Role of shear stress and endothelial prostaglandins in flow- and viscosity-induced dilation of arterioles in vitro

Mechanistic, participant, and movement-related factors that contribute to low-flow-mediated constriction

Endothelial function is commonly determined via the ultrasound-based flow-mediated dilation (FMD) technique which assesses arterial dilation in response to a hyperemia response following distal cuff occlusion. However, the low-flow-mediated constriction (L-FMC) response during cuff-induced ischemia is often overlooked. L-FMC provides unique information regarding endothelial function, but vascular researchers may be unclear on what this metric adds. Therefore, the objective of this review was to examine the mechanistic determinants and participant-level factors of L-FMC. Existing mechanistic studies have demonstrated that vasoreactivity to low flow may be mediated via non-nitric oxide vasodilators (i.e., endothelial hyperpolarizing factors and/or prostaglandins), inflammatory markers, and enhancement of vasoconstriction via endothelin-1. In general, participant-level factors such as aging and presence of cardiovascular conditions generally are associated with attenuated L-FMC responses. However, the influence of sex on L-FMC is unclear with divergent results between L-FMC in upper versus lower limb vessels. The ability of aerobic exercise to augment L-FMC (i.e., make more negative) is well supported, but there is a major gap in the literature concerning the mechanistic underpinnings of this observation. This review summarizes that while larger L-FMC responses are generally healthy, the impact of interventions to augment/attenuate L-FMC has not included mechanistic measures that would provide insight into non-nitric oxide-based endothelial function. Clarifications to terminology and areas of further inquiry as it relates to the specific pharmacological, individual-level factors, and lifestyle behaviors that impact L-FMC are highlighted. A greater integration of mechanistic work alongside applied lifestyle interventions is required to better understand endothelial cell function to reductions in local blood flow.

CAPILLARY LEAK AND EDEMA AFTER RESUSCITATION: THE POTENTIAL CONTRIBUTION OF REDUCED ENDOTHELIAL SHEAR STRESS CAUSED BY HEMODILUTION

ABSTRACT

Normal shear stress is essential for the normal structure and functions of the microcirculation. Hemorrhagic shock leads to reduced shear stress due to reduced tissue perfusion. Although essential for the urgent restoration of cardiac output and systemic blood pressure, large volume resuscitation with currently available solutions causes hemodilution, further reducing endothelial shear stress. In this narrative review, we consider how the use of currently available resuscitation solutions results in persistent reduction in endothelial shear stress, despite successfully increasing cardiac output and systemic blood pressure. We consider how this reduced shear stress causes (1) a failure to restore normal vasomotor function and normal tissue perfusion thus leading to persistent tissue hypoxia and (2) increased microvascular endothelial permeability resulting in edema formation and impaired organ function. We discuss the need for clinical research into resuscitation strategies and solutions that aim to quickly restore endothelial shear stress in the microcirculation to normal.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Malondialdehyde as an Important Key Factor of Molecular Mechanisms of Vascular Wall Damage under Heart Diseases Development

This mini review is devoted to a specific issue: the role of malondialdehyde (MDA)-a secondary product of free radical lipid peroxidation-in the molecular mechanisms of the formation of primary atherosclerotic vascular wall lesions. The principal difference between this review and the available literature is that it discusses in detail the important role in atherogenesis not of "oxidized" LDL (i.e., LDL particles containing lipohydroperoxides), but of LDL particles chemically modified by the natural low-molecular weight dicarbonyl MDA. To confirm this, we consider the data obtained by us earlier, indicating that "atherogenic" are not LDL oxidized as a result of free radical lipoperoxidation and containing lipohydroperoxy derivatives of phospholipids in the outer layer of particles, but LDL whose apoprotein B-100 has been modified due to the chemical reaction of terminal lysine residue amino groups of the apoB-100 with the aldehyde groups of the MDA (Maillard reaction). In addition, we present our original data proving that MDA injures endothelial glycocalyx that suppress the ability of the endothelium to control arterial tone according to changes in wall shear stress. In summary, this mini review for the first time exhaustively discloses the key role of MDA in atherogenesis.

Impact of impaired cerebral blood flow autoregulation on cognitive impairment

Although the causes of cognitive impairment are multifactorial, emerging evidence indicates that cerebrovascular dysfunction plays an essential role in dementia. One of the most critical aspects of cerebrovascular dysfunction is autoregulation of cerebral blood flow (CBF), mainly mediated by the myogenic response, which is often impaired in dementia individuals with comorbidities, such as diabetes and hypertension. However, many unsolved questions remain. How do cerebrovascular networks coordinately modulate CBF autoregulation in health and disease? Does poor CBF autoregulation have an impact on cognitive impairment, and what are the underlying mechanisms? This review summarizes the cerebral vascular structure and myogenic (a three-phase model), metabolic (O2, CO2, adenosine, and H+), and endothelial (shear stress) factors in the regulation of CBF; and the consequences of CBF dysautoregulation. Other factors contributing to cerebrovascular dysfunction, such as impaired functional hyperemia and capillary abnormalities, are included as well. Moreover, this review highlights recent studies from our lab in terms of novel mechanisms involved in CBF autoregulation and addresses a hypothesis that there is a three-line of defense for CBF autoregulation in the cerebral vasculature.

Epigenetic Regulation of the Vascular Endothelium by Angiogenic LncRNAs

The functional properties of the vascular endothelium are diverse and heterogeneous between vascular beds. This is especially evident when new blood vessels develop from a pre-existing closed cardiovascular system, a process termed angiogenesis. Endothelial cells are key drivers of angiogenesis as they undergo a highly choreographed cascade of events that has both exogenous (e.g., hypoxia and VEGF) and endogenous regulatory inputs. Not surprisingly, angiogenesis is critical in health and disease. Diverse therapeutics target proteins involved in coordinating angiogenesis with varying degrees of efficacy. It is of great interest that recent work on non-coding RNAs, especially long non-coding RNAs (lncRNAs), indicates that they are also important regulators of the gene expression paradigms that underpin this cellular cascade. The protean effects of lncRNAs are dependent, in part, on their subcellular localization. For instance, lncRNAs enriched in the nucleus can act as epigenetic modifiers of gene expression in the vascular endothelium. Of great interest to genetic disease, they are undergoing rapid evolution and show extensive inter- and intra-species heterogeneity. In this review, we describe endothelial-enriched lncRNAs that have robust effects in angiogenesis.

Angiotensin II type 1 receptor is involved in flow-induced vasomotor responses of isolated middle cerebral arteries: role of oxidative stress

This study aimed to determine the mechanosensing role of angiotensin II type 1 receptor (AT1R) in flow-induced dilation (FID) and oxidative stress production in middle cerebral arteries (MCA) of Sprague-Dawley rats. Eleven-week old, healthy male Sprague-Dawley rats on a standard diet were given the AT1R blocker losartan (1 mg/mL) in drinking water (losartan group) or tap water (control group) ad libitum for 7 days. Blockade of AT1R attenuated FID and acetylcholine-induced dilation was compared with control group. Nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) and cyclooxygenase inhibitor indomethacin (Indo) significantly reduced FID in control group. The attenuated FID in losartan group was further reduced by Indo only at Δ100 mmHg, whereas l-NAME had no effect. In losartan group, Tempol (a superoxide scavenger) restored dilatation, whereas Tempol + l-NAME together significantly reduced FID compared with restored dilatation with Tempol alone. Direct fluorescence measurements of NO and reactive oxygen species (ROS) production in MCA, in no-flow conditions revealed significantly reduced vascular NO levels with AT1R blockade compared with control group, whereas in flow condition increased the NO and ROS production in losartan group and had no effect in the control group. In losartan group, Tempol decreased ROS production in both no-flow and flow conditions. AT1R blockade elicited increased serum concentrations of ANG II, 8-iso-PGF2α, and TBARS, and decreased antioxidant enzyme activity (SOD and CAT). These results suggest that in small isolated cerebral arteries: 1) AT1 receptor maintains dilations in physiological conditions; 2) AT1R blockade leads to increased vascular and systemic oxidative stress, which underlies impaired FID.NEW & NOTEWORTHY The AT1R blockade impaired the endothelium-dependent, both flow- and acetylcholine-induced dilations of MCA by decreasing vascular NO production and increasing the level of vascular and systemic oxidative stress, whereas it mildly influenced the vascular wall inflammatory phenotype, but had no effect on the systemic inflammatory response. Our data provide functional and molecular evidence for an important role of AT1 receptor activation in physiological conditions, suggesting that AT1 receptors have multiple biological functions.

