Decrease In Renal Sympathetic Nerve Activity Research Articles

Abstract 4121234: Macrophage activation in aorticorenal ganglion as a potential trigger to induce renal sympathetic overactivation-related ventricular arrhythmias after acute myocardial infarction

Background: Ischemia-related ventricular arrhythmia is the leading cause of death in patients after acute myocardial infarction (AMI). Although the therapeutic potential of renal denervation (RDN) for ventricular arrhythmias has been reported, RDN-induced adverse complications partially limit its use in the clinic. Since neuroinflammation has recently emerged as a key factor for sympathetic overactivation in the central and peripheral nervous systems, this study aims to test if macrophage activation in renal sympathetic postganglionic (RSP) neurons located in the aorticorenal ganglion (ARG) contributes to renal sympathetic overactivation-related ventricular arrhythmogenesis post-AMI. Methods and Results: AMI was induced by surgical ligation of the left anterior descending artery in rats. Data from immunofluorescence staining showed that the expression of Iba1, TNF-α, and IL-1β was increased in ARGs at 3 days post-AMI rats compared with sham rats, suggesting that macrophage activation and neuroinflammation occurred in the ARG at an early stage after AMI. Direct recording of renal sympathetic nerve activity (RSNA) found that the RSNA was much higher in the 3 days post-AMI rats than in sham rats. To validate the contribution of macrophage activation to AMI-elevated RSNA, we depleted macrophages in ARGs by local microinjection of clodronate liposomes (CL) into ARGs. Immunofluorescence staining data showed that microinjection of CL into ARGs attenuated the AMI-induced macrophage activation (Iba1) and neuroinflammation (TNF-α and IL-1β) in ARGs. Flow cytometry data also confirmed that macrophage depletion in ARGs reduced the AMI-elevated macrophage infiltration in ARGs, which was accompanied by a significant decrease in RSNA in CL-treated AMI rats. To test the ventricular arrhythmogenesis post-AMI, we measured ventricular tachycardia/fibrillation (VT/VF) from 24-hour telemetry ECG recording in conscious rats. Our data showed that macrophage depletion in ARGs alleviated AMI-induced ventricular arrhythmogenesis, as evidenced by a significant reduction in the occurrence of VT/VF in CL-treated AMI rats. Conclusion: These data suggest that macrophage activation and neuroinflammation in the ARG might be critical factors responsible for the renal sympathetic overactivation-related ventricular arrhythmia post-AMI. Inhibiting macrophage activation and neuroinflammation in the ARG could be an effective strategy to achieve the antiarrhythmic effect of RDN after AMI.

Mechanical Circulatory Support Reduces Directly Recorded Renal Sympathetic Nerve Activity in an Ovine Model of Cardiogenic Shock

Cardiogenic shock (CS) following myocardial infarction carries a high mortality risk despite advancements in management. The use of mechanical circulatory support in CS has become an effective strategy to improve haemodynamics and prevent acute kidney injury. However, the mechanism of how kidney function improves is unclear. We hypothesised that mechanical support would inhibit renal sympathetic nerve activity (RSNA), mediating renal protection in CS. Experiments were conducted in two groups of anaesthetised female sheep. In one group, CS was induced (n=8) using injections of polystyrene microspheres into the left coronary artery under fluoroscopic guidance. After a 30-minute baseline period (post -embolisation), intravenous phenylephrine infusion at incremental doses was used to assess the baroreflex control of RSNA before pump insertion. When the pressures recovered to baseline values, the circulatory assist Impella pump was inserted into the left ventricle and run randomly at different levels (P0 min-P6 max) with two minutes at each pump level. The controls underwent the same protocol without embolisation (n=6). Coronary artery embolisation resulted in a drop in mean arterial pressure (MAP) of 21 ± 4 mmHg compared to baseline values (n=8). This was associated with a 67% increase in renal sympathetic nerve activity (RSNA) from 16 ± 5 to 21 ± 5 spikes/s (p=0.038; n=7). Circulatory support using Impella significantly increased MAP from 55 ± 4 mmHg to 68 ± 5 mmHg at pump level P6 (one-way ANOVA, p<0.001). Incremental pump support resulted in a significant decrease in RSNA (p<0.001). At pump level P6, RSNA was decreased by 25 ± 5 % compared to P0 (p<0.001). Baroreflex curves predicted a 2% reduction in RSNA at P6 equivalent pressures in CS, which was significantly less than that observed with circulatory support (p=0.006). In the control cohort with no CS, the changes in MAP and RSNA from P0 to P6 were qualitatively similar to the CS cohort. MAP increased from 84 ± 9 to 94 ± 8 mmHg (one-way ANOVA, p<0.001) and RSNA decreased by 13 ± 5 % at P6 (one-way ANOVA, p<0.001). The expected reduction in RSNA with baroreflex assessment at equivalent pressures to P6 was 5% in controls, which was not significantly different to what was observed (p=0.065). Our data indicate that the use of Impella pumps in CS reduces RSNA, which may mediate improvements in kidney function. Interestingly, the renal sympathoinhibition with the Impella pump is greater than that with vasoactive agents alone. We gratefully acknowledge the granting support from Abiomed and the University of Auckland Faculty Research Development Fund. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

