Characteristics of the response of the iliac artery to wall shear stress in the anaesthetized pig

Guiding the prostatic artery embolization procedure with computational uid dynamics

Benign prostatic hyperplasia (BPH) is a common disease associated with lower urinary tract symptoms and the most frequent benign tumor in men. To reduce BPH therapy complications, prostatic artery embolization (PAE) was developed to replace the surgical options. PAE is a minimally invasive technique in which emboli are injected into the prostate arteries (PA), obstructing the blood flow in the hypervascular nodules. In this work, a personalized PAE treatment strategy was proposed using patient-specific computational fluid dynamics (CFD). First, the hemodynamics environment in the iliac arterial tree considering a large network of bifurcations was studied. The results showed complex blood flow patterns in the iliac arterial network. Subsequently, the transport of embolic particulates during PAE for the standard horizontal and a hypothetical vertical patient positioning was simulated using Lagrangian particle tracking. Emboli with different sizes were released at various locations across the iliac arterial tree. The emboli entering the PA were mapped back to their initial location to create emboli release maps (ERMs). The obtained ERMs during the standard patient positioning for smaller emboli at certain release locations showed distinct regions in which if the emboli were released within these regions, all of them would reach the PA without non-target embolization. During the hypothetical vertical patient positioning, the larger emboli formed a larger coherent region in the ERMs. Our patient-specific model can be used to find the best spatial location for emboli injection and perform the embolization procedure with minimal off-target delivery.

The Development of an ex vivo Flow System to Assess Acute Arterial Drug Retention of Cardiovascular Intravascular Devices

Purpose: The goal of this study was to develop an ex vivo system capable of rapidly evaluating arterial drug levels in living, isolated porcine carotid arteries.

Methods: A vascular bioreactor system was developed that housed a native porcine carotid artery under physiological flow conditions. The ex vivo bioreactor system was designed to quantify the acute drug transfer of catheter-based drug delivery devices into explanted carotid arteries. To evaluate our ex vivo system, a paclitaxel-coated balloon and a perfusion catheter device delivering liquid paclitaxel were utilized. At 1-h post-drug delivery, arteries were removed, and paclitaxel drug levels measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Parallel experiments were performed in a pig model to validate ex vivo measurements.

Results: LC-MS/MS analysis demonstrated arterial paclitaxel levels of the drug-coated balloon-treated arteries to be 48.49 ± 24.09 ng/mg and the perfusion catheter-treated arteries to be 25.42 ± 9.74 ng/mg at 1 h in the ex vivo system. Similar results were measured in vivo, as arterial paclitaxel concentrations were measured at 59.23 ± 41.27 ng/mg for the drug-coated balloon-treated arteries and 23.43 ± 20.23 ng/mg for the perfusion catheter-treated arteries. Overall, no significant differences were observed between paclitaxel measurements of arteries treated ex vivo vs. in vivo.

Conclusion: This system represents the first validated ex vivo pulsatile system to determine pharmacokinetics in a native blood vessel. This work provides proof-of-concept of a quick, inexpensive, preclinical tool to study acute drug tissue concentration kinetics of drug-releasing interventional vascular devices.

Endothelial permeability, LDL deposition, and cardiovascular risk factors-a review

Early atherosclerosis features functional and structural changes in the endothelial barrier function that affect the traffic of molecules and solutes between the vessel lumen and the vascular wall. Such changes are mechanistically related to the development of atherosclerosis. Proatherogenic stimuli and cardiovascular risk factors, such as dyslipidaemias, diabetes, obesity, and smoking, all increase endothelial permeability sharing a common signalling denominator: an imbalance in the production/disposal of reactive oxygen species (ROS), broadly termed oxidative stress. Mostly as a consequence of the activation of enzymatic systems leading to ROS overproduction, proatherogenic factors lead to a pro-inflammatory status that translates in changes in gene expression and functional rearrangements, including changes in the transendothelial transport of molecules, leading to the deposition of low-density lipoproteins (LDL) and the subsequent infiltration of circulating leucocytes in the intima. In this review, we focus on such early changes in atherogenesis and on the concept that proatherogenic stimuli and risk factors for cardiovascular disease, by altering the endothelial barrier properties, co-ordinately trigger the accumulation of LDL in the intima and ultimately plaque formation.

