Physical Activity in Prevention and Treatment of Coronary Disease: The Battle Line is in Exercise Vascular Cell Biology

Joseph Wolffe was a pioneer in several areas during the early years of the American College of Sports Medicine. One of his areas of interest was the hypothesis that physical activity, throughout life, affords protection against degenerative diseases, especially cardiovascular diseases and arthritis (57). Recent reports confirm that Dr. Wolffe and colleagues were on the right track as reflected in the results presented in Figure 1. The relative risk of death examined in relation to categories of exercise capacity or fitness reveal that the most fit individuals have the lowest risk (1) (33). All-cause mortality is greater in unfit subjects whereas being highly fit is associated with substantially lower mortality (Fig. 1). There are a number of studies that have found that the incidence of coronary artery disease is inversely related to the level of fitness and/or activity of the individual (1).FIGURE 1: Mortality rates of men, without known coronary disease, categorized by level of fitness (Categories of Exercise Capacity) and adjusted for age. The range of exercise capacity values (METs) for each category are presented within each bar. Modified from Myers (33) with permission of the American Heart Association.In 1995, the National Institutes of Health held a conference on physical activity and cardiovascular health which reported, among a number of conclusions, that physical activity decreases known risk factors and has other beneficial effects that impede the development of atherosclerosis and are beneficial in treatment of coronary disease (36). As shown in Figure 2, Myers et al. (34) reported that highly fit individuals or individuals with a high aerobic capacity consistently had about half the risk of disease, even when considering the major known risks of cardiovascular disease including: hypertension, COPD, diabetes, smoking, BMI, and total cholesterol. These results indicate that an individual’s level of fitness is a more important predictor of death than established risk factors such as high blood pressure, COPD, diabetes, smoking, and high cholesterol. Stated another way, these results indicate that physical activity produces beneficial effects that are independent of the effects of physical activity on established risk factors (34). The research in our laboratory is driven by a quest to determine the mechanisms responsible for these beneficial effects of exercise that are not mediated by effects on known risk factors. Establishing these mechanisms is important because knowledge of the mechanisms whereby physical activity has these beneficial effects is important for progress in scientifically designing exercise prescriptions for individuals suffering from a spectrum of diseases. The central thesis of this paper is that the mechanisms responsible for these beneficial effects of physical activity on cardiovascular health will be found through application of understanding of the effects of physical activity on vascular cells, i.e., the battle line for physical activity in prevention and treatment of cardiovascular disease is in exercise vascular cell biology.FIGURE 2: Relative risk of mortality from any cause among subjects with various risk factors and with exercise capacities as shown in the legend. Numbers in parentheses are 95% confidence intervals for the relative risks. BMI denotes body mass index; COPD, chronic obstructive pulmonary disease; modified from Myers and colleagues (34).Before outlining the rationale for the proposal that the battle line is exercise vascular cell biology, we focus on what we know about mechanisms responsible for the beneficial effects of exercise on vascular cells and how this may influence atherosclerosis. It is not widely appreciated that the vasculature is the largest organ in the body or that blood vessels are more than just tubes that distribute blood (although that is clearly their most important function). Figure 3 is a photomicrograph of a vascular cast of the arterial circulation of a small portion of the rat gastrocnemius muscle that illustrates the diversity of the vascular tree. The blood vessels that make up the vascular tree are composed of three different types of cells: endothelial cells, which line the blood vessel as a single cell layer; smooth muscle cells, which are in the blood vessel wall and responsible for controlling the diameter of the arteries and therefore blood flow; and fibroblasts, which are in the adventitia. Fibroblasts are important because they, working in concert with smooth muscle cells, produce connective tissue in the vessel walls. This connective tissue in the blood vessel walls contributes to the compliance characteristics of blood vessels. The field of vascular cell biology and vascular medicine has experienced explosive growth in the past decade (28). For example, PubMed searches for vascular cells reveals 90,000 references, for endothelial cells 67,000 references, and 21,000 for smooth muscle cells. In contrast to the explosive growth in the field of vascular cell biology, growth in exercise vascular cell biology has been and remains modest. Most work