety of causes and with marked variation in age and pre-morbid health and fitness. Animal models of heart failure provide diseased groups of animals in which experimental procedures, not possible in humans, can be evaluated and in which new treatments can be tested before their safety is established in humans. An ideal model should have a common human counterpart and should closely mimic heart failure in humans. Thus the haemodynamic changes should include increased cardiac filling pressures and low cardiac output. There should be evidence of activation of the sympathetic nervous system and increased secretion of hormones such as renin, angiotensin, aldosterone, vasopressin, atrial natriuretic factor and endothelin. The clinical features of the human syndrome such as cardiomegaly, lung and peripheral oedema and decreased exercise tolerance should be present. Lastly, the model should be inexpensive and technically simple to produce and study. This paper reviews some commonly used models of heart failure in relation to the criteria listed above. There is no perfect animal model of heart failure and in practice one should match the model to the purpose of the study.

Brain renin-angiotensin system and sympathetic hyperactivity in rats after myocardial infarction

Blockade of brain "ouabain" prevents the sympathetic hyperactivity and impairment of baroreflex function in rats with congestive heart failure (CHF). Because brain "ouabain" may act by activating the brain renin-angiotensin system (RAS), the aim of the present study was to assess whether chronic treatment with the AT1-receptor blocker losartan given centrally normalizes the sympathetic hyperactivity and impairment of baroreflex function in Wistar rats with CHF postmyocardial infarction (MI). After left coronary artery ligation (2 or 6 wk), rats received either intracerebroventricular losartan (1 mg. kg-1. day-1, CHF-Los) or vehicle (CHF-Veh) by osmotic minipumps. To assess possible peripheral effects of intracerebroventricular losartan, one set of CHF rats received the same rate of losartan subcutaneously. Sham-operated rats served as control. After 2 wk of treatment, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) at rest and in response to air-jet stress and intracerebroventricular injection of the alpha2-adrenoceptor-agonist guanabenz were measured in conscious animals. Arterial baroreflex function was evaluated by ramp changes in MAP. Compared with sham groups, CHF-Veh groups showed impaired arterial baroreflex control of HR and RSNA, increased sympathoexcitatory and pressor responses to air-jet stress, and increased sympathoinhibitory and hypotensive responses to guanabenz. The latter is consistent with decreased activity in sympathoinhibitory pathways. Chronic intracerebroventricular infusion of losartan largely normalized these abnormalities. In CHF rats, the same rate of infusion of losartan subcutaneously was ineffective. In sham-operated rats, losartan intracerebroventricularly or subcutaneously did not affect sympathetic activity. We conclude that the chronic increase in sympathoexcitation, decrease in sympathoinhibition, and desensitized baroreflex function in CHF all appear to depend on the brain RAS, since this whole pattern of changes can be normalized by chronic central AT1-receptor blockade with losartan.

Altered frequency characteristics of sympathetic nerve activity after sustained elevation in arterial pressure

We tested the hypothesis that sustained elevation in mean arterial pressure (MAP) alters the frequency-domain characteristics of efferent sympathetic nerve discharge (SND) after the return of MAP to control levels. Renal, lumbar, and splanchnic SND were recorded before, during, and after a 30-min increase in MAP produced by phenylephrine (PE) infusion in alpha-chloralose-anesthetized, spontaneously hypertensive (SH) rats. The following observations were made. 1) The basic cardiac-locked pattern of renal, lumbar, and splanchnic SND bursts was altered after sustained elevation in MAP, demonstrating prolonged effects on the neural circuits involved in entraining efferent SND to the cardiac cycle. Importantly, discharge bursts in afferent baroreceptor nerve activity remained pulse-synchronous after sustained increases in arterial pressure. 2) The frequency-domain relationships between the activity in sympathetic nerve pairs were altered after sustained elevation in MAP, suggesting a transformation from a system of tightly coupled neural circuits to one of multiple generators exerting selective control over SND. 3) The most prominent reduction in SND power after sustained elevation in MAP occurred in the frequency band containing the cardiac cycle, indicating that the prolonged suppression of SND after sustained increases in arterial pressure is due primarily to the selective inhibition of cardiac-related SND bursts. We conclude that sustained elevation in MAP profoundly affects the neural circuits responsible for the frequency components of basal SND in SH rats.