Ramp and step increases in shear stress result in a similar magnitude of brachial artery flow-mediated dilation

PURPOSE

There is evidence that the endothelium is responsive to both the rate and magnitude of increases in shear stress. However, whether flow-mediated dilation stimulated by sustained increases in shear stress (SS-FMD) is rate sensitive in humans is unknown. The purpose of this investigation was to test whether ramp (gradual) and step (instantaneous) increases in shear stress elicit disparate SS-FMD.

METHODS

Young, healthy men (n = 18, age = 22 ± 2 years, body mass index = 25 ± 3 kg m^-2) performed two 11-min bouts of rhythmic handgrip exercise; one with a 5.5-min ramp-increase in shear stress and one with an immediate step increase in shear stress. Ramp increases in shear stress were achieved through incremental increases in handgrip exercise intensity [increases of 4% maximum voluntary contraction (MVC) every 30 s for 5.5 min, ending at 44% MVC] and step increases in shear stress were achieved through a combination of arterial compression and commencing handgrip exercise at 44% MVC.

RESULTS

Shear rate was greater in the step versus ramp protocol in minutes 1-6, but not different thereafter. Similarly, SS-FMD was greater in the step versus ramp protocol during minutes 2-6, but similar in minutes 7-11 (minute 11: ramp 8.7 ± 4.6%; step 9.4 ± 3.6%; P = 0.343). SS-FMD continued to increase over time with maintenance of a steady shear stress stimulus (step minutes 2-11: 0.51 ± 0.36% min^-1; ramp minutes 7-11: 0.64 ± 0.57% min^-1; P = 0.259).

CONCLUSIONS

These findings indicate that in the brachial artery of humans, the magnitude of SS-FMD is determined by the magnitude and duration, but not the rate, of increases in shear stress.

Oral Administration of Glucosamine Improves Vascular Endothelial Function by Modulating Intracellular Redox State

Glucosamine, used to treat osteoarthritis, has been shown to have anti-inflammatory and anti-atherosclerotic effects in experimental studies. A recent cohort study has demonstrated that the use of glucosamine was significantly associated with decreased total mortality. Vascular endothelial function is a potent surrogate marker of atherosclerosis and cardiovascular mortality where oxidative stress could participate. Therefore, we investigated whether glucosamine improves vascular endothelial function and intracellular redox state. We examined the effects of oral glucosamine administration (3000 mg/day) for 4 weeks on flow-mediated vasodilation (FMD) and intraerythrocyte glutathione parameters in 20 volunteers. Nineteen age-matched volunteers served as controls. Glucosamine administration significantly increased FMD (from 7.0 ± 2.3 to 8.7 ± 2.3%, P = 0.022). In the control group, FMD did not change. Glucosamine administration significantly increased intraerythrocyte total glutathione levels (from 212.9 ± 46.2 to 240.6 ± 49.4 μmol/L, P = 0.006), intraerythrocyte reduced form of glutathione (GSH) levels (from 124.7 ± 42.6 to 155.2 ± 47.7 μmol/L; P = 0.004) and intraerythrocyte GSH/oxidized form of glutathione (GSSG) ratios (from 3.18 ± 1.64 to 3.88 ± 1.61, P = 0.04). In the control group, any glutathione parameters did not change. Moreover, a stepwise multivariate analysis revealed percent change of GSH/GSSG is the only independent predictor for those of FMD (standardized β = 0.58, P = 0.007) in the glucosamine group. Glucosamine administration improved FMD in association with amelioration of intraerythrocyte GSH/GSSG ratios. These results suggest that oral glucosamine administration might improve vascular endothelial function by modulating intracellular redox state.

Responses of iliac conduit artery and hindlimb resistance vessels to luminal hyperfructosemia in the anaesthetized pig

AIMS

High fructose levels are found in diabetes mellitus, associated with high corn syrup diets, and have been claimed to cause hypertension. As the direct effects on conduit and resistance arteries have not been previously reported, we measured these in vivo in the anaesthetized pig with instrumented iliac arteries.

METHODS

Experiments were performed on the iliac artery preparation in the anaesthetized pig: blood flow, diameter and pressure were measured in the iliac.

