An endothelium-derived hyperpolarizing factor distinct from NO and prostacyclin is a major endothelium-dependent vasodilator in resistance vessels of wild-type and endothelial NO synthase knockout mice

Role of TRPV4 on vascular tone regulation in pathophysiological states

Vascular tone regulation is a key event in controlling blood flow in the body. Endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) help regulate the vascular tone. Abnormal vascular responsiveness to various stimuli, including constrictors and dilators, has been observed in pathophysiological states although EC and VSMC coordinate to maintain the exquisite balance between contraction and relaxation in vasculatures. Thus, investigating the mechanisms underlying vascular tone abnormality is very important in maintaining vascular health and treating vasculopathy. Increased intracellular free Ca2+ concentration ([Ca2+]i) is one of the major triggers initiating each EC and VSMC response. Transient receptor potential vanilloid family member 4 (TRPV4) is a Ca2+-permeable non-selective ion channel, which is activated by several stimuli, and is presented in both ECs and VSMCs. Therefore, TRPV4 plays an important role in vascular responses. Emerging evidence indicates the role of TRPV4 on the functions of ECs and VSMCs in various pathophysiological states, including hypertension, diabetes, and obesity. This review focused on the link between TRPV4 and the functions of ECs/VSMCs, particularly its role in vascular tone and responsiveness to vasoactive substances.

Moderate-Intensity Exercise Improves Mesenteric Arterial Function in Male UC Davis Type-2 Diabetes Mellitus (UCD-T2DM) Rats: A Shift in the Relative Importance of Endothelium-Derived Relaxing Factors (EDRF)

The beneficial cardiovascular effects of exercise are well documented, however the mechanisms by which exercise improves vascular function in diabetes are not fully understood. This study investigates whether there are (1) improvements in blood pressure and endothelium-dependent vasorelaxation (EDV) and (2) alterations in the relative contribution of endothelium-derived relaxing factors (EDRF) in modulating mesenteric arterial reactivity in male UC Davis type-2 diabetes mellitus (UCD-T2DM) rats, following an 8-week moderate-intensity exercise (MIE) intervention. EDV to acetylcholine (ACh) was measured before and after exposure to pharmacological inhibitors. Contractile responses to phenylephrine and myogenic tone were determined. The arterial expressions of endothelial nitric oxide (NO) synthase (eNOS), cyclooxygenase (COX), and calcium-activated potassium channel (K_Ca) channels were also measured. T2DM significantly impaired EDV, increased contractile responses and myogenic tone. The impairment of EDV was accompanied by elevated NO and COX importance, whereas the contribution of prostanoid- and NO-independent (endothelium-derived hyperpolarization, EDH) relaxation was not apparent compared to controls. MIE 1) enhanced EDV, while it reduced contractile responses, myogenic tone and systolic blood pressure (SBP), and 2) caused a shift away from a reliance on COX toward a greater reliance on EDH in diabetic arteries. We provide the first evidence of the beneficial effects of MIE via the altered importance of EDRF in mesenteric arterial relaxation in male UCD-T2DM rats.

Acid-sensing ion channel 1a activates IKCa/SKCa channels and contributes to endothelium-dependent dilation

Acid-sensing ion channel 1a (ASIC1a) belongs to a novel family of proton-gated cation channels that are permeable to both Na+ and Ca2+. ASIC1a is expressed in vascular smooth muscle and endothelial cells in a variety of vascular beds, yet little is known regarding the potential impact of ASIC1a to regulate local vascular reactivity. Our previous studies in rat mesenteric arteries suggest ASIC1a does not contribute to agonist-induced vasoconstriction but may mediate a vasodilatory response. The objective of the current study is to determine the role of ASIC1a in systemic vasodilatory responses by testing the hypothesis that the activation of endothelial ASIC1a mediates vasodilation of mesenteric resistance arteries through an endothelium-dependent hyperpolarization (EDH)-related pathway. The selective ASIC1a antagonist psalmotoxin 1 (PcTX1) largely attenuated the sustained vasodilatory response to acetylcholine (ACh) in isolated, pressurized mesenteric resistance arteries and ACh-mediated Ca2+ influx in freshly isolated mesenteric endothelial tubes. Similarly, basal tone was enhanced and ACh-induced vasodilation blunted in mesenteric arteries from Asic1a knockout mice. ASIC1a colocalizes with intermediate- and small-conductance Ca2+-activated K+ channels (IKCa and SKCa, respectively), and the IKCa/SKCa-sensitive component of the ACh-mediated vasodilation was blocked by ASIC1a inhibition. To determine the role of ASIC1a to activate IKCa/SKCa channels, we measured whole-cell K+ currents using the perforated-patch clamp technique in freshly isolated mesenteric endothelial cells. Inhibition of ASIC1a prevented ACh-induced activation of IKCa/SKCa channels. The ASIC1 agonist, α/β-MitTx, activated IKCa/SKCa channels and induced an IKCa/SKCa-dependent vasodilation. Together, the present study demonstrates that ASIC1a couples to IKCa/SKCa channels in mesenteric resistance arteries to mediate endothelium-dependent vasodilation.

17β-Estradiol Treatment Improves Acetylcholine-Induced Relaxation of Mesenteric Arteries in Ovariectomized UC Davis Type 2 Diabetes Mellitus Rats in Prediabetic State

We recently reported sex differences in mesenteric arterial function of the UC Davis type-2 diabetes mellitus (UCD-T2DM) rats as early as the prediabetic state. We reported that mesenteric arteries (MA) from prediabetic male rats exhibited a greater impairment compared to that in prediabetic females. However, when females became diabetic, they exhibited a greater vascular dysfunction than males. Thus, the aim of this study was to investigate whether the female sex hormone, estrogen preserves mesenteric arterial vasorelaxation in UCD-T2DM female rats at an early prediabetic state. Age-matched female Sprague Dawley and prediabetic (PD) UCD-T2DM rats were ovariectomized (OVX) and subcutaneously implanted with either placebo or 17β-estradiol (E_2, 1.5 mg) pellets for 45 days. We assessed the contribution of endothelium-derived relaxing factors (EDRF) to acetylcholine (ACh)-induced vasorelaxation, using pharmacological inhibitors. Responses to sodium nitroprusside (SNP) and phenylephrine (PE) were also measured. Additionally, metabolic parameters and expression of some targets associated with vascular and insulin signaling were determined. We demonstrated that the responses to ACh and SNP were severely impaired in the prediabetic state (PD OVX) rats, while E₂ treatment restored vasorelaxation in the PD OVX + E₂. Moreover, the responses to PE was significantly enhanced in MA of PD OVX groups, regardless of placebo or E₂ treatment. Overall, our data suggest that 1) the impairment of ACh responses in PD OVX rats may, in part, result from the elevated contractile responses to PE, loss of contribution of endothelium-dependent hyperpolarization (EDH) to vasorelaxation, and a decreased sensitivity of MA to nitric oxide (NO), and 2) the basis for the protective effects of E₂ may be partly attributed to the elevation of the NO contribution to vasorelaxation and its interaction with MA as well as potential improvement of insulin signaling. Here, we provide the first evidence of the role of E₂ in protecting MA from early vascular dysfunction in prediabetic female rats.

Membrane estrogen receptor alpha (ERα) participates in flow-mediated dilation in a ligand-independent manner

Estrogen receptor alpha (ERα) activation by estrogens prevents atheroma through its nuclear action, whereas plasma membrane-located ERα accelerates endothelial healing. The genetic deficiency of ERα was associated with a reduction in flow-mediated dilation (FMD) in one man. Here, we evaluated ex vivo the role of ERα on FMD of resistance arteries. FMD, but not agonist (acetylcholine, insulin)-mediated dilation, was reduced in male and female mice lacking ERα (Esr1-/- mice) compared to wild-type mice and was not dependent on the presence of estrogens. In C451A-ERα mice lacking membrane ERα, not in mice lacking AF2-dependent nuclear ERα actions, FMD was reduced, and restored by antioxidant treatments. Compared to wild-type mice, isolated perfused kidneys of C451A-ERα mice revealed a decreased flow-mediated nitrate production and an increased H₂O₂ production. Thus, endothelial membrane ERα promotes NO bioavailability through inhibition of oxidative stress and thereby participates in FMD in a ligand-independent manner.

Endothelial TRPV4-eNOS coupling as a vital therapy target for treatment of hypertension

BACKGROUND AND PURPOSE

Reduced nitric oxide (NO) level and activity are signs of endothelial dysfunction, which is important in mediating blood pressure up-regulation. Previously, we demonstrated that transient receptor potential channel V4 (TRPV4) could form functional complex with other proteins to mediate vasodilation in the Endothelial cells (ECs). But how TRPV4 interacts with the NO pathway in larger arteries requires further exploration.

