Age-related changes in muscle sympathetic nerve activity in essential hypertension

Muscle sympathetic nervous activity in depressed patients before and after treatment with sertraline

BACKGROUND

Sympathetic hyperactivity is one of the mechanisms involved in the increased cardiovascular risk associated with depression, and there is evidence that antidepressants decrease sympathetic activity.

OBJECTIVES

We tested the following two hypotheses: patients with major depressive disorder with high scores of depressive symptoms (HMDD) have augmented muscle sympathetic nervous system activity (MSNA) at rest and during mental stress compared with patients with major depressive disorder with low scores of depressive symptoms (LMDD) and controls; sertraline decreases MSNA in depressed patients.

METHODS

Ten HMDD, nine LMDD and 11 body weight-matched controls were studied. MSNA was directly measured from the peroneal nerve using microneurography for 3 min at rest and 4 min during the Stroop color word test. For the LMDD and HMDD groups, the tests were repeated after treatment with sertraline (103.3 +/- 40 mg).

RESULTS

Resting MSNA was significantly higher in the HMDD [29.1 bursts/min (SE 2.9)] compared with LMDD [19.9 (1.6)] and controls [22.2 (2.0)] groups (P = 0.026 and 0.046, respectively). There was a significant positive correlation between resting MSNA and severity of depression. MSNA increased significantly and similarly during stress in all the studied groups. Sertraline significantly decreased resting MSNA in the LMDD group and MSNA during mental stress in LMDD and HMDD groups. Sertraline significantly decreased resting heart rate and heart rate response to mental stress in the HMDD group.

CONCLUSION

Moderate-to-severe depression is associated with increased MSNA. Sertraline treatment reduces MSNA at rest and during mental challenge in depressed patients, which may have prognostic implications in this group.

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.

Is kidney ischemia the central mechanism in parallel activation of the renin and sympathetic system?

In chronic kidney disease simultaneous activation of the renin-angiotensin and sympathetic systems occurs. Kidney ischemia seems to play a key role in the pathogenesis. This review firstly summarizes experimental and clinical evidence in chronic kidney disease supporting this idea and addresses the possibility that this mechanism is also relevant in some other disease conditions.

Sympathetic neural mechanisms in human cardiovascular health and disease

The sympathetic nervous system plays a key role in regulating arterial blood pressure in humans. This review provides an overview of sympathetic neural control of the circulation and discusses the changes that occur in various disease states, including hypertension, heart failure, and obstructive sleep apnea. It focuses on measurements of sympathetic neural activity (SNA) obtained by microneurography, a technique that allows direct assessment of the electrical activity of sympathetic nerves in conscious human beings. Sympathetic neural activity is tightly linked to blood pressure via the baroreflex for each individual person. However, SNA can vary greatly among individuals and that variability is not related to resting blood pressure; that is, the blood pressure of a person with high SNA can be similar to that of a person with much lower SNA. In healthy normotensive persons, this finding appears to be related to a set of factors that balance the variability in SNA, including cardiac output and vascular adrenergic responsiveness. Measurements of SNA are very reproducible in a given person over a period of several months to a few years, but SNA increases progressively with healthy aging. Cardiovascular disease can be associated with substantial increases in SNA, as seen for example in patients with hypertension, obstructive sleep apnea, or heart failure. Obesity is also associated with an increase in SNA, but the increase in SNA among patients with obstructive sleep apnea appears to be independent of obesity per se. For several disease states, successful treatment is associated with both a decrease in sympathoexcitation and an improvement in prognosis. This finding points to an important link between altered sympathetic neural mechanisms and the fundamental processes of cardiovascular disease.

Sympathetic neural mechanisms in human cardiovascular health and disease

The sympathetic nervous system plays a key role in regulating arterial blood pressure in humans. This review provides an overview of sympathetic neural control of the circulation and discusses the changes that occur in various disease states, including hypertension, heart failure, and obstructive sleep apnea. It focuses on measurements of sympathetic neural activity (SNA) obtained by microneurography, a technique that allows direct assessment of the electrical activity of sympathetic nerves in conscious human beings. Sympathetic neural activity is tightly linked to blood pressure via the baroreflex for each individual person. However, SNA can vary greatly among individuals and that variability is not related to resting blood pressure; that is, the blood pressure of a person with high SNA can be similar to that of a person with much lower SNA. In healthy normotensive persons, this finding appears to be related to a set of factors that balance the variability in SNA, including cardiac output and vascular adrenergic responsiveness. Measurements of SNA are very reproducible in a given person over a period of several months to a few years, but SNA increases progressively with healthy aging. Cardiovascular disease can be associated with substantial increases in SNA, as seen for example in patients with hypertension, obstructive sleep apnea, or heart failure. Obesity is also associated with an increase in SNA, but the increase in SNA among patients with obstructive sleep apnea appears to be independent of obesity per se. For several disease states, successful treatment is associated with both a decrease in sympathoexcitation and an improvement in prognosis. This finding points to an important link between altered sympathetic neural mechanisms and the fundamental processes of cardiovascular disease.

Age- and fitness-related alterations in vascular sympathetic control

In the current study we explored (1) if there were differences in sympathetic activity and baroreflex function by age, sex, or physical activity status, (2) if any aspect of baroreflex function related to differences in resting sympathetic activity, and (3) if mechanical and/or neural baroreflex components related to differences in integrated baroreflex gain. Electrocardiogram, blood pressure, carotid diameter and muscle sympathetic nerve activity were recorded continuously at rest and during sequential bolus injections of sodium nitroprusside and phenylephrine in 22 young, 21 older sedentary and 10 older trained individuals. Analyses of co-variance were used to examine age, sex and training status differences and to explore the explanatory power of integrated baroreflex gain and its mechanical and neural components. Training status and sex influenced neither resting sympathetic outflow nor sympathetic baroreflex gain components. Older subjects had a smaller mechanical component and a strong tendency towards a greater neural component of the sympathetic baroreflex during both pressure falls and pressure rises. Opposing age-related changes in mechanical and neural components resulted in a smaller integrated gain during pressure falls, but a greater integrated gain during pressure rises in older subjects. Thus, in older individuals, compromised sympathetic activation to pressure falls was owing to the stiffening of barosensory vessels, whereas the more sensitive sympathoinhibition to pressure rise was due to an increased neural control. Enhanced neural control with age, however, did not contribute the increased resting sympathetic outflow, which indicates that these two changes are probably driven by distinct neural mechanisms.

Limb venous tone and responsiveness in hypertensive humans

Hypertensive (HTN) animal models demonstrate lower venous compliance as well as increased venous tone and responsiveness compared with normotensive (NTN) controls. However, the extent to which findings in experimental animals can be extended to humans is unknown. Forearm and calf venous compliance were quantified in 9 NTN (23 +/- 1 yr) and 9 HTN (24 +/- 1 yr) men at baseline, after administration of nitroglycerin (NTG), during a cold pressor test (CP), and post-handgrip exercise ischemia (PEI). Individual pressure-volume relationships from a cuff deflation protocol (1 mmHg/s) were modeled with a quadratic regression. Regression parameters beta(1) and beta(2) were used to calculate compliance. A one-way ANOVA was used to compare the beta parameters and a repeated-measures ANOVA was used to compare volumes across all pressures (between groups at baseline and within groups during perturbations). Limb venous compliance was similar between groups (forearm: NTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00097 +/- 0.0001, HTN beta(1) = 0.10 +/- 0.01 and beta(2) = -0.00088 +/- 0.0001; calf: NTN beta(1) = 0.12 +/- 0.01 and beta(2) = -0.00102 +/- 0.0001, HTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00090 +/- 0.0001). However, at baseline, volume across all pressures (i.e., capacitance) was lower in the forearm (P < or = 0.01) and tended to be lower in the calf (P = 0.08) in HTN subjects. Venous compliance was not altered by any perturbation in either group. Forearm volume was increased during NTG in HTN subjects only. While venous compliance was similar between NTN and HTN adults, HTN adults have lower forearm venous capacitance (volume) which is increased with NTG. These data suggest that young HTN adults may have augmented venous smooth muscle tone compared with NTN controls.

