Skeletal muscle sympathetic activity at rest in trained and untrained subjects

Neurocirculatory regulation and adaptations to exercise in chronic kidney disease

Chronic kidney disease (CKD) is characterized by pronounced exercise intolerance and exaggerated blood pressure reactivity during exercise. Classic mechanisms of exercise intolerance in CKD have been extensively described previously and include uremic myopathy, chronic inflammation, malnutrition, and anemia. We contend that these classic mechanisms only partially explain the exercise intolerance experienced in CKD and that alterations in cardiovascular and autonomic regulation also play a key contributing role. The purpose of this review is to examine the physiological factors that contribute to neurocirculatory dysregulation during exercise and discuss the adaptations that result from regular exercise training in CKD. Key neurocirculatory mechanisms contributing to exercise intolerance in CKD include augmentation of the exercise pressor reflex, aberrations in neurocirculatory control, and increased neurovascular transduction. In addition, we highlight how some contributing factors may be improved through exercise training, with a specific focus on the sympathetic nervous system. Important areas for future work include understanding how the exercise prescription may best be optimized in CKD and how the beneficial effects of exercise training may extend to the brain.

Influence of sex and age on the relationship between aerobic fitness and muscle sympathetic nerve activity in healthy adults

We examined the influence of sex and age on the relationship between aerobic fitness and muscle sympathetic nerve activity (MSNA) in healthy adults. Data were assessed from 224 volunteers (88 females), aged 18-76 yr, in whom resting MSNA (microneurography) and peak oxygen uptake (V̇o_2peak; incremental exercise test) were evaluated. When separated into younger (<50 yr) and older (≥50 yr) subgroups, there were inverse relationships between relative V̇o_2peak (mL·kg^-1·min^-1) and MSNA burst frequency in younger males (R² = 0.21, P < 0.0001) and older females (R² = 0.36, P < 0.01), but not older males (R² = 0.05, P = 0.08) or younger females (R² = 0.03, P = 0.14). Similar patterns were observed with absolute V̇o_2peak (L·min^-1) and percent-predicted (based on age, sex, weight, height, and modality), and with burst incidence. Sex and age influence the relationship between aerobic fitness and resting MSNA, and, thus, must be considered as key variables when studying these potential associations; inverse relationships are strongest in younger males and older females.NEW & NOTEWORTHY Our data reveal for the first time that associations between aerobic fitness and resting muscle sympathetic nerve activity are sex and age specific; inverse relationships are evident in younger males (<50 yr) and older females (≥50 yr), but absent in younger females (<50 yr) and older males (≥50 yr).

Sympathetic vasoconstriction in skeletal muscle: Modulatory effects of aging, exercise training, and sex

The sympathetic nervous system (SNS) is a critically important regulator of the cardiovascular system. The SNS controls cardiac output and its distribution, as well as peripheral vascular resistance and blood pressure at rest and during exercise. Aging is associated with increased blood pressure and decreased skeletal muscle blood flow at rest and in response to exercise. The mechanisms responsible for the blunted skeletal muscle blood flow response to dynamic exercise with aging have not been fully elucidated; however, increased muscle sympathetic nerve activity (MSNA), elevated vascular resistance and a decline in endothelium-dependent vasodilation are commonly reported in older adults. In contrast to aging, exercise training has been shown to reduce blood pressure and enhance skeletal muscle vascular function. Exercise training has been shown to enhance nitric oxide-dependent vascular function and may improve the vasodilatory capacity of the skeletal muscle vasculature; however, surprisingly little is known about the effect of exercise training on the neural control of circulation. The control of blood pressure and skeletal muscle blood flow also differs between males and females. Blood pressure and MSNA appear to be lower in young females compared to males. However, females experience a larger increase in MSNA with aging compared to males. The mechanism(s) for the altered SNS control of vascular function in females remain to be determined. Novelty: • This review will summarize our current understanding of the effects of aging, exercise training and sex on sympathetic vasoconstriction at rest and during exercise. • Areas where additional research is needed are also identified.

Greater Influence of Aerobic Fitness on Autonomic Support of Blood Pressure in Young Women Than in Older Women

Aging increases autonomic support of blood pressure; however, the impact of aerobic fitness on autonomic support of blood pressure has not been addressed in women. As such, we hypothesized that aerobic fitness would be related to the change in blood pressure during ganglionic blockade such that women with greater aerobic fitness would have a blunted fall in blood pressure during ganglionic blockade due to increased vagal tone. Thirteen young premenopausal and 13 older postmenopausal women completed a screening visit where aerobic fitness (maximal oxygen consumption, VO_2max) was measured. On a separate study day, participants were instrumented for assessment of muscle sympathetic nerve activity, heart rate (electrocardiography), and beat by beat blood pressure (arterial catheter and pressure transducer) and underwent pharmacological blockade of the autonomic ganglia using trimethaphan camyslate. Heart rate, blood pressure, and muscle sympathetic nerve activity were analyzed before and during ganglionic blockade. In young women, there was a significant relationship between aerobic fitness and the change in blood pressure during ganglionic blockade (r=0.761, P=0.003). In older women, there was no relationship between aerobic fitness and the change in blood pressure during ganglionic blockade (r=-0.106, P=0.73). Measures of heart rate variability were related to fitness in young women, but not older women (root mean square of successive differences between normal heartbeats, r=0.713, P=0.006 versus r=-0.172, P=0.575). Our data suggest that in young women, autonomic support of blood pressure is attenuated in those that are highly fit; however, this relationship is not significant in older women.