EXPERIMENTAL APPROACH

We used single-cell RNA-sequencing to find the CD106⁺ TRPV4^high NOS3^high ECs. The TRPV4-eNOS interaction was verified by co-immunoprecipitation and Immunofluorescence resonance energy transfer (FRET), and their binding site was found by site-directed mutagenesis. Endothelium-specific TRPV4 knockout (TRPV4_EC ^-/- ) mice were used to study the effect of the TRPV4-eNOS interaction on blood pressure. A small molecule, JNc-463 was designed through molecular docking technology.

KEY RESULTS

We uncovered CD106⁺ TRPV4^high NOS3^high ECs in the mouse aorta, which they could regulate vasodilation via a TRPV4-eNOS interaction, and they were essential to regulate blood pressure. The TRPV4-eNOS interaction markedly decreased during the process of hypertension. We further attempted to identify the molecules re-join the TRPV4-eNOS interaction and develop a small-molecule drug, JNc-463, which could increase the TRPV4-eNOS interaction to enhance vasodilation, and exert antihypertensive effects in mice.

CONCLUSION AND IMPLICATIONS

This is the first study integrating single-cell RNA-Seq, single-cell functional study and drug screening in aorta. We identified a subpopulation of CD106⁺ TRPV4^high NOS3^high ECs, in which an impaired TRPV4-eNOS interaction was important in the progress of hypertension and we designed a small molecule, JNc-463 to improve the impaired TRPV4-eNOS interaction in hypertension.

Potentiation of Acetylcholine-Induced Relaxation of Aorta in Male UC Davis Type 2 Diabetes Mellitus (UCD-T2DM) Rats: Sex-Specific Responses

Previous reports suggest that diabetes may differentially affect the vascular beds of females and males. The objectives of this study were to examine whether there were (1) sex differences in aortic function and (2) alterations in the relative contribution of endothelium-derived relaxing factors in modulating aortic reactivity in UC Davis Type 2 Diabetes Mellitus (UCD-T2DM) rats. Endothelium-dependent vasorelaxation (EDV) in response to acetylcholine (ACh) was measured in aortic rings before and after exposure to pharmacological inhibitors. Relaxation responses to sodium nitroprusside were assessed in endothelium-denuded rings. Moreover, contractile responses to phenylephrine (PE) were measured before and after incubation of aortic rings with a nitric oxide synthase (NOS) inhibitor in the presence of indomethacin. Metabolic parameters and expression of molecules associated with vascular and insulin signaling as well as reactive oxygen species generation were determined. Diabetes slightly but significantly impaired EDV in response to ACh in aortas from females but potentiated the relaxation response in males. The potentiation of EDV in diabetic male aortas was accompanied by a traces of nitric oxide (NO)- and prostanoid-independent relaxation and elevated aortic expression of small- and intermediate conductance Ca2+-activated K+ channels in this group. The smooth muscle sensitivity to NO was not altered, whereas the responsiveness to PE was significantly enhanced in aortas of diabetic groups in both sexes. Endothelium-derived NO during smooth muscle contraction, as assessed by the potentiation of the response to PE after NOS inhibition, was reduced in aortas of diabetic rats regardless of sex. Accordingly, decreases in pAkt and peNOS were observed in aortas from diabetic rats in both sexes compared with controls. Our data suggest that a decrease in insulin sensitivity via pAkt-peNOS-dependent signaling and an increase in oxidative stress may contribute to the elevated contractile responses observed in diabetic aortas in both sexes. This study demonstrates that aortic function in UCD-T2DM rats is altered in both sexes. Here, we provide the first evidence of sexual dimorphism in aortic relaxation in UCD-T2DM rats.

Taurine treatment reverses protein malnutrition-induced endothelial dysfunction of the pancreatic vasculature: The role of hydrogen sulfide

BACKGROUND

Protein malnutrition in childhood predisposes individuals to vascular and pancreatic endocrine dysfunction, thus increasing the risk of diabetes and hypertension. Because taurine may reduce cardiometabolic risk, we hypothesized that taurine treatment has a beneficial effect on the pancreatic vasculature during protein restriction.

METHODS AND RESULTS

Weaned mice were fed a normal or a low-protein diet and were treated with or without taurine for 3 months. The lieno-pancreatic artery (LPA) from low-protein diet-treated mice exhibited impaired endothelium-dependent relaxation to acetylcholine that was associated with decreased endothelium-derived hyperpolarization (EDH), hydrogen sulfide (H₂S) production, and H₂S-synthesizing CBS expression and impaired vasorelaxation to an H₂S-donor, NaHS. These changes were prevented by taurine treatment. We compared the effects of taurine with the effects of the direct vasodilator hydralazine and found that both normalized blood pressure and the endothelial vasodilator function of the LPA in the mice fed a protein-restricted diet. However, only taurine restored the CBS expression in the LPA and insulin secretion in response to high glucose. The LPA supplies the pancreas and shares morphometry with the mesenteric resistance artery (MRA). However, in the MRA, low-protein diet-induced endothelial dysfunction is driven by impaired NOS-derived NO with no changes in H₂S signaling.

CONCLUSIONS

The results suggest that taurine protects against protein malnutrition-induced endothelial dysfunction in the LPA by upregulating the CBS-H₂S pathway. Considering the importance of the pancreatic vasculature for endocrine islet activity, taurine may be a potential therapy for the vascular and metabolic dysfunction associated with malnutrition and comorbidities.

Chronic Low-Level Lead Exposure Increases Mesenteric Vascular Reactivity: Role of Cyclooxygenase-2-Derived Prostanoids

Lead (Pb) exposure causes hazardous effects as hypertension and other cardiovascular diseases. We evaluated whether chronic Pb exposure alters the peripheral vascular resistance measuring the vascular reactivity of mesenteric resistance arteries in rats to identify the underlying mechanisms that are associated to the development of Pb-induced hypertension. Mesenteric resistance arteries from lead-treated and untreated Wistar rats (1st dose: 10 μg/100 g; subsequent doses: 0.125 μg/100 g, intramuscular, 30 days) were used. Contractile responses to phenylephrine increased, while acetylcholine and sodium nitroprusside-induced relaxation was not affected by lead treatment. Endothelium removal and inhibition of NO synthase by L-NAME similarly enhanced the response to phenylephrine in untreated and lead-treated rats. The antioxidants apocynin and superoxide dismutase (SOD) did not affect vasoconstriction in either group. The vascular expression of cyclooxygenase-2 (COX-2) protein increased after lead exposure. The respective non-specific or specific COX-2 inhibitors indomethacin and NS398 reduced more strongly the response to phenylephrine in treated rats. Antagonists of EP1 (SC19220), TP (SQ29548), IP (CAY10441) and angiotensin II type 1 (losartan) receptors reduced vasoconstriction only in treated rats. These conclusions present further evidence that lead, even in small concentration, produces cardiovascular hazards being an environmental contaminant that account for lead-induced hypertension.

Involvement of Gap Junctions in Acetylcholine-Induced Endothelium-Derived Hyperpolarization-Type Dilation of Retinal Arterioles in Rats

An electrical communication between the endothelial and smooth muscle cells via gap junctions, which provides the signaling pathway known as endothelium-dependent hyperpolarization (EDH), plays a crucial role in controlling the vascular tone. In this study, we investigated the role of gap junctions in the acetylcholine (ACh)-induced EDH-type dilation of rat retinal arterioles in vivo. The dilator response was evaluated by measuring the diameter of retinal arterioles. Intravitreal injection of gap junction blockers (18β-glycyrrhetinic acid and carbenoxolone) reduced the ACh-induced dilation of retinal arterioles. Moreover, the retinal arteriolar response to ACh was attenuated by 18β-glycyrrhetinic acid under treatment with a combination of NG-nitro-L-arginine methyl ester (a nitric oxide (NO) synthase inhibitor; 30 mg/kg) and indomethacin (a cyclooxygenase inhibitor; 5 mg/kg). The NO- and prostaglandin-independent, EDH-related component of ACh-induced dilation of retinal arterioles was prevented by intravitreal injection of iberiotoxin, which inhibits large-conductance Ca2+-activated K+ channels. Furthermore, the combination of 18β-glycyrrhetinic acid and iberiotoxin produced greater attenuation in the EDH-related response than that by the individual agent. Treatment with 18β-glycyrrhetinic acid revealed no significant effect on NOR3 (an NO donor)-induced retinal vasodilator response. These results suggest that gap junctions contribute to the ACh-induced, EDH-type dilation of rat retinal arterioles in vivo.