Metabolic syndrome and coronary artery disease in adults with congenital heart disease

In adults with congenital heart disease (ACHD), conditions acquired with aging, such as metabolic syndrome, hypertension, diabetes mellitus, and obesity, can negatively influence the original cardiovascular disease. Metabolic syndrome has a higher prevalence in ACHD than in the general population. In contrast, coronary artery disease shows a similar prevalence in adults with acyanotic CHD and the general population, while adults with cyanotic CHD, even after repair, have an even lower incidence of coronary artery disease than the general population/adults with acyanotic CHD. However, even in those with cyanotic CHD, coronary artery disease can develop when they have risk factors such as obesity, dyslipidemia, hypertension, diabetes mellitus, smoking habit, or limited exercise. The prevalence of risk factors for cardiovascular disease is similar between ACHD and the general population, but an increased risk of coronary atherosclerosis has been observed for congenital coronary artery anomalies, dextro-transposition of the great arteries after arterial switch operation, Ross procedure, and coarctation of the aorta. Aortopathy may be an additional risk factor for cardiovascular disease. As ACHD have other abnormalities that may make the heart more vulnerable to both the development of atherosclerosis and adverse cardiovascular sequelae, regular evaluation of their cardiovascular disease risk status is recommended. Metabolic syndrome is more common among ACHD than in the general population, and may therefore increase the future incidence of atherosclerotic coronary artery disease even in ACHD. Thus, ACHD should be screened for metabolic syndrome to eliminate risk factors for atherosclerotic coronary artery disease.

Quantification of Retinal Vessel Myogenic Constriction in Response to Blood Pressure Peaks: Implications for Flicker Light-Induced Dilatation

Background/Aims

Flicker-light induced retinal vessel dilatation (FID), a marker of microvascular endothelial function, has been shown to be blunted in sedentary cardiovascular risk patients (SR) as well as healthy physically active individuals (HA). This study aimed to quantify the retinal myogenic response to blood pressure (BP) peaks and its effects on consecutive FID for differentiation of microvascular health.

Methods

Ten HA and eleven SR with a previously established restriction of arteriolar FID (aFID) (<2.2%) were invited in order to assess BP-induced myogenic constriction following a standardized handgrip task and a consecutive FID. BP was measured beat-to-beat.

Results

The complete dataset of nine HA (3 female, mean age 65 years) and nine SR (5 female, mean age 61 years) individuals was analyzed. The central retinal arteriolar diameter equivalent (CRAE) was 183 ± 11 μm for HA and 176 ± 20 μm for SR. Initial baseline aFID was 1.6 ± 0.4% in HA and 1.6 ± 0.7% in SR. Systolic (p = 0.334) and diastolic (p = 0.245) BP increase following the handgrip task was in the range of 20–30% and comparable in both groups. BP increase was followed by a significantly higher arteriolar (−2.9 ± 1.3% vs. −1.3 ± 0.6%, p < 0.01) myogenic constriction in HA compared to SR. Moreover, in the consecutive assessment of FID directly after the BP-induced vessel constriction, aFID (4.1 ± 2.0% vs. 1.6 ± 0.9%, p < 0.01) was higher in HA compared to SR.

Conclusion

Initial baseline aFID was blunted in HA and SR. Retinal myogenic constriction was impaired in SR compared to HA. The consecutive aFID after BP-induced myogenic constriction recovered in HA but remained blunted in SR. Additional assessment of retinal myogenic constriction needs to be considered to improve CV risk stratification and reduce false-positive findings of endothelial dysfunction in otherwise healthy active individuals.

Clinical Trial Registration

ClinicalTrials.gov: NCT03986892 (https://clinicaltrials.gov/ct2/show/NCT03986892).

Altered Retinal Hemodynamics and Mean Circulation Time in Spontaneously Hypertensive Rats

Purpose

Although it is known that the retinal arteriolar vasculature is constricted in hypertension, the details of retinal hemodynamics and perfusion of the retinal circulation have yet to be adequately characterized.

Methods

Male and female spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls were anesthetized before measurements of mean arterial blood pressure and preparation for intravital microscopy of the retinal microcirculation. Retinal vascular velocities were measured with the use of fluorescent microspheres, and diameters and mean circulation times were measured after the infusion of fluorescent dextran. Arteriolar and venular shear rates were calculated from the ratio of velocity to diameter.

Results

In the retinas of SHR, velocities were elevated (compared with control WKY) in arterioles, but not in venules. Both arteriolar and venular diameters were significantly smaller in SHR versus WKY, with substantial increases in shear rates. Despite a tendency toward lower retinal blood flow rates, the mean circulation time through the SHR retina was much faster than can be explained by the measured arteriolar and venular velocities.

Conclusions

The pattern of hypertension-induced increases in blood velocity, dissipating from the arteriolar to venular side of the retinal circulation, indicates a potential transfer of the extra kinetic energy through the vasculature. The combination of elevated velocities through narrower retinal arterioles resulted in a markedly higher level of wall shear rate that may induce changes in the vessel wall. Finally, significantly more rapid transits through the hypertensive retina could be a result of altered blood flow distribution.

Metabolic Syndrome in Adult Congenital Heart Disease

In adult congenital heart disease (ACHD), residua and sequellae after initial repair develop late complications such as cardiac failure, arrhythmias, thrombosis, aortopathy, pulmonary hypertension and others. Acquired lesions with aging such as hypertension, diabetes mellitus, obesity can be negative influence on original cardiovascular disease (CVD). Also, atherosclerosis may pose an additional health problem to ACHD when they grow older and reach the age at which atherosclerosis becomes clinically relevant. In spite of the theoretical risk of atherosclerosis in ACHD due to above mentioned factors, cyanotic ACHDs even after repair are noted to have minimal incidence of coronary artery disease (CAD). Acyanotic ACHD has similar prevalence of CAD as the general population. However, even in cyanotic ACHD, CAD can develop when they have several risk factors for CAD. The prevalence of risk factor is similar between ACHD and the general population. Risk of premature atherosclerotic CVD in ACHD is based, 3 principal mechanisms: lesions with coronary artery abnormalities, obstructive lesions of left ventricle and aorta such as coarctation of the aorta and aortopathy. Coronary artery abnormalities are directly affected or altered surgically, such as arterial switch in transposition patients, may confer greater risk for premature atherosclerotic CAD. Metabolic syndrome is more common among ACHD than in the general population, and possibly increases the incidence of atherosclerotic CAD even in ACHD in future. Thus, ACHD should be screened for metabolic syndrome and eliminating risk factors for atherosclerotic CAD.