Effect of short-term endurance training on venous compliance in the calf and forearm differs between continuous and interval exercise in humans

We examined whether the effect of short-term endurance exercise training on venous compliance in the calf and forearm differed between continuous and interval workloads. Young healthy volunteers (10 women and 16 men) were randomly assigned to continuous (C-TRA; n = 8) and interval (I-TRA; n = 9) exercise training groups, and a control group (n = 9). Subjects in the C-TRA group performed a continuous cycling exercise at 60% of heart rate reserve (HRR), and subjects in the I-TRA group performed a cycling exercise consisting of alternating 2-min intervals at 40% HRR and 80% HRR. Training programs were performed for 40 min/day, 3 days/week for 8 weeks. Before and after training, limb volume in the calf and forearm was measured with subjects in the supine position by venous occlusion plethysmography using a venous collecting cuff placed around the thigh and upper arm. Cuff pressure was held at 60 mmHg for 8 min and then decreased to 0 mmHg at a rate of 1 mmHg/s. Venous compliance was calculated as the numerical derivative of the cuff pressure-limb volume curve. Calf venous compliance was increased after I-TRA, but not C-TRA. Forearm venous compliance was unchanged after C-TRA or I-TRA. These results suggest that the adaptation of venous compliance in response to endurance training for 8 week may occur in interval but not continuous exercise bouts and may be specific to the exercising limb.

Upward resetting of the vascular sympathetic baroreflex in middle-aged male runners

This study focused on the influence of habitual endurance exercise training (i.e., committed runner or nonrunner) on the regulation of muscle sympathetic nerve activity (MSNA) and arterial pressure in middle-aged (50 to 63 yr, n = 23) and younger (19 to 30 yr; n = 23) normotensive men. Hemodynamic and neurophysiological assessments were performed at rest. Indices of vascular sympathetic baroreflex function were determined from the relationship between spontaneous changes in diastolic blood pressure (DBP) and MSNA. Large vessel arterial stiffness and left ventricular stroke volume also were measured. Paired comparisons were performed within each age category. Mean arterial pressure and basal MSNA bursts/min were not different between age-matched runners and nonrunners. However, MSNA bursts/100 heartbeats, an index of baroreflex regulation of MSNA (vascular sympathetic baroreflex operating point), was higher for middle-aged runners (P = 0.006), whereas this was not different between young runners and nonrunners. The slope of the DBP-MSNA relationship (vascular sympathetic baroreflex gain) was not different between groups in either age category. Aortic pulse wave velocity was lower for runners of both age categories (P < 0.03), although carotid β-stiffness was lower only for middle-aged runners (P = 0.04). For runners of both age categories, stroke volume was larger, whereas heart rate was lower (both P < 0.01). In conclusion, we suggest that neural remodeling and upward setting of the vascular sympathetic baroreflex compensates for cardiovascular adaptations after many years committed to endurance exercise training, presumably to maintain arterial blood pressure stability. NEW & NOTEWORTHY Exercise training reduces muscle sympathetic burst activity in disease; this is often extrapolated to infer a similar effect in health. We demonstrate that burst frequency of middle-aged and younger men committed to endurance training is not different compared with age-matched casual exercisers. Notably, well-trained, middle-aged runners display similar arterial pressure but higher sympathetic burst occurrence than untrained peers. We suggest that homeostatic plasticity and upward setting of the vascular sympathetic baroreflex maintains arterial pressure stability following years of training.

Vascular characteristics in young women-Effect of extensive endurance training or a sedentary lifestyle

AIM

To explore whether high-level endurance training in early age has an influence on the arterial wall properties in young women.

METHODS

Forty-seven athletes (ATH) and 52 controls (CTR), all 17-25 years of age, were further divided into runners (RUN), whole-body endurance athletes (WBA), sedentary controls (SC) and normally active controls (AC). Two-dimensional ultrasound scanning of the carotid arteries was conducted to determine local common carotid artery (CCA) geometry and wall distensibility. Pulse waves were recorded with a tonometer to determine regional pulse wave velocity (PWV) and pulse pressure waveform.

RESULTS

Carotid-radial PWV was lower in WBA than in RUN (P < .05), indicating higher arterial distensibility along the arm. Mean arterial pressure was lower in ATH than in CTR and in RUN than in WBA (P < .05). Synthesized aortic augmentation index (AI@75) was lower among ATH than among CTR (-12.8 ± 1.6 vs -2.6 ± 1.2%, P < .001) and in WBA than in RUN (-16.4 ± 2.5 vs -10.7 ± 2.0%, P < .05), suggesting a diminished return of reflection waves to the aorta during systole. Carotid-femoral PWV and intima-media thickness (IMT), lumen diameter and radial distensibility of the CCA were similar in ATH and CTR.

CONCLUSION

Elastic artery distensibility and carotid artery IMT are not different in young women with extensive endurance training over several years and in those with sedentary lifestyle. On the other hand, our data suggest that long-term endurance training is associated with potentially favourable peripheral artery adaptation, especially in sports where upper body work is added. This adaptation, if persisting later in life, could contribute to lower cardiovascular risk.

Potential role of endurance training in altering renal sympathetic nerve activity in CKD?

Chronic kidney disease (CKD), is characterized by a progressive loss of renal function and increase in cardiovascular risk. In this review paper, we discuss the pathophysiology of increased sympathetic nerve activity in CKD patients and raise the possibility of endurance exercise being an effective countermeasure to address this problem. We specifically focus on the potential role of endurance training in altering renal sympathetic nerve activity as increased renal sympathetic nerve activity negatively impacts kidney function as well indirectly effects multiple other systems and organs. Recent technological advances in device based therapy have highlighted the detrimental effect of elevated renal sympathetic nerve activity in CKD patients, with kidney function and blood pressure being improved post renal artery nerve denervation in selected patients. These developments provide optimism for the development of alternative and/or complementary strategies to lower renal sympathetic nerve activity. However, appropriately designed studies are required to confirm preliminary observations, as the widespread use of the renal denervation approach to lower sympathetic activity presently has limited feasibility. Endurance training may be one alternative strategy to reduce renal sympathetic nerve activity. Here we review the role of endurance training as a potential alternative or adjunctive to current therapy in CKD patients. We also provide recommendations for future research to assist in establishing an evidence base for the use of endurance training to lower renal sympathetic activity in CKD patients.

