Biomedical subjects

D R Seals

Publications and source records attributed to D R Seals.

At least 91 records · Page 5Linked to original sources

PubMed1995

Hemostatic, metabolic, and androgenic risk factors for coronary heart disease in physically active and less active postmenopausal women.

Physically active postmenopausal women have a lower incidence of coronary heart disease (CHD) than their more sedentary peers, but little information is available concerning the responsible mechanisms. The primary aim of this study was to test the hypothesis that physically active post-menopausal women demonstrate more favorable levels of hemostatic, metabolic, and androgenic CHD risk factors than less active control subjects. If so, a secondary aim was to determine which of the characteristics associated with a physically active lifestyle, ie, low body fat, a high-carbohydrate/low-fat diet, high maximal aerobic capacity (aerobic fitness), and high levels of physical activity, are most closely related to this lower risk profile. To address these aims, we compared CHD risk factors in physically very active women (n = 14; age, 55 +/- 2 years) with those in healthy, nonobese sedentary control subjects (n = 17; age, 56 +/- 1 years). Maximal aerobic capacity (fitness) was 83% higher (P < .001) in the physically active women. Concentrations of plasminogen activator inhibitor type 1 activity and tissue plasminogen activator antigen were lower (more favorable) (P < .005) in the physically active women versus control subjects, whereas plasma fibrinogen levels did not differ. The physically active women had lower (P < .01) fasting plasma insulin and glucose concentrations as well as smaller responses to an oral glucose challenge. Both total-body and abdominal fat levels were lower (P < .001) and lipid and lipoprotein profiles were generally more favorable (P < .05) in the physically active women.(ABSTRACT TRUNCATED AT 250 WORDS)

Body Composition↗

PubMed1994

Active forearm blood flow adjustments to handgrip exercise in young and older healthy men.

1. Our purpose was to test the hypothesis that ageing impairs the active muscle hyperaemia consequent to dynamic exercise in humans. 2. Eleven young (19-29 years) and eleven older (60-74 years) healthy, non-obese men with similar chronic physical activity levels and forearm size performed two protocols of dynamic handgrip exercise: (a) brief (1 min), incremental loads to exhaustion, and (b) sustained (8 min), submaximal loads. Active forearm blood flow (FBF) was measured at rest and during a brief period of relaxation at the end of each minute of exercise. Arterial blood pressure was recorded to calculate active forearm vascular conductance (FVC). Sustained forearm ischaemia plus handgrip was used to elicit a peak forearm vasodilatatory response. 3. There were no differences in pre-exercise levels of any variable between the young and older men. During exercise, ratings of perceived effort, the peak workload attained, and the ability to sustain submaximal workloads were all similar for the two groups. 4. During brief exercise, both submaximal and peak levels of FBF were similar in the two groups; however, the peak increases in FVC were greater in the older men. During sustained exercise, FBF and FVC were not different in the two groups at the lowest loads, but the increases became relatively greater in the older men with increasing workloads. 5. Peak levels of FBF and FVC in response to the peak vasodilatatory stimulus were similar in the young and older men. 6. These findings fail to support the postulate that ageing results in impaired active muscle hyperaemia and vasodilatation during small-muscle dynamic exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Sympathetic neural reactivity to stress does not increase with age in healthy humans.