A sympathetic view of the sympathetic nervous system and human blood pressure regulation

New ideas about the relative importance of the autonomic nervous system (and especially its sympathetic arm) in long-term blood pressure regulation are emerging. It is well known that mean arterial blood pressure is normally regulated in a fairly narrow range at rest and that blood pressure is also able to rise and fall 'appropriately' to meet the demands of various forms of mental, emotional and physical stress. By contrast, blood pressure varies widely when the autonomic nervous system is absent or when key mechanisms that govern it are destroyed. However, 24 h mean arterial pressure is still surprisingly normal under these conditions. Thus, the dominant idea has been that the kidney is the main long-term regulator of blood pressure and the autonomic nervous system is important in short-term regulation. However, this 'renocentric' scheme can be challenged by observations in humans showing that there is a high degree of individual variability in elements of the autonomic nervous system. Along these lines, the level of sympathetic outflow, the adrenergic responsiveness of blood vessels and individual haemodynamic patterns appear to exist in a complex, but appropriate, balance in normotension. Furthermore, evidence from animals and humans has now clearly shown that the sympathetic nervous system can play an important role in longer term blood pressure regulation in both normotension and hypertension. Finally, humans with high baseline sympathetic traffic might be at increased risk for hypertension if the 'balance' among factors deteriorates or is lost. In this context, the goal of this review is to encourage a comprehensive rethinking of the complexities related to long-term blood pressure regulation in humans and promote finer appreciation of physiological relationships among the autonomic nervous system, vascular function, ageing, metabolism and blood pressure.

Attenuation of autonomic nervous system functions in hypertensive patients at rest and during orthostatic stimulation

The effect of age on autonomic nervous system was assessed at rest and while standing using systolic blood pressure (SBP) and diastolic blood pressure (DBP), heart rate, and power spectral analysis of the time duration between 2 consecutive R waves of an electrocardiogram (RR) interval variability, as well as on plasma norepinephrine and epinephrine levels in mild to moderate hypertensive patients (DBP, 90-110 mm Hg). Patients younger than 60 years (n=57) and older than 60 years (n=32), were evaluated after a 3- to 4-week placebo period. Plasma catecholamines were measured in the supine position at rest and after 10 minutes of standing. Power spectral analysis of the RR interval variability was performed in each condition using the high-frequency (HF) band (0.15-0.4 Hz) as an index of parasympathetic activity and the low-frequency (LF) band (0.05-0.15 Hz) and LF-HF ratio to estimate sympathetic activity. The total power was calculated as the sum of LF and HF power. supine SBP was significantly higher in older patients (P<.05). SBP and DBP increased significantly only in younger patients during standing (P<.05), while the changes were smaller and nonsignificantly lower in older patients. HR was similar in both groups at rest and increased similarly during standing. Norepinephrine and epinephrine levels were similar at rest and increased similarly in both groups of patients during standing. At rest, lower LF and HF components were observed in older patients. The LF component increased less and the HF component decreased less in older patients during standing. A lower sympathetic and parasympathetic basal cardiac tone was observed at rest in older hypertensive patients. Moreover, reduced hemodynamic and sympathetic responses to standing as assessed by SBP, DBP, and the LF component of HR variability were observed in older hypertensives in the presence of a normal catecholamine response. These observations could reflect a decreased sensitivity of cardiac beta-adrenoceptors with aging.

Differing pattern of sympathoexcitation in normal-weight and obesity-related hypertension

Hypertension in normal-weight and obese individuals is characterized by activation of the sympathetic nervous system. Measurement of spillover of the sympathetic transmitter, norepinephrine, to plasma indicates that the regional pattern of sympathetic activation in the 2 "variants" of essential hypertension differs, excluding the heart in obesity-related hypertension. Whether sympathetic nerve firing characteristics also differ is unknown. We studied multiunit and single fiber sympathetic nerve firing properties in patients with normal-weight hypertension and obesity-related hypertension, comparing these with nerve characteristics in normal-weight and obese people with normal blood pressure. Both normal-weight hypertensive (n=10) and obese hypertensive (n=14) patients had increased total multiunit muscle sympathetic nerve activity compared with the normal-weight (n=11) and obese (n=11) people with normal blood pressure (65+/-4 versus 47+/-6 bursts per 100 heartbeats, P<0.01 in the normal-weight groups and 68+/-4 versus 53+/-3 bursts per 100 beats, P<0.01 in the obese groups). Sympathetic activation in normal-weight hypertension was characterized by increased firing rate of single vasoconstrictor fibers (70+/-8 versus 28+/-3 spikes per 100 beats; P<0.001), increased firing probability per heartbeat (39+/-3% versus 20+/-3%; P<0.001), and higher incidence of multiple spikes per heartbeat (30+/-4% versus 17+/-4%; P<0.05). Sympathetic activation in obesity-related hypertension differed, involving recruitment of previously silent fibers, which fired at a normal rate. The pattern of sympathetic activation in normal-weight and obesity-related hypertension differs in terms of both the firing characteristics of individual sympathetic fibers and the sympathetic outflows involved. The underlying central nervous system mechanism and the adverse consequences of the 2 modes of sympathetic activation may differ.

Role of long-term potentiation of sympathetic ganglia (gLTP) in hypertension

Ganglionic long-term potentiation (gLTP) is an activity-dependent sustained increase in the synaptic efficacy of the nicotinic pathway that has been demonstrated in autonomic ganglia. Sustained enhancement in ganglionic transmission as in chronic mental stress may affect the activity of autonomic functions, including blood pressure and heart rate. An increase in sympathetic activity associated with psychosocial stress and stress-prone conditions such as obesity and aging could result in in vivo expression of gLTP leading to hypertension of a neural origin. Recent reports indicated that the prevention of the expression of gLTP in animal models of hypertension prevented or reduced high blood pressure. Although stress-induced hypertension normalizes within a few days of stress relief, prolonged mild-moderate hypertension may contribute to atherosclerotic cardiovascular diseases. The relation between hypertension and enhanced ganglionic transmission as a result of in vivo expression of gLTP is discussed in this review.

Selective cardiac neuroadrenergic abnormalities in hypertensive patients with left ventricular hypertrophy

BACKGROUND

Increased sympathetic drive to the heart might contribute to the development and progression of myocardial damage in hypertensive patients (HTs). This study assessed the possible presence of abnormalities in myocardial uptake of (123)I-metaiodobenzylguanidine (MIBG), a marker of sympathetic activity, in HTs with left ventricular hypertrophy (LVH).

METHODS

Eleven HTs with LVH and 10 matched normotensive controls underwent clinical and laboratory examination, as well as LVH determination by echocardiography. The presence of myocardial ischemia was ruled out by exercise stress testing. Global and regional myocardial uptake of (123)I-MIBG was determined in both groups using planar and single proton emission tomography scintigraphy. In addition, thallium-201 (Tl-201) myocardial scintigraphy was performed in HTs. The heart/mediastinum (H/M) ratio on planar (123)I-MIBG images at different time points was compared between HTs and controls. Moreover, regional cardiac uptake of (123)I-MIBG was compared between groups and, within the HTs group, with regional Tl-201 uptake.

RESULTS

At all study times, the H/M ratio was lower in HTs than in controls (all p <0.05). A significant reduction in (123)I-MIBG uptake in the mid-inferolateral and mid-inferior segments was observed in HTs compared to controls. Also, a significant reduction in (123)I-MIBG uptake compared to Tl-210 uptake was observed in non-septal segments of HTs.

CONCLUSIONS

Cardiac abnormalities in global and regional uptake of (123)I-MIBG, as well as impaired (123)I-MIBG compared to Tl-201 uptake, are present in HTs with LVH. Given the effect of sympathetic nervous system on the heart, these abnormalities might play a role in hypertension-related cardiac damage.

