Nitric oxide is significantly reduced in ex vivo porcine arteries during reverse flow because of increased superoxide production

Elevated oxidative stress and endothelial dysfunction in right coronary artery of right ventricular hypertrophy

Remodeling of right coronary artery (RCA) occurs during right ventricular hypertrophy (RVH) induced by banding of the pulmonary artery (PA). The effect of RVH on RCA endothelial function and reactive oxygen species (ROS) in vessel wall remains unclear. A swine RVH model (n = 12 pigs) induced by PA banding was used to study RCA endothelial function and ROS level. To obtain longitudinal coronary hemodynamic and geometric data, digital subtraction angiography was used during the progression of RVH. Blood flow in the RCA increased by 82% and lumen diameter of RCA increased by 22% over a 4-wk period of RVH. The increase in blood flow and the commensurate increase in diameter resulted in a constant wall shear stress in RCA throughout the RVH period. ROS was elevated by ∼100% in RCA after 4 wk of PA banding. The expressions of p47(phox), NADPH oxidase (NOX1, NOX2, and NOX4) were upregulated in the range of 20-300% in RCA of RVH. The endothelial function was compromised in RCA of RVH as attributed to insufficient endothelial nitric oxide synthase cofactor tetrahydrobiopterin. In vivo angiographic analysis suggests an increased basal tone in the RCA during RVH. In conclusion, stretch due to outward remodeling of RCA during RVH (at constant wall shear stress), similar to vessel stretch in hypertension, appears to induce ROS elevation, endothelial dysfunction, and an increase in basal tone.

Effect of blood pressure on vascular hemodynamics in acute tachycardia

Paroxysmal supraventricular tachycardia is accompanied by hypotension, which can affect vascular hemodynamics. Here, we hypothesized that a fall in blood flow as a result of hypotension has a larger effect on hemodynamics in medium-sized peripheral arteries compared with increased pulsatility in rapid pacing. To test this hypothesis, we experimentally and theoretically investigated hemodynamic changes in femoral, carotid, and subclavian arteries at heart rates of 95-170 beats/min after acute pacing. The arterial pressure, blood flow, and other hemodynamic parameters remained statistically unchanged for heart rates ≤ 135 beats/min. Systemic pressure and flow velocities, however, showed an abrupt decrease, resulting in larger alteration of hemodynamic parameters for heart rates ≥ 155 beats/min after pacing (initial period) and then recovered close to baseline after several minutes of pacing (recovery period). During the initial period, the pressure dropped from 88 mmHg (baseline) to 44 mmHg, and the flow velocity decreased to about one-third of baseline at heart rate of 170 beats/min. A hemodynamic analysis showed a velocity profile with a near-wall retrograde flow or a fully reversed flow during the initial period, which vanished at the recovery period. It was concluded that the initial fall of blood flow due to pressure drop led to transient flow reversal and negative wall shear stress because this phenomena was not observed at the recovery period. This study underscores the significant effects of hypotension on vascular hemodynamics, which may have relevance to physiology and chronic pathophysiology in paroxysmal supraventricular tachycardia.

Shear-induced endothelial cell-cell junction inclination

Atheroprone regions of the arterial circulation are characterized by time-varying, reversing, and oscillatory wall shear stress. Several in vivo and in vitro studies have demonstrated that flow reversal (retrograde flow) is atherogenic and proinflammatory. The molecular and structural basis for the sensitivity of the endothelium to flow direction, however, has yet to be determined. It has been hypothesized that the ability to sense flow direction is dependent on the direction of inclination of the interendothelial junction. Immunostaining of the mouse aorta revealed an inclination of the cell-cell junction by 13 degrees in direction of flow in the descending aorta where flow is unidirectional. In contrast, polygonal cells of the inner curvature where flow is disturbed did not have any preferential inclination. Using a membrane specific dye, the angle of inclination of the junction was dynamically monitored using live cell confocal microscopy in confluent human endothelial cell monolayers. Upon application of shear the junctions began inclining within minutes to a final angle of 10 degrees in direction of flow. Retrograde flow led to a reversal of junctional inclination. Flow-induced junctional inclination was shown to be independent of the cytoskeleton or glycocalyx. Additionally, within seconds, retrograde flow led to significantly higher intracellular calcium responses than orthograde flow. Together, these results show for the first time that the endothelial intercellular junction inclination is dynamically responsive to flow direction and confers the ability to endothelial cells to rapidly sense and adapt to flow direction.

