Age-associated Arterial Remodelling and Cardiovascular Diseases

Arterial remodelling is a major risk factor for a variety of age-related diseases and represents a potential target for therapeutic development. During ageing, the structural, mechanical and functional changes of arteries predispose individuals to the development of diseases related to vascular abnormalities in vital organs such as the brain, heart, eye and kidney. For example, aortic stiffness increases nonlinearly with advancing age – a few percent prior to 50 years of age but over 70% after 70 years of age. The elevated stiffness in large elastic arteries leads to increased transmission of high pressure to downstream smaller blood vessels, in turn affecting the microcirculation and end-organ functions. Meanwhile, the augmented remodelling of small arteries accelerates central arterial stiffening. This chapter is to provide an overview of age-associated changes in the arterial wall and their contributions to both central and peripheral vascular abnormalities associated with ageing. Therapeutics that specially target the different aspects of arterial remodelling are expected to be more effective than the traditional medications, particularly for the treatment and management of vascular ageing-related diseases.published_or_final_versio

Cerebrovascular Dysfunction in Metabolic Syndrome and Depression: Mechanisms and Implications for Neurovascular Disease

Cerebrovascular diseases are any pathological conditions that are caused by disruptions or alterations in the blood supply to the brain. The brain, which is only 2% of body mass, constantly requires a minimum of 20% of cardiac output to meet its unique metabolic demands. Cerebral tissue relies on aerobic respiration and therefore requires a constant, steady supply of blood to provide oxygen and glucose to make ATP. Insufficient perfusion of the brain, which occurs as a result of reduced or obstructed blood flow, results in cerebral ischemia; ischemic neurons quickly deplete their available ATP and die if blood flow is not restored. Disruptions in the delivery of blood to the brain, either acutely or chronically, can lead to impairment of neurological function, neuronal cell death, and even death.;Metabolic Syndrome (MetSyn) is the comorbid presence of three or more risk factors, including: central obesity, hypertriglyceridemia, high cholesterol, hypertension, and hyperglycemia. Data from the NHANES study reports that over 35% of all adults, and over 50% of adults over 60, are estimated to have MetSyn. MetSyn is a risk factor for almost every major cardiovascular disease, and causes well-defined impairments in the peripheral circulation. However, its effects on regulation of cerebral blood flow are not well understood. The primary aim of this dissertation is to study the functional and structural alterations cause by MetSyn in the cerebral circulation, utilizing the obese Zucker rat (OZR), a translationally relevant model for studying the vascular complications of MetSyn, and to interrogate how vascular and neurological outcomes to cerebrovascular disruptions are influenced by pre-existing cerebral vasculopathies associated with MetSyn in OZR. The specific aims are as follows:;1. Define and characterize structural and functional changes in vascular reactivity, vessel wall mechanics, and vascular remodeling in middle cerebral arteries (MCA) of OZR, and evaluate changes in microvascular density throughout the cerebral microcirculation.;2. Determine the contribution of each constituent pathology of MetSyn on the development of cerebrovascular impairments in OZR, and identify mechanisms through which these pathologies are progressing in the cerebral circulation.;3. Interrogate the mechanisms of stroke-induced vascular dysfunction in lean Zucker (LZR) and OZR, and determine how pre-existing vascular dysfunction in OZR changes outcomes and pathological mechanisms following ischemic stroke.;4. Investigate the effects of chronic stress/ depression on cerebrovascular function in LZR and OZR, assess efficacy of exercise as a prophylactic anti-depressant treatment, and evaluate mechanisms through which impairments, and exercise-based recovery, are occurring.;The results of these studies have helped to establish the prominent driving mechanism of cerebrovascular dysfunction in MetSyn, and defined the functional impacts these impairments have on outcomes to ischemic stroke and stress-induced depression. By thoroughly interrogating these mechanisms and evaluating functional outcomes of relevant disease states, this dissertation hopes to lay the groundwork for a new approach to understanding the relationship between MetSyn, cerebrovascular dysfunction, and neurological outcomes

Inositol 1,4,5-Trisphosphate Receptors in Hypertension.