Muscle sympathetic activity in resting and exercising humans with and without heart failure

The sympathetic nervous system is critical for coordinating the cardiovascular response to various types of physical exercise. In a number of disease states, including human heart failure with reduced ejection fraction (HFrEF), this regulation can be disturbed and adversely affect outcome. The purpose of this review is to describe sympathetic activity at rest and during exercise in both healthy humans and those with HFrEF and outline factors, which influence these responses. We focus predominately on studies that report direct measurements of efferent sympathetic nerve traffic to skeletal muscle (muscle sympathetic nerve activity; MSNA) using intraneural microneurographic recordings. Differences in MSNA discharge between subjects with and without HFrEF both at rest and during exercise and the influence of exercise training on the sympathetic response to exercise will be discussed. In contrast to healthy controls, MSNA increases during mild to moderate dynamic exercise in the presence of HFrEF. This increase may contribute to the exercise intolerance characteristic of HFrEF by limiting muscle blood flow and may be attenuated by exercise training. Future investigations are needed to clarify the neural afferent mechanisms that contribute to efferent sympathetic activation at rest and during exercise in HFrEF.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Assessment of resting electrocardiogram, P wave dispersion and duration in different genders applying for registration to the School of Physical Education and Sports - results of a single centre Turkish Trial with 2093 healthy subjects

BACKGROUND

The 12-lead electrocardiogram shows a broad range of abnormal patterns in trained athletes. The primary end point of this study was to investigate P wave dispersion, and P wave durations and related factors in different genders applying for registration to the School of Physical Education and Sports.

METHODS AND RESULTS

From 2006 to 2009, a total of 2093 students - 1674 boys with a mean age of 19.8 plus or minus 1.9 years and 419 girls with a mean age of 19.1 plus or minus 1.8 years - were included in the study. All 12 leads of the resting electrocardiogram were evaluated for P wave dispersion and electrocardiogram abnormalities. Baseline parameters such as age, body weight, body height, and body mass index, as well as electrocardiogram findings such as P wave maximal duration and P wave dispersion, were significantly higher in boys than in girls. Of all the parameters tested with correlation analysis, only gender (p = 0.03) (r = 0.04), body weight (p < 0.001) (r = 0.07), body height (p = 0.004) (r = 0.06), and body mass index (p = 0.01) (p = 0.05) were correlated with P wave dispersion.

CONCLUSION

The frequencies of all electrocardiogram abnormalities, P wave dispersion, and P wave maximal duration were higher in boys as compared with girls in an unselected student population applying for registration to the School of Physical Education and Sports; in addition, P wave dispersion was correlated with gender, body weight, body height, and body mass index.

Exercise and vascular adaptation in asymptomatic humans

Beneficial effects of exercise training on the vasculature have been consistently reported in subjects with cardiovascular risk factors or disease, whereas studies in apparently healthy subjects have been less uniform. In this review, we examine evidence pertaining to the impact of exercise training on conduit and resistance vessel function and structure in asymptomatic subjects. Studies of arterial function in vivo have mainly focused on the endothelial nitric oxide dilator system, which has generally been shown to improve following training. Some evidence suggests that the magnitude of benefit depends upon the intensity or volume of training and the relative impact of exercise on upregulation of dilator pathways versus effects of inflammation and/or oxidation. Favourable effects of training on autonomic balance, baroreflex function and brainstem modulation of sympathetic control have been reported, but there is also evidence that basal vasoconstrictor tone increases as a result of training such that improvements in intrinsic vasodilator function and arterial remodelling are counterbalanced at rest. Studies of compliance suggest increases in both the arterial and the venous sides of the circulation, particularly in older subjects. In terms of mechanisms, shear stress appears to be a key signal to improvement in vascular function, whilst increases in pulse pressure and associated haemodynamics during bouts of exercise may transduce vascular adaptation, even in vascular beds which are distant from the active muscle. Different exercise modalities are associated with idiosyncratic patterns of blood flow and shear stress, and this may have some impact on the magnitude of exercise training effects on arterial function and remodelling. Other studies support the theory that that there may be different time course effects of training on specific vasodilator and constrictor pathways. A new era of understanding of the direct impacts of exercise and training on the vasculature is evolving, and future studies will benefit greatly from technological advances which allow direct characterization of arterial function and structure.

Physical (in)activity-dependent alterations at the rostral ventrolateral medulla: influence on sympathetic nervous system regulation

A sedentary lifestyle is a major risk factor for cardiovascular disease, and rates of inactivity and cardiovascular disease are highly prevalent in our society. Cardiovascular disease is often associated with overactivity of the sympathetic nervous system, which has both direct and indirect effects on multiple organ systems. Although it has been known for some time that exercise positively affects the brain in terms of memory and cognition, only recently have changes in how the brain regulates the cardiovascular system been examined in terms of physical activity and inactivity. This brief review will discuss the evidence for physical activity-dependent neuroplasticity related to control of sympathetic outflow. It will focus particularly on recent studies from our laboratory and others that have examined changes that occur in the rostral ventrolateral medulla (RVLM), considered one of the primary brain regions involved in the regulation and generation of sympathetic nervous system activity.

Effects of aerobic exercise training on sympathetic and renal responses to mental stress in humans

The effects of aerobic exercise training (ET) on muscle sympathetic nerve activity (MSNA) and renal vascular responses to mental stress (MS) have not been determined in humans. We hypothesized that aerobic ET would reduce MSNA and renal vasoconstriction during MS. MSNA, mean arterial pressure (MAP), heart rate, renal blood flow velocity (RBFV), and peak oxygen uptake (V(O2peak)) were recorded in 23 healthy adults. Fourteen subjects participated in 8 wk of aerobic ET, while nine subjects served as sedentary controls (Con). ET significantly increased V(O2peak) (Delta18 +/- 1%; P < 0.001) and decreased RBFV at rest (60 +/- 4 to 48 +/- 3 cm/s; P < 0.01), whereas Con did not alter V(O2peak) or RBFV. ET did not alter resting MSNA (11 +/- 1 to 9 +/- 1 bursts/min) or MAP (84 +/- 2 to 83 +/- 2 mmHg), and these findings were similar in the Con group. MS elicited similar increases in MSNA (approximately Delta2 bursts/min; P < 0.05), MAP (approximately Delta15 mmHg; P < 0.001), and heart rate (approximately Delta20 beats/min; P < 0.001) before and after ET, and the responses were not different between ET and Con. Likewise, MS elicited similar decreases in RBFV and renal vascular conductance before and after ET, and the responses were not different between ET and Con. Perceived stress levels during MS were similar before and after the 8-wk study in both ET and Con. In conclusion, ET does not alter MSNA and renal vascular responses to MS in healthy humans.