Vasoresponsiveness of collateral vessels in the rat hindlimb: influence of training

Exercise training is known to be an effective means of improving functional capacity and quality of life in patients with peripheral arterial insufficiency (PAI). However, the specific training-induced physiological adaptations occurring within collateral vessels remain to be clearly defined. The purpose of this study was to determine the effect of exercise training on vasomotor properties of isolated peripheral collateral arteries. We hypothesized that daily treadmill exercise would improve the poor vasodilatory capacity of collateral arteries isolated from rats exposed to surgical occlusion of the femoral artery. Following femoral artery ligation, animals were either kept sedentary or exercise trained daily for a period of 3 weeks. Hindlimb collateral arteries were then isolated, cannulated and pressurized via hydrostatic reservoirs to an intravascular pressure of either 45 or 120 cmH(2)O. Non-occluded contralateral vessels of the sedentary animals served as normal Control. Vasodilatory responses to acetylcholine (ACh; 1 x 10(9)-1 x 10(5)m) and sodium nitroprusside (SNP; 1 x 10(9)-1 x 10(4)m), constrictor responses to phenylephrine (PE; 1 x 10(9)-1 x 10(4)m), and flow-induced vasodilatation were determined. Endothelium-mediated vasodilatation responses were significantly greater to either ACh (P < 0.02) or intravascular flow (P < 0.001) in collateral arteries of trained rats. Neither blockade of cyclooxygenase with indomethacin (Indo; 5 microm) nor blockade of endothelial nitric oxide synthase with N(G)-nitro-L-arginine methyl ester (L-NAME; 300 microm) eliminated this ACh- or flow-induced vasodilatation. The depressed vasodilatory response to SNP caused by vascular occlusion was reversed with training. These data indicate that exercise training improves endothelium-mediated vasodilatory capacity of hindlimb collateral arteries, apparently by enhanced production of the putative endothelium-derived hyperpolarizing factor(s). If these findings were applicable to patients with PAI, they could contribute to an improved collateral vessel function and enhance exercise tolerance during routine physical activity.

Vasoresponsiveness of collateral vessels in the rat hindlimb: influence of training

Exercise training is known to be an effective means of improving functional capacity and quality of life in patients with peripheral arterial insufficiency (PAI). However, the specific training-induced physiological adaptations occurring within collateral vessels remain to be clearly defined. The purpose of this study was to determine the effect of exercise training on vasomotor properties of isolated peripheral collateral arteries. We hypothesized that daily treadmill exercise would improve the poor vasodilatory capacity of collateral arteries isolated from rats exposed to surgical occlusion of the femoral artery. Following femoral artery ligation, animals were either kept sedentary or exercise trained daily for a period of 3 weeks. Hindlimb collateral arteries were then isolated, cannulated and pressurized via hydrostatic reservoirs to an intravascular pressure of either 45 or 120 cmH(2)O. Non-occluded contralateral vessels of the sedentary animals served as normal Control. Vasodilatory responses to acetylcholine (ACh; 1 x 10(9)-1 x 10(5)m) and sodium nitroprusside (SNP; 1 x 10(9)-1 x 10(4)m), constrictor responses to phenylephrine (PE; 1 x 10(9)-1 x 10(4)m), and flow-induced vasodilatation were determined. Endothelium-mediated vasodilatation responses were significantly greater to either ACh (P < 0.02) or intravascular flow (P < 0.001) in collateral arteries of trained rats. Neither blockade of cyclooxygenase with indomethacin (Indo; 5 microm) nor blockade of endothelial nitric oxide synthase with N(G)-nitro-L-arginine methyl ester (L-NAME; 300 microm) eliminated this ACh- or flow-induced vasodilatation. The depressed vasodilatory response to SNP caused by vascular occlusion was reversed with training. These data indicate that exercise training improves endothelium-mediated vasodilatory capacity of hindlimb collateral arteries, apparently by enhanced production of the putative endothelium-derived hyperpolarizing factor(s). If these findings were applicable to patients with PAI, they could contribute to an improved collateral vessel function and enhance exercise tolerance during routine physical activity.

Neurohumoral response and clinical effectiveness of continuous aortic flow augmentation in patients with decompensated heart failure

The increasing number of patients with progressive or exacerbated heart failure that is refractory to medical treatment necessitates the development of innovative cardiac assist devices. The aim of this study was to investigate whether a new percutaneously inserted system, which allows continuous aortic flow augmentation (CAFA), could be shown to be clinically effective with neurohormonal benefit in patients admitted with decompensated heart failure. Patients with exacerbations of chronic heart failure were recruited for the study. A percutaneous circulation assist device (Cancion system) promoting CAFA was implanted for up to 4 days in each patient. Clinical improvement was evaluated by measuring the clinical status according to the New York Heart Association (NYHA) classification and biochemical parameters including troponin and B-type natriuretic peptide (BNP) as markers of cardiac necrosis and cardiac overload; these parameters were measured before, during, and after CAFA treatment. The decrease in BNP was determined after implantation, reaching, on average, a maximum decrease of 57% at 72 h (P = 0.04). The neurohumoral response remained significant (P < 0.05) up to 120 h after implantation, with a decrease in BNP levels of 37%, on average, compared to baseline values. Troponin I did not show any significant change during mechanical assistance (P > 0.2). All patients had improved clinical status according to the NYHA classification, and the improvement lasted for more than 1 week. Percutaneous heart-assist devices promoting CAFA offer clinical improvement and a neurohumoral response, with a significant BNP reduction in severe exacerbation of chronic heart failure that is refractory to medical treatment.

