Abstract

Background:

Long-term moderate-intensity exercise augments endothelium-dependent vasodilation through an increase in nitric oxide (NO) production. The purpose of this study was to determine the effects of different intensities of acute exercise on hemodynamics in humans.

Methods:

We evaluated forearm blood flow (FBF) responses to different intensities of exercise (mild, 25% maximum oxygen consumption [O2max]; moderate, 50% O2max; and high, 75% O2max; bicycle ergometer, for 30 min) in eight healthy young men. The FBF was measured by using a strain–gauge plethysmography.

Results:

After exercise began, moderate-intensity exercise, but not mild-intensity exercise, promptly increased FBF from 2.8 ± 1.1 mL/min/100 mL to a plateau at 5.4 ± 1.6 mL/min/100 mL at 5 min (P < .01) and increased mean arterial pressure from 84.7 ± 11.8 mm Hg to a plateau at 125.7 ± 14.3 mm Hg at 5 min (P < .01). Moderate-intensity exercise decreased forearm vascular resistance (FVR) from 29.2 ± 5.4 to 16.8 ± 3.2 mm Hg/mL/min/100 mL tissue (P < .01). The administration of NG-monomethyl-L-arginine, an NO synthase inhibitor, abolished moderate exercise–induced augmentation of vasodilation. Although we were not able to measure FBF during high-intensity exercise because of large body motion, high-intensity exercise markedly increased mean arterial pressure from 82.6 ± 12.2 to 146.8 ± 19.8 mm Hg. High-intensity exercise, but not mild-intensity or moderate-intensity exercise, increased plasma concentration of 8-isoprostane, an index of oxidative stress, from 24.1 ± 10.8 to 40.2 ± 16.7 pg/mL (P < .05) at 10 min after the end of exercise.

Conclusions:

These findings suggest that acute moderate-intensity exercise induces vasodilation through an increase in NO bioavailability in humans and that high-intensity exercise increases oxidative stress. Am J Hypertens 2007;20: 825–830 © 2007 American Journal of Hypertension, Ltd.

Recent epidemiologic studies have shown that physical exercise reduces cardiovascular morbidity and mortality in the general population, including healthy subjects.1,2 Physical training has been shown to enhance endothelium-dependent vasodilation in patients with hypertension, coronary artery disease, and chronic heart failure, as well as in healthy individuals.3–6 One possible mechanism by which aerobic exercise reduces cardiovascular outcomes is augmentation or improvement in endothelial function. Endothelial dysfunction is the initial step in the pathogenesis of atherosclerosis, resulting in cardiovascular complications.7–9 Therefore, from a clinical perspective, it is important to select an appropriate intervention that is effective in improving endothelial dysfunction in patients with cardiovascular diseases and in augmenting endothelial function in healthy individuals.

It is clinically important to select the appropriate intensity, duration, frequency, and kind of exercise, because intense exercise can be hazardous to human vessels. Intensity of exercise is particularly important. Several guidelines for prevention of atherosclerosis recommend aerobic exercise of moderate intensity. Recently, we have shown that 12 weeks of moderate-intensity exercise (50% maximum oxygen consumption [O2max]), but not mild-intensity (25% O2max) or high-intensity (75% O2max) exercise, has beneficial effects on endothelial function in healthy young men.6 In addition, long-term high-intensity exercise increased oxidative stress in these subjects. A balance of nitric oxide (NO) and reactive oxygen species (ROS) plays an important role in the maintenance of endothelial function.

Long-term aerobic exercise is resulted in accumulation of acute exercise. It has been found by using a respiratory gas analysis system during exercise in healthy subjects that the amount of NO production increases in relation to exercise intensity.10 However, there is no information concerning the role of different intensities of acute exercise on vascular function and oxidative stress in humans.

