Inhibition of nitric oxide formation in the nucleus tractus solitarius increases renal sympathetic nerve activity in rabbits

Synaptic mechanisms underlying the elevated sympathetic outflow in fructose-induced hypertension

Metabolic syndrome is associated with cardiovascular dysfunction, including elevated sympathetic outflow. However, the underlying brain mechanisms are unclear. The nucleus tractus solitarius (NTS) critically regulates autonomic reflexes related to cardiovascular function and contains neurons projecting to the caudal ventrolateral medulla (CVLM). Nitric oxide (NO) is a diffusible free-radical messenger in the vascular, immune, and nervous systems. In this study, we determine if NO in the NTS is involved in the synaptic plasticity underlying the elevated sympathetic outflow in fructose-induced hypertension. We retrogradely labeled CVLM-projecting NTS neurons through the injection of FluoSpheres into the CVLM in a fructose-fed rat model to determine the cellular mechanism involved in increased sympathetic outflow. Fructose feeding increased the blood pressure and glucose levels, which represent metabolic syndrome. We found that fructose feeding reduces the NO precursor L-arginine-induced increase in the firing activity of CVLM-projecting NTS neurons. Furthermore, fructose feeding reduces the L-arginine-induced increase in presynaptic spontaneous glutamatergic synaptic inputs to NTS neurons, while NO donor DEA/NO produces an increase in glutamatergic synaptic inputs in fructose-fed rats similar to that in vehicle-treated rats. In addition, fructose feeding reduces the NO-induced depressor response and sympathoinhibition. These data suggested that fructose feeding reduced NO production and, thus, the subsequent NO-induced glutamate releases in the NTS and depressor response. The findings of this study provide new insights into the central mechanisms involved in the neural control of cardiovascular and autonomic functions in the NTS in metabolic syndrome.

Analytic and Integrative Framework for Understanding Human Sympathetic Arterial Baroreflex Function: Equilibrium Diagram of Arterial Pressure and Plasma Norepinephrine Level

Background

The sympathetic arterial baroreflex is a closed-loop feedback system for stabilizing arterial pressure (AP). Identification of unique functions of the closed system in humans is a challenge. Here we propose an analytic and integrative framework for identifying a static operating point and open-loop gain to characterize sympathetic arterial baroreflex in humans.

Methods and Results

An equilibrium diagram with two crossing functions of mechanoneural (MN) and neuromechanical (NM) arcs was analyzed during graded tilt maneuvers in seven healthy subjects. AP and plasma norepinephrine level (PNE), as a surrogate for sympathetic nerve activity, and were recorded after vagal modulation of heart function was blocked by atropine. The MN-arc curve was described as a locus of operating points during –7, 0, 15, and 60° head-up tilting (HUT) on a PNE-AP plane. The NM-arc curve was drawn as a line between operating points before and after ganglionic blockade (trimethaphan, 0.1 mg⋅ml^–1⋅kg^–1) during 0° or 15° HUT. Gain values were estimated from the slopes of these functional curves. Finally, an open-loop gain, which is a most important index for performance of arterial baroreflex, was given by a product of the gain values of MN (G_MN) and NM arcs (G_NM). Gain values of MN was 8.92 ± 3.07 pg⋅ml⁻¹⋅mmHg⁻¹; and G_NM at 0° and 15° HUT were 0.61 ± 0.08 and 0.36 ± 0.05 mmHg⋅ml⋅pg^–1, respectively. A postural change from supine to 15° HUT significantly reduced the open-loop gain from 5.62 ± 0.98 to 3.75 ± 0.62. The effects of HUT on the NM arc and open-loop gain seemed to be similar to those of blood loss observed in our previous animal studies.

Conclusion

An equilibrium-diagram analysis contributes to a quantitative and integrative understanding of function of human sympathetic arterial baroreflex.

Anatomical evidence of non-parasympathetic cardiac nitrergic nerve fibres in rat

Neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) plays a major role in the neural control of circulation and in many cardiovascular diseases. However, the exact mechanism of how NO regulates these processes is still not fully understood. This study was designed to determine the possible sources of nitrergic nerve fibres supplying the heart attempting to imply their role in the cardiac neural control. Sections of medulla oblongata, vagal nerve, its rootlets and nodose ganglia, vagal cardiac branches, Th₁ -Th₅ spinal cord segments, dorsal root ganglia of C₈ -Th₅ spinal nerves, and stellate ganglia from 28 Wistar rats were examined applying double immunohistochemical staining for nNOS combined with choline acetyltransferase (ChAT), peripherin, substance P, calcitonin gene-related peptide, tyrosine hydroxylase or myelin basic protein. Our findings show that the most abundant population of purely nNOS-immunoreactive (IR) neuronal somata (NS) was observed in the nodose ganglia (37.4 ± 1.3%). A high number of nitrergic NFs spread along the vagal nerve and entered its cardiac branches. All nitrergic neuronal somata (NS) in the nucleus ambiguus were simultaneously immunoreactive (IR) to ChAT and composed only a small subset of neurons (6%). In the dorsal nucleus of vagal nerve, biphenotypic nNOS-IR/ChAT-IR neurons composed 7.0 ± 1.0%, while small purely nNOS-IR neurons were scarce. Nitrergic NS were plentifully distributed within the nuclei of solitary tract. In the examined dorsal root and stellate ganglia, a few nitrergic NS were sporadically present. The majority of sympathetic NS in the intermediolateral nucleus were simultaneously immunoreactive for nNOS and ChAT. In conclusion, an abundant population of nitrergic NS in the nodose ganglion implies that neuronal NO is involved in afferent cardiac innervation. Nevertheless, nNOS-IR neurons identified within vagal nuclei may play a role in the transmission of preganglionic parasympathetic nerve impulses.

Arterial Baroreflex Resetting During Exercise in Humans: Underlying Signaling Mechanisms

The arterial baroreflex (ABR) resets during exercise in an intensity-dependent manner to operate around a higher blood pressure with maintained sensitivity. This review provides a historical perspective of ABR resetting and the involvement of other neural reflexes in mediating exercise resetting. Furthermore, we discuss potential underlying signaling mechanisms that may contribute to exercise ABR resetting in physiological and pathophysiological conditions.

Chronic Intermittent Hypobaric Hypoxia Ameliorates Renal Vascular Hypertension Through Up-regulating NOS in Nucleus Tractus Solitarii

Chronic intermittent hypobaric hypoxia (CIHH) is known to have an anti-hypertensive effect, which might be related to modulation of the baroreflex in rats with renal vascular hypertension (RVH). In this study, RVH was induced by the 2-kidney-1-clip method (2K1C) in adult male Sprague-Dawley rats. The rats were then treated with hypobaric hypoxia simulating 5000 m altitude for 6 h/day for 28 days. The arterial blood pressure (ABP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were measured before and after microinjection of L-arginine into the nucleus tractus solitarii (NTS) in anesthetized rats. Evoked excitatory postsynaptic currents (eEPSCs) and spontaneous EPSCs (sEPSCs) were recorded in anterogradely-labeled NTS neurons receiving baroreceptor afferents. We measured the protein expression of neuronal nitric oxide synthase (nNOS) and endothelial NOS (eNOS) in the NTS. The results showed that the ABP in RVH rats was significantly lower after CIHH treatment. The inhibition of ABP, HR, and RSNA induced by L-arginine was less in RVH rats than in sham rats, and greater in the CIHH-treated RVH rats than the untreated RVH rats. The eEPSC amplitude in NTS neurons receiving baroreceptor afferents was lower in the RVH rats than in the sham rats and recovered after CIHH. The protein expression of nNOS and eNOS in the NTS was lower in the RVH rats than in the sham rats and this decrease was reversed by CIHH. In short, CIHH treatment decreases ABP in RVH rats via up-regulating NOS expression in the NTS.