C-type natriuretic peptide (CNP) in the paraventricular nucleus-mediated renal sympatho-inhibition.

Volume reflex produces sympatho-inhibition that is mediated by the hypothalamic paraventricular nucleus (PVN). However, the mechanisms for the sympatho-inhibitory role of the PVN and the neurochemical factors involved remain to be identified. In this study, we proposed C-type natriuretic peptide (CNP) as a potential mediator of this sympatho-inhibition within the PVN. Microinjection of CNP (1.0μg) into the PVN significantly decreased renal sympathetic nerve activity (RSNA) (-25.8% ± 1.8% vs. -3.6% ± 1.5%), mean arterial pressure (-15.0 ± 1.9 vs. -0.1 ± 0.9mmHg) and heart rate (-23.6 ± 3.5 vs. -0.3 ± 0.9 beats/min) compared with microinjection of vehicle. Picoinjection of CNP significantly decreased the basal discharge of extracellular single-unit recordings in 5/6 (83%) rostral ventrolateral medulla (RVLM)-projecting PVN neurons and in 6/13 (46%) of the neurons that were not antidromically activated from the RVLM. We also observed that natriuretic peptide receptor type C (NPR-C) was present on the RVLM projecting PVN neurons detected by dual-labeling with retrograde tracer. Prior NPR-C siRNA microinjection into the PVN significantly blunted the decrease in RSNA to CNP microinjections into the PVN. Volume expansion-mediated reduction in RSNA was significantly blunted by prior administration of NPR-C siRNA into the PVN. These results suggest a potential role for CNP within the PVN in regulating RSNA, specifically under physiological conditions of alterations in fluid balance.

Altered Neuronal Discharge in the Organum Vasculosum of the Lamina Terminalis Contributes to Dahl Salt-Sensitive Hypertension.

Salt-sensitive hypertension in humans and experimental models is associated with higher plasma and cerebrospinal fluid sodium chloride (NaCl) concentrations. Changes in extracellular NaCl concentrations are sensed by specialized neurons in the organum vasculosum of the lamina terminalis (OVLT). Stimulation of OVLT neurons increases sympathetic nerve activity (SNA) and arterial blood pressure (ABP), whereas chronic activation produces hypertension. Therefore, the present study tested whether OVLT neuronal activity was elevated and contributed to SNA and ABP in salt-sensitive hypertension. Male Dahl salt-sensitive (Dahl S) and Dahl salt-resistant (Dahl R) rats were fed 0.1% or 4.0% NaCl diets for 3 to 4 weeks and used for single-unit recordings of OVLT neurons or simultaneous recording of multiple sympathetic nerves during pharmacological inhibition of the OVLT. Plasma and cerebrospinal fluid Na+ and Cl- concentrations were higher in Dahl S rats fed 4% versus 0.1% or Dahl R rats fed either diet. In vivo single-unit recordings revealed a significantly higher discharge of NaCl-responsive OVLT neurons in Dahl S rats fed 4% versus 0.1% or Dahl R rats. Interestingly, intracarotid infusion of hypertonic NaCl evoked greater increases in OVLT neuronal discharge of Dahl S versus Dahl R rats regardless of NaCl diet. The activity of non-NaCl-responsive OVLT neurons was not different across strain or diets. Finally, inhibition of OVLT neurons by local injection of the gamma-aminobutyric acid agonist muscimol produced a greater decrease in renal SNA, splanchnic SNA, and ABP of Dahl S rats fed 4% versus 0.1% or Dahl R rats. A high salt diet activates NaCl-responsive OVLT neurons to increase SNA and ABP in salt-sensitive hypertension.