Blockade of brain 'ouabain' prevents the impairment of baroreflexes in rats after myocardial infarction

BACKGROUND

The purpose of this study was to test whether increased brain "ouabain" contributes to impairment of both arterial and cardiopulmonary baroreceptor reflexes in congestive heart failure (CHF).

METHODS AND RESULTS

Two to 5 days after coronary artery ligation (MI) or sham surgery in male Wistar rats, chronic intracerebroventricular (ICV) infusion was started with either antibody Fab fragments, which bind ouabain and related steroids with high affinity, or gamma-globulins as control (200 microg x 12 microL[-1] x d[-1] for both) with osmotic minipumps implanted subcutaneously. After 8 weeks of infusion, in conscious rats, mean arterial pressure (MAP), heart rate (HR), central venous pressure (CVP), and renal sympathetic nerve activity (RSNA) were recorded at rest and in response to ramp changes in blood pressure (BP) induced by intravenous phenylephrine and nitroprusside and to changes in CVP elicited by acute volume expansion with 5% dextrose. Compared with sham rats, in MI rats with ICV gamma-globulins, resting MAP was significantly lower and CVP increased, and both arterial and cardiopulmonary baroreflex control of RSNA and HR were attenuated. ICV Fab fragments prevented the decrease in resting BP and largely prevented impairment of arterial and cardiopulmonary baroreflex control of both RSNA and HR.

CONCLUSIONS

These data indicate that increased brain ouabain plays a major role in the impairment of baroreflexes in rats with CHF after myocardial infarction.

Angiotensin II blockade [corrected] enhances baroreflex control of sympathetic outflow in heart failure

Enhanced sympathetic outflow is seen in both patients with congestive heart failure and animals with experimental heart failure. In a previous study, we demonstrated that the baroreflex control of heart rate was impaired in conscious rabbits with pacing-induced heart failure and that this impairment was partially restored by blockade of angiotensin II type 1 (AT1) receptors. In the present study, we determined the interaction between the renin-angiotensin system and baroreflex control of renal sympathetic nerve activity in normal conscious rabbits and conscious rabbits with pacing-induced heart failure before and after AT1 receptor blockade. Heart failure was induced by rapid ventricular pacing at a rate of 360 to 380 beats per minute for an average of 16.7 +/- 0.6 days. To generate baroreflex curves, we altered arterial pressure by administering phenylephrine and sodium nitroprusside. A sigmoidal logistic function was fit to renal sympathetic nerve activity-mean arterial pressure relationships for analysis of several components of baroreflex function. AT1 receptors were blocked by intravenous administration of the specific antagonist L-158,809. In normal rabbits, there was no significant difference in any parameter of baroreflex function before and after blockade of AT1 receptors. In contrast, blockade of AT1 receptors enhanced baroreflex sensitivity in heart failure rabbits. The maximal gain increased to 5.0 +/- 0.7% renal sympathetic nerve activity/mm Hg from 2.6 +/- 0.3 (P < .05). Although L-158,809 had no effect on baseline renal sympathetic nerve activity in normal rabbits, analysis of the data in the heart failure rabbits indicated that baseline renal sympathetic nerve activity was reduced from 33 +/- 5% to 17 +/- 4% after L-158,809 administration after adjustment for changes in arterial pressure. These data suggest that angiotensin II plays a role in baroreflex impairment in this model of heart failure and may be in part responsible for the depressed baroreflex sensitivity observed in heart failure.