Inflammatory monocyte response due to altered wall shear stress in an isolated femoral artery model

Arteriogenesis (collateral formation) is accompanied by a pro-inflammatory state that may be related to the wall shear stress (WSS) within the neo-collateral vessels. Examining the pro-inflammatory component in situ or in vivo is complex. In an ex vivo mouse femoral artery perfusion model, we examined the effect of wall shear stress on pro-arteriogenic inflammatory markers and monocyte adhesion. In a femoral artery model with defined pulsatile flow, WSS was controlled (at physiological stress, 1.4×, and 2× physiological stress) during a 24 h perfusion before gene expression levels and monocyte adhesion were assessed. Significant upregulation of expression was found for the cytokine TNFα, adhesion molecule ICAM-1, growth factor TGFβ, and the transcription factor Egr-1 at varying levels of increased WSS compared to physiological control. Further, trends toward upregulation were found for FGF-2, the cytokine MCP-1 and adhesion molecules VCAM-1 and P-selectin with increased WSS. Finally, monocytes adhesion increased in response to increased WSS. We have developed a murine femoral artery model for studying changes in WSS ex vivo and show that the artery responds by upregulating inflammatory cytokines, adhesion molecules and growth factors consistent with previous in vivo findings.

Inositol 1,4,5-Trisphosphate Receptors in Hypertension

Chronic hypertension remains a major cause of global mortality and morbidity. It is a complex disease that is the clinical manifestation of multiple genetic, environmental, nutritional, hormonal, and aging-related disorders. Evidence supports a role for vascular aging in the development of hypertension involving an impairment in endothelial function together with an alteration in vascular smooth muscle cells (VSMCs) calcium homeostasis leading to increased myogenic tone. Changes in free intracellular calcium levels ([Ca²⁺] _i ) are mediated either by the influx of Ca²⁺ from the extracellular space or release of Ca²⁺ from intracellular stores, mainly the sarcoplasmic reticulum (SR). The influx of extracellular Ca²⁺ occurs primarily through voltage-gated Ca²⁺ channels (VGCCs), store-operated Ca²⁺ channels (SOC), and Ca²⁺ release-activated channels (CRAC), whereas SR-Ca²⁺ release occurs through inositol trisphosphate receptor (IP₃R) and ryanodine receptors (RyRs). IP₃R-mediated SR-Ca²⁺ release, in the form of Ca²⁺ waves, not only contributes to VSMC contraction and regulates VGCC function but is also intimately involved in structural remodeling of resistance arteries in hypertension. This involves a phenotypic switch of VSMCs as well as an alteration of cytoplasmic Ca²⁺ signaling machinery, a phenomena tightly related to the aging process. Several lines of evidence implicate changes in expression/function levels of IP₃R isoforms in the development of hypertension, VSMC phenotypic switch, and vascular aging. The present review discusses the current knowledge of these mechanisms in an integrative approach and further suggests potential new targets for hypertension management and treatment.

Protein tyrosine phosphatase 1B inactivation limits aging-associated heart failure in mice

We have previously shown that protein tyrosine phosphatase 1B (PTP1B) inactivation in mice [PTP1B-deficient (PTP1B−/−) mice] improves left ventricular (LV) angiogenesis, perfusion, remodeling, and function and limits endothelial dysfunction after myocardial infarction. However, whether PTP1B inactivation slows aging-associated cardiovascular dysfunction remains unknown. Wild-type (WT) and PTP1B−/− mice were allowed to age until 18 mo. Compared with old WT mice, in which aging increased the LV mRNA expression of PTP1B, old PTP1B−/− mice had 1) reduced cardiac hypertrophy with decreased LV mRNA levels of hypertrophic markers and atrial and brain natriuretic peptides, 2) lower LV fibrosis (collagen: 16 ± 3% in WT mice and 5 ± 3% in PTP1B−/− mice, P < 0.001) with decreased mRNA levels of transforming growth-factor-β1 and matrix metalloproteinase-2, and 3) higher LV capillary density and lower LV mRNA level of hypoxic inducible factor-1α, which was associated over time with a higher rate of proangiogenic M2 type macrophages and a stable LV mRNA level of VEGF receptor-2. Echocardiography revealed an age-dependent LV increase in end-diastolic volume in WT mice together with alterations of fractional shortening and diastole (transmitral Doppler E-to-A wave ratio). Invasive hemodynamics showed better LV systolic contractility and better diastolic compliance in old PTP1B−/− mice (LV end-systolic pressure-volume relation: 13.9 ± 0.9 in WT mice and 18.4 ± 1.6 in PTP1B−/− mice; LV end-diastolic pressure-volume relation: 5.1 ± 0.8 mmHg/relative volume unit in WT mice and 1.2 ± 0.3 mmHg/relative volume unit in PTP1B−/− mice, P < 0.05). In addition, old PTP1B−/− mice displayed a reduced amount of LV reactive oxygen species. Finally, in isolated resistance mesenteric arteries, PTP1B inactivation reduced aging-associated endothelial dysfunction (flow-mediated dilatation: −0.4 ± 2.1% in WT mice and 8.2 ± 2.8% in PTP1B−/− mice, P < 0.05). We conclude that PTP1B inactivation slows aging-associated LV remodeling and dysfunction and reduces endothelial dysfunction in mesenteric arteries.

NEW & NOTEWORTHY The present study shows that protein tyrosine phosphatase 1B inactivation in aged mice improves left ventricular systolic and diastolic function associated with reduced adverse cardiac remodeling (hypertrophy, fibrosis, and capillary rarefaction) and limits vascular endothelial dysfunction. This suggests that protein tyrosine phosphatase 1B inhibition could be an interesting treatment approach in age-related cardiovascular dysfunction.

Dynamic remodeling of arteriolar collaterals after acute occlusion in chick chorioallantoic membrane

OBJECTIVE

After arteriolar occlusion, collaterals enlarge and initially elevated WSS normalizes. While most previous studies focused on endpoints of such adaptive changes in larger collaterals, the present investigation aimed to continuously determine the relation between WSS and diameter in microvascular collaterals during adaptive reactions.

METHODS

In Hamburger-Hamilton stage 40 CAMs, junction points between arteriolar segments were identified and the third upstream segment on one side was occluded. Intravital microscopy recordings were taken for 24 hours post-occlusion. Segment diameter and blood velocity were measured: WSS and capillary density were calculated.

RESULTS

After occlusion, vascular diameters exhibited an immediate decrease, then increased with a time constant of 2.5 ± 0.8 hours and reached a plateau of up to 60% above baseline after about 7 hours. Vascular tone showed no significant change. WSS exhibited an immediate increase post-occlusion and linearly returned to baseline after about 12 hours. Local WSS change and diameter change rate showed similar patterns during the initial but not the later phase of post-occlusive adaptation.

CONCLUSIONS

CAM collaterals undergo fast structural remodeling within 24 hours post-occlusion. This remodeling might be driven by local WSS and by other regulators within the vascular network.