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.

Evaluation of the autonomic response in healthy subjects during treadmill training with assistance of a robot-driven gait orthosis

Body weight supported treadmill training assisted with a robotic driven gait orthosis is an emerging clinical tool helpful to restore gait in individuals with loss of motor skills. However, the autonomic response during this rehabilitation protocol is not known. The aim of the study was to evaluate the autonomic response during a routine protocol of motor rehabilitation through spectral and symbolic analyses of short-term heart rate variability in a group of 20 healthy subjects (11 men, mean age 25+/-3.8 years). The protocol included the following phases: (1) sitting position; (2) standing position; (3) suspension during subject instrumentation; (4 and 5) robotic-assisted treadmill locomotion at 1.5km/h and 2.5km/h respectively with partial body weight support; (6) standing recovery after exercise. Results showed a significant tachycardia associated with the reduction in variance during the suspended phase of the protocol compared to the sitting position. Spectral analysis did not demonstrate any significant autonomic response during the entire protocol, while symbolic analysis detected an increase in sympathetic modulation during body suspension and an increase of vagal modulation during walking. These results could be used to improve understanding of the cardiovascular effects of rehabilitation in subjects undergoing robotic driven gait orthosis treadmill training.

No Effect of Training State on Ambulatory Measures of Cardiac Autonomic Control

We examined the effect of training state on cardiac autonomic control in a naturalistic setting. Twenty-four vigorous exercisers were compared to age- and sex-matched sedentary controls. The regular exercisers were subjected to a 6-week training program after which they were randomized to 2 weeks of continued training or 2 weeks of detraining. Cardiac autonomic control was measured over a 24-h period by ambulatory recording, using the preejection period (PEP) and respiratory sinus arrhythmia (RSA). Nonexercising controls had a significantly higher ambulatory heart rate (HR) compared to the regular exercisers but comparable 24-h levels of PEP and RSA. In regular exercisers, 2 weeks of detraining did not significantly change the 24-h levels of HR, PEP, or RSA. We conclude that the bradycardia in healthy regular exercisers is the result of a lower intrinsic heart rate rather than a shift in cardiac autonomic balance from sympathetic to vagal control.

Acute sympathetic vasoconstriction at rest and during dynamic exercise in cyclists and sedentary humans

The impact of exercise training on sympathetic activation is not well understood, especially across untrained and trained limbs in athletes. Therefore, in eight sedentary subjects (maximal oxygen consumption = 40 ± 2 ml·kg−1·min−1) and eight competitive cyclists (maximal oxygen consumption= 64 ± 2 ml·kg−1·min−1), we evaluated heart rate, blood pressure, blood flow, vascular conductance, and vascular resistance in the leg and arm during acute sympathetic stimulation [cold pressor test (CPT)]. The CPT was also performed during dynamic leg (knee extensor) or arm (handgrip) exercise at 50% of maximal work rate (WRmax) with measurements in the exercising limb. At rest, the CPT decreased vascular conductance similarly in the leg and arm of sedentary subjects (−33 ± 8% leg, −38 ± 6% arm) and cyclists (−34 ± 4% leg, −31 ± 9% arm), and during exercise CPT-induced vasoconstriction was blunted (i.e., sympatholysis) in both the leg and arm of both groups. However, the magnitude of sympatholysis was significantly different between the arm and leg of the sedentary group (−47 ± 11% arm, −25 ± 8% leg), and it was less in the arm of cyclists (−28 ± 11%) than sedentary controls. Taken together, these data provide evidence that sympathetically mediated vasoconstriction is expressed equally and globally at rest in both sedentary and trained individuals, with a differential pattern of vasoconstriction during acute exercise according to limb and exercise training status.

Cardiac response to robotic assisted locomotion in normal subjects: a preliminary study

Robotic assisted locomotion systems are recently gaining appreciation and diffusion as useful methods to rehabilitate individuals with lost sensorimotor function. Our aim was to evaluate potential changes in the autonomic nervous system activity (by ECG and spectral analysis), due to the experimental protocol, which include suspension of the subject to be instrumented on the system. A group of 10 normal subjects was studied during the rehabilitation protocol. Results showed a significant tachycardia and a reduced variance, during orthostatic stress induced by the suspension phase in comparison with sitting baseline condition but no significant increase of LF normalized power as it would be expected during a sympathetic activation.

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.

Sympathetic neural regulation in endurance-trained humans: fitness vs. fatness

We tested the hypothesis that muscle sympathetic nerve activity (MSNA) would be higher in endurance-trained (ET) compared with sedentary (Sed) men with similar levels of total body and abdominal adiposity. We further hypothesized that sympathetic baroreflex gain would be augmented in ET compared with Sed men independent of the level of adiposity. To address this, we measured MSNA (via microneurography), sympathetic and vagal baroreflex responses (the modified Oxford technique), body composition (dual-energy X-ray absorptiometry), and waist circumference (Gulick tape) in Sed ( n = 22) and ET men ( n = 8). The ET men were also compared with a subgroup of Sed men ( n = 6) with similar levels of total body and abdominal adiposity. Basal MSNA was greater in the ET compared with Sed men with similar levels of total body and abdominal adiposity (28 ± 2.0 vs. 21 ± 2.0 bursts/min; P < 0.05) but similar to the larger group of Sed men ( n = 22) with higher total body and abdominal adiposity (vs. 26 ± 3 bursts/min; P > 0.05). In contrast to our hypothesis, sympathetic baroreflex gain was lower in the ET compared with Sed men (−6.4 ± 0.8 vs. −8.4 ± 0.4 arbitrary integrative units·beat−1·mmHg−1; P < 0.05) regardless of the level of adiposity. Taken together, the results of the present study suggest that MSNA is higher in ET compared with Sed men with similar levels of total body and abdominal adiposity. In addition, sympathetic baroreflex gain is lower in ET compared with Sed men. That sympathetic baroreflex gain was lower in ET compared with Sed men regardless of the level of adiposity suggests an influence of the ET state per se.