Sympathetic nervous system reactivity to stress is though to increase with age in humans. We tested this hypothesis by recording postganglionic sympathetic nerve activity to skeletal muscle (MSNA) (peroneal microneurography) and by measuring plasma norepinephrine concentrations (PNE), heart rate, and arterial pressure before (prestress control) and during cognitive challenge (mental arithmetic and colored word test), thermal stress (i.e., the cold pressor test), and exhaustive isometric handgrip exercise (40% of maximum voluntary force)/postexercise ischemia in 15 older (60-74 yr, mean +/- SE = 64 +/- 1) and 15 young (19-30 yr, mean +/- SE = 25 +/- 1) healthy men and women (8 males, 7 females each). The initial prestress control level of MSNA was higher in the older subjects (P < 0.01 vs. young), but there were no significant differences for PNE, heart rate, or arterial pressure. The MSNA and PNE responses to mental stress were small and not different in the two groups. MSNA and PNE increased markedly in response to the cold pressor test and isometric handgrip exercise/post exercise ischemia in both groups. The absolute unit increases in MSNA were similar in the two groups, but the relative (percentage) increases were actually smaller in the older subjects (P < 0.05 vs. young) due to their elevated baseline levels. The stress-evoked increases in arterial pressure were similar in the groups, but the older subjects tended to demonstrate smaller increases in heart rate. In general, no gender differences were noted in either age group. These findings fail to support the long-held concept that stress-induced sympathetic nervous system stimulation becomes exaggerated with age. Thus, sympathetic neural hyperreactivity does not appear to be a fundamental property of the aging process in humans.

Adult↗

PubMed1994

Total blood volume in healthy young and older men.

The primary aim of this investigation was to test the general hypothesis that aging does not influence total blood volume in humans. To accomplish this, we measured supine resting plasma volume (modified Evans blue dye technique) and hematocrit in healthy nonobese young (24.7 +/- 1.2 yr, n = 7) and older (66.1 +/- 1.8 yr, n = 7) men carefully matched for body size and daily energy expenditure. Estimated body fat was greater (23 +/- 2 vs. 17 +/- 1%, P < 0.01) and maximal O2 consumption was lower (32.4 +/- 2.1 vs. 48.7 +/- 1.5 ml.kg-1.min-1, P < 0.001) in the older men than in the young controls. Absolute levels of total blood volume were 24% lower (P < 0.01) in the older men than in the young controls (4,749 +/- 333 vs. 6,234 +/- 318 ml). The lower absolute level of total blood volume in the older men was due to a 21% lower (P < 0.01) plasma volume (2,956 +/- 194 vs. 3,745 +/- 176 ml) and a 28% lower (P < 0.01) erythrocyte volume (1,792 +/- 148 vs. 2,488 +/- 158 ml) than in the young controls. Total blood volume, plasma volume, and erythrocyte volume also were lower (P < 0.01) in the older than in the young men when expressed relative to body weight, body surface area, or estimated fat-free mass. These findings suggest that total blood volume decreases with age in healthy men of similar size and chronic physical activity levels.

Adult↗

PubMed1994

Endurance exercise training is associated with elevated basal sympathetic nerve activity in healthy older humans.

We tested the hypothesis that endurance training is associated with altered basal levels of muscle sympathetic nerve activity (MSNA) and responses to acute stress in healthy older adults. MSNA (peroneal microneurography) and plasma norepinephrine (NE) concentrations were measured during supine rest, a cold pressor test, and isometric handgrip (40% maximal voluntary force to exhaustion) in 16 older masters endurance athletes [10 men, 6 women; 66 +/- 1 (SE) yr] and 15 healthy normotensive untrained control subjects (9 men, 6 women; 65 +/- 1 yr). The athletes had higher levels of estimated daily energy expenditure and maximal oxygen uptake and lower levels of resting heart rate and body fat than the control subjects (all P < 0.05). MSNA during supine rest was elevated in the athletes whether expressed as burst frequency (43 +/- 2 vs. 32 +/- 3 bursts/min, respectively; P < 0.05) or burst incidence (75 +/- 4 vs. 52 +/- 5 bursts/100 heartbeats, respectively; P < 0.01). These whole group differences were due primarily to markedly higher levels of MSNA in the athletic vs. untrained women (48 +/- 4 vs. 25 +/- 3 bursts/min, 82 +/- 3 vs. 38 +/- 3 bursts/100 heartbeats, respectively, P < 0.001). In contrast, basal plasma NE concentrations were not significantly different in the athletes vs. control subjects. The MSNA and plasma NE responses to acute stress tended to be greater in the athletes. These findings indicate that vigorous regular aerobic exercise is associated with an elevated level of MSNA at rest and a tendency for an enhanced response to acute stress in healthy normotensive older humans.

