The sympathetic nervous system is involved in the maintenance but not initiation of the hypertension induced by N(omega)-nitro-L-arginine methyl ester

Nitric oxide and salt resistance in Dahl rats: no role of inducible NO synthase

Inducible NO synthase (NOS II) was proposed to play an important role in salt resistance of Dahl salt-resistant (SR/Jr) rats. Its chronic inhibition by specific inhibitors was accompanied by blood pressure (BP) elevation in animals subjected to high salt intake. The aim of our study was to evaluate 1) whether such inhibitors affect BP and/or its particular components (sympathetic tone and NO-dependent vasodilation) only under the conditions of high salt intake, and 2) whether similar BP effects are elicited after systemic or intracerebroventricular (icv) application of these inhibitors. Wistar rats fed Altromin diet (0.45 % NaCl) and SR/Jr rats fed either a low-salt (LS, 0.3 % NaCl) or a high-salt (HS, 4 % NaCl) diet were studied. Aminoguanidine (AMG) and 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT) were used as NOS II inhibitors. BP and its responses to acute blockade of renin-angiotensin system (captopril), sympathetic nervous system (pentolinium) and NO synthase (L-NAME) were measured in conscious cannulated rats. There were no significant changes of BP or its components in either Wistar rats or SR/Jr rats subjected to chronic inhibition of NOS II by peroral aminoguanidine administration (50 mg/kg/day for 4 weeks). This was true for SR/Jr rats fed either LS or HS diets. Furthermore, we have studied BP effects of chronic icv administration of both NOS II inhibitors in SR/Jr rats fed HS diet, but we failed to find any BP changes elicited by such treatment. In conclusion, inducible NO synthase does not participate in the resistance of SR/Jr rats to hypertensive effects of excess salt intake.

The restorative effect of apocynin and catalase in l-arginine induced hypotension on normotensive subjects – the role of oxidative stress

L-arginine is a substrate for nitric oxide synthase (NOS) responsible for the production of NO. This investigation studied the effect of apocynin, an NADPH oxidase inhibitor and catalase, an H2O2 scavenger on L-arginine induced oxidative stress and hypotension. Forty Wistar-Kyoto rats were treated for 14 days with vehicle, L-arginine (12.5mg/ml p.o.), L-arginine+apocynin (2.5mmol/L p.o.), L-arginine+catalase (10000U/kg/day i.p.) and L-arginine plus apocynin+catalase respectively. Weekly renal functional and hemodynamic parameters were measured and kidneys harvested at the end of the study for histopathological and renal NADPH oxidase 4 (Nox4) assessments. L-arginine administration in normotensive rats decreased systolic blood pressure (120±2 vs 91±2mmHg) and heart rate (298±21 vs 254±15b/min), enhanced urinary output (21.5±4.2 vs 32±1.9ml/24h , increased creatinine clearance (1.72±0.56 vs 2.62±0.40ml/min/kg), and fractional sodium excretion (0.88±0.16 vs 1.18±0.16 %), caused proteinuria (28.10±1.93 vs 35.26±1.69mg/kg/day) and a significant decrease in renal cortical blood perfusion (292±3 vs 258±5bpu) and pulse wave velocity (3.72±0.20 vs 2.84±0.13m/s) (all P<0.05). L-arginine increased plasma malondialdehyde (by ~206 % P<0.05) and NO (by ~51 %, P<0.05) but decreased superoxide dismutase (by ~31 %, P<0.05) and total antioxidant capacity (by ~35 %, P<0.05) compared to control. Renal Nox4 mRNA activity was approximately 2.1 fold higher (P<0.05) in the L-arginine treated rats but was normalized by apocynin and apocynin plus catalase treatment. Administration of apocynin and catalase, but not catalase alone to rats fed L-arginine, restored the deranged renal function and structure, prevented hypotension and enhanced the antioxidant capacity and suppressed Nox4 expression. These findings suggest that apocynin and catalase might be used prophylactically in states of oxidative stress.

Pattern of sympathetic vasomotor activity in a model of hypertension induced by nitric oxide synthase blockade

We aimed to investigate the effects of nitric oxide (NO) synthesis inhibition by NO synthase inhibitor N‐nitro‐L‐arginine‐methyl ester (L‐NAME) treatment on the sympathetic vasomotor nerve activity (SNA) on two sympathetic vasomotor nerves, the renal and splanchnic. NO plasma level and systemic oxidative stress were assessed. Hypertension was induced by L‐NAME (20 mg/kg per day, by gavage, for seven consecutive days) in male Wistar rats. At the end of the treatment, blood pressure, heart rate, arterial baroreflex sensitivity, renal SNA (rSNA), and splanchnic SNA (sSNA) were assessed in urethane anesthetized rats. L‐NAME‐treated rats presented increased blood pressure (152 ± 2 mmHg, n = 17) compared to the control group (101 ± 2 mmHg, n = 15). Both rSNA (147 ± 10, n = 15 vs. 114 ± 5 Spikes/s, n = 9) and sSNA (137 ± 13, n = 14 vs. 74 ± 13 spikes/s, n = 9) were significantly increased in the L‐NAME‐treated compared to the control group. A differential response on baroreflex sensitivity was found, with a significant reduction for rSNA but not for sSNA arterial baroreceptor sensitivity in L‐NAME‐treated rats. The adjusted regression model revealed that the reduction of systemic NO levels partially explains the variation in sSNA and blood pressure, but not rSNA. Taken together, our data show that hypertension induced by NO synthase blockade is characterized by increased SNA to the rSNA and sSNA. In addition, we found that the rats that had the greatest reduction in NO levels in plasma by L‐NAME were those that developed higher blood pressure levels. The reduction in the NO level partially explains the variations in sSNA but not in rSNA.

Electrical stimulation of the carotid sinus lowers arterial pressure and improves heart rate variability in L-NAME hypertensive conscious rats

We evaluated the effects of long-term (48 h) electrical stimulation of the carotid sinus (CS) in hypertensive rats. L-NAME-treated (10 days) Wistar rats were implanted with a catheter in the femoral artery and a miniaturized electrical stimulator attached to electrodes positioned around the left CS, encompassing the CS nerve. One day after implantation, arterial pressure (AP) was directly recorded in conscious animals for 60 min. Square pulses (1 ms, 3 V, 30 Hz) were applied intermittently (20/20 s ON/OFF) to the CS for 48 h. After the end of stimulation, AP was recorded again. Nonstimulated rats (control group) and rats without electrodes around the CS (sham-operated) were also studied. Next, the animals were decapitated, and segments of mesenteric resistance arteries were removed to study vascular function. After the stimulation period, AP was 16 ± 5 mmHg lower in the stimulated group, whereas sham-operated and control rats showed similar AP between the first and second recording periods. Heart rate variability (HRV) evaluated using time and frequency domain tools and a nonlinear approach (symbolic analysis) suggested that hypertensive rats with electrodes around the CS, stimulated or not, exhibited a shift in cardiac sympathovagal balance towards parasympathetic tone. The relaxation response to acetylcholine in endothelium-intact mesenteric arteries was enhanced in rats that underwent CS stimulation for 48 h. In conclusion, long-term CS stimulation is effective in reducing AP levels, improving HRV and increasing mesenteric vascular relaxation in L-NAME hypertensive rats. Moreover, only the presence of electrodes around the CS is effective in eliciting changes in HRV similar to those observed in stimulated rats.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

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.