Fluid-structure interaction in aortic cross-clamping: implications for vessel injury

Vascular cross-clamping is applied in many cardiovascular surgeries such as coronary bypass, aorta repair and valve procedures. Experimental studies have found that clamping of various degrees caused damage to arteries. This study examines the effects of popular clamps on vessel wall. Models of the aorta and clamp were created in Computer Assisted Design and Finite Element Analysis packages. The vessel wall was considered as a non-linear anisotropic material while the fluid was simulated as Newtonian with pulsatile flow. The clamp was applied through displacement time function. Fully coupled two-way solid-fluid interaction models were developed. It was found that the clamp design significantly affected the stresses in vessel wall. The clamp with a protrusion feature increased the overall Von Mises stress by about 60% and the compressive stress by more than 200%. Interestingly, when the protrusion clamp was applied, the Von Mises stress at the lumen (endothelium) side of artery wall was about twice that of the outer wall. This ratio was much higher than that of the plate-like clamp which was about 1.3. The flow reversal process was demonstrated during clamping. Vibrations, flow and wall shear stress oscillations were detected immediately before total vessel occlusion. The commonly used protrusion clamp increased stresses in vessel wall, especially the compressive stress. This design also significantly increased the stresses on endothelium, detrimental to vessel health. The present findings are relevant to surgical clamp design as well as the transient mechanical loading on the endothelium and potential injury. The deformation and stress analysis may provide valuable insights into the mode of tissue injury during cross-clamping.

Role of glycocalyx in flow-induced production of nitric oxide and reactive oxygen species

Although the glycocalyx has been implicated in wall shear stress (WSS) mechanotransduction, the role of glycocalyx components in nitric oxide (NO) and reactive oxygen species (ROS) production remains unclear. Here, we tested the hypothesis that glycocalyx is implicated in both endothelial NO and O(2)(-) production. Specifically, we evaluated the role of hyaluronic acid (HA), heparan sulfate (HS), and sialic acid (SA) in NO and O(2)(-) mechanotransduction. Twenty-seven ex vivo porcine superficial femoral arteries were incubated with heparinase III, hyaluronidase, or neuraminidase, to remove HS, HA, or SA, respectively, from glycocalyx. The arteries were then subjected to steady-state flow and the effluent solution was measured for nitrites and the vessel diameter was tracked to quantify the degree of vasodilation. Our results show that removal of HA decreased both nitrites and vasodilation, and tempol treatment had no reversing effect. Degradation of HS proteoglycans decreased NO bioavailability through an increase in O(2)(-) production as indicated by fluorescent signals of dihydroethidium (DHE) and its area fraction (209+/-24% increase) and also removed extracellular O(2)(-) dismutase (ecSOD) (67+/-9% decrease). The removal of SA also increased O(2)(-) production as indicated by DHE fluorescent signals (86+/-17% increase) and the addition of tempol, a mimic O(2)(-) scavenger, restored both NO availability and vasodilation in both heparinase- and neuraminidase-treated vessels. This implies that HS and SA are not directly involved in WSS-mediated NO production. This study implicates HA in WSS-mediated NO mechanotransduction and underscores the role of HS and SA in ROS regulation in vessel walls in response to WSS stimulation.

A simulation of vessel-clamp interaction: transient closure dynamics

Cross-clamping of aorta is routinely performed in cardiac surgery. The objective of this study was to simulate cross-clamping of the aorta to elucidate the perturbation of stresses in the wall (solid mechanics) and lumen of the vessel (fluid mechanics). Models of the aorta and clamp were created in Computer Assisted Design and Finite Element Analysis packages. The vessel wall was considered as a non-linear anisotropic material while the fluid was simulated as Newtonian with pulsatile flow. The clamp was applied to produce total occlusion in approximately 1 s. A cylindrical and rectangular geometry for the clamp were considered. High jet speed and flow reversal were demonstrated during clamping. It was found that the clamp design and vessel wall anisotropy affected both the fluid wall shear stress (WSS) and solid stresses in vessel wall. The maximum wall stresses increased by about 170 and 220% during closure in the cases of plate and cylindrical clamps, respectively. The plate clamp design was superior for reduction of both solid stresses as well as fluid shear stresses. The cylindrical clamp causes much larger stresses than the plate clamp in each of the stress components; e.g., radial compression of -180 vs. -50 kPa. Vibrations, flow and WSS oscillations were detected immediately before total vessel occlusion. The present findings provide valuable insights into the mode of tissue injury during clamping and may also be useful for improving surgical clamp designs.

Effects of stent sizing on endothelial and vessel wall stress: potential mechanisms for in-stent restenosis

Stent sizing and apposition have been shown to be important determinants of clinical outcome. This study evaluates the mechanical effects of undersizing and oversizing of stents on endothelial wall shear stress (WSS) and vessel wall stress to determine a possible biomechanical mechanism of in-stent restenosis and thrombosis. Three-dimensional computational models of stents, artery, and internal fluid were created in a computer-assisted design package, meshed, and solved in finite element and computational fluid dynamic packages. The simulation results show that the effects of various degrees of undersizing on WSS, WSS gradient, and oscillatory shear index were highly nonlinear. As the degree of undersizing increased, the heterogeneity of WSS became smaller. The WSS distribution for the 20% undersizing was smooth and uniform, whereas the 5% case was very heterogeneous. The combination of lower WSS and higher WSS gradient and oscillatory shear index in the 5% undersized case may induce neointimal hyperplasia or thrombosis. Additionally, the oversizing simulation results show that the maximum intramural wall stress of the 20% oversizing case is significantly larger than the maximum stress for the 10% and zero oversizing cases. Edge stress concentration was observed, consistent with the restenosis typically observed in this region. This study demonstrates that proper sizing of stent is important for reducing the hemodynamic and mechanical disturbances to the vessel wall. Furthermore, the present findings may be used to improve stent design to reduce endothelial flow disturbances and intramural wall stress concentrations.