Methods

Subjects

The subjects were eight young healthy men (mean age, 26.3 ± 2.3 years). After measurement of O2max to decide each intensity of exercise, all of the subjects performed three types of exercise: mild-intensity exercise (25% O2max), moderate-intensity exercise (50% O2max), and high-intensity exercise (75% O2max). None of the subjects had been regularly doing exercise. None of the subjects had a history of serious disease or had taken any medications for at least 4 weeks before the study. The study protocol was approved by the Ethics Committees of Hiroshima University Graduate School of Biomedical Sciences. Written informed consent for participation was obtained from all subjects.

Exercise

All subjects performed each exercise for 30 min on a bicycle ergometer in the sitting position. Each 30-min exercise was performed after a 5-min rest period, and the exercise was followed by a 10-min recovery period. We explained the method of aerobic exercise in detail (exercise type, frequency, duration, and intensity) and demonstrated the operation of the bicycle ergometer for the subjects. The studies were carried out in a randomized fashion at intervals of 1 week in all subjects. Participants were asked to maintain their original behavioral and dietary habits, especially their intake of sodium, potassium, calories, and alcohol.

Measurement of Forearm Blood Flow

The forearm blood flow (FBF) was measured with a mercury-filled Silastic strain–gauge plethysmography (EC-5R; D.E. Hokanson, Inc., Bellevue, WA), as previously described.11 The FBF is expressed as milliliters per minute per 100 milliliters of forearm tissue volume. The forearm vascular resistance (FVR) was calculated as the mean blood pressure (BP) divided by FBF and is expressed as mm Hg per milliliter per minute per 100 milliliter of forearm tissue volume. Four plethysmographic measurements were averaged to obtain FBF at baseline, during the exercise period, and during the recovery period. The intra-assay coefficient of variation was 5.1%.

All subjects were instructed to remain in the supine position for 30 min in an air-conditioned room (constant temperature of 22° to 25°C) before the study. After a 5-min rest period, basal FBF was measured. The FBF during the 30-min exercise was measured at 5-min intervals, and the FBF during the 10-min recovery period was measured at 5-min intervals.

After a 30-min rest period, to examine the effect of exercise on release of NO, we measured FBF during acute exercise in the presence of the NO synthase (NOS) inhibitor NG-monomethyl-L-arginin (L-NMMA; CLINALFA Co., Läufelfiger, Switzerland; 8 μmol/min, intra-arterial infusion for 5 min) in all subjects.

Measurement of Blood Pressure and Heart Rate

Blood pressure and heart rate were measured by using a Portapres model-2 (TNO Institute of Applied Physics Bio Medical Instrumentation, The Netherlands, Amsterdam), as previously described.12

Measurement of Norepinephrine

Samples of venous blood were obtained before exercise, at 20 min after the start of exercise, and at 10 min after the end of exercise. The plasma concentration of norepinephrine, an index of sympathetic nervous activity, was measured by HPLC. The intra-assay coefficient of variation was 5.9%.

Measurement of 8-Isoprostane

Samples of venous blood were obtained before exercise, at 20 min after the start of exercise, and at 10 min after the end of exercise. The plasma concentration of 8-isoprostane, an index of oxidative stress, was measured by ELISA (Cayman Chemical Co., Ann Arbor, MI). The intra-assay coefficient of variation was 6.2%.

Statistical Analysis

Results are presented as the mean ± SD. Values of P < .05 were considered significant. The paired t-test was used to evaluate differences between parameters in each exercise group. Comparisons of parameters before and after exercise were performed with adjusted means by ANCOVA using baseline date as covariates. Comparisons of time curves of parameters in exercise were analyzed by two-way ANOVA for repeated measures. The data were analyzed using the software package StatView V (SAS Institute, Cary, NC) and Super ANOVA (Abacus Concepts, Berkley, CA).

