Chronic heart failure is often associated with sympathoexcitation and blunted arterial baroreflex function. These phenomena have been causally linked to elevated central ANG II mechanisms. Recent studies have shown that NAD(P)H oxidase-derived reactive oxygen species (ROS) are important mediators of ANG II signaling and therefore might play an essential role in these interactions. The aims of this study were to determine whether central subchronic infusion of ANG II in normal animals has effects on O2- production and expression of NAD(P)H oxidase subunits as well as ANG II type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM). Twenty-four male New Zealand White rabbits were divided into four groups and separately received a subchronic intracerebroventricular infusion of saline alone, ANG II alone, ANG II with losartan, and losartan alone for 1 wk. On day 7 of intracerebroventricular infusion, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) values were recorded, and arterial baroreflex sensitivity was evaluated while animals were in the conscious state. We found that ANG II significantly increased baseline RSNA (161.9%; P< 0.05), mRNA and protein expression of AT1 receptors (mRNA, 66.7%; P < 0.05; protein, 85.1%; P < 0.05), NAD(P)H oxidase subunits (mRNA, 120.0-200.0%; P < 0.05; protein, 90.9-197.0%; P < 0.05), and O2- production (83.2%; P < 0.05) in the RVLM. In addition, impaired baroreflex control of HR (Gain(max) reduced by 48.2%; P < 0.05) and RSNA (Gain(max) reduced by 53.6%; P < 0.05) by ANG II was completely abolished by losartan. Losartan significantly decreased baseline RSNA (-49.5%; P < 0.05) and increased baroreflex control of HR (Gain(max) increased by 64.8%; P < 0.05) and RSNA (Gain(max) increased by 67.9%; P < 0.05), but had no significant effects on mRNA and protein expression of AT1 receptor and NAD(P)H oxidase subunits and O2- production in the RVLM. These data suggest that in normal rabbits, NAD(P)H oxidase-derived ROS play an important role in the modulation of sympathetic activity and arterial baroreflex function by subchronic central treatment of exogenous ANG II via AT1 receptors.

These results indicate that elevation of Ang II and concomitant upregulation of AT1 receptor in the CB contribute to the increased CB chemoreceptor activity and enhanced peripheral chemoreflex function in CHF.

Chronic exposure to intermittent hypoxia (CIH) increases carotid sinus nerve activity in normoxia and in response to acute hypoxia. We hypothesized that CIH augments basal and chemoreflex-stimulated sympathetic outflow through an angiotensin receptor-dependent mechanism. Rats were exposed to CIH for 28 days: a subset was treated with losartan. Then, lumbar sympathetic activity was recorded under anesthesia during 20-second apneas, isocapnic hypoxia, and potassium cyanide. We measured carotid body superoxide production and expression of angiotensin II type-1 receptor, neuronal nitric oxide synthase, and NADPH oxidase. Sympathetic activity was higher in CIH vs. control rats at baseline, during apneas and isocapnic hypoxia, but not cyanide. Carotid body superoxide production and expression of angiotensin II type 1 receptor and gp91phox subunit of NADPH oxidase were elevated in CIH rats, whereas expression of neuronal nitric oxide synthase was reduced. None of these differences were evident in animals treated with losartan. CIH-induced augmentation of chemoreflex sensitivity occurs, at least in part, via the renin-angiotensin system.

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Peripheral chemoreflex sensitivity is potentiated in clinical and experimental chronic heart failure (CHF). Blood supply to tissues is inevitably reduced in CHF. However, it remains poorly understood whether the reduced blood flow is the cause of increased peripheral chemoreflex sensitivity in CHF. This work highlights the effect of chronically reduced blood flow to the carotid body (CB) on peripheral chemoreflex function in rabbits. In pacing-induced CHF rabbits, blood flow in the carotid artery was reduced by 36.4 ± 5.2% after 3 weeks of pacing. For comparison, a similar level of blood flow reduction was induced by carotid artery occlusion (CAO) over a similar 3 week time course without pacing. CB blood supply was reduced by similar levels in both CHF and CAO rabbits as measured with fluorescent microspheres. Compared with sham rabbits, CAO enhanced peripheral chemoreflex sensitivity in vivo, increased CB chemoreceptor activity in an isolated CB preparation and decreased outward potassium current (Ik) in CB glomus cells to levels similar to those that were observed in CHF rabbits. In CAO CB compared to sham, neural nitric oxide (NO) synthase (nNOS) expression and NO levels were suppressed, and angiotensin II (Ang II) type 1 receptor (AT1-R) protein expression and Ang II concentration were elevated; these changes were similar to those seen in the CB from CHF rabbits. A NO donor and AT1-R antagonist reversed CAO-enhanced chemoreflex sensitivity. These results suggest that a reduction of blood flow to the CB is involved in the augmentation of peripheral chemoreflex sensitivity in CHF.