Acute left ventricular unloading in dogs with chronic heart failure: continuous aortic flow augmentation versus intra-aortic balloon pumping

BACKGROUND

Continuous aortic flow augmentation (CAFA) therapy with the Cancion System (Orqis Medical, Inc) was shown to effectively unload the left ventricle in dogs with chronic heart failure (HF). This study compared the extent of acute left ventricular (LV) unloading elicited by CAFA to that elicited by intra-aortic balloon counterpulsation (IABP) in normotensive dogs with coronary microembolization-induced HF.

METHODS AND RESULTS

Seven HF dogs were studied with both CAFA and IABP in random order and 1 week apart. In both instances, active therapy was maintained for 4 hours. The Cancion system was positioned using a dual femoral approach configuration with a constant pump flow of 250 mL/min. In all dogs and with both devices, LV end-diastolic pressure (EDP), LV end-systolic volume (ESV), and LV ejection fraction (EF) were measured at baseline and at 2 and 4 hours after instituting CAFA or IABP. Plasma samples obtained at the end of 4 hours of therapy were used to measure a host of circulating biomarkers that included neurohormones, cytokines, and A-type and B-type natriuretic peptides. IABP had no significant effects on LVEDP, LVESV, and LVEF. In contrast, CAFA significantly decreased LVEDP and LVESV and increased LVEF. Compared with IABP, CAFA was accompanied by significant improvements in circulating levels of neurohormones, cytokines, and natriuretic peptides.

CONCLUSIONS

The results indicate that CAFA is more effective than IABP in achieving acute global LV unloading in dogs with chronic HF not complicated by ongoing myocardial ischemia or cardiogenic shock.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

NADPH oxidase has a directional response to shear stress

Vessel regions with predilection to atherosclerosis have negative wall shear stress due to flow reversal. The flow reversal causes the production of superoxides (O(2)(-)), which scavenge nitric oxide (NO), leading to a decrease in NO bioavailability and endothelial dysfunction. Here, we implicate NADPH oxidase as the primary source of O(2)(-) during full flow reversal. Nitrite production and the degree of vasodilation were measured in 46 porcine common femoral arteries in an ex vivo system. Nitrite production and vasodilation were determined before and after the inhibition of NADPH oxidase, xanthine oxidase, or mitochondrial oxidase. NADPH oxidase inhibition with gp91ds-tat or apocynin restored nitrite production and vasodilation during reverse flow. Xanthine oxidase inhibition increased nitrite production at the highest flow rate, whereas mitochondrial oxidase inhibition had no effect. These findings suggest that the NADPH oxidase system can respond to directional changes of flow and is activated to generate O(2)(-) during reverse flow in a dose-dependent fashion. These findings have important clinical implications for oxidative balance and NO bioavailability in regions of flow reversal in a normal and compromised cardiovascular system.

Relation of nitrite to structural and mechanical adaptation of arteries during postnatal development

Mammalian arteries undergo rapid remodeling during postnatal growth and development. The high wall shear stress at birth is an important mediator of postnatal endothelial nitric oxide (NO) and consequently of growth and remodeling. The objective of this study was to quantify the NO production in relation to geometric and mechanical remodeling of aorta and pulmonary artery during postnatal development. Fifty-one C57BL/6 mice aged from 1 to 33 days were divided into 8 age groups for measurements of nitrite (NO(x)). Systematic measurements of NO(x) in each rings were made in the main pulmonary artery and primary branch as well as along the length of aorta using the combination of a diazo coupling method and high-performance liquid chromatography. The NO(x) data on the aorta were correlated with data on the geometry (diameter, wall thickness) and mechanical properties (stress, strain, elastic modulus) in the same strain of mice under the same conditions. Our findings show postnatal age and vessel size affects the NO production; i.e., the NO(x) decreased with age and diameter. Furthermore, there is a significant positive correlation between strain and NO(x) but negative correlation between both wall thickness and elastic modulus and NO(x) levels. These findings suggest an important interplay between NO(x) and geometric and mechanical remodeling during postnatal growth and development.