in vascular cell biology of normal blood vessels is focused on two major functional cell types: endothelial cells (the single cell layer that lines the arteries) and the smooth muscle cells. In the smallest arteries and veins, there is a single cell layer of smooth muscle cells, but in larger arteries, there are several layers of smooth muscle. We will consider the biology of each of these types of vascular cells.FIGURE 3: Vascular cast of the arterial tree of the medial head of the gastrocnemius muscle of a rat.ENDOTHELIAL CELL BIOLOGY Just 20 years ago, endothelial cells were considered to be quiescent cells, with no function except passive lining of blood vessels and thereby inhibition of platelet adhesion. Now, it is clearly established that endothelial cells play key roles in a large number of vascular functions, including regulating vascular permeability, regulating interactions of platelets and maintenance of normal hemostasis, regulating leukocyte or white blood cell adhesion to blood vessel walls, participating in immune responses and inflammatory mechanisms, modulating lipid oxidation, regulating vascular structure, and maintaining vascular tone and therefore determining how much blood flow is distributed to each tissue of the body (21,23,29). A function or process that is only in operation if the endothelium is intact is often referred to as an endothelial-dependent process. For example, dilation induced by a drug or hormone that is abolished by removal of the endothelium is referred to as endothelium-dependent dilation (23). Many of these endothelium-dependent processes are mediated by the endothelium releasing signaling molecules such as illustrated in Figure 4.FIGURE 4: A simplified model receptor/signal transduction processes in endothelial cells that contribute to relaxation of vascular smooth muscle cell and endothelium-dependent dilation. DAG, diacylglycerol. IP3, inositol trisphosphate; P450, enzymes that are proposed to produce endothelium-derived hyperpolarizating factor (EDHF); SAC, stretch activated channel.As shown in Figure 4, endothelial cells release a number of vasoconstrictor substances as well as vasodilator substances (21,23,29). The three presented in Figure 4 are considered among the most important dilator molecules: prostacyclin (PGI2), endothelium hyperpolarizing factor (EDHF), and nitric oxide (NO). These signaling molecules are released by the endothelial cells (top) and have their actions on smooth muscle cells and other cells, as these molecules are released into the blood stream (above the endothelial cell at the top of the figure). In general, endothelium-dependent responses are initiated either by physical/mechanical signals such as sheer stress or deformation of the endothelial cell, or by hormones and drugs that act through receptors and/or receptor second messenger signaling processes. As shown, one of the most common methods of turning on release of one or more of these substances is to cause an increase in intracellular calcium. These same molecules (PGI2, EDHF, and NO) communicate with platelets and have interactions with other cells in the body. Nitric oxide is now recognized as a common mediator in many responses in the circulation. Endothelial function is often assessed by measuring endothelium-dependent dilation (EDD), either in vivo or in vitro (21,23,29). In vivo it is an advantage to examine EDD because measurements can be made noninvasively in human subjects using transcutaneous Doppler principle measures of flow velocity and ultrasound imaging techniques. Physical activity can stimulate increased EDD, that is, improve endothelial function. This has been demonstrated in conduit arteries, skeletal muscle arterioles, and coronary arterioles of animals and humans (7,9,15,18,21,23,29). For example, Muller and colleagues (32) examined EDD in arterioles isolated from the hearts of pigs and found that arterioles from trained pigs had much greater EDD to bradykinin (BK) than did arterioles isolated from inactive pigs (Fig. 5). In the same animals, we found that slightly larger arteries, coronary resistance arteries, isolated from trained pigs also exhibited increased BK-induced EDD than arteries from inactive pigs (39). Using drugs to block each of the pathways summarized in Figure 4 revealed that when NO synthase (NOS) was blocked there was no longer a difference between the responses of arterioles from trained and sedentary pigs (32). An increase in NO release from NOS could result from several mechanisms. We first evaluated the hypothesis that exercise training stimulates increased NOS protein content in endothelial cells through increased expression of the gene for endothelial NOS (eNOS). Results revealed that training did increase messenger RNA for eNOS gene in coronary arterioles (58), and protein content in arterioles/small arteries (25).FIGURE 5: Endothelium-dependent vasodilator responses of coronary arterioles to bradykinin. Responses expressed as a percentage of the relaxation achieved in response to 100 μM sodium nitroprusside. EX, arterioles from exercise-trained pigs; SED, arterioles from sedentary pigs. Data are from Muller et al. (32) with permission of the American Heart Association.There