The Role of Angiotensin II Infusion on the Baroreflex Sensitivity and Renal Function in Intact and Bilateral Renal Denervation Rats

Background

The role of renin-angiotensin system (RAS) in communication between renal system and cardiovascular system is extremely important. Baroreflex sensitivity (BRS) index defines as heart rate (HR) alteration versus mean arterial pressure (MAP) change ratio . Sympathetic nerve is arm of the baroreflexes and any change in its activity will lead to change in the BRS. The role of angiotensin II (Ang II) infusion in systemic circulation accompanied with bilateral renal denervation (RDN) on BRS index and renal function was studied.

Materials and Methods

Seventy-two male and female Wistar rats in 12 groups were anesthetized and catheterized. The alteration of MAP and HR responses to phenylephrine infusion compared to control groups was determined in bilateral RDN rats subjected to treat with Ang II (300 or 1000 ng/kg/min) administration.

Results

The BRS index was elevated in Ang II-treated non-RDN (normal) male rats gradually and dose dependently (P < 0.05), while this index was significantly different when compared with RDN male rats (P < 0.05). Accordingly, the BRS index was significantly lower in RDN than non-RDN male rats, and such observation was not observed in female rats. The creatinine clearance (insignificantly) and urine flow (significantly; P < 0.05) were decreased in both non-RDN and RDN male and female rats treated with Ang II. In RDN model, the serum nitrite levels were decreased in male and increased in female by Ang II infusion when compared with vehicle infusion.

Conclusion

The Ang II infusion could increase the BRS index in non-RDN (normal) male rats which is significantly greater than BRS index in RDN rats.

Simulated weightlessness affects the expression and activity of neuronal nitric oxide synthase in the rat brain

Spaceflight induces pathophysiological alterations in various organs. To study pathophysiological adaptations to weightlessness on the ground, the tail suspension (TS) rat model has been used to simulate the effects of weightlessness. There is currently little information on the effect of TS on the expression and activity of nitric oxide synthase (NOS) in the brain. In this study, we examined time-dependent alterations in the expression and activity of neuronal NOS (nNOS) in the brains of TS rats. Male Sprague-Dawley rats were tail-suspended for 1 (TS1), 7 (TS7), and 14 (TS14) days or rested on the ground for 3 days after 14 days of TS. TS1 and TS7 rats exhibited no significant alterations in the expression of nNOS compared to control rats, whereas nNOS expression in TS14 rats was significantly upregulated compared to control rats. Normalized expression of nNOS mRNA and protein in TS14 rats (1.86 ± 0.48 and 1.84 ± 0.29, respectively) were significantly higher than that of control rats (P < 0.001 and P < 0.001, respectively). Consistent with these results, significant elevations in NOS activity and NO production were observed in TS14 rats. Thus, we demonstrated a significant upregulation of nNOS expression, accompanied by significant increases in NOS activity and NO production, in the brain of rats exposed to simulated weightlessness.

Superoxide Anions and NO in the Paraventricular Nucleus Modulate the Cardiac Sympathetic Afferent Reflex in Obese Rats

This study was conducted to explore the hypothesis that the endogenous superoxide anions (O₂⁻) and nitric oxide (NO) system of the paraventricular nucleus (PVN) regulates the cardiac sympathetic afferent reflex (CSAR) contributing to sympathoexcitation in obese rats induced by a high-fat diet (42% kcal as fat) for 12 weeks. CSAR was evaluated by monitoring the changes of renal sympathetic nerve activity (RSNA) and the mean arterial pressure (MAP) responses to the epicardial application of capsaicin (CAP) in anaesthetized rats. In obese rats with hypertension (OH group) or without hypertension (OB group), the levels of PVN O₂⁻, angiotensinII (Ang II), Ang II type 1 receptor (AT1R), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase were elevated, whereas neural NO synthase (nNOS) and NO were significantly reduced. Moreover, CSAR was markedly enhanced, which promoted the elevation of plasma norepinephrine levels. The enhanced CSAR was attenuated by PVN application of the superoxide scavenger polyethylene glycol-superoxide dismutase (PEG-SOD) and the NO donor sodium nitroprusside (SNP), and was strengthened by the superoxide dismutase inhibitor diethyldithiocarbamic acid (DETC) and the nNOS inhibitor N(ω)-propyl-l-arginine hydrochloride (PLA); conversely, there was a smaller CSAR response to PLA or SNP in rats that received a low-fat (12% kcal) diet. Furthermore, PVN pretreatment with the AT1R antagonist losartan or with PEG-SOD, but not SNP, abolished Ang II-induced CSAR enhancement. These findings suggest that obesity alters the PVN O₂⁻ and NO system that modulates CSAR and promotes sympathoexcitation.

Sympathetic Overactivity in Chronic Kidney Disease: Consequences and Mechanisms

The incidence of chronic kidney disease (CKD) is increasing worldwide, with more than 26 million people suffering from CKD in the United States alone. More patients with CKD die of cardiovascular complications than progress to dialysis. Over 80% of CKD patients have hypertension, which is associated with increased risk of cardiovascular morbidity and mortality. Another common, perhaps underappreciated, feature of CKD is an overactive sympathetic nervous system. This elevation in sympathetic nerve activity (SNA) not only contributes to hypertension but also plays a detrimental role in the progression of CKD independent of any increase in blood pressure. Indeed, high SNA is associated with poor prognosis and increased cardiovascular morbidity and mortality independent of its effect on blood pressure. This brief review will discuss some of the consequences of sympathetic overactivity and highlight some of the potential pathways contributing to chronically elevated SNA in CKD. Mechanisms leading to chronic sympathoexcitation in CKD are complex, multifactorial and to date, not completely understood. Identification of the mechanisms and/or signals leading to sympathetic overactivity in CKD are crucial for development of effective therapeutic targets to reduce the increased cardiovascular risk in this patient group.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction - manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre-sympathetic but also vasopressin-secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide-dependent effects on neural activity, depletion of nitric oxide and induction of low-grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide-dependent and pro-inflammatory pathways.

Role of Phosphodiesterase 5 and Cyclic GMP in Hypertension

Cyclic GMP (cGMP) is a ubiquitous intracellular second messenger that mediates a wide spectrum of physiologic processes in multiple cell types within the cardiovascular and nervous systems. Synthesis of cGMP occurs either by NO-sensitive guanylyl cyclases in response to nitric oxide or by membrane-bound guanylyl cyclases in response to natriuretic peptides and has been shown to regulate blood pressure homeostasis by influencing vascular tone, sympathetic nervous system, and sodium and water handling in the kidney. Several cGMPs degrading phosphodiesterases (PDEs), including PDE1 and PDE5, play an important role in the regulation of cGMP signaling. Recent findings revealed that increased activity of cGMP-hydrolyzing PDEs contribute to the development of hypertension. In this review, we will summarize recent research findings regarding the cGMP/PDE signaling in the vasculature, the central nervous system, and the kidney which are associated with the development and maintenance of hypertension.

Cardiovascular Autonomic Dysfunction in Chronic Kidney Disease: a Comprehensive Review

Cardiovascular autonomic dysfunction is a major complication of chronic kidney disease (CKD), likely contributing to the high incidence of cardiovascular mortality in this patient population. In addition to adrenergic overdrive in affected individuals, clinical and experimental evidence now strongly indicates the presence of impaired reflex control of both sympathetic and parasympathetic outflow to the heart and vasculature. Although the principal underlying mechanisms are not completely understood, potential involvements of altered baroreceptor, cardiopulmonary, and chemoreceptor reflex function, along with factors including but not limited to increased renin-angiotensin-aldosterone system activity, activation of the renal afferents and cardiovascular structural remodeling have been suggested. This review therefore analyzes potential mechanisms underpinning autonomic imbalance in CKD, covers results accumulated thus far on cardiovascular autonomic function studies in clinical and experimental renal failure, discusses the role of current interventional and therapeutic strategies in ameliorating autonomic deficits associated with chronic renal dysfunction, and identifies gaps in our knowledge of neural mechanisms driving cardiovascular disease in CKD.