Chronic hypertension remains a major cause of global mortality and morbidity. It is a complex disease that is the clinical manifestation of multiple genetic, environmental, nutritional, hormonal, and aging-related disorders. Evidence supports a role for vascular aging in the development of hypertension involving an impairment in endothelial function together with an alteration in vascular smooth muscle cells (VSMCs) calcium homeostasis leading to increased myogenic tone. Changes in free intracellular calcium levels ([Ca] ) are mediated either by the influx of Ca from the extracellular space or release of Ca from intracellular stores, mainly the sarcoplasmic reticulum (SR). The influx of extracellular Ca occurs primarily through voltage-gated Ca channels (VGCCs), store-operated Ca channels (SOC), and Ca release-activated channels (CRAC), whereas SR-Ca release occurs through inositol trisphosphate receptor (IPR) and ryanodine receptors (RyRs). IPR-mediated SR-Ca release, in the form of Ca waves, not only contributes to VSMC contraction and regulates VGCC function but is also intimately involved in structural remodeling of resistance arteries in hypertension. This involves a phenotypic switch of VSMCs as well as an alteration of cytoplasmic Ca signaling machinery, a phenomena tightly related to the aging process. Several lines of evidence implicate changes in expression/function levels of IPR isoforms in the development of hypertension, VSMC phenotypic switch, and vascular aging. The present review discusses the current knowledge of these mechanisms in an integrative approach and further suggests potential new targets for hypertension management and treatment.This publication was made possible by an MPP fund (#320133) from the American University of Beirut to AE

Endothelin-1 and Hypoxic Vascular Remodeling in Ovine Fetal Cerebral Arteries

Intrauterine hypoxia resulting from decreased maternal oxygen uptake, insufficient oxygen carrying capacity, or compromised oxygen delivery to the fetus jeopardizes fetal oxygen delivery, detrimentally affecting growth and development of the immature vasculature. Hypoxia transiently increases Hypoxia Inducible Factor-1α (HIF- 1α), which complexes with HIF-1β to form the active HIF-1 dimer that can affect transcription. This temporary rise in HIF-1 can promote gene transcription of ligands such as Vascular Endothelial Growth Factor (VEGF) and Endothelin-1 (ET-1), which rises and falls with HIF levels. The absence of chronic elevation of these ligands prompted the question of how long-term effects of hypoxia is sustained. Results suggest that in addition to stimulating transient rises in ligand levels, hypoxia alters receptor expression and coupling of these ligands to the intracellular kinases. Endothelin-1 (ET-1) is an established vasoconstrictor that can activate ETA or ETB receptors, respectively stimulating vasoconstriction and vasodilation. ET-1 activates pathways such as Protein Kinase C (PKC), Ca2+/Calmodulin-Dependent Protein Kinase (CaMK), p38, and MEK/ERK, which are involved in cellular growth, proliferation, and differentiation. Our results demonstrate that chronic hypoxia altered ovine fetal cerebrovascular reactivity to ET-1 but not plasma ET-1 levels or ETA receptor cerebrovascular expression. However, chronic hypoxia enhances ET-1-induced contractility in an ETAdependent manner in Middle Cerebral Arteries (MCAs). ET-1 also exerts trophic effects on ovine fetal cerebrovasculature in organ culture in a PKC-dependent manner by inducing hypertrophy and increasing medial thicknesses, more in normoxic than hypoxic MCAs. ET-1-induced increase in arterial wall thickness is mediated by CaMKII and p38 dependent pathways in normoxic but not hypoxic arteries. Additionally, Myosin Light Chain Kinase (MLCK) and Smooth Muscle Alpha Actin (SMαA) colocalization data shows that ET-1 promotes contractile dedifferentiation in normoxic but not hypoxic MCAs in a PKC, CaMKII, and p38 dependent manner. These results support the notion that chronic hypoxia has long term effects mediated by altered receptor expression levels and intracellular coupling. A better understanding of how chronic hypoxia affects ET-1- induced intracellular coupling will help identify potential targets for future therapies to prevent and potentially treat remodeling of cerebral arteries in infants exposed to intrauterine hypoxia

Altered Hemodynamic Control in the Skeletal Muscle Microcirculation in the Metabolic Syndrome: The Emergence of a new Attractor