Aged↗

PubMed1994

Arm muscle sympathetic nerve activity during preparation for and initiation of leg-cycling exercise in humans.

We tested the hypothesis that sympathetic vasoconstrictor nerve activity to nonactive skeletal muscle (MSNA) decreases immediately before and remains suppressed during initiation of conventional large muscle upright dynamic exercise in humans. In 11 healthy young subjects, adequate recordings of MSNA from the radial nerve in the arm were obtained during upright seated rest (control) and throughout 1 min of leg-cycling exercise at one or more submaximal workloads (range 33-266 W; approximately 10-80% of peak power output). MSNA was analyzed during four consecutive time intervals; control, preparation for cycling (end of control to onset of pedal movement), initiation of cycling (onset of pedal movement to attainment of target power output), and the initial 60 s of cycling at target power output. MSNA decreased (P < 0.05) abruptly and markedly in all subjects [to 19 +/- 4% (SE) of control levels] during the preparation period before the 33-W load and remained suppressed throughout the period of initiation of cycling in 8 of 11 subjects; MSNA increased during the initiation period in three subjects in whom diastolic arterial pressure fell below control levels. This general pattern was observed at all loads. MSNA remained at or below control levels throughout the 1 min of cycling exercise at 33-166 W. MSNA increased above control levels during the latter portion of the 1 min of cycling only at loads > or = 60% of peak power output.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Maximal aerobic capacity and total blood volume in highly trained middle-aged and older female endurance athletes.

The primary aim of this study was to test the hypothesis that, relative to age-matched sedentary controls, middle-aged and older female endurance athletes exhibit levels of maximal rate of O2 consumption (VO2max) similar to those previously reported in male endurance athletes in the same age group and, furthermore, that these high levels of VO2max are associated with elevated and total blood volume in 13 highly trained female runners and 17 healthy untrained women 50-70 yr of age. The runners had lower (P < 0.001) body mass index and estimated body fat relative to the controls. They also had higher levels of VO2max (2.70 +/- 0.08 vs. 1.74 +/- 0.06 l/min, 48.6 +/- 1.9 vs. 26.5 +/- 0.8 ml.kg-1.min-1, 58.0 +/- 1.8 vs. 38.8 +/- 1.2 ml.kg fat-free mass-1.min-1; all P < 0.001) and total blood volume (4,964 +/- 226 vs. 4,336 +/- 130 ml, P < 0.02; 89 +/- 4 vs. 64 +/- 2 ml/kg, P < 0.001; 106 +/- 4 vs. 96 +/- 3 ml/kg fat-free mass, P < 0.05). The elevated total blood volumes in the runners were due to both higher plasma and erythrocyte volumes. In addition, hematocrit and hemoglobin concentrations were higher in the runners (P < 0.007). VO2max was strongly (P < 0.001) related to total blood volume (r = 0.79), plasma volume (r = 0.76), and erythrocyte volume (r = 0.78) when all were expressed relative to body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed1994

Enhanced left ventricular performance in endurance trained older men.