RESULTS

The change in diameter of an occluded iliac artery segment filled with hyperfructosemic (15 μm) blood was 89.5 ± 22.1 μm (mean ± SE), contrasted with 7.7 ± 13.06 μm control (P = 0.005, paired t-test, n = 6). There was no significant difference when compared with blood containing both hyperfructosemic blood and the nitric oxide synthesis inhibitor, N(G)-nitro-l-arginine methyl ester (250 μg mL(-1)). Step changes in pressure and flow were achieved by progressive arterial stenosis during control saline and 15 μm min(-1) fructose downstream intra-arterial infusions. Linear regression of the step changes in blood pressure versus the instantaneous step changes in blood flow showed a statistically significant decrease in slope of the conductance (P < 0.001, analysis of covariance), indicating an increase in instantaneous peripheral vascular resistance. Peripheral autoregulation and conduit artery shear-stress-mediated dilatation were not significantly altered.

CONCLUSION

An elevated level of fructose caused dilatation of a conduit artery but constriction of resistance vessels. The latter effect could account, if maintained long-term, for the hypertension claimed to be due to hyperfuctosemia.

The impact of baseline artery diameter on flow-mediated vasodilation: a comparison of brachial and radial artery responses to matched levels of shear stress

An inverse relationship between baseline artery diameter (BAD) and flow-mediated vasodilation (FMD) has been identified using reactive hyperemia (RH) to create a shear stress (SS) stimulus in human conduit arteries. However, RH creates a SS stimulus that is inversely related to BAD. The purpose of this study was to compare FMD in response to matched levels of SS in two differently sized upper limb arteries [brachial (BA) and radial (RA) artery]. With the use of exercise, three distinct, shear rate (SR) stimuli were created (SR = blood velocity/vessel diameter; estimate of SS) in the RA and BA. Artery diameter and mean blood velocity were assessed with echo and Doppler ultrasound in 15 healthy male subjects (19–25 yr). Data are means ± SE. Subjects performed 6 min of adductor pollicis and handgrip exercise to increase SR in the RA and BA, respectively. Exercise intensity was modulated to achieve uniformity in SR between arteries. The three distinct SR levels were as follows: steady-state exercise 39.8 ± 0.6, 57.3 ± 0.7, and 72.4 ± 1.2 s−1 ( P < 0.001). %FMD and AbsFMD (mm) at the end of exercise were greater in the RA vs. the BA at each shear level [at the highest level: RA = 15.7 ± 1.5%, BA = 5.4 ± 0.8% ( P < 0.001)]. The mean slope of the within-subject SR-%FMD regression line was greater in the RA (RA = 0.33 ± 0.04, BA = 0.13 ± 0.02, P < 0.001), and a strong within-subjects relationship between %FMD and SR was observed in both arteries (RA: r2 = 0.92 ± 0.02; BA: r2 = 0.90 ± 0.03). Within the RA, there was a significant relationship between baseline diameter and %FMD; however, this relationship was not present in the BA (RA: r2 = 0.76, P < 0.001; BA: r2 = 0.03, P = 0.541). These findings suggest that the response to SS is not uniform across differently sized vessels, which is in agreement with previous studies.

The effect of arterial wall shear stress on the incremental elasticity of a conduit artery

AIMS

The purpose of this investigation was to determine the effects of flow mediated dilatation on arterial incremental elasticity (E(inc) ).

METHODS

  In four female anaesthetized pigs, the iliac artery and vein were connected by a shunt with a variable resistance which allowed blood flow and therefore shear stress to be regulated. E(inc) was calculated from simultaneous records of diameter and pressure throughout a minimum of four cardiac cycles.

RESULTS

Passive increases in diameter (∼1-2%) throughout a cardiac cycle, brought about by pressure, resulted in a two- to threefold increase in E(inc) . In contrast, increases in shear stress caused active smooth muscle relaxation and a significant increase in diameter from 3.663 ± 0.215 mm to 4.488 ± 0.163 mm (mean ± SEM, P < 0.05) equivalent to a fractional increase in diameter (fD) of 1.5 with no significant change in mean arterial pressure, 108 ± 2 mmHg to 106 ± 1 mmHg (mean ± SEM). The average value of E(inc) per cardiac cycle at baseline was 2.17 ± 0.10 × 10(3) kPa and remained relatively constant until fD exceeded 1.3 thereafter increasing to a maximum of 9.23 ± 1.0 × 10(3) kPa.