Acute and chronic role of nitric oxide, renin-angiotensin system and sympathetic nervous system in the modulation of calcium sensitization in Wistar rats

Principal vasoactive systems - renin-angiotensin system (RAS), sympathetic nervous system (SNS), nitric oxide (NO) and prostanoids - exert their vascular effects through the changes in calcium levels and/or calcium sensitization. To estimate a possible modulation of calcium sensitization by the above vasoactive systems, we studied the influence of acute and chronic blockade of particular vasoactive systems on blood pressure (BP) changes elicited in conscious normotensive rats by acute dose-dependent administration of Rho-kinase inhibitor fasudil. Adult male chronically cannulated Wistar rats were used throughout this study. The acute inhibition of NO synthase (NOS) by L-NAME enhanced BP response to fasudil, the effect being considerably augmented in rats deprived of endogenous SNS. The acute inhibition of prostanoid synthesis by indomethacin modified BP response to fasudil less than the acute NOS inhibition. The chronic NOS inhibition caused moderate BP elevation and a more pronounced augmentation of fasudil-induced BP changes compared to the effect of acute NOS inhibition. This indicates both short-term and long-term NO-dependent attenuation of calcium sensitization. Long-term inhibition of RAS by captopril caused a significant attenuation of BP changes elicited by fasudil. In contrast, a long-term attenuation of SNS by chronic guanethidine treatment (in youth or adulthood) had no effect on BP response to fasudil, suggesting the absence of SNS does not affect calcium sensitization in vascular smooth muscle of normotensive rats. In conclusion, renin-angiotensin system contributes to the long-term increase of calcium sensitization and its effect is counterbalanced by nitric oxide which decreases calcium sensitization in Wistar rats.

The Effect of Nitric Oxide Inhibition in Spinal Cord Injured Humans with and without Preserved Sympathetic Control of the Vasculature

Systemic pharmacological inhibition of nitric oxide (NO) causes a hypertensive response, which has been attributed both to inhibition of peripheral NO-mediated vasodilatation and to inhibition of central nervous NO-production leading to a later onset sympathetic vasoconstriction. In the present study we aimed to test the importance of these two mechanisms by comparing the time-courses of the hypertensive responses in spinal cord injured (SCI) subjects with varying degrees of loss of sympathetic vascular control depending on level of injury as well as able-bodied controls. We hypothesized that high level SCI with no sympathetic vasoconstrictor control would have an abbreviated time-course of the hypertensive response to the NO-inhibitor L-NAME, because they would lack the late onset sympathetic component to the hypertensive response. NO production was blocked in 12 subjects with SCI and 6 controls by intravenous infusion of L-NAME (1.55–2.7 mg/kg). We measured blood pressure, heart rate, and vascular conductance in the carotid, brachial, and femoral arteries before, during, and after 1 h of L-NAME in a 4-h protocol. Peak increases in mean arterial pressure were significantly larger in high level SCI vs. controls: 32 ± 6 vs. 12 ± 2 mmHg (both groups received 1.55 mg/kg). The decreases in vascular conductance in the brachial and femoral vascular beds were also larger in the high level SCI group, whereas decreases in heart rate and carotid conductance were not significantly different between the groups. There were no indications of any abbreviated responses in blood pressure or vascular conductance in the high level SCI compared to control. The mid level and low-level SCI subject had responses similar to controls. These data confirm previous reports that NO inhibition causes a larger increase in blood pressure in high level SCI, and extend these data by providing evidence for differences in vascular conductance in the limbs. The current data do not support an obligatory important role for sympathetic vasoconstriction in maintaining the hypertensive response to L-NAME in humans.

Chemical Sympathectomy Restores Baroreceptor-Heart Rate Reflex and Heart Rate Variability in Rats With Chronic Nitric Oxide Deficiency

Nitric oxide (NO) plays a crucial role not only in regulation of blood pressure but also in maintenance of cardiac autonomic tone and its deficiency induced hypertension is accompanied by cardiac autonomic dysfunction. However, underlying mechanisms are not clearly defined. We hypothesized that sympathetic activation mediates hemodynamic and cardiac autonomic changes consequent to deficient NO synthesis. We used chemical sympathectomy by 6-hydroxydopamine to examine the influence of sympathetic innervation on baroreflex sensitivity (BRS) and heart rate variability (HRV) of chronic NG-nitro-L-arginine methyl ester (L-NAME) treated adult Wistar rats. BRS was determined from heart rate responses to changes in systolic arterial pressure achieved by intravenous administration of phenylephrine and sodium nitroprusside. Time and frequency domain measures of HRV were calculated from 5-min electrocardiogram recordings. Chronic L-NAME administration (50 mg/kg per day for 7 days orally through gavage) in control rats produced significant elevation of blood pressure, tachycardia, attenuation of BRS for bradycardia and tachycardia reflex and fall in time as well as frequency domain parameters of HRV. Sympathectomy completely abolished the pressor as well as tachycardic effect of chronic L-NAME. In addition, BRS and HRV improved after removal of sympathetic influence in chronic L-NAME treated rats. These results support the concept that an exaggerated sympathetic activity is the principal mechanism of chronic L NAME hypertension and associated autonomic dysfunction.

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.

Passive limb movement: evidence of mechanoreflex sex specificity

Previous studies have determined that premenopausal women exhibit an attenuated metaboreflex; however, little is known about sex specificity of the mechanoreflex. Thus, we sought to determine if sex differences exist in the central and peripheral hemodynamic responses to passive limb movement. Second-by-second measurements of heart rate, stroke volume, cardiac output (CO), mean arterial pressure, and femoral artery blood flow (FBF) were recorded during 3 min of supine passive knee extension in 24 young healthy subjects (12 women and 12 men). Normalization of CO and stroke volume to body surface area, expressed as cardiac index and stroke index, eliminated differences in baseline central hemodynamics, whereas, peripherally, basal FBF and femoral vascular conductance were similar between the sexes. In response to passive limb movement, women displayed significantly attenuated peak central hemodynamic responses compared with men (heart rate: 9.0 ± 1 vs. 14.8 ± 2% change, stroke index: 4.5 ± 0.6 vs. 7.8 ± 1.2% change, cardiac index: 9.6 ± 1 vs. 17.2 ± 2% change, all P < 0.05), whereas movement induced similar increases in peak FBF (167 ± 32 vs. 193 ± 17% change) and femoral vascular conductance (172 ± 31 vs. 203 ± 16% change) in both sexes (women vs. men, respectively). Additionally, there was a significant positive relationship between individual peak FBF and peak CO response to passive movement in men but not in women. Thus, although both sexes exhibited similar movement-induced hyperemia and peripheral vasodilatory function, the central hemodynamic response was blunted in women, implying an attenuated mechanoreflex. Therefore, this study reveals that, as already recognized with the metaboreflex, there is likely a sex-specific attenuation of the mechanoreflex in women.