Muscle afferent contributions to the cardiovascular response to isometric exercise

The cardiovascular response to isometric exercise is governed by both central and peripheral mechanisms. Both metabolic and mechanical stresses on the exercising skeletal muscle produce cardiovascular change, yet it is often overlooked that the afferent signal arising from the muscle can be modified by factors other than exercise intensity. This review discusses research revealing that muscle fibre type, muscle mass and training status are important factors in modifying this peripheral feedback from the active muscles. Studies in both animals and humans have shown that the pressor response resulting from exercise of muscle with a faster contractile character and isomyosin content is greater than that from a muscle of slower contractile character. Athletic groups participating in training programmes that place a high anaerobic load on skeletal muscle groups show attenuated muscle afferent feedback. Similarly, longitudinal studies have shown that specific local muscle training also blunts the pressor response to isometric exercise. Thus it appears that training may decrease the metabolic stimulation of muscle afferents and in some instances chronic exposure to the products of anaerobic metabolism may blunt the sensitivity of the muscle metaboreflex. There may be surprising parallels between the local muscle conditions induced in athletes training for longer sprint events (e.g. 400 m) and by the low-flow conditions in, for example, the muscles of chronic heart failure patients. Whether their similar attenuations in muscle afferent feedback during exercise are due to decreased metabolite accumulation or to a desensitization of the muscle afferents is not yet known.

The effects of exercise training on sympathetic neural activation in advanced heart failure: a randomized controlled trial

OBJECTIVES

The goal of this study was to test the hypothesis that exercise training reduces resting sympathetic neural activation in patients with chronic advanced heart failure.

BACKGROUND

Exercise training in heart failure has been shown to be beneficial, but its mechanisms of benefit remain unknown.

METHODS

Sixteen New York Heart Association class II to III heart failure patients, age 35 to 60 years, ejection fraction < or =40% were divided into two groups: 1) exercise-trained (n = 7), and 2) sedentary control (n = 9). A normal control exercise-trained group was also studied (n = 8). The four-month supervised exercise training program consisted of three 60 min exercise sessions per week, at heart rate levels that corresponded up to 10% below the respiratory compensation point. Muscle sympathetic nerve activity (MSNA) was recorded directly from peroneal nerve using the technique of microneurography. Forearm blood flow was measured by venous plethysmography.

RESULTS

Baseline MSNA was greater in heart failure patients compared with normal controls; MSNA was uniformly decreased after exercise training in heart failure patients (60 +/- 3 vs. 38 +/- 3 bursts/100 heart beats), and the mean difference in the change was significantly (p < 0.05) greater than the mean difference in the change in sedentary heart failure or trained normal controls. In fact, resting MSNA in trained heart failure patients was no longer significantly greater than in trained normal controls. In heart failure patients, peak VO(2) and forearm blood flow, but not left ventricular ejection fraction, increased after training.

CONCLUSIONS

These findings demonstrate that exercise training in heart failure patients results in dramatic reductions in directly recorded resting sympathetic nerve activity. In fact, MSNA was no longer greater than in trained, healthy controls.

Conversion from vagal to sympathetic predominance with strenuous training in high-performance world class athletes

BACKGROUND

Benefits of moderate endurance training include increases in parasympathetic activity and baroreflex sensitivity (BRS) and a relative decrease in sympathetic tone. However, the effect of very intensive training load on neural cardiovascular regulation is not known. We tested the hypothesis that strenuous endurance training, like in high-performance athletes, would enhance sympathetic activation and reduce vagal inhibition.

METHODS AND RESULTS

We studied the entire Italian junior national team of rowing (n=7) at increasing training loads up to 75% and 100% of maximum, the latter approximately 20 days before the Rowing World Championship. Autoregressive power spectral analysis was used to investigate RR interval and blood pressure (BP) variabilities. BRS was assessed by the sequences method. Increasing training load up to 75% of maximum was associated with a progressive resting bradycardia and increased indexes of cardiac vagal modulation and BRS. However, at 100% training load these effects were reversed, with increases in resting heart rate, diastolic BP, low-frequency RR interval, and BP variabilities and decreases in high-frequency RR variability and BRS. Three athletes later won medals in the World Championship.

CONCLUSIONS

This study indicates that very intensive endurance training shifted the cardiovascular autonomic modulation from a parasympathetic toward a sympathetic predominance. This finding should be interpreted within the context of the substantial role played by the sympathetic nervous system in increasing cardiovascular performance at peak training. Whether the altered BP and autonomic function shown in this study might be in time hazardous to human cardiovascular system remains to be established.

Tonic sympathetic support of metabolic rate is attenuated with age, sedentary lifestyle, and female sex in healthy adults

We recently demonstrated in young adult humans that the sympathetic nervous system contributes to the control of resting metabolic rate via tonic beta-adrenergic receptor stimulation. In the present follow-up study we determined the respective effects of age, habitual exercise status, and sex on this regulatory mechanism. Resting metabolic rate (ventilated hood, indirect calorimetry) was determined in 55 healthy sedentary or endurance exercise-trained adults, aged 18-35 or 60-75 yr (29 men and 26 women), before (baseline) and during the infusion of either a nonselective beta-adrenergic receptor antagonist (propranolol) or saline (control). Relative to baseline values, during beta-adrenergic receptor antagonism resting metabolic rate adjusted for fat-free mass was reduced to a lesser extent in older (mean +/- SE, -130 +/- 46 kJ/d) compared with young (-297 +/- 46) adults, sedentary (-151 +/- 50) compared with endurance exercise-trained (-268 +/- 46) adults, and women (-105 +/- 33) compared with men (-318 +/- 50; all P < 0.01). Reductions in resting metabolic rate during beta-adrenergic receptor antagonism were positively related to higher baseline resting metabolic rate and plasma catecholamine concentrations and negatively related to adiposity (all P < 0.05). Resting metabolic rate was unchanged in response to saline control in all groups. These results provide experimental support for the hypothesis that aging, sedentary living, and female sex are associated with attenuated sympathetic nervous system support of resting metabolic rate in healthy adult humans.