CONCLUSION

These results show that in a conduit artery during the dilatory response to shear stress, the interaction between smooth muscle and collagen operates so as to maintain E(inc) relatively constant over much of the working range of dilatation. This is consistent with a model of the arterial wall in which collagen is recruited both by passive stretch, in response to an increase in pressure and therefore wall stress, and also by active contraction of smooth muscle.

Review: Hyperglycaemia and the vascular glycocalyx: the key to microalbuminuria and cardiovascular disease in diabetes mellitus?

The vascular glycocalyx is a gel layer between endothelium and the blood, 0.5 µm thick. Evidence is presented from published studies to indicate that hyperglycaemia causes damage to the vascular glycocalyx. This damage results in microalbuminuria, excess fluid transfer to the interstitium, reduction in nitric oxide (NO) production by arterial endothelium, and leukocyte and platelet adhesion to endothelium leading to atherothrombosis. The lack of NO production proceeds from the fact that glycocalyx is the mechanotransducer transmitting the signal for increased shear stress between blood and arterial wall, and this function is inhibited by hyperglycaemia. When hyperinsulinaemia is also present, the problem is compounded by general arterial dilatation leading to low shear rates throughout the arterial tree. These findings explain the predisposition to atherothrombosis in the pre-diabetic condition of insulin resistance/metabolic syndrome/obesity and diabetes mellitus. It is proposed that greater efforts than ever are required to detect occult insulin resistance, to treat such patients and diabetics with ever more strict blood glucose control while minimising insulin levels, and to carry out further research into how glycocalyx structure and function can be preserved.

eNOS activation by physical forces: from short-term regulation of contraction to chronic remodeling of cardiovascular tissues

Nitric oxide production in response to flow-dependent shear forces applied on the surface of endothelial cells is a fundamental mechanism of regulation of vascular tone, peripheral resistance, and tissue perfusion. This implicates the concerted action of multiple upstream "mechanosensing" molecules reversibly assembled in signalosomes recruiting endothelial nitric oxide synthase (eNOS) in specific subcellular locales, e.g., plasmalemmal caveolae. Subsequent short- and long-term increases in activity and expression of eNOS translate this mechanical stimulus into enhanced NO production and bioactivity through a complex transcriptional and posttranslational regulation of the enzyme, including by shear-stress responsive transcription factors, oxidant stress-dependent regulation of transcript stability, eNOS regulatory phosphorylations, and protein-protein interactions. Notably, eNOS expressed in cardiac myocytes is amenable to a similar regulation in response to stretching of cardiac muscle cells and in part mediates the length-dependent increase in cardiac contraction force. In addition to short-term regulation of contractile tone, eNOS mediates key aspects of cardiac and vascular remodeling, e.g., by orchestrating the mobilization, recruitment, migration, and differentiation of cardiac and vascular progenitor cells, in part by regulating the stabilization and transcriptional activity of hypoxia inducible factor in normoxia and hypoxia. The continuum of the influence of eNOS in cardiovascular biology explains its growing implication in mechanosensitive aspects of integrated physiology, such as the control of blood pressure variability or the modulation of cardiac remodeling in situations of hemodynamic overload.

Dilatation in the femoral vascular bed does not cause retrograde relaxation of the iliac artery in the anaesthetized pig

AIM

We tested the hypothesis that dilatation of a feeding artery may be elicited by transmission of a signal through the tissue of the arterial wall from a vasodilated peripheral vascular bed.

METHODS

In eight pentobarbital anaesthetized pigs, acetylcholine (ACh, an endothelium-dependent vasodilator) was injected intra-arterially above (upstream) and below (downstream) a test segment of the left iliac artery, the diameter of which was measured continuously by sonomicrometry.

