An integrated set-up for ex vivo characterisation of biaxial murine artery biomechanics under pulsatile conditions.

Ex vivo characterisation of arterial biomechanics enables detailed discrimination of the various cellular and extracellular contributions to arterial stiffness. However, ex vivo biomechanical studies are commonly performed under quasi-static conditions, whereas dynamic biomechanical behaviour (as relevant in vivo) may differ substantially. Hence, we aim to (1) develop an integrated set-up for quasi-static and dynamic biaxial biomechanical testing, (2) quantify set-up reproducibility, and (3) illustrate the differences in measured arterial stiffness between quasi-static and dynamic conditions. Twenty-two mouse carotid arteries were mounted between glass micropipettes and kept fully vasodilated. While recording pressure, axial force (F), and inner diameter, arteries were exposed to (1) quasi-static pressure inflation from 0 to 200 mmHg; (2) 300 bpm dynamic pressure inflation (peaking at 80/120/160 mmHg); and (3) axial stretch (λz) variation at constant pressures of 10/60/100/140/200 mmHg. Measurements were performed in duplicate. Single-point pulse wave velocities (PWV; Bramwell-Hill) and axial stiffness coefficients (cax = dF/dλz) were calculated at the in vivo value of λz. Within-subject coefficients of variation were ~ 20%. Dynamic PWVs were consistently higher than quasi-static PWVs (p < 0.001); cax increased with increasing pressure. We demonstrated the feasibility of ex vivo biomechanical characterisation of biaxially-loaded murine carotid arteries under pulsatile conditions, and quantified reproducibility allowing for well-powered future study design.

Functional ex-vivo Imaging of Arterial Cellular Recruitment and Lipid Extravasation.

The main purpose of this sophisticated and highly versatile method is to visualize and quantify structural vessel wall properties, cellular recruitment, and lipid/dextran extravasation under physiological conditions in living arteries. This will be of interest for a broad range of researchers within the field of inflammation, hypertension, atherosclerosis, and even the pharmaceutical industry. Currently, many researchers are using in vitro techniques to evaluate cellular recruitment, like transwell or flow chamber systems with cultured cells, with unclear physiological comparability. The here introduced method describes in detail the use of a sophisticated and flexible method to study arterial wall properties and leukocyte recruitment in fresh and viable murine carotid arteries ex vivo under arterial flow conditions. This model mimics the in vivo situation and allows the use of cells and arteries isolated from two different donors (for example, wildtype vs. specific knockouts) to be combined into one experiments, thereby providing information on both leukocyte and/or endothelial cell properties of both donors. As such, this model can be considered an alternative for the complicated and invasive in vivo studies, such as parabiotic experiments.

Wall irregularity rather than intima-media thickness is associated with nearby atherosclerosis.

In addition to intima-media thickness (IMT), IMT inhomogeneity may carry information about atherosclerosis progression. In 147 vascular diseased patients (mean 66 y, 48% male), we determined the carotid bulb stenosis degree based on local Doppler blood flow velocities. Common carotid artery (CCA) morphologic characteristics, i.e. IMT, IMT-inhomogeneity (intraregistration variation) and IMT uni- and bilateral intrasubject variation (DeltaIMT), were measured using multiple M-mode. Associations of morphologic characteristics, stenosis degree and Framingham score were evaluated with Pearson correlation (r) and multiple regression analysis. The IMT distributions for subjects without and with stenosis were not similar. The stenosis degree score correlated significantly to unilateral (r=0.68) and bilateral DeltaIMT (r=0.62), IMT (r=0.41) and IMT-inhomogeneity (r=0.45). The averaged IMT and IMT-inhomogeneity increased slightly for singular stenosis and abruptly for multiple stenoses. Mean uni- and bilateral DeltaIMT per stenosis degree increased linearly with this degree, reaching a correlation close to 1 (r=0.98 and r=0.97). Interestingly, the majority of the subjects with a moderate to severe bulb stenosis exhibited a carotid IMT lower than the considered critical threshold of 0.9 mm. In conclusion, although CCA is not prone to plaques, its morphologic characteristics are positively correlated with stenosis degree score and other risk scores. DeltaIMT can be more reliable derived from inter-registration rather than from intra-registration variation. In the CCA, DeltaIMT substantiates vascular alteration better than IMT.

Circulating calcification inhibitors and vascular properties in children after renal transplantation.