Exercise and autonomic function in health and cardiovascular disease

Autonomic nervous system activity contributes to the regulation of cardiac output during rest, exercise, and cardiovascular disease. Measurement of HRV has been particularly useful in assessing parasympathetic activity, while its utility for assessing sympathetic function and overall sympathovagal balance remains controversial. Studies have revealed that parasympathetic tone dominates the resting state, while exercise is associated with prompt withdrawal of vagal tone and subsequent sympathetic activation. Conversely, recovery is characterized by parasympathetic activation followed by sympathetic withdrawal, although clarification of the normal trajectory and autonomic basis of heart rate decay following exercise is needed. Abnormalities in autonomic physiology--especially increased sympathetic activity, attenuated vagal tone, and delayed heart rate recovery--have been associated with increased mortality. Exercise training is associated with a relative enhancement of vagal tone, improved heart rate recovery after exercise, and reduced morbidity in patients with cardiovascular disease. However, whether exercise training leads to reduced mortality in this population because of its ability to specifically modulate autonomic function is unknown at the present time. Although the results of a recent randomized study in patients with CHF and a meta-analysis in the setting of a recent myocardial infarction determined that exercise training leads to improved outcomes in these populations, neither study measured autonomic function. Improved autonomic function due to exercise training is a promising rationale for explaining improvements in outcome, although more research is needed to confirm this hypothesis.

Isometric handgrip training reduces arterial pressure at rest without changes in sympathetic nerve activity

The purpose of this study was to determine whether isometric handgrip (IHG) training reduces arterial pressure and whether reductions in muscle sympathetic nerve activity (MSNA) mediate this drop in arterial pressure. Normotensive subjects were assigned to training (n = 9), sham training (n = 7), or control (n = 8) groups. The training protocol consisted of four 3-min bouts of IHG exercise at 30% of maximal voluntary contraction (MVC) separated by 5-min rest periods. Training was performed four times per week for 5 wk. Subjects' resting arterial pressure and heart rate were measured three times on 3 consecutive days before and after training, with resting MSNA (peroneal nerve) recorded on the third day. Additionally, subjects performed IHG exercise at 30% of MVC to fatigue followed by muscle ischemia. In the trained group, resting diastolic (67 +/- 1 to 62 +/- 1 mmHg) and mean arterial pressure (86 +/- 1 to 82 +/- 1 mmHg) significantly decreased, whereas systolic arterial pressure (116 +/- 3 to 113 +/- 2 mmHg), heart rate (67 +/- 4 to 66 +/- 4 beats/min), and MSNA (14 +/- 2 to 15 +/- 2 bursts/min) did not significantly change following training. MSNA and cardiovascular responses to exercise and postexercise muscle ischemia were unchanged by training. There were no significant changes in any variables for the sham training and control groups. The results indicate that IHG training is an effective nonpharmacological intervention in lowering arterial pressure.

Sympathetic adaptations to one-legged training

The purpose of the present study was to determine the effect of leg exercise training on sympathetic nerve responses at rest and during dynamic exercise. Six men were trained by using high-intensity interval and prolonged continuous one-legged cycling 4 day/wk, 40 min/day, for 6 wk. Heart rate, mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA; peroneal nerve) were measured during 3 min of upright dynamic one-legged knee extensions at 40 W before and after training. After training, peak oxygen uptake in the trained leg increased 19 +/- 2% (P < 0.01). At rest, heart rate decreased from 77 +/- 3 to 71 +/- 6 beats/min (P < 0.01) with no significant changes in MAP (91 +/- 7 to 91 +/- 11 mmHg) and MSNA (29 +/- 3 to 28 +/- 1 bursts/min). During exercise, both heart rate and MAP were lower after training (108 +/- 5 to 96 +/- 5 beats/min and 132 +/- 8 to 119 +/- 4 mmHg, respectively, during the third minute of exercise; P < 0.01). MSNA decreased similarly from rest during the first 2 min of exercise both before and after training. However, MSNA was significantly less during the third minute of exercise after training (32 +/- 2 to 22 +/- 3 bursts/min; P < 0.01). This training effect on MSNA remained when MSNA was expressed as bursts per 100 heartbeats. Responses to exercise in five untrained control subjects were not different at 0 and 6 wk. These results demonstrate that exercise training prolongs the decrease in MSNA during upright leg exercise and indicates that attenuation of MSNA to exercise reported with forearm training also occurs with leg training.

Differential effects of training on the control of skeletal muscle perfusion

Endurance and high-intensity sprint training have been shown to alter skeletal muscle blood flow and factors that govern muscle perfusion under various conditions. Neither endurance nor sprint training alter skeletal muscle perfusion at rest but can result in an increase in muscle blood flow during the anticipation of exercise. The magnitude of the anticipatory increases in muscle blood flow is dependent on the intensity and duration of the prior training bouts and results from elevations in mean arterial pressure and decreases in vascular resistance in skeletal muscle. The decrements in skeletal muscle vascular resistance appear to be mediated through increases in muscle sympathetic cholinergic nerve activity or decreases in muscle sympathetic adrenergic nerve activity. During submaximal exercise, total muscle blood flow is either unchanged or slightly lower. However, a redistribution of muscle blood flow may occur following aerobic training, resulting in an enhanced perfusion of high-oxidative skeletal muscles and less flow going to low-oxidative muscles. The increased perfusion of the high-oxidative muscles may result from various factors including: a) increased recruitment of high-oxidative motor units, b) increased local release of metabolic vasodilator substances, c) qualitative changes in the metabolic substances released, d) decreased muscle sympathetic nerve activity, e) diminished sensitivity of the arterial vasculature to norepinephrine or other vasoconstrictor agents, f) enhanced endothelium-mediated dilation in the resistance vasculature, and g) an increased effectiveness of the skeletal muscle pump. Conversely, the decreases in blood flow to low-oxidative muscles may result from an enhanced autoregulatory responsiveness of the resistance vasculature. Endurance and sprint training increase muscle perfusion during exercise at VO2max: this primarily appears to be the result of an enhanced pumping capacity of the heart to increase in maximal cardiac output. Many of the training-induced alterations in muscle blood flow and vascular structure are localized in the muscles that are most active during the training bouts. Therefore, differences in muscle recruitment patterns that occur with low-intensity endurance exercise and high-intensity sprint exercise may account for differences observed between these two training regimens.