Role of endothelial nitric oxide in control of peripheral vascular conductance during muscle metaboreflex activation

The muscle metaboreflex is a powerful pressor reflex induced by the activation of chemically sensitive muscle afferents as a result of metabolite accumulation. During submaximal dynamic exercise, the rise in arterial pressure is primarily due to increases in cardiac output, since there is little systemic vasoconstriction. Indeed, in normal animals, we have often shown a small, but significant, peripheral vasodilation during metaboreflex activation, which is mediated, at least in part, by release of epinephrine and activation of vascular β₂-receptors. We tested whether this vasodilation is in part due to increased release of nitric oxide caused by the rise in cardiac output eliciting endothelium-dependent flow-mediated vasodilation. The muscle metaboreflex was activated via graded reductions in hindlimb blood flow during mild exercise with and without nitric oxide synthesis blockade [N^G-nitro-l-arginine methyl ester (l-NAME); 5 mg/kg]. We assessed the role of increased cardiac output in mediating peripheral vasodilation via the slope of the relationship between the rise in nonischemic vascular conductance (conductance of all vascular beds excluding hindlimbs) vs. the rise in cardiac output. l-NAME increased mean arterial pressure at rest and during exercise. The metaboreflex-induced increases in mean arterial pressure were unaltered by l-NAME, whereas the increases in cardiac output and nonischemic vascular conductance were attenuated. However, the slope of the relationship between nonischemic vascular conductance and cardiac output was not affected by l-NAME, indicating that the rise in cardiac output did not elicit vasodilation via increased release of nitric oxide. Thus, although nitric oxide is intrinsic to the vascular tonus, endothelial-dependent flow-mediated vasodilation plays little role in the small peripheral vasodilation observed during muscle metaboreflex activation.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Vascular Adaptation to Exercise in Humans: Role of Hemodynamic Stimuli

On the 400th anniversary of Harvey's Lumleian lectures, this review focuses on "hemodynamic" forces associated with the movement of blood through arteries in humans and the functional and structural adaptations that result from repeated episodic exposure to such stimuli. The late 20th century discovery that endothelial cells modify arterial tone via paracrine transduction provoked studies exploring the direct mechanical effects of blood flow and pressure on vascular function and adaptation in vivo. In this review, we address the impact of distinct hemodynamic signals that occur in response to exercise, the interrelationships between these signals, the nature of the adaptive responses that manifest under different physiological conditions, and the implications for human health. Exercise modifies blood flow, luminal shear stress, arterial pressure, and tangential wall stress, all of which can transduce changes in arterial function, diameter, and wall thickness. There are important clinical implications of the adaptation that occurs as a consequence of repeated hemodynamic stimulation associated with exercise training in humans, including impacts on atherosclerotic risk in conduit arteries, the control of blood pressure in resistance vessels, oxygen delivery and diffusion, and microvascular health. Exercise training studies have demonstrated that direct hemodynamic impacts on the health of the artery wall contribute to the well-established decrease in cardiovascular risk attributed to physical activity.

Glycocalyx Degradation Induces a Proinflammatory Phenotype and Increased Leukocyte Adhesion in Cultured Endothelial Cells under Flow

Leukocyte adhesion to the endothelium is an early step in the pathogenesis of atherosclerosis. Effective adhesion requires the binding of leukocytes to their cognate receptors on the surface of endothelial cells. The glycocalyx covers the surface of endothelial cells and is important in the mechanotransduction of shear stress. This study aimed to identify the molecular mechanisms underlying the role of the glycocalyx in leukocyte adhesion under flow. We performed experiments using 3-D cell culture models, exposing human abdominal aortic endothelial cells to steady laminar shear stress (10 dynes/cm2 for 24 hours). We found that with the enzymatic degradation of the glycocalyx, endothelial cells developed a proinflammatory phenotype when exposed to uniform steady shear stress leading to an increase in leukocyte adhesion. Our results show an up-regulation of ICAM-1 with degradation compared to non-degraded controls (3-fold increase, p<0.05) and we attribute this effect to a de-regulation in NF-κB activity in response to flow. These results suggest that the glycocalyx is not solely a physical barrier to adhesion but rather plays an important role in governing the phenotype of endothelial cells, a key determinant in leukocyte adhesion. We provide evidence for how the destabilization of this structure may be an early and defining feature in the initiation of atherosclerosis.

Age-dependent impact of CaV 3.2 T-type calcium channel deletion on myogenic tone and flow-mediated vasodilatation in small arteries

KEY POINTS

Blood pressure and flow exert mechanical forces on the walls of small arteries, which are detected by the endothelial and smooth muscle cells, and lead to regulation of the diameter (basal tone) of an artery. CaV 3.2 T-type calcium channels are expressed in the wall of small arteries, although their function remains poorly understood because of the low specificity of T-type blockers. We used mice deficient in CaV 3.2 channels to study their role in pressure- and flow-dependent tone regulation and the possible impact of ageing on this role. In young mice, CaV 3.2 channels oppose pressure-induced vasoconstriction and participate in endothelium-dependent, flow-mediated dilatation. These effects were not seen in mature adult mice. The results of the present study demonstrate an age-dependent impact of CaV 3.2 T-type calcium channel deletion in rodents and suggest that the loss of CaV 3.2 channel function leads to more constricted arteries, which is a risk factor for cardiovascular disease.

ABSTRACT

The myogenic response and flow-mediated vasodilatation are important regulators of local blood perfusion and total peripheral resistance, and are known to entail a calcium influx into vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), respectively. CaV 3.2 T-type calcium channels are expressed in both VSMCs and ECs of small arteries. The T-type channels are important drug targets but, as a result of the lack of specific antagonists, our understanding of the role of CaV 3.2 channels in vasomotor tone at various ages is scarce. We evaluated the myogenic response, flow-mediated vasodilatation, structural remodelling and mRNA + protein expression in small mesenteric arteries from CaV 3.2 knockout (CaV 3.2KO) vs. wild-type mice at a young vs. mature adult age. In young mice only, deletion of CaV 3.2 led to an enhanced myogenic response and a ∼50% reduction of flow-mediated vasodilatation. Ni2+ had both CaV 3.2-dependent and independent effects. No changes in mRNA expression of several important K+ and Ca2+ channel genes were induced by CaV 3.2KO However, the expression of the other T-type channel isoform (CaV 3.1) was reduced at the mRNA and protein level in mature adult compared to young wild-type arteries. The results of the present study demonstrate the important roles of the CaV 3.2 T-type calcium channels in myogenic tone and flow-mediated vasodilatation that disappear with ageing. Because increased arterial tone is a risk factor for cardiovascular disease, we conclude that CaV 3.2 channels, by modulating pressure- and flow-mediated vasomotor responses to prevent excess arterial tone, protect against cardiovascular disease.

The Beta-1-Receptor Blocker Nebivolol Elicits Dilation of Cerebral Arteries by Reducing Smooth Muscle [Ca2+]i

Rationale

Nebivolol is known to have beta-1 blocker activity, but it was also suggested that it elicits relaxation of the peripheral arteries in part via release of nitric oxide (NO). However, the effect of nebivolol on the vasomotor tone of cerebral arteries is still unclear.

Objective

To assess the effects of nebivolol on the diameter of isolated rat basilar arteries (BA) in control, in the presence of inhibitors of vasomotor signaling pathways of know action and hemolysed blood.

Methods and Results

Vasomotor responses were measured by videomicroscopy and the intracellular Ca²⁺ by the Fura-2 AM ratiometric method. Under control conditions, nebivolol elicited a substantial dilation of the BA (from 216±22 to 394±20 μm; p<0.05) in a concentration-dependent manner (10⁻⁷ to 10⁻⁴ M). The dilatation was significantly reduced by endothelium denudation or by L-NAME (inhibitor of NO synthase) or by SQ22536 (adenylyl cyclase blocker). Dilatation of BA was also affected by beta-2 receptor blockade with butoxamine, but not by the guanylate cyclase blocker ODQ. Interestingly, beta-1 blockade by atenolol inhibited nebivolol-induced dilation. Also, the BK_Ca channel blocker iberiotoxin and K_Ca channel inhibitor TEA significantly reduced nebivolol-induced dilation. Nebivolol significantly reduced smooth muscle Ca²⁺ level, which correlated with the increases in diameters and moreover it reversed the hemolysed blood-induced constriction of BA.

