(Circulation. 2001;103:2773.)
© 2001 American Heart Association, Inc.
Editorial |
Go With the Flow
From the Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, Calif.
Correspondence to John P. Cooke, MD, PhD, Section of Vascular Medicine, Stanford University School of Medicine, Stanford, CA 94305.
Key Words: Editorials • blood flow • nitric oxide • exercise • nitric oxide synthase
Physiologists have long recognized that blood vessels vasodilate in response to an increase of flow through their lumen.1 This flow-mediated vasodilation is not due to changes in luminal pressure, but rather to the tractive force of fluid flow, or shear stress.2 3 4 The endothelium senses shear stress by means of a mechanotransducer apparatus that is not fully defined but that may involve cytoskeletal deformation, activation of intracellular signaling molecules (such as the Smad proteins, mitogen-activated protein kinases, G proteins, and endothelial ion channels), and/or the release of endothelial agonists such as bradykinin.5 6 7 8 The endothelial mechanotransducer links the stimulus of flow to a response: vasodilation. This response results from the release of endothelial factors such as prostacyclin, endothelium-derived hyperpolarizing factor, and nitric oxide (NO).9 10 11 The latter has received much attention lately because it seems to be of primary importance in the flow-mediated vasodilation of most conduit vessels, most notably the human brachial artery.12 Flow-mediated vasodilation in the brachial artery is relatively simple to induce and easy to observe by duplex ultrasonography. Studies of flow-mediated vasodilation in the brachial artery have provided many insights into human endothelial physiology, including the adverse effects of risk factors on endothelial vasodilator function and the benefits of various interventions.13 14
The Role of NO in Exercise Physiology
The flow-stimulated release of endothelium-derived NO plays a critical role in the response to exercise. During exercise, the delivery of nutrients and oxygen to metabolically active skeletal muscle is critically dependent on flow-mediated vasodilation. When the NO synthase (NOS) pathway is impaired, either by hypercholesterolemia or by the administration of exogenous NOS antagonists, exercise-induced redistribution of blood flow to skeletal muscle is attenuated and exercise capacity is impaired.15 16 Specifically, in normal mice during peak exertion, the portion of the cardiac output that is directed to the hindlimb doubles. This doubling of the portion of cardiac output to the exercising muscles underestimates the absolute increase in blood flow to these muscles by not taking into account the increase in cardiac output, which, in a normal human subject, can increase 4-fold during exercise. In contrast, in apoE-deficient hypercholesterolemic mice or in mice treated with L-nitroarginine (an antagonist of NOS), there is no increase in the portion of cardiac output directed to the running muscles. Furthermore, we have observed that hypercholesterolemic mice manifest a reduction in indices of aerobic exercise capacity during treadmill testing, including a decline in maximum O2, anaerobic threshold, distance run to exhaustion, and aerobic work capacity.15 16 This is true of both diet-induced and genetically prone (ie, apoE-deficient) hypercholesterolemic mice. These abnormalities are associated with reduced aortic NO production ex vivo and an abrogation of exercise-induced NO synthesis (as assessed by post-exercise urinary nitrogen oxides).
These data reveal the importance of endothelium-delivered NO in determining physiological regional shifts in blood flow and are consistent with the notion that vascular transport of oxygen can be rate-limiting to metabolic capacity.17 In addition to its importance in flow-mediated vasodilation, NO has been proposed to regulate skeletal muscle function. Skeletal muscle is known to express 2 different isoforms of NOS, neuronal NOS (nNOS) and endothelial NOS (eNOS). In humans, nNOS is a splice variant that incorporates an additional 102 bp and is termed nNOSµ.18 In rodents, nNOSµ is expressed mainly in type II (fast twitch) muscle, whereas in humans, it is more generally distributed in type I and II fibers. Immunohistochemical association of nNOS with the sarcolemma suggests that it can influence blood delivery, glucose uptake, and excitation-contraction coupling. eNOS is more homogenously distributed in type I and II fibers. Its expression is correlated with mitochondria and may therefore influence oxidative metabolism.19
Long-term exercise increases the expression of NOS in the vessel wall and augments endothelium-dependent vasodilation.20 21 In addition, both nNOS and eNOS are elevated in skeletal muscle by exercise. The article by Kojda and colleagues22 in the current issue of Circulation confirms the effect of long-term exercise on eNOS expression. This effect of exercise may be mediated by shear stress. When endothelial cells are exposed to fluid flow in culture, NOS mRNA increases.23 This increased expression of eNOS in response to fluid flow may be mediated by shear stress and/or oxidant responsive elements in the promoter region of the gene.24 25 The increased expression of NOS in the vessel wall has physiological effects. Endothelium-dependent vasodilation is enhanced by long-term exercise.19 20 Furthermore, in individuals with impaired endothelium-dependent vasodilation (as in heart failure), exercise restores endothelial response.26
NO Mediates the Salutary Effects of Exercise
Regular aerobic exercise reduces cardiovascular morbidity and mortality.27 Physically active people are less likely to manifest symptoms of coronary artery disease.28 Although vigorous physical activity is associated with myocardial infarction, this adverse outcome is most usually incurred by individuals who otherwise lead a sedentary existence.29 The salutary effects of regular exercise include reductions in percentage of body fat, atherogenic lipids, insulin resistance, neurohormonal activation, blood pressure, and heart rate.30