BACKGROUND: The age-associated decline in aerobic exercise capacity is partially reversible by endurance exercise training. Moderate-intensity endurance exercise training increases aerobic exercise capacity mediated, in part, by improvement of stroke volume and left ventricular performance in older men. The present study was designed to characterize the nature of cardiovascular adaptations to strenuous endurance exercise of long duration and to delineate the mechanisms underlying increased stroke volume and cardiac output in highly trained older endurance athletes. METHODS AND RESULTS: Nine male master athletes (MA: 64 +/- 2 years old, mean +/- SEM) and 9 older sedentary healthy men (controls: 63 +/- 1 year) were studied. Left ventricular systolic function was evaluated with the use of cardiac blood pool imaging and echocardiography. Maximal O2 uptake (VO2max) was 50.4 +/- 1.7 mL.kg-1 x min-1 in the MA and 29.6 +/- 1.4 mL.kg-1 x min-1 (P = .0001) in controls. Systolic and mean blood pressures at rest and during exercise were not different in the two groups. Left ventricular systolic function at peak exercise was higher in the MA than in sedentary controls as evidenced by (1) a higher left ventricular functional reserve (delta EF: 12.4 +/- 2 versus 5.6 +/- 2.5, P = .05), (2) a large decrease in end-systolic volume during exercise (MA: 56 +/- 4 mL at rest and 42 +/- 5 mL at peak exercise, P = .007; controls: 43 +/- 2 mL at rest and 42 +/- 6 mL at peak exercise, P = .35) with no differences in systolic blood pressure, (3) a higher left ventricular fractional shortening at peak exercise (MA: 52 +/- 2.6%; controls: 45 +/- 1%, P = .046) at comparable values for end-systolic wall stress (MA: 56 +/- 12 g/cm2; controls: 53 +/- 7 g/cm2, P = .50), and (4) a greater decrease in end-systolic diameter at peak exercise in the MA than in controls (MA: -1.2 +/- 0.16 cm versus -0.57 +/- 0.13 cm, P = .014) despite no significant differences between the changes in end-systolic wall stress during exercise (MA: -15.5 +/- 7.5 g/cm2, controls: -11.0 +/- 9.0 g/cm2, P = .6). MA had a larger end-diastolic volume at rest (153 +/- 6 versus 132 +/- 4 mL, P = .009) with a normal wall thickness-to-radius ratio (0.34 +/- 0.02). Peak exercise stroke volume was higher (P = .023) in the MA (132 +/- 6 mL/min) than in the sedentary controls (111 +/- 6 mL/min). Changes in stroke volume correlated strongly with changes in ejection fraction in the MA (r = .80, P = .010) but not in sedentary controls (r = .59, P = .097). Further, changes in stroke volume from rest to exercise correlated strongly with changes in end-diastolic volume in both MA (r = .78, P = .013) and sedentary controls (r = .73, P = .026), suggestive of reliance of stroke volume on end-diastolic volume and preload. However, for a given increase in end-diastolic volume, the rise in stroke volume during exercise was significantly larger in the MA than in controls, which, in the absence of differences in mean blood pressures, indicates that enhanced left ventricular systolic function independent of preload plays an additional role in maintaining a higher stroke volume at peak exercise in the highly trained older men. CONCLUSIONS: Cardiac adaptations in older endurance trained men are characterized by volume-overload left ventricular hypertrophy and enhancement of left ventricular systolic performance at peak exercise. These adaptive responses contribute to enhanced stroke volume at peak exercise in older endurance trained men.

Aging↗

PubMed1994

Exercise and aging: autonomic control of the circulation.

This review describes age-related changes in autonomic control of the circulation during exercise and the associated effects on exercise capacity. The increase in heart rate during exercise becomes smaller with aging probably due to both less withdrawal of cardiac vagal tone and diminished beta-adrenergic responsiveness. The latter also appears to contribute to an attenuation in the left ventricular contractile response to exercise despite greater beta-adrenergic stimulation. At rest, muscle sympathetic nerve activity and arterial plasma norepinephrine spillover rates are elevated in older humans. With aging, sympathetically mediated vasoconstriction in nonactive muscle is augmented during brief dynamic exercise. Paradoxically, during more prolonged exercise increases in plasma norepinephrine concentrations/spillover rates are not greater with age. These age-related changes do not adversely affect submaximal exercise performance at a particular % maximal oxygen consumption. However, the lower peak heart rate and attenuated left ventricular contractile response reduce maximal cardiac output, oxygen consumption, and exercise capacity. In older humans, aerobic exercise training lowers heart rate at rest, reduces levels of heart rate and plasma catecholamines at the same absolute submaximal workload, and, at least in men, improves left ventricular performance during peak exercise, but does not reduce, and may even increase, basal sympathetic nerve activity.

Adult↗

PubMed1993

Influence of force on muscle and skin sympathetic nerve activity during sustained isometric contractions in humans.