Nitric oxide-mediated central sympathetic excitation promotes CNS and pulmonary O₂ toxicity

In hyperbaric oxygen (HBO(2)) at or above 3 atmospheres absolute (ATA), autonomic pathways link central nervous system (CNS) oxygen toxicity to pulmonary damage, possibly through a paradoxical and poorly characterized relationship between central nitric oxide production and sympathetic outflow. To investigate this possibility, we assessed sympathetic discharges, catecholamine release, cardiopulmonary hemodynamics, and lung damage in rats exposed to oxygen at 5 or 6 ATA. Before HBO(2) exposure, either a selective inhibitor of neuronal nitric oxide synthase (NOS) or a nonselective NOS inhibitor was injected directly into the cerebral ventricles to minimize effects on the lung, heart, and peripheral circulation. Experiments were performed on both anesthetized and conscious rats to differentiate responses to HBO(2) from the effects of anesthesia. EEG spikes, markers of CNS toxicity in anesthetized animals, were approximately four times as likely to develop in control rats than in animals with central NOS inhibition. In inhibitor-treated animals, autonomic discharges, cardiovascular pressures, catecholamine release, and cerebral blood flow all remained below baseline throughout exposure to HBO(2). In control animals, however, initial declines in these parameters were followed by significant increases above their baselines. In awake animals, central NOS inhibition significantly decreased the incidence of clonic-tonic convulsions or delayed their onset, compared with controls. The novel findings of this study are that NO produced by nNOS in the periventricular regions of the brain plays a critical role in the events leading to both CNS toxicity in HBO(2) and to the associated sympathetic hyperactivation involved in pulmonary injury.

Cardiac autonomic function in acutely nitric oxide deficient hypertensive rats: role of the sympathetic nervous system and oxidative stress

We evaluated the role of the sympathetic nervous system and oxidative stress in hemodynamic and autonomic control after acute inhibition of the synthesis of nitric oxide, using intravenous (i.v.) injection of 30 mg·kg(-1) N(G)-nitro-l-arginine methyl ester (L-NAME) in adult Wistar rats. Baroreflex sensitivity (BRS) and heart rate variability (HRV) were measured as indices of cardiac autonomic control, before and after L-NAME treatment in rats with intact autonomic innervation, and in rats with chemical sympathectomy by 6-hydroxydopamine. Serum malondialdehyde (MDA) was measured as a marker of oxidative stress. In control rats, L-NAME treatment resulted in a significant rise in blood pressure, augmentation of BRS, and enhanced serum MDA. HRV showed an attenuation of total spectral power and high frequency spectral power, along with a rise of the low to high frequency ratio (LF:HF). Administration of L-NAME produced a pressor response even in sympathectomised rats, but augmented BRS was not observed, and the high frequency spectral power showed an increase, in addition to a significant decline of LF:HF and serum MDA. We therefore conclude that even though pressor response was unaffected, reversal of cardiac autonomic responses and decline in oxidative stress following sympathectomy in L-NAME-treated rats reflects a significant role for sympathetic innervation in acute L-NAME-induced hypertension.

Sympathetic activity is not increased in L-NAME hypertensive rats

The role played by the sympathetic drive in the development of N(G)-nitro-l-arginine methyl ester (l-NAME)-induced hypertension is not firmly established. Therefore, the present study was undertaken in conscious rats in which hypertension was induced by treatment with l-NAME over the course of either 2 or 14 days. Mean arterial pressure (MAP) was measured via a catheter placed in the femoral artery, drugs were administered via a cannula placed in the femoral vein, and renal sympathetic nerve activity (RSNA) was monitored using an implanted electrode. Despite the remarkable increase in arterial pressure, heart rate did not change after treatment with l-NAME. RSNA was similar in l-NAME-induced hypertensive rats treated over the course of 2 or 14 days, as well as in normotensive rats. It was also demonstrated that l-NAME-induced hypertensive rats displayed a resetting of the baroreflex control of RSNA to hypertensive levels, with decreased sensitivity over the course of 2 or 14 days. Furthermore, the sympathetic-vagal balance examined in the time and frequency domain and the renal and plasma norepinephrine content did not differ between groups. In conclusion, the evaluation of the sympathetic drive in conscious rats demonstrated that the arterial hypertension induced by l-NAME treatment over the course of 2 and 14 days does not show sympathetic overactivity.

Inhibition of nitric oxide synthase evokes central sympatho-excitation in healthy humans

Animal studies have indicated that nitric oxide is a key signalling molecule involved in the tonic restraint of central sympathetic outflow from the brainstem. Extension of these findings to humans has been difficult because systemic infusion of nitric oxide synthase (NOS) inhibitors increases blood pressure due to inhibition of endothelial NOS, resulting in activation of the arterial baroreflex and subsequent inhibition of central sympathetic outflow. To overcome this confounding inhibitory influence of the baroreflex, in the current study we directly measured skin sympathetic nerve activity (SNA), which is not under baroreceptor control. Healthy, normotensive humans were studied before, during a 60 min intravenous infusion of the NOS inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME; 4 mg kg(1)), and for 120 min following the infusion (i.e. 180 min total). Skin SNA and arterial blood pressure (BP) were continuously measured. BP was increased from baseline at the end of the l-NAME infusion (14 +/- 2 mmHg; P < 0.05) and remained significantly elevated for the remainder of the experiment (18 +/- 3 mmHg; P < 0.05). Similarly, systemic NOS inhibition produced time-dependent increases in skin SNA, such that skin SNA was elevated at the end of the l-NAME infusion (total activity, 200 +/- 22% baseline; P = 0.08) and was further increased at the end of the study protocol (total activity, 350 +/- 41% baseline; P < 0.05). Importantly, skin SNA remained unchanged during time and hypertensive (phenylephrine) control experiments. These findings indicate that pharmacological inhibition of NOS causes sympathetic activation and support a role of nitric oxide in central sympathetic control in humans.

Sympathetic nerves and the progression of chronic kidney disease during 5/6 nephrectomy: studies in sympathectomized rats

1. Chronically increased sympathetic nerve activity is present during chronic kidney disease (CKD); however, its role in contributing to hypertension or the progression of CKD remains poorly understood. The aim of the present study was to determine whether neonatal sympathectomy attenuates hypertension in 5/6 nephrectomized rats and affects renal structure and function in a blood pressure-independent manner. 2. We performed 5/6 nephrectomy (referred to as CKD) in both sympathetically intact and sympathectomized (injected neonatally with guanethidine; referred to as CKD + Sympath) male Sprague-Dawley rats. Sham-operated sympathetically intact and sympathectomized rats (Sham and Sham + Sympath, respectively) were used as controls. Radiotelemetry was used to monitor blood pressure throughout the 6 week duration of the study, after which renal function and histology were assessed. 3. Overall average systolic arterial pressure and final urinary protein excretion were significantly lower in CKD + Sympath compared with CKD rats (168 +/- 7 mmHg and 33 +/- 5 mg/24 h vs. 184 +/- 6 mmHg and 66 +/- 7 mg/24 h, respectively). However, the level of proteinuria in the CKD + Sympath group was reduced to a greater extent than what would be expected solely on the basis of lower blood pressure. All other indices of renal function and histology were comparable between both CKD groups. All measurements were comparable between Sham and Sham + Sympath groups. 4. In conclusion, sympathectomy attenuated hypertension by approximately one-third in 5/6 nephrectomized rats. Furthermore, sympathetic nerves to the kidney during 5/6 nephrectomy may contribute to proteinuria in a blood pressure-independent manner.