Endothelial ATP-Sensitive Potassium Channel Protects Against the Development of Hypertension and Atherosclerosis

In the endothelium, ATP-sensitive potassium (KATP) channels are thought to couple cellular metabolism with membrane excitability, calcium entry, and endothelial mediator release. We hypothesized that endothelial KATP channels have a broad role protecting against high blood pressure and atherosclerosis. Endothelial-specific Kir6.1 KO mice (eKO) and eKO mice on an apolipoprotein E KO background were generated (A-eKO) to investigate the role of KATP channels in the endothelium. Basal blood pressure was not elevated in eKO mice. However, when challenged with a high-salt diet and the eNOS inhibitor L-NAME, eKO mice became more hypertensive than their littermate controls. In aorta, NO release at least partly contributes to the endothelium-dependent vasorelaxation induced by pinacidil. In A-eKO mice atherosclerotic plaque density was significantly greater than in their littermate controls when challenged with a high-fat diet, particularly in the aortic arch region. Levels of endothelial dysfunction markers were higher in eKO compared with WT mice; however, these were not significant for A-eKO mice compared with their littermate controls. Furthermore, decreased vascular reactivity was observed in the mesenteric arteries of A-eKO mice, but not in aorta when on a high-fat diet. Our data support a role for endothelial Kir6.1-containing KATP channels in the endothelial protection against environmental stressors: the maintenance of blood pressure homeostasis in response to high salt and endothelial integrity when challenged with a high-fat diet.

Beetroot juice lowers blood pressure and improves endothelial function in pregnant eNOS-/- mice: importance of nitrate-independent effects

KEY POINTS

Maternal hypertension is associated with increased rates of pregnancy pathologies, including fetal growth restriction, due at least in part to reductions in nitric oxide (NO) bioavailability and associated vascular dysfunction. Dietary nitrate supplementation, from beetroot juice (BRJ), has been shown to increase NO bioavailability and improve cardiovascular function in both preclinical and clinical studies. This study is the first to investigate effects of dietary nitrate supplementation in a pregnant animal model. Importantly, the effects of nitrate-containing BRJ were compared with both 'placebo' (nitrate-depleted) BRJ as well as water to control for potential nitrate-independent effects. Our data show novel, nitrate-independent effects of BRJ to lower blood pressure and improve vascular function in endothelial nitric oxide synthase knockout (eNOS^-/- ) mice. These findings suggest potential beneficial effects of BRJ supplementation in pregnancy, and emphasize the importance of accounting for nitrate-independent effects of BRJ in study design and interpretation.

ABSTRACT

Maternal hypertension is associated with adverse pregnancy outcomes, including fetal growth restriction (FGR), due in part to reductions in nitric oxide (NO) bioavailability. We hypothesized that maternal dietary nitrate administration would increase NO bioavailability to reduce systolic blood pressure (SBP), improve vascular function and increase fetal growth in pregnant endothelial NO synthase knockout (eNOS^-/- ) mice, which exhibit hypertension, endothelial dysfunction and FGR. Pregnant wildtype (WT) and eNOS^-/- mice were supplemented with nitrate-containing beetroot juice (BRJ+) from gestational day (GD) 12.5. Control mice received an equivalent dose of nitrate-depleted BRJ (BRJ-) or normal drinking water. At GD17.5, maternal SBP was measured; at GD18.5, maternal nitrate/nitrite concentrations, uterine artery (UtA) blood flow and endothelial function were assessed, and pregnancy outcomes were determined. Plasma nitrate concentrations were increased in both WT and eNOS^-/- mice supplemented with BRJ+ (P < 0.001), whereas nitrite concentrations were increased only in eNOS^-/- mice (P < 0.001). BRJ- did not alter nitrate/nitrite concentrations. SBP was lowered and UtA endothelial function was enhanced in eNOS^-/- mice supplemented with either BRJ+ or BRJ-, indicating nitrate-independent effects of BRJ. Improvements in endothelial function in eNOS^-/- mice were abrogated in the presence of 25 mm KCl, implicating enhanced EDH signalling in BRJ- treated animals. At GD18.5, eNOS^-/- fetuses were significantly smaller than WT animals (P < 0.001), but BRJ supplementation did not affect fetal weight. BRJ may be a beneficial intervention in pregnancies associated with hypertension, endothelial dysfunction and reduced NO bioavailability. Our data showing biological effects of non-nitrate components of BRJ have implications for both interpretation of previous findings and in the design of future clinical trials.

NADPH oxidase in the vasculature: Expression, regulation and signalling pathways; role in normal cardiovascular physiology and its dysregulation in hypertension

The last 20-25 years have seen an explosion of interest in the role of NADPH oxidase (NOX) in cardiovascular function and disease. In vascular smooth muscle and endothelium, NOX generates reactive oxygen species (ROS) that act as second messengers, contributing to the control of normal vascular function. NOX activity is altered in response to a variety of stimuli, including G-protein coupled receptor agonists, growth-factors, perfusion pressure, flow and hypoxia. NOX-derived ROS are involved in smooth muscle constriction, endothelium-dependent relaxation and smooth muscle growth, proliferation and migration, thus contributing to the fine-tuning of blood flow, arterial wall thickness and vascular resistance. Through reversible oxidative modification of target proteins, ROS regulate the activity of protein tyrosine phosphatases, kinases, G proteins, ion channels, cytoskeletal proteins and transcription factors. There is now considerable, but somewhat contradictory evidence that NOX contributes to the pathogenesis of hypertension through oxidative stress. Specific NOX isoforms have been implicated in endothelial dysfunction, hyper-contractility and vascular remodelling in various animal models of hypertension, pulmonary hypertension and pulmonary arterial hypertension, but also have potential protective effects, particularly NOX4. This review explores the multiplicity of NOX function in the healthy vasculature and the evidence for and against targeting NOX for antihypertensive therapy.

Increased inward rectifier K+ current of coronary artery smooth muscle cells in spontaneously hypertensive rats; partial compensation of the attenuated endothelium-dependent relaxation via Ca2+ -activated K+ channels

Endothelium-dependent vasorelaxation is partly mediated by small-conductance (SK3) and intermediate-conductance Ca²⁺ -activated K⁺ channels (SK4) in the endothelium that results in endothelium-dependent hyperpolarization (EDH). Apart from the electrical propagation through myoendothelial gap junctions, the K⁺ released from the endothelium facilitates EDH by increasing inward rectifier K⁺ channel (Kir) conductance in smooth muscle cells. The EDH-dependent relaxation of coronary artery (CA) and Kir current in smooth muscle cells (CASMCs) of hypertensive animals are poorly understood despite the critical role of coronary flow in the hypertrophic heart. In spontaneously hypertensive (SHR) and control (WKY) rats, we found attenuation of the CA relaxation by activators of SK3 and SK4 (NS309 and 1-EBIO) in SHR. In isolated CASMCs, whole-cell patch-clamp study revealed larger I_Kir in SHR than WKY, whereas the myocytes of skeletal and cerebral arteries showed smaller I_Kir in SHR than WKY. While the treatment with I_Kir inhibitor (0.1 mmol/L Ba²⁺ ) alone did not affect the WKY-CA, the SHR-CA showed significant contractile response, suggesting relaxing influence of the higher I_{K ir} in the CASMCs of SHR. Furthermore, the attenuation of NS309-induced relaxation of CA by the combined treatment with 0.1 mmol/L Ba²⁺ was more prominent in SHR than WKY. Our study firstly shows a distinct increase of I_{K ir} in the CASMCs of SHR, which could partly compensate for the attenuated relaxation via endothelial SK3 and SK4.

Morin induces endothelium-dependent relaxation by activating TRPV4 channels in rat mesenteric arteries

Morin, a natural flavonol, has been reported to have beneficial pharmacological effects. Although its vascular protective effects have been studied, little is known about its effects on the mesenteric artery and the underlying mechanisms. Transient receptor potential vanilloid type 4 (TRPV4) channels are one of the most important Ca²⁺-permeable cation channels in vascular endothelial cells and play an important role in regulating rat mesenteric vascular tone. In the present study, the myogenic effects of morin were investigated using wire and pressure myography in the isolated mesenteric artery. Morin induced endothelium-dependent relaxation of isolated rat mesenteric arteries in a concentration-dependent manner. In addition, morin stimulated relaxation by activating TRPV4-mediated Ca²⁺ influx without affecting the nitric oxide (NO), hydrogen peroxide (H₂O₂), cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) pathways. In primary cultured rat mesenteric artery endothelial cells and over-expressing TRPV4 HEK 293 cells, the TRPV4 inhibitor HC067047 significantly reduced the morin-induced increase in intracellular Ca²⁺ concentration. Furthermore, in rats with hypertension induced by N^Ꞷ-nitro-L-arginine methyl ester (L-NAME), oral administration of morin (50 mg/kg/day) decreased systolic blood pressure. In L-NAME-induced hypertensive rats, morin significantly improved the relaxation response of the arteries to acetylcholine. Thus, we demonstrated that morin induces endothelium-dependent relaxation in the rat mesenteric artery by acting on TRPV4 channels to mediate Ca²⁺ influx and attenuate blood pressure in L-NAME-induced hypertension, thereby highlighting the potential of morin in the treatment of hypertension.