NAD(P)H oxidase-derived peroxide mediates elevated basal and impaired flow-induced NO production in SHR mesenteric arteries in vivo

Nitric oxide (NO) and reactive oxygen species (ROS) have fundamentally important roles in the regulation of vascular tone and remodeling. Although arterial disease and endothelial dysfunction alter NO and ROS levels to impact vasodilation and vascular structure, direct measurements of these reactive species under in vivo conditions with flow alterations are unavailable. In this study, in vivo measurements of NO and H2O2 were made on mesenteric arteries to determine whether antioxidant therapies could restore normal NO production in spontaneously hypertensive rats (SHR). Flow was altered from approximately 50-200% of control in anesthetized Wistar-Kyoto rats (WKY) and SHR by selective placement of microvascular clamps on adjacent arteries while NO and H2O2 were directly measured with microelectrodes. Relative to WKY, SHR had significantly increased baseline NO and H2O2 concentrations (2,572 +/- 241 vs. 1,059 +/- 160 nM, P < 0.01; and 26 +/- 7 vs. 7 +/- 1 microM, P < 0.05, respectively). With flow elevation, H2O2 but not NO increased in SHR; NO but not H2O2 was elevated in WKY. Apocynin and polyethylene-glycolated catalase decreased baseline SHR NO and H2O2 to WKY levels and restored flow-mediated NO production. Suppression of NAD(P)H oxidase with gp91ds-tat decreased SHR H2O2 to WKY levels. Addition of topical H2O2 to increase peroxide to the basal concentration measured in SHR elevated WKY NO to levels observed in SHR. The results support the hypothesis that increased vascular peroxide in SHR is primarily derived from NAD(P)H oxidase and increases NO concentration to levels that cannot be further elevated with increased flow. Short-term and even acute administration of antioxidants are able to restore normal flow-mediated NO signaling in young SHR.

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.

Increased reversal and oscillatory shear stress cause smooth muscle contraction-dependent changes in sheep aortic dynamics: role in aortic balloon pump circulatory support

AIMS

The intra-aortic balloon pumping (IABP) changes pressure and increases the aorta shear stress reversal (SS(R)) and oscillatory (SS(O)) components. Hence, IABP-dependent changes in aortic biomechanics would be expected, because of vascular smooth muscle (VSM) tone (i.e. flow-induced endothelium-dependent response, related to SS(R) and SS(O) variations) and/or pressure changes. To characterize: (i) the IABP effects on the aortic and global (systemic circulation) biomechanics, analysing their dependence on pressure and VSM basic tone changes and (ii) the relation between the SS(R) and SS(O) and the aortic biomechanical changes associated with the VSM tone variations.

METHODS

Aortic flow, pressure and diameter were measured in eight sheep during basal, augmented and assisted beats (1 : 1 and 1 : 2 IABP modalities). Calculations: (i) aortic effective and isobaric elasticity, viscosity, circumferential stress, pulse wave velocity, shear stress and buffer and conduit functions, (ii) peripheral resistance, global compliance, reflection coefficient and wave propagation times and (iii) the relation between SS(R) and SS(O) and biomechanical changes associated with variations in the aortic VSM tone.

RESULTS

Augmented and assisted beats showed: global VSM relaxation pattern (reduced peripheral resistance and reflection coefficient; increased propagation times) and local VSM contraction pattern (increased viscosity; reduced diameter, elasticity and circumferential stress), associated with SS(R) and SS(O), levels and changes. The vascular changes reduced the ventricle afterload determinants, increased the vascular buffer performance and kept the conduit capability.

CONCLUSION

In addition to pressure-dependent changes, IABP determined biomechanical changes related to variations in the VSM tone. The increased SS(R) and SS(O) were associated with the aortic VSM contraction pattern and biomechanical changes.

Acute increase in reversal blood flow during counterpulsation is associated with vasoconstriction and changes in the aortic mechanics

While the effects of increases in forward blood flow on the arterial diameter and elasticity are known, the effects of reversal flow on the arterial properties remain to be characterized. The intra-aortic balloon pumping (IABP), the device most frequently used in circulatory support, acts generating changes in aortic flow (i.e. increasing reversal flow). Recently, in vitro studies showed that flow reversion reduces the endothelial release of relaxing factors. Hence, vascular smooth muscle (VSM) dependent changes in the aortic properties would be expected during IABP. The aim was to analyze the changes in flow during IABP and to characterize the potential effects of reversal blood flow on the aortic biomechanics. Pressure, flow and diameter were measured in sheep, before and during IABP circulatory support. Potential effects of IABP-dependent high reversal flow conditions on viscous and elastic aortic modulus were analyzed, using isobaric analysis. Flow and pressure waveforms were analyzed in the time domain, and the contribution of oscillatory forward and backward waves to the IABP-dependent changes in flow patterns were evaluated. We found that IABP changed mainly diastolic blood flow, with an increase in the reversal flow, secondary to an increase in the oscillatory backward wave amplitude. The acute increase in reversal flow during IABP was associated with vasoconstriction and changes in the aortic mechanics, possibly due to VSM activation.

Flow patterns in three-dimensional porcine epicardial coronary arterial tree

The branching pattern of epicardial coronary arteries is clearly three-dimensional, with correspondingly complex flow patterns. The objective of the present study was to perform a detailed hemodynamic analysis using a three-dimensional finite element method in a left anterior descending (LAD) epicardial arterial tree, including main trunk and primary branches, based on computed tomography scans. The inlet LAD flow velocity was measured in an anesthetized pig, and the outlet pressure boundary condition was estimated based on scaling laws. The spatial and temporal wall shear stress (WSS), gradient of WSS (WSSG), and oscillatory shear index (OSI) were calculated and used to identify regions of flow disturbances in the vicinity of primary bifurcations. We found that low WSS and high OSI coincide with disturbed flows (stagnated, secondary, and reversed flows) opposite to the flow divider and lateral to the junction orifice of the main trunk and primary branches. High time-averaged WSSG occurs in regions of bifurcations, with the flow divider having maximum values. Low WSS and high OSI were found to be related through a power law relationship. Furthermore, zones of low time-averaged WSS and high OSI amplified for larger diameter ratio and high inlet flow rate. Hence, different focal atherosclerotic-prone regions may be explained by different physical mechanism associated with certain critical levels of low WSS, high OSI, and high WSSG, which are strongly affected by the diameter ratio. The implications of the flow patterns for atherogenesis are enumerated.