Conclusions

Nebivolol seems to have an important dilator effect in cerebral arteries, which is mediated via several vasomotor mechanisms, converging on the reduction of smooth muscle Ca²⁺ levels. As such, nebivolol may be effective to improve cerebral circulation in various diseased conditions, such as hemorrhage.

Vascular flow reserve as a link between long-term blood pressure level and physical performance capacity in mammals

Mean arterial pressure (MAP) is surprisingly similar across different species of mammals, and it is, in general, not known which factors determine the arterial pressure level. Mammals often have a pronounced capacity for sustained physical performance. This capacity depends on the vasculature having a flow reserve that comes into play as tissue metabolism increases. We hypothesize that microvascular properties allowing for a large vascular flow reserve is linked to the level of the arterial pressure.To study the interaction between network properties and network inlet pressure, we developed a generic and parsimonious computational model of a bifurcating microvascular network where diameter and growth of each vessel evolves in response to changes in biomechanical stresses. During a simulation, the network develops well-defined arterial and venous vessel characteristics. A change in endothelial function producing a high precapillary resistance and thus a high vascular flow reserve is associated with an increase in network inlet pressure. Assuming that network properties are independent of body mass, and that inlet pressure of the microvascular network is a proxy for arterial pressure, the study provides a conceptual explanation of why high performing animals tend to have a high MAP.

Heme Oxygenases in Cardiovascular Health and Disease

Heme oxygenases are composed of two isozymes, Hmox1 and Hmox2, that catalyze the degradation of heme to carbon monoxide (CO), ferrous iron, and biliverdin, the latter of which is subsequently converted to bilirubin. While initially considered to be waste products, CO and biliverdin/bilirubin have been shown over the last 20 years to modulate key cellular processes, such as inflammation, cell proliferation, and apoptosis, as well as antioxidant defense. This shift in paradigm has led to the importance of heme oxygenases and their products in cell physiology now being well accepted. The identification of the two human cases thus far of heme oxygenase deficiency and the generation of mice deficient in Hmox1 or Hmox2 have reiterated a role for these enzymes in both normal cell function and disease pathogenesis, especially in the context of cardiovascular disease. This review covers the current knowledge on the function of both Hmox1 and Hmox2 at both a cellular and tissue level in the cardiovascular system. Initially, the roles of heme oxygenases in vascular health and the regulation of processes central to vascular diseases are outlined, followed by an evaluation of the role(s) of Hmox1 and Hmox2 in various diseases such as atherosclerosis, intimal hyperplasia, myocardial infarction, and angiogenesis. Finally, the therapeutic potential of heme oxygenases and their products are examined in a cardiovascular disease context, with a focus on how the knowledge we have gained on these enzymes may be capitalized in future clinical studies.

Enhancement of arterial pulsation during flow-mediated dilation is impaired in the presence of ischemic heart disease

Purpose

The aim of this study is to investigate the relationship between arterial pulse amplitude change under increased shear stress and the presence of ischemic heart disease (IHD).

Methods

This study comprised 31 subjects, including 14 subjects with IHD. We investigated the change in brachial artery pulse amplitude during flow-mediated dilation (FMD) using ultrasonography.

Results

The arterial pulse amplitude increased during FMD in 19 subjects, whereas it decreased in 12 subjects. There was a marked difference in the change in arterial pulse amplitude (the maximum amplitude of the arterial pulse amplitude during FMD/the arterial pulse amplitude at baseline) between subjects with and without IHD (0.98 ± 0.53 and 1.37 ± 0.53, p = 0.028). Furthermore, decreased arterial pulse amplitude during FMD was a significant predictor of IHD after adjustment of age, blood pressure, the presence of each type of coronary risks, the value of FMD and sex (p = 0.0001).

Conclusions

The decrease of arterial pulsation amplitude during FMD was a useful predictive parameter for IHD.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-2794-0) contains supplementary material, which is available to authorized users.

Acquired Von Willebrand Factor Abnormalities in Adults with Congenital Heart Disease: Dependence Upon Cardiopulmonary Pathophysiological Subtype

Hemostasis is often abnormal in adults with con genital heart disease, and von Willebrand factor abnormalities have been reported in this patient population. We sought to determine the prevalence, type, and severity of the von Wil lebrand factor abnormality, and its relationship to three patho physiological variables; cyanosis, pulmonary vascular disease, and turbulent blood flow. This prospective study comprised 76 unoperated congenital heart disease patients aged 20 to 68 years (mean = 34 years). There were 44 cyanotic and 32 acyanotic patients. Twenty-seven cyanotic and 6 acyanotic pa tients had pulmonary vascular disease, 31 cyanotic and 16 acyanotic patients had turbulent blood flow, and 11 patients were acyanotic without pulmonary vascular disease or turbu lent flow. The largest plasma von Willebrand factor multimers were relatively decreased or absent in 77% of cyanotic versus 41 % of acyanotic patients ( p < .001); in 76% of patients with, versus 51 % without, pulmonary vascular disease ( p < .029); and in 72% of patients with, versus 45% without, turbulent flow ( p <.016). Von Willebrand factor multimers were normal in all 11 acyanotic patients without pulmonary vascular disease or turbulent blood flow. Von Willebrand factor multimer ab normalities normalized after reparative surgery in five patients. Depletion of the largest plasma von Willebrand factor multim ers is common in adults with congenital heart disease. Cyano sis, pulmonary vascular disease, and turbulent flow are deter minants of the abnormality that is acquired and reversible.

Attenuated flow-induced dilatation of middle cerebral arteries is related to increased vascular oxidative stress in rats on a short-term high salt diet

KEY POINTS

Recent studies have shown that high salt (HS) intake leads to endothelial dysfunction and impaired vascular reactivity in different vascular beds in both animal and human models, due to increased oxidative stress. The objective of this study was to assess vascular response to flow-induced dilatation (FID) and to elucidate the role of vascular oxidative stress/antioxidative capacity in middle cerebral arteries (MCAs) of HS-fed rats in vitro. The novelty of this study is in demonstrating impaired flow-induced dilatation of MCAs and down-regulation of vascular antioxidant genes with HS intake, leading to increased levels of oxidative stress in blood vessels and peripheral lymph organs, which together contribute to impaired FID. In addition, results show increased oxidative stress in leukocytes of peripheral lymph organs, suggesting the occurrence of inflammatory processes due to HS intake. Recirculation of leukocytes might additionally increase vascular oxidative stress in vivo.

ABSTRACT

The aim of this study was to determine flow-induced dilatation (FID) and the role of oxidative stress/antioxidative capacity in isolated, pressurized middle cerebral arteries (MCAs) of high salt (HS)-fed rats. Healthy male Sprague-Dawley rats (11 weeks old) were fed low salt (0.4% NaCl; LS group) or high salt (4% NaCl; HS group) diets for 1 week. Reactivity of MCAs in response to stepwise increases in pressure gradient (Δ10-Δ100 mmHg) was determined in the absence or presence of the superoxide dismutase (SOD) mimetic TEMPOL and/or the nitric oxide synthases (NOS) inhibitor N(ω) -nitro-l-arginine methyl ester (l-NAME). mRNA levels of antioxidative enzymes, NAPDH-oxidase components, inducible (iNOS) and endothelial nitric oxide synthases (eNOS) were determined by quantitative real-time PCR. Blood pressure (BP), antioxidant enzymes activity, oxidative stress in peripheral leukocytes, lipid peroxidation products and the antioxidant capacity of plasma were measured for both groups. FID was reduced in the HS group compared to the LS group. The presence of TEMPOL restored dilatation in the HS group, with no effect in the LS group. Expression of glutathione peroxidase 4 (GPx4) and iNOS in the HS group was significantly decreased; oxidative stress was significantly higher in the HS group compared to the LS group. HS intake significantly induced basal reactive oxygen species production in the leukocytes of mesenteric lymph nodes and splenocytes, and intracellular production after stimulation in peripheral lymph nodes. Antioxidant enzyme activity and BP were not affected by HS diet. Low GPx4 expression, increased superoxide production in leukocytes, and decreased iNOS expression are likely to underlie increased oxidative stress and reduced nitric oxide bioavailability, leading to impairment of FID in the HS group without changes in BP values.