RESULTS

Under control conditions, ACh injections upstream and downstream of the test segment caused dilatation. Downstream injection dilated the peripheral arterioles, resulting in increased blood flow and proximal dilatation. This is a shear stress, nitric oxide (NO)-dependent response. The experiment was then repeated after applying a stenosis to prevent the increased flow caused by downstream injection of ACh; the stenosis was placed either above the site of diameter measurement to allow retrograde conduction, or below that site to prevent distally injected ACh reaching the measurement site. Under these conditions, downstream injection of ACh had a minimal effect on the shear stress of the test segment with no increase in test segment diameter. This was not due to endothelial damage or dysfunction as injection of ACh upstream still caused a large increase in test segment diameter.

CONCLUSIONS

Our results indicate that dilatation of the feeding artery of a vasodilated bed is caused by increased shear stress within the feeding artery and not via a signal transmitted through the arterial wall from below.

Hypothesis: arterial glycocalyx dysfunction is the first step in the atherothrombotic process

We present evidence that the 0.5 microm thick gel layer, lining the inner wall of healthy blood vessels, the glycocalyx, is the first line of defence against atherothrombotic disease. All blood vessel linings are coated with this gel, a highly negatively charged structure, rich in anionic sites mostly represented by the sialic acid moieties of glycoproteins and the sulphate and carboxyl groups of heparan-sulphate proteoglycans. Blood flow in arteries is associated with a shear stress at the glycocalyx, which signals the underlying endothelial cells to release nitric oxide (NO), an anti-atherogenic factor. Sites of low shear stress in the arterial tree are more susceptible to atheroma due to lack of NO generation through this mechanism, whereas exercise, by increasing blood flow and shear stress, is protective. We postulate that risk factors for atherothrombosis act by impairing glycocalyx function. That luminal hyperglycaemia causes glycocalyx dysfunction has already been shown; we postulate this to be the first step in the atherothrombotic process in patients with diabetes mellitus and metabolic syndrome (insulin resistance). There is also evidence of glycocalyx defects from exposure to oxidized low-density lipoprotein. We postulate that other risk factors will have a similar action on the glycocalyx as the initiating factor in the disease process, e.g. smoking, hyperlipidaemias and hyperhomocystenaemia. These predictions can now be tested in a large animal model of shear-stress-mediated arterial dilatation.

Brachial artery flow-mediated dilation during handgrip exercise: evidence for endothelial transduction of the mean shear stimulus

Exercise elevates shear stress in the supplying conduit artery. Although this is the most relevant physiological stimulus for flow-mediated dilation (FMD), the fluctuating pattern of shear that occurs may influence the shear stress-FMD stimulus response relationship. This study tested the hypothesis that the brachial artery FMD response to a step increase in shear is influenced by the fluctuating characteristics of the stimulus, as evoked by forearm exercise. In 16 healthy subjects, we examined FMD responses to step increases in shear rate in three conditions: stable shear upstream of heat-induced forearm vasodilation (FHStable); fluctuating shear upstream of heat-induced forearm vasodilation and rhythmic forearm cuff inflation/deflation (FHFluctuating); and fluctuating shear upstream of exercise-induced forearm vasodilation (FEStep Increase). The mean increase in shear rate (±SD) was the same in all trials (FHFluctuating: 51.69 ± 15.70 s−1; FHStable: 52.16 ± 14.10 s−1; FEStep Increase: 50.14 ± 13.03 s−1 P = 0.131). However, the FHFluctuating and FEStep Increase trials resulted in a fluctuating shear stress stimulus with rhythmic high and low shear periods that were 96.18 ± 24.54 and 11.80 ± 7.30 s−1, respectively. The initial phase of FMD (phase I) was followed by a second, delayed-onset FMD and was not different between conditions (phase I: FHFluctuating: 5.63 ± 2.15%; FHStable: 5.33 ± 1.85%; FEStep Increase: 5.30 ± 2.03%; end-trial: FHFluctuating: 7.76 ± 3.40%; FHStable: 7.00 ± 3.03%; FEStep Increase: 6.68 ± 3.04%; P = 0.196). Phase I speed also did not differ ( P = 0.685). In conclusion, the endothelium transduced the mean shear when exposed to shear fluctuations created by a typical handgrip protocol. Muscle activation did not alter the FMD response. Forearm exercise may provide a viable technique to investigate brachial artery FMD in humans.