is interest in the hypothesis that exercise causes changes in endothelial gene expression through effects of exercise on sheer stress on endothelial cells. Results from endothelial cells this hypothesis in that of cells to flow of expression of eNOS and there have been studies in and that increased eNOS expression in arteries to an increased blood flow through this of stress on eNOS gene expression in coronary arterioles as revealed by isolated coronary arterioles to flow for 4 which increased eNOS in these coronary arterioles the effects of exercise training on endothelium in normal conduit arteries and skeletal muscle arterioles in animals and humans an increase in EDD as coronary These changes are associated with endothelial gene It is to that if increased physical activity has these physical cause decreases in endothelial-dependent dilation and gene this we examined EDD in muscle arteries and arterioles and of inactive We the muscle of the rat because it is a which has blood of about 100 of tissue the rat is physical activity by for produce a in EDD, if the hypothesis is As shown in Figure results indicate that arteries from the inactive animals exhibited a in the of EDD by we found that arterioles of the had EDD In from had EDD to normal as shown at the of Figure we the of eNOS protein in these arteries and found a in eNOS protein associated with endothelial-dependent dilation in arteries and In the arterioles no difference in eNOS protein and no in endothelial function (Fig. These results are with the hypothesis that a or of physical activity is to normal endothelial function in these As in a physical activity is a major in our this to be at in to endothelial function. we in our from the that being inactive or sedentary is Physical is of the disease process for a large number of important chronic disease Endothelium-dependent vasodilator responses of arteries and first and second arterioles of rat muscle. Responses expressed as a percentage of the dilation arteries) or relative diameter and protein from are shown at the for and inactive of Results for the arteries are from and Results for the arterioles are from et al. important in presented in Figure is that there is a of on endothelial function and eNOS expression in the arterial tree of the muscle. is, arteries that blood to the muscle have a in EDD and endothelial gene expression whereas other in the same tree did There is that the effects of exercise physical activity are also throughout the arterial circulation. For example, exercise training eNOS protein content in arteries in the coronary arterial tree but not in (Fig. as in a number of several responses of coronary arteries the coronary arterial including and response of the small arteries to increased Physical activity produces in several of these vascular but these changes are also distributed in a throughout the coronary arterial tree. For example, exercise training produces an increased response in the not in the arteries, whereas exercise produces a in in the arteries, no in the Exercise training produces an increase in to in the large coronary arteries, not the and EDD responses are in the coronary arteries but not in the conduit coronary arteries it is that physical activity has effects throughout the arterial circulation This that in eNOS protein content are not distributed in the coronary arterial circulation in response to exercise training (Fig. to the coronary endothelial cells have the same of eNOS is, an endothelial cell from a conduit coronary artery have the same eNOS protein content of endothelial cell as the endothelial cell in a small coronary As shown in Figure the endothelial cell to smooth muscle cell for coronary arteries as from large artery to arterioles, there is only one endothelial cell layer and one smooth muscle cell We that if endothelial cells the same of eNOS there be more eNOS protein total artery protein in these small arteries than in the large arteries because the large arteries more smooth muscle As shown in Figure we found that the large arteries had as much or more protein of total artery than did the smallest arterioles, in contrast to our hypothesis These results that endothelial cells of large coronary arteries have more eNOS protein cell than the endothelial cells of small coronary Exercise changes in eNOS content of coronary arteries of different conduit coronary are small arteries and to and arterioles with of and with is expressed relative to values of the same arteries of sedentary pigs. Modified from et al. with permission of the American of coronary on smooth muscle cell to endothelial cell and eNOS protein content in are presented in In artery are the A are arteries for and eNOS content than in other coronary Results are from et al. physical activity is important for and endothelial function and artery These beneficial effects of physical activity to be the result of in the content of eNOS as well as other in endothelial cells. The signals by exercise that these may be sheer stress as well as stretch physical activity not these effects on arteries but produces effects throughout the circulation. There is a body of that physical activity causes changes in the functional of coronary vascular