Results

Effect of Exercise on Systemic Hemodynamics

After moderate-intensity and high-intensity exercises, but not mild-intensity exercise, mean arterial pressure (MAP) promptly increased from 84.7 ± 11.8 mm Hg and reached a plateau at 125.7 ± 14.3 mm Hg and from 82.6 ± 12.2 mm Hg and reached a plateau at 146.8 ± 19.8 mm Hg at 5 min, respectively (P < .01). A prompt return to the baseline level occurred after the end of exercise. The magnitude of change in MAP increased with increase in intensity of exercise (Fig. 1, top). Changes in systolic and diastolic BP were exactly paralleled by changes in MAP (data not shown). After mild-intensity, moderate-intensity, and high-intensity exercises, heart rate gradually increased from 71.2 ± 5.2 beats/min to a plateau at 110.9 ± 8.6 beats/min, from 71.9 ± 4.0 beats/min to a plateau at 141.7 ± 10.6 beats/min, and from 72.1 ± 3.8 beats/min to a plateau at 179.4 ± 17.4 beats/min at 20 min, respectively (P < .01). Heart rate gradually returned to the baseline level after the end of exercise. Magnitude of change in heart rate increased with increase in intensity of exercise (Fig. 1, bottom).

Line graphs show the changes in mean arterial pressure (top) and heart rate (bottom) during mild-intensity, moderate-intensity, and high-intensity exercises. *P < .05 v rest; †P < .05 v mild-intensity exercise; +P < .05 v moderate-intensity exercise. Re, recovery.

Effects of Exercise on FBF and FVR

Moderate-intensity exercise significantly increased FBF from 2.8 ± 1.1 mL/min/100 mL tissue to 5.4 ± 1.7 mL/min/100 mL tissue (P < .01) and decreased FVR from 29.2 ± 5.4 mm Hg/mL/min/100 mL tissue to 16.8 ± 3.2 mm Hg/mL/min/100 mL tissue (P < .01) in the plateau period after exercise began (Fig. 2). Mild-intensity exercise did not alter FBF or FVR (Fig. 2). The FBF could not be measured during high-intensity exercise because of large body movement. Therefore, FVR was also not calculated during high-intensity exercise.

Line graphs show the changes in forearm blood flow (FBF) (top) and forearm vascular resistance (FVR) (bottom) during mild-intensity and moderate-intensity exercises. *P < .05 v rest; †P < .05 v mild-intensity exercise. Re, recovery.

Intra-arterial infusion of the NOS inhibitor L-NMMA significantly decreased basal FBF from 3.1 ± 1.0 to 2.3 ± 0.8 mL/min/100 mL (P < .01) in the moderate-intensity exercise group. The L-NMMA reduced moderate-intensity exercise-induced reduction in FVR (Fig. 3). Changes in BP or heart rate during exercise were not altered by L-NMMA infusion in any of the subjects.

Line graphs show the changes in forearm vascular resistance (FVR) in the absence and presence of NG-monomethyl-L-arginin (L-NMMA) during moderate-intensity exercise. *P < .05 v rest; †P < .05 v L-NMMA (−). Re, recovery.

Effect of Exercise on Norepinephrine

High-intensity exercise, but not mild-intensity or moderate-intensity exercise, significantly increased plasma concentrations of norepinephrine in the exercise period and recovery period from 267 ± 58 to 905 ± 110 and to 473 ± 57 pg/mL, respectively (P < .01) (Fig. 4). Plasma concentrations of norepinephrine in the exercise period and recovery period in the high-intensity exercise group were significantly higher than those in the mild-intensity and moderate-intensity exercise groups.

Line graphs show the changes in norepinephrine during mild-intensity, moderate-intensity, and high-intensity exercises. *P < .05 v rest; †P < .05 v mild-intensity exercise; +P < .05 v moderate-intensity exercise.

Effect of Exercise on 8-Isoprostane

High-intensity exercise, but not mild-intensity or moderate-intensity exercise, significantly increased plasma concentrations of 8-isoprostane in the exercise period and recovery period from 24.1 ± 10.8 to 40.2 ± 16.7 and to 32.5 ± 15.1 pg/mL, respectively (P < .05) (Fig. 5).

Line graphs show the changes in 8-isoprostane during mild-intensity, moderate-intensity, and high-intensity exercises. *P < .05 v rest.

