Altered cardiovascular responsiveness to adrenaline in endurance-trained subjects

Maturation of the Cardiac Autonomic Nervous System Activity in Children and Adolescents

Background

Despite the increasing interest in cardiac autonomic nervous activity, the normal development is not fully understood. The main aim was to determine the maturation of different cardiac sympathetic‐(SNS) and parasympathetic nervous system (PNS) activity parameters in healthy patients aged 0.5 to 20 years. A second aim was to determine potential sex differences.

Methods and Results

Five studies covering the 0.5‐ to 20‐year age range provided impedance‐ and electrocardiography recordings from which heart rate, different PNS‐parameters (eg, respiratory sinus arrhythmia) and an SNS‐parameter (pre‐ejection period) were collected. Age trends were computed in the mean values across 12 age‐bins and in the age‐specific variances. Age was associated with changes in mean and variance of all parameters. PNS‐activity followed a cubic trend, with an exponential increase from infancy, a plateau phase during middle childhood, followed by a decrease to adolescence. SNS‐activity showed a more linear trend, with a gradual decrease from infancy to adolescence. Boys had higher SNS‐activity at ages 11 to 15 years, while PNS‐activity was higher at 5 and 11 to 12 years with the plateau level reached earlier in girls. Interindividual variation was high at all ages. Variance was reasonably stable for SNS‐ and the log‐transformed PNS‐parameters.

Conclusions

Cardiac PNS‐ and SNS‐activity in childhood follows different maturational trajectories. Whereas PNS‐activity shows a cubic trend with a plateau phase during middle childhood, SNS‐activity shows a linear decrease from 0.5 to 20 years. Despite the large samples used, clinical use of the sex‐specific centile and percentile normative values is modest in view of the large individual differences, even within narrow age bands.

Ambulatory measurement of the ECG T-wave amplitude

Ambulatory recording of the preejection period (PEP) can be used to measure changes in cardiac sympathetic nervous system (SNS) activity under naturalistic conditions. Here, we test the ECG T-wave amplitude (TWA) as an alternative measure, using 24-h ambulatory monitoring of PEP and TWA in a sample of 564 healthy adults. The TWA showed a decrease in response to mental stress and a monotonic decrease from nighttime sleep to daytime sitting and more physically active behaviors. Within-participant changes in TWA were correlated with changes in the PEP across the standardized stressors (r = .42) and the unstandardized naturalistic conditions (mean r = .35). Partialling out changes in heart rate and vagal effects attenuated these correlations, but they remained significant. Ambulatory TWA cannot replace PEP, but simultaneous recording of TWA and PEP provides a more comprehensive picture of changes in cardiac SNS activity in real-life settings.

Aerobic exercise and strength training effects on cardiovascular sympathetic function in healthy adults: a randomized controlled trial

OBJECTIVE

Exercise has widely documented cardioprotective effects, but the mechanisms underlying these effects are not entirely known. Previously, we demonstrated that aerobic but not strength training lowered resting heart rate and increased cardiac vagal regulation, changes that were reversed by sedentary deconditioning. Here, we focus on the sympathetic nervous system and test whether aerobic training lowers levels of cardiovascular sympathetic activity in rest and that deconditioning would reverse this effect.

METHODS

We conducted a randomized controlled trial contrasting the effects of aerobic (A) versus strength (S) training on indices of cardiac (preejection period, or PEP) and vascular (low-frequency blood pressure variability, or LF BPV) sympathetic regulation in 149 young, healthy, and sedentary adults. Participants were studied before and after conditioning, as well as after 4 weeks of sedentary deconditioning.

RESULTS

As previously reported, aerobic capacity increased in response to conditioning and decreased after deconditioning in the aerobic, but not the strength, training group. Contrary to prediction, there was no differential effect of training on either PEP (A: mean [SD] -0.83 [7.8] milliseconds versus S: 1.47 [6.69] milliseconds) or LF BPV (A: mean [SD] -0.09 [0.93] ln mm Hg(2) versus S: 0.06 [0.79] ln mm Hg(2)) (both p values > .05).

CONCLUSIONS

These findings, from a large randomized controlled trial using an intent-to-treat design, show that moderate aerobic exercise training has no effect on resting state cardiovascular indices of PEP and LF BPV. These results indicate that in healthy, young adults, the cardioprotective effects of exercise training are unlikely to be mediated by changes in resting sympathetic activity.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT00358137.