Cardiovascular responses and interactions by neuropeptide Y in rats with congestive heart failure

Neuropeptide Y (NPY) has been shown to potentiate the effects of various vasoactive agents in both in vitro and in vivo experiments. The present study was designed to investigate the potentiation effects of NPY on various vasoconstrictive agents and the influence of NPY on the dilatation effects of endothelin-1 in rats with congestive heart failure (CHF). CHF was induced by left coronary artery ligation. Sham-operated rats subjected to thoracotomy served as normal controls. Experiments in conscious rats were performed 4-6 weeks after coronary artery ligation or sham operation. The pressor responses of intravenous phenylephrine (12.5 nmol/kg), endothelin-1 (400 pmol/kg) and angiotensin II (10 ng) but not tyramine (40 micrograms) were significantly decreased in CHF rats compared with sham-operated rats (p < 0.01). In sham-operated rats, subthreshold dose of NPY (0.1 microgram/kg/min) potentiated the pressor responses of all the agonists (p < 0.05). In CHF rats, significant enhancement of mean arterial pressure (MAP) by subthreshold dose of NPY was observed with angiotensin II (p < 0.05). The MAP was enhanced by 45.4% in CHF and 40.6% in sham-operated rats respectively. NPY did not enhance the pressor responses induced by phenylephrine, endothelin-1 or tyramine in CHF rats. The initial decrease of MAP after bolus injection of endothelin-1 was observed in both CHF and sham-operated control rats, and magnitude of this response was similar in both groups. Subthreshold dose of NPY significantly reduced the vasodilatation effect of endothelin-1 in CHF (p < 0.05) but not in normal control rats. We conclude that NPY potentiates pressor effects of angiotensin II and reduces vasodilatation effects of endothelin-1 in rats with CHF. These effects of NPY may contribute to the increased vascular tone in CHF.

Brain 'ouabain' mediates sympathetic hyperactivity in congestive heart failure

In congestive heart failure (CHF), endogenous compounds with ouabainlike activity (OLA) may contribute to the maintenance of the circulatory homeostasis by peripheral as well as central effects. In the present study, we assessed changes in peripheral (plasma and left ventricle) and central (pituitary, hypothalamus, pons, and cortex) OLA in two animal models of CHF and determined whether brain OLA mediates sympathetic hyperactivity in CHF. Cardiomyopathic hamsters with their controls were studied at 9 months of age for tissue OLA. Rats were studied 4 weeks after acute coronary artery ligation for tissue OLA and sympathetic activity. In both models, left ventricular end-diastolic pressure was markedly increased. CHF was associated with significant increases in both plasma and tissue OLA in both models. In the brain, the most marked (twofold to threefold) increases occurred in the hypothalamus. In vitro, all OLA measured could be blocked by antibody Fab fragments (Digibind). Conscious rats with CHF showed elevated plasma catecholamines and enhanced responses of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) to air stress and to intracerebroventricular (ICV) injection of the alpha 2-adrenergic receptor agonist guanabenz compared with sham-operated rats. ICV administration of the Fab fragments did not change resting RSNA or responses to air stress at 1 hour. However, 18 hours after injection of the Fab fragments, resting RSNA levels had significantly decreased compared with the control values, and plasma catecholamine levels had decreased to control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and renal effects of alpha-trinositol in ischemic heart failure rats

Previous studies have demonstrated that alpha-trinositol (D-myo-inositol-1.2.6-trisphosphate; PP56) may act as a functional neuropeptide Y (NPY) inhibitor. Because NPY is known to be a potent vasoconstrictor, the effects of alpha-trinositol on renal function, vascular responses and the potentiating effects of NPY were investigated in rats with congestive heart failure (CHF) induced by ligation of the left coronary artery. Incremental doses of alpha-trinositol were given to conscious rats (bolus 2, 4 or 10 mg/kg i.v. followed by a 15-minute infusion 20, 40 and 100 mg/kg/h, respectively). Urinary volume, sodium and potassium excretions were significantly increased in both CHF and sham-operated control animals after alpha-trinositol administration compared with saline. Diuresis and natriuresis were observed also during co-administration of alpha-trinositol with NPY but not with norepinephrine (NE). In the pithed CHF rats, threshold doses of NPY potentiated the pressor effects of endothelin-1 (ET-1) and angiotensin II (AII), but not preganglionic nerve stimulation or phenylephrine administration. Alpha-trinositol antagonized both the pressor response to NPY and the potentiation by NPY of pressor responses to effects of ET-1 and AII. Our data show that alpha-trinositol exhibis diuretic and natriuretic effects as well as vascular antagonistic effects on NPY in normal and CHF rats. These effects of alpha-trinositol may be due to an interaction with NPY mediated antidiuresis and antinatriuresis.