Central sympathetic overactivity: maladies and mechanisms

There is growing evidence to suggest that many disease states are accompanied by chronic elevations in sympathetic nerve activity. The present review will specifically focus on central sympathetic overactivity and highlight three main areas of interest: 1) the pathological consequences of excessive sympathetic nerve activity; 2) the potential role of centrally derived nitric oxide in the genesis of neural dysregulation in disease; and 3) the promise of several novel therapeutic strategies targeting central sympathetic overactivity. The findings from both animal and human studies will be discussed and integrated in an attempt to provide a concise update on current work and ideas in these important areas.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Direct and reflexive effects of nitric oxide synthase inhibition on blood pressure

Direct effects of vasoactive substances on blood pressure can be examined in individuals with tetraplegia due to disruption of descending spinal pathways to sympathetic preganglionic neurons, as cervical lesions interfere with baroreceptor reflex buffering of sympathetic outflow. In this study, we assessed effects of the nitric oxide synthase inhibitor nitro-L-arginine methyl ester (L-NAME) on mean arterial pressure, heart rate, and plasma norepinephrine concentrations in individuals with tetraplegia vs. effects shown in a neurologically intact control group. Seven individuals with tetraplegia and seven age-matched controls received, on separate visits and in the following order, placebo (30 ml normal saline) and 0.5, 1, 2, and 4 mg/kg L-NAME intravenously over 60 min. Supine hemodynamic data were collected, and blood was sampled at the end of each infusion and at 120, 180, and 240 min thereafter. L-NAME increased mean arterial pressure, and the relative increase was greater in the tetraplegia group than in the control group. Heart rate was reduced after L-NAME administration in both groups. L-NAME decreased plasma norepinephrine in the control group but not in the group with tetraplegia. These findings suggest that reflexive sympathoinhibition normally buffers the pressor response to nitric oxide synthase inhibition, an effect that is not evident in individuals with tetraplegia as a result of decentralized sympathetic vasomotor control.

Renal sodium-glucose cotransporter activity and aquaporin-2 expression in rat kidney during chronic nitric oxide synthase inhibition

BACKGROUND/AIMS

The renal sodium glucose cotransporter (SGLT2) and the water channel aquaporin-2 (AQP2) play a critical role in tubular sodium and water reabsorption and in the regulation of extracellular fluid volume both in physiologic and pathophysiologic conditions. However, there is little information about SGLT2 and AQP2 expression and/or activity in hypertension and there are no reports during hypertension induced by chronic nitric oxide synthase (NOS) inhibition.

METHODS

Hypertension was induced in rats by oral administration of N(G)-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg/24 h) for 6 (H6) or 12 (H12) weeks. SGLT2 activity was measured using alpha-(14)C-methylglucose active uptake. The expression level of transporters was assessed by immunohistochemistry and/or immunoblotting.

RESULTS

SGLT2 activity was reduced in both H6 and H12; this was due neither to a decrease in SGLT2 expression nor to a change in membrane phospholipid composition. In H6, AQP2 expression diminished only in the inner medulla (IM), while in H12 it diminished in both outer (OM) and IM. This reduced expression of AQP2 may partially account for the increased urinary volume and decreased urinary osmolality in H12, since we obtained a strong correlation between AQP2 expression and these urinary parameters in both OM and IM.

CONCLUSION

We propose that in rats in which hypertension is induced by NOS inhibition, SGLT2 activity and AQP2 expression are modified to compensate for the elevated arterial pressure. However, we cannot discount the possibility that the observed changes are due to the decrease in NO production itself.

Nitric oxide in hypertension

Hypertension is a major risk factor for cardiovascular disease, and reduction of elevated blood pressure significantly reduces the risk of cardiovascular events. Endothelial dysfunction, which is characterized by impairment of nitric oxide (NO) bioavailability, is an important risk factor for both hypertension and cardiovascular disease and may represent a major link between the conditions. Evidence suggests that NO plays a major role in regulating blood pressure and that impaired NO bioactivity is an important component of hypertension. Mice with disruption of the gene for endothelial NO synthase have elevated blood pressure levels compared with control animals, suggesting a genetic component to the link between impaired NO bioactivity and hypertension. Clinical studies have shown that patients with hypertension have a blunted arterial vasodilatory response to infusion of endothelium-dependent vasodilators and that inhibition of NO raises blood pressure. Impaired NO bioactivity is also implicated in arterial stiffness, a major mechanism of systolic hypertension. Clarification of the mechanisms of impaired NO bioactivity in hypertension could have important implications for the treatment of hypertension.

Role of renal sympathetic nerve activity in hypertension induced by chronic nitric oxide inhibition

Nitric oxide levels are diminished in hypertensive patients, suggesting nitric oxide might have an important role to play in the development of hypertension. Chronic blockade of nitric oxide leads to hypertension that is sustained throughout the period of the blockade in baroreceptor-intact animals. It has been suggested that the sympathetic nervous system is involved in the chronic increase in blood pressure; however, the evidence is inconclusive. We measured renal sympathetic nerve activity and blood pressure via telemetry in rabbits over 7 days of nitric oxide blockade. Nitric oxide blockade via Nω-nitro-l-arginine methyl ester (l-NAME) in the drinking water (50 mg·kg−1·day−1) for 7 days caused a significant increase in arterial pressure (7 ± 1 mmHg above control levels; P < 0.05). While the increase in blood pressure was associated with a decrease in heart rate (from 233 ± 6 beats/min before the l-NAME to 202 ± 6 beats/min on day 7), there was no change in renal sympathetic nerve activity (94 ± 4 %baseline levels on day 2 and 96 ± 5 %baseline levels on day 7 of l-NAME; baseline nerve activity levels were normalized to the maximum 2 s of nerve activity evoked by nasopharyngeal stimulation). The lack of change in renal sympathetic nerve activity during the l-NAME-induced hypertension indicates that the renal nerves do not mediate the increase in blood pressure in conscious rabbits.

Normalization of supine blood pressure after nitric oxide synthase inhibition in persons with tetraplegia

BACKGROUND/OBJECTIVE

Orthostatic hypotension is a well-defined clinical consequence of spinal cord injury (SCI), particularly in those with tetraplegia. The etiology of orthostatic hypotension is thought to be loss of sympathetic vasomotor control, although other factors may play a role. There is evidence of up-regulation of nitric oxide synthase (NOS) activity after hind-limb suspension in rats, a condition of antigravity that may have similar vascular effects as shown in persons with tetraplegia caused by paralysis. The study objective was to determine the effect of a NOS inhibitor (nitro-L-arginine methyl ester [L-NAME]) on supine mean arterial pressure in persons with chronic tetraplegia compared with non-SCI controls.

METHODS

Fourteen individuals participated (7 with tetraplegia and 7 controls). Subjects visited the laboratory twice for placebo on day 1 and L-NAME (1 mg/kg) on day 2; both were infused intravenously over 60 minutes. Blood pressure was monitored for 3 hours after infusion at the brachial artery using a standard manual cuff.

RESULTS

Mean arterial pressure (MAP) was lower at baseline (P < 0.05) and after placebo administration (P < 0.0001) in the tetraplegia group compared with the control group. L-NAME increased MAP in both groups; however, the relative increase was greater in the tetraplegia group compared with the control group, such that group differences for MAP were eliminated. Supine MAP was normalized with L-NAME, and there was an increased sensitivity to NOS inhibition in the group with tetraplegia.

CONCLUSIONS

These findings indicate that blood pressure dysregulation in persons with tetraplegia may reflect increased vascular NO and suggest a novel treatment of hypotension using NOS inhibition in this population.