Background-In a previous study, we showed that simvastatin (SIM) therapy normalized sympathetic outflow and cardiovascular reflex regulation in chronic heart failure (CHF). However, the precise neural and cellular pathways for these effects are unknown. We hypothesized that SIM exerts its beneficial effect on autonomic function in CHF by downregulating central angiotensin II (Ang II) and superoxide mechanisms. Methods and Results-Experiments were carried out on 36 male New Zealand White rabbits, 13 normal and 23 CHF. All rabbits were identically instrumented to record mean arterial pressure, heart rate, and renal sympathetic nerve activity (RSNA

Objective: A previous study from this laboratory showed that elevation of endogenous angiotensin II (Ang II) and upregulation of the angiotensin II type 1 (AT 1 ) receptor in the carotid body (CB) are involved in the enhanced peripheral chemoreceptor sensitivity in rabbits with chronic heart failure (CHF). NADPH oxidase-derived superoxide anion mediates the effects of Ang II in many organs. We investigated whether this signaling pathway may mediate the enhanced peripheral chemoreceptor sensitivity induced by Ang II in CHF rabbits. Methods and results: By recording single-unit activity from the carotid sinus nerve in isolated preparations, we found that phenylarsine oxide 2 μM (PAO, NADPH oxidase inhibitor) and TEMPOL 1 mM (superoxide dismutase mimetic) significantly decreased not only the Ang II-enhanced CB chemoreceptor responses to different levels of hypoxia in sham rabbits (Δ-12.5 ± 0.8 and Δ-12.8 ± 0.9 imp/s at 40.7 ± 2.3 mm Hg of PO 2 , and Δ-5.6 ± 0.5 and Δ-5.3 ± 0.4 imp/s at 60.2 ± 3.1 mm Hg of PO 2 , p b 0.05, respectively) but also the CHF-induced elevation of CB chemoreceptor responses to different levels of hypoxia (Δ-13.6 ± 1.1 and Δ-13.7 ± 0.9 imp/s at 40.9 ± 3.1 mm Hg of PO 2 , and Δ-6.7 ± 1.2 and Δ-6.6 ± 0.8 imp/s at 59.8 ± 3.5 mm Hg of PO 2 , p b 0.05). In addition, mRNA and protein expressions of NADPH oxidase components (gp91 phox , p40 phox and p47 phox ) were higher in the CB from CHF rabbits compared to sham rabbits. Furthermore, 100 pM Ang II induced an increase in superoxide production in CB homogenates from sham rabbits, which was similar to that in CB homogenate from CHF rabbits. PAO and Tempol inhibited the Ang II-and CHF-enhanced superoxide anion production. Conclusions: These results suggest that the enhanced peripheral chemoreceptor sensitivity mediated by Ang II in CHF rabbits occurs via a NADPH oxidase-superoxide signaling pathway.

An exaggerated exercise pressor reflex (EPR) contributes to exercise intolerance and excessive sympatho-excitation in the chronic heart failure (CHF) state. However, the components of this reflex that are responsible for the exaggerated EPR in CHF remain unknown. To determine whether muscle afferent function is altered in CHF, we recorded the discharge of group III and IV afferents in response to static contraction, passive stretch and hindlimb intra-arterial injection of capsaicin in sham and CHF rats. We also investigated the roles of purinergic 2X receptor (P2X) and the transient receptor potential vanilloid 1 (VR1) in mediating the altered sensitivity of muscle afferents. Compared with sham rats, CHF rats exhibited greater responses of group III afferents to contraction and stretch whereas the responses of group IV afferents to contraction and capsaicin were blunted. Hindlimb intra-arterial infusion of pyridoxal phosphate-6-azophenyl-2 ,4 -disulfonic acid (PPADS), a P2X antagonist, attenuated the responses of group III afferents to contraction and stretch in CHF rats to a greater extent than in sham rats. Western blot data showed that P2X3 receptors were significantly upregulated in doral root ganglion (DRG) of CHF rats whereas VR1 receptors were significantly downregulated. Immunohistochemical evidence showed that immunostaining of the P2X3 receptors was more intense in both IB4-positive (C-fibre) and NF200-positive (A-fibre) neurons in DRG of CHF rats whereas the immunostaining of the VR1 receptors was decreased in IB4-positive neurons. These data suggest that group III afferents are sensitized whereas group IV afferents are desensitized in CHF, which is related to the dysfunction of P2X and VR1 receptors.