smooth muscle in humans and animals For example, et al. reported that coronary arteries of a greater for dilation in response to sodium than the arteries of normal This increased response could be to larger coronary arteries) or to increased of the coronary artery smooth muscle cells to There are also a number of studies that physical activity can characteristics of coronary vascular smooth muscle in animals and colleagues a number of studies the past of in smooth muscle cells from conduit coronary arteries of trained pigs. Using and of coronary smooth muscle cells, have a number of important changes in the of and of intracellular In exercise-trained smooth there is of an increase in application of a number of This increase in to be the result of changes in and release by the Results a larger of is released into the the between the of the and of the cell and is of the In exercise-trained coronary smooth muscle cells have an increased the through also to increase through and of the cells through these to make the cell more the cells to It that these changes in and smooth muscle cell results in in coronary arteries of trained animals exercise training changes how smooth muscle is with the result being to from the also produces decreases in content of the of smooth muscle cells by such as This may be important in the of gene expression in smooth muscle and may in of and vascular in vascular these indicate that exercise training produces the changes in coronary smooth it dilator responses and decreases responses of conduit coronary artery smooth muscle cells it in the arterioles it through well as through and in the cell and it the of and just as was of exercise training produces effects on coronary vascular smooth muscle throughout the arterial tree. we have been focused on changes in function of endothelial cells and vascular smooth muscle cells, exercise training can also increase capacity of the coronary arterial tree through vascular The that exercise training the number of as well as the number and of arteries and arterioles not in of vascular it is important to that these processes are important of exercise vascular cell CELL BIOLOGY this we from of exercise vascular cell biology in normal subjects to application of exercise vascular cell biology to vascular how to the of exercise vascular cell biology to vascular disease, it is to three physical activity improve endothelial function in arteries effects of vascular physical activity have effects on smooth muscle of normal and coronary are the effects of exercise training on endothelial and/or smooth muscle cells modified as the arteries through the of atherosclerosis from disease to coronary artery these an understanding of the of atherosclerosis. a is presented because several are for more in of of vascular disease A normal artery not have or to the an early in the of atherosclerosis to be increased expression of for adhesion molecules by endothelial cells. This in gene expression to be by lipid the artery The adhesion molecules on the endothelial cells or which to the endothelium of the blood vessel wall into the of the these the to into or and up The high content of lipid these cells small cells, which are referred to as cells. this of the disease platelets also to adhesion factors expressed by the endothelial cells. The endothelial cells also to and release inflammatory into the tissue and into the blood As the disease lipid in the of the These more have a of connective tissue and smooth muscle cells. These smooth muscle cells to play an important in of connective tissue in the to and/or the connective tissue on the of the blood to into the of the The of and in the of the is highly of the of an can to and related is a disease, and et al. has different in the of atherosclerosis that a of disease not have to consistently and progress to coronary artery can or even the of atherosclerosis Exercise is one important in with other can have beneficial effects on coronary artery often throughout much of the of vascular with of coronary artery disease only in of atherosclerosis. that endothelial is and throughout the process of that the endothelium a in atherosclerosis it has been proposed that endothelial-dependent dilation and endothelial release of nitric oxide may with increased expression of adhesion factors and therefore may be key factors in determining or not this important related to the of atherosclerosis is that the effects of exercise training or physical activity on vascular cells and on coronary artery disease may at different of the There are a number of studies that examined the effects of exercise in with coronary disease For example, and colleagues reported that physical activity has beneficial effects on the endothelium of coronary arteries in subjects that have coronary artery coronary artery disease were into two one and one were exercise trained for 4 A of this is that endothelium-dependent dilation was in conduit arteries and resistance As shown in Figure of of the conduit arteries of of a in i.e., a than the dilation. that the sedentary had about the same of 4 of no In the that was exercise trained exhibited a in (Fig. This in of the conduit arteries could endothelial function