The Role of Endothelial Dysfunction in Peripheral Blood Nerve Barrier: Molecular Mechanisms and Pathophysiological Implications

The exchange of solutes between the blood and the nerve tissue is mediated by specific and high selective barriers in order to ensure the integrity of the different compartments of the nervous system. At peripheral level, this function is maintained by the Blood Nerve Barrier (BNB) that, in the presence, of specific stressor stimuli can be damaged causing the onset of neurodegenerative processes. An essential component of BNB is represented by the endothelial cells surrounding the sub-structures of peripheral nerves and increasing evidence suggests that endothelial dysfunction can be considered a leading cause of the nerve degeneration. The purpose of this review is to highlight the main mechanisms involved in the impairment of endothelial cells in specific diseases associated with peripheral nerve damage, such as diabetic neuropathy, erectile dysfunction and inflammation of the sciatic nerve.

Higenamine exerts an antispasmodic effect on cold-induced vasoconstriction by regulating the PI3K/Akt, ROS/α2C-AR and PTK9 pathways independently of the AMPK/eNOS/NO axis

The present study aimed to investigate the antispasmodic effect of higenamine on cold-induced cutaneous vasoconstriction and the underlying molecular mechanisms. A cold-induced cutaneous vasoconstriction rat model was established and different doses of higenamine were delivered by intravenous injection. The changes of cutaneous regional blood flow (RBF) between groups were analyzed. In vitro, the proliferation of human dermal microvascular endothelial cells was measured by MTT. The NO concentration was detected by a nitrate reductase assay. Flow cytometry was applied to measure reactive oxygen species (ROS) levels. The protein expression levels were detected by western blotting. The results demonstrated that in the model group, RBF declined compared with the normal control group, but was reversed by treatment with higenamine. The expression of endothelial nitric oxide synthase (eNOS), phosphorylated (p)-eNOS, protein kinase B (Akt1), p-Akt1, AMP-activated protein kinase (AMPK) α1 and p-AMPKα1 was upregulated by hypothermic treatment but was reversed by higenamine treatment. Treatment with higenamine significantly reduced the level of intracellular α2C-adrenoreceptor (AR) compared with the hypothermia group (P<0.05). Furthermore, the expression of twinfilin-1 (PTK9) was downregulated in the higenamine and positive control groups compared with the hypothermia group (P<0.05). Compared with the hypothermia group, the levels of ROS and α2C-AR (intracellular & membrane) were decreased in higenamine and the positive control group (P<0.05 and P<0.01, respectively). This study, to the best of our knowledge, is the first to assess the effects of higenamine on cold-induced vasoconstriction in vivo and its molecular mechanisms on the PI3K/Akt, AMPK/eNOS/nitric oxide, ROS/α2C-AR and PTK9 signaling pathways under hypothermia conditions. Higenamine may be a good therapeutic option for Raynaud's phenomenon (RP) and cold-induced vasoconstriction.

Endothelium-dependent Hyperpolarization-Mediated Relaxation Pathway in Bovine Mesenteric Lymph Nodes

Endothelium-dependent relaxation mechanisms have been studied in phenylephrine-precontracted capsules of bovine mesenteric lymph nodes studied in vitro. Tetraethylammonium chloride and TRAM-34 in a solution with L-NAME and Indomethacin, which suppress the production NO and prostacyclin of endothelium, increased the tone of the lymph nodes. We believe that in bovine mesenteric lymph nodes, the dilation mechanism is mediated by hyperpolarization of the endothelium, which is associated with activation of large- and intermedium conductance Ca²⁺-activated potassium channels.

Bioactive lipid metabolism in platelet "first responder" and cancer biology

Platelets can serve as "first responders" in cancer and metastasis. This is partly due to bioactive lipid metabolism that drives both platelet and cancer biology. The two primary eicosanoid metabolites that maintain platelet rapid response homeostasis are prostacyclin made by endothelial cells that inhibits platelet function, which is counterbalanced by thromboxane produced by platelets during activation, aggregation, and platelet recruitment. Both of these arachidonic acid metabolites are inherently unstable due to their chemical structure. Tumor cells by contrast predominantly make more chemically stable prostaglandin E₂, which is the primary bioactive lipid associated with inflammation and oncogenesis. Pharmacological, clinical, and epidemiologic studies demonstrate that non-steroidal anti-inflammatory drugs (NSAIDs), which target cyclooxygenases, can help prevent cancer. Much of the molecular and biological impact of these drugs is generally accepted in the field. Cyclooxygenases catalyze the rate-limiting production of substrate used by all synthase molecules, including those that produce prostaglandins along with prostacyclin and thromboxane. Additional eicosanoid metabolites include lipoxygenases, leukotrienes, and resolvins that can also influence platelets, inflammation, and carcinogenesis. Our knowledge base and technology are now progressing toward identifying newer molecular and cellular interactions that are leading to revealing additional targets. This review endeavors to summarize new developments in the field.

Functional Interaction among KCa and TRP Channels for Cardiovascular Physiology: Modern Perspectives on Aging and Chronic Disease

Effective delivery of oxygen and essential nutrients to vital organs and tissues throughout the body requires adequate blood flow supplied through resistance vessels. The intimate relationship between intracellular calcium ([Ca²⁺]_i) and regulation of membrane potential (V_m) is indispensable for maintaining blood flow regulation. In particular, Ca²⁺-activated K⁺ (K_Ca) channels were ascertained as transducers of elevated [Ca²⁺]_i signals into hyperpolarization of V_m as a pathway for decreasing vascular resistance, thereby enhancing blood flow. Recent evidence also supports the reverse role for K_Ca channels, in which they facilitate Ca²⁺ influx into the cell interior through open non-selective cation (e.g., transient receptor potential; TRP) channels in accord with robust electrical (hyperpolarization) and concentration (~20,000-fold) transmembrane gradients for Ca²⁺. Such an arrangement supports a feed-forward activation of V_m hyperpolarization while potentially boosting production of nitric oxide. Furthermore, in vascular types expressing TRP channels but deficient in functional K_Ca channels (e.g., collecting lymphatic endothelium), there are profound alterations such as downstream depolarizing ionic fluxes and the absence of dynamic hyperpolarizing events. Altogether, this review is a refined set of evidence-based perspectives focused on the role of the endothelial K_Ca and TRP channels throughout multiple experimental animal models and vascular types. We discuss the diverse interactions among K_Ca and TRP channels to integrate Ca²⁺, oxidative, and electrical signaling in the context of cardiovascular physiology and pathology. Building from a foundation of cellular biophysical data throughout a wide and diverse compilation of significant discoveries, a translational narrative is provided for readers toward the treatment and prevention of chronic, age-related cardiovascular disease.

Sexually Dimorphic Regulation of EET Synthesis and Metabolism: Roles of Estrogen

Epoxyeicosatrienoic acids (EETs) are metabolites of arachidonic acid via cytochrome P450 (CYP)/epoxygenase and are hydrolyzed by soluble epoxide hydrolase (sEH). Circulating and tissue levels of EETs are controlled by CYP (EET synthesis) and sEH (EET degradation). Therefore, both increases in CYP activity and decreases in sEH expression potentiate EET bioavailability, responses that prevail in the female sex as a function of estrogen. This mini review, based on subtitles listed, briefly summarizes studies focusing specifically on (1) female-specific potentiation of CYP/epoxygenase activity to compensate for the endothelial dysfunction; and (2) estrogen-dependent downregulation of sEH expression, which yields divergent actions in both systemic and pulmonary circulation, respectively.

Estrogen-Potentiating EET Synthesis in Response to Endothelial Dysfunction: This section summarizes the current understanding regarding the roles of estrogen in facilitating EET synthesis in response to endothelial dysfunction. In this regard, estrogen recruitment of EET-driven signaling serves as a back-up mechanism, which compensates for NO deficiency to preserve endothelium-dependent vasodilator responses and maintain normal blood pressure.

Estrogen-Dependent Downregulation of Ephx2/sEH Expression: This section focuses on molecular mechanisms responsible for the female-specific downregulation of sEH expression.

Roles of EETs in Systemic Circulation, as a Function of Estrogen-Dependent Downregulation of sEH: This section summarizes studies conducted on animals that are either deficient in the Ephx2 gene (sEH-KO) or have been treated with sEH inhibitors (sEHIs), and exhibit EET-mediated cardiovascular protections in the cerebral, coronary, skeletal, and splanchnic circulations. In particular, the estrogen-inherent silencing of the Ephx2 gene duplicates the action of sEH deficiency, yielding comparable adaptations in attenuated myogenic vasoconstriction, enhanced shear stress-induced vasodilation, and improved cardiac contractility among female WT mice, male sEH-KO and sEHI-treated mice.