Black Raspberry Extract Increased Circulating Endothelial Progenitor Cells and Improved Arterial Stiffness in Patients with Metabolic Syndrome: A Randomized Controlled Trial

Administration of black raspberry (Rubus occidentalis) is known to improve vascular endothelial function in patients at a high risk for cardiovascular (CV) disease. We investigated short-term effects of black raspberry on circulating endothelial progenitor cells (EPCs) and arterial stiffness in patients with metabolic syndrome. Patients with metabolic syndrome (n = 51) were prospectively randomized into the black raspberry group (n = 26, 750 mg/day) and placebo group (n = 25) during the 12-week follow-up. Central blood pressure, augmentation index, and EPCs, such as CD34/KDR(+), CD34/CD117(+), and CD34/CD133(+), were measured at baseline and at 12-week follow-up. Radial augmentation indexes were significantly decreased in the black raspberry group compared to the placebo group (-5% ± 10% vs. 3% ± 14%, P < .05). CD34/CD133(+) cells at 12-week follow-up were significantly higher in the black raspberry group compared to the placebo group (19 ± 109/μL vs. -28 ± 57/μL, P < .05). Decreases from the baseline in interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) were significantly greater in the black raspberry group compared to the placebo group (-0.5 ± 1.4 pg/mL vs. -0.1 ± 1.1 pg/mL, P < .05 and -5.4 ± 4.5 pg/mL vs. -0.8 ± 4.0 pg/mL, P < .05, respectively). Increases from the baseline in adiponectin levels (2.9 ± 2.1 μg/mL vs. -0.2 ± 2.5 μg/mL, P < .05) were significant in the black raspberry group. The use of black raspberry significantly lowered the augmentation index and increased circulating EPCs, thereby improving CV risks in patients with metabolic syndrome during the 12-week follow-up.

Mathematical model for shear stress dependent NO and adenine nucleotide production from endothelial cells

We developed a mass transport model for a parallel-plate flow chamber apparatus to predict the concentrations of nitric oxide (NO) and adenine nucleotides (ATP, ADP) produced by cultured endothelial cells (ECs) and investigated how the net rates of production, degradation, and mass transport for these three chemical species vary with changes in wall shear stress (τw). These simulations provide an improved understanding of experimental results obtained with parallel-plate flow chambers and allows quantitative analysis of the relationship between τw, adenine nucleotide concentrations, and NO produced by ECs. Experimental data obtained after altering ATP and ADP concentrations with apyrase were analyzed to quantify changes in the rate of NO production (RNO). The effects of different isoforms of apyrase on ATP and ADP concentrations and nucleotide-dependent changes in RNO could be predicted with the model. A decrease in ATP was predicted with apyrase, but an increase in ADP was simulated due to degradation of ATP. We found that a simple proportional relationship relating a component of RNO to the sum of ATP and ADP provided a close match to the fitted curve for experimentally measured changes in RNO with apyrase. Estimates for the proportionality constant ranged from 0.0067 to 0.0321 μM/s increase in RNO per nM nucleotide concentration, depending on which isoform of apyrase was modeled, with the largest effect of nucleotides on RNO at low τw (<6 dyn/cm(2)).

The use of shear rate-diameter dose-response curves as an alternative to the flow-mediated dilation test

The brachial artery flow-mediated dilation test (FMD) is the non-invasive gold-standard used to test endothelial function. Reduced FMD precedes the development of atherosclerosis and provides an early marker for predicting future cardiovascular disease events. Although, this test is of high potential, it is somewhat limited by poor reproducibility. By utilizing hand warming and grip exercise combined with hierarchical linear modeling, shear rate-diameter dose-response curves may provide a novel and more accurate way to assess endothelial function in humans. Shear rate-diameter dose-response curves could potentially improve upon the traditional FMD measurement and serve as a superior clinical and research tool for assessing cardiovascular disease risk in a variety of populations. The current paper presents testable hypotheses and methodology for assessing the validity and reliability of an alternative to the current FMD test.

Hydrogen peroxide mediates endothelium-dependent dilation of coronary arterioles in obese rats on a low-carbohydrate diet

OBJECTIVE

Endothelium-dependent vasodilation of coronary arterioles is impaired in obese rats and may be improved by a LCD. The aim of this study is to elucidate the mechanism by which this improvement occurs.

METHODS

We used four groups of male Zucker rats: lean and obese on either SD or LCD. Coronary arterioles were cannulated and pressurized for diameter measurements during administration of acetylcholine or sodium nitroprusside or during flow. Real-time PCR was performed to quantify mRNA expression of CuZnSOD and catalase.

RESULTS

The LCD significantly increased endothelium-dependent dilation in the obese rats. l-NAME and indomethacin reduced responses to flow and acetylcholine in the lean rats without any effect on the obese on either diet. In contrast, TEA and catalase blocked flow-dependent and acetylcholine-induced dilation in the obese on either diet, while no effect was observed on the lean. The LCD in the obese significantly up-regulated catalase mRNA expression and slightly increased CuZnSOD mRNA levels.

CONCLUSIONS

A LCD improves endothelium-dependent vasodilation of coronary arterioles in obese rats through the production of H2 O2 which acts as a hyperpolarizing factor, independent of nitric oxide and PGI2 .

Peroxynitrite disrupts endothelial caveolae leading to eNOS uncoupling and diminished flow-mediated dilation in coronary arterioles of diabetic patients

Peroxynitrite (ONOO(-)) contributes to coronary microvascular dysfunction in diabetes mellitus (DM). We hypothesized that in DM, ONOO(-) interferes with the function of coronary endothelial caveolae, which plays an important role in nitric oxide (NO)-dependent vasomotor regulation. Flow-mediated dilation (FMD) of coronary arterioles was investigated in DM (n = 41) and non-DM (n = 37) patients undergoing heart surgery. NO-mediated coronary FMD was significantly reduced in DM patients, which was restored by ONOO(-) scavenger, iron-(III)-tetrakis(N-methyl-4'pyridyl)porphyrin-pentachloride, or uric acid, whereas exogenous ONOO(-) reduced FMD in non-DM subjects. Immunoelectron microscopy demonstrated an increased 3-nitrotyrosine formation (ONOO(-)-specific protein nitration) in endothelial plasma membrane in DM, which colocalized with caveolin-1 (Cav-1), the key structural protein of caveolae. The membrane-localized Cav-1 was significantly reduced in DM and also in high glucose-exposed coronary endothelial cells. We also found that DM patients exhibited a decreased number of endothelial caveolae, whereas exogenous ONOO(-) reduced caveolae number. Correspondingly, pharmacological (methyl-β-cyclodextrin) or genetic disruption of caveolae (Cav-1 knockout mice) abolished coronary FMD, which was rescued by sepiapterin, the stable precursor of NO synthase (NOS) cofactor, tetrahydrobiopterin. Sepiapterin also restored coronary FMD in DM patients. Thus, we propose that ONOO(-) selectively targets and disrupts endothelial caveolae, which contributes to NOS uncoupling, and, hence, reduced NO-mediated coronary vasodilation in DM patients.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Role of endogenous hydrogen peroxide during angiotensin type 1 receptor blockers administration in pacing-induced metabolic coronary vasodilatation in dogs in vivo

BACKGROUND

We have previously demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor (EDHF) in canine coronary microcirculation in vivo. However, the role of H2O2/EDHF during angiotensin type 1 receptor blockers (ARB) administration in metabolic coronary dilatation in vivo remains to be examined. We examined whether H2O2 during ARB administration is involved in pacing-induced metabolic coronary vasodilatation in dogs in vivo and if so, whether such beneficial effects of ARB administration acutely improve coronary vasodilatation in diabetes mellitus (DM).