Angiotensin II-Induced Hypertension Is Attenuated by Reduction of Sympathetic Output in NO-Sensitive Guanylyl Cyclase 1 Knockout Mice

In the regulation of vascular tone, the dilatory nitric oxide (NO)/cGMP pathway balances vasoconstriction induced by the renin-angiotensin and sympathetic nervous systems. NO-induced cGMP formation is catalyzed by two guanylyl cyclases (GC), NO-sensitive guanylyl cyclase 1 (NO-GC1) and NO-GC2, with indistinguishable enzymatic properties. In vascular smooth muscle cells, NO-GC1 is the major isoform and is responsible for more than 90% of cGMP formation. Despite reduced vasorelaxation, NO-GC1-deficient mice are not hypertensive. Here, the role of NO-GC1 in hypertension provoked by contractile agonists angiotensin II (Ang II) and norepinephrine (NE) was evaluated in NO-GC1-deficient mice. Hypertension induced by chronic Ang II treatment did not differ between wild-type (WT) and NO-GC1 knockout mice (KO). Also, attenuation of NO-dependent aortic relaxation induced by the Ang II treatment was similar in both genotypes and was most probably attributable to an increase of phosphodiesterase 1 expression. Analysis of plasma NE content-known to be influenced by Ang II-revealed lower NE in the NO-GC1 KO under Ang II-treated- and nontreated conditions. The finding indicates reduced sympathetic output and is underlined by the lower heart rate in the NO-GC1 KO. To find out whether the lack of higher blood pressure in the NO-GC1 KO is a result of reduced sympathetic activity counterbalancing the reduced vascular relaxation, mice were challenged with chronic NE application. As the resulting blood pressure was higher in the NO-GC1 KO than in WT, we conclude that the reduced sympathetic activity in the NO-GC1 KO prevents hypertension and postulate a possible sympatho-excitatory action of NO-GC1 counteracting NO-GC1's dilatory effect in the vasculature.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

Blood pressure regulation VIII: resistance vessel tone and implications for a pro-atherogenic conduit artery endothelial cell phenotype

Dysfunction of the endothelium is proposed as the primary initiator of atherosclerotic peripheral artery disease, which occurs mainly in medium- to large-sized conduit arteries of the lower extremities (e.g., iliac, femoral, popliteal arteries). In this review article, we propose the novel concept that conduit artery endothelial cell phenotype is determined, in part, by microvascular tone in skeletal muscle resistance arteries through both changes in arterial blood pressure as well as upstream conduit artery shear stress patterns. First, we summarize the literature supporting the involvement of sympathetic nerve activity (SNA) and nitric oxide (NO) in the modulation of microvascular tone and arterial blood pressure. We then focus on the role of elevated blood pressure and shear stress profiles in modulating conduit artery endothelial cell phenotype. Last, we discuss findings from classic and emerging studies indicating that increased vascular resistance, as it occurs in the context of increased SNA and/or reduced NO bioavailability, is associated with greater oscillatory shear stress (e.g., increased retrograde shear) in upstream conduit arteries. The ideas put forth in this review set the stage for a new paradigm concerning the mechanistic link between increased microvascular tone and development of conduit artery endothelial dysfunction and thus increased risk for peripheral artery disease. Indeed, a vast amount of evidence supports the notion that excessive blood pressure and oscillatory shear stress are potent pro-atherogenic signals to the endothelium.

Sympathetic regulation of vascular function in health and disease

The sympathetic nervous system (SNS) is known to play a pivotal role in short- and long-term regulation of different functions of the cardiovascular system. In the past decades increasing evidence demonstrated that sympathetic neural control is involved not only in the vasomotor control of small resistance arteries but also in modulation of large artery function. Sympathetic activity and vascular function, both of which are key factors in the development and prognosis of cardiovascular events and disease, are linked at several levels. Evidence from experimental studies indicates that the SNS is critically influenced, at the central and also at the peripheral level, by the most relevant factors regulating vascular function, such as nitric oxide (NO), reactive oxygen species (ROS), endothelin (ET), the renin-angiotensin system. Additionally, there is indirect evidence of a reciprocal relationship between endothelial function and activity of the SNS. A number of cardiovascular risk factors and diseases are characterized both by increased sympathetic outflow and decreased endothelial function. In healthy subjects, muscle sympathetic nerve activity (MSNA) appears to be related to surrogate markers of endothelial function, and an acute increase in sympathetic activity has been associated with a decrease in endothelial function in healthy subjects. However, direct evidence of a cause-effect relationship from human studies is scanty. In humans large artery stiffness has been associated with increased sympathetic discharge, both in healthy subjects and in renal transplant recipients. Peripheral sympathetic discharge is also able to modulate wave reflection. On the other hand, large artery stiffness can interfere with autonomic regulation by impairing carotid baroreflex sensitivity.

Imbalance of central nitric oxide and reactive oxygen species in the regulation of sympathetic activity and neural mechanisms of hypertension

Nitric oxide (NO) and reactive oxygen species (ROS) play important roles in blood pressure regulation via the modulation of the autonomic nervous system, particularly in the central nervous system (CNS). In general, accumulating evidence suggests that NO inhibits, but ROS activates, the sympathetic nervous system. NO and ROS, however, interact with each other. Our consecutive studies and those of others strongly indicate that an imbalance between NO bioavailability and ROS generation in the CNS, including the brain stem, activates the sympathetic nervous system, and this mechanism is involved in the pathogenesis of neurogenic aspects of hypertension. In this review, we focus on the role of NO and ROS in the regulation of the sympathetic nervous system within the brain stem and subsequent cardiovascular control. Multiple mechanisms are proposed, including modulation of neurotransmitter release, inhibition of receptors, and alterations of intracellular signaling pathways. Together, the evidence indicates that an imbalance of NO and ROS in the CNS plays a pivotal role in the pathogenesis of hypertension.

Sympathetic renal innervation and resistant hypertension

Hypertension in chronic renal disease and renovascular disease is often resistant to therapy. Understanding the pathogenic mechanisms responsible for hypertension in these conditions may lead to improved and more targeted therapeutic interventions. Several factors have been implicated in the pathogenesis of hypertension associated with renal disease and/or renal failure. Although the role of sodium retention, total body volume expansion, and hyperactivity of the renin-angiotensin-aldosterone system (RAAS) are well recognized, increasing evidence suggests that afferent impulses from the injured kidney may increase sympathetic nervous system activity in areas of the brain involved in noradrenergic regulation of blood pressure and contribute to the development and maintenance of hypertension associated with kidney disease. Recognition of this important pathogenic factor suggests that antiadrenergic drugs should be an essential component to the management of hypertension in patients with kidney disease, particularly those who are resistant to other modalities of therapy.

Oral L-histidine exerts antihypertensive effects via central histamine H3 receptors and decreases nitric oxide content in the rostral ventrolateral medulla in spontaneously hypertensive rats

1. L-histidine is generally found in meat, poultry and fish. To investigate its effects on blood pressure, L-histidine was administered to 9-week-old spontaneously hypertensive rats (SHR). 2. Oral administration of L-histidine (100 mg / kg) increased histamine content in cerebrospinal fluid and reduced mean arterial pressure (MAP) in SHR. Intracerebroventricular injection of L-histidine (0.01 microg / 5 microL) also caused a decrease in MAP, which was reversed by cotreatment with the histamine H3 receptor antagonist thioperamide (20.4 microg / 5 microL, i.c.v.). There was a significant, time-dependent increase (over 6 h) in the NOx (NO2- + NO3-) content of the dialysate from the rostral ventrolateral medulla (RVLM), a major vasomotor centre, after oral administration of L-histidine. 3. In another experiment, SHR were treated with l-histidine (100 mg / kg) twice a day for 4 weeks. Chronic treatment with L-histidine inhibited the age-dependent increases in systolic blood pressure and urinary noradrenaline excretion seen in vehicle-treated SHR. Conversely, intracerebroventricular injection of thioperamide (20.4 microg / 5 microL, i.c.v.) reversed the decrease in MAP in response to L-histidine in SHR. 4. Reverse transcription-polymerase chain reaction analysis revealed that the aortic expression of angiotensin-converting enzyme mRNA was suppressed by chronic treatment with L-histidine. 5. These results suggest that L-histidine decreases blood pressure by attenuating sympathetic output via the central histamine H3 receptor in SHR. In addition, the antihypertensive effects of L-histidine appear to be associated with an increase in nitric oxide in the RVLM.