Roles of Estrogen-Driven EET Production in Pulmonary Circulation: This section reviews epidemiological and clinical studies that provide the correlation between the polymorphism, or mutation of gene(s) involving estrogen metabolism and female predisposition to pulmonary hypertension, and specifically addresses an intrinsic causation between the estrogen-dependent downregulation of Ephx2 gene/sEH expression and female-susceptibility of being pulmonary hypertensive, a topic that has never been explored before. Additionally, the issue of the “estrogen paradox” in the incidence and prognosis of pulmonary hypertension is discussed.

The role of potassium channels in the endothelial dysfunction induced by periodontitis

OBJECTIVE

Periodontitis is associated with endothelial dysfunction, which is clinically characterized by a reduction in endothelium-dependent relaxation. However, we have previously shown that impairment in endothelium-dependent relaxation is transient. Therefore, we evaluated which mediators are involved in endothelium-dependent relaxation recovery.

MATERIAL AND METHODS

Rats were subjected to ligature-induced experimental periodontitis. Twenty-one days after the procedure, the animals were prepared for blood pressure recording, and the responses to acetylcholine or sodium nitroprusside were obtained before and 30 minutes after injection of a nitric oxide synthase inhibitor (L-NAME), cyclooxygenase inhibitor (Indomethacin, SC-550 and NS- 398), or calcium-dependent potassium channel blockers (apamin plus TRAM- 34). The maxilla and mandible were removed for bone loss analysis. Blood and gingivae were obtained for C-reactive protein (CRP) and myeloperoxidase (MPO) measurement, respectively.

RESULTS

Experimental periodontitis induces bone loss and an increase in the gingival MPO and plasmatic CRP. Periodontitis also reduced endothelium-dependent vasodilation, a hallmark of endothelial dysfunction, 14 days after the procedure. However, the response was restored at day 21. We found that endothelium-dependent vasodilation at day 21 in ligature animals was mediated, at least in part, by the activation of endothelial calcium-activated potassium channels.

CONCLUSIONS

Periodontitis induces impairment in endothelial-dependent relaxation; this impairment recovers, even in the presence of periodontitis. The recovery is mediated by the activation of endothelial calcium-activated potassium channels in ligature animals. Although important for maintenance of vascular homeostasis, this effect could mask the lack of NO, which has other beneficial properties.

Treatment of hypertension by increasing impaired endothelial TRPV4-KCa2.3 interaction

The currently available antihypertensive agents have undesirable adverse effects due to systemically altering target activity including receptors, channels, and enzymes. These effects, such as loss of potassium ions induced by diuretics, bronchospasm by beta‐blockers, constipation by Ca²⁺ channel blockers, and dry cough by ACEI, lead to non‐compliance with therapies (Moser, 1990). Here, based on new hypertension mechanisms, we explored a new antihypertensive approach. We report that transient receptor potential vanilloid 4 (TRPV4) interacts with Ca²⁺‐activated potassium channel 3 (KCa2.3) in endothelial cells (ECs) from small resistance arteries of normotensive humans, while ECs from hypertensive patients show a reduced interaction between TRPV4 and KCa2.3. Murine hypertension models, induced by high‐salt diet, N(G)‐nitro‐l‐arginine intake, or angiotensin II delivery, showed decreased TRPV4‐KCa2.3 interaction in ECs. Perturbation of the TRPV4‐KCa2.3 interaction in mouse ECs by overexpressing full‐length KCa2.3 or defective KCa2.3 had hypotensive or hypertensive effects, respectively. Next, we developed a small‐molecule drug, JNc‐440, which showed affinity for both TRPV4 and KCa2.3. JNc‐440 significantly strengthened the TRPV4‐KCa2.3 interaction in ECs, enhanced vasodilation, and exerted antihypertensive effects in mice. Importantly, JNc‐440 specifically targeted the impaired TRPV4‐KCa2.3 interaction in ECs but did not systemically activate TRPV4 and KCa2.3. Together, our data highlight the importance of impaired endothelial TRPV4‐KCa2.3 coupling in the progression of hypertension and suggest a novel approach for antihypertensive drug development.

Genetically increased angiotensin I-converting enzyme alters peripheral and renal vascular reactivity to angiotensin II and bradykinin in mice

Angiotensin I-converting enzyme (ACE) levels in humans are under strong genetic influence. Genetic variation in ACE has been linked to risk for and progression of cardiovascular and renal diseases. Causality has been documented in genetically modified mice, but the mechanisms underlying causality are not completely elucidated. To further document the vascular and renal consequences of a moderate genetic increase in ACE synthesis, we studied genetically modified mice carrying three copies of the ACE gene (three-copy mice) and littermate wild-type animals (two-copy mice). We investigated peripheral and renal vascular reactivity to angiotensin II and bradykinin in vivo by measuring blood pressure and renal blood flow after intravenous administration and also reactivity of isolated glomerular arterioles by following intracellular Ca2+ mobilization. Carrying three copies of the ACE gene potentiated the systemic and renal vascular responses to angiotensin II over the whole range of peptide concentration tested. Consistently, the response of isolated glomerular afferent arterioles to angiotensin II was enhanced in three-copy mice. In these mice, signaling pathways triggered by endothelial activation by bradykinin or carbachol in glomerular arterioles were also altered. Although the nitric oxide (NO) synthase (NOS)/NO pathway was not functional in arterioles of two-copy mice, in muscular efferent arterioles of three-copy mice NOS3 gene expression was induced and NO mediated the effect of bradykinin or carbachol. These data document new and unexpected vascular consequences of a genetic increase in ACE synthesis. Enhanced vasoconstrictor effect of angiotensin II may contribute to the risk for cardiovascular and renal diseases linked to genetically high ACE levels.

NEW & NOTEWORTHY A moderate genetic increase in angiotensin I-converting enzyme (ACE) in mice similar to the effect of the ACE gene D allele in humans unexpectedly potentiates the systemic and renal vasoconstrictor responses to angiotensin II. It also alters the endothelial signaling pathways triggered by bradykinin or carbachol in glomerular efferent arterioles.

DOCA-salt hypertension impairs artery function in rat middle cerebral artery and parenchymal arterioles

OBJECTIVE

Chronic hypertension induces detrimental changes in the structure and function of surface cerebral arteries. Very little is known about PAs, which perfuse distinct neuronal populations in the cortex and may play a role in cerebrovascular disorders. We investigated the effect of DOCA-salt induced hypertension on endothelial function and artery structure in PAs and MCAs.

METHODS

Uninephrectomized male Sprague-Dawley rats were implanted with a subcutaneous pellet containing DOCA (150 mg/kg b.w.) and drank salt water (1% NaCl and 0.2% KCl) for 4 weeks. Sham rats were uninephrectomized and drank tap water. Vasoreactivity and passive structure in the MCAs and the PAs were assessed by pressure myography.

RESULTS

Both MCAs and PAs from DOCA-salt rats exhibited impaired endothelium-dependent dilation (P<.05). In the PAs, addition of NO and COX inhibitors enhanced dilation in DOCA-salt rats (P<.05), suggesting that dysfunctional NO and COX-dependent signaling could contribute to impaired endothelium-mediated dilation. MCAs from DOCA-salt rats exhibited inward remodeling (P<.05).

CONCLUSIONS

Hypertension-induced MCA remodeling coupled with impaired endothelium-dependent dilation in both the MCAs and PAs may exacerbate the risk of cerebrovascular accidents and the associated morbidity and mortality.

Differences in the Thoracic Aorta by Region and Sex in a Murine Model of Marfan Syndrome

Marfan syndrome (MFS) is a hereditary disorder of the connective tissue that causes life-threatening aortic aneurysm, which initiates at the aortic root and can progress into the ascending portion. However, analysis of ascending aorta reactivity in animal models of MFS has remained elusive. Epidemiologic evidence suggests that although MFS is equally prevalent in men and women, men are at a higher risk of aortic complications than non-pregnant women. Nevertheless, there is no experimental evidence to support this hypothesis. The aim of this study was to explore whether there are regional and sex differences in the thoracic aorta function of mice heterozygous for the fibrillin 1 (Fbn1) allele encoding a missense mutation (Fbn1^C1039G/+), the most common class of mutation in MFS. Ascending and descending thoracic aorta reactivity was evaluated by wire myography. Ascending aorta mRNA and protein levels, and elastic fiber integrity were assessed by qRT-PCR, Western blotting, and Verhoeff-Van Gieson histological staining, respectively. MFS differently altered reactivity in the ascending and descending thoracic aorta by either increasing or decreasing phenylephrine contractions, respectively. When mice were separated by sex, contractions to phenylephrine increased progressively from 3 to 6 months of age in MFS ascending aortas of males, whereas contractions in females were unchanged. Endothelium-dependent relaxation was unaltered in the MFS ascending aorta of either sex; an effect related to augmented endothelium-dependent hyperpolarization-type dilations. In MFS males, the non-selective cyclooxygenase (COX) inhibitor indomethacin prevented the MFS-induced enhancement of phenylephrine contractions linked to increased COX-2 expression. In MFS mice of both sexes, the non-selective nitric oxide synthase inhibitor L-NAME revealed negative feedback of nitric oxide on phenylephrine contractions, which was associated with upregulation of eNOS in females. Finally, MFS ascending aortas showed a greater number of elastic fiber breaks than the wild-types, and males exhibited more breaks than females. These results show regional and sex differences in Fbn1^C1039G/+ mice thoracic aorta contractility and aortic media injuries. The presence of more pronounced aortic alterations in male mice provides experimental evidence to support that male MFS patients are at increased risk of suffering aortic complications.