Estimated preejection period (PEP) based on the detection of the R-wave and dZ/dt-min peaks does not adequately reflect the actual PEP across a wide range of laboratory and ambulatory conditions

The current study evaluates the validity of the PEP computed from a fixed value for the Q-wave onset to R-wave peak (QR) interval and an R-wave peak to B-point (RB) interval that is estimated from the R-peak to dZ/dt-min peak (ISTI) interval. Ninety-one subjects participated in a 90min laboratory experiment in which a variety of often employed physical and mental stressors were presented and 31 further subjects participated in a structured 2hour ambulatory recording in which they partook in natural activities that induced large variation in posture and physical activity. PEP, QR interval, and ISTI were scored and rigorously checked by interactive inspection. Across the very diverse laboratory and ambulatory conditions the QR interval could be approximated by a fixed interval of 40ms but 95% confidence intervals were large (25.5 to 54.5ms). Multilevel analysis showed that 79% to 81% of the within and between-subject variation in the RB interval could be predicted by the ISTI with a simple linear regression equation. However, the optimal intercept and slope values in this equation varied significantly across subjects and study setting. Bland Altman plots revealed a large discrepancy between the estimated PEP using the R-wave peak and dZ/dt-min peak and the actual PEP based on the Q-wave onset and B-point. We conclude that the PEP estimated from a fixed QR interval and the ISTI could be a useful addition to the psychophysiologist's toolbox, but that it cannot replace the actual PEP to index cardiac sympathetic control.

Effects of antidepressants, but not psychopathology, on cardiac sympathetic control: a longitudinal study

Increased sympathetic activity has been hypothesized to have a role in the elevated somatic disease risk in persons with depressive or anxiety disorders. However, it remains unclear whether increased sympathetic activity reflects a direct effect of anxiety or depression or an indirect effect of antidepressant medication. The aim of this study was to test longitudinally whether cardiac sympathetic control, measured by pre-ejection period (PEP), was increased by depression/anxiety status and by antidepressant use. Cross-sectional and longitudinal data were from a depression and anxiety cohort: the Netherlands Study of Depression and Anxiety (NESDA). Baseline data of 2838 NESDA subjects (mean age 41.7 years, 66.7% female) and 2-year follow-up data of 2226 subjects were available for analyses. Included were subjects with and without depressive/anxiety disorders, using or not using different antidepressants at baseline or follow-up. The PEP was measured non-invasively by 1.5 h of ambulatory impedance cardiography. Cross-sectional analyses compared PEP across psychopathology and antidepressant groups. Longitudinal analyses compared 2-year changes in PEP in relation to changes in psychopathology and antidepressant use. Cross-sectional analyses showed that antidepressant-naïve depressive/anxious subjects had comparable PEP as controls, whereas subjects using tricyclic (TCA) or combined serotonergic/noradrenergic antidepressants (SNRI) had significantly shorter PEP compared with controls. In contrast, subjects using selective serotonin re-uptake inhibitors (SSRIs) had longer PEP than controls. Longitudinal results confirmed these findings: compared with 2-year change in PEP in continuous non-users (+2 ms), subjects who started TCA or SNRI treatment showed significantly shortened PEP (-11 ms, p=0.005 and p<0.001), whereas subjects who started SSRI treatment showed significant prolongation of PEP (+9 ms, p=0.002). Reversed findings were observed among those who stopped antidepressant use. These findings suggest that depressive and anxiety disorders are not associated with increased cardiac sympathetic control. However, results pose that TCA and SNRI use increases sympathetic control, whereas SSRI use decreases sympathetic control.

Adrenaline during mental stress in relation to fitness, metabolic risk factors and cardiovascular responses in young men

We studied plasma adrenaline (A) in relation to physical fitness, metabolic cardiovascular risk factors and cardiovascular responses. Men (age 21-24 years) with high and normal (both n=19) screening blood pressure (BP) were studied cross-sectionally. We measured peak oxygen uptake (VO2peak) (treadmill exercise), and plasma catecholamines, heart rate (HR), finger systolic (SBP) and diastolic (DBP) BP, and insulin-adjusted glucose disposal rate (GDR/I) during a hyperinsulinaemic glucose clamp (rest) and mental arithmetic stress test (MST). By multiple regression, A at rest (Arest) (beta=0.37, p<0.05) and during MST (Amst) (beta=0.40, p<0.01) were associated with high screening BP. In the respective models, Arest was negatively related to body mass index (BMI) (beta=-0.56, p<0.001) and Amst positively to VO2peak (beta=0.54, p<0.001). BP and HR responses correlated positively with VO2peak, but were determined by Amst in multiple regression models. Independently of BMI and VO2peak, serum high-density lipoprotein cholesterol was positively related to A levels, whereas GDR/I was independently related only to VO2peak. Increased adrenaline secretion may be related to high BP, but may at the same time be associated with a beneficial metabolic profile.