Sympathetic neural control of integrated cardiovascular function: Insights from measurement of human sympathetic nerve activity

Sympathetic neural control of cardiovascular function is essential for normal regulation of blood pressure and tissue perfusion. In the present review we discuss sympathetic neural mechanisms in human cardiovascular physiology and pathophysiology, with a focus on evidence from direct recordings of sympathetic nerve activity using microneurography. Measurements of sympathetic nerve activity to skeletal muscle have provided extensive information regarding reflex control of blood pressure and blood flow in conditions ranging from rest to postural changes, exercise, and mental stress in populations ranging from healthy controls to patients with hypertension and heart failure. Measurements of skin sympathetic nerve activity have also provided important insights into neural control, but are often more difficult to interpret since the activity contains several types of nerve impulses with different functions. Although most studies have focused on group mean differences, we provide evidence that individual variability in sympathetic nerve activity is important to the ultimate understanding of these integrated physiological mechanisms.

The effect of gender on the sympathetic nerve hyperactivity of essential hypertension

We planned to determine whether or not there is a difference in the level of muscle sympathetic nerve activity (MSNA) between hypertensive women and hypertensive men. Sympathetic activation of essential hypertension (EHT) has been associated with increased cardiovascular events, which are known to be less likely to occur in women than in men. Normal women have been reported to have less sympathetic nerve activity than men, but no reported data are available regarding gender differences in sympathetic activity in hypertensive subjects. We examined 36 patients with untreated and uncomplicated EHT comprising 18 women and 18 men, and 36 normal controls comprising 18 women and 18 men. MSNA was quantified as the mean frequency of single units and as multiunit bursts using the technique of microneurography. The hypertensive groups had greater sympathetic nerve activity than the control groups. Female hypertensives had lower (P<0.001) single unit hyperactivity (56+/-1.7 impulses/100 cardiac beats) than male hypertensives (72+/-1.7 impulses/100 cardiac beats). Normotensive females had lower (P<0.01) single unit activity (42+/-3.6 impulses/100 cardiac beats) than normotensive males (56+/-4.6 impulses/100 cardiac beats). Similar results were obtained for the frequency of multiunit burst activity. Hypertension in women is associated with a lower level of central sympathetic hyperactivity than in men. It is suggested that this may at least partly explain the observed lower hypertension-related cardiovascular events in women than in men. In addition, the findings may have implications for gender-specific management of hypertension.

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

The norepinephrine transporter in physiology and disease

The norepinephrine transporter (NET) terminates noradrenergic signalling by rapid re-uptake of neuronally released norepinephrine (NE) into presynaptic terminals. NET exerts a fine regulated control over NE-mediated behavioural and physiological effects including mood, depression, feeding behaviour, cognition, regulation of blood pressure and heart rate. NET is a target of several drugs which are therapeutically used in the treatment or diagnosis of disorders among which depression, attention-deficit hyperactivity disorder and feeding disturbances are the most common. Individual genetic variations in the gene encoding the human NET (hNET), located at chromosome 16q12.2, may contribute to the pathogenesis of those diseases. An increasing number of studies concerning the identification of single nucleotide polymorphisms in the hNET gene and their potential association with disease as well as the functional investigation of naturally occurring or induced amino acid variations in hNET have contributed to a better understanding of NET function, regulation and genetic contribution to disorders. This review will reflect the current knowledge in the field of NET from its initial discovery until now.

Regulation of sympathetic nerve traffic to skeletal muscle in resting humans

An overview is given of microneurographic studies of resting vasoconstrictor traffic in human muscle nerves (muscle sympathetic nerve activity = MSNA). In multiunit recordings, the activity consists of synchronized bursts of vasoconstrictor impulses, the outflow of which is under potent arterial baroreflex control. In agreement with this, the bursts always display cardiac rhythmicity and occur during temporary reductions of blood pressure. Burst occurrence shows a close inverse correlation to variations of diastolic blood pressure whereas the correlation to the strength of the bursts is weak or absent, suggesting that the mechanisms controlling the two parameters are not identical. These dynamic characteristics are similar in all subjects despite large, reproducible, interindividual differences in number of bursts. Such interindividual differences probably have a genetic origin, and since discharge frequencies in single vasoconstrictor fibers are similar in subjects with few and many bursts, the differences in multiunit activity are likely to be due to a higher number of active fibers in subjects with many bursts. The interindividual differences in multiunit activity are not associated with differences in resting blood pressure levels. Recent studies have revealed (a) an inverse relationship between resting levels of cardiac output and MSNA and (b) evidence of reduced vascular responsiveness to noradrenaline in subject with many sympathetic bursts at rest. These findings suggest that the vasoconstriction induced by the sympathetic impulses is balanced or reduced by these factors, which thereby contribute to the poor relationship between the mean number of sympathetic bursts and the blood pressure level.

Age and ethnicity differences in short-term heart-rate variability

OBJECTIVE

Hypertension is more frequent and more severe in older individuals and in African Americans. Differences in autonomic nervous system activity might contribute to these differences. Autonomic effects on the heart can be studied noninvasively through analysis of heart rate variability (HRV). We examined the effects of age and ethnicity on HRV.

METHODS

We studied 135 subjects (57 African Americans and 78 Caucasian Americans), aged 23 to 54 years. Using their surface electrocardiogram (ECG) data, we calculated the HRV indices with spectral analyses. High frequency (HF) power was used to index parasympathetic activity, whereas the ratio of low to high frequency power (LF/HF) was used to index sympathovagal balance.

RESULTS

Three HRV indices (HF, LF power, and LF/HF) were significantly related to age in Caucasian Americans but not in African Americans. The effect of age, ethnicity, and the age-by-ethnicity interaction on HF and LF power was significant, even after controlling for gender, body mass index, and blood pressure.

CONCLUSIONS

Young African Americans manifested a pattern of HRV response similarly to older Caucasian Americans. These results suggest that young African American individuals might show signs of premature aging in their autonomic nervous system.

Baroreceptors, baroreceptor unloading, and the long-term control of blood pressure

Whether arterial baroreceptors play a role in setting the long-term level of mean arterial pressure (MAP) has been debated for more than 75 years. Because baroreceptor input is reciprocally related to efferent sympathetic nerve activity (SNA), it is obvious that baroreceptor unloading would cause an increase in MAP. Experimental proof of concept is evident acutely after baroreceptor denervation. Chronically, however, baroreceptor denervation is associated with highly variable changes in MAP but not sustained hypertension. The ability of baroreceptors to buffer imposed increases in MAP appears limited by a process termed "resetting," in which the threshold to fire shifts in the direction of the pressure change and if the pressure elevation is maintained, it leads to a rightward shift in the relationship between baroreceptor firing and MAP. The most common hypothesis linking baroreceptors to changes in MAP proposes that reduced vascular distensibility in baroreceptive areas would cause reduced firing at the same pulsatile pressure and, thus, reflexively increase SNA. This review focuses on effects of baroreceptor denervation in the regulation of MAP in human subjects compared with animal studies; the relationship between vascular compliance, MAP, and baroreceptor resetting; and, finally, the effect of chronic baroreceptor unloading on the regulation of MAP.

Muscle sympathetic nerve activity during isometric exercise in patients with cerebrovascular accidents

OBJECTIVES

To define isometric exercise-induced pressor responses in patients with cerebrovascular accidents (CVAs) and to assess potential cardiovascular and sympathetic nervous system abnormalities during isometric exercise in CVA.

DESIGN

Nonrandomized study.

SETTING

University laboratory setting.

PARTICIPANTS

Eight men with CVA who had documented damage of subcortical structures and 8 sex-matched controls.

INTERVENTIONS

A 2-minute sustained contraction of elbow flexor muscles in the unaffected side at 35% of maximal voluntary contraction (MVC; isometric exercise).

MAIN OUTCOME MEASURES

Heart rate, arterial blood pressure, and muscle sympathetic nerve activity (MSNA), recorded from the peroneal nerve on the affected side.

RESULTS

The percent changes in total MSNA, heart rate, and mean blood pressure in patients with CVA increased during isometric exercise but were attenuated compared with the controls. Total MSNA (mean burst amplitude per minute times burst rate) increased significantly in CVA and control subjects during isometric exercise by 18.7%+/-6.3% and 95.8%+/-25.2%, respectively.