1. Our purpose was to test the hypothesis that efferent sympathetic nerve activity to non-active skeletal muscle (MSNA) and skin (SSNA) is independent of the level of force during sustained submaximal isometric contractions in humans. 2. In twelve healthy subjects, arterial blood pressure, heart rate, and MSNA (n = 6) or SSNA (n = 6) (peroneal microneurography) were recorded before and during isometric handgrip contractions sustained to exhaustion at 20, 40 and 60% of maximal force. Responses were examined at similar percentages of endurance time at each level of force. 3. Contraction duration decreased progressively with increasing force (495 +/- 54, 140 +/- 13, 73 +/- 8 s, respectively), but peak ratings of perceived effort were similar for the three force levels. 4. The peak increases in systolic pressure were not different among the three levels of force. The increases in diastolic and mean pressure were similar at 40 and 60% of maximal force, but were smaller at the end of 20% of maximal force. The contraction-induced rise in heart rate was directly related to the level of force. 5. The contraction-evoked stimulation of both MSNA and SSNA was similar during handgrip at 40 and 60% of maximal force, but was much less during handgrip at 20% of maximal force. The increases in SSNA were associated with increases in both skin blood flow and skin electrical conductance suggesting primarily sudomotor fibre activation. 6. These findings indicate that there is a minimum force necessary to elicit peak levels of MSNA and SSNA during sustained isometric contractions in humans. When normalized to endurance time, however, the regulation of these sympathetic outflows appears to be independent of force above this minimum level. The results also indicate that during this type of muscle activity the relationship between force and heart rate is different to that between force and peripheral sympathetic discharge.

Adult↗

PubMed1993

Tissue-specific noradrenergic activity during acute heat stress in rats.

The purpose of this study was to determine whether nonexertional heat stress alters the behavior of internal organ and skeletal muscle sympathetic nervous system (SNS) activity in unrestrained conscious rats. Norepinephrine (NE) synthesis was blocked with alpha-methyl-p-tyrosine, and the rate of decline in tissue NE concentration after synthesis blockade was used to estimate SNS activity in the left ventricle, kidney, liver, adrenal gland, and soleus and extensor digitorum longus muscles of the hindlimb. Male Sprague-Dawley rats (250-320 g) were maintained in thermoneutral control conditions (ambient temperature = 24 degrees C, n = 10) or exposed to an ambient temperature of 42 degrees C until a colonic temperature (Tc) of 39.5 or 41.0 degrees C (n = 10 each) was attained. During heating, as Tc rose from control levels (approximately 38 degrees C) to 41.0 degrees C, mean arterial pressure and heart rate increased from 120 +/- 3 to 141 +/- 3 mmHg and from 381 +/- 4 to 420 +/- 5 beats/min, respectively (P < 0.05). There was a strong trend for increased NE turnover rates in the left ventricle, liver, and adrenal gland in NE synthesis-blocked rats attaining a Tc of 39.5 degrees C compared with the normothermic values, whereas the turnover rate in the kidney was significantly elevated at this level of hyperthermia (126%) vs. the control condition. In animals heated to a Tc of 41.0 degrees C, the NE turnover rate was markedly increased in the left ventricle (590%), kidney (531%), liver (262%), and adrenal gland (602%) compared with normothermic control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Hypoxic potentiation of the ventilatory response to dynamic forearm exercise.

The slope of the relationship between ventilation (VI) and O2 consumption, as derived in progressive-intensity exercise tests, is increased markedly by systemic hypoxia. The mechanisms underlying the hypoxic potentiation of the ventilatory response to exercise have not been established, partly because several factors that can increase respiratory drive (e.g., metabolic rate, cardiac output, circulating catecholamine levels) change significantly and simultaneously under these conditions. In an effort to avoid these confounding changes, we sought to determine whether hypoxia potentiates the ventilatory response to dynamic forearm exercise in humans. Forearm exercise increased the O2 consumption by only 80-90 ml/min; nevertheless, hypoxia resulted in a significant potentiation of VI that was mediated by a marked increase in breathing frequency. These observations led us to hypothesize that the hypoxic potentiation of VI is due to an exaggerated stimulation of chemosensitive afferent nerve endings within the exercising muscles ("muscle chemoreceptors"). We tested this hypothesis in separate experiments under conditions of forearm ischemia so that the stimulus to the muscle chemoreceptors in normoxic and hypoxic exercise would be the same. The magnitude of the change in VI evoked by hypoxic ischemic exercise was significantly greater than the sum of the separate changes evoked by normoxic ischemic exercise and hypoxic ischemic rest. We conclude that the combination of dynamic forearm exercise and hypoxia potentiates VI and that this effect is mediated by neural structures that govern respiratory frequency. Moreover the potentiated ventilatory response cannot be attributed to an exaggerated stimulation of intramuscular chemoreceptors.