Hypercapnia causes cellular oxidation and nitrosation in addition to acidosis: implications for CO2 chemoreceptor function and dysfunction

Cellular mechanisms of CO2 chemoreception are discussed and debated in terms of the stimuli produced during hypercapnic acidosis and their molecular targets: protons generated by the hydration of CO2 and dissociation of carbonic acid, which target membrane-bound proteins and lipids in brain stem neurons. The CO2 hydration reaction, however, is not the only reaction that CO2 undergoes that generates molecules capable of modifying proteins and lipids. Molecular CO2 also reacts with peroxynitrite (ONOO-), a reactive nitrogen species (RNS), which is produced from nitric oxide (*NO) and superoxide (*O2-). The CO2/ONOO- reaction, in turn, produces additional nitrosative and oxidative reactive intermediates. Furthermore, protons facilitate additional redox reactions that generate other reactive oxygen species (ROS). ROS/RNS generated by these redox reactions may act as additional stimuli of CO2 chemoreceptors since neurons in chemosensitive areas produce both *NO and *O2- and, therefore, ONOO-. Perturbing *NO, *O2-, and ONOO- activities in chemosensitive areas modulates cardiorespiration. Moreover, neurons in at least one chemosensitive area, the solitary complex, are stimulated by cellular oxidation. Together, these data raise the following two questions: 1) do pH and ROS/RNS work in tandem to stimulate CO2 chemoreceptors during hypercapnic acidosis; and 2) does nitrosative stress and oxidative stress contribute to CO2 chemoreceptor dysfunction? To begin considering these two issues and their implications for central chemoreception, this minireview has the following three goals: 1) summarize the nitrosative and oxidative reactions that occur during hypercapnic acidosis and isocapnic acidosis; 2) review the evidence that redox signaling occurs in chemosensitive areas; and 3) review the evidence that neurons in the solitary complex are stimulated by cellular oxidation.

Involvement of tumor necrosis factor-alpha in natriuretic response to systemic infusion of nitric oxide synthase inhibitor in anesthetized mice

Systemic infusion of TNF-alpha exerts renal vasoconstriction but caused marked natriuresis in mice. Similar renal responses were also observed during systemic infusion of nitric oxide (NO) synthase inhibitors as opposed to their usual antinatriuretic responses when administered intrarenally. In the present study, we examined the hypothesis that acute NO blockade systemically induces TNF-alpha generation. which induces this natriuretic response. Renal responses to intravenous infusion of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 0.2 microg x min(-1) x g body wt(-1) for 85 min) and its impact on the plasma level of TNF-alpha were evaluated in anesthetized mice. Plasma TNF-alpha was undetected in untreated mice (n = 7) but was elevated in L-NAME-treated mice (109 +/- 22 pg/ml; P < 0.01 vs. untreated group; n = 7) along with an increase in TNF-alpha protein expression in kidney tissue. L-NAME infusion caused a usual increase in mean arterial pressure (MAP; 98 +/- 3 to 122 +/- 3 mmHg; P < 0.01) and decreases in renal blood flow (RBF; 8.6 +/- 0.3 to 4.4 +/- 0.2 ml x min(-1) x g(-1); P < 0.01) and glomerular filtration rate (GFR; 1.14 +/- 0.07 to 0.77 +/- 0.04 ml x min(-1) x g(-1); P < 0.01) with a marked increase in sodium excretion (U(Na)V; 0.48 +/- 0.10 to 3.52 +/- 0.85 micromol x min(-1) x g(-1); P < 0.01). Interestingly, in mice (n = 7) pretreated with the TNF-alpha blocker etanercept (5 mg/kg sc), the U(Na)V response to l-NAME infusion was markedly blunted (0.58 +/- 0.08 to 1.22 +/- 0.28 micromol x min(-1) x g(-1); P = NS) although responses for MAP, RBF, and GFR were mostly unchanged. However, pretreatment with the superoxide scavenger tempol in mice (n = 7) did not alter the U(Na)V response to L-NAME. These data demonstrate that L-NAME-induced natriuresis is mediated, at least in part, by concomitant generation of TNF-alpha during NO blockade.

Muscle sympathetic nerve activity is related to a surrogate marker of endothelial function in healthy individuals

BACKGROUND

Evidence from animal studies indicates the importance of an interaction between the sympathetic nervous system and the endothelium for cardiovascular regulation. However the interaction between these two systems remains largely unexplored in humans. The aim of this study was to investigate whether directly recorded sympathetic vasoconstrictor outflow is related to a surrogate marker of endothelial function in healthy individuals.

METHODS AND RESULTS

In 10 healthy normotensive subjects (3 f/7 m), (age 37+/-11 yrs), (BMI 24+/-3 kg/m(2)) direct recordings of sympathetic action potentials to the muscle vascular bed (MSNA) were performed and endothelial function estimated with the Reactive Hyperaemia- Peripheral Arterial Tonometry (RH-PAT) technique. Blood samples were taken and time spent on leisure-time physical activities was estimated. In all subjects the rate between resting flow and the maximum flow, the Reactive Hyperemic index (RH-PAT index), was within the normal range (1.9-3.3) and MSNA was as expected for age and gender (13-44 burst/minute). RH-PAT index was inversely related to MSNA (r = -0.8, p = 0.005). RH-PAT index and MSNA were reciprocally related to time (h/week) spent on physical activity (p = 0.005 and p = 0.006 respectively) and platelet concentration (PLT) (p = 0.02 and p = 0.004 respectively).

CONCLUSIONS

Our results show that sympathetic nerve activity is related to a surrogate marker of endothelial function in healthy normotensive individuals, indicating that sympathetic outflow may be modulated by changes in endothelial function. In this study time spent on physical activity is identified as a predictor of sympathetic nerve activity and endothelial function in a group of healthy individuals. The results are of importance in understanding mechanisms underlying sympathetic activation in conditions associated with endothelial dysfunction and emphasise the importance of a daily exercise routine for maintenance of cardiovascular health.

Cardiovascular responses to cold exposure

The prevalence of hypertension is increased in winter and in cold regions of the world. Cold temperatures make hypertension worse and trigger cardiovascular complications (stroke, myocardial infarction, heart failure, etc.). Chronic or intermittent exposure to cold causes hypertension and cardiac hypertrophy in animals. The purpose of this review is to provide the recent advances in the mechanistic investigation of cold-induced hypertension (CIH). Cold temperatures increase the activities of the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). The SNS initiates CIH via the RAS. Cold exposure suppresses the expression of eNOS and formation of NO, increases the production of endothelin-1 (ET-1), up-regulates ETA receptors, but down-regulates ETB receptors. The roles of these factors and their relations in CIH will be reviewed.