Peripheral vascular disease is a pathological disease state whereby the peripheral vascular system becomes progressively limited in its ability to adequately perfuse extremities despite increases in metabolic demand. One of the primary risk factors for development of peripheral disease is the Metabolic Syndrome. It is the presentation of three simultaneous comorbidities, with obesity being the most common denominator in most metabolic syndrome patients, but also including a pro-inflammatory state, pro-oxidant state, pro-thrombotic state, increased blood pressure, atherogenic dyslipidemia, or insulin resistance. Currently, the population of the United States is approximately 75% overweight, obese, or extremely obese and the prevalence of obesity related consequences is rising, including but not limited to diabetes, cardiovascular disease, and peripheral vascular disease.;Our laboratory has demonstrated that a defining characteristic of metabolic syndrome is microvascular dysfunction. Increasing evidence has indicated that the mechanisms responsible for microvascular dysfunction in metabolic syndrome are oxidant stress-based alterations to arachidonic acid metabolism and increased sensitivity to adrenergic stimulation. However, it remains unclear if the altered microvascular reactivity that presents with metabolic syndrome plays a role in the perfusion: demand mismatch that accompanies and defines peripheral vascular disease. This dissertation seeks to understand the altered hemodynamic control found in metabolic syndrome. It set out to examine: 1. The effect of altered microvascular reactivity in metabolic syndrome on vascular tone and performance outcomes. 2. Identify the spatial and temporal alterations to perfusion distribution in metabolic syndrome. 3. Determine the effect of metabolic syndrome on erythrocyte distribution in the microvasculature and capillaries.;The results of these studies determined that a key contributor to the development, maintenance, and progression of peripheral vascular disease is microvascular dysfunction. This dysfunction increases the microvascular perfusion distribution by spatially distinct mechanisms, with adrenergic dysfunction dominating in larger microvessels and oxidant stress-based increases in thromboxane A2 dominating in smaller microvessels. Additionally, metabolic syndrome blunts the temporal compensation that would serve to attenuate the increased perfusion distribution. Combined, the spatial and temporal impairments to microvasculature serve to alter erythrocyte distribution on a network level in skeletal muscle. These data shed light on how metabolic syndrome insidiously alters microvascular control of blood flow, leading to a perfusion: demand mismatch, and ultimately contributing to pathological disease states like peripheral vascular disease

Basic and Clinical Understanding of Microcirculation

Microcirculation is key to providing enough nutrition and oxygen from head to toe. This is possible only through an extensive network of blood vessels spread around the body. Effect of microcirculation abnormalities stretch beyond one’s comprehension. The effects could be felt at any age, from the foetal life to the adulthood. The chapters present in this book describe how these abnormalities could lead to diseases such as atherosclerosis, thrombosis, diabetes, hypertension. Disorders of microcirculation could be related to the structural and/or functional damage to the inner lining of the blood vessels. Early identification of these disorders could benefit many ailments including cardiovascular and cerebrovascular diseases such as heart attack and stroke

Microvascular Function and Remodeling Due to Chronic Changes Within the Skeletal Muscle Microenvironment