CONCLUSIONS

The attenuated pressor responses during isometric exercise in subjects with CVA relative to the controls indicated damage to subcortical structures; such damage lowered sympathetic nervous response to isometric exercise. Our findings suggest that isometric exercise at 35% of MVC does not put patients with CVA at risk for serious tachycardia or hypertension.

Persistent sympathetic activation during chronic antihypertensive therapy: a potential mechanism for long term morbidity?

Previous studies have demonstrated that antihypertensive treatment resets baroreflex control of heart rate (HR) and increases cardiac vagal baroreflex sensitivity. However, it is uncertain whether baroreflex control of muscle sympathetic nerve activity (MSNA) also resets after treatment. We tested the hypothesis that chronic antihypertensive therapy alters baroreflex regulation of MSNA in patients with untreated moderate hypertension. Seven newly diagnosed patients with systolic blood pressure (BP) of 159+/-5 mm Hg (mean+/-SE) and diastolic BP of 103+/-4 mm Hg were studied before and after 1 to 2 weeks ' and 3 months (chronic) of antihypertensive treatment with losartan-hydrochlorothiazide (Hyzaar). MSNA and hemodynamics were measured supine, during a Valsalva maneuver (VM), and at 70 degrees head-up tilt (HUT) for 10 minutes. Data were compared with those obtained in 7 age-matched healthy controls. We found that Hyzaar lowered mean BP acutely and chronically by 20+/-4 and 23+/-3 mm Hg (both P<0.01) but did not change HR. Supine MSNA increased by 43+/-11% and 34+/-11% after acute and chronic treatment (both P<0.01). However, MSNA responses to VM and HUT did not differ after treatment compared with before treatment, indicating unchanged reflex control. These data indicate that sympathetic neural activity was augmented substantially by antihypertensive treatment with Hyzaar, consistent with an ongoing baroreflex unloading, and did not return to baseline or "reset" after 3 months of therapy. We speculate that persistent and marked sympathetic activation by the baroreflex may be a potential mechanism for hypertension that is refractory to antihypertensive therapy and may provide a target mechanism for persistent morbidity despite adequate BP control.

Collateral damage: cardiovascular consequences of chronic sympathetic activation with human aging

Adult aging in humans is associated with marked and sustained increases in sympathetic nervous system (SNS) activity to several peripheral tissues, including the heart, the gut-liver circulation, and skeletal muscle. This chronic activation of the peripheral SNS likely is, at least in part, a primary response of the central nervous system to stimulate thermogenesis to prevent further fat storage in the face of increasing adiposity with aging. However, as has been proposed in obesity hypertension, this tonic activation of the peripheral SNS has a number of adverse secondary cardiovascular consequences. These include chronic reductions in leg blood flow and vascular conductance, increased tonic support of arterial blood pressure, reduced limb and systemic α-adrenergic vasoconstrictor responsiveness, impaired baroreflex buffering, large conduit artery hypertrophy, and decreased vascular and cardiac responsiveness to β-adrenergic stimulation. These effects of chronic age-associated SNS activation on the structure and function of the cardiovascular system, in turn, may have important implications for the maintenance of physiological function and homeostasis, as well as the risk of developing clinical cardiovascular and metabolic diseases in middle-aged and older adults.

Age-related decline in autonomic control of blood pressure: implications for the pharmacological management of hypertension in the elderly

Autonomic control of blood pressure appears to decline with age giving rise to an increased risk of orthostatic hypotension and major hypotensive reactions to antihypertensive drugs. In the past few years, many workers have assessed autonomic function in the elderly and sometimes found controversial results. Baroreflex sensitivity, as measured by the steepness of the heart rate/mean pressure curve, decreases with age. However, this phenomenon does not correlate well with orthostatic impairment. Sympathetic dysfunction might be more responsible for syncopal symptoms in the elderly, a finding supported by the fact that elderly with orthostatic symptoms never collapse within a few seconds, but do so after 1 or more minutes of standing. However, the results of sympathetic function testing in the elderly indicate that sympathetic function in most elderly is not impaired and that sympathetic activity, as measured by circulating levels of catecholamines, is usually increased rather than decreased. In various populations with increased sympathetic activity, but not in the elderly, beta-adrenoceptor antagonists (beta-blockers) have been demonstrated to cause pressor effects, presumably due to alpha-adrenoceptor-mediated vasoconstriction unopposed by beta-receptor-mediated vasodilation. In the past year, large studies have been completed indicating that the same is true for the elderly, and that the depressor effect on pulse pressure upon standing in this category of patients can be offset and turned into a pressor effect by long-term beta-blocker treatment. This phenomenon could not be demonstrated with non-beta-blocker antihypertensive drugs, including ACE inhibitors, calcium channel antagonists, diuretics and angiotensin II receptor antagonists. In elderly patients beta-blockers may, therefore, be the most appropriate antihypertensive agents as they protect the elderly from orthostatic impairment.

Impairment of thermoregulatory control of skin sympathetic nerve traffic in the elderly

BACKGROUND

Human aging is characterized by a marked increase in muscle sympathetic nerve traffic (MSNA). No information exists, however, on the effects of aging on skin sympathetic nerve traffic (SSNA) and on its reflex modulation by thermoregulatory mechanisms.

METHODS AND RESULTS

In 13 young, 11 middle-aged, and 12 elderly healthy subjects, we measured arterial blood pressure (Finapres), skin temperature (thermocouples), and resting MSNA and SSNA (microneurography). Measurements also included the SSNA responses to (1) an acute increase and reduction (+/-8 degrees C) in room temperature, each lasting 45 minutes and (2) an acoustic stimulus capable to trigger an emotional arousal. Although resting MSNA was progressively and significantly (P<0.05) increased from young to middle-aged and elderly groups, SSNA was significantly (P<0.05) reduced in the latter compared with the former 2 groups. Cold exposure induced a SSNA increase that was significantly (P<0.01) smaller in the elderly than in young and middle-aged subjects. Conversely, heat exposure induced a SSNA reduction that was significantly (P<0.05) smaller in elderly than in young and middle-aged subjects. Compared with SSNA in young individuals, the SSNA change from cold to warm temperature was reduced by 61% in the elderly group. This was not the case, however, for the SSNA responses to the arousal stimulus, which were superimposable in the 3 groups.

CONCLUSIONS

These data provide the first demonstration of a dichotomy in the MSNA and SSNA responses to aging. They also show that aging markedly impairs thermoregulatory control of SSNA and that this impairment might participate at the age-related SSNA decrease.

Host and environmental determinants of heart rate and heart rate variability in four European populations

OBJECTIVE

In a population-based sample of nuclear families recruited in the framework of the European Project on Genes in Hypertension (EPOGH), we investigated the association between heart rate (HR) and its variability (HRV), and gender, age, posture, breathing frequency, body mass index, systolic blood pressure, family history of hypertension and various lifestyle factors, such as smoking, alcohol and coffee consumption and physical activity.

METHODS

RR interval and respiration were registered in the supine and standing positions (15 min each) in 1208 subjects in Bucharest (Romania, n= 267), Cracow (Poland, n= 323), Mirano (Italy, n= 323) and Novosibirsk (Russian Federation, n= 295). After exclusion of 199 participants on antihypertensive treatment and/or patients with diabetes mellitus (n= 40) or myocardial infarction (n= 4), 993 subjects were eligible for analysis. We evaluated 858 participants with high-quality recordings. Using fast Fourier transform, we decomposed HRV into low-frequency (LF: 0.04-0.15 Hz) and high-frequency (HF: 0.15-0.40 Hz) components, which were expressed in normalized units.