Effect of Nitric Oxide Synthase Inhibition and Saline Administration on Blood Pressure and Renal Sodium Handling During Experimental Sepsis in Rats

Much effort has been made in recent years to clarify metabolic and renal function changes in sepsis. A number of studies performed in different models of sepsis have been described. One such model that is frequently used is cecal ligation and puncture (CLP) in rats. This model resembles human sepsis in several important aspects, such as an early phase of hyperdynamic, hypermetabolic sepsis followed by a late hypodynamic, hypometabolic phase. The present study evaluated the blood pressure (n = 5) and renal function changes during development of CLP renal failure and to determine the effects of NOS inhibition (L-NAME) and 0.15 M NaCl administration on tail blood pressure and renal function in randomly assigned five groups (n = 10 each): (1) Sham-operated, (2) Sham-operated L-NAME-treated, (3) CLP rats, (4) CLP L-NAME-treated, and (5) CLP 0.15 M NaCl-treated rats. The basal tail blood pressure was not significantly different among the four groups. One week later, arterial pressure was significantly increased in sham-operated L-NAME-treated rats (159 +/- 12 mmHg) compare with the other groups (118 +/- 9.0 mmHg in nontreated rats, p < 0.05). Blood pressure shows a slightly and not significant decrease up to 12h in L-NAME and 0.15 M NaCl treated rats, which in turn was followed by a significant reduced arterial pressure 18h after CLP in both groups (L-NAME: 96.0 +/- 3.6 mmHg, p < 0.05) and NaCl: 82.3 +/- 2.4 mmHg, p < 0.05) compared to sham-operated groups. The glomerular filtration rate estimated by CCr decreases significantly in the CLP untreated group (p < 0.001) and did not significantly differ from the sham-operated and L-NAME-treated groups (p = 0.4) during the studies of renal tubule sodium handling. On the other hand, subcutaneous 0.15 M NaCl administration prevented CCr decreases in CLP rats (p = 0.25). CLP increased the FENa in the sham-operated from: 857.2 +/- 85.1 delta%min(-1) to CLP: 1197.8 +/- 119.0 delta%min(-1). The high FENa to CLP was blunted and significantly reduced by previous systemic treatment of animals with L-NAME from sham-operated+L-NAME: 1368.0 +/- 72.0 delta%min(-1) to CLP+L-NAME: 1148.0 +/- 60.4 delta%min(-1) (p < 0.01). The enhanced FENa in the CLP group were accompanied by a significant increase in proximal sodium reabsorption rejection. The salient findings of the present study suggest that a decrease in the blood pressure and creatinine clearance caused by CLP may benefit from L-NAM and fluid resuscitation during initial bacteremia (first 12 h) by promoting an additional increase of tubule sodium reabsorption in the post-proximal segments of nephrons, but these therapies could not prevent acute renal failure after established endotoxemia.

Sympathoinhibitory mechanism of moxonidine: role of the inducible nitric oxide synthase in the rostral ventrolateral medulla

AIMS

The central antihypertensive drug moxonidine lowers blood pressure (BP) through stimulating an imidazoline receptor within the rostral ventrolateral medulla (RVLM). Nitric oxide (NO) generated by the inducible NO synthase (iNOS) in the RVLM has been suggested to be involved in tonic sympathetic inhibition. The aim of this study was to determine the role of NO generated by iNOS in mediating moxonidine-induced cardiovascular inhibition in rats.

METHODS AND RESULTS

In anaesthetized rats, the cardiovascular response to local or systemic injection of moxonidine was observed after treatment with the selective iNOS inhibitor S-methylisothiourea (SMT) in the brain. Using immunohistochemical staining and western blot techniques, the protein expression of iNOS in the RVLM was measured in the moxonidine-infused rats. Intracerebroventricular (ICV) injection of SMT (1-100 nmol) dose-dependently attenuated the moxonidine (20 nmol, ICV)-induced decrease in BP and heart rate. Prior injection of SMT (20 and 200 pmol) into the RVLM also dose-dependently prevented the decrease in BP and renal sympathetic nerve activity evoked by RVLM microinjection of moxonidine (5 nmol) or intravenous injection of moxonidine (50 microg/kg). We further found that expression of iNOS protein following chronic ICV infusion of moxonidine (20 nmol, 2 weeks) is selectively upregulated in the RVLM but not in the nucleus tractus solitarius.

CONCLUSION

The present data suggest that an NO mechanism generated by iNOS in the RVLM plays an important role in mediating the sympathetic inhibition of the centrally acting drug moxonidine.

Glutamatergic neurons say NO in the nucleus tractus solitarii

Both glutamate and nitric oxide (NO) may play an important role in cardiovascular reflex and respiratory signal transmission in the nucleus tractus solitarii (NTS). Pharmacological and physiological data have shown that glutamate and NO may be linked in mediating cardiovascular regulation by the NTS. Through tract tracing, multiple-label immunofluorescent staining, confocal microscopic, and electronic microscopic methods, we and other investigators have provided anatomical evidence that supports a role for glutamate and NO as well as an interaction between glutamate and NO in cardiovascular regulation in the NTS. This review article focuses on summarizing and discussing these anatomical findings. We utilized antibodies to markers of glutamatergic neurons and to neuronal NO synthase (nNOS), the enzyme that synthesizes NO in NTS neurons, to study the anatomical relationship between glutamate and NO in rats. Not only were glutamatergic markers and nNOS both found in similar subregions of the NTS and in vagal afferents, they were also frequently colocalized in the same neurons and fibers in the NTS. In addition, glutamatergic markers and nNOS were often present in fibers that were in close apposition to each other. Furthermore, N-methyl-d-aspartate (NMDA) type glutamate receptors and nNOS were often found on the same NTS neurons. Similarly, alpha-amino-3-hydroxy-5-methylisoxozole-proprionic acid (AMPA) type glutamate receptors also frequently colocalized with nNOS in NTS neurons. These findings support the suggestion that the interaction between glutamate and NO may be mediated both through NMDA and AMPA receptors. Finally, by applying tracer to the cut aortic depressor nerve (ADN) to identify nodose ganglion (NG) neurons that transmit cardiovascular signals to the NTS, we observed colocalization of vesicular glutamate transporters (VGluT) and nNOS in the ADN neurons. Thus, taken together, these neuroanatomical data support the hypothesis that glutamate and NO may interact with each other to regulate cardiovascular and likely other visceral functions through the NTS.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Transport and peripheral bioactivities of nitrogen oxides carried by red blood cell hemoglobin: role in oxygen delivery

The biology of NO (nitric oxide) is poorly explained by the activity of the free radical NO ((.)NO) itself. Although (.)NO acts in an autocrine and paracrine manner, it is also in chemical equilibrium with other NO species that constitute stable stores of NO bioactivity. Among these species, S-nitrosylated hemoglobin (S-nitrosohemoglobin; SNO-Hb) is an evolved transducer of NO bioactivity that acts in a responsive and exquisitely regulated manner to control cardiopulmonary and vascular homeostasis. In SNO-Hb, O(2) sensing is dynamically coupled to formation and release of vasodilating SNOs, endowing the red blood cell (RBC) with the capacity to regulate its own principal function, O(2) delivery, via regulation of blood flow. Analogous, physiological actions of RBC SNO-Hb also contribute to central nervous responses to blood hypoxia, the uptake of O(2) from the lung to blood, and baroreceptor-mediated control of the systemic flow of blood. Dysregulation of the formation, export, or actions of RBC-derived SNOs has been implicated in human diseases including sepsis, sickle cell anemia, pulmonary arterial hypertension, and diabetes mellitus. Delivery of SNOs by the RBC can be harnessed for therapeutic gain, and early results support the logic of this approach in the treatment of diseases as varied as cancer and neonatal pulmonary hypertension.

Roles of nitric oxide and angiotensin II in the impaired baroreflex gain of pregnancy

The present study tested the hypothesis that nitric oxide (NO) contributes to impaired baroreflex gain of pregnancy and that this action is enhanced by angiotensin II. To test these hypotheses, we quantified baroreflex control of heart rate in nonpregnant and pregnant conscious rabbits before and after: 1) blockade of NO synthase (NOS) with Nomega-nitro-L-arginine (20 mg/kg iv); 2) blockade of the angiotensin II AT1 receptor with L-158,809 (5 microg x kg(-1) x min(-1) iv); 3) infusion of angiotensin II (1 ng x kg(-1) x min(-1) nonpregnant, 1.6-4 ng x kg(-1) x min(-1) pregnant iv); 4) combined blockade of angiotensin II AT(1) receptors and NOS; and 5) combined infusion of angiotensin II and blockade of NOS. To determine the potential role of brain neuronal NOS (nNOS), mRNA and protein levels were measured in the paraventricular nucleus, nucleus of the solitary tract, caudal ventrolateral medulla, and rostral ventrolateral medulla in pregnant and nonpregnant rabbits. The decrease in baroreflex gain observed in pregnant rabbits (from 23.3 +/- 3.6 to 7.1 +/- 0.9 beats x min(-1) x mmHg(-1), P < 0.05) was not reversed by NOS blockade (to 8.3 +/- 2.5 beats x min(-1) x mmHg(-1)), angiotensin II blockade (to 5.0 +/- 1.1 beats x min(-1) x mmHg(-1)), or combined blockade (to 12.3 +/- 4.8 beats x min(-1) x mmHg(-1)). Angiotensin II infusion with (to 5.7 +/- 1.0 beats x min(-1) x mmHg(-1)) or without (to 8.4 +/- 2.4 beats x min(-1) x mmHg(-1)) NOS blockade also failed to improve baroreflex gain in pregnant or nonpregnant rabbits. In addition, nNOS mRNA and protein levels in cardiovascular brain regions were not different between nonpregnant and pregnant rabbits. Therefore, we conclude that NO, either alone or via an interaction with angiotensin II, is not responsible for decrease in baroreflex gain during pregnancy.