Sympathetic neural adaptations to exercise training in humans: insights from microneurography

Sympathetic nerve activity has long been regarded as an important regulator of blood flow and blood pressure. Its importance has been especially recognized during exercise. The present review examines sympathetic neural adaptations to exercise training in humans obtained by sympathetic nerve recordings to nonactive skeletal muscle. Little evidence exists from both cross-sectional and longitudinal studies indicating that training alters resting muscle sympathetic nerve activity (MSNA). However, MSNA responses during exercise appear to be attenuated after training. This attenuation of MSNA seems to be specific to the trained muscle and not generalizable to other muscle groups. The mechanisms for the decrease in exercise-induced MSNA have been attributed to changes in both the muscle metaboreflex and muscle mechanoreflex. In addition to exercise, training has generally not altered MSNA responses to other stressors such as cold pressor test, lower body negative pressure, and upright tilting. However, the effect of training on baroreflex control of MSNA is equivocal. These conclusions are based on few studies. More comprehensive training studies are needed to better understand the role of training on sympathetic neural outflow.

Power spectrum analysis contribution to the detection of cardiovascular dysautonomia in multiple sclerosis

In multiple sclerosis (MS) autonomic cardiovascular dysfunction is an uncommon, but potentially dangerous event, to which studies of spectral analysis of heart rate variability have not been applied, yet. MATERIAL AND METHODS--We studied 16 patients with definite MS (11 women and 5 men, mean age 30.3 +/- 7.4 yrs., mean EDSS 2.06 +/- 1.42) and 16 sex- and age-matched healthy controls. Besides cardiovascular reflex tests (valsalva manoeuvre, deep breathing, lying to standing, Blood Pressure response to standing and sustained handgrip), each underwent spectral analysis of the R-R interval short-term variability at rest and after tilting, to detect three components: very low frequency (VLF), low frequency (LF) and high frequency (HF). A recent brain MRI was obtained from patients, to compare plaque characteristics with spectral parameters. RESULTS--At cardiovascular reflexes, only four patients (25%) showed an impairment, mostly of a mild degree. VLF and LF at rest were lower in MS subjects than in controls (p < 0.01). No significant correlation was found between spectral parameters and lesion area or localization as detected on MRI. CONCLUSIONS--Spectral analysis could usefully flank reflex tests to detect autonomic subclinical cardiovascular abnormalities.

Sympathetic neural mechanisms in obstructive sleep apnea

Blood pressure, heart rate, sympathetic nerve activity, and polysomnography were recorded during wakefulness and sleep in 10 patients with obstructive sleep apnea. Measurements were also obtained after treatment with continuous positive airway pressure (CPAP) in four patients. Awake sympathetic activity was also measured in 10 age- and sex-matched control subjects and in 5 obese subjects without a history of sleep apnea. Patients with sleep apnea had high levels of nerve activity even when awake (P < 0.001). Blood pressure and sympathetic nerve activity did not fall during any stage of sleep. Mean blood pressure was 92 +/- 4.5 mmHg when awake and reached peak levels of 116 +/- 5 and 127 +/- 7 mmHg during stage II sleep (n = 10) and rapid eye movement (REM) sleep (n = 5), respectively (P < 0.001). Sympathetic activity increased during sleep (P = 0.01) especially during stage II (133 +/- 9% above wakefulness; P = 0.006) and REM (141 +/- 13%; P = 0.007). Peak sympathetic activity (measured over the last 10 s of each apneic event) increased to 299 +/- 96% during stage II sleep and to 246 +/- 36% during REM sleep (both P < 0.001). CPAP decreased sympathetic activity and blood pressure during sleep (P < 0.03). We conclude that patients with obstructive sleep apnea have high sympathetic activity when awake, with further increases in blood pressure and sympathetic activity during sleep. These increases are attenuated by treatment with CPAP.

Physical training and baroreceptor control of sympathetic nerve activity in humans

In nine sedentary subjects (16.5 +/- 0.4 years, mean +/- SEM) we measured blood pressure (Finapres device), heart rate (electrocardiogram), and postganglionic muscle sympathetic nerve activity (microneurography from the peroneal nerve) at rest and during intravenous infusion of phenylephrine and nitroprusside. These measurements were performed before and after 10 weeks of endurance training (2 h/d, 5 d/wk) that increased maximum oxygen consumption from 34.8 +/- 2.1 to 40.4 +/- 1.8 mL/kg per minute (P < .02). Basal mean blood pressure and muscle sympathetic nerve activity were lower after than before endurance training (86.5 +/- 2.6 versus 97.5 +/- 1.8 mm Hg, P < .05, and 14.0 +/- 1.8 versus 21.2 +/- 2.3 bursts per minute, P < .02), and the changes in these variables were closely related (r = .95, P < .01). Similar mean blood pressure increases induced by phenylephrine caused greater reductions in heart rate and muscle sympathetic nerve activity after than before endurance training (-8.6 +/- 0.8 versus -6.1 +/- 1.1 beats per minute, P = NS, and -78.0 +/- 4.6% versus -53.6 +/- 4.8%, P < .05). Likewise, similar mean blood pressure reductions induced by nitroprusside caused greater increases in heart rate and muscle sympathetic nerve activity after than before endurance training (18.6 +/- 3.0 versus 12.4 +/- 2.4 beats per minute, P < .05, and 128.1 +/- 26% versus 63.2 +/- 11%, P < .02). No alteration in hemodynamics, oxygen consumption, muscle sympathetic nerve activity, and baroreceptor reflex sensitivity occurred in four other age-matched sedentary subjects studied before and after a 10-week observation period without endurance training.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of muscle sympathetic nerve activity during exercise in dominant and nondominant forearm