Abstract MP22: Dietary Salt Restriction And High Salt Intake Exaggerate Sympathetic Reflexes And Sensitize Central Autonomic Networks: Potential Mechanisms Of The J-shaped Curve

Observational cohort studies suggest that severe salt restriction increases cardiovascular morbidity/mortality, and the relationship between cardiovascular morbidity and dietary salt intake resembles a J-shaped curve. A high salt diet exaggerates sympathetic nerve activity (SNA) and arterial blood pressure (ABP) responses to several cardiovascular reflexes in salt-resistant animals. This study assessed whether salt restriction also exaggerates cardiovascular reflex responses and sensitizes central autonomic networks. To test this hypothesis, male Sprague-Dawley rats were fed low (0.01% NaCl), normal (0.1% NaCl), and high (4.0% NaCl) salt diet for 14-21 days. Baseline mean ABP was not different across groups (low: 104±4, normal: 107±4, high: 107±4mmHg). Activation of sciatic afferents (1ms pulse, 500uA, 5s duration, 2-20Hz) produced significantly greater increases in renal SNA (5Hz; low: 196±12, normal: 136±9, high: 177±8%, n=8, P<0.05) and ABP (5Hz; low: 29±3, normal: 16±1, high: 24±2 mmHg, n=8, P<0.05) of rats fed low and high versus normal NaCl diets. Activation of the aortic depressor nerve (2ms pulse, 500uA, 15s duration, 2-20Hz) produced significantly greater decreases in renal SNA (5Hz; low: -55±9, normal: -34±8, high: -63±13%, n=7-8, P<0.05) and ABP (5Hz; low: -31±3, normal: -15±5, high: -32±5 mmHg, n=7-8, P<0.05) of rats fed low and high versus normal NaCl diets. To test whether dietary salt intake sensitized central sympathetic circuits, microinjection of L-glutamate (0.1-1nmol, 30nL) in the rostral ventrolateral medulla produced significantly greater increases in renal SNA (0.1nmol; low: 212±15, normal: 149±8, high: 183±17%, n=7-8, P<0.05) and ABP (0.1Hz; low: 20±2, normal: 12±2, high: 22±2 mmHg, n=7-8, P<0.05) of rats fed low and high versus normal NaCl diets. Finally, rats fed low or high NaCl versus normal NaCl diets displayed exaggerated cardiovascular responses to cage switch or mild restraint and increased 24-h blood pressure variability. The present findings show that severe salt restriction and excess dietary salt intake exaggerate sympathetic and cardiovascular responses, and may be explained by a parallel change in the sensitivity of central autonomic networks to resemble a J-shaped curve.

Chronic intermittent hypoxia impairs diuretic and natriuretic responses to volume expansion in rats with preserved low-pressure baroreflex control of the kidney.

We examined the effects of exposure to chronic intermittent hypoxia (CIH) on baroreflex control of renal sympathetic nerve activity (RSNA) and renal excretory responses to volume expansion (VE) before and after intrarenal transient receptor potential vanilloid 1 (TRPV1) blockade by capsaizepine (CPZ). Male Wistar rats were exposed to 96 cycles of hypoxia per day for 14 days (CIH) or normoxia. Urine flow and absolute Na+ excretion during VE were less in CIH-exposed rats, but the progressive decrease in RSNA during VE was preserved. Assessment of the high-pressure baroreflex revealed an increase in the operating and response range of RSNA and decreased slope in CIH-exposed rats with substantial hypertension [+19 mmHg basal mean arterial pressure (MAP)] but not in a second cohort with modest hypertension (+12 mmHg). Intrarenal CPZ caused diuresis, natriuresis, and a reduction in MAP in sham-exposed (sham) and CIH-exposed rats. After intrarenal CPZ, diuretic and natriuretic responses to VE in CIH-exposed rats were equivalent to those of sham rats. TRPV1 expression in the renal pelvic wall was similar in both experimental groups. Exposure to CIH did not elicit glomerular hypertrophy, renal inflammation, or oxidative stress. We conclude that exposure to CIH 1) does not impair the low-pressure baroreflex control of RSNA; 2) has modest effects on the high-pressure baroreflex control of RSNA, most likely indirectly due to hypertension; 3) can elicit hypertension in the absence of kidney injury; and 4) impairs diuretic and natriuretic responses to fluid overload. Our results suggest that exposure to CIH causes renal dysfunction, which may be relevant to obstructive sleep apnea.