PULMONARY VASCULAR REACTIVITY OF SPONTANEOUSLY HYPERTENSIVE RATS IS EXACERBATED IN RESPONSE TO THE CENTRAL ADMINISTRATION OF EXOGENOUS NITRIC OXIDE

1. Centrally, nitric oxide (NO) is a sympathoinhibitory substance. Spontaneously hypertensive rats (SHR) have an impaired central nitroxidergic system and, consequently, NO-mediated decrease in sympathetic activity is exacerbated in SHR compared with Wistar-Kyoto (WKY) rats. We have demonstrated previously that acute hypoxic pulmonary vasoconstriction (HPV) is enhanced by central NO administration. Therefore, in the present study, we hypothesized that accentuation of the HPV by NO would be exacerbated in SHR compared with WKY rats. 2. Mean pulmonary arterial pressure, systemic mean arterial blood pressure, cardiac output and heart rate were measured in pentobarbitone-anaesthetized, artificially ventilated, male SHR and WKY rats. The brief, transient response to a bolus intracerebroventricular (i.c.v.) dose of N(G)-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) was recorded in all rats. Upon recovery, rats were exposed to acute hypoxia (10% O(2) for 4 min) before and after the i.c.v. administration of the NO donor 3-[4-morpholinyl]-sydnonimine-hydrochloride (SIN-1; 100 microg in 10 microL). 3. In WKY rats, central inhibition of NO synthesis by L-NAME caused a mild increase in tonic pulmonary vascular tone and induced a large systemic pressor response. These responses were not observed in SHR. In contrast, SIN-1 failed to alter tonic pulmonary vascular tone, although it enhanced the HPV in WKY rats and, significantly more so, in SHR. 4. These results confirm that accentuation of the HPV by NO is exacerbated in SHR compared with WKY rats. The mechanism(s) by which the HPV is accentuated by central NO remains to be fully elucidated, but is likely to be associated with the sympathoinhibitory effects of NO and, if so, supports the idea that the nitroxidergic system of the SHR is impaired. Further electrophysiological studies are essential to confirm these assumptions.

Contribution of central nitric oxide to the regulation of blood pressure and sodium balance in DOCA-salt hypertension

We examined whether central nitric oxide is involved in blood pressure (BP) regulation in deoxycorticosterone acetate (DOCA)-salt hypertension. DOCA-salt rats were intracerebroventricularly infused (ICV) NG-monomethyl L-arginine (L-NMMA) for 4 weeks at either low (0.08 mg/kg/d; n = 8) or high (0.16 mg/kg/d; n = 8) dose. Saline ICV (n = 9) and intraperitoneal infused L-NMMA (low, n = 6; high dose, n = 6) were served as controls. Also, L-NMMA ICV (low, n = 6; high dose, n = 6) was conducted in normal rats. At week 3 and after, DOCA-salt with low L-NMMA ICV showed a higher BP than saline ICV (at week 4: 167.4 +/- 3.6 vs. 150.3 +/- 3.9 mm Hg, P < 0.01); this difference of BP was cancelled after ganglionic block. High L-NMMA ICV did not affect the trend of BP; however, it caused a reduced amount of saline drinking and a less estimated sodium retention than saline or low L-NMMA ICV (for 3 wk; 47.5 +/- 1.1 vs. 66.0 +/- 3.7 and 61.7 +/- 2.5 mmol, P < 0.01). In normal rats, high, but not low, L-NMMA ICV elevated BP with no effect on drinking behavior. Intraperitoneal infused L-NMMA did not affect the development of hypertension and/or sodium balance. These data suggested that, in DOCA-salt, central nitric oxide is involved in BP regulation through the dual action on sympathetic nervous activity and sodium balance.

NO-cGMP pathway at ventrolateral medullary cardiac inhibitory sites enhances the baroreceptor reflex bradycardia in the rat

The neuronal isoform of the enzyme nitric oxide synthase (nNOS) has been identified in the caudal ventrolateral medulla of the rat close to the location of cardiac vagal motoneurones. Therefore in this study we tested identified ventral medulla cardioinhibitory sites for the involvement of nitric oxide (NO) in the baroreceptor-heart rate reflex pathway. In rats anaesthetised with a mixture of urethane (650 mg kg(-1)) and chloralose (50 mg kg(-1)) i.v., blood pressure and heart rate were monitored continuously and using stereotaxic coordinates the ventrolateral caudal brainstem within and around the nucleus ambiguus was systematically explored for sites producing a bradycardia of >50 bpm, without a change in blood pressure, using D,L homocysteic acid (DLH, 0.2 M) microinjections (50 nl) from a glass micropipette. Identified sites were marked with pontamine sky blue. Microinjection of the NO donor sodium nitroprusside (SNP, 1 mM, 50 nl) at a cardioinhibitory site also produced a significant bradycardia (68+/-14 bpm) while the NOS inhibitor N(G)-nitro-l-arginine (l-NNA) (3 mM, 50 nl) caused a small significant increase in heart rate (5+/-1 bpm). Baroreceptor reflex gain measured by the response in heart rate to a change in blood pressure induced by phenylephrine i.v. was significantly increased (610+/-171%, p<0.05) during the steady state of the response to SNP, whereas it was significantly reduced (73+/-5%, p<0.01) by l-NNA injection at a medullary cardioinhibitory site. An inhibitor of soluble guanylyl cyclase, (1)H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ, 1 mM, 50 nl) also significantly reduced the baroreceptor reflex gain (63+/-8%, p<0.05). The results suggest that a NOS-cGMP signalling system in the baroreceptor reflex pathway distal to the NTS and closer to cardiac vagal motoneurones in the caudal ventral medulla contributes to enhancement of cardiac vagal tone.

Central nitric oxide modulates hindquarter vasodilation elicited by AMPA receptor stimulation in the NTS of conscious rats

Microinjection of S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the nucleus of the solitary tract (NTS) of conscious rats causes hypertension, bradycardia, and vasoconstriction in the renal, mesenteric, and hindquarter vascular beds. In the hindquarter, the initial vasoconstriction is followed by vasodilation with AMPA doses >5 pmol/100 nl. To test the hypothesis that this vasodilation is caused by activation of a nitroxidergic pathway in the NTS, we examined the effect of pretreatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10 nmol/100 nl, microinjected into the NTS) on changes in mean arterial pressure, heart rate, and regional vascular conductance (VC) induced by microinjection of AMPA (10 pmol/100 nl in the NTS) in conscious rats. AMPA increased hindquarter VC by 18 +/- 4%, but after pretreatment with L-NAME, AMPA reduced hindquarter VC by 16 +/- 7% and 17 +/- 9% (5 and 15 min after pretreatment, P < 0.05 compared with before pretreatment). Pretreatment with L-NAME reduced AMPA-induced bradycardia from 122 +/- 40 to 92 +/- 32 beats/min but did not alter the hypertension induced by AMPA (35 +/- 5 mmHg before pretreatment, 43 +/- 6 mmHg after pretreatment). Control injections with D-NAME did not affect resting values or the response to AMPA. The present study shows that stimulation of AMPA receptors in the NTS activates both vasodilatatory and vasoconstrictor mechanisms and that the vasodilatatory mechanism depends on production of nitric oxide in the NTS.

