Biomechanical Properties of Mouse Carotid Arteries With Diet-Induced Metabolic Syndrome and Aging

Renal denervation restores biomechanics of carotid arteries in a rat model of hypertension

The prevalence of hypertension increases with aging and is associated with increased arterial stiffness. Resistant hypertension is presented when drug treatments fail to regulate a sustained increased blood pressure. Given that the mechanisms between the sympathetic nervous system and the kidney play an important role in blood regulation, renal denervation (RDN) has emerged as a therapeutic potential in resistant hypertension. In this study, we investigated the effects of RDN on the biomechanical response and microstructure of elastic arteries. Common carotid arteries (CCA) excised from 3-month, 8-month, and 8-month denervated rats were subjected to biaxial extension-inflation test. Our results showed that hypertension developed in the 8-month-old rats. The sustained elevated blood pressure resulted in arterial remodeling which was manifested as a significant stress increase in both axial and circumferential directions after 8 months. RDN had a favorable impact on CCAs with a restoration of stresses in values similar to control arteries at 3 months. After biomechanical testing, arteries were imaged under a multi-photon microscope to identify microstructural changes in extracellular matrix (ECM). Quantification of multi-photon images showed no significant alterations of the main ECM components, elastic and collagen fibers, indicating that arteries remained intact after RDN. Regardless of the experimental group, our microstructural analysis of the multi-photon images revealed that reorientation of the collagen fibers might be the main microstructural mechanism taking place during pressurization with their straightening happening during axial stretching.

Compromised homeostasis in aged carotid arteries revealed by microstructural studies of elastic lamellae

Healthy arteries are continuously subjected to diverse mechanical stimuli and adapt in order to maintain a mechanical homeostasis which is characterized by a uniform distribution of wall stresses. However, aging may compromise the homeostatic microenvironment within arteries. Structural heterogeneity has been suggested as a potential microstructural mechanism that could lead to homogeneous stress distribution across the arterial wall. Our previous study on the unfolding and stretching of the elastic lamellae revealed the underlying microstructural mechanism for equalizing the circumferential stresses through wall; inner elastic layers are wavier and unfold more than the outer layers which helps to evenly distribute lamellar stretching (Yu et al., 2018). In this study, we investigated the effect of aging on lamellar deformation and its implications for tissue homeostasis. Common carotid arteries from aged mice were imaged under a multi-photon microscope while subjected to biaxial extension and inflation at five different pressures ranging from 0 up to 120 mmHg. Lamellar unfolding during pressurization was then determined from the reconstructed cross-sectional images of elastic lamellae. Tissue-level circumferential stretch was combined with the lamellar unfolding to calculate lamellar stretching. Our results revealed that the straightness gradient of aged elastic lamellae is similar to the young ones. However, during pressurization, the inner elastic lamella of the aged mice unfolded significantly more than the inner layer in young arteries. An important finding of our study is the uneven increase in inter-lamellar space which contributed to a nonuniform stretching of the elastic lamellae of aged mice arteries, elevated stress gradient, and a shifting of the load-bearing component to adventitia. Our results shed light into the complex microstructural mechanisms that take place in aging and adversely affect arterial mechanical behavior and homeostasis.

Renal denervation restores biomechanics of carotid arteries in a rat model of hypertension

The prevalence of hypertension increases with aging and is associated with increased arterial stiffness. Resistant hypertension is presented when drug treatments fail to regulate a sustained increased blood pressure. Given that the mechanisms between the sympathetic nervous system and the kidney play an important role in blood regulation, renal denervation (RDN) has emerged as a therapeutic potential in resistant hypertension. In this study, we investigated the effects of RDN on the biomechanical response and microstructure of elastic arteries. Common carotid arteries (CCA) were excised from 3-, 8- and 8-month-old denervated rats, and subjected to biaxial extension-inflation test. Our results showed that hypertension developed in the 8-month-old rats. The sustained elevated blood pressure resulted in arterial remodeling which was manifested as a significant stress increase in both axial and circumferential directions after 8 months. RDN had a favorable impact on CCAs with a restoration of stresses in values similar to control arteries at 3 months. After biomechanical testing, arteries were imaged under a multi-photon microscope to identify microstructural changes in extracellular matrix (ECM). Quantification of multi-photon images showed no significant alterations of the main ECM components, elastic and collagen fibers, indicating that arteries remained intact after RDN. Regardless of the experimental group, our microstructural analysis of the multi-photon images revealed that reorientation of the collagen fibers might be the main microstructural mechanism taking place during pressurization with their straightening happening during axial stretching.

Effect of Aging, Sex, and Gene (Fbln5) on Arterial Stiffness of Mice: 20 Weeks Adult Fbln5-knockout Mice Have Older Arteries than 100 Weeks Wild-Type Mice