Intermedin in Paraventricular Nucleus Attenuates Sympathoexcitation and Decreases TLR4-Mediated Sympathetic Activation via Adrenomedullin Receptors in Rats with Obesity-Related Hypertension.

Intermedin/adrenomedullin-2 (IMD/AM2), a member of the calcitonin gene-related peptide/AM family, plays an important role in protecting the cardiovascular system. However, its role in the enhanced sympathoexcitation in obesity-related hypertension is unknown. In this study, we investigated the effects of IMD in the paraventricular nucleus (PVN) of the hypothalamus on sympathetic nerve activity (SNA), and lipopolysaccharide (LPS)-induced sympathetic activation in obesity-related hypertensive (OH) rats induced by a high-fat diet for 12 weeks. Acute experiments were performed under anesthesia. The dynamic alterations of sympathetic outflow were evaluated as changes in renal SNA and mean arterial pressure (MAP) in response to specific drugs. Male rats were fed a control diet (12% kcal as fat) or a high-fat diet (42% kcal as fat) for 12 weeks to induce OH. The results showed that IMD protein in the PVN was downregulated, but Toll-like receptor 4 (TLR4) and plasma norepinephrine (NE, indicating sympathetic hyperactivity) levels, and systolic blood pressure were increased in OH rats. LPS (0.5 µg/50 nL)-induced enhancement of renal SNA and MAP was greater in OH rats than in obese or control rats. Bilateral PVN microinjection of IMD (50 pmol) caused greater decreases in renal SNA and MAP in OH rats than in control rats, and inhibited LPS-induced sympathetic activation, and these were effectively prevented in OH rats by pretreatment with the AM receptor antagonist AM22-52. The mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) inhibitor U0126 in the PVN partially reversed the LPS-induced enhancement of SNA. However, IMD in the PVN decreased the LPS-induced ERK activation, which was also effectively prevented by AM22-52. Chronic IMD administration resulted in significant reductions in the plasma NE level and blood pressure in OH rats. Moreover, IMD lowered the TLR4 protein expression and ERK activation in the PVN, and decreased the LPS-induced sympathetic overactivity. These results indicate that IMD in the PVN attenuates SNA and hypertension, and decreases the ERK activation implicated in the LPS-induced enhancement of SNA in OH rats, and this is mediated by AM receptors.

Do the carotid body chemoreceptors mediate cardiovascular and sympathetic adjustments induced by sodium overload in rats?

Acute plasma hypernatremia induces several cardiovascular and sympathetic responses. It is conceivable that these responses contribute to rapid sodium excretion and restoration of normal conditions. Afferent pathways mediating these responses are not entirely understood. The present study analyses the effects of acute carotid chemoreceptor inactivation on cardiovascular and sympathetic responses induced by infusion of hypertonic saline (HS). All experiments were performed on anesthetized male Wistar rats instrumented for recording of arterial blood pressure (ABP), renal blood flow (RBF) and renal sympathetic nerve activity (RSNA). Animals were subjected to sham surgery or carotid chemoreceptor inactivation by bilateral ligation of the carotid body artery (CBA). In sham rats (n=8), intravenous infusion of HS (3 M NaCl, 1.8 ml/kg b.wt.) elicited a transient increase (9±2mmHg) in ABP, and long lasting (30 min) increases in RBF (138±5%) and renal vascular conductance (RVC) (128±5%) with concurrent decrease in RSNA (-19±4%). In rats submitted to CBA ligation (n=8), the pressor response to HS was higher (24±2mmHg; p<0.05). However, RBF and RVC responses to HS infusion were significantly reduced (113±5% and 93±4%, respectively) while RSNA was increased (13±2%). When HS (3M NaCl, 200μl) was administrated into internal carotid artery (ICA), distinct sympathetic and cardiovascular responses were observed. In sham-group, HS infusion (3M NaCl, 200μl) into ICA promoted an increase in ABP (26±8mmHg) and RSNA (29±13%). In CBA rats, ABP (-3±5.6mmHg) remained unaltered despite sympathoinhibition (-37.6±5.4%). These results demonstrate that carotid body chemoreceptors play a role in the development of hemodynamic and sympathetic responses to acute HS infusion.