Adult↗

PubMed1993

Influence of active muscle size on sympathetic nerve discharge during isometric contractions in humans.

We tested the hypothesis that sympathetic nerve discharge to nonactive skeletal muscle (MSNA) is influenced by active muscle size during isometric contractions performed at a constant submaximal force in humans. In six subjects, MSNA (peroneal microneurography), arterial pressure, heart rate, and ratings of perceived effort were recorded before (resting control) and during isometric contractions of either a small hand muscle [1st dorsal interosseus (FDI); 2 trials (FDI1 and FDI2)] or the fore- and upper arm muscles (handgrip; 1 trial) sustained to the point of exhaustion and normalized to endurance time (te). There were no differences in resting control levels. MSNA, heart rate, arterial pressure, and perceived effort all increased similarly during the two FDI contractions. During handgrip, the rates of rise and peak increases MSNA, heart rate, and arterial pressure were much greater than during FDI (all P < 0.05), but peak levels of perceived effort were not different. These findings indicate that during voluntary isometric efforts sustained to the same performance and perceptual end points the rates of increase and final levels of MSNA, heart rate, and arterial pressure are greater during contraction of a larger muscle mass and that these responses appear to be similar during successive trials of a task when normalized to te. Thus, active muscle size can be an important factor in the regulation of sympathetic nervous system discharge and cardiovascular function during isometric muscle activity in the human.

Adult↗

PubMed1993

Age and gender influence muscle sympathetic nerve activity at rest in healthy humans.

Muscle sympathetic nerve activity at rest increases with age in humans. The respective influences of the aging process per se and gender on this increase and whether age and gender effects on muscle sympathetic nerve activity can be identified with plasma norepinephrine concentrations, however, have not been established. To examine these issues, nine young women (aged 24 +/- 1 years; mean +/- SEM), eight young men (aged 26 +/- 1 years), seven older women (aged 63 +/- 1 years), and eight older men (aged 66 +/- 1 years) were studied. All were healthy, normotensive (blood pressure < 140/90 mm Hg), nonobese (< 20% above ideal weight), unmedicated, nonsmokers engaged in minimal to recreational levels of chronic physical activity. Arterial blood pressure (manual sphygmomanometry, brachial artery), heart rate, muscle sympathetic nerve activity (peroneal microneurography), and antecubital venous plasma norepinephrine concentrations (radioenzymatic assay) were determined during quiet supine resting conditions. Body weight was higher in men, but there were no age-related differences, whereas estimated body fat (sum of skinfolds) was higher in women and in the older groups (p < 0.05). Estimated daily energy expenditure, arterial blood pressure, and heart rate were not different among the groups. Both muscle sympathetic nerve activity burst frequency and burst incidence at rest were progressively higher in the young women, young men, older women, and older men (10 +/- 1 versus 18 +/- 2 versus 25 +/- 3 versus 39 +/- 5 bursts/min and 16 +/- 1 versus 30 +/- 4 versus 40 +/- 3 versus 61 +/- 6 bursts/100 heartbeats, respectively; all p < 0.05 versus each other).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Postexercise hypotension. Key features, mechanisms, and clinical significance.