RESULTS

Mean values were 35.3 years for age, 24.3 kg/m for body mass index (BMI) and 121.0/77.2 mmHg for blood pressure. The group included 462 (53.8%) women. Across four centres, HR and HRV were similarly and independently associated with gender, age and postural position (P <0.001). In the supine position, HR was higher in women than men (67.2 versus 63.7 bpm). Men had higher normalized LF power than women (48.8 versus 41.5), but lower HF power (40.6 versus 47.4). The normalized HF power decreased with age (r = -0.43), whereas LF power increased (r = 0.32). On standing, HR increased (83.3 versus 65.6 bpm), normalized HF power declined (19.2 versus 44.3) and LF power increased (67.4 versus 44.9). The independent effects of respiration frequency, systolic blood pressure, family history of hypertension, body mass index and lifestyle factors on HRV differed between populations, and explained no more than 8% of the total variance.

CONCLUSIONS

Across four European populations, gender, age and posture were consistent and independent correlates of HR and HRV. Lifestyle seems to have small but varying influences on HR and/or HRV, probably depending on the environmental and cultural background of the population under study.

The activity of single vasoconstrictor nerve units in hypertension

AIM

It has long been established from controlled experiments in anaesthetized animals that it is more accurate to quantify the mean frequency of efferent sympathetic nerve activity from single unit than from multi-unit bursts recordings. More recently, sympathetic nerve hyperactivity has been reported in patients with essential hypertension (EHT) when using microneurographic recordings from peripheral efferent nerves. This review will focus on the mean frequency of single unit of muscle sympathetic nerve activity (s-MSNA) in relation to that of multi-unit bursts (MSNA) as obtained by microneurography in EHT.

RESULTS

We have shown that the resting levels of s-MSNA and MSNA were increased in uncomplicated EHT, white coat hypertension and in EHT complicated by left ventricular hypertrophy. There was a relatively greater increase in s-MSNA than in MSNA in mild hypertension and in complicated EHT. We also found that both s-MSNA and MSNA were increased to a similar extent in conditions known to affect reflexes emanating from the heart and influencing sympathetic output, such as acute myocardial infarction. In other preliminary studies, the increase of s-MSNA in response to the discomfort of cold pressor test was greater than that of MSNA and this difference was abolished by the centrally sympatholytic agent moxonidine.

CONCLUSION

These results are consistent with the hypothesis that an increase in the mean frequency of central sympathetic discharge to the periphery (greater s-MSNA than MSNA) is involved in the pathogenesis and complications of EHT. Target organ damage may in turn lead to an increase in overall sympathetic output (excessive MSNA increase) through the operation of peripheral reflex mechanisms.

Sympathetic nerve activity in metabolic control--some basic concepts

A role for the sympathetic nervous system in hypertension has been looked for in relation to the 'metabolic syndrome' with associations between body weight, insulin sensitivity and hypertension. By use of microneurography human sympathetic responses to hypoglycaemia, normoglycaemic hyperinsulinaemia and food intake have been studied. A strong but differentiated influence of insulin-induced hypoglycaemia comprises increase in muscle sympathetic nerve activity (MSNA) and the sudomotor part of skin sympathetic nerve activity (SSNA), whereas vasoconstrictor SSNA is inhibited. Responses to infusion of 2-deoxy-D-glucose are identical, suggesting central nervous system glucopenia and not insulin to be the causative factor. Insulin infusion during normoglycaemia evokes a moderate increase in MSNA; SSNA and blood pressure does not change. After glucose ingestion MSNA displays a sustained increase, which is only partly elicited by insulin. A significant albeit weaker increase occurs after pure protein or fat meals, and after glucose ingestion in C-peptide-negative diabetic patients, with no insulin secretion. In healthy elderly people the MSNA response to food intake is weak, because of a high outflow already at rest; this is suggested to explain postprandial hypotension in the elderly, a paradoxical mechanism behind clinical autonomic failure. A pathophysiological role of MSNA in the metabolic syndrome with hypertension has been speculated. An association between obesity and elevated level of MSNA at rest is established; observed relationships to chronic insulin levels and hypertension are less unanimous. The adipose tissue regulating hormone leptin has become one focus of interest in ongoing attempts to elucidate a possible role of the human sympathetic nervous system in the 'metabolic syndrome' and hypertension.

Sympathetic response to ventricular extrasystolic beats in hypertension and heart failure

Provoked premature ventricular contractions (PVCs) evoke, in concomitance with an early and late blood pressure fall and overshoot, an early sympathoexcitation and a later period of sympathoinhibition, respectively. The present study was designed to examine whether in healthy subjects this is the case for spontaneous PVCs. Because of their pathophysiological relevance for arrhythmogenesis, it was also designed to determine whether the sympathetic responses are different from those seen in essential hypertension and congestive heart failure. In 14 untreated mild essential hypertensives (EH; age, 53.8+/-2.6 years; mean+/-SEM), 20 untreated congestive heart failure patients (CHF; age, 56.7+/-2.5 years; New York Heart Association class, II or III), and 16 age-matched healthy subjects (control) in Lown class <II, we evaluated the blood pressure (Finapres), heart rate (ECG), and muscle sympathetic nerve traffic (MSNA; by microneurography) responses to isolated monofocal PVCs. MSNA, quantified as bursts/100 heart beats, was significantly increased in EH (57.8+/-3.8, P<0.05) and CHF patients (77.7+/-4.0, P<0.01) compared with controls (44.6+/-4.4). In controls, the PVC-induced blood pressure fall and overshoot were accompanied by a sympathoexcitation (144.2+/-14%), followed by a period of sympathoinhibition (average duration, 12043+/-985 ms). The responses were similar in EH but not in CHF, in whom the magnitude of the sympathoexcitation and particularly the duration of the subsequent sympathoinhibition were strikingly reduced (average reduction, -46.1 and -72.8%, respectively). The most important factor accounting for this reduction appeared to be an altered baroreflex response to the PVC-induced BP changes. These data demonstrate that the MSNA responses to spontaneous PVCs are similar in controls and EH but markedly impaired in CHF, presumably because of the baroreflex alteration. This may represent an important factor for the genesis of the life-threatening ventricular arrhythmias that characterize CHF.

The influence of aging on the human sympathetic nervous system and brain norepinephrine turnover

Investigating aging effects on the sympathetic nervous system and ascertaining underlying central nervous system (CNS) mechanisms mediating sympathetic stimulation is clinically pertinent because of the possible interconnection of cardiovascular disease development with age-dependent sympathetic nervous changes. Because of previous evidence linking human CNS neuronal noradrenergic function and sympathetic activity, we investigated the influence of aging on brain norepinephrine turnover in 22 healthy men aged 20-30 yr and 16 healthy men aged 60-75 yr by measuring the internal jugular venous overflow of norepinephrine and its lipophilic metabolites. Sympathoneural and adrenal medullary function was also studied, using plasma catecholamine isotope dilution methodology and regional central venous sampling. In the older men there was increased norepinephrine turnover in suprabulbar subcortical brain regions, 317 +/- 50 ng/min compared with 107 +/- 18 ng/min in younger men. A differentiated sympathetic nervous activation was also present in older men. Overall, levels of both cardiac and hepatomesenteric norepinephrine spillover were directly correlated with subcortical norepinephrine turnover. These findings suggest that in sympathetic nervous activation accompanying aging, as has previously been demonstrated with the sympathetic nervous stimulation in human hypertension and heart failure, there is an underlying sympathoexcitatory influence of noradrenergic projections to suprabulbar subcortical regions.

Hypertensive left ventricular hypertrophy: relation to peripheral sympathetic drive

OBJECTIVES

This study was designed to examine whether the occurrence of left ventricular hypertrophy (LVH) in moderate to severe essential hypertension (EHT) was associated with alteration in peripheral sympathetic drive.

BACKGROUND

In hypertension, LVH is an independent predictor of increased morbidity and mortality. The reported mechanisms leading to LVH remain unclear but include hemodynamic and humoral factors. The sympathetic nervous system may be important, particularly as catecholamines have been shown to have trophic properties. We tested the hypothesis that sympathetic activity measured using microneurography could be different in patients with hypertension depending on the presence of LVH.

METHODS

We examined 28 subjects with moderate to severe EHT (stages 2 to 3; Joint National Committee [JNC]-VI classification). Fourteen had echocardiographic evidence of LVH (EHT + LVH), while the other 14 subjects (EHT) did not. Subjects were matched in terms of age, body mass index and levels of arterial blood pressure. Peripheral muscle sympathetic nerve activity was measured from both multiunit bursts (MSNA) and single unit (s-MSNA) vasoconstrictor impulses via the peroneal nerve.