Ergonomics, user comfort, and performance in standard and robot-assisted laparoscopic surgery

Background

Robot-assisted surgical systems have been introduced to improve the outcome of minimally invasive surgery. These systems also have the potential to improve ergonomics for the surgeon during endoscopic surgery. This study aimed to compare the user’s mental and physical comfort in performing standard laparoscopic and robot-assisted techniques. Surgical performance also was analyzed.

Methods

In this study, 16 surgically inexperienced participants performed three tasks using both a robotic system and standard laparoscopic instrumentation. Distress was measured using questionnaires and an ambulatory monitoring system. Surgical performance was analyzed with time-action analysis.

Results

The physiologic parameters (p = 0.000), the questionnaires (p = 0.000), and the time-action analysis (p = 0.001) favored the robot-assisted group in terms of lower stress load and an increase in work efficiency.

Conclusion

In this experimental setup, the use of a robot-assisted surgical system was of value in both cognitive and physical stress reduction. Robotic assistance also demonstrated improvement in performance.

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.

Effects of variation in posture and respiration on RSA and pre-ejection period

The extent to which variation in posture and respiration can confound pre-ejection period and respiratory sinus arrhythmia (RSA) as indices of cardiac sympatho-vagal activity was examined. Within-subjects changes in these measures were assessed in 36 subjects during different postures and (paced) respiratory frequencies. Changes from supine to sitting to standing led to reduced RSA values and longer pre-ejection periods, reflecting the known decrease in vagal but not the increase of sympathetic activity. Multilevel path analysis showed that within-subjects changes in sympatho-vagal balance were faithfully reflected by changes in interbeat interval, but imperfectly by changes in RSA and pre-ejection period. It was concluded that pre-ejection period should be stratified for posture and RSA for respiratory frequency to reliably index changes in sympatho-vagal balance when these factors are prone to change (e.g., during 24-h ambulatory recording).

Overcommitment to work is associated with changes in cardiac sympathetic regulation

OBJECTIVE

Work stress is associated with an increased risk for cardiovascular disease (CVD). Exaggerated cardiovascular reactivity to work-related stressors or incomplete recovery after work is a proposed mechanism underlying this increase in risk. This study examined the effects of work stress on 24-hour profiles of the pre-ejection period (PEP), a measure of cardiac sympathetic activity, obtained from ambulatory measurement of the impedance cardiogram.

METHODS

A total of 67 male white-collar workers (age 47.1 +/- 5.2) underwent ambulatory monitoring on 2 workdays and 1 non-workday. Work stress was defined according to Siegrist's model as 1) a combination of high effort and low reward at work (high imbalance) or 2) an exhaustive work-related coping style (high overcommitment).

RESULTS

High overcommitment was associated with shorter absolute PEP levels during all periods on all 3 measurement days, reduced wake-to-sleep PEP differences and reduced PEP variability, as indexed by the SD.

CONCLUSIONS

Overcommitment to work was associated with an increase in basal sympathetic drive and a reduction in the dynamic range of cardiac sympathetic regulation. Both findings are compatible with the hypothesis that overcommitment induces beta-receptor down-regulation.

Large-scale ensemble averaging of ambulatory impedance cardiograms

Impedance cardiography has been used increasingly to measure human physiological responses to emotional and mentally engaging stimuli. The validity of large-scale ensemble averaging of ambulatory impedance cardiograms was evaluated for preejection period (PEP), interbeat interval, and dZ/dt(min) amplitude. We tested whether the average of "classical" 60-sec ensemble averages across periods with fixed activity, posture, physical load, social situation, and location could be accurately estimated from a single large-scale ensemble average spanning these entire periods. Impedance and electrocardiograms were recorded for about 24-h from 21 subjects. Recordings were scored by seven raters, using both methods for each subject. Good agreement (average intraclass correlation coefficient was .91) between both ensemble averaging methods was found for all three cardiac function measures. The results indicate that for unambiguous ambulatory impedance cardiograms, large-scale ensemble averaging is valid, which makes measuring prolonged changes in cardiac sympathetic activity by measuring ambulatory PEP feasible even in large epidemiological samples.