Discussion

In the present study, acute moderate-intensity exercise increased FBF and BP but did not alter plasma concentrations of norepinephrine and 8-isoprostane. Intra-arterial infusion of L-NMMA abolished moderate-intensity exercise-induced augmentation of vasodilation. Acute high-intensity exercise increased BP and plasma concentrations of norepinephrine and 8-isoprostane. Unfortunately, we were not able to measure FBF during high-intensity exercise because of large body motion. Acute mild-intensity exercise did not alter FBF, BP, FVR, and plasma concentrations of norepinephrine and 8-isoprostane.

Epidemiologic studies have demonstrated that daily physical aerobic exercise prevents cardiovascular mortality and morbidity.1,2,13 Physical inactivity, a sedentary state, per se is a risk factor for cardiovascular diseases. The World Health Organization/International Society of Hypertension and the seventh report of the Joint National Committee of High Blood Pressure recommend exercise at an intensity of approximately 50% of maximum oxygen consumption for 30 min per time and five to seven times per week for patients with mild to moderate essential hypertension.14,15 In accordance with these guidelines, the beneficial effects of exercise appear after 10 weeks, when patients perform exercise for at least 30 min per time and at least three times per week. It is clinically important to select the appropriate intensity, duration, frequency, and kind of exercise, because intense exercise can be hazardous to human vessels.16,17 Exercise intensity is the most important.

Several investigators, including us, have shown that regular physical training enhances endothelium-dependent vasodilation in the forearm and coronary circulation in patients with hypertension and patients with coronary heart disease as well as in healthy individuals.3–6 Recently, we reported that long-term moderate-intensity (50% O2max) exercise, but not mild-intensity (25% O2max) or high-intensity (75% O2max) exercise, augmented endothelium-dependent vasodilation in healthy subjects.6 This moderate-intensity exercise fits the index of exercise training that is recommended from the general viewpoint of prevention of cardiovascular diseases. In the present study, acute moderate-intensity exercise induced vasodilation and decreased vascular resistance in peripheral resistance arteries, although exercise of this intensity increases BP and heart rate in healthy young men. The repetition of daily, acute moderate-intensity exercise should contribute to the beneficial effects of long-term aerobic exercise.

There are several possible explanations for the increase in FBF during acute moderate-intensity exercise. One possible mechanism of moderate-intensity exercise-induced vasodilation is an increase in NO bioavailability through an increase in NO production or decrease in NO inactivation. Long-term aerobic exercise improves endothelial function in patients with hypertension and patients with coronary heart disease and augments endothelial function in healthy subjects through an increase in NO bioavailability.3–6 We found that the increase in FBF in the acute moderate-intensity exercise group was inhibited by L-NMMA, suggesting that augmentation of NO production or inhibition of NO degradation is involved in exercise-enhanced FBF. These findings suggest that acute exercise-induced endothelium-dependent vasodilation is also predominately due to an increase in NO bioavailability. However, intra-arterial infusion of L-NMMA did not completely abolish the moderate-intensity exercise-induced reduction of FVR. The remaining moderate-intensity exercise-induced vasodilation may be due to factors other than NO, such as endothelium-derived hyperpolarizing factor and prostaglandins.

It has been suggested that acute moderate-intensity exercise-induced increase in FBF is due to an increase in vascular shear stress resulting from increased blood flow. Increase in shear stress stimulates the production of NO in isolated vessels and cultured cells.18,19 The main reason for vasodilation during acute moderate-intensity exercise is thought to be an increase in shear stress–induced NO production. Experimental studies have shown that shear stress–mediated mechanotransductions, such as the phosphatidyl-inositol-3-kinase/Akt pathway, c-Src, heat shock protein/hypoxia-inducible factor-1 pathway, and Ras/Raf/MEK/ERK pathway, contribute to the upregulation of endothelial NOS (eNOS) mRNA and eNOS protein during exercise training.20,21 However, it is unlikely that a bout of acute exercise of 30 min moderates the eNOS mRNA levels and eNOS protein levels through the activation of shear stress–mediated mechanotransductions.