Involvement of sinoaortic afferents in renal sympathoinhibition and vasodilation induced by acute hypernatremia.

Despite the abundance of evidence that supports the important role of aortic and carotid afferents to short-term regulation of blood pressure and detection of variation in the arterial PO2 , PCO2 and pH, relatively little is known regarding the role of these afferents during changes in the volume and composition of extracellular compartments. The present study sought to determine the involvement of these afferents in the renal vasodilation and sympathoinhibition induced by hypertonic saline (HS) infusion. Sinoaortic-denervated and sham male Wistar rats were anaesthetised with intravenous (i.v.) urethane (1.2g/kg body weight (bw)) prior to the measurement of the mean arterial pressure (MAP), renal vascular conductance (RVC) and renal sympathetic nerve activity (RSNA). In the sham group, the HS infusion (3mol/L NaCl, 1.8mL/kg bw, i.v.) induced transient hypertension (12±4 mmHg from baseline, peak at 10min; P<0.05), an increase in RVC (127±9% and 150±13% from baseline, at 20 and 60min respectively; P<0.05) and a decrease in RSNA (-34±10% and -29±5% from baseline, at 10 and 60min respectively; P<0.05). In sinoaortic-denervated rats, HS infusion promoted a sustained pressor response (30±5 and 17±6mmHg of baseline values, at 10 and 30min respectively; P<0.05) and abolished the increase in RVC (85±8% from baseline, at 10min) and decrease in RSNA (-4±3% from baseline, at 10min). These results suggest that aortic and carotid afferents are involved in cardiovascular and renal sympathoinhibition responses induced by acute hypernatremia.

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla.

Elevated NaCl concentrations of the cerebrospinal fluid increase sympathetic nerve activity (SNA) in salt-sensitive hypertension. Neurons of the rostral ventrolateral medulla (RVLM) play a pivotal role in the regulation of SNA and receive mono- or polysynaptic inputs from several hypothalamic structures responsive to hypernatremia. Therefore, the present study investigated the contribution of RVLM neurons to the SNA and pressor response to cerebrospinal fluid hypernatremia. Lateral ventricle infusion of 0.15 mol/L, 0.6 mol/L, and 1.0 mol/L NaCl (5 µL/10 minutes) produced concentration-dependent increases in lumbar SNA, adrenal SNA, and arterial blood pressure, despite no change in splanchnic SNA and a decrease in renal SNA. Ganglionic blockade with chlorisondamine or acute lesion of the lamina terminalis blocked or significantly attenuated these responses, respectively. RVLM microinjection of the gamma-aminobutyric acid (GABAA) agonist muscimol abolished the sympathoexcitatory response to intracerebroventricular infusion of 1 mol/L NaCl. Furthermore, blockade of ionotropic glutamate, but not angiotensin II type 1, receptors significantly attenuated the increase in lumbar SNA, adrenal SNA, and arterial blood pressure. Finally, single-unit recordings of spinally projecting RVLM neurons revealed 3 distinct populations based on discharge responses to intracerebroventricular infusion of 1 mol/L NaCl: type I excited (46%; 11/24), type II inhibited (37%; 9/24), and type III no change (17%; 4/24). All neurons with slow conduction velocities were type I cells. Collectively, these findings suggest that acute increases in cerebrospinal fluid NaCl concentrations selectively activate a discrete population of RVLM neurons through glutamate receptor activation to increase SNA and arterial blood pressure.

Abstract P191: Increases in Cerebrospinal Fluid NaCl Concentration Excites Neurons of the Organum Vasculosum of the Lamina Terminalis to Elevate Sympathetic Outflow and Blood Pressure