Antenatal dexamethasone treatment decreases plasma catecholamine levels in preterm infants

Antenatal corticosteroid therapy (ACT) has many beneficial effects on preterm infants. The cellular mechanisms of action of ACT include beta-adrenergic receptor-mediated cAMP generation. This study investigated the effects of ACT on sympathoadrenal mechanisms during immediate postnatal adaptation of preterm infants. Plasma epinephrine, norepinephrine, 3,4-dihydroxyphenylglycol, and cAMP were measured within 12 h after birth in 103 preterm infants (gestational age 24-36 wk), who were divided into two groups (non-ACT and ACT group) according to whether the mother had received dexamethasone treatment. Infants in the ACT group had significantly lower concentrations of plasma catecholamines than infants in the non-ACT group; plasma epinephrine was 38% lower, and plasma norepinephrine was 20-40% lower in the ACT group, depending on gestational age (r = -0.37 in the non-ACT group and r = -0.28 in the ACT group, p < 0.05). Plasma cAMP concentrations were similar in the two groups. Antihypertensive treatment of the mother was associated with low plasma cAMP (p < 0.001), whereas tocolytic treatment was associated with high plasma cAMP (p = 0.001) of the infant. The results indicate that ACT attenuates the birth-related increase in plasma catecholamines. Still, plasma cAMP levels remain high, which suggests enhanced beta-adrenoceptor signaling after ACT.

Opposing effects of plasma epinephrine and norepinephrine on coronary thrombosis in vivo

BACKGROUND

It is well known that plasma catecholamines and myocardial infarction have a close relation and that coronary artery thrombosis is a major cause of myocardial infarction. In addition, epinephrine is known to be a prothrombogenic agent in vivo. However, the role of the other major circulating catecholamine, norepinephrine, in the development of coronary thrombosis is somewhat uncertain, although the role of norepinephrine is often considered analogous to the role of epinephrine. Therefore, the present study was designed to investigate the effect of norepinephrine and its interaction with epinephrine on coronary thrombosis.

METHODS AND RESULTS

To compare the effects of epinephrine and norepinephrine on coronary thrombosis, we analyzed the frequency of cyclic blood flow reductions (CFRs) in an anesthetized canine model of coronary thrombosis (n = 25). Three experiments were used in the present study. In the first experiment with epinephrine infusion, plasma epinephrine was elevated from 0.46 +/- 0.25 to 27.7 +/- 1.85 nmol/L. The frequency of CFRs increased by more than 60%, from 7.1 +/- 0.5 to 11.5 +/- 0.7 in 40 minutes (P < .01). The second experiment included three experimental periods: control, norepinephrine infusion, and norepinephrine infusion plus epinephrine infusion. Norepinephrine was infused to raise plasma norepinephrine from 1.3 +/- 0.2 to 32.4 +/- 4.3 nmol/L. The frequency of CFRs in the dogs was markedly reduced, from 7.89 +/- 0.42 to 2.41 +/- 1.08 in 40 minutes (P < .01), whereas arterial pressure was elevated from 88 +/- 3 to 118 +/- 5 mm Hg (P < .01). However, when epinephrine infusion was added to the norepinephrine infusion, the frequency of CFRs increased from 2.41 +/- 1.08 to 7.74 +/- 1.12 in 40 minutes (P < .01). In the third experiment, a servocontrol device was used during the norepinephrine infusion to prevent rises in coronary arterial pressure. As a result of the norepinephrine infusion, the frequency of CFRs was reduced from 7.47 +/- 0.71 to 0.83 +/- 0.65 in 40 minutes (P < .01), even though the coronary arterial pressure was not altered.

CONCLUSIONS

The present study demonstrated that infusion of epinephrine stimulated coronary artery thrombosis, whereas infusion of norepinephrine inhibited coronary artery thrombosis. In addition, the inhibitory effect of norepinephrine on coronary thrombosis is independent of increases in coronary arterial pressure. Therefore, the present findings suggest that epinephrine and norepinephrine have opposing effects on coronary thrombosis in dogs.