The endocrine system in chronic nitric oxide deficiency

The experimental model of chronic inhibition of nitric oxide (NO) production has proven to be a useful tool to study cardiovascular and renal lesions produced by this type of hypertension, which are similar to those found in human hypertension. It also offers a unique opportunity to study the interaction of NO with the humoral systems, known to have a role in the normal physiology of vascular tone and renal function. This review provides a thorough and updated analysis of the interactions of NO with the endocrine system. There is special focus on the main vasoactive factors, including the renin-angiotensin-aldosterone system, catecholamines, vasopressin, and endothelin among others. Recent discoveries of crosstalk between the endocrine system and NO are also reported. Study of these humoral interactions indicates that NO is a molecule with ubiquitous function and that its inhibition alters virtually to all other known regulatory systems. Thus, hypothyroidism attenuates the pressor effect of NO inhibitor N-nitro-L-arginine methyl ester, whereas hyperthyroidism aggravates the effects of NO synthesis inhibition; the sex hormone environment determines the blood pressure response to NO blockade; NO may play a homeostatic role against the prohypertensive effects of mineralocorticoids, thyroid hormones and insulin; and finally, NO deficiency affects not only blood pressure but also glucose and lipid homeostasis, mimicking the human metabolic syndrome X, suggesting that NO deficiency may be a link between metabolic and cardiovascular disease.

Effect of acute inhibition of nitric oxide synthesis by l-NAME on cardiovascular responses following peripheral autonomic blockade in rabbits

The pressor and chronotropic responses to acute inhibition of nitric oxide synthase enzyme by N(G)-nitro-L-arginine methyl ester (L-NAME) were studied in anaesthetized rabbits with intact autonomic nervous system (ANS) activity. Also, they were investigated when administration of L-NAME was preceded by peripheral autonomic blockade. Autonomic blockade had different forms: ganglionic (hexamethonium-induced), post-ganglionic beta-adrenergic blockade (propranolol induced), parasympathetic blockade (atropine induced), and complete autonomic blockade by coadministration of hexamethonium and atropine simultaneously. L-NAME injected intravenously (10 mg/kg) in animals with intact and blocked autonomic activity induced a pressor response. This pressor response was accompanied by bradycardia in rabbits with either intact autonomic activity or hexamethonium-induced ganglionic blockade. L-NAME exerted no effect on heart rate in animals with beta-adrenergic blockade or parasympathetic blockade. In rabbits with complete autonomic blockade, L-NAME evoked tachycardia. These experiments indicate that L-NAME-induced hypertension is not relying only on ANS. Also, L-NAME-induced tachycardia in rabbits treated with atropine plus hexamethonium suggests other humoral mechanisms that may be involved in the L-NAME induced chronotropic response.

Modulated hemodynamic response to clonidine in bile duct-ligated rats: the role of nitric oxide

Despite the well-known involvement of the peripheral sympathetic abnormalities in the development of cardiovascular complications of cholestasis, the role of the central sympathetic system is still elusive. The goal of this study was to evaluate the effects of central sympathetic tone reduction, through clonidine administration, on hemodynamic parameters of 7-day bile duct-ligated rats. The contributions of nitric oxide and endogenous opioids were also examined by acute intravenous (10 min before clonidine) or chronic daily subcutaneous administrations of N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 mg/kg) or naltrexone (20 mg/kg). Seven days after bile duct ligation or sham operation, animals were anesthetized with sodium pentobarbital. After hemodynamic stabilization, clonidine (10 microg/kg) was injected intravenously, which elicited an initial hypertension (the peripheral effect) followed by persistent hypotension and bradycardia (the central effects). Cholestatic rats demonstrated significant basal bradycardia (P<0.001) and hypotension (P<0.05), which were corrected by chronic naltrexone but not L-NAME treatment. While the peripheral effect of clonidine was blunted, the central effects were exaggerated in cholestatic rats (P<0.01). Acute L-NAME treatment accentuated the hypertensive phase in sham-operated and cholestatic rats (P<0.05). However, the difference between the two groups was preserved (P<0.01). This treatment attenuated the central effects in both sham-operated and cholestatic rats to the same level (P<0.001). Chronic L-NAME treatment resulted in exaggeration of the peripheral response in cholestatic and central responses in sham-operated rats (P<0.05), and abolished the difference between the groups. Naltrexone treatment had no significant effect on either the central or the peripheral responses to clonidine. This study shows that both central and peripheral hemodynamic responses to clonidine are altered in cholestasis. It also provides evidence that nitric oxide contributes to the development of these abnormalities.

Antihypertensive Mechanisms of Chronic Captopril or N-Acetylcysteine Treatment in L-NAME Hypertensive Rats

Hypertension due to chronic inhibition of NO synthase (NOS) by Nomega-nitro-L-arginine methyl ester (L-NAME) administration is characterized by both impaired NO-dependent vasodilation and enhanced sympathetic vasoconstriction. The aim of our study was to evaluate changes in the participation of major vasoactive systems in L-NAME-treated rats which were subjected to simultaneous antihypertensive (captopril) or antioxidant (N-acetylcysteine, NAC) treatment. Three-month-old Wistar males treated with L-NAME (60 mg/kg/day) for 5 weeks were compared to rats in which L-NAME treatment was combined with simultaneous chronic administration of captopril or NAC. Basal blood pressure (BP) and its acute responses to consecutive i.v. injections of captopril (10 mg/kg), pentolinium (5 mg/kg), L-NAME (30 mg/kg), tetraethylammonium (TEA, 16 mg/kg) and nitroprusside (NP, 20 microg/kg) were determined in conscious rats at the end of the study. The development of L-NAME hypertension was prevented by captopril treatment, whereas NAC treatment caused only a moderate BP reduction. Captopril treatment normalized the sympathetic BP component and significantly reduced residual BP (measured at full NP-induced vasodilation). In contrast, chronic NAC treatment did not modify the sympathetic BP component or residual BP, but significantly enhanced NO-dependent vasodilation. Neither captopril nor NAC treatment influenced the compensatory increase of TEA-sensitive vasodilation mediated by endothelium-derived hyperpolarizing factor in L-NAME-treated rats. Chronic captopril treatment prevented L-NAME hypertension by lowering of sympathetic tone, whereas chronic NAC treatment attenuated L-NAME hypertension by reduction in the vasodilator deficit due to enhanced NO-dependent vasodilation.

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.

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.

Potentiation of L-NAME-induced systemic and renal vasoconstrictor responses by alpha1-adrenoceptor antagonism

BACKGROUND

Acute inhibition of nitric oxide synthase results in systemic and renal vasoconstriction, which might be attributable to unopposed activity of the sympathetic nervous (SNS) and the renin-angiotensin system (RAS). We studied the effects of N-nitro-L-arginine methyl ester (L-NAME) during alpha1-adrenoceptor blockade and concomitant angiotensin II type 1 (AT1)-receptor blockade in hypertensive individuals pretreated with hydrochlorothiazide (Hct; 25 mg once daily).

METHODS

Thirteen individuals (47 +/- 9 years) were studied during administration of placebo, and after pretreatment with Hct + doxazosin (Dox; 8 mg twice daily for 9 days), with Hct + Dox + losartan (Los; 50 mg twice daily for 9 days), or (n = 5) with doxazosin or Dox + Los without hydrochlorothiazide. Mean arterial pressure (MAP) and cardiac output were derived from the finger blood pressure signal recorded by Finapres. Systemic vascular resistance (SVR) was calculated as MAP/cardiac output. Five renal clearance studies of 40 min were performed. Renal vascular resistance (RVR) was calculated as MAP divided by renal blood flow (RBF). L-NAME (12.5 microg/kg per min intravenously) was given during the third clearance period.