Recent investigations have demonstrated that there is a sustained reduction in arterial blood pressure after a single bout of exercise, ie, postexercise hypotension (PEH). The purpose of this discussion is to integrate the available information on this topic and to review studies using sustained stimulation of somatic afferents in experimental rats as a model to study the role of somatic afferents in PEH. PEH occurs in response to several types of large-muscle dynamic exercise (ie, walking, running, leg cycling, and swimming) at submaximal intensities greater than 40% of peak aerobic capacity and exercise durations generally between 20 and 60 minutes. PEH is observed in both normotensive and hypertensive humans and in spontaneously hypertensive rats but is generally greater in magnitude in hypertensive subjects. The maximal exercise-induced reductions in systolic and diastolic arterial blood pressures have been on average 18 to 20 and 7 to 9 mm Hg, respectively, in hypertensive humans and 8 to 10 and 3 to 5 mm Hg, respectively, in normotensive humans. PEH has been reported to persist for 2 to 4 hours under laboratory conditions. Whether PEH is sustained for a prolonged period of time under free-living conditions remains controversial, although the results of one study indicate that PEH can persist for up to 13 hours. Possible mechanisms involved in mediating postexercise and poststimulation reductions in arterial blood pressure include decreased stroke volume and cardiac output; reductions in limb vascular resistance, total peripheral resistance, and muscle sympathetic nerve discharge; group III somatic afferent activation; altered baroreceptor reflex circulatory control; reduced vascular responsiveness to alpha-adrenergic receptor-mediated stimulation; and activation of endogenous opioid and serotonergic systems. It appears that the magnitude of PEH in hypertensive subjects is clinically significant; however, more investigation is required to determine if the duration is sufficient under real-life conditions to contribute to the reduction in blood pressure observed with chronic exercise conditioning.

Adult↗

PubMed1993

Respiratory modulation of muscle sympathetic nerve activity in intact and lung denervated humans.

We determined the influences of breathing-induced changes in intrathoracic and intravascular pressures, central respiratory drive, and pulmonary vagal feedback on the within-breath variation in skeletal muscle sympathetic nerve activity (MSNA) in humans. MSNA (peroneal microneurography), arterial blood pressure (Finapres finger monitor), and tidal volume (VT) were recorded continuously in six normal subjects and four heart-lung transplant patients during: 1) spontaneous air breathing; 2) increased FICO2; 3) voluntary augmentation of VT with and without inspiratory resistance; and 4) positive pressure, passive mechanical ventilation. During conditions 3 and 4, which were performed under isocapnic conditions with a high MSNA background (either high resting activity or nonhypotensive lower body suction), subjects breathed at control or elevated VT with normal or prolonged inspiratory time (TI); breathing frequency was 12 breaths per minute. During control breathing in normal subjects there was a distinct within-breath pattern of MSNA, with approximately 70% of the activity occurring during low lung volumes (initial half of inspiration and latter half of expiration). This within-breath variation of MSNA was potentiated with increased VT breathing (> 85% of activity occurring during low lung volumes; p < 0.05 versus control breathing) and was similar during the voluntary and CO2-induced hyperpneas. MSNA decreased progressively and markedly from onset to late inspiration; fell slightly further, reaching its nadir at end-inspiration/onset-expiration; and rose sharply during mid-late expiration. Only the nadir of MSNA was associated with any change in arterial pressure. Resistive breathing, especially at elevated VT, caused a fall in arterial pressure and increased respiratory drive during inspiration, yet MSNA still declined as lung volume increased. Normal within-breath modulation of MSNA also was observed during control and elevated VT induced via positive pressure with passive ventilation, which reversed lung inflation/deflation-induced intrathoracic pressure changes and reduced or removed respiratory motor output. During control breathing in transplant patients the specific within-breath pattern of MSNA was somewhat different than that of the normal subjects, but on average, the overall low lung volume to high lung volume MSNA ratio was similar to normal subjects. In contrast to the normal subjects, however, there was no potentiation of the within-breath variation of MSNA with elevated tidal breathing. These findings indicate that during normal levels of tidal breathing most of the respiratory phase influence on muscle sympathetic outflow observed in normal conscious humans is independent of baroreceptor-sensed fluctuations in intrathoracic or intravascular pressures and of lung inflation-stimulated vagal afferent activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1992

Sympathetic nervous system activity during skin cooling in humans: relationship to stimulus intensity and pain sensation.