Effect of epinephrine infusion on chest pain in syndrome X in the absence of signs of myocardial ischemia

Eight female patients (aged 51 to 65 years) with New York Heart Association class II angina pectoris, normal coronary angiograms, normal hyperventilation, and abnormal exercise stress tests (chest pain and ST depression), and 5 sex- and age-matched controls participated in this study. Epinephrine was given intravenously to both patients and controls at 5-minute intervals in doses of 0.1, 0.2, 0.3, 0.4, and 0.6 nmol/kg/min. After rest (15 minutes), the alpha-adrenoceptor antagonist phentolamine or placebo was administered intravenously to patients in a double-blind, crossover study on 2 separate occasions in doses of 250 micrograms/min for 5 minutes and 500 micrograms/min for the next 10 minutes; the epinephrine infusion was repeated. Blood pressure, heart rate, and electrocardiogram were monitored continuously and pain was estimated on the Borg CR-10 scale. On a third occasion, chest pain was induced in patients using the same epinephrine protocol during echocardiographic monitoring. In the control group, all patients received the maximal epinephrine dose. No chest discomfort or pain developed. In the patient group, the maximal tolerable epinephrine dose (0.39 +/- 0.19 nmol/kg/min) decreased diastolic pressure (-14 +/- 9 mm Hg, p < 0.01) and increased heart rate (+24 +/- 10 beats/min, p < 0.01), not statistically different from the control group. Pulse pressure increased in the patient group (27 +/- 17 mm Hg, p < 0.01) but not in the controls. Left ventricular ejection fraction at baseline was within reference limits (58% to 75%) and did not change during epinephrine infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobic exercise and Type A behaviour

The Type A behaviour pattern is a well-documented, if controversial, risk factor for coronary heart disease. Surprisingly, relatively little work has been reported on ways of modifying this behaviour pattern. Aerobic exercise, with its demonstrated benefits for both cardiovascular reactivity and psychological 'well-being', is a promising treatment. The literature is reviewed and recommendations are made for practical applications and future research.

Norepinephrine-induced human platelet activation in vivo is only partly counteracted by aspirin

BACKGROUND

Epinephrine and mental stress may, via platelet stimulation, enhance the risk of thrombus formation. Norepinephrine is more likely than epinephrine to activate platelets in vivo because of higher levels in plasma but is less well studied in this respect. The antiplatelet drug of choice for patients with coronary artery disease, aspirin, may be less effective during sympathoadrenal activation. We therefore investigated platelet responses in vivo to exogenous norepinephrine with and without aspirin pretreatment.

METHODS AND RESULTS

Platelet aggregability in vivo was assessed in 11 healthy male subjects, by filtragometry ex vivo (which reflects platelet aggregability in vivo) and by measurements of plasma beta-thromboglobulin (beta-TG, which reflects platelet secretion). Norepinephrine infusions elevated venous plasma norepinephrine from 1.5 to 4 and 15 nmol/L, respectively, and enhanced platelet aggregability (filtragometry) concentration dependently (P < .001). Platelet secretion (beta-TG levels) increased during high-dose infusion (P < .01). Aspirin pretreatment (500 mg orally 12 hours earlier) reduced the excretion of 11-dehydrothromboxane B2 by 62 +/- 5% (P < .001) and attenuated platelet aggregability at rest (P < .05) but not the effect of norepinephrine infusion on platelet aggregability. Conversely, resting plasma beta-TG levels and the urinary excretion of high-molecular-weight beta-TG were not altered by aspirin pretreatment, whereas the norepinephrine-induced increase in plasma beta-TG was abolished.

CONCLUSIONS

Norepinephrine, at plasma levels easily attained during exercise, enhances platelet aggregability and platelet secretion in vivo in healthy humans. Aspirin may be less effective as an antithrombotic drug during sympathoadrenal activation in humans.

Epinephrine sensitizes human platelets in vivo and in vitro as studied by fibrinogen binding and P-selectin expression

Epinephrine (Epi) infusion influences platelet activation markers in vivo, but in vitro studies have mainly examined supraphysiological Epi concentrations and have yielded conflicting results. In this study whole-blood flow-cytometric measurements of platelet fibrinogen binding and P-selectin expression were used to compare enhancement of ADP (0.1 to 10 mumol/L)-induced platelet activation by Epi infusion in vivo (0.1 and 0.4 nmol.kg-1.min-1) and by Epi in vitro (10 and 50 nmol/L) in nine healthy volunteers. ADP caused concentration-dependent increases in the percentage of platelets that bound fibrinogen (from 4.4 +/- 0.9% to 69.9 +/- 4.2%) and that expressed P-selectin (from 4.5 +/- 0.5% to 44.2 +/- 3.8%). Fibrinogen and P-selectin binding indices (FgBI and PSBI; calculated from mean fluorescence intensity and percentage of positive cells) also increased from 0.18 +/- 0.03 to 11.70 +/- 1.99 for FgBI and from 0.22 +/- 0.03 to 2.34 +/- 0.29 for PSBI. Epi concentration-dependently enhanced fibrinogen binding and P-selectin expression in vitro (by approximately 30% at the midportion of the ADP curve at 10 nmol/L Epi; P < .001 for both by ANOVAs). High-dose Epi infusion enhanced FgBI similarly and increased maximal P-selectin expression by 38%. Epi (50 nmol/L in vitro) enhanced platelet activation further, whether samples were taken with or without prior Epi infusion. Total expression of glycoprotein IIb/IIIa was unaffected by Epi infusion, but glycoprotein Ib expression per platelet was reduced (P < .05). These in vivo and in vitro effects of Epi on platelet responses to agonist stimulation indicate a prothrombotic potential for sympathoadrenal activation in humans.