Angiotensin II--nitric oxide interactions in the control of sympathetic outflow in heart failure

Activation of the sympathetic nervous system is a compensatory mechanism which initially provides support for the circulation in the face of a falling cardiac output. It has been recognized for some time that chronic elevation of sympathetic outflow with the consequent increase in plasma norepinephrine, is counterproductive to improving cardiac function. Indeed, therapeutic targeting to block excessive sympathetic activation in heart failure is becoming a more accepted modality. The mechanism(s) by which sympathetic excitation occurs in the heart failure state are not completely understood. Components of abnormal cardiovascular reflex regulation most likely contribute to this sympatho-excitation. However, central mechanisms which relate to the elaboration of angiotensin II (Ang II) and nitric oxide (NO) may also play an important role. Ang II has been shown to be a sympatho-excitatory peptide in the central nervous system while NO is sympatho-inhibitory. Recent studies have demonstrated that blockade of Ang II receptors of the AT(1) subtype augments arterial baroreflex control of sympathetic nerve activity in the heart failure state, thereby predisposing to a reduction in sympathetic tone. Ang II and NO interact to regulate sympathetic outflow. Blockade of NO production in normal conscious rabbits was only capable of increasing sympathetic outflow when accompanied by a background infusion of Ang II. Conversely, providing a source of NO to rabbits with heart failure reduced sympathetic nerve activity when accompanied by blockade of AT(1) receptors. Chronic heart failure is also associated with a decrease in NO synthesis in the brain as indicated by a reduction in the mRNA for the neuronal isoform (nNOS). Chronic blockade of Ang II receptors can up regulate nNOS expression. In addition, exercise training of rabbits with developing heart failure has been shown to reduce sympathetic tone, decrease plasma Ang II, improve arterial baroreflex function and increase nNOS expression in the central nervous system. This review summarizes a large number of studies which have concentrated on the mechanisms of sympatho-excitation in heart failure. It now seems clear that one mechanism which is important in regulating sympathetic outflow in this disease state depends upon a central interaction between Ang II and NO at the cellular and nuclear levels.

Exogenous nitric oxide centrally enhances pulmonary reactivity in the normal and hypertensive rat

1. Chronic hypoxia causes sustained pulmonary hypertension and, although impairment of the pulmonary endothelial nitric oxide (NO) pathway has been implicated, no study has described the central role of NO in modulating pulmonary vascular tone and reactivity. Centrally, NO inhibits sympathetic outflow, so we hypothesised that central NO would modulate pulmonary vascular tone and its reactivity to acute hypoxia, especially in the hypertensive state. 2. Male adult Sprague-Dawley rats were exposed to normoxia (N) or chronic hypoxia (CH; 12% O2) for 14 days. Mean pulmonary arterial pressure (MPAP), systemic mean arterial blood pressure (MABP), cardiac output and heart rate were then measured in pentobarbitone-anaesthetized, artificially ventilated rats. The N and CH rats were exposed to acute hypoxia (10% O2 for 4 min) after the intracerebroventricular (i.c.v.) administration of artificial cerebrospinal fluid (control) and then again after either i.c.v. NG-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) or 3-morpholino-sydnonimine hydrochloride (SIN-1; 100 microg in 10 microL). 3. Chronic hypoxia caused pulmonary hypertension (MPAP 20+/-1 vs 30+/-1 mmHg in N and CH rats, respectively) and attenuated acute hypoxic pulmonary vasoconstriction (HPV). Central inhibition of NO synthesis (by l-NAME) did not alter baseline MPAP or the acute HPV in either N or CH rats, but it did elevate MABP. The NO donor SIN-1 did not alter baseline MPAP, but it did enhance (N rats) or restore (CH rats) the HPV and decreased MABP. 4. The results of the present study indicate that central NO has a limited role in the tonic modulation of MPAP during normoxia and after chronic hypoxia. However, the acute HPV seems to be enhanced by exogenous NO.

L-NAME- and ADMA-induced sympathetic neural activation in conscious rats

Although studies in anesthetized, sino-aortic denervated animals indicate that inhibition of central nitric oxide (NO) causes an excitatory influence on efferent sympathetic nerve activity (SNA) that is normally offset by baroreflex activation, studies in conscious animals have not provided clear-cut evidence for a sympathoexcitatory effect of N(omega)-nitro-l-arginine methyl ester (L-NAME) or the endogenous circulating NO synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Thus our goals were to 1) use surgical sino-aortic denervation to test for a sympathoexcititatory effect of intravenous l-NAME in conscious rats, and 2) to determine whether SNA responses to intravenous L-NAME can be extrapolated directly to intravenous ADMA. We recorded mean arterial blood pressure and renal SNA in both intact and sino-aortic-denervated conscious rats during 3 h of continuous intravenous infusion with either L-NAME or ADMA. When we eliminated the confounding influence of the sino-aortic baroreceptors, L-NAME produced a progressive increase in SNA with the peak response exceeding the baseline level of nerve firing by 150%. The same type of frank sympathetic activation was observed with intravenous ADMA. Taken together, these data offer straightforward evidence for l-NAME, as well as ADMA-induced sympathetic activation with direct recordings of SNA in conscious animals. These data confirm and extend the concept that circulating endogenous NOS inhibitors can constitute an excitatory signal to SNA.

Evidence of differential control of renal and lumbar sympathetic nerve activity in conscious rabbits

We have explored the possibility that renal sympathetic nerve activity (RSNA) and vasomotor sympathetic nerve activity are differentially regulated. We measured sympathetic nerve activity (SNA) to the kidney and the hind limb vasculature in seven conscious rabbits 6-8 days after the implantation of recording electrodes. Acute infusion of N(G)-nitro-L-arginine methyl ester (L-NAME) (6 mg.kg(-1).min(-1) for 5 min) led to an increase in blood pressure (from 66 +/- 1 to 82 +/- 3 mmHg) and a decrease in heart rate (from 214 +/- 15 to 160 +/- 13 bpm). L-NAME administration caused a significantly greater decrease in RSNA than lumbar sympathetic nerve activity (LSNA) (to 68 +/- 14% vs. 84 +/- 4% of control values, respectively). Volume expansion (1.5 ml.kg(-1).min(-1)) resulted in a significant decrease in RSNA to 66 +/- 7% of control levels but no change in LSNA (127 +/- 20%). There was no difference in the gain of the baroreflex curves between the LSNA and RSNA [maximum gain of -7.6 +/- 0.4 normalized units (nu)/mmHg for LSNA vs. -7.9 +/- 0.75 nu/mmHg for RSNA]. A hypoxic stimulus (10% O2 and 3% CO2) led to identical increases in both RSNA and LSNA (195 +/- 40% and 158 +/- 21% of control values, respectively). Our results indicate tailored differential control of RSNA and LSNA in response to acute stimuli.

Overexpression of eNOS in brain stem reduces enhanced sympathetic drive in mice with myocardial infarction

Reduced nitric oxide (NO) in the brain might contribute to enhanced sympathetic drive in heart failure (HF). The aim of this study was to determine whether increased NO production induced by local overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarius (NTS) of the brain stem reduces the enhanced sympathetic drive in mice with HF. Myocardial infarction (MI) was induced in mice by ligating the left coronary artery. MI mice exhibited left ventricular dilatation and a reduced left ventricular ejection fraction. Urinary norepinephrine excretion in MI mice was greater than that in sham-operated mice, indicating that sympathetic drive was enhanced in this model. Thus this model has features that are typical of HF. Western blot analysis and immunohistochemical staining for neuronal NOS (nNOS) indicated that nNOS protein expression was significantly reduced in the brain stem of MI mice. MI mice had a significantly smaller increase in blood pressure evoked by intracisternal injection of N(G)-monomethyl-L-arginine than sham-operated mice. Adenoviral vectors encoding either eNOS (AdeNOS) or beta-galactosidase (Adbeta gal) were transfected into the NTS to examine the effect of increased NO production in the NTS on the enhanced sympathetic drive in HF. After the gene transfer, urinary norepinephrine excretion was reduced in AdeNOS-transfected MI mice but not in Adbeta gal-transfected MI mice. These results indicate that nNOS expression in the brain stem, especially in the NTS, is reduced in the MI mouse model of HF, and increased NO production induced by overexpression of eNOS in the NTS attenuates the enhanced sympathetic drive in this model.