Flow-induced shear strain in intima of porcine coronary arteries

The in vivo circumferential strain has a small variation throughout the vascular system (aorta to arterioles). The axial strain has also been shown to be nearly the same as the circumferential strain under physiological loading. Since the endothelium is mechanically much softer than the media-adventitia in healthy arteries, the porcine intima was considered as a mechanically distinct layer from the media-adventitia in a two-layer computational model. Based on the simulation result, we hypothesize that the flow-induced shear strain in intima can be of similar value as the pressure-induced circumferential strain in healthy coronary arteries, even though the shear stress is orders of magnitude smaller than the circumferential stress. The nearly isotropic deformation (circumferential, axial, and shear strains) may have important implications for mechanical homeostasis of endothelial cells, mechanotransduction, growth, and remodeling of blood vessels.

Nitric oxide regulation of microvascular oxygen

The role of nitric oxide (NO) as a highly diffusible free radical gaseous vasodilator is intrinsically linked to the control of blood flow and oxygen (O(2)) delivery to tissue. NO also is involved in regulating mitochondrial O(2) metabolism, growth of new blood vessels, and blood oxygenation through control of respiratory ventilation. Hemoglobin and myoglobin may help to conserve NO for subsequent release of a NO-related vasoactive species under hypoxic conditions. NO has a major role in regulating microvascular O(2), and dysfunctional NO signaling is associated with the pathogenesis of metabolic and cardiovascular diseases.

Short-term effects of di-(2-ethylhexyl) phthalate on testes, liver, kidneys and pancreas in mice

AIM

To determine the biochemical effect of di-(2-ethylhexyl) phthalate (DEHP) on testes, liver, kidneys and pancreas on day 10 in the process of degeneration of the seminiferous epithelium.

METHODS

Diets containing 2% DEHP were given to male Crlj:CD1(ICR) mice for 10 days. The dose of DEHP was 0.90 +/- 0.52 mg/mouse/day. Their testes, livers, kidneys and pancreata were examined for detection of mono-(2-ethylhexyl) phthalate (MEHP), nitrogen oxides (NOx) produced by peroxidation of nitric oxide (NO) with free radicals, and lipid peroxidation induced by the chain reaction of free radicals.

RESULTS

Histological observation and serum analysis showed the presence of severe spermatogenic disturbance, Leydig cell dysfunction, liver dysfunction and dehydration. Unexpectedly, the concentration of MEHP in the testes was extremely low compared with that in the liver. However, the concentration of the NOx in the testes was as high as the hepatic concentration. Furthermore, free radical-induced lipid peroxidation was histochemically detected in the testes but not in the liver.

CONCLUSION

The results indicate that DEHP-induced aspermatogenesis is caused by the high sensitivity of the testicular tissues to MEHP rather than the specific accumulation or uptake of circulating MEHP into the testes.

BIOMECHANICAL CONSIDERATIONS IN THE DESIGN OF GRAFT: THE HOMEOSTASIS HYPOTHESIS

Since its inception in the 1960s, coronary artery bypass graft (CABG) evolved as one of the most common, best documented, and most effective of all major surgical treatments for ischemic heart disease. Despite its widespread use, however, the outcome is not always completely satisfactory. The objective of this review is to highlight the physical determinants of biomechanical design of CABG so that future procedures would have prolonged patency and better outcome. Our central axiom postulates the existence of a mechanical homeostatic state of the blood vessel, i.e., the variation in vessel wall stresses and strains are relatively small under physiological conditions. Any perturbation of mechanical homeostasis leads to growth and remodeling. In this sense, stenosis and failure of a graft may be viewed as an adaptation process gone awry. We outline the principles of engineering design and discuss the biofluid and biosolid mechanics principles that may have the greatest bearing on mechanical homeostasis and the long-term outcome of CABG.

Effect of cigarette smoking on nitric oxide, structural, and mechanical properties of mouse arteries

Cigarette smoking (CS) is a major risk factor for vascular disease. The aim of this study was to quantitatively assess the influence of CS on mouse arteries. We studied the effect of short-term (6 wk) and long-term (16 wk) CS exposure on structural and mechanical properties of coronary arteries compared with that of control mice. We also examined the reversibility of the deleterious effects of CS on structural [e.g., wall thickness (WT)], mechanical (e.g., stiffness), and biochemical [e.g., nitric oxide (NO) by-products] properties with the cessation of CS. The left and right coronary arteries were cannulated in situ and mechanically distended. The stress, strain, elastic modulus, and WT of coronary arteries were determined. Western blot analysis was used to analyze endothelial NO synthase (eNOS) in the femoral and carotid arteries of the same mice, and NO by-products were determined by measuring the levels of nitrite. Our results show that the mean arterial pressure was increased by CS. Furthermore, CS significantly increased the elastic modulus, decreased stress and strain, and increased the WT and WT-to-radius ratio compared with those of control mice. The reduction of eNOS protein expression was found only after long-term CS exposure. Moreover, the NO metabolite was markedly decreased in CS mice after short- and long-term exposure of CS. These findings suggest that 16 wk of CS exposure can cause an irreversible deterioration of structural and elastic properties of mouse coronary arteries. The decrease in endothelium-derived NO in CS mice was seen to significantly correlate with the remodeling of arterial wall.