RESULTS

The mean frequency of s-MSNA and MSNA was greater in the EHT + LVH group than it was in the EHT group (mean +/- SEM; 75.9 +/- 6.9 impulses/100 beats vs. 52.1 +/- 2.9 impulses/100 beats, p < 0.001 and 64.2 +/- 5.7 bursts/100 beats vs. 48.9 +/- 2.8 bursts/100 beats, p < 0.05).

CONCLUSIONS

These results indicate that, in subjects with moderate to severe hypertension, the presence of LVH is associated with higher sympathetic discharge, evidenced by an increase in unitary firing frequency and also by fiber recruitment.

Altered autonomic support of arterial blood pressure with age in healthy men

BACKGROUND

Primary aging is associated with changes in the autonomic nervous system (ANS), but the functional significance of these changes for systemic circulatory control of arterial blood pressure (BP) is unknown. We tested the hypothesis that ANS support of BP is altered in healthy older humans.

METHODS AND RESULTS

A total of 23 young (aged 24+/-1 years; systolic/diastolic BP, 126+/-2/66+/-1 mm Hg) and 16 older (aged 65+/-1 years; systolic/diastolic BP, 125+/-3/62+/-2 mm Hg) healthy men were studied before and during ganglionic blockade (intravenous trimethaphan). The reduction in mean BP (radial artery catheter) with trimethaphan was almost twice as great in the older men (-33+/-2 versus -19+/-2 mm Hg; -40% versus -22% of baseline; P<0.01) due to a lack of increase in heart rate (3+/-2 versus 25+/-2 bpm; P<0.001) and cardiac output (-0.42+/-0.19 versus 1.01+/-0.26 L/min; P<0.001); the decreases in systemic vascular resistance were not different. The absence of tachycardia in the older men was associated with reduced baseline heart rate variability (HRV, P<0.05); the change in heart rate with trimethaphan correlated with the standard deviation of the R-R intervals (HRV(SD R-R interval); r=0.57, P<0.001). Among individual subjects (pooled groups), the reductions in mean BP with trimethaphan were most strongly related to measures of sympathetic activity (r=0.58 to 0.67, P<0.005), change in mean BP with intravenous phenylephrine (r=0.57, P<0.001), and HRV(SD R-R interval) (r=-0.40, P<0.01).

CONCLUSIONS

ANS support of BP is altered with age in healthy men due to less cardiac vagal inhibition of heart rate and cardiac output. Basal sympathetic activity and alpha-adrenergic vascular sensitivity are also key physiological correlates of ANS support of BP in healthy men.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Muscle sympathetic nerve activity in blood pressure control against gravitational stress

Muscle sympathetic nerve activity (MSNA) can be directly recorded from human peripheral nerves in situ using microneurography. MSNA plays an essential role to control systemic blood pressure against gravitational stress. MSNA was enhanced by changing posture against terrestrial gravity from lying to sitting, and from sitting to standing. This activity was enhanced by head-up tilt depending on the gravitational input from the head to the leg (+Gz) in the human body. Orthostatic hypotension occurred when MSNA response to gravitational stress was impaired both in high and low responders of this sympathetic outflow. Syncope was preceded and/or associated by a withdrawal of MSNA. MSNA was suppressed by short-term exposure to microgravity but was enhanced after long-term exposure to microgravity. Orthostatic intolerance after exposure to prolonged microgravity was associated with a reduction of increased MSNA response to gravitational stress. Aging influenced gravity-related responses of MSNA.

The process of aging changes the interplay of the immune, endocrine and nervous systems

The immune, endocrine and nervous systems interact with each other by means of cytokines, hormones and neurotransmitters. Interaction is dependent on specific receptors and respective signaling pathways in target cells. During aging, changes occur on many levels of these global systems which depend on oxidative damage, non-enzymatic glycosylation, mitochondrial mutations, defects in cell cycle control, mitotic dysregulation, genome instability, telomere shortening and other chromosomal pathologies. An alteration of the numerical value of a parameter of one system can lead to changes of the numerical value of a variable of another system. In a non-linear dynamic process these changes can contribute to the aging phenotype. Although it is extremely difficult to dissect linear interrelations of three global systems during aging, this review attempts to identify some simple linear pathways. Furthermore, it is demonstrated that chronic inflammatory diseases may accelerate the aging process. This review also reveals that new statistical and computational methods are necessary to unravel the complexity of the aging process.

Marked sympathetic activation in patients with chronic respiratory failure

The autonomic nervous system may be disturbed in chronic respiratory failure. We tested the hypothesis that there is increased sympathetic activity in patients with chronic hypoxemia. Furthermore, we examined the effect of short-term oxygen on muscle sympathetic nerve activity (MSNA) in these patients. We performed microneurography of the peroneal nerve in 11 patients with hypoxemia due to chronic obstructive pulmonary disease (COPD, n = 6) or lung fibrosis (n = 5) and in 11 healthy subjects matched for age and sex. MSNA was measured during normal breathing in all subjects. In eight patients and in seven control subjects, MSNA was also measured during nasal oxygen (4 L/min). MSNA was higher in the patients with chronic respiratory failure compared with the healthy subjects during normal breathing (61 +/- 5 versus 34 +/- 2 bursts/min, mean +/- SEM; p = 0.0002, paired t test). During oxygen administration, MSNA decreased from 63 +/- 6 to 56 +/- 6 bursts/min in the patients (p = 0.0004, ANOVA); there was no change in sympathetic activity in the control subjects. For the first time, there is direct evidence of marked sympathetic activation in patients with chronic respiratory failure. This is partly explained by arterial chemoreflex activation and may play an important role in the pathogenesis of the disease.

Antihypertensive drugs and the sympathetic nervous system

The sympathetic nervous system (SNS) plays an important role in the regulation of blood pressure homeostasis and cardiac function. Furthermore, the increased SNS activity is a predictor of mortality in patients with hypertension, coronary artery disease and congestive heart failure. Experimental data and a few clinical trials suggest that there are important interactions between the main pressor systems, i.e. the SNS, the renin-angiotensin system and the vascular endothelium with the strongest vasoconstrictor, endothelin. The main methods for the assessment of SNS activity are described. Cardiovascular drugs of different classes interfere differently with the SNS and the other pressor systems. Pure vasodilators including nitrates, alpha-blockers and dihydropyridine (DHP)-calcium channel blockers increase SNS activity. Finally, central sympatholytics and possibly phenylalkylamine-type calcium channel blockers reduce SNS activity. The effects of angiotensin-II receptor antagonists on SNS activity in humans is not clear; experimental data are discussed in this review. There are important interactions between the pressor systems under experimental conditions. Recent studies in humans suggest that an activation of the SNS with pure vasodilators in parallel increases plasma endothelin. It can be assumed that, in cardiovascular diseases with already enhanced SNS activity, drugs which do not increase SNS activity or even lower it are preferable. Whether this reflects in lower mortality needs to be investigated in intervention trials.

Aging and baroreflex control of RSNA and heart rate in rats

Aging is associated with altered autonomic control of cardiovascular function, but baroreflex function in animal models of aging remains controversial. In this study, pressor and depressor agent-induced reflex bradycardia and tachycardia were attenuated in conscious old (24 mo) rats [57 and 59% of responses in young (10 wk) Wistar rats, respectively]. The intrinsic heart rate (HR, 339 +/- 5 vs. 410 +/- 10 beats/min) was reduced in aged animals, but no intergroup differences in resting mean arterial blood pressure (MAP, 112 +/- 3 vs. 113 +/- 5 mmHg) or HR (344 +/- 9 vs. 347 +/- 9 beats/min) existed between old and young rats, respectively. The aged group also exhibited a depressed (49%) parasympathetic contribution to the resting HR value (vagal effect) but preserved sympathetic function after intravenous methylatropine and propranolol. An implantable electrode revealed tonic renal sympathetic nerve activity (RSNA) was similar between groups. However, old rats showed impaired baroreflex control of HR and RSNA after intravenous nitroprusside (-0.63 +/- 0. 18 vs. -1.84 +/- 0.4 bars x cycle(-1) x mmHg(-1) x s(-1)). Therefore, aging in rats is associated with 1) preserved baseline MAP, HR, and RSNA, 2) impaired baroreflex control of HR and RSNA, and 3) altered autonomic control of resting HR.