Furthermore, beneficial alterations in vascular structure are achieved. Experimental studies and clinical observations indicate that there is a significant correlation between regular exercise and an increase in the luminal diameter of the coronary arteries.31 32 33 Men who have more vigorous lifestyles have larger coronary arteries. This beneficial effect of long-term exercise on vascular structure is mediated in large part by exercise-induced increases in endothelial shear stress. Long-term changes in blood flow cause a remodeling of the vessel wall that is endothelium-dependent.29 The endothelial factors that are involved in flow-induced vascular remodeling likely include platelet-derived growth factor-ß, transforming growth factor-ß, tissue plasminogen activator, matrix metalloproteinases, endothelin, prostacyclin, and NO.34
The flow-induced release of NO also opposes several atherogenic processes. NO inhibits platelet aggregation, monocyte adherence, and the proliferation of vascular smooth muscle.35 Flow-stimulated endothelial cells are less adhesive for monocytes.36 This antiatherogenic effect is due to the release of NO and is related to the NO-induced suppression of adhesion molecules and chemokines mediating monocyte adherence and entry into the vessel wall. In hypercholesterolemic animals, the enhancement of NO synthesis markedly reduces the progression of atheroma and can even induce regression of vascular lesions.37 38
NOS Polymorphisms and the Response to Exercise
In this context, the work of Kojda et al22 takes on potential significance. These investigators studied the response of eNOS to exercise in normal mice (eNOS+/+) and in those that have one dysfunctional eNOS allele (eNOS+/–). They found that the heterozygotes have normal endothelium-dependent vasodilation under basal conditions. When mice are exposed to daily exercise long-term, the normal mice manifest the expected increase in the endothelial expression of NOS and an enhancement of NOS activity in response to acetylcholine. Unexpectedly, the heterozygotes did not manifest any increase in NOS expression or activity with exercise. This is surprising because one may have expected an intermediate response to exercise but not a complete absence of response.
The mechanism underlying this observation remains to be elucidated. The investigators surmise that when 2 functioning alleles are present, under basal conditions both are being transcribed at a submaximal rate. In contrast, with one dysfunctional allele, the opposite allele may be activated at a near-peak transcription rate under basal conditions to maintain normal levels of eNOS. Accordingly, in the case of 2 normal alleles, there could be an exercise-induced increase in transcriptional rate that would not be achievable in the case of one functional allele. However, this scenario implies that under basal conditions, transcriptional activity must be regulated by the product of the enzyme. Indeed, this could have important ramifications in general for transcriptional regulation. An alternative hypothesis that the authors suggest is that in normal mice, one gene is silenced and becomes activated during exercise training. There is evidence that the expression of certain genes is regulated by methylation and demethylation.39
There are some possible explanations not considered by the authors that are just as likely, if not as interesting. Specifically, the heterozygote mice may not have been able to exercise with the same efficiency as wild-type mice. Accordingly, the exercise stimulus may have been reduced in the heterozygotes. Although the authors used a submaximal exercise training program where no obvious differences in exercise tolerance were observed, they did not assess parameters of aerobic exercise capacity. Accordingly, the proportion of time spent in aerobic versus anaerobic exercise training in the 2 groups is not known. Another methodological problem may occur with the assessment of eNOS expression. The exercise regimen used in this study only induced a 1.75-fold increase in eNOS message. This is a rather small increase in message that challenges the limits of the technology. It is therefore possible that an intermediate increase in eNOS expression occurred in the eNOS heterozygotes that was undetected by the investigators.
However, should additional investigations confirm the observations made in the current study, the implication would be that some polymorphisms of NOS in humans may be associated with normal endothelial function under basal conditions but could manifest abnormal responses under certain conditions of stress. In fact, this possibility has already been anticipated by the work of Wang and colleagues,40 who described individuals with a polymorphism of eNOS that increases their susceptibility to tobacco-induced coronary artery disease. Furthermore, there is accumulating evidence from human genetic association studies that certain polymorphisms of eNOS are associated with coronary artery disease.41 42 Therefore, the article by Kojda and colleagues22 supports the concept that the clinical characterization of such polymorphisms should include an assessment of the response to physiologically relevant stimuli. Careful phenotyping that includes physiological interventions will be necessary to fully understand and use the deluge of information that is becoming available regarding the human genome.
Acknowledgments
This work was supported in part by a grant from the National Heart, Lung, and Blood Institute (RO1 HL58638) and funding from the Tobacco-Related Diseases Research Program. Dr Cooke is an Established Investigator of the American Heart Association. Dr Tsao is a recipient of a Scientist Development Grant from the American Heart Association.
Footnotes
The opinions expressed in this editorial do not necessarily reflect the view of the editors or of the American Heart Association.
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