Accumulating evidence suggests salt-sensitive hypertension is mediated partly by an increase in cerebrospinal fluid (CSF) NaCl concentration and elevation in sympathetic nerve activity (SNA). Increased NaCl concentration or osmolality is sensed by specialized neurons in the organum vasculosum of the lamina terminalis (OVLT). The present study investigated the contribution of these neurons to the SNA and arterial blood pressure (ABP) responses during acute increases in CSF NaCl concentrations. Male Sprague-Dawley rats were anesthetized with Inactin (120mg/kg, IV) and prepared for SNA and ABP recordings. Lateral ventricle infusion of 1M NaCl (5uL over 10 min) increased CSF [Na+] by 5±1 mM and elevated mean ABP (9±1 mmHg), lumbar SNA (125±3%) and adrenal SNA (121±5%) but decreased renal SNA (-9±1%, n=8) and did not alter splanchnic SNA (102±3%). Inhibition of the OVLT with injection of the GABAA agonist muscimol (2.5mM per 20nL, n=5) significantly attenuated the NaCl-induced increase in ABP (2±1 mmHg), lumbar SNA (102±1%), adrenal SNA (103±2%) and decrease in renal SNA (-3±1%). In vivo single-unit recordings demonstrate that lateral ventricular infusion of 1M NaCl (5uL per 10 min) significantly increased the firing rate in 75% (3/4) of OVLT neurons from 1.2±0.4Hz to 5.1±1.2Hz (P&lt;0.05). Furthermore, direct injection of NaCl (100nL over 20s, n=3) into the OVLT produced dose-dependent increases in mean ABP (0.15M: 0±1mmHg; 0.5M: 2±1mmHg; 1.0M: 4±1mmHg; 2.0M: 7±1mmHg) and lumbar SNA (0.15M: 101±2%; 0.5M: 107±1%; 1.0M: 112±3%; 2.0M: 118±4%). Altogether, these findings suggest that increases in CSF NaCl concentrations excite OVLT neurons to elevate lumbar and adrenal SNA and ABP.

Angiotensin AT2 receptors and the baroreflex control of renal sympathetic nerve activity

Angiotensin AT2 receptors and the baroreflex control of renal sympathetic nerve activity

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.

Effects of exercise training on SFO-mediated sympathoexcitation during chronic heart failure

Exercise training (ExT) has been shown to reduce sympathetic drive during heart failure (HF). The subfornical organ (SFO) is involved in the neural control of sympathetic drive. We hypothesized that an activated SFO contributes to enhanced sympathetic activity in HF. We also postulated that ExT would reduce the activation of the SFO and its contribution to the sympathetic drive during HF. Sprague-Dawley rats were subjected to coronary artery ligation to induce HF. Rats were assigned to ExT for 3-4 wk. Rats with HF had a 2.5-fold increase in FosB-positive cells in the SFO compared with sham-operated rats, and this was normalized by ExT. Microinjection of ANG II (100 pmol) into the SFO resulted in a greater increase in renal sympathetic nerve activity (RSNA), blood pressure, and heart rate in the HF group than in the sham-operated group. These responses were normalized after ExT (change in RSNA: 23 ± 3% vs. 8 ± 2%). ExT also abolished the decrease in RSNA in HF rats after the microinjection of losartan (200 pmol) into the SFO (-21 ± 4% vs. -2 ± 3%). Finally, there was elevated mRNA (5-fold) and protein expression (43%) of ANG II type 1 receptors in the SFO of rats with HF, which were reversed after ExT. These data suggest that the enhanced activity of the SFO by elevated tonic ANG II contributes to the enhanced sympathoexcitation exhibited in HF. The decrease in ANG II type 1 receptor expression in the SFO by ExT may be responsible for reversing the neuronal activation in the SFO and SFO-mediated sympathoexcitation in rats with HF.

Role of angiotensin AT2 receptors and nitric oxide in the cardiopulmonary baroreflex control of renal sympathetic nerve activity in rats.

