Aging is associated with a progressive decline in numerous physiological processes, leading to an increased risk of health complications and disease. By delivering oxygenated blood to all tissues in the body, the health of the cardiovascular system is vital for health of every tissue and longevity of the organism as a whole. Aging has a remarkable effect on the heart and arterial system, leading to an increase in CVD including atherosclerosis, hypertension, myocardial infarction, and stroke.3
Aging cardiovascular tissues are exemplified by pathological alterations including hypertrophy, altered left ventricular (LV) diastolic function, and diminished LV systolic reverse capacity, increased arterial stiffness, and impaired endothelial function3,4
(). However, the health of the arterial and cardiac systems is not mutually exclusive, as each system greatly affects the other. For instance, an increase in arterial stiffness leads to compensatory mechanisms by the myocardium including LV hypertrophy and fibroblast proliferation, resulting in decreased cardiac output and increase in fibrotic tissue.3,4
Heart rate modulation is also affected by age with a decrease in both rate variability and maximum heart rate.5
Heart rate is influenced not only by the loss of cells in the sinoatrial node (responsible for controlling heart rate) but also by structural changes in the heart, including fibrosis and hypertrophy, which slow propagation of electric impulse throughout the heart.
Age-dependent changes to cardiovascular tissues
At early ages, the LV diastolic filling rate begins to decline, which is compensated for by increasing arterial contraction to sustain stroke volume and workload, maintaining sufficient ejection fraction.6,7
However, with age, the LV contractility and ejection fraction, as well as sympathetic modulation of heart rate, and response to β-adrenergic receptor activation all decrease.4
A reduction in cardiac output due to decline in function with age stimulates the myocardium to compensate by increasing muscle mass by undergoing cardiac hypertrophy; although this may provide short-term enhancement of cardiac output, the long-term effect of hypertrophy diminishes cardiac function.8
Ventricular hypertrophy is the result of an increase in size of individual cardiomyocytes and can be either physiological, which is reversible (eg, exercise-induced), or pathological which is irreversible (disease-based).9
Hypertrophy in mammalian models such as mouse and rat can be measured with a variety of techniques including echocardiography to measure LV size, histological analysis of heart tissue, or isolation of cardiomyocytes for cell size measurements. The noninvasive nature of echocardiography allows for the unique opportunity to measure alterations in cardiac size and functional parameters longitudinally in an aging study cohort.
Aging of the vasculature results in increased arterial thickening and stiffness as well as dysfunctional endothelium. Clinically, these changes result in increased systolic pressure and present major risk factors for development of atherosclerosis, hypertension and stroke, and arterial fibrillation.4
Vascular dysfunction associated with aging leads to a variety of age-related pathologies, including loss of adequate tissue perfusion (resulting in ischemia), insufficient vascular growth or regression (resulting in hypertension), or excessive growth and remodeling (resulting in age-related macular degeneration). The vasculature undergoes structure and function alterations with age that are well documented, such as luminal enlargement with wall thickening and a decline in endothelial cell function negatively affecting endothelium-dependent dilation and promoting vascular stiffness.10
In addition, endothelial cells lose their ability to proliferate and migrate after tissue injury.11
Furthermore, endothelial barriers become porous and vascular smooth muscle cells migrate into subendothelial spaces and deposit extracellular matrix proteins that result in intimal thickening. At the molecular level, as endothelial cells age, they exhibit a reduction in endothelial nitric oxide synthetase (eNOS) activity, reducing the abundance of NO.12
NO is a critical vasodilator produced by endothelial cells, regulating vascular tone, in addition to inhibiting vascular inflammation, thrombotic events, and aberrant cellular proliferation.13
Loss of NO also promotes endothelial cell senescence.14
Numerous mechanisms can modulate eNOS activity. However, hemodynamic shear stress, the frictional force acting on endothelial cell surface as a result of blood flow, is one of the most potent inducers of eNOS activity.12
As vessels age, they are exposed to less hemodynamic stress due to reduced blood flow caused by decline in heart function; in addition, endothelial cells become less responsive to shear stress, resulting in a decline in the protective NO.15
Therefore, measuring arterial thickness and stiffness in aged models can lead to further understanding of the role various longevity genes influence vasculature aging. Elastic properties of arteries can be directly measured, as well as histological analysis of luminal enlargement, arterial thickness, and deposition of vascular smooth muscle cells. In addition, measuring eNOS activity in vessels, as well as endothelial senescence, can also serve as a marker of vascular aging.