Pediatric transplant patients are known to have vascular abnormalities. The calcification inhibitors matrix Gla protein (MGP) and fetuin-A play an important role in the pathophysiology of vascular calcification. In the cross-sectional study reported here, we examined the circulating levels of fetuin-A and MGP in children after renal transplantation compared to healthy children and the association of these factors with vascular properties of the carotid artery. Levels of MGP and fetuin-A together with vascular properties of the carotid artery were determined in 29 pediatric renal transplant recipients and 54 healthy controls. The level of fetuin-A was decreased in the transplant group relative to the control group (P=0.005), whereas the level of MGP (both non-phosphorylated MGP and non-carboxylated MGP) did not differ between groups. The intima-media thickness (P<0.001) and the elasticity (P=0.002) of the carotid artery were significantly increased in children after renal transplantation compared to healthy children. No associations between vascular parameters and calcification inhibitors were found in either group. Circulating levels of MGP and fetuin-A could not be identified as independent predictors of vascular stiffness or other carotid artery parameters in pediatric renal transplant recipients. Future prospective studies in pediatric ESRD and transplant patients are needed to learn more about the role of calcification inhibitors in relation to the prevention of vascular damage.

The effect of flow changes on the arterial system proximal to an arteriovenous fistula for hemodialysis.

Arterial remodeling in response to flow changes is controlled by the endothelium, sensing wall shear stress (SS) changes. The present study focuses on the remodeling capacities of the brachial (BA) and radial artery (RA) of 16 renal failure patients after arteriovenous fistula creation. Pre- and postoperatively at predetermined time-points, diameter, wall thickness and peak and mean SS were assessed. After arteriovenous fistula creation, acute increases in BA SS (p = 0.018) and lumen diameter (p = 0.028) were observed. The diameter further increased in the next year (p = 0.023), whereas BA SS remained unchanged. RA SS and diameter increased acutely (p = 0.005) and remained unaltered after 1 y. RA wall thickness tended to decrease acutely (p = 0.059) and increased steadily during 1 y (p = 0.008). BA and RA diameter acutely increased after an acute SS rise and remained augmented after 1 y. Also, the RA vessel wall thickness enlarged, indicating structural remodeling. After 1 y, however, these changes did not result in SS restoration.

Shear stress depends on vascular territory: comparison between common carotid and brachial artery.

Shear stress (SS) is thought to be constant throughout the vascular system. Evidence for this supposition is scarce, however. To verify this hypothesis in vivo, we assessed common carotid (CCA) and brachial artery (BA) peak and mean wall shear rate (SR) noninvasively in 10 healthy volunteers (23.7 +/- 3.4 yr) with an ultrasound SR estimation system. SS was estimated from SR and calculated whole blood viscosity. SR was higher (P < 0.05) in the CCA (mean: 359 +/- 111 s(-1); peak: 1,047 +/- 345 s(-1)) than in the BA (mean: 95 +/- 24 s(-1); peak: 770 +/- 170 s(-1)). Whole blood viscosity was higher in the BA than in the CCA (5.1 +/- 0.7 vs. 3.3 +/- 0.6 mPa. s; P < 0.001). Peak SS did not differ between the CCA and the BA, whereas mean SS was significantly higher in the CCA (1.15 +/- 0.21 Pa) than in the BA (0.48 +/- 0.15 Pa; P < 0.001). These results demonstrate that BA SS strongly deviates from CCA SS in vivo.

Brachial artery shear stress is independent of gender or age and does not modify vessel wall mechanical properties.

The objective of the present study was to obtain brachial artery (BA) baseline shear stress (SS) values in healthy volunteers and to relate this to gender and age. Peak and mean wall shear rate (SR) were noninvasively measured in 30 healthy subjects using an SR estimation system. Arterial diameter and wall characteristics were obtained with a wall tracking system. SS was estimated from SR and calculated whole blood viscosity. Intrasubject variability and the effects of age and gender were assessed. Intrasubject variability of BA peak and mean SR were 16.2% and 28.6%. Baseline peak ( approximately 3.0 +/- 0.7 Pa) and mean SS ( approximately 0.5 +/- 0.2 Pa) were not gender-dependent, nor were they influenced by age. No vessel wall parameter related to BA SS. No differences in BA SS were observed between the genders and no influence of age was apparent. Our results indicate that the BA adequately responds to chronic changes in blood flow.