This study investigated the hypothesis that angiotensin II (type 2) (AT2) receptor activation to modulate the renal sympatho-inhibition to saline volume expansion was dependent on nitric oxide production. Renal sympatho-inhibition to a saline volume expansion (VEP, 0.25% body weight/min i.v. for 30 min) was studied following intracerebroventricular (ICV) saline, CGP42112 (CGP, AT2 agonist), PD123319 (AT2 antagonist), and losartan (AT1 antagonist), and then in combination with N-nitro-L-arginine methyl ester (L-NAME) (nitric oxide synthase inhibitor). ICV saline, PD123319, CGP, and losartan did not change baseline mean arterial pressure, heart rate, or renal sympathetic nerve activity (RSNA). VEP decreased RSNA in all groups by 58-62% (P<0.05). CGP enhanced the decrease in RSNA compared to saline (74 vs. 60%; P<0.05), whereas PD123319 was without effect (58 vs. 57%). L-NAME only increased baseline RSNA when co-administered with PD123319 (P<0.05). VEP-induced reduction in RSNA following L-NAME was less than during ICV saline (46 vs. 62%; P<0.05). In the group where PD123319 preceded L-NAME, the fall in RSNA was smaller than when PD123319 was infused alone (40 vs. 63%; P<0.05), but not if PD123319 followed L-NAME (52 vs. 44%). L-NAME did not change the magnitude of VEP-induced sympatho-inhibition following CGP (67 vs. 60%). Losartan enhanced the renal sympatho-inhibition to VEP (70 vs. 62%; P<0.05), the magnitude of which was unchanged when L-NAME was present (70 vs. 65%). AT2 receptor activation enhances the VEP-induced reduction in RSNA. Although nitric oxide is important in allowing the normal renal sympatho-inhibitory response to VEP, this is not dependent on AT2 receptors.

Exercise training normalizes the blunted central component of the baroreflex in rats with heart failure: role of the PVN

Exercise training (ExT) normalizes the increased sympathetic outflow in chronic heart failure (HF). The underlying mechanisms are not clearly understood. We hypothesized that ExT normalized the blunted central component of the baroreflex control of renal sympathetic nerve activity (RSNA) in HF. Four groups of rats [sham operated (sham)-sedentary (Sed), sham-ExT, HF-Sed, and HF-ExT] were used. HF was induced by left coronary artery ligation, and ExT consisted of 3 wk of treadmill running. In anesthetized rats, the decrease in RSNA in response to aortic depressor nerve stimulation (5-40 Hz) in the HF-Sed group was significantly lower than that in the sham-Sed group (-37 ± 7% vs. -63 ± 8% at 40 Hz, P < 0.05). In the HF-ExT group, responses in RSNA, mean arterial pressure (MAP), and heart rate (HR) were not significantly different from those in the sham-Sed or sham-ExT groups. ExT normalized blunted RSNA, MAP, and HR responses to bicuculline microinjections into the paraventricular nucleus (PVN) in rats with HF. Activation of the PVN by blockade of GABA receptors with bicuculline in normal control rats blunted the centrally component of the baroreflex arc. GABAA-α1 and -β1 receptor protein expression were significantly lower (by 48% and 30%) in the HF-Sed group, but ExT normalized this difference between the HF and sham groups. These data suggest that one mechanism by which ExT alleviates elevated sympathetic outflow in HF may be through normalization of central integrative mechanisms, perhaps via improving the inhibitory GABAergic mechanism within the PVN, on the baroreflex arc.

Organ Selective Regulation of Sympathetic Outflow by the Brain Angiotensin System

Angiotensin II (Ang II) has actions on the sympathetic nervous system both as a circulating hormone acting on the circumventricular organs and also as a neurotransmitter/ neuromodulator acting within the brain. Administration of Ang II into the cerebral ventricles has diverse effects on sympathetic nerve activity (SNA), causing an increase in cardiac and splanchnic and a decrease in renal SNA. Similar contrasting effects on cardiac and renal SNA are seen with administration of hypertonic saline, which is thought to act centrally through angiotensinergic pathways. In heart failure there is compelling evidence that central angiotensinergic mechanisms contribute to the increases in cardiac and renal SNA, which have numerous detrimental effects. Although there is evidence that Ang II regulates sympathetic activity, and contributes to excess SNA in disease, the exact sites in the brain at which Ang II acts to selectively control SNA to individual organs are not well defined.

Exercise Training (ExT) Normalizes Subfornical Organ (SFO)‐ Mediated Sympathoexcitation in Chronic Heart Failure (HF)