1. Our aim was to determine the relationship between efferent sympathetic nervous system activity to skeletal muscle (MSNA) and both the dynamics of the stimulus and pain sensation during localized skin cooling in humans. MSNA in the lower leg (peroneal microneurography), heart rate, arterial blood pressure, hand skin and muscle temperatures and perceptions of pain were recorded in ten healthy subjects before, during and after immersion (3 min) of a hand in water of different temperatures ranging from non-noxious to extremely noxious (28, 21, 14, 7 and 0 degrees C). 2. Immersion produced an abrupt, water temperature-dependent fall in hand skin temperature (initial 30 s) followed by a more gradual decline. In contrast, the fall in hand muscle temperature was almost linear during immersion. Throughout immersions at the 28, 21, and 14 degrees C water temperatures and during the initial phase of the 7 degrees C level, sensations ranged from not painful to somewhat painful; however, the latter phase of the 7 degrees C immersion and the entire 0 degrees C level were perceived as intensely painful. 3. During the initial 15-30 s of immersion at the 21-7 degrees C water temperatures, MSNA decreased from control levels in all subjects (47-58% on average, P < 0.05), whereas mean arterial blood pressure did not change. MSNA then returned to and remained at control levels throughout the 28-14 degrees C immersions, although arterial pressure, primarily systolic, rose slightly. 4. After some delay, MSNA increased during immersion at both the 7 degrees C (P < 0.05 at 90 s) and 0 degrees C (P < 0.05 at 60 s) levels in a progressive, water temperature-dependent manner, achieving peak values of approximately 200 and 300% of control, respectively, by 2.0-2.5 min. These elevations in MSNA were associated with parallel increases in arterial pressure. 5. Heart rate rose during the onset of immersion at all water temperatures (P < 0.05), but fell rapidly to control levels after 60-90 s. The increases were small (approximately 5 beats/min) and similar at the 28-7 degrees C levels, but were twice as great at the coldest water temperature.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1992

Sympathetic activity is influenced by task difficulty and stress perception during mental challenge in humans.

1. Our aim was to determine the influence of the type of task, the absolute and relative difficulty of the task, and the perceived stress associated with performance of the task on sympathetic circulatory regulation during cognitive challenge in humans. 2. Sympathetic nerve activity to skeletal muscle (MSNA) determined from peroneal microneurography, heart rate and arterial blood pressure were recorded continuously in twelve subjects during a modified Stroop colour word test (CWT) and mental arithmetic (MA), each performed over six levels of increasing absolute task difficulty. Performance (percentage correct) on each task was assessed and ratings of perceived stress obtained. Responses to CWT and MA were compared at similar levels of performance and perceived stress. 3. MSNA decreased at task onset, remained below baseline levels at low levels of difficulty which were not perceived as stressful, increased above baseline levels at higher levels of difficulty which were perceived as stressful, and increased further during recovery. Thus, the regulation of MSNA was stress dependent. At similar levels of stress perception there were no differences in MSNA between CWT and MA. Although performance declined as task difficulty increased, there was no particular 'threshold' level of performance associated with the stimulation of MSNA. 4. Arterial pressure and heart rate were elevated above baseline levels throughout the mental tasks. Arterial pressure increased over the first 3-4 levels of each task and then plateaued whereas heart rate did not vary across increasing levels of task difficulty. Heart rate and arterial pressure responses to CWT were higher than those to MA. 5. These data demonstrate that during cognitive challenge the stimulation of MSNA is governed primarily by perceived stress which is dependent, in part, on the absolute level of task difficulty. In contrast, neither performance nor the type of cognitive task appear to be important determinants of MSNA. Arterial pressure is influenced by the task and level of difficulty. Heart rate is independent of task difficulty but may be task dependent.

Adult↗