A balance between ambient levels of ROS and released NO plays a critical role in the maintenance of endothelial function. In the present study, high-intensity exercise, but not mild-intensity or moderate-intensity exercise, increased circulating 8-isopeostane levels, an index of oxidative stress. Lovlin et al22 demonstrated that a bout of 40% O2max exercise slightly decreases plasma levels of malonaldehyde as an index of oxidative stress and that a bout of 100% O2max exercise markedly increases plasma levels of malonaldehyde in healthy men. Davies et al23 reported that the massive increase in oxygen uptake that occurs in skeletal muscle during exercise is associated with an increase in the generation of free radicals. These findings suggest that high-intensity exercise increases oxidative stress. It is thought that increased oxidative stress induced by high-intensity exercise will diminish vasodilation during exercise. However, unfortunately, we could not measure FBF during high-intensity exercise due to a technical issue (ie, large body motion). Matsumoto et al10 reported that the production of NO progressively increases as exercise intensity increases. Although we did not assess the production of NO, it is possible that acute exercise of high intensity increases NO production. The action of increased ROS that inactivates NO is removed by increased NO production, resulting in maintenance of endothelial function. Acute exercise of moderate intensity may predominately increase NO production compared with ROS production, leading to vasodilation during exercise in healthy subjects.

Protective antioxidant mechanisms are complex and multifactorial. Antioxidant defense system, such as superoxide dismutase, glutathione peroxidase, and catalase, scavenges ROS in the vasculature, resulting in inhibition of NO degradation. The susceptibility of vascular cells to oxidative stress is a function of the overall balance between the degree of oxidative stress and the antioxidant defense capability. Interestingly, Ohno24 has shown that high-intensity exercise of 75% O2max decreases the activity of Cu/Zn superoxide dismutase at 15 min after exercise in healthy young men. The antioxidant defense system may act to clear oxidative stress up to acute moderate exercise. However, high-intensity exercise may terminate the dysfunction of the antioxidant defense system.

The beneficial effect of physical exercise on oxidative stress might be related to increased antioxidant defenses.25 A major benefit of moderate-intensity exercise is to induce a mild oxidative stress that stimulates the expression of certain antioxidant enzymes mediated by the activation of a redox-sensitive signaling pathway. Interestingly, Ji et al26 have shown that gene expression of muscle mitochondrial superoxide dismutase is enhanced after an acute bout of exercise preceded by an elevated level of nuclear factor-kappaB. Repeated bouts of exercise would increase de novo protein synthesis of an antioxidant enzyme. Franzoni et al27 have demonstrated that chronic endurance training is associated with increased endothelium-dependent vasodilation and plasma antioxidant activity in young and older men.

Although norepinephrine is not released from the vascular endothelium, norepinephrine is a major vasoconstricting factor. In the present study, because all intensities of exercise increased heart rate, moderate-intensity and high-intensity exercises increased BP, and high-intensity exercise increased plasma norepinephrine concentrations, it is thought that sympathetic nervous activity is enhanced during acute exercise, especially high-intensity exercise. These findings are consistent with results of previous studies showing that acute bouts of high-intensity exercise increase circulating norepinephrine levels and augment sympathetic nervous activation in humans.28–30 It is well known that there is an interaction between NO and norepinephrine, an index of the sympathetic nervous system.31 Increase in norepinephrine during acute exercise may attenuate endothelium-dependent vasodilation. During acute moderate-intensity exercise, various vasodilators and vasoconstrictors act highly complex and interact with each other, resulting in vasodilation.

Study Limitations

Measurement of FBF during acute high-intensity exercise may enable us to draw more specific conclusions concerning the relationships between exercise intensity, oxidative stress, and NO production. In addition, although this was a randomized study, the number of subjects in this study was small.

In conclusion, acute moderate-intensity exercise induces vasodilation through an increase in NO bioavailability in humans. The beneficial effect of acute bouts of moderate-intensity exercise on endothelial function should, at least in part, contribute to the antihypertensive and antiatherogenic effects of long-term moderate-intensity aerobic exercise.

We thank Satoko Michiyama and Megumi Wakisaka for their secretarial assistance.

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Author notes

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This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.