Carotid artery stenosis in hypertensive rats impairs dilatory pathways in parenchymal arterioles

Hypertension is a leading risk factor for vascular cognitive impairment and is strongly associated with carotid artery stenosis. In normotensive rats, chronic cerebral hypoperfusion induced by bilateral common carotid artery stenosis (BCAS) leads to cognitive impairment that is associated with impaired endothelium-dependent dilation in parenchymal arterioles (PAs). The aim of this study was to assess the effects of BCAS on PA function and structure in stroke-prone spontaneously hypertensive rats, a model of human essential hypertension. Understanding the effects of hypoperfusion on PAs in a hypertensive model could lead to the identification of therapeutic targets for cognitive decline in a model that reflects the at-risk population. We hypothesized that BCAS would impair endothelium-dependent dilation in PAs and induce artery remodeling compared with sham rats. PAs from BCAS rats had endothelial dysfunction, as assessed using pressure myography. Inhibition of nitric oxide and prostaglandin production had no effect on PA dilation in sham or BCAS rats. Surprisingly, inhibition of epoxyeicosatrienoic acid production increased dilation in PAs from BCAS rats but not from sham rats. Similar results were observed in the presence of inhibitors for all three dilatory pathways, suggesting that epoxygenase inhibition may have restored a nitric oxide/prostaglandin-independent dilatory pathway in PAs from BCAS rats. PAs from BCAS rats underwent remodeling with a reduced wall thickness. These data suggest that marked endothelial dysfunction in PAs from stroke-prone spontaneously hypertensive rats with BCAS may be associated with the development of vascular cognitive impairment. NEW & NOTEWORTHY The present study assessed the structure and function of parenchymal arterioles in a model of chronic cerebral hypoperfusion and hypertension, both of which are risk factors for cognitive impairment. We observed that impaired dilation and artery remodeling in parenchymal arterioles and abolished cerebrovascular reserve capacity may mediate cognitive deficits.

The Role of Adenosine A2A Receptor, CYP450s, and PPARs in the Regulation of Vascular Tone

Adenosine is an endogenous mediator involved in a myriad of physiologic functions, including vascular tone regulation. It is also implicated in some pathologic conditions. Four distinct receptor subtypes mediate the effects of adenosine, such as its role in the regulation of the vascular tone. Vascular tone regulation is a complex and continuous process which involves many mechanisms and mediators that are not fully disclosed. The vascular endothelium plays a pivotal role in regulating blood flow to and from all body organs. Also, the vascular endothelium is not merely a physical barrier; it is a complex tissue with numerous functions. Among adenosine receptors, A2A receptor subtype (A2AAR) stands out as the primary receptor responsible for the vasodilatory effects of adenosine. This review focuses on important effectors of the vascular endothelium, including adenosine, adenosine receptors, EETs (epoxyeicosatrienoic acids), HETEs (hydroxyeicosatetraenoic acids), PPARs (peroxisome proliferator-activated receptors), and KATP channels. Given the impact of vascular tone regulation in cardiovascular physiology and pathophysiology, better understanding of the mechanisms affecting it could have a significant potential for developing therapeutic agents for cardiovascular diseases.

Endothelial dysfunction and vascular disease - a 30th anniversary update

The endothelium can evoke relaxations of the underlying vascular smooth muscle, by releasing vasodilator substances. The best-characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO) which activates soluble guanylyl cyclase in the vascular smooth muscle cells, with the production of cyclic guanosine monophosphate (cGMP) initiating relaxation. The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDH-mediated responses). As regards the latter, hydrogen peroxide (H₂ O₂ ) now appears to play a dominant role. Endothelium-dependent relaxations involve both pertussis toxin-sensitive G_i (e.g. responses to α₂ -adrenergic agonists, serotonin, and thrombin) and pertussis toxin-insensitive G_q (e.g. adenosine diphosphate and bradykinin) coupling proteins. New stimulators (e.g. insulin, adiponectin) of the release of EDRFs have emerged. In recent years, evidence has also accumulated, confirming that the release of NO by the endothelial cell can chronically be upregulated (e.g. by oestrogens, exercise and dietary factors) and downregulated (e.g. oxidative stress, smoking, pollution and oxidized low-density lipoproteins) and that it is reduced with ageing and in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively lose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and EDH, in particular those due to H₂ O₂ ), endothelial cells also can evoke contraction of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factors. Recent evidence confirms that most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells and that prostacyclin plays a key role in such responses. Endothelium-dependent contractions are exacerbated when the production of nitric oxide is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive and diabetic patients. In addition, recent data confirm that the release of endothelin-1 can contribute to endothelial dysfunction and that the peptide appears to be an important contributor to vascular dysfunction. Finally, it has become clear that nitric oxide itself, under certain conditions (e.g. hypoxia), can cause biased activation of soluble guanylyl cyclase leading to the production of cyclic inosine monophosphate (cIMP) rather than cGMP and hence causes contraction rather than relaxation of the underlying vascular smooth muscle.

Inwardly rectifying K+ channels are major contributors to flow-induced vasodilatation in resistance arteries

KEY POINTS

Endothelial inwardly rectifying K⁺ (Kir2.1) channels regulate flow-induced vasodilatation via nitric oxide (NO) in mouse mesenteric resistance arteries. Deficiency of Kir2.1 channels results in elevated blood pressure and increased vascular resistance. Flow-induced vasodilatation in human resistance arteries is also regulated by inwardly rectifying K⁺ channels. This study presents the first direct evidence that Kir channels play a critical role in physiological endothelial responses to flow.

ABSTRACT

Inwardly rectifying K⁺ (Kir) channels are known to be sensitive to flow, but their role in flow-induced endothelial responses is not known. The goal of this study is to establish the role of Kir channels in flow-induced vasodilatation and to provide first insights into the mechanisms responsible for Kir signalling in this process. First, we establish that primary endothelial cells isolated from murine mesenteric arteries express functional Kir2.1 channels sensitive to shear stress. Then, using the Kir2.1^+/- heterozygous mouse model, we establish that downregulation of Kir2.1 results in significant decrease in shear-activated Kir currents and inhibition of endothelium-dependent flow-induced vasodilatation (FIV) assayed in pressurized mesenteric arteries pre-constricted with endothelin-1. Deficiency in Kir2.1 also results in the loss of flow-induced phosphorylation of eNOS and Akt, as well as inhibition of NO generation. All the effects are fully rescued by endothelial cell (EC)-specific overexpression of Kir2.1. A component of FIV that is Kir independent is abrogated by blocking Ca²⁺ -sensitive K⁺ channels. Kir2.1 has no effect on endothelium-independent and K⁺ -induced vasodilatation in denuded arteries. Kir2.1^+/- mice also show increased mean blood pressure measured by carotid artery cannulation and increased microvascular resistance measured using a tail-cuff. Importantly, blocking Kir channels also inhibits flow-induced vasodilatation in human subcutaneous adipose microvessels. Endothelial Kir channels contribute to FIV of mouse mesenteric arteries via an NO-dependent mechanism, whereas Ca²⁺ -sensitive K⁺ channels mediate FIV via an NO-independent pathway. Kir2 channels also regulate vascular resistance and blood pressure. Finally, Kir channels also contribute to FIV in human subcutaneous microvessels.

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.