The arterial stiffening is a strong independent predictor of cardiovascular risk and has been used to characterize the biological age of arteries ('arterial age'). Here we revealed that the Fbln5 gene knockout (Fbln5 ^-/- ) significantly increases the arterial stiffening for both male and female mice. We also showed that the arterial stiffening increases with natural aging, but the stiffening effect of Fbln5 ^-/- is much more severe than aging. The arterial stiffening of 20 weeks old mice with Fbln5 ^-/- is much higher than that at 100 weeks in wild-type (Fbln5 ^+/+ ) mice, which indicates that 20 weeks mice (equivalent to ∼26 years old humans) with Fbln5 ^-/- have older arteries than 100 weeks wild-type mice (equivalent to ∼77 years humans). Histological microstructure changes of elastic fibers in the arterial tissue elucidate the underlying mechanism of the increase of arterial stiffening due to Fbln5-knockout and aging. These findings provide new insights to reverse 'arterial age' due to abnormal mutations of Fbln5 gene and natural aging. This work is based on a total of 128 biaxial testing samples of mouse arteries and our recently developed unified-fiber-distribution (UFD) model. The UFD model considers the fibers in the arterial tissue as a unified distribution, which is more physically consistent with the real fiber distribution of arterial tissues than the popular fiber-family-based models (e.g., the well-know Gasser-Ogden-Holzapfel [GOH] model) that separate the fiber distribution into several fiber families. Thus, the UFD model achieves better accuracies with less material parameters. To our best knowledge, the UFD model is the only existing accurate model that could capture the property/stiffness differences between different groups of the experimental data discussed here.

Increased pulse wave velocity is related to impaired working memory and executive function in older adults with metabolic syndrome

Age-related vascular alterations promote the pathogenesis of vascular cognitive impairment (VCI). Cardiovascular risk factors that accelerate vascular aging exacerbate VCI. Metabolic syndrome (MetS) constitutes a cluster of critical cardiovascular risk factors (abdominal obesity, hypertension, elevated triglycerides, elevated fasting glucose, reduced HDL cholesterol), which affects nearly 37% of the adult US population. The present study was designed to test the hypotheses that MetS exacerbates cognitive impairment and that arterial stiffening moderates the association between cognitive dysfunction and MetS in older adults. MetS was defined by the NCEP ATP III guidelines. Cognitive function (digit span and trail-making tests) and brachial-ankle pulse wave velocity (baPWV; a non-invasive clinical measurement of arterial stiffness) were assessed in older adults with MetS and age- and sex-matched controls. Multiple linear regression models were applied to test for the main effects of MetS, baPWV, and their interaction on cognitive performance. Fifty-three participants with MetS (age: 68 ± 8 years) and 39 age-matched individuals without MetS (age: 66 ± 9 years) were enrolled into the study. In adjusted multivariable regression analyses of the digit span backward length score, both MetS (ß = 1.97, p = 0.048) and MetS by baPWV interaction (ß =  - 0.001, p = 0.026) were significant predictors. In participants with MetS, higher baPWV was associated with poorer performance on digit span backward length score, a test of working memory (R =  - 0.44, p = 0.0012), but there was no association in those without MetS (R = 0.035, p = 0.83). MetS was negatively associated with performance on the digit span backward length score, baPWV was negatively associated with multiple neuropsychological outcomes, and baPWV moderated the association between digit span backward length score and MetS, as individuals with both MetS and higher baPWV had the most impaired cognitive function. Our findings add to the growing body of evidence that individuals with MetS and higher baPWV may be prone to VCI.

Sex Differences in Vascular Aging and Impact of GPER Deletion

Aging is a nonmodifiable risk factor for cardiovascular disease associated with arterial stiffening and endothelial dysfunction. We hypothesized that sex differences exist in vascular aging processes and would be attenuated by global deletion of the G protein-coupled estrogen receptor. Blood pressure was measured by tail cuff plethysmography, pulse wave velocity (PWV) and echocardiography were assessed with high resolution ultrasound, and small vessel reactivity was measured using wire myography in adult (25 weeks) and middle-aged (57 weeks) male and female mice. Adult female mice displayed lower blood pressure and PWV, but this sex difference was absent in middle-aged mice. Aging significantly increased PWV but not blood pressure in both sexes. Adult female carotids were more distensible than males, but this sex difference was lost during aging. Acetylcholine-induced relaxation was greater in female than male mice at both ages, and only males showed aging-induced changes in cardiac hypertrophy and function. GPER deletion removed the sex difference in PWV as well as ex vivo stiffness in adult mice. The sex difference in blood pressure was absent in KO mice and was associated with endothelial dysfunction in females. These findings indicate that the impact of aging on arterial stiffening and endothelial function is not the same in male and female mice. Moreover, nongenomic estrogen signaling through GPER impacted vascular phenotype differently in male and female mice. Delineating sex differences in vascular changes during healthy aging is an important first step in improving early detection and sex-specific treatments in our aging population.

Harmonic Distortion of Blood Pressure Waveform as a Measure of Arterial Stiffness

Aging and disease alter the composition and elastic properties of the aortic wall resulting in shape changes in blood pressure waveform (BPW). Here, we propose a new index, harmonic distortion (HD), to characterize BPW and its relationship with other in vitro and in vivo measures. Using a Fourier transform of the BPW, HD is calculated as the ratio of energy above the fundamental frequency to that at the fundamental frequency. Male mice fed either a normal diet (ND) or a high fat, high sucrose (HFHS) diet for 2–10 months were used to study BPWs in diet-induced metabolic syndrome. BPWs were recorded for 20 s hourly for 24 h, using radiotelemetry. Pulse wave velocity (PWV), an in vivo measure of arterial stiffness, was measured in the abdominal aorta via ultrasound sonography. Common carotid arteries were excised from a subset of mice to determine the tangent modulus using biaxial tension-inflation test. Over a 24-h period, both HD and systolic blood pressure (SBP) show a large variability, however HD linearly decreases with increasing SBP. HD is also linearly related to tangent modulus and PWV with slopes significantly different between the two diet groups. Overall, our study suggests that HD is sensitive to changes in blood pressure and arterial stiffness and has a potential to be used as a noninvasive measure of arterial stiffness in aging and disease.