Human eNOS gene delivery attenuates cold-induced elevation of blood pressure in rats

We previously showed that chronic cold exposure inhibits endothelial nitric oxide synthase (eNOS) expression and decreases nitric oxide (NO) production. The aim of the present study was to evaluate the possible role of the NO system in the development of cold-induced hypertension (CIH) by testing the hypothesis that adenoviral delivery of human eNOS gene increases NO production and attenuates CIH in rats. The effect of in vivo delivery of adenovirus carrying human eNOS full-length cDNA (rAdv.heNOS) on CIH was tested using four groups of Sprague-Dawley rats (6 rats/group). Blood pressure (BP) did not differ among the four groups during the control period at room temperature (24 degrees C). Two groups of rats received intravenous injection of rAdv.heNOS (1 x 10(9) plaque-forming units/rat), and the other two groups received the same dose of rAdv.LacZ to serve as controls. After gene delivery, one rAdv.heNOS-treated group and one rAdv.LacZ-treated group were exposed to cold (6 degrees C) while the remaining groups were kept at 24 degrees C. We found that the BP of the rAdv.LacZ group increased significantly within 1 wk of exposure to cold and reached a peak level at week 5 (152.2 +/- 6.4 mmHg). In contrast, BP (118.7 +/- 8.4 mmHg) of the cold-exposed rAdv.heNOS group did not increase until 5 wk after exposure to cold. The rAdv.heNOS increased plasma and urine levels of NO significantly in cold-exposed rats, which indicates that eNOS gene transfer increased NO production. Notably, rAdv.heNOS decreased plasma levels of norepinephrine and plasma renin activity in cold-exposed rats, which suggests that eNOS gene transfer may decrease the activities of the sympathetic nervous system and the renin-angiotensin system. Immunohistochemical analysis showed that the transferred human eNOS was expressed in both endothelium and adventitia of mesenteric arteries. We conclude that 1) eNOS gene transfer attenuates CIH by increasing NO production and inhibiting the sympathetic nervous system and the renin-angiotensin system; and 2) the NO system appears to mediate this nongenetic, nonpharmacological, nonsurgical model of hypertension.

NITRIC OXIDE and SYMPATHETIC NERVE ACTIVITY IN THE CONTROL OF BLOOD PRESSURE

1. Endothelial dysfunction marked by impairment in the release of nitric oxide (NO) is seen very early in the development of hypertension and is considered important in mediating the impaired vascular tone evident in essential hypertensive patients. 2. Recently, a hypothesis has emerged that NO acting as a neurotransmitter in the brain can modulate levels of sympathetic nerve activity and thereby blood pressure. The NO inhibition model of hypertension has been used to explore the possibility that a decrease in levels of NO can cause an increase in levels of sympathetic nerve activity that can mediate the hypertension. 3. In the present review, we examine the literature regarding the role of NO in setting the mean level of sympathetic nerve activity and blood pressure. Although the acute effects of NO inhibition are well understood, the chronic interaction between the sympathetic nervous system and NO has only been investigated using indirect measures of sympathetic nerve activity, such as ganglionic blockade. This has led to inconsistent results regarding the role of NO in modulating sympathetic nerve activity chronically. 4. Some of the conflicting results may be explained by differences in the 'background' levels of angiotensin (Ang) II. Evidence suggests that NO may interact with AngII and baroreceptor afferent inputs in the central nervous system to set the mean level of sympathetic nerve activity. 5. We suggest chronic NO inhibition can increase sympathetic nerve activity if baroreceptor input is intact and AngII levels are elevated. Although studies exploring the actions of NO or AngII in isolation are useful for gathering initial information, future studies should focus on their interactions and their role in setting the long-term levels of sympathetic activity and blood pressure.

Brain neuronal nitric oxide synthase neuron-mediated sympathoinhibition is enhanced in hypertensive Dahl rats

OBJECTIVE

To elucidate the role of central neurons containing neuronal nitric oxide synthase (nNOS neurons) in the sympathetic nervous system in hypertensive Dahl salt-sensitive (DS) rats.

DESIGN AND METHODS

Dahl rats were fed either a regular-salt (0.4% NaCl) or high-salt (8% NaCl) diet for 4 weeks. The effect of intracerebroventricular administration of S-methyl-L-thiocitrulline, a selective nNOS inhibitor, on renal sympathetic nerve activity was examined in chronically instrumented conscious DS rats. The activity and protein amount of brain nNOS was evaluated by enzyme assay and western blot analysis. The distribution and number of nNOS neurons in the brainstem were examined immunohistochemically in hypertensive and normotensive DS rats.

RESULTS

S-methyl-L-thiocitrulline induced a larger increase in tonic renal sympathetic nerve activity generated before baroreflex-mediated inhibition in hypertensive DS rats than normotensive DS rats. Hypertensive DS rats showed increased nNOS activity in the brainstem, but not in the diencephalon or cerebellum. High nNOS activity was confirmed by an increase in the amount of nNOS protein. nNOS Neurons were localized in several nuclei throughout the brainstem; the dorsolateral periaqueductal gray, pedunculopontine tegmental nucleus, dorsal raphe nucleus, laterodorsal tegmental nucleus, lateral parabrachial nucleus, rostral ventrolateral medulla, nucleus tractus solitarius and raphe magnus. The number of nNOS neurons in these nuclei, except for the two raphes, was significantly greater in hypertensive than in normotensive DS rats.

CONCLUSIONS

These findings suggest that central nNOS-mediated sympathoinhibition may be enhanced in salt-sensitive hypertensive Dahl rats. The upregulated nNOS-mediated inhibition may occur in the central sympathetic control system generated before baroreflex-mediated inhibition.

Involvement of nitric oxide in both central and peripheral haemodynamic effect of d/l-nebivolol and its enantiomers in rats

The cardiovascular profile of the racemate D/L-nebivolol and its enantiomers administered by intravenous (i.v.) or by intracerebroventricular (i.c.v.) route was investigated in anaesthetized normotensive rats. D/L-Nebivolol (0.1-0.5 mg/kg) induced a dose-related reduction in blood pressure when administered by i.c.v. route. These hypotensive effects were more marked as compared to those achieved by peripheral administration of D/L-nebivolol (0.1-1 mg/kg i.v.). Both enantiomers contributed to the hypotensive effect of D/L-nebivolol by i.c.v. route, while the effects of the drug on blood pressure by i.v. route were due to the d-enantiomer. The bradycardic effect of the racemic form given i.v. was dose-related and, at the highest dose (1 mg/kg), was more pronounced as compared to i.c.v. route. D-Nebivolol was responsible for chronotropic effects by both the i.v. and i.c.v. route, although by i.c.v. route L-nebivolol also induced a reduction in heart rate. The nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) administered at 5 mg/kg i.v. bolus + 0.1 mg/kg/min infusion or at 2.5 mg/kg i.c.v. counteracted the effects of D/L-nebivolol (either 1 mg/kg i.v. or 0.5 mg/kg i.c.v.) on blood pressure, while it did not inhibit the cardiovascular changes induced by isoprenaline (300 ng/kg i.v.) or calcitonin gene-related peptide (CGRP; 400 ng/kg i.v.). In addition, i.c.v. effects of D/L-nebivolol on blood pressure and heart rate were not affected by pre-treatment with atropine (2 mg/kg i.v.). The present findings demonstrate that D/L-nebivolol produced haemodynamic changes following both peripheral and central administration; these latter findings are mainly due to its L-enantiomer and these effects involve the L-arginine/nitric oxide pathway.