To determine whether or not muscle endurance training alters exercise-induced sympathetic nerve response, we recorded muscle sympathetic nerve activity (MSNA) microneurographically during forearm exercise and compared MSNA between dominant (D) and non-dominant (ND) forearms of players of racket sports. Three kinds of forearm exercise were conducted on each side; static (SHG) and dynamic (DHG, at a rate of 1 Hz) handgrip exercise at a loading of 25% of maximal voluntary contraction until exhaustion, and 10-min submaximal dynamic handgrip (at a rate of 1 Hz) at an intensity of 0.9 W. Heart rate, ventilation and blood pressure were also monitored at rest and during SHG and DHG exercises. During the last minute of SHG exercise, MSNA burst rate had increased on average by 290 (SEM 46)% in D and 330 (SEM 46)% in ND, while during DHG it increased by 288 (SEM 38)% in D and 344 (SEM 36)% in ND, respectively. There were no significant differences in the MSNA responses between D and ND forearms in either exercise modes. Significant increases in heart rate, ventilation and blood pressure during the last minute of fatiguing SHG and DHG were observed, but there were no significant differences between the two forearms. During submaximal DHG, while MSNA increased significantly above control values in both D and ND, the MSNA response was less in D than that in ND forearm. The results would suggest that exercise-induced MSNA responsiveness is influenced little by muscle endurance training but the intensity of response may be due to the magnitude of metaboreceptor stimulation in the exercising muscle.

Comparison of 24-hour parasympathetic activity in endurance-trained and untrained young men

OBJECTIVES

This study compares 24-h parasympathetic activity in aerobically trained and untrained healthy young men.

BACKGROUND

Higher values of parasympathetic nervous system activity are associated with a low mortality rate in patients after myocardial infarction, but it remains uncertain what therapeutic interventions can be used to increase parasympathetic activity. Although it is thought that exercise training can increase parasympathetic activity, studies have reported conflicting results, perhaps because this variable was measured for only brief intervals and usually inferred from changes in reflex responses induced by pharmacologic blockade.

METHODS

Parasympathetic activity was assessed noninvasively from 24-h ECG recordings by calculating high frequency (0.15 to 0.40 Hz) beat to beat heart period variability in eight endurance-trained men (maximal oxygen consumption greater than or equal to 55 ml/kg per min) and eight age-matched (mean = 29 yr) untrained men (maximal oxygen consumption less than or equal to 40 ml/kg per min). The data were analyzed separately for sleeping hours when parasympathetic activity is dominant and also for waking hours.

RESULTS

The geometric mean of high frequency power was greater in the trained than in the untrained men during the day (852 vs. 177 ms2, p less than 0.005), during the night (1,874 vs. 427 ms2, p less than 0.005) and over the entire 24 h (1,165 vs. 276 ms2, p less than 0.001).

CONCLUSIONS

Parasympathetic activity is substantially greater in trained than in untrained men, and this effect is present during both waking and sleeping hours. These data suggest that exercise training may increase parasympathetic activity over the entire day and may therefore prove to be a useful adjunct or alternative to drug therapy in lessening the derangements of autonomic balance found in many cardiovascular diseases.

Sympathetic neural adjustments to stress in physically trained and untrained humans

The purpose of this study was to determine if the state of physical training influences sympathetic neural activation during acute stress in humans. We recorded muscle sympathetic nerve activity (microneurography of the peroneal nerve), arterial blood pressure, and heart rate in 12 highly trained, endurance athletes (25 +/- 1 years, mean +/- SEM) and 12 untrained subjects (27 +/- 1 years) before (supine rest control) and during: 1) lower body negative pressure at -5, -10, -15, and -20 mm Hg (orthostatic stress); 2) isometric handgrip at 30% of maximum (exercise stress); and 3) hand immersion in ice water, that is, the cold pressor test (thermal stress). Body weight was not different in the two groups, but the athletes had a lower body fat content (8.9 +/- 1.3% versus 16.1 +/- 2.0%, p less than 0.05). During supine rest, muscle sympathetic nerve burst frequency (24 +/- 3 versus 24 +/- 2 bursts/min, athletes versus untrained subjects) and burst incidence (36 +/- 3 versus 44 +/- 4 bursts/100 heart beats) and arterial blood pressure were not different in the two groups, but heart rate was lower in the athletes (54 +/- 2 versus 67 +/- 3 beats/min, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Responses in muscle sympathetic nerve activity to sustained hand-grips of different tensions in humans

To clarify whether sympathetic nerve activity increases in relation to the tension of a sustained muscle contraction, muscle sympathetic nerve activity (MSA) was recorded directly from the peroneal nerve fascicle at the popliteal fossa by means of tungsten microelectrodes in five healthy male subjects. A sustained muscle contraction was performed by handgrip for two minutes in a supine position at tensions of 10, 30 and 45% of maximal grip strength (MGS). MSA, electrocardiogram (ECG) using bipolar electrodes from the chest and surface electromyogram (EMG) from the extensor pollicis longus were recorded simultaneously before and during the sustained handgrip. Arterial blood pressure was measured at the resting upper arm by auscultation. During handgrip with tensions of 10, 30 and 45% MGS, average MSA burst rate (bursts X min-1) increased to 122, 152 and 230% of the resting value, respectively. During the same experimental procedures with tensions of 10, 30 and 45% MGS, average heart rate increased to 105, 110 and 111% of the resting value. These results confirm that sympathetic outflow to a resting muscle is increased with elevation of tension in an active muscle. This process would promote perfusion pressure in the active muscle.