: Vascular Function in Health and Disease

Overview

Many control mechanisms exist which successfully match the supply of blood with the metabolic demand of various tissues under wide-ranging conditions. One primary regulator of vasomotion and thus perfusion to the muscle tissue is the host of chemical factors originating from the vascular endothelium and the muscle tissue, which collectively sets the level of vascular tone. With advancing age and in many disease states, deleterious adaptations in the production and sensitivity of these vasodilator and vasoconstrictor substances may be observed, leading to a reduction in skeletal muscle blood flow and compromised perfusion to the muscle tissue. Adequate perfusion is particularly important during exercise to meet the increased metabolic demand of the exercising tissue, and thus any condition that reduces tissue perfusion may limit the capacity for physical activity. As it is now well established that regular physical activity is a key component in maintaining cardiovascular health with advancing age, there is a clear need for further studies in populations where vascular dysfunction is compromised, with the goal of identifying the mechanisms responsible for the dysfunction and exploring whether these maladaptations may be remediable. Thus, to better understand the etiology of these vascular adaptations in health and disease, the current proposal is designed to study changes in vascular function with advancing age, and also examine peripheral vascular changes in patients suffering from chronic obstructive pulmonary disease (COPD), Sepsis, Pulmonary Hypertension, and cardiovascular disease. While there are clearly a host of vasoactive substances which collectively act to govern vasoconstriction both at rest and during exercise, four specific pathways that may be implicated have been identified in these populations: Angiotensin-II (ANG-II), Endothelin-1 (ET-1), Nitric Oxide (NO), and oxidative stress.

Angiotensin-II (ANG-II) is the end-product of the renin-angiotensin cascade, and acts as a potent endogenous vasoconstrictor through binding to the angiotensin receptor (AT1) on arteriolar vascular smooth muscle. With advancing age, there is a notable decline in plasma renin activity accompanied by decrements in circulating ANG-II and an increase in AT1 receptor density. However, the functional consequence of this age-related adaptation of the renin-angiotensin system (RAS) on the peripheral circulation is not well understood. Likewise, in recent years it has become apparent that cardiovascular disease is a major cause of morbidity in COPD, which may be related to vascular dysfunction and associated adoption of a sedentary lifestyle. In these patients, changes in RAS activity have been linked to peripheral vascular dysfunction, with compelling evidence for improvements in peripheral oxygen use following angiotensin-converting enzyme (ACE) inhibition. Like the aging population, systematic studies evaluating AT1 receptor sensitivity and the efficacy of AT1 receptor blockade on peripheral hemodynamics have not been undertaken. The ubiquitous substance nitric oxide (NO) is now recognized as a key pathway for endothelium-dependent vasodilation, with the bioavailability of NO serving as an indicator for overall vascular health. Cardiac risk factors have been shown to cause impairment in endothelial vasodilator function in both the peripheral and coronary arteries. Coronary vascular dysfunction is an important phase in atherogenesis and is associated with myocardial ischemia. Furthermore, peripheral vascular function has been linked to coronary vascular dysfunction which could have important clinical implications in terms of health screening. Impaired endothelium-dependent vasodilation has been associated with the elderly, patients with COPD, and most cardiovascular diseases including pulmonary hypertension, and heart failure (HF), though the functional consequence of this adaptation on peripheral blood flow regulation remains unclear. Thus, we propose the use of a compound which inhibits the enzyme responsible for NO production in endothelial cells, N-monomethyl-L-arginine (L-NMMA), to temporarily block production of NO and thus determine the importance of this pathway at rest and during physical activity. Additionally, we propose the use of acetylcholine (Ach) to determine endothelial-dependent vasodilation and sodium nitroprusside (SNP) and nitroglycerin (NTG) to determine the endothelial-independent vasodilation in the coronary arteries and the periphery. Oxidative stress associated with aging has been shown to reduce vascular function and antioxidant supplementation restores vascular function to levels that are indistinguishable from healthy young adults. The manner by which this improvement in vascular function occurs is not known by may be acting through a NO dependent mechanism. Histamine has been reported to mediate sustained post-exercise vasodilation through histamine-1 (H1) and histamine-2 (H2) receptor activity, which results in a \~50% elevation in femoral artery blood flow (above resting levels) that lasts for more than 100 minutes after a single bout of moderate-intensity dynamic exercise. Vasodilation can be markedly reduced by giving either fexofenadine (Allegra, a selective H1-receptor antagonist) or ranitidine (Zantac, a selective H2-receptor antagonist). The combination of H1/H2 blockade abolishes \~80% of the post-exercise vasodilation seen after whole-body exercise such as cycling and this observation has been observed in multiple studies in young sedentary, recreationally active, and endurance trained men and women. The impact of histamine on the post-exercise vasodilatory response is substantial; however, the role of H1/H2 receptors in regulating skeletal muscle blood flow during exercise is unknown. Thus, we intend to investigate the role of H1/H2 receptors in the regulation of skeletal muscle blood during exercise as this may be an important pathway in age and disease related reductions in blood flow during exercise. Exercise training and rehabilitation can be used as an alternative approach to combat the deleterious effects oxidative stress on aging and disease. An effective exercise training intervention can decrease sympathetic nervous system activity, improve arterial compliance and vascular endothelial function, and alter the pro- and antioxidant balance resulting in improved endogenous antioxidant defense mechanisms. Moreover, exercise training concomitantly improves musculoskeletal strength and function, glucose regulation and insulin sensitivity, cardiovascular function, body composition, blood chemistry (decreased triglyceride and cholesterol levels), and overall well-being. The physiologic effect of an exercise rehabilitation program in diseases such as COPD, and pulmonary arterial hypertension (PAH) is incompletely understood. However, recent studies suggest that exercise training in this patient population is well tolerated and associated with clinically significant physiologic improvements as well as improvements in various quality of life scores. A unique feature of the proposed studies identified herein is the inclusion of a novel methodological approach to comprehensively evaluate the functional outcome of the proposed pharmacologic interventions. The recent development of a unique combination of nuclear magnetic resonance (NMR) techniques by members of our group enables near-simultaneous measurements of both muscle perfusion and metabolism in vivo. The arterial spin labeling (ASL) technique allows the measurement of both spatially and temporally resolved quantification of perfusion, while the kinetics of phosphocreatine (PCr) depletion and recovery provide high resolution measurements of muscle energetics. The interweaving of these imaging and spectroscopic modules provides the opportunity for determination of skeletal muscle perfusion and metabolism kinetics during and following the stress of physical exercise. Thus, this NMR-based approach, combined with direct measures of muscle fatigue, offers the potential to further define the individual and collective contribution of these variables to the attenuated limb blood flow in the elderly and in patients with COPD and PAH. We propose that each of these pathways outlined above represent an avenue by which vascular function is compromised in the elderly and in patients with COPD, PAH and cardiovascular disease. However, because these pathways are not mutually exclusive, the proposed studies are designed to systematically evaluate hemodynamic responses to intra-arterial or intravenous administration of pharmacologic agents specific for the AT1 receptor (ANG-II and Diovan, AT1 agonist and antagonist, respectively), the Endothelin receptor Type-A (ETA receptor) (BQ-123, ETA antagonist), and the NO pathway (L-NMMA, Ach, and SNP) both before and after exercise training.