Adrenaline and the relationship between neurosomatism, aerobic fitness and mental task performance

The influence of neurotic instability as manifested by functional somatic complaints (neurosomatism) and aerobic fitness on responses to mental stress and to intravenous adrenaline infusions were investigated in 44 university students. Adrenaline-induced changes from resting levels in state anxiety and somatic anxiety were significantly more pronounced in high than in low neurosomatic subjects and all anxiety ratings were generally negatively related to aerobic fitness. Cardiovascular reactivity was induced by mental stress and by adrenaline infusions, but was not altered by neurosomatism. In individuals assumed to be characterized by a susceptibility to adrenergic effects, interference of adrenaline-induced arousal with cognitive performance may not occur. In contrast, a further increase in performance may occur when adrenaline is infused. Performance measures correlated negatively with anxiety during the baseline task and the placebo task, but this negative relation was absent during the adrenaline infusion and was replaced by positive relations between performance and aerobic power. The complex relations between bodily symptoms of anxiety, aerobic fitness and mental stress are discussed.

Differences in cardiovascular responses to isoproterenol in relation to age and exercise training in healthy men

BACKGROUND

Cardiac aging is characterized by a reduced heart rate response to beta-agonist stimulation with isoproterenol, but whether the ejection fraction and other cardiovascular responses are reduced in humans is largely unknown. In addition, whether reduced beta-agonist responses can be improved with exercise training has not been determined in humans.

METHODS AND RESULTS

Cardiovascular responses to graded isoproterenol infusions (3.5, 7, 14, and 35 ng/kg/min for 14 minutes each) were assessed in 15 older (age, 60-82 years) and 17 young (age, 24-32 years) rigorously screened healthy men. Thirteen older and 11 young subjects completed 6 months of endurance training and were retested. At baseline, the older group had reduced responses to isoproterenol for heart rate (+65% older versus +92% young, p less than 0.001), systolic blood pressure (+9% versus +24%, p less than 0.001), diastolic blood pressure (-12% versus -24%, p less than 0.05), ejection fraction (+12 versus +20 ejection fraction units, p less than 0.001), and cardiac output (+70% versus +100%, p less than 0.001). The mean plasma isoproterenol concentrations achieved during the infusions were marginally higher (p = 0.07) in the older group (128 +/- 58, 227 +/- 64, 354 +/- 114, and 700 +/- 125 pg/ml) than in the young (79 +/- 20, 178 +/- 49, 273 +/- 79, and 571 +/- 139 pg/ml). Intensive training increased maximal oxygen consumption by 21% in the older group (28.9 +/- 4.6 to 35.1 +/- 3.8 ml/kg/min, p less than 0.001) and by 17% in the young (44.5 +/- 5.1 to 52.1 +/- 6.3 ml/kg/min, p less than 0.001), but training did not augment any of the cardiovascular responses to isoproterenol in either group. The mean plasma isoproterenol concentrations at the four infusion doses were unchanged after training in both groups.

CONCLUSIONS

We conclude that there is an age-associated decline in heart rate, blood pressure, ejection fraction, and cardiac output responses to beta-adrenergic stimulation with isoproterenol in healthy men. Altered beta-adrenergic responses probably contribute to the reduced cardiac responses to maximal exercise that also occur with aging. Furthermore, intensive exercise training does not increase cardiac responses to beta-adrenergic stimulation with isoproterenol in either young or older men. The reduced beta-adrenergic response appears to be a primary age-associated change that is not caused by disease or inactivity.