RESULTS

MAP was 113 +/- 11 mmHg at baseline and decreased to 99 +/- 10 mmHg during the administration of Hct + Dox and to 92 +/- 10 mmHg during Hct + Dox + Los. This decrease in MAP was caused by a decrease in SVR (P = 0.0009). Pretreatment with Hct + Dox or Hct + Dox + Los had no effect on glomerular filtration rate or RBF. Infusion of L-NAME during the administration of Hct + Dox resulted in an augmented (P < 0.0001) increase in MAP (18%), SVR (61%) and RVR (70%) compared with those observed with placebo (8, 30 and 49%, respectively). This augmentation was abolished by losartan.

CONCLUSION

L-NAME-induced systemic and renal vasoconstrictor responses are potentiated during alpha1-adrenoceptor blockade. This potentiation was abolished by AT1-receptor antagonism. In man, unopposed activity of the SNS or SNS and RAS is not involved in the systemic and renal vasoconstriction induced by L-NAME.

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.

Differential effects of antihypertensive drugs on renal and glomerular hemodynamics and injury in the chronic nitric-oxide-suppressed rat

BACKGROUND/AIMS

Prolonged nitric oxide synthase (NOS) inhibition with N(omega)-nitro-L-arginine methylester in normotensive and hypertensive rats has been demonstrated to produce severe systemic and glomerular hypertension with glomerular sclerosis, and these changes have become a useful experimental model of hypertensive nephrosclerosis. This review summarizes data from our serial studies as well as work of others who are also investigating the effects of the commonly used antihypertensive drugs (including calcium antagonist, angiotensin-converting enzyme inhibitor, angiotensin II type 1 receptor blocker, aldosterone antagonist and thiazide diuretic) on renal and glomerular hemodynamics, renal function and glomerular histopathology using this model.

METHODS

A Medline search was performed to identify the relevant literature describing renal effects of antihypertensive drugs in models of hypertension and nephrosclerosis produced or exacerbated by NOS inhibition.

RESULTS

Existing data have indicated that most of these drug classes have produced dramatic renoprotective effects, structurally or functionally, on nephrosclerosis induced by prolonged NOS inhibition.

CONCLUSION

This review of experimental studies has provided strong evidence supporting the clinical benefits of antihypertensive drugs for hypertensive patients with renal impairment particularly those with endothelial dysfunction associated with NOS deficiency.

Impact of combined NO and PG blockade on rapid vasodilation in a forearm mild-to-moderate exercise transition in humans

We tested the hypothesis that nitric oxide (NO) and prostaglandins (PGs) contribute to the rapid vasodilation that accompanies a transition from mild to moderate exercise. Nine healthy volunteers (2 women and 7 men) lay supine with forearm at heart level. Subjects were instrumented for continuous brachial artery infusion of saline (control condition) or combined infusion of NG-nitro-l-arginine methyl ester (l-NAME) and ketorolac (drug condition) to inhibit NO synthase and cyclooxygenase, respectively. A step increase from 5 min of steady-state mild (5.4 kg) rhythmic, dynamic forearm handgrip exercise (1 s of contraction followed by 2 s of relaxation) to moderate (10.9 kg) exercise for 30 s was performed. Steady-state forearm blood flow (FBF; Doppler ultrasound) and forearm vascular conductance (FVC) were attenuated in drug compared with saline (control) treatment: FBF = 196.8 ± 30.8 vs. 281.4 ± 34.3 ml/min and FVC = 179.3 ± 29.4 vs. 277.8 ± 34.8 ml·min−1·100 mmHg−1 (both P < 0.01). FBF and FVC increased from steady state after release of the initial contraction at the higher workload in saline and drug conditions: ΔFBF = 72.4 ± 8.7 and 52.9 ± 7.8 ml/min, respectively, and ΔFVC = 66.3 ± 7.3 and 44.1 ± 7.0 ml·min−1·100 mmHg−1, respectively (all P < 0.05). The percent ΔFBF and ΔFVC were not different during saline infusion or combined inhibition of NO and PGs: ΔFBF = 27.2 ± 3.1 and 28.1 ± 3.8%, respectively ( P = 0.78) and ΔFVC = 25.7 ± 3.2 and 26.0 ± 4.0%, respectively ( P = 0.94). The data suggest that NO and vasodilatory PGs are not obligatory for rapid vasodilation at the onset of a step increase from mild- to moderate-intensity forearm exercise. Additional vasodilatory mechanisms not dependent on NO and PG release contribute to the immediate and early increase in blood flow in an exercise-to-exercise transition.

Vasoactive systems in L-NAME hypertension: the role of inducible nitric oxide synthase

OBJECTIVES

The contribution of the renin-angiotensin system (RAS) and the sympathetic nervous system (SNS) to blood pressure (BP) maintenance was evaluated in rats with N(omega)-nitro-L-arginine methyl ester (L-NAME) hypertension. Furthermore, we studied the extent of nitric oxide (NO) synthesis inhibition and the participation of remaining NO in the counterbalance of pressor systems, with a special reference to inducible nitric oxide synthase (iNOS).

METHODS

Wistar rats subjected to chronic L-NAME treatment (40 mg/kg per day for 4 weeks) were used. A consecutive blockade of RAS (captopril) and SNS (pentolinium) was followed by acute L-NAME injection. Dimethylguanidine or aminoguanidine were used to affect NO synthesis by iNOS.

RESULTS

L-NAME hypertensive rats had borderline augmentation of depressor response to captopril injection, but their BP fall after pentolinium was considerably enhanced compared with controls. Residual BP (recorded after simultaneous blockade of the RAS and the SNS) was elevated by 20-40% in hypertensive rats. Pronounced inhibition of NO synthase activity (50% reduction in the aorta and myocardium) was detected in L-NAME hypertensive rats in which the BP rise elicited by acute L-NAME injection was considerably attenuated (by 60-80%). In contrast, acute administration of dimethylguanidine [mixed endothelial NO synthase (eNOS)/iNOS inhibitor] to hypertensive rats induced a major BP rise similar to that caused by L-NAME injection in controls. Aminoguanidine (a selective iNOS inhibitor) caused a substantial BP rise in L-NAME hypertensive rats only.

CONCLUSION

The contribution of SNS to BP maintenance in L-NAME hypertension is more important than that of RAS. In L-NAME hypertensive rats the iNOS becomes a major source of hemodynamically important NO production, which is still insufficient to compensate prevailing vasoconstriction.