Differential contribution of endothelium-derived relaxing factors to vascular reactivity in conduit and resistance arteries from normotensive and hypertensive rats

The endothelium contributes to the maintenance of vasodilator tone by releasing nitric oxide (NO), prostacyclin (PGI2), and endothelium-derived hyperpolarizing factor (EDHF). In hypertension, endothelium-dependent relaxation is attenuated (a phenomenon referred to as endothelial dysfunction) and contributes to the increased peripheral resistance. However, which vasodilator among NO, PGI2, and EDHF is impaired in hypertension remains largely unknown. The present study was designed to study the exact contribution of NO, PGI2, and EDHF to vascular reactivity in conduit and resistance artery, under physiological and pathological conditions. The aorta and the second-order mesenteric artery from spontaneous hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were used to measure the vasorelaxation with myograph technology, in the presence or absence of different inhibitors. The results showed that the endothelium-dependent vasodilatation in the conduit artery was mediated mainly by NO, whereas the resistant artery by NO, PGI2, and EDHF together. In hypertension, both NO-mediated relaxation in the conduit artery and NO-, PGI2-, and EDHF-mediated dilation in the resistant artery were markedly impaired. Furthermore, the endothelium-dependent and the endothelium-independent vasorelaxation in conduit artery was attenuated more pronouncedly than that in the resistant artery from hypertensive rats, suggesting that the conduit artery is more vulnerable to hypertensive condition. In conclusion, vasodilators including NO, PGI2, and EDHF contribute distinctively to endothelium-dependent relaxation in conduit and resistance artery under physiological and pathological conditions.

Functional Tuning of Intrinsic Endothelial Ca2+ Dynamics in Swine Coronary Arteries

RATIONALE

Recent data from mesenteric and cerebral beds have revealed spatially restricted Ca(2+) transients occurring along the vascular intima that control effector recruitment and vasodilation. Although Ca(2+) is pivotal for coronary artery endothelial function, spatial and temporal regulation of functional Ca(2+) signals in the coronary endothelium is poorly understood.

OBJECTIVE

We aimed to determine whether a discrete spatial and temporal profile of Ca(2+) dynamics underlies endothelium-dependent relaxation of swine coronary arteries.

METHODS AND RESULTS

Using confocal imaging, custom automated image analysis, and myography, we show that the swine coronary artery endothelium generates discrete basal Ca(2+) dynamics, including isolated transients and whole-cell propagating waves. These events are suppressed by depletion of internal stores or inhibition of inositol 1,4,5-trisphosphate receptors but not by inhibition of ryanodine receptors or removal of extracellular Ca(2+). In vessel rings, inhibition of specific Ca(2+)-dependent endothelial effectors, namely, small and intermediate conductance K(+) channels (K(Ca)3.1 and K(Ca)2.3) and endothelial nitric oxide synthase, produces additive tone, which is blunted by internal store depletion or inositol 1,4,5-trisphosphate receptor blockade. Stimulation of endothelial inositol 1,4,5-trisphosphate-dependent signaling with substance P causes idiosyncratic changes in dynamic Ca(2+) signal parameters (active sites, event frequency, amplitude, duration, and spatial spread). Overall, substance P-induced vasorelaxation corresponded poorly with whole-field endothelial Ca(2+) measurements but corresponded precisely with the concentration-dependent change in Ca(2+) dynamics (linearly translated composite of dynamic parameters).

CONCLUSIONS

Our findings show that endothelium-dependent control of swine coronary artery tone is determined by spatial and temporal titration of inherent endothelial Ca(2+) dynamics that are not represented by tissue-level averaged Ca(2+) changes.

Bilateral common carotid artery stenosis in normotensive rats impairs endothelium-dependent dilation of parenchymal arterioles

Chronic cerebral hypoperfusion is a risk factor for cognitive impairment. Reduced blood flow through the common carotid arteries induced by bilateral carotid artery stenosis (BCAS) is a physiologically relevant model of chronic cerebral hypoperfusion. We hypothesized that BCAS in 20-wk-old Wistar-Kyoto (WKY) rats would impair cognitive function and lead to reduced endothelium-dependent dilation and outward remodeling in the parenchymal arterioles (PAs). After 8 wk of BCAS, both short-term memory and spatial discrimination abilities were impaired. In vivo assessment of cerebrovascular reserve capacity showed a severe impairment after BCAS. PA endothelial function and structure were assessed by pressure myography. BCAS impaired endothelial function in PAs, as evidenced by reduced dilation to carbachol. Addition of nitric oxide synthase and cyclooxygenase inhibitors did not change carbachol-mediated dilation in either group. Inhibiting CYP epoxygenase, the enzyme that produces epoxyeicosatrienoic acid (EETs), a key determinant of endothelium-derived hyperpolarizing factor (EDHF)-mediated dilation, abolished dilation in PAs from Sham rats, but had no effect in PAs from BCAS rats. Expression of TRPV4 channels, a target for EETs, was decreased and maximal dilation to a TRPV4 agonist was attenuated after BCAS. Together these data suggest that EET-mediated dilation is impaired in PAs after BCAS. Thus impaired endothelium-dependent dilation in the PAs may be one of the contributing factors to the cognitive impairment observed after BCAS.

The Gatekeepers in the Mouse Ophthalmic Artery: Endothelium-Dependent Mechanisms of Cholinergic Vasodilation

Cholinergic regulation of arterial luminal diameter involves intricate network of intercellular communication between the endothelial and smooth muscle cells that is highly dependent on the molecular mediators released by the endothelium. Albeit the well-recognized contribution of nitric oxide (NO) towards vasodilation, the identity of compensatory mechanisms that maintain vasomotor tone when NO synthesis is deranged remain largely unknown in the ophthalmic artery. This is the first study to identify the vasodilatory signalling mechanisms of the ophthalmic artery employing wild type mice. Acetylcholine (ACh)-induced vasodilation was only partially attenuated when NO synthesis was inhibited. Intriguingly, the combined blocking of cytochrome P₄₅₀ oxygenase (CYP450) and lipoxygenase (LOX), as well as CYP450 and gap junctions, abolished vasodilation; demonstrating that the key compensatory mechanisms comprise arachidonic acid metabolites which, work in concert with gap junctions for downstream signal transmission. Furthermore, the voltage-gated potassium ion channel, K_v1.6, was functionally relevant in mediating vasodilation. Its localization was found exclusively in the smooth muscle. In conclusion, ACh-induced vasodilation of mouse ophthalmic artery is mediated in part by NO and predominantly via arachidonic acid metabolites, with active involvement of gap junctions. Particularly, the K_v1.6 channel represents an attractive therapeutic target in ophthalmopathologies when NO synthesis is compromised.

NOX4-dependent Hydrogen peroxide promotes shear stress-induced SHP2 sulfenylation and eNOS activation

Laminar shear stress (LSS) triggers signals that ultimately result in atheroprotection and vasodilatation. Early responses are related to the activation of specific signaling cascades. We investigated the participation of redox-mediated modifications and in particular the role of hydrogen peroxide (H2O2) in the sulfenylation of redox-sensitive phosphatases. Exposure of vascular endothelial cells to short periods of LSS (12 dyn/cm(2)) resulted in the generation of superoxide radical anion as detected by the formation of 2-hydroxyethidium by HPLC and its subsequent conversion to H2O2, which was corroborated by the increase in the fluorescence of the specific peroxide sensor HyPer. By using biotinylated dimedone we detected increased total protein sulfenylation in the bovine proteome, which was dependent on NADPH oxidase 4 (NOX4)-mediated generation of peroxide. Mass spectrometry analysis allowed us to identify the phosphatase SHP2 as a protein susceptible to sulfenylation under LSS. Given the dependence of FAK activity on SHP2 function, we explored the role of FAK under LSS conditions. FAK activation and subsequent endothelial NO synthase (eNOS) phosphorylation were promoted by LSS and both processes were dependent on NOX4, as demonstrated in lung endothelial cells isolated from NOX4-null mice. These results support the idea that LSS elicits redox-sensitive signal transduction responses involving NOX4-dependent generation of hydrogen peroxide, SHP2 sulfenylation, and ulterior FAK-mediated eNOS activation.

Spironolactone treatment attenuates vascular dysfunction in type 2 diabetic mice by decreasing oxidative stress and restoring NO/GC signaling

Type 2 diabetes (DM2) increases the risk of cardiovascular disease. Aldosterone, which has pro-oxidative and pro-inflammatory effects in the cardiovascular system, is positively regulated in DM2. We assessed whether blockade of mineralocorticoid receptors (MR) with spironolactone decreases reactive oxygen species (ROS)-associated vascular dysfunction and improves vascular nitric oxide (NO) signaling in diabetes. Leptin receptor knockout [LepR(db)/LepR(db) (db/db)] mice, a model of DM2, and their counterpart controls [LepR(db)/LepR(+), (db/+) mice] received spironolactone (50 mg/kg body weight/day) or vehicle (ethanol 1%) via oral per gavage for 6 weeks. Spironolactone treatment abolished endothelial dysfunction and increased endothelial nitric oxide synthase (eNOS) phosphorylation (Ser(1177)) in arteries from db/db mice, determined by acetylcholine-induced relaxation and Western Blot analysis, respectively. MR antagonist therapy also abrogated augmented ROS-generation in aorta from diabetic mice, determined by lucigenin luminescence assay. Spironolactone treatment increased superoxide dismutase-1 and catalase expression, improved sodium nitroprusside and BAY 41-2272-induced relaxation, and increased soluble guanylyl cyclase (sGC) β subunit expression in arteries from db/db mice. Our results demonstrate that spironolactone decreases diabetes-associated vascular oxidative stress and prevents vascular dysfunction through processes involving increased expression of antioxidant enzymes and sGC. These findings further elucidate redox-sensitive mechanisms whereby spironolactone protects against vascular injury in diabetes.