The inhibition of cardiovascular responses to mental stress following aerobic exercise

The effects of exercise on subsequent psychophysiological responses to mental stress were assessed in a study of 30 normotensive male volunteers. Participants were randomly allocated to three experimental conditions--20-min exercise at 100 Watts (high exercise), 20-min exercise at 25 Watts (low exercise), or 20-min no exercise (control). After a recovery period of 20 min, all subjects performed a mental arithmetic task for four 5-min trials. Blood pressure and heart rate were monitored continuously using a Finapres, and respiration and electrodermal activity were also recorded. Baroreceptor reflex control of heart rate was assessed using power spectrum analysis. Exercise produced consistent increases in systolic blood pressure, heart rate, and subjective tension, together with reductions in systemic resistance and baroreflex sensitivity. The systolic and diastolic blood pressure and heart rate reactions to mental arithmetic were significantly blunted in the high exercise compared with control conditions, with the low exercise group showing an intermediate pattern. Subjective responses to mental stress were unaffected by prior exercise. The pattern of hemodynamic response was not a result of changes in baroreflex sensitivity. The mechanisms underlying this result are discussed in relation to the discrepancies between subjective and physiological responses to mental stress, and the implications for the use of exercise in stress management.

Effects of prolonged exercise at a similar percentage of maximal oxygen consumption in trained and untrained subjects

Six trained male cyclists and six untrained but physically active men participated in this study to test the hypothesis that the use of percentage maximal oxygen consumption (%VO2max) as a normalising independent variable is valid despite significant differences in the absolute VO2max of trained and untrained subjects. The subjects underwent an exercise test to exhaustion on a cycle ergometer to determine VO2max and lactate threshold. The subjects were grouped as trained (T) if their VO2max exceeded 60 ml.kg-1.min-1, and untrained (UT) if their VO2max was less than 50 ml.kg-1.min-1. The subjects were required to exercise on the ergometer for up to 40 min at power outputs that corresponded to approximately 50% and 70% VO2max. The allocation of each exercise session (50% or 70% VO2max) was random and each session was separated by at least 5 days. During these tests venous blood was taken 10 min before exercise (- 10 min), just prior to the commencement of exercise (0 min), after 20 min of exercise (20 min), at the end of exercise and 10 min postexercise (+ 10 min) and analysed for concentrations of cortisol, [Na+], [K+], [Cl-], glucose, free fatty acid, lactate [la-], [NH3], haemoglobin [Hb] and for packed cell volume. The oxygen consumption (VO2) and related variables were measured at two time intervals (14-15 and 34-35 min) during the prolonged exercise tests. Rectal temperature was measured throughout both exercise sessions. There was a significant interaction effect between the level of training and exercise time at 50% VO2max for heart rate (fc) and venous [la-].(ABSTRACT TRUNCATED AT 250 WORDS)

Existing and training induced differences in aerobic fitness: their relationship to physiological response patterns during different types of stress

Aerobic fitness has been associated with various desirable psychological and physiological characteristics. Recently, attenuation of physiological reactivity during stressful situations was added to this list, although comparison of the stress responses of sportsmen and sedentary subjects has yielded equivocal results. The present study examined cardiovascular patterns rather than single variables, and tried to clarify these matters. Tasks were used that were known to increase blood pressure through different combinations of changes in cardiac output and vascular resistance. Autonomic nervous system dynamics underlying these response patterns were studied using preejection period as an index of beta-adrenergic activity, and respiratory sinus arrhythmia as an index of vagal activity. Pre-existing differences in aerobic fitness in a sample of sedentary subjects were related to their responses during the stressful tasks and the recovery periods afterwards. This approach prevented confounding of the relationship between fitness and stress-reactivity with the psychological effects of regular exercise. Furthermore, it excluded the bias in psychological makeup that is introduced when subjects spontaneously engaged in sports are compared to non-exercising persons. To rule out a third (hereditary?) factor underlying both stress-reactivity and fitness, physiological responses before and after a seven-week training program were compared to those of subjects in a waiting list control group. Substantial individual differences in aerobic fitness were found in spite of the fact that all subjects reported low levels of habitual activity. During two active coping tasks, diastolic blood pressure reactivity and vagal withdrawal were negatively related to these pre-existing differences in fitness. No such relation was seen during a cold pressor test or during recovery from the tasks. Neither beta-adrenergic cardiac reactivity nor heart rate responses were related to fitness, but the absolute heart rate during the tasks was lower in the more fit subjects. Seven weeks of training were not effective in changing either reactivity or recovery of any of the variables. The discrepancy between cross-sectional and longitudinal results in the present study suggests that training of longer duration is necessary to induce the psychological or physiological changes underlying reduced reactivity. The latter may include changes in cardiac vagal/sympathetic balance or in adrenoceptor sensitivity. Alternatively, both psychological and physiological determinants of stress-reactivity may be related to aerobic fitness at a dispositional level.