Antioxidant effects and the therapeutic mode of action of calcium channel blockers in hypertension and atherosclerosis

Drugs currently known as calcium channel blockers (CCB) were initially called calcium antagonists because of their ability to inhibit calcium-evoked contractions in depolarized smooth muscles. Blocking the entry of calcium reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological property has been the basis for the use of CCBs in the management of hypertension and coronary heart disease. A major question is whether drugs reducing blood pressure have other effects that help prevent the main complications of hypertension, such as atherosclerosis, stroke, peripheral arterial disease, heart failure and end-state renal disease. Experimental studies that focus on this question are reviewed in the present paper.

AMP-Activated Protein Kinase Is Involved in Endothelial NO Synthase Activation in Response to Shear Stress

Objective— The regulation of AMP-activated protein kinase (AMPK) is implicated in vascular biology because AMPK can phosphorylate endothelial NO synthase (eNOS). In this study, we investigate the regulation of the AMPK–eNOS pathway in vascular endothelial cells (ECs) by shear stress and the activation of aortic AMPK in a mouse model with a high level of voluntary running (High-Runner).

Methods and Results— By using flow channels with cultured ECs, AMPK Thr172 phosphorylation was increased with changes of flow rate or pulsatility. The activity of LKB1, the upstream kinase of AMPK, and the phosphorylation of eNOS at Ser1179 were concomitant with AMPK activation responding to changes in flow rate or pulsatility. The blockage of AMPK by a dominant-negative mutant of AMPK inhibited shear stress-induced eNOS Ser1179 phosphorylation and NO production. Furthermore, aortic AMPK activity and level of eNOS phosphorylation were significantly elevated in the aortas of High-Runner mice.

Conclusions— Our results suggest that shear stress activates AMPK in ECs, which contributes to elevated eNOS activity and subsequent NO production. Hence, AMPK, in addition to serving as an energy sensor, also plays an important role in regulating vascular tone.

Proper Orientation of the Graft Artery Is Important to Ensure Physiological Flow Direction

Arterial grafts such as right internal mammary artery (RIMA), radial artery (RA) or epigastric artery are being used with increasing frequency as free grafts or as composite grafts with left internal mammary artery (LIMA). Currently, there is no consideration of the orientation of the free artery graft to mimic the in vivo state. Hence, some grafts may be oriented such that the direction of blood flow exerted on the endothelium is reversed relative to the in vivo condition. Previous studies have shown that transient flow reversal lead to atherogenesis. A recent study demonstrated that nitric oxide is significantly reduced during reverse flow and the reduction is mediated through an increase in superoxide production. In light of these data, we suggest that the flow direction is important and recommend the assurance of proper orientation of the free arterial graft. Furthermore, we propose a new surgical procedure to modify the composite LIMA-RA or LIMA-RIMA configuration as a horseshoe or K composite graft to ensure proper orientation of the flow direction relative to the endothelium.

Estrogen modulates the mechanical homeostasis of mouse arterial vessels through nitric oxide

We have recently shown that estrogen causes vessel dilation through receptor-mediated stimulation of nitric oxide (NO) production. Here, we hypothesize that estrogen modulates the mechanical homeostasis in the blood vessel wall through NO production. The mechanical properties of female ovariectomized (ovx) mice, female mice lacking the gene for endothelial NO synthase (eNOS(-/-)), and control female and male mice were studied to test the hypothesis. The femoral and carotid arteries and aorta were cannulated in situ and mechanically distended. The stress, strain, elastic modulus, and wall thickness of vessels in ovx and eNOS(-/-) mice, as well as intact female and male mice, were determined. Western blot and immunohistochemistry were used to assess eNOS protein expression in the aorta. Moreover, NO by-products of the femoral and carotid artery were determined by measuring the levels of nitrite and nitrate. Our results show that ovariectomy and eNOS(-/-) significantly decrease the strain in all arteries. Furthermore, the eNOS protein was significantly reduced in ovx mice. Finally, the NO metabolites were significantly decreased both in ovx and eNOS(-/-) mice. We found statistically significant correlations between the structural (wall thickness), mechanical (stress, strain, and elastic modulus), and biochemical parameters (NO by-products). These novel results connect NO to the structural and mechanical properties of the vessel wall. Hence, the effect of endogenous estrogen on the arterial mechanical properties is mediated by the regulation of NO derived from eNOS.

Continuous Aortic Flow Augmentation

Background— Diminished aortic flow may induce adverse downstream vascular and renal signals. Investigations in a heart failure animal model have shown that continuous aortic flow augmentation (CAFA) achieves hemodynamic improvement and ventricular unloading, which suggests a novel therapeutic approach to patients with heart failure exacerbation that is inadequately responsive to medical therapy.