Human ageing and the sympathoadrenal system

Over the past three decades the changes in sympathoadrenal function that occur with age in healthy adult humans have been systematically studied using a combination of neurochemical, neurophysiological and haemodynamic experimental approaches. The available experimental evidence indicates that tonic whole-body sympathetic nervous system (SNS) activity increases with age. The elevations in SNS activity appear to be region specific, targeting skeletal muscle and the gut, but not obviously the kidney. The SNS tone of the heart is increased, although this appears to be due in part to reduced neuronal reuptake of noradrenaline (norepinephrine). In contrast to SNS activity, tonic adrenaline (epinephrine) secretion from the adrenal medulla is markedly reduced with age. This is not reflected in plasma adrenaline concentrations because of reduced plasma clearance. Despite widely held beliefs to the contrary, sympathoadrenal responsiveness to acute stress is not exaggerated with age in healthy adults. Indeed, adrenaline release in response to acute stress is substantially attenuated in older men. The mechanisms underlying the age-associated increases in SNS activity have not been established, but our preliminary data are consistent with increased subcortical central nervous system (CNS) sympathetic drive. These changes in sympathoadrenal function with advancing age may have a number of important physiological and pathophysiological consequences for human health and disease.

Adrenergic and reflex abnormalities in obesity-related hypertension

Previous studies have shown that essential hypertension and obesity are both characterized by sympathetic activation coupled with a baroreflex impairment. The present study was aimed at determining the effects of the concomitant presence of the 2 above-mentioned conditions on sympathetic activity as well as on baroreflex cardiovascular control. In 14 normotensive lean subjects (aged 33. 5+/-2.2 years, body mass index 22.8+/-0.7 kg/m(2) [mean+/-SEM]), 16 normotensive obese subjects (body mass index 37.2+/-1.3 kg/m(2)), 13 lean hypertensive subjects (body mass index 24.0+/-0.8 kg/m(2)), and 16 obese hypertensive subjects (body mass index 37.5+/-1.3 kg/m(2)), all age-matched, we measured beat-to-beat arterial blood pressure (by Finapres device), heart rate (HR, by ECG), and postganglionic muscle sympathetic nerve activity (MSNA, by microneurography) at rest and during baroreceptor stimulation and deactivation induced by stepwise intravenous infusions of phenylephrine and nitroprusside, respectively. Blood pressure values were higher in lean hypertensive and obese hypertensive subjects than in normotensive lean and obese subjects. MSNA was significantly (P:<0.01) greater in obese normotensive subjects (49.1+/-3.0 bursts per 100 heart beats) and in lean hypertensive subjects (44.5+/-3.3 bursts per 100 heart beats) than in lean normotensive control subjects (32.2+/-2.5 bursts per 100 heart beats); a further increase was detectable in individuals with the concomitant presence of obesity and hypertension (62.1+/-3. 4 bursts per 100 heart beats). Furthermore, whereas in lean hypertensive subjects, only baroreflex control of HR was impaired, in obese normotensive subjects, both HR and MSNA baroreflex changes were attenuated, with a further attenuation being observed in obese hypertensive patients. Thus, the association between obesity and hypertension triggers a sympathetic activation and an impairment in baroreflex cardiovascular control that are greater in magnitude than those found in either of the above-mentioned abnormal conditions alone.

Ethnic differences in insulinemia and sympathetic tone as links between obesity and blood pressure

Hyperinsulinemia and increased sympathetic nervous system (SNS) activity are thought to be pathophysiological links between obesity and hypertension. In the present study, we examined the relation among heart rate (HR), blood pressure (BP), and percent body fat (hydrodensitometry or DEXA), fasting plasma insulin concentration, and muscle sympathetic nerve activity (MSNA, microneurography) in male, normotensive whites (n=42) and Pima Indians (n=77). Pima Indians have a high prevalence of obesity and hyperinsulinemia but a relatively low prevalence of hypertension. Compared with whites, Pima Indian men had a higher percent body fat (28% versus 21%) and higher fasting insulin concentrations (210 versus 132 pmol/L) but lower MSNA (27 versus 33 bursts/min) (all P<0.001). In both ethnic groups, HR and BP were positively related to percent body fat and MSNA, and both were significant independent determinants of HR and BP in multiple regression analyses. However, MSNA was positively related to percent body fat and the fasting insulin concentration in whites (r=0.60 and r=0.47, both P<0.01) but not in Pima Indians (r=0.15 and r=0.03, NS) (P<0.01 for ethnic differences in the slope of the regression lines). These results confirm the physiological importance of the SNS in normal BP regulation but indicate that the roles of hyperinsulinemia and increased SNS activity as mediators for the relation between obesity and hypertension can differ between different ethnic groups. The lack of an increase in SNS activity with increasing adiposity and insulinemia in Pima Indians may contribute to the low prevalence of hypertension in this population.

Relation of leptin and insulin to adiposity-associated elevations in sympathetic activity with age in humans

OBJECTIVE

To determine whether plasma leptin and insulin concentrations are related to adiposity-associated elevations in muscle sympathetic nerve activity (MSNA) with age in healthy adult humans.

DESIGN

Cross-sectional investigation of young and older adult men.

SUBJECTS

Thirty healthy adult men, 16 young (25+/-1 y, mean+/-s.e.) and 14 older (61+/-1 y).

MEASUREMENTS/RESULTS

The older men had higher (P<0.05) levels of body mass, BMI, total fat mass and truncal fat mass (dual energy X-ray absorptiometry) than the young men. MSNA burst frequency (microneurography) was approximately 75% higher in the older men (P<0.001). Plasma leptin concentrations were approximately 150% higher (P<0.01), whereas plasma insulin concentrations were approximately 70% higher (P<0.05) in the older subjects. MSNA was related to both total (r=0.51, P<0.01) and truncal (r=0.56, P<0.01) fat mass. Plasma leptin concentrations were related to total and truncal fat mass (both r=0.83, P<0.001), and to MSNA (r=0.49, P<0.01). Plasma insulin concentrations were related to MSNA (r=0.38, P<0.05). We used partial correlation analyses to assess whether leptin and/or insulin are potential contributors to the relation between body fat and MSNA. Adjusting for the effects of plasma leptin, but not insulin, concentrations eliminated the significant relations between MSNA and total and truncal fat mass.

CONCLUSION

Our results: (1) demonstrate a positive relation between MSNA and plasma leptin concentrations in young and older healthy men; and (2) support the concept that circulating leptin concentrations may act as a humoral signal contributing to adiposity-associated elevations in MSNA with age in adult humans.

Chronic I(1)-imidazoline agonism : sympathetic mechanisms in hypertension

Evidence exists for a state of sympathetic hyperactivity in essential hypertension, and moxonidine, a new central sympathetic inhibitor, has been introduced for its treatment. Acute administration of moxonidine lowers peripheral sympathetic neural output. This study examined the effect of chronic moxonidine therapy, at increasing therapeutic doses, on resting peripheral sympathetic activity and vascular resistance and their responses to physiological reflex maneuvers. Twelve newly diagnosed patients with essential hypertension were studied sequentially at least 1 month apart, initially on no therapy, then on 200 microg, and finally on 400 microg of oral moxonidine daily. Changes in heart rate, arterial blood pressure, calf vascular resistance, and peripheral sympathetic drive were assessed at rest and during reflex maneuvers. Peroneal microneurography was used to quantify peripheral sympathetic vasoconstrictor activity by single-unit and multiunit techniques. Moxonidine therapy progressively reduced resting mean arterial pressure (P<0.0001) without affecting heart rate. At 200 microg daily, there was a significant reduction in sympathetic nerve activity (P<0.001) and calf vascular resistance (P<0.01). At 400 microg daily, further reductions were smaller and insignificant. Responses to cold stimulus and isometric handgrip exercise showed a similar pattern, with the greatest magnitude of change at 200 microg daily. In patients with essential hypertension, chronic moxonidine therapy inhibited resting sympathetic vasoconstrictor drive and also its reflex responses. The magnitude of inhibition became less as the therapeutic dose was increased, suggesting that moxonidine may be more effective under conditions of high sympathetic activity.