The skeletal muscle microcirculation is a key regulator of local blood distribution, vascular resistance and overall blood pressure (BP). Arterioles and capillaries are two important components of the microcirculation, which can undergo remodeling such as arteriogenesis, angiogenesis, or capillary rarefaction. Vascular remodeling requires the coordinated action of several factors within the microenvironment. These include: matrix metalloproteinases (MMPs) and their endogenous inhibitors, tissue inhibitor of metalloproteinases (TIMPs), vascular endothelial growth factor-A (VEGF-A) and thrombospondin-1 (TSP-1). The objective of this dissertation was to examine how alterations to the microenvironment impacted the appropriate microvascular remodeling responses to alterations in flow. The skeletal muscle microenvironment was altered through manipulation of TIMP1 expression or glucocorticoid (GC) levels. Furthermore, blood flow was altered via femoral artery (FA) ligation or prazosin treatment. This dissertation includes three primary hypotheses and corresponding findings to examine the importance of alterations to the microenvironment on microvascular remodeling. Firstly, the loss of TIMP1 would enhance both ischemia and flow-induced vascular remodeling by increasing MMP activity. Using TIMP1 deficient mice (Timp1-/-), we demonstrated that TIMP1 is integral for vascular network maturation. Additionally, TIMP1 is required for microvascular adaptations to alterations in flow. This was proven by the absence of arteriogenesis and/or angiogenesis in Timp1-/- mice in response to elevations in flow despite an increase in both VEGF-A and eNOS mRNA. Secondly, Corticosterone (CORT) treatment would inhibit endothelial mediated shear stress signaling and subsequently, the microvascular remodeling responses to prazosin administration. Lastly, CORT mediated hypertension and microcirculatory rarefaction would be prevented with 2 weeks of concurrent prazosin or Tempol (a ROS scavenger) administration. Endothelial cell responsiveness to shear stress was partially blunted by CORT pre-treatment. The lack of vascular remodeling (angiogenesis and arteriogenesis) and prevention of GC-mediated capillary rarefaction in CORT-prazosin animals supports this finding. The maintenance of vascular tone and skeletal muscle blood flow, more so then lowering circulating levels of ROS, was responsible for mitigating CORT-induced capillary rarefaction and hypertension. Taken together, these three studies demonstrate that perturbations of the microenvironment, due to the loss of TIMP1 or elevated GCs, results in impaired microvascular remodeling to alterations in flow. Furthermore, alterations to the skeletal muscle microcirculation can impact overall cardiovascular health

Exercise Training Improves Cerebrovascular Oxidative Stress Regulation and Insulin Stimulated Vasodilation in Juvenile and Mature Pigs

Background: Selective insulin resistance in the cerebrovasculature, characterized by augmented vasoconstriction in response to insulin, may relate to enhanced sensitivity to endothelin-1 (ET1) or increased oxidative stress, culminating in attenuated nitric oxide (NO) signalling. Regular exercise has been shown to enhance vascular responses to insulin, but the mechanisms remain unclear. This study tested the hypothesis that exercise training improves oxidative stress regulation and cerebrovascular insulin-stimulated vasodilation in juvenile and mature pigs. Methods: Twenty juvenile (n=10F/10M; 3±1 months; mass=11±3 kg) and 17 mature (n=9F/7M; 14±1 months; mass=83±9 kg) Ossabaw miniature-pigs were divided into sedentary or exercise training groups. Pigs in the exercise training groups completed high intensity interval training three times per week for eight weeks. All animals were group housed with access to 1 kg of feed per pig per day, as well as sugar water (~5 L per pig of 10% solution). At euthanasia, cerebral arteries were dissected for pressure myography experiments. Vascular diameter was tracked continuously and vasomotor responses to insulin (1e-9-1e-6 M) and ET1 (1e-12-1e-7 M) were studied under three conditions: 1) untreated (vehicle); 2) superoxide dismutase (SOD) mimetic (TEMPOL; 1e-4 M) and 3) NAD(P)H Oxidase (NOX) inhibition (Apocynin; 1e-4 M). Physiologic maximum and cumulative change in diameter (AUC) for all groups were compared using a two-way ANOVA (independent variables: age and exercise training). Results: Whereas sedentary pigs displayed insulin-stimulated vasoconstriction, exercise trained pigs exhibited insulin-stimulated vasodilation. Indices of insulin-stimulated vasodilation were significantly greater in exercise trained vs. sedentary controls (main effect: P<0.001). Pretreatment with the SOD mimetic or NOX inhibitor abolished between group differences (P≥0.85). Indices of ET1-induced vasoconstriction were not significantly different between groups under any experimental condition (P≥0.11). Conclusion: That insulin-stimulated vasoconstriction was reversible with a SOD mimetic or NOX inhibition in sedentary pigs implicates impaired oxidative stress regulation in the manifestation of selective insulin resistance. Exercise training coincided with improved oxidative stress regulation conjunctional with augmented insulin-stimulated cerebral vasodilation. Given vasoreactivity to ET1 was similar between groups, greater insulin-stimulated vasodilation in exercise trained pigs was likely the result of enhanced oxidative stress regulation yielding improvements in NO signalling