Methods and Results— We studied 24 patients (12 in Europe and 12 in the United States) with heart failure exacerbation and persistent hemodynamic derangement despite intravenous diuretic and inotropic and/or vasodilator treatment. CAFA (mean±SD 1.34±0.12 L/min) was achieved through percutaneous (n=19) or surgical (n=5) insertion of the Cancion system, which consists of inflow and outflow cannulas and a magnetically levitated and driven centrifugal pump. Hemodynamic improvement was observed within 1 hour. Systemic vascular resistance decreased from 1413±453 to 1136±381 dyne · s · cm −5 at 72 hours ( P =0.0008). Pulmonary capillary wedge pressure decreased from 28.5±4.9 to 19.8±7.0 mm Hg ( P <0.0001), and cardiac index (excluding augmented aortic flow) increased from 1.97±0.44 to 2.27±0.43 L · min −1 · m −2 ( P =0.0013). Serum creatinine trended downward during treatment (overall P =0.095). There were 8 complications during treatment, 7 of which were self-limited. Hemodynamics remained improved 24 hours after CAFA discontinuation.

Conclusions— In patients with heart failure and persistent hemodynamic derangement despite intravenous inotropic and/or vasodilator therapy, CAFA improved hemodynamics, with a reduction in serum creatinine. CAFA represents a promising, novel mode of treatment for patients who are inadequately responsive to medical therapy. The clinical impact of the observed hemodynamic improvement is currently being explored in a prospective, randomized, controlled trial.

Calcium channel blocker inhibits Western-type diet-evoked atherosclerosis development in ApoE-deficient mice

Calcium channel blockers slow the progression of atherosclerosis. The purpose of the present experiments was to examine the action of lacidipine in a condition that accelerates the development of atherosclerosis in order to test the hypothesis that the protective action of lacidipine in atherosclerosis is unrelated to the reduction of blood pressure. Male ApoE-deficient mice (6 weeks old) were exposed either to normal chow (ND) or to a Western-type diet (WD, adjusted calorie diet containing 42% from fat) for 8 weeks. Western-type diet induced a reduction of nitric oxide (NO)-mediated endothelium-dependent relaxation to acetylcholine (Max relaxation % = 55.8 +/- 2 for ND and 46.6 +/- 2 for WD, n = 8, p < 0.05). Dose-relaxation curves to S-nitroso-N-acetylpenicillamine (SNAP) NO donor were also significantly rightward-shifted (n = 7, ANOVA, p < 0.01) in WD compared with ND arteries. Chronic treatment of WD mice with lacidipine (1 and 3 mg/kg/day) increased significantly the acetylcholine-evoked relaxation (to 76.6 +/- 3.5%, n = 6, ANOVA, p < 0.001) and prevented the loss of responsiveness to SNAP in mice exposed to WD. Plasma renin activity and endothelin-1 plasma levels as well as thiobarbituric acid-reactive substance levels in kidneys were significantly lower in WD mice treated with lacidipine than in untreated ones. In mice exposed to WD lacidipine reduced extension of atherosclerotic lesions, renal injury and increase in blood pressure. Experimental data indicate that inhibition of Western-type diet-evoked alterations is related to both antioxidant and vasoactive properties of lacidipine.

Differential responses of the Nrf2-Keap1 system to laminar and oscillatory shear stresses in endothelial cells

The Nrf2-Keap1 system coordinately regulates cytoprotective gene expression via the antioxidant responsive element (ARE). The expression of several ARE-regulated genes was found to be up-regulated in endothelial cells by laminar shear stress, suggesting that Nrf2 contributes to the anti-atherosclerosis response via the ARE. To gain further insight into the roles that Nrf2 plays in the development of atherosclerosis, we examined how Nrf2 regulates gene expression in response to anti-atherogenic laminar flow (L-flow) or pro-atherogenic oscillatory flow (O-flow). Exposure of human aortic endothelial cells (HAECs) to L-flow, but not to O-flow, induced the expression of cytoprotective genes, such as NAD(P)H quinone oxidoreductase 1 (NQO1) by 5-fold and heme oxygenase-1 by 8-fold. The critical contribution of Nrf2 to the expression induced by L-flow was ascertained in siRNA-mediated knock-down experiments. Two cyclooxygenase-2 (COX-2) specific inhibitors attenuated Nrf2 nuclear accumulation in the acute phase of L-flow exposure. A downstream product of COX-2, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), activated the Nrf2 regulatory pathway in HAECs through binding to the cysteines of Keap1. These results demonstrate that 15d-PGJ2 is essential for L-flow to activate Nrf2 and induce anti-atherosclerotic gene expression. Whereas both L-flow and O-flow induced the nuclear accumulation of Nrf2 to comparable levels, chromatin immunoprecipitation analysis revealed that Nrf2 binding to the NQO1 ARE was significantly diminished in the case of O-flow compared with that of L-flow. These results suggest that O-flow inhibits Nrf2 activity at the DNA binding step, thereby suppressing athero-protective gene expression and hence predisposing the blood vessels to the formation of atherosclerosis.