METHODS

Canine subepicardial coronary small arteries (CSA,≥ 100 μm) and arterioles (CA, <100 μm) in left anterior descending artery area were continuously observed by an intravital microscope under cyclooxygenase blockade(ibuprofen, 12.5 mg/kg, intravenous infusion, iv). Experiments were performed during paired right ventricular pacing under the following 4 conditions (n=5 each); (i) control, (ii) DM(alloxan 40 mg/ kg, iv, 1 week prior to study), (iii) DM+ARB(olmesartan, 10 μg/kg/min, 10 min, intracoronary infusion,ic)+L-NMMA (NOS inhibitor, 2 !mol/min, ic) and (iv)DM+ARB+catalase (H2O2 discomposer, 1000 U/ml, 5 min, ic).

RESULTS

Cardiac tachypacing (60 to 120 bpm) caused coronary vasodilatation in both-sized arteries under control conditions. DM significantly decreased the vasodilatation compared with control in CSA and there was a residual vasodilatation for the loss of NO in CA, whereas DM+ARB+L-NMMA improved the vasodilatation compared with DM alone in CA and was significantly decreased by DM+ARB+catalase in CA.

CONCLUSIONS

These results indicate that H2O2 during ARB administration is involved in pacing-induced metabolic coronary vasodilatation in DM in vivo and that there are substantial compensatory interactions between NO and H2O2.

The impact of crystalloidal and colloidal infusion preparations on coronary vascular integrity, interstitial oedema and cardiac performance in isolated hearts

INTRODUCTION

Recent data suggested an interaction between plasma constituents and the endothelial glycocalyx to be relevant for vascular barrier function. This might be negatively influenced by infusion solutions, depending on ionic composition, pH and binding properties. The present study evaluated such an influence of current artificial preparations.

METHODS

Isolated guinea pig hearts were prepared in a modified Langendorff mode and perfused with Krebs-Henseleit buffer augmented with 1g% human albumin. After equilibration the perfusion was switched to replacement of one half buffer by either isotonic saline (NaCl), ringer's acetate (Ri-Ac), 6% and 10% hydroxyethyl starch (6% and 10% HES, resp.), or 4% gelatine (Gel), the artificial colloids having been prepared in balanced solution. We analysed glycocalyx shedding, functional integrity of the vascular barrier and heart performance.

RESULTS

While glycocalyx shedding was not observed, diluting albumin concentration towards 0.5g% by artificial solutions was associated with a marked functional breakdown of vascular barrier competence. This effect was biggest with isotonic saline and significantly attenuated with artificial colloids, the difference in the pressure dependent transvascular fluid filtration (basal vs. during infusion in groups NaCl, Ri-Ac, 6% HES, 10% HES and Gel, n = 6 each) being 0.31 ± 0.03 vs. 1.00 ± 0.04; 0.27 ± 0.03 vs. 0.81 ± 0.03; 0.29 ± 0.03 vs. 0.68 ± 0.02; 0.32 ± 0.03 vs. 0.59 ± 0.08 and 0.31 ± 0.04 vs. 0.61 ± 0.03 g/5min, respectively. Heart performance was directly related to pH value (7.38 ± 0.06, 7.33 ± 0.03, 7.14 ± 0.04, 7.08 ± 0.04, 7.25 ± 0.03), the change in the rate pressure product being 21,702 ± 1969 vs. 21,291 ± 2,552; 22,098 ± 2,115 vs. 14,114 ± 3,386; 20,897 ± 2,083 vs. 10,671 ± 1,948; 21,822 ± 2,470 vs. 10,047 ± 2,320 and 20,955 ± 2,296 vs. 15,951 ± 2,755 mmHg × bpm, respectively.

CONCLUSIONS

It appears important to maintain the pH value within a physiological range to maintain optimal myocardial contractility. Using colloids prepared in calcium-containing, balanced solutions for volume replacement therapy may attenuate the breakdown of vascular barrier competence in the critically ill.

One-year results of a prospective, randomized trial comparing two machine perfusion devices used for kidney preservation

Studies have shown beneficial effects of machine perfusion (MP) on early kidney function and long-term graft survival. The aim of this study was to investigate whether the type of perfusion device could affect outcome of transplantation of deceased donor kidneys. A total of 50 kidneys retrieved from 25 donors were randomized to machine perfusion using a flow-driven (FD) device (RM3; Waters Medical Inc) or a pressure-driven (PD) device (LifePort; Organ Recovery Systems), 24 of these kidneys (n = 12 pairs; 48%) were procured from expanded criteria donors (ECD). The primary endpoints were kidney function after transplantation defined using the incidence of delayed graft function (DGF), the number of hemodialysis sessions required, graft function at 12 months, and analyses of biopsy. DGF was similar in both groups (32%; 8/25). Patients with DGF in the FD group required a mean of 4.66 hemodialysis sessions versus 2.65 in the PD group (P = 0.005). Overall, 1-year graft survival was 80% (20/25) vs. 96% (24/25) in the FD and PD groups. One-year graft survival of ECD kidneys was 66% (8/12) in the FD group versus 92% (11/12) in the PD group. Interstitial fibrosis and tubular atrophy were significantly more common in the FD group - 45% (5/11) vs. 0% (0/9) (P = 0.03) in PD group. There were no differences in creatinine levels between the groups. Machine perfusion using a pressure-driven device generating lower pulse stress is superior to a flow-driven device with higher pulse stress for preserving kidney function.

Contribution of nitric oxide to brachial artery vasodilation during progressive handgrip exercise in the elderly

The reduction in nitric oxide (NO)-mediated vascular function with age has largely been determined by flow-mediated dilation (FMD). However, in light of recent uncertainty surrounding the NO dependency of FMD and the recognition that brachial artery (BA) vasodilation during handgrip exercise is predominantly NO-mediated in the young, we sought to determine the contribution of NO to BA vasodilation in the elderly using the handgrip paradigm. BA vasodilation during progressive dynamic (1 Hz) handgrip exercise performed at 3, 6, 9, and 12 kg was assessed with and without NO synthase (NOS) inhibition [intra-arterial N(G)-monomethyl-l-arginine (l-NMMA)] in seven healthy older subjects (69 ± 2 yr). Handgrip exercise in the control condition evoked significant BA vasodilation at 6 (4.7 ± 1.4%), 9 (6.5 ± 2.2%), and 12 kg (9.5 ± 2.7%). NOS inhibition attenuated BA vasodilation, as the first measurable increase in BA diameter did not occur until 9 kg (4.0 ± 1.8%), and the change in BA diameter at 12 kg was reduced by ∼30% (5.1 ± 2.2%), with unaltered shear rate (

CONTROL

407 ± 57, l-NMMA: 427 ± 67 s(-1)). Although shifted downward, the slope of the relationship between BA diameter and shear rate during handgrip exercise was unchanged (

CONTROL

0.0013 ± 0.0004, l-NMMA: 0.0011 ± 0.007, P = 0.6) as a consequence of NOS inhibition. Thus, progressive handgrip exercise in the elderly evokes a robust BA vasodilation, the magnitude of which was only minimally attenuated following NOS inhibition. This modest contribution of NO to BA vasodilation in the elderly supports the use of the handgrip exercise paradigm to assess NO-dependent vasodilation across the life span.