Effects of age and level of physical activity on plasma norepinephrine kinetics

We examined the effects of age and physical activity on plasma norepinephrine (NE) kinetics in active and inactive older and younger men. NE kinetics were estimated by calculation of the rates of NE appearance and clearance during infusion of tritiated norepinephrine [( 3H]-NE). Older active men had the highest level of plasma NE (pg/ml) and NE appearance (microgram.min-1.FFW-1, where FFW is fat-free weight) relative to younger men and inactive older men. NE clearance (l.min-1.FFW-1) was not influenced by age or physical activity. No significant associations were noted between percent body fat, supine blood pressure with resting levels of NE, NE appearance, and NE clearance. We concluded that 1) chronic participation in physical activity by older men, but not younger men, is associated with higher resting levels of plasma NE and NE appearance and 2) the degree of adiposity does not influence plasma NE kinetics in healthy younger and older men. The level of physical activity is an influencing factor on plasma NE and NE appearance into circulation in healthy older men.

Skeletal muscle mitochondria: the aerobic gate?

At an animal's maximum aerobic capacity (VO2max), the O2 flowing through the respiratory system is consumed by a functionally exclusive sink, skeletal muscle mitochondria. Thus, O2 consumption will never exceed the muscles O2 demand. If the system is ideally designed, structures upstream to the skeletal muscle O2 sink must be built to insure adequate O2 delivery to the working muscle. There are a number of structure-function solutions available to supply the demanded O2 to the muscle; these have been found to vary, often ontogenetically, with hypoxia, training, etc. But there is one relationship that is invariant: Total O2 uptake can be predicted by the total (active) skeletal muscle mitochondrial volume. In aerobic and sedentary animals, across a range of body sizes, maximum (in vivo) mitochondrial O2 consumption is constant among mammals (at approximately 2000 O2 molecules per square micron of inner mitochondrial membrane per second). Because the volume of mitochondria is one of the most plastic of all respiratory structures, we interpret this relationship as suggesting that skeletal muscle mitochondria alone sets the demand for O2 and, thus, the volume of skeletal muscle mitochondria dictates an animal's maximum aerobic capacity.

Aerobic fitness and the physiological stress response: a critical evaluation

Aerobically fit persons need less sympathetic activation to perform the same absolute workload than less fit persons. This led to the idea that aerobic fitness might reduce the physiological activation during psychological stress as well. Several experiments showed inconsistent results with regard to this supposed effect of fitness. The comparability of the results is hampered by the differences in operationalization of aerobic fitness and by the confusion of the terms aerobic fitness, training and habitual physical exercise. The expectancy of an effect of fitness on the physiological stress response is based on the assumption that this response resembles the response to exercise. The tenability of this assumption was examined for cardiac, vascular and hormonal responses respectively. It was concluded that the two types of responses only superficially have similarity. So a simple analogy between the stress and the exercise response does not allow a reliable prediction concerning the effect of fitness on the stress response. There are however other reasons to expect an effect. Especially the effect of fitness on adrenoceptor sensitivity suggests that the most important effect of fitness might be found in the vascular part of the stress response. It is argued that the measurement of complete response patterns, instead of isolated parameters, is a prerequisite for progress in this field. Future studies should address the question what the relative contribution of psychological and physiological factors is to the effect of fitness on the physiological stress response.

Reduction of premature mortality by high physical activity: a 20-year follow-up of middle-aged Finnish men

The association of physical activity level with the risk of death was analysed for a cohort of 636 healthy Finnish men aged 45-64 years followed up for 20 years. 39% of the cohort were classed as highly active physically at baseline in 1964. Up to 1984 there were 287 deaths, 106 of them due to coronary heart disease (CHD). During the first-two thirds of the follow-up, men with high physical activity had a lower risk of death than did men with low physical activity. During the last third, the survival curves of the men with high and low physical activity gradually converged. Of the men who died, those with high physical activity lived 2.1 years longer (p = 0.002) than those with low physical activity, after adjustment for age, smoking, blood pressure, serum cholesterol, and body mass index. This difference was due mainly to fewer CHD deaths among the highly active group. Low physical activity was clearly weaker than smoking as a predictor of risk of death. High physical activity may thus independently prevent premature death among middle-aged men, but it probably does not prolong the maximum achievable life-span.