Effect of I1 Imidazoline Receptor Agonist, Moxonidine, in Nitric Oxide-Deficient Hypertension in Pregnant Rats

Decreased nitric oxide production has been reported in preeclampsia, which is also frequently associated with glucose intolerance. It was thus considered of interest to investigate the effects of moxonidine, a centrally acting antihypertensive drug that reduces insulin resistance, in a rat model of preeclampsia. Hypertension was induced in Wistar rats by dietary l-NNA (N(omega)-nitro-L-arginine, 0.063%, 31 mg/kg/d, days 13-19 of gestation) and, over the same period, moxonidine or vehicle was administered orally (2 mg/kg/d by gavage). On day 20, blood pressure was measured in the pentobarbital anesthetized animals, glucose tolerance was tested (2 g/kg glucose i.p.), and morphologic studies were conducted on the litter to determine the benefits with respect to fetal outcome. Hypertension was reduced with daily moxonidine treatment (P < 0.05). Basal plasma insulin and insulin/glucose index were decreased with moxonidine treatment evidencing improved insulin sensitivity in the control and l-NNA-treated pregnant rats (P < 0.05). After glucose challenge, plasma insulin increased in all the groups as expected and plasma insulin and insulin/glucose index were significantly higher in the l-NNA group than in the control, moxonidine, or l-NNA + moxonidine groups (P < 0.05 for time 60 minutes). Thus, moxonidine improved glucose tolerance in l-NNA-treated pregnant rats. Moreover, moxonidine treatment very effectively decreased the number of necroses (1 necrosis in 71 fetuses in the l-NNA + moxonidine group versus 15 necroses in 79 fetuses in the l-NNA group, P < 0.01). In conclusion, the 7-day treatment with moxonidine suppressed hypertension and reduced glucose intolerance and fetal necrosis, thus demonstrating the effectiveness of moxonidine in the preeclamptic model.

The origin of sympathetic outflow in heart failure: the roles of angiotensin II and nitric oxide

The regulation of sympathetic nerve activity in chronic heart failure (CHF) has been an area of renewed investigation. Understanding the central mechanisms that are responsible for sympatho-excitation in this disease state may help in reducing the deleterious effects of chronic sympatho-excitation. This review will summarize our understanding of abnormal reflex control of the circulation in CHF. The roles of the arterial baroreflex, the chemoreflex, the cardiac sympathetic afferent reflex and the cardiopulmonary reflex are discussed. New experimental techniques that allow genetic manipulation of substances such as nitric oxide synthase in discrete areas of the brain aid in clarifying the role of NO in the modulation of sympathetic tone in the CHF state. Lastly, clinical implications of this work are discussed.

Cyclooxygenase pathway is involved in the vascular reactivity and inhibition of the Na+, K+-ATPase activity in the tail artery from L-NAME-treated rats

L-NAME (LN) induces hypertension by blocking nitric oxide (NO) synthesis. It produces vascular hyperreactivity to phenylephrine (PHE) associated with a reduced vascular Na+, K+-ATPase activity. The aim of this work was to investigate whether products of the cyclooxygenase pathway are involved in alterations of vascular reactivity and Na+-pump activity in the tail artery from LN-induced hypertension rats. Four groups of rats were used: Control (CT, normotensive), LN (50 mg/kg/day, hypertensive), indomethacin (Indo-4 mg/kg/day, normotensive), and LN plus Indo (LN + Indo, partially prevented hypertension). All drugs were administered in drinking water during 7 days. In isolated rat tail vascular beds; the reactivity to PHE, acetylcholine (ACh), sodium nitroprusside (SNP), the functional activity of the Na+, K+-ATPase (K+-induced relaxation) and the modulation of PHE-induced vasoconstriction by constitutively available NO were evaluated. LN increased vascular sensitivity (pD2) and reactivity (Emax) to PHE and Indo blocked the effect of LN on Emax without changing pD2. Emax and pD2 values for ACh were reduced by LN and partially reverted by Indo. SNP-induced vasodilatation was similar in all groups. LN reduced the activity of Na+, K+-ATPase and Indo prevented LN effects. LN also abolished NO ability to modulate PHE-induced contractions. This effect was partially prevented by Indo suggesting that products from the cyclooxygenase pathway might reduce NO actions. Indo itself did not affect vascular reactivity to PHE, ACh or SNP or the Na+,K+-ATPase activity. Results suggested that products from cyclooxygenase pathway are involved in the genesis or maintenance of LN-induced hypertension, playing a role in the increased vascular reactivity, in the reduction of the endothelium-dependent relaxation and in the inhibition of the functional activity of the Na+, K+-ATPase.

Central and peripheral haemodynamic effects of L-NAME infusion in healthy volunteers

AIMS

To evaluate the effects of the intravenous administration of the nitric oxide synthesis inhibitor N(g)nitro-L-arginine methyl ester (L-NAME) in healthy volunteers.

METHODS

L-NAME (0.25, 0.5 and 0.75 mg/kg over 8 min) was infused in 13 healthy male volunteers. Finally, subjects were infused with either L- or D-arginine.

RESULTS

L-NAME resulted in dose-dependent falls in heart rate 60 bpm (55-64 bpm) to 49 bpm (46-52 bpm) (P<0.01) and increased mean arterial pressure 77.0 mmHg (73.2-80.8 mmHg) to 90.0 mmHg (87.1-92.8 mmHg) (P<0.01). The cardiac output was significantly reduced after each L-NAME infusion, and systemic vascular resistance increased linearly over the dosage range. Cardiac stroke volume was significantly reduced only following 0.75 mg/kg/min L-NAME: from 100 ml (91.3-108.7 ml) to 83 ml (74.7-91.4 ml); P<0.01. Forearm blood flow was unchanged at any dosage. L-arginine but not D-arginine infusion reversed the haemodynamic effects of L-NAME.

CONCLUSIONS

Contrasting with the profound dose-dependent effects of L-NAME had significant effects on central haemodynamics but no discernible effects on peripheral blood flow.

Exaggerated response to restraint stress in rats congenic for the chromosome 1 blood pressure quantitative trait locus

1. To understand the roles of a putative hypertension gene in the chromosome 1 quantitative trait locus (QTL) region, the response to restraint stress was studied in strains congenic for this QTL. 2. To establish congenic strains, the QTL region was introgressed from stroke-prone spontaneously hypertensive rats (SHRSP)/Izm to Wistar-Kyoto/Izm (WKY/Izm) rats by repeated backcrossing. Two congenic strains (WKYpch1.0 and WKYpch1.1) were established to cover the whole QTL region between D1Wox29 and D1Arb21 (approximately 40 cM) and a smaller region between D1Smu11 and D1Arb21 (approximately 10 cM), respectively. After telemetry probes were implanted, rats were exposed to restraint stress to investigate the blood pressure response. 3. Basal blood pressure measured by radiotelemetry differed significantly between WKY rats and WKYpch1.0 (103 +/- 10 and 116 +/- 4 mmHg, respectively; P = 0.002 by anova). When exposed to restraint stress, WKYpch1.0 showed a greater increase in blood pressre than did WKY rats. The exaggerated response in the WKYpch1.0 strain was abolished by chemical sympathectomy using guanethidine. The WKYpch1.1 rats did not differ significantly from WKY rats either in basal blood pressure or in the response to restraint stress. 4. In conclusion, a QTL for high blood pressure was successfully introgressed in the established congenic strain, WKYpch1.0. A gene (or genes) in the chromosome 1 QTL region modulates the cardiovascular responses to restraint stress in these congenic rats, probably through the sympathetic nervous system.