The heart undergoes complex changes during aging that affect the cellular composition, marked by a decrease in absolute number of cardiomyocytes due to increased apoptosis and necrosis and a decrease in repopulation of cardiomyocytes from cardiac stem cell reserves.16,17
With age, cardiomyocytes become more susceptible to stress, including oxidative stress. Therefore the increase in oxidative stress due to the increase in reactive oxygen species (ROS) production with age results in an overall enhancement in the rate of cardiomyocyte death with age. In cases when cardiomyocytes undergo necrosis, the release of cellular components can affect survival of neighboring cardiomyocytes, in addition to promoting the development of proinflammatory and profibrotic environments in the aging heart. Cardiomyocyte senescence, defined by the increased expression of senescence markers and decreased telomere length, also increases with age.18
However, a recent study suggests that removal of senescence cells alone may not be sufficient to improve cardiac defects observed in some aging models.19
Measuring cardiac-specific senescence, DNA damage, as well as levels of apoptosis and necrosis, coupled with fibrosis measurements in animal models of aging, will lead to a better understanding of the link between aging and CVD. Senescence is coupled with expression of such factors as p53, p21, p16, and senescence-associated β-galactosidase activity, which can be measured by immunoblotting or histological techniques. In addition, the levels of damaged nuclear and mitochondrial DNA can be assessed by quantifying levels of 8-oxoguanine, a common DNA lesion associated with oxidative stress, and thus a marker for fundamental damaged DNA pathways in the heart.20
DNA damage and can also be assessed by determining phosphorylation status of γ-H2Ax, a histone variant that is phosphorylated near DNA break sites and thus forms readily detectable foci in cells.21
Fibrosis is measured primarily by histological methods, using staining techniques such as Masson trichrome. These biomarkers of aging can be used in cardiac tissue to assess how modulation of longevity genes influences the rate and degree of cardiovascular aging at the cellular level.
Although long thought to be postmitotic, cardiomyocytes undergo division and regeneration, and recent work has discovered that cardiac regeneration is a vital mechanism of maintaining cardiovascular health. However, the rate of regeneration in the aged may not be adequate to maintain cardiomyocyte numbers in response to cardiomyocyte loss.22
Regenerated cardiac tissue is thought to involve a small pool of cardiac stem cells and a subset of small incompletely differentiated cardiomyocytes that can reenter the cell cycle.23,24
Regenerative capacity can be measured by assessing the proliferative index, using markers of proliferation such as Ki67 or proliferating cell nuclear antigen. Therefore, long-lived or short-lived progeroid animal models can be tested for alterations in their regenerative capacity to determine how regulatory mechanisms controlling regeneration are involved in the aging cardiovascular system.
Other less understood changes in the heart with age are a decrease in the number and function of sinoatrial nodal pacemaker cells and a concomitant increase in conduction abnormalities. The aging heart undergoes a decrease in heart rate variability and maximum heart rate.5
Consistent with this notion, the aging models budding uninhibited by benzimidazoles related 1 (BubR1) and Klotho have both been described to contain abnormalities leading to arrhythmias and altered conduction system, respectively.19,25
Very few aging models have been assessed for alterations in cardiac conduction. Therefore the use of electrophysiology (EP) studies as well as testing aging models for alterations in EP parameters is a future area of research that could lead to a prominent understanding for the increased incidence of cardiovascular disease with age. EP studies can be carried out in cultured cardiomyocytes and in vivo by intracardiac and surface ECG analysis. Like echocardiography, the noninvasive nature of measuring surface ECGs can allow for longitudinal studies in aging cohorts.