Sympathetic and reflex alterations in systo-diastolic and systolic hypertension of the elderly

BACKGROUND

Previous studies have shown that young and middle-aged essential hypertensives are characterized by a sympathetic activation coupled with an impaired baroreflex-heart rate control. The present study aimed to determine whether these neuroadrenergic and reflex alterations also characterize systo-diastolic and systolic hypertension of the elderly.

SUBJECTS AND METHODS

In 20 untreated elderly essential hypertensive subjects [10 with a systo-diastolic and 10 with an isolated systolic hypertension, aged 67.2 +/- 1.5 years and 66.9 +/- 1.7 years (mean +/- SEM)], we measured beat-to-beat arterial blood pressure (finger photoplethysmographic device), heart rate (electrocardiogram) and efferent postganglionic muscle sympathetic nerve activity (microneurography) at rest and during baroreceptor stimulation and deactivation induced by stepwise intravenous infusions of phenylephrine and nitroprusside, respectively. Data were compared with those obtained in 11 age-matched normotensive control subjects.

RESULTS

Compared to the elderly normotensive group, muscle sympathetic nerve activity was increased to a similar degree in the group of systo-diastolic and systolic hypertension (50.8 +/- 4.2 versus 75.2 +/- 5.2 and 70.4 +/- 5.1 bursts per 100 heart beats, respectively, P< 0.01 for both). In the control group, the stepwise increase in arterial pressure induced by phenylephrine caused progressive bradycardia and sympathoinhibition, while the stepwise decrease in arterial pressure had opposite effects. While baroreceptor-heart rate control was markedly impaired (average reduction 41.6%), in both systo-diastolic and systolic hypertensive patients, baroreceptor modulation of sympathetic nerve traffic was similar to that seen in normotensive individuals.

CONCLUSIONS

These data demonstrate that sympathetic activation is not only a feature of young and middle-aged, but also of elderly hypertensives, regardless of whether both systolic and diastolic or only systolic blood pressure is increased. They also show that hypertension of the elderly is not accompanied by an impaired baroreceptor modulation of sympathetic nerve traffic.

Interactive effect of heart rate and muscle sympathetic nerve activity on blood pressure

BACKGROUND

Sympathetic traffic to the peripheral vasculature and sympathetic discharge to the heart have complementary effects on blood pressure. Although faster heart rates have been linked to higher blood pressures, the relationship between muscle sympathetic nerve activity (MSNA) and long-term regulation of blood pressure is not clear. We tested the hypothesis that MSNA and heart rate are linked to blood pressure levels in normotensive subjects.

METHODS AND RESULTS

We studied normal young male (n=120) and female (n=48) subjects subdivided according to tertiles of heart rate and MSNA distributions. Systolic, diastolic, and pulse pressures were significantly different across the heart rate tertiles in male subjects, with the highest blood pressure values in the upper tertile of heart rate. No significant differences in blood pressure across the tertiles of MSNA were found. The relationship between MSNA and blood pressure, however, was affected by heart rate. MSNA did not influence blood pressure in the first and second heart rate tertiles. However, within the upper heart rate tertile, subjects with higher levels of MSNA had significantly higher systolic (P=0. 02) and pulse (P=0.004) pressures than subjects with lower levels of MSNA. In female subjects, blood pressure was not different across the tertiles of heart rate or MSNA.

CONCLUSIONS

MSNA and heart rate have interactive effects on systolic blood pressure and pulse pressure in normotensive male but not female subjects. No relationship between MSNA and blood pressure or pulse pressure is evident in subjects with slower heart rate. In male subjects with faster heart rates, higher levels of MSNA are associated with higher systolic and pulse pressures.

Angiotensin II blunts, while an angiotensin-converting enzyme inhibitor augments, reflex sympathetic inhibition in humans

1. The role of angiotensin (Ang)II in and the effects of angiotensin-converting enzyme (ACE) inhibitors on the regulation of sympathetic neural activity were examined in humans. 2. We measured baseline values of muscle sympathetic nerve activity (MSNA) and its reflex inhibition in 28 patients with essential hypertension with elevated plasma renin activity (PRA; > 1.0 ng/mL per h = 0.28 ng/L per s) before and after either acute or chronic oral administration of an ACE inhibitor or placebo and in 20 normotensive subjects before and after infusion of either AngII (5 ng/kg per min = 4.8 pmol/kg per min) or vehicle (5% dextrose). Muscle sympathetic nerve activity was recorded from the tibial nerve and its reflex inhibition was evaluated during pressor responses to bolus injection of phenylephrine (2 micrograms/kg, i.v.). 3. Blood pressure was significantly decreased (P < 0.01) after the acute oral administration of captopril (25 mg), accompanied by a slight increase in MSNA in patients with essential hypertension compared with control patients who received placebo administration. Reflex changes in MSNA were significantly augmented after oral administration of captopril (-4.1 +/- 0.5 vs -6.2 +/- 0.6%/mmHg, respectively; P < 0.01), with a significant reduction of plasma AngII, while they were not affected by placebo administration. 4. In contrast, acute AngII infusion was accompanied by decreases in both PRA and MSNA in normotensive subjects. Reflex changes in MSNA were significantly reduced after AngII infusion (-11.0 +/- 0.8 vs -7.4 +/- 1.0%/mmHg, respectively; P < 0.01) but not after vehicle alone. 5. Chronic ACE inhibition by 12 week oral imidapril administration (5-10 mg/day) significantly (P < 0.05) decreased baseline values of MSNA, which were accompanied by a significant (P < 0.05) increase in the reflex inhibition of MSNA, while plasma concentrations of noradrenaline were unaffected. 6. These results indicate that AngII blunts reflex inhibition of sympathetic neural activity and that inhibition of the renin-angiotensin system by an ACE inhibitor augments reflex regulation of sympathetic neural activity and reduces baseline values in patients with essential hypertension.

Single-unit sympathetic discharge : quantitative assessment in human hypertensive disease

Background-Reports demonstrating sympathetic hyperactivity in hypertension with the use of microneurography have been inconsistent. One possible reason is that previous studies have assessed muscle sympathetic nerve activity (MSNA) from integrated voltage waves ("bursts") recorded from multiunit discharges. We studied single units with defined vasoconstrictor properties (s-MSNA) to further characterize sympathetic output in hypertensive disease. Methods and Results-We examined 74 subjects with a wide range of arterial blood pressure that were considered to be either normal (NT), high normal (HN), or stages 1 to 3 essential hypertension (EHT-1, EHT-2/3). All had their peripheral sympathetic activity measured from both multiunit bursts and single-unit vasoconstrictor impulses. There was a significant correlation between s-MSNA and MSNA, and results of variability studies were similar. The EHT-1 and EHT-2/3 groups had greater s-MSNA and MSNA than did the matched NT group (always P<0. 01). The HN group also had greater s-MSNA and MSNA than did the NT group (mean+/-SEM; 43+/-5 vs 29+/-2 impulses/100 beats, P<0.05; 36+/-4 vs 24+/-2 bursts/100 beats, P<0.05). In addition, the EHT-1 group had significantly greater s-MSNA than did the EHT-2/3 group (63+/-6 vs 51+/-3 impulses/100 beats, P<0.05), which could not be demonstrated with MSNA bursts. Conclusions-Quantification from single vasoconstrictor units has provided additional evidence in established essential hypertension of increased central sympathetic output. Furthermore, in the mild or early stages of hypertension, this technique has provided new evidence of augmented sympathetic output compared with more severe hypertension.