Chemical sympathectomy alters regulation of body weight during prolonged ICV leptin infusion

To assess the importance of the sympathetic nervous system in regulating body weight during prolonged leptin infusion, we evaluated food intake, body weight, and physical activity in conscious, unrestrained rats. Initial studies illustrated that prolonged intracerebroventricular (ICV) infusion of leptin enhanced substrate oxidation so that adipose tissue lipid stores were completely ablated, and muscle triglyceride and liver glycogen stores were depleted. After neonatal chemical sympathectomy, changes in weight and food intake were compared in groups of sympathectomized (SYM) and control (CON) adult animals during ICV infusion of leptin. CON animals lost 60 +/- 9 g over 10 days vs. 25 +/- 3 g in the SYM animals when food intake was matched between the two groups. Greater weight loss despite similar energy intake points to an important role of the sympathetic nervous system in stimulating energy expenditure during ICV leptin infusion by increasing the resting metabolic rate, since no differences in physical activity were observed between CON and SYM groups. In conclusion, activation of the SNS by leptin increases energy expenditure by augmenting the resting metabolic rate.

Effects of angiotensin blockade in the rostral ventrolateral medulla on maintenance of hypertension induced by chronic L-NAME treatment

The major aim of the present study was to evaluate the role of the angiotensin II receptors located within the rostral ventrolateral medulla (RVLM) in the maintenance of high blood pressure following chronic nitric oxide inhibition. Rats were treated orally with L-NAME (70 mg/kg/day) for 1 week. We inhibited the RVLM neurons using drugs such as Sarthran, Losartan and glycine in urethane-anesthetized rats (1.2 to 1.4 g/kg, i.v.). (1) Bilateral microinjection of Sarthran into the RVLM decreased BP in the hypertensive and normotensive groups, but the depressor effect of the drug was lower in hypertensive than in normotensive rats. (2) The decrease in BP in response to Sarthran in the RVLM compared with glycine was significantly smaller in the hypertensive group, but not in the normotensive group. (3) No change in MAP was observed in response to bilateral microinjection of the Ang AT(1) receptor antagonist Losartan into the RVLM in either group. These results suggest that (1) the endogenous angiotensin receptors of the RVLM are involved in the maintenance of high BP in L-NAME-treated animals and (2) the tonic action of Ang II is not dependent on AT(1) receptors within the RVLM. The possibility of an action via other Ang II receptors remains to be investigated.

Blood pressure variability in established L-NAME hypertension in rats

METHODS

Blood pressure variability was evaluated in conscious Wistar control rats and rats with established L-NAME hypertension (20 mg/kg per 24 h, 4 weeks).

RESULTS

Final systolic arterial pressure was 185+/-5 and 132+/-4 mm Hg in the Nomega-nitro-L-arginine methyl ester (L-NAME)-treated and control rats, respectively. The standard deviation of systolic arterial pressure in the L-NAME group was 70% greater than in the control rats, indicating a significant increase in the overall variability. Arterial pressure in the L-NAME rats exhibited aperiodical, abrupt rises and falls and data was grossly non-stationary. Blood pressure variability was therefore evaluated using Poincaré plot analysis. The variance of the difference (delta) between two successive values of systolic arterial pressure, determined for time intervals of 0.2 to 5 s (0.2 s increment), was always significantly higher in the L-NAME group compared with untreated animals. The variance of delta systolic arterial pressure increased with the time interval and plateaued for time intervals of 2.4 and 1.4 s in hypertensive and normotensive rats, respectively. These differences vanished when the sudden events oberved in L-NAME rats were omitted in the construction of Poincaré plots. Acute administration of prazosin (1 mg/kg), but not losartan (10 mg/kg) markedly reduced the variance of delta systolic arterial pressure in hypertensive rats.

CONCLUSIONS

Nitric oxide participates in the control of arterial pressure variability. The sympathetic nervous system seems to be a major determinant of the increased short-term variability of arterial pressure in this model.

Rostral ventrolateral medulla : A source of sympathetic activation in rats subjected to long-term treatment with L-NAME

The major aim of the present study was to evaluate the role of the rostral ventrolateral medulla (RVLM) in the maintenance of hypertension in rats subjected to long-term treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) (70 mg/kg orally for 1 week). We inhibited or stimulated RVLM neurons with the use of drugs such as glycine, L-glutamate, or kynurenic acid in urethane-anesthetized rats (1.2 to 1.4 g/kg IV). Bilateral microinjection of glycine (50 nmol, 100 nL) into the RVLM of hypertensive rats produced a decrease in mean arterial blood pressure (MAP) from 158+/-4 to 71+/-4 mm Hg (P<0.05), which was similar to the decrease produced by intravenous administration of hexamethonium. In normotensive rats, glycine microinjection reduced MAP from 106+/-4 to 60+/-3 mm Hg (P<0.05). Glutamate microinjection into the RVLM produced a significant increase in MAP in both hypertensive rats (from 157+/-3 to 201+/-6 mm Hg) and normotensive rats (from 105+/-5 to 148+/-9 mm Hg). No change in MAP was observed in response to kynurenic acid microinjection into the RVLM in either group. These results suggest that hypertension in response to long-term L-NAME treatment is dependent on an increase in central sympathetic drive, mediated by RVLM neurons. However, glutamatergic synapses within RVLM are probably not involved in this response.

A large blood pressure-raising effect of nitric oxide synthase inhibition in humans

In experimental animals, systemic administration of nitric oxide synthase (NOS) inhibitors causes large increases in blood pressure that are in part sympathetically mediated. The aim of this study was to determine the extent to which these conclusions can be extrapolated to humans. In healthy normotensive humans, we measured blood pressure in response to two NOS inhibitors, NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), the latter of which recently became available for use in humans. The major new findings are 3-fold. First, L-NAME produced robust increases in blood pressure that were more than 2 times larger than those previously reported in humans with L-NMMA and approximated those seen in experimental animals. L-NAME (4 mg/kg) raised mean arterial pressure by 24+/-2 mm Hg (n=27, P<0.001), whereas in subjects who received both inhibitors, a 12-fold higher dose of L-NMMA (50 mg/kg) raised mean arterial pressure by 15+/-2 mm Hg (n=4, P<0.05 vs L-NAME). Second, the L-NAME-induced increases in blood pressure were caused specifically by NOS inhibition because they were reversed by L-arginine (200 mg/kg, n=12) but not D-arginine (200 mg/kg, n=6) and because NG-nitro-D-arginine methyl ester (4 mg/kg, n=5) had no effect on blood pressure. Third, in humans, there is an important sympathetic component to the blood pressure-raising effect of NOS inhibition. alpha-Adrenergic blockade with phentolamine (0.2 mg/kg, n=9) attenuated the L-NAME-induced increase in blood pressure by 40% (P<0.05). From these data, we conclude that pharmacological inhibition of NOS causes large increases in blood pressure that are in part sympathetically mediated in humans as well as experimental animals.

Neural mechanisms in nitric-oxide-deficient hypertension

Nitric oxide is hypothesized to be an inhibitory modulator of central sympathetic nervous outflow, and deficient neuronal nitric oxide production to cause sympathetic overactivity, which then contributes to nitric-oxide-deficient hypertension. The biochemical and neuroanatomical basis for this concept revolves around nitric oxide modulation of glutamatergic neurotransmission within brainstem vasomotor centers. The functional consequence of neuronal nitric oxide in blood pressure regulation is, however, marked by an apparent conflict in the literature. On one hand, conscious animal studies using sympathetic blockade suggest a significant role for neuronal nitric oxide deficiency in the development of nitric-oxide-deficient hypertension, and on the other hand, there is evidence against such a role derived from 'knock-out' mice lacking nitric-oxide synthase 1, the major source of neuronal nitric oxide.