ExT normalizes sympathoexcitation (SymEx) exhibited in HF, however the underlying mechanisms are not clear. The SFO is involved in dictating SymEx and is influenced by peptides such as angiotensin (Ang) II due to its weak blood‐brain barrier. We hypothesized that neuronal activity in the SFO would be enhanced in HF and contribute to increased Ang II‐mediated SymEx, and that ExT would reverse this process. HF induced by coronary artery ligation in Sprague‐Dawley rats was confirmed by echocardiography 4 weeks after surgery. HF rats displayed increased neuronal activity in the SFO, as assessed by FosB immunohistochemistry (101 ± 9 vs. 29 ± 2 cells). This increase was attenuated after 4 weeks of treadmill ExT (36 ± 7 cells). In anesthetized rats, microinjection of Ang II into the SFO increased renal sympathetic nerve activity (RSNA) to a greater extent in HF (23 ± 3 vs. 9 ± 1%; 100 pmol), and was normalized after ExT (8 ± 2%). ExT also abolished the decrease in RSNA in HF rats after losartan microinjection into the SFO (−21 ± 4 vs. −1 ± 3%; 200 pmol), implying higher endogenous Ang II tone in HF. ExT reversed the elevated mRNA and protein expression of AT1R in the SFO of rats with HF. The enhanced activation of the SFO by elevated tonic Ang II contributes to the enhanced SymEx exhibited in HF. The reduction in AT1 receptors in the SFO by ExT may be responsible for restoring the neuronal activation in the SFO and SFO‐mediated SymEx. Supp. by NIH HL62222.

P52 Perfusion of isolated carotid sinus with hydrogen sulfide attenuated the renal sympathetic nerve activity in anesthetized male rats

The present study was to define the effect of H 2 S on central region which regulates sympathetic outflow by perfusion of isolated carotid sinus with H 2 S. Sodium hydrogen sulfide (NaHS), a H 2 S donor, was perfused into isolated carotid sinus; the functional curve of the carotid sinus baroreflex was measured by recording changes in renal sympathetic nerve activity (RSNA) in anesthetized male rats. (1) Perfusion of isolated carotid sinus with NaHS (25, 50, and 100 μmol/L) dose-dependently inhibited sympathetic outflow by enhancing the response of RSNA to the increased intrasinus pressure (ISP). RSNA was decreased to 84.95 ± 3.58% ( P < 0.01), 63.89 ± 2.53% ( P < 0.01) and 48.70 ± 4.16% ( P < 0.01) compare with control. (2) Pretreatment with glibenclamide (20 μmol/L), a K ATP channels blocker, the above effect of NaHS was abolished. (3) Prior perfusion of Bay K8644 (500 nmol/L), an agonist of calcium channels, the effect of NaHS was eliminated. (4) Perfusion of the cystathionine γ-lyase (CSE) inhibitor, DL -propargylglycine (PPG, 200 μmol/L) increased sympathetic outflow. The results suggest that perfusion of isolated carotid sinus with NaHS inhibits sympathetic outflow as evidenced by the decrease of RSNA. The effect is mediated by opening a K ATP channels and further closing the calcium channel in smooth muscle cells. Endogenous H 2 S may tonically suppress the sympathetic vasomotor by activating the activity of the carotid sinus baroreflex (CSB).

288 EFFECTS OF OVARIAN HORMONES ON THE CARDIAC AFFERENT REFLEX

Background: In disease states, such as hypertension and heart failure, the cardiac afferent reflex has been implicated in driving pathophysiological increases in sympathetic nerve activity (SNA). The current study set out to investigate the impact that ovarian hormones have on the cardiac afferent reflex. Methods: On the day of experiment blood pressure (BP), heart rate (HR) and renal SNA were recorded in chloralose-urethane anesthetized, open-chest and baroreceptor denervated female Wistar rats with ovaries-intact or -removed. The cardiac afferent reflex was investigated by placing capsaicin (1, 5 and 10 μg per 10 μl) via a piece of filter paper onto the anterior surface of the left ventricle for 40 seconds. Results: Removal of ovaries abolished the initial vagal mediated decrease in renal SNA that was observed in females with ovaries-intact at 4 seconds post capsaicin application. At 40 seconds post-capsaicin application the increase in renal SNA was significantly greater in ovary-removed females compared to ovary-intact females. The estrous cycle in ovary-intact females also significantly impacted on the cardiac afferent reflex; females in the proestrous phase displayed greater increases in renal SNA compared to females in the estrous/metestrous phase. Discussion: The current findings suggest that circulating ovarian hormones affect the cardiac afferent reflex, with removal of circulating ovarian hormones resulting in greater reflex sympathetic excitation. These findings provide evidence of a mechanism that may contribute to greater sympathetic excitation in cardiovascular disorders post-menopause, compared to pre-menopausal, which may relate to higher rates of morbidity and mortality in women following menopause.