Cardiac-like flow generator for long-term imaging of endothelial cell responses to circulatory pulsatile flow at microscale

In vitro models of circulatory hemodynamics are required to mimic the microcirculation for study of endothelial cell responses to pulsatile shear stress by live cell imaging. This study reports the design, fabrication and characterisation of a microfluidic device that generates cardiac-like flow in a continuous culture system with a circulatory volume of only 2-3 μL. The device mimics a single chamber heart, with the following cardiac phases: (1) closure of the ventricle inlet valve, (2) contraction of the ventricle (systole) followed by opening of the outlet valve and (3) relaxation of the ventricle (diastole) with opening of the inlet valve whilst the outlet valve remains closed. Periodic valve states and ventricular contractions were actuated by microprocessor controlled pneumatics. The time-dependent velocity-field was characterised by micro-particle image velocimetry (μ-PIV). μ-PIV observations were used to help tune electronic timing of valve states and ventricular contractions for synthesis of an arterial pulse waveform to study the effect of pulsatile shear stress on bovine artery endothelial cells (BAECs). BAECs elongated and aligned with the direction of shear stress after 48 h of exposure to a pulsatile waveform with a maximum shear stress of 0.42 Pa. The threshold for BAECs alignment and elongation under steady (non-pulsatile) flow reported by Kadohama et al. (2006) is 0.7-1.4 Pa. These cells respond to transient shear stress because the time average shear stress of the pulse waveform to generate this morphological response was only 0.09 Pa, well below the steady flow threshold. The microfluidic pulse generator can simulate circulatory hemodynamics for live cell imaging of shear-induced signalling pathways.

Postocclusive reactive hyperemia occurs in the rat retinal circulation but not in the choroid

PURPOSE

We tested the hypothesis that retinal blood flow has a postocclusive reactive hyperemia response modulated by occlusion duration and metabolic activity, and that choroidal blood flow does not.

METHODS

Anesthetized and paralyzed rats (n = 34) were studied. Retinal and choroidal blood flow was measured by laser speckle imaging and laser Doppler flowmetry, respectively. Blood oxygenation level-dependent functional magnetic resonance imaging (BOLD fMRI) was used to measure changes in relative blood oxygenation of the retinal and choroidal circulations. Transient carotid occlusion was elicited with a hydraulic occluder on the common carotid artery. Several occlusion durations were tested during dark, constant light, and flicker light conditions to modulate metabolic demand. The hyperemia response magnitude was quantified by integrating the area above the blood flow baseline for the 3 minutes after release of the occlusion.

RESULTS

Systemic arterial pressure (108.2 ± 1.4 mm Hg) was unaffected by the carotid occlusions, and was similar among animals and conditions. Retinal blood flow had a reactive hyperemia, but choroidal blood flow did not (e.g., 14 ± 2%.sec versus 0.5 ± 4%. sec after 60-second occlusion). The hyperemia magnitude increased as a nonlinear function of occlusion duration and reached a plateau at occlusion durations < 60 second. The hyperemia magnitude was not altered by different lighting conditions at occlusion durations of 15 and 60 seconds. BOLD fMRI results were similar to the laser-based blood flow measurements.

CONCLUSIONS

The results indicate that metabolic local control has a negligible role in choroidal blood flow regulation and only partially accounts for the blood flow behavior in the retinal circulation.

Biomechanical force in blood development: extrinsic physical cues drive pro-hematopoietic signaling

The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

Role of caveolae in shear stress-mediated endothelium-dependent dilation in coronary arteries

AIMS

Caveolae are membrane microdomains where important signalling pathways are assembled and molecular effects transduced. In this study, we hypothesized that shear stress-mediated vasodilation (SSD) of mouse small coronary arteries (MCA) is caveolae-dependent.

METHODS AND RESULTS

MCA (80-150 μm) isolated from wild-type (WT) and caveolin-1 null (Cav-1(-/-)) mice were subjected to physiological levels of shear stress (1-25 dynes/cm(2)) with and without pre-incubation of inhibitors of nitric oxide synthase (L-NAME), cyclooxygenase (indomethacin, INDO), or cytochrome P450 epoxygenase (SKF 525A). SSD was endothelium-dependent in WT and Cav-1(-/-) coronaries but that in Cav-1(-/-) was significantly diminished compared with WT. Pre-incubation with L-NAME, INDO, or SKF 525A significantly reduced SSD in WT but not in Cav-1(-/-) mice. Vessels from the soluble epoxide hydrolase null (Ephx2(-/-)) mice showed enhanced SSD, which was further augmented by the presence of arachidonic acid. In donor-detector-coupled vessel experiments, Cav-1(-/-) donor vessels produced diminished dilation in WT endothelium-denuded detector vessels compared with WT donor vessels. Shear stress elicited a robust intracellular Ca(2+) increase in vascular endothelial cells isolated from WT but not those from Cav-1(-/-) mice.

CONCLUSION

Integrity of caveolae is critical for endothelium-dependent SSD in MCA. Cav-1(-/-) endothelium is deficient in shear stress-mediated generation of vasodilators including NO, prostaglandins, and epoxyeicosatrienoic acids. Caveolae plays a critical role in endothelial signal transduction from shear stress to vasodilator production and release.

Cyanotic congenital heart disease the coronary arterial circulation

Background:

The coronary circulation in cyanotic congenital heart disease (CCHD) includes the extramural coronary arteries, basal coronary blood flow, flow reserve, the coronary microcirculation, and coronary atherogenesis.

Methods:

Coronary arteriograms were analyzed in 59 adults with CCHD. Dilated extramural coronaries were examined histologically in six patients. Basal coronary blood flow was determined with N-13 positron emission tomography in 14 patients and in 10 controls. Hyperemic flow was induced by intravenous dipyridamole pharmacologic stress. Immunostaining against SM alpha-actin permitted microcirculatory morphometric analysis. Non-fasting total cholesterols were retrieved in 279 patients divided into four groups: Group A---143 cyanotic unoperated, Group B---47 rendered acyanotic by reparative surgery, Group C---41 acyanotic unoperated, Group D---48 acyanotic before and after operation.

Results:

Extramural coronary arteries were mildly or moderately dilated to ectatic in 49/59 angiograms. Histologic examination disclosed loss of medial smooth muscle, increased medial collagen, and duplication of internal elastic lamina. Basal coronary flow was appreciably increased. Hyperemic flow was comparable to controls. Remodeling of the microcirculation was based upon coronary arteriolar length, volume and surface densities. Coronary atherosclerosis was absent in both the arteriograms and the necropsy specimens.

Conclusions:

Extramural coronary arteries in CCHD dilate in response to endothelial vasodilator substances supplemented by mural attenuation caused by medial abnormalities. Basal coronary flow was appreciably increased, but hyperemic flow was normal. Remodeling of the microcirculation was responsible for preservation of flow reserve. The coronaries were atheroma-free because of the salutory effects of hypocholesterolemia, hypoxemia, upregulated nitric oxide, low platelet counts, and hyperbilirubinrmia.