Vascular dysfunctions in the isolated aorta of double-transgenic hypertensive mice developing aortic aneurysm

Angiotensin-II and oxidative stress are involved in the genesis of aortic aneurysms, a phenomenon exacerbated by endothelial nitric oxide synthase (eNOS) deletion or uncoupling. The purpose of this work was to study the endothelial function in wild-type C57BL/6 (BL) and transgenic mice expressing the h-angiotensinogen and h-renin genes (AR) subjected to either a control, or a high-salt diet plus a treatment with a NO-synthase inhibitor, N-ω-nitro-L-arginine-methyl-ester (L-NAME; BLSL and ARSL). BLSL showed a moderate increase in blood pressure, while ARSL became severely hypertensive. Seventy-five percent of ARSL developed aortic aneurysms, characterized by major histo-morphological changes and associated with an increase in NADP(H) oxidase-2 (NOX2) expression. Contractile responses (KCl, norepinephrine, U-46619) were similar in the four groups of mice, and relaxations were not affected in BLSL and AR. However, in ARSL, endothelium-dependent relaxations (acetylcholine, UK-14304) were significantly reduced, and this dysfunction was similar in aortae without or with aneurysms. The endothelial impairment was unaffected by catalase, superoxide-dismutase mimetic, radical scavengers, cyclooxygenase inhibition, or TP-receptor blockade and could not be attributed to sGC oxidation. Thus, ARSL is a severe hypertension model developing aortic aneurysm. A vascular dysfunction, involving both endothelial (reduced role of NO) and smooth muscle cells, precedes aneurysms formation and, paradoxically, does not appear to involve oxidative stress.

New approach to measure cutaneous microvascular function: an improved test of NO-mediated vasodilation by thermal hyperemia

Cutaneous hyperemia in response to rapid skin local heating to 42°C has been used extensively to assess microvascular function. However, the response is dependent on both nitric oxide (NO) and endothelial-derived hyperpolarizing factors (EDHFs), and increases cutaneous vascular conductance (CVC) to ∼90-95% maximum in healthy subjects, preventing the study of potential means to improve cutaneous function. We sought to identify an improved protocol for isolating NO-dependent dilation. We compared nine heating protocols (combinations of three target temperatures: 36°C, 39°C, and 42°C, and three rates of heating: 0.1°C/s, 0.1°C/10 s, 0.1°C/min) in order to select two protocols to study in more depth (protocol 1; N = 6). Then, CVC was measured at four microdialysis sites receiving: 1) lactated Ringer solution (Control), 2) 50-mM tetraethylammonium (TEA) to inhibit EDHFs, 3) 20-mM nitro-L-arginine methyl ester (L-NAME) to inhibit NO synthase, and 4) TEA+L-NAME, in response to local heating either to 39°C at 0.1°C/s (protocol 2; N = 10) or 42°C at 0.1°C/min (protocol 3; N = 8). Rapid heating to 39°C increased CVC to 43.1 ± 5.2%CVCmax (Control), which was attenuated by L-NAME (11.4 ± 2.8%CVCmax; P < 0.001) such that 82.8 ± 4.2% of the plateau was attributable to NO. During gradual heating, 81.5 ± 3.3% of vasodilation was attributable to NO at 40°C, but at 42°C only 32.7 ± 7.8% of vasodilation was attributable to NO. TEA+L-NAME attenuated CVC beyond L-NAME at temperatures >40°C (43.4 ± 4.5%CVCmax at 42°C, P < 0.001 vs. L-NAME), suggesting a role of EDHFs at higher temperatures. Our findings suggest local heating to 39°C offers an improved approach for isolating NO-dependent dilation and/or assessing perturbations that may improve microvascular function.

Inhibition of calpain reduces oxidative stress and attenuates endothelial dysfunction in diabetes

Aims

The present study was to investigate the role of calpain in reactive oxygen species (ROS) production in endothelial cells and endothelium-dependent vascular dysfunction under experimental conditions of diabetes.

Methods and results

Exposure to high glucose activated calpain, induced apoptosis and reduced nitric oxide (NO) production without changing eNOS protein expression, its phosphorylation and dimers formation in primary human umbilical vein endothelial cells (HUVECs). These effects of high glucose correlated with intracellular ROS production and mitochondrial superoxide generation. Selectively scavenging mitochondrial superoxide increased NO production in high glucose-stimulated HUVECs. Inhibition of calpain using over-expression of calpastatin or pharmacological calpain inhibitor prevented high glucose-induced ROS production, mitochondrial superoxide generation and apoptosis, which were concurrent with an elevation of NO production in HUVECs. In mouse models of streptozotocin-induced type-1 diabetes and OVE26 type-1 diabetic mice, calpain activation correlated with an increase in ROS production and peroxynitrite formation in aortas. Transgenic over-expression of calpastatin reduced ROS production and peroxynitrite formation in diabetic mice. In parallel, diabetes-induced reduction of endothelium-dependent relaxation in aortic ring was reversed by over-expression of calpastatin in mouse models of diabetes. However, the protective effect of calpastatin on endothelium-dependent relaxation was abrogated by eNOS deletion in diabetic mice.

Conclusions

This study suggests that calpain may play a role in vascular endothelial cell ROS production and endothelium-dependent dysfunction in diabetes. Thus, calpain may be an important therapeutic target to overcome diabetes-induced vascular dysfunction.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Vasodilator responses to acetylcholine are not mediated by the activation of soluble guanylate cyclase or TRPV4 channels in the rat

The effects of 1H-[1,2,4]-oxadizaolo[4,3-]quinoxaline-1-one (ODQ), an inhibitor of the activation of soluble guanylate cyclase (sGC) on responses to NO donors acetylcholine (ACh) and bradykinin (BK) were investigated in the pulmonary and systemic vascular beds of the rat. In these studies the administration of ODQ in a dose of 5 mg/kg iv attenuated vasodilator responses to five different NO donors without inhibiting responses to ACh and BK in the systemic and pulmonary vascular beds of the rat. Vasodilator responses to ACh were not inhibited by l-NAME or the transient receptor vanilloid type 4 (TRPV4) antagonist GSK-2193874, which attenuated vasodilator responses to the TRPV4 agonist GSK-1016790A. ODQ did not inhibit vasodilator responses to agents reported to act in an NO-independent manner or to vasoconstrictor agents, and ODQ did not increase blood methemoglobin levels, suggesting that off target effects were minimal. These results show that ODQ in a dose that inhibited NO donor-mediated responses did not alter vasodilator responses to ACh in the pulmonary and systemic vascular beds and did not alter systemic vasodilator responses to BK. The present results indicate that decreases in pulmonary and systemic arterial pressures in response to ACh are not mediated by the activation of sGC or TRPV4 channels and that ODQ can be used to study the role of the activation of sGC in mediating vasodilator responses in the rat.

Endothelial nitric oxide synthase uncoupling and microvascular dysfunction in the mesentery of mice deficient in α-galactosidase A

A defect in the gene for the lysosomal enzyme α-galactosidase A (Gla) results in globotriaosylceramide (Gb3) accumulation in Fabry disease and leads to premature death from cardiac and cerebrovascular events. However, gastrointestinal symptoms are often first observed during childhood in these patients and are not well understood. In this study, we demonstrate an age-dependent microvasculopathy of the mesenteric artery (MA) in a murine model of Fabry disease (Gla-knockout mice) resulting from dysregulation of the vascular homeostatic enzyme endothelial nitric oxide synthase (eNOS). The progressive accumulation of Gb3 in the MA was confirmed by thin-layer chromatographic analysis. A total absence of endothelium-dependent dilation was observed in MAs from mice at 8 mo of age, while suppression of ACh-mediated vasodilation was evident from 2 mo of age. Endothelium-independent dilation with sodium nitroprusside was normal compared with age-matched wild-type mice. The microvascular defect in MAs from Fabry mice was endothelium-dependent and associated with suppression of the active homodimer of eNOS. Phosphorylation of eNOS at the major activation site (Ser(1179)) was significantly downregulated, while phosphorylation at the major inhibitory site (Thr(495)) was remarkably enhanced in MAs from aged Fabry mice. These profound alterations in eNOS bioavailability at 8 mo of age were observed in parallel with high levels of 3-nitrotyrosine, suggesting increased reactive oxygen species along with eNOS uncoupling in this vascular bed. Overall, the mesenteric microvessels in the setting of Fabry disease were observed to have an early and profound endothelial dysfunction associated with elevated reactive nitrogen species and decreased nitric oxide bioavailability.

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.