and/or of the coronary smooth muscle to effects of As shown in Figure total coronary blood flow was with a Doppler in the coronary about a increase in coronary flow velocity in at the of the 4 of exercise there was a increase in coronary blood flow in response to in the trained and no in the subjects (Fig. These results indicate that in the of the coronary there was an increase in the endothelium-dependent dilation in the exercise-trained These also a for the of the effects of exercise training on and the of training for with coronary artery endothelium-dependent dilation of conduit coronary arteries and in coronary blood flow of with coronary artery was trained for 4 the Figure from et al. with permission of the of have examined the effects of exercise training on endothelial function in two of coronary artery One model coronary that is not by associated with This model an to produce total of the coronary artery a of 4 an of the (the and an of (the hypothesis was that chronic endothelium-dependent dilation and that exercise training of pigs with chronic coronary to endothelium-dependent dilation. were isolated from sedentary and trained pigs. from the arteries in the from the were found to have endothelium-dependent dilation. In arteries in the of exercise-trained pigs normal endothelial-dependent dilation Results that the endothelium-dependent dilation was in to eNOS expression in arteries of trained pigs. The second model we are using is a model of early of coronary artery disease et al. in which pigs coronary disease in response to a which produces and cells and early were in the coronary arteries of these pigs We examined endothelium-dependent dilation in the in the coronary artery from normal and pigs that were exercise trained or sedentary As shown in Figure induced relaxation in from exercise-trained and sedentary pigs on normal In sedentary pigs that were on the clearly had a endothelium-dependent dilation. exercise training endothelium-dependent dilation of from pigs was to normal (Fig. Results revealed that EDD is in to release of NO and the of an exercise training increased the of endothelium-dependent dilation and to release of the by relaxation of coronary arteries bradykinin are pigs exercise exercise relaxation was as in from induced BK-induced relaxation was in at and high of was by that were not different from and with permission of the work is to determine the mechanisms whereby endothelial function and mechanisms whereby exercise training or these eNOS protein content is not the only whereby endothelial cells can increase the of nitric oxide to An increase in the activity of eNOS will also cause increased release of NO by and there are a number of substances that influence NO For example, can increase expression of and increased of eNOS by in endothelium to eNOS activity a number of have been reported to protein such as that to have the to influence eNOS activity without gene in to gene protein interactions can be by atherosclerosis and/or by physical training and thereby influence endothelial function. in human subjects with coronary artery disease, exercise training to improve endothelium-dependent dilation. exercise training to improve endothelium-dependent dilation in coronary arteries early such as cells and/or the beneficial effects of exercise training to be greater early in the disease process. The exercise in the early of disease to be the result of increased dilation as well as a in a The endothelial throughout the of atherosclerosis may indicate that the endothelium in vascular disease or endothelial may be one result of vascular of the of endothelial in the of disease, exercise training to beneficial effects on endothelial Exercise training also has beneficial effects on vascular smooth muscle cells. remains is what are the signals by exercise that the changes in endothelial and smooth muscle cells There is that are by the effects of exercise on the artery cells, i.e., stress and/or There is also that exercise signals inflammatory that are released by skeletal and/or These signals may expression of the in the endothelium and smooth muscle cells and expression of in vascular cells. It is important to determine the effects of physical activity on artery health are mediated through factors released by these other or through effects on the arteries because this will the and of exercise to and normal understanding of atherosclerosis has the past This has from studies of lipid and the of in the disease, especially cholesterol. This has also been the result of understanding of vascular cell biology to determine mechanisms in atherosclerosis Exercise vascular cell biology is the of the In the on the of in atherosclerosis has many to that atherosclerosis is an inflammatory Endothelial cells can in and inflammatory and and may therefore play a key in these processes. work these to exercise vascular cell biology is It is that many of the of this will to in this studies with understanding of vascular cell biology are The battle with coronary artery disease will be through the application of understanding of exercise vascular cell This work was presented at the American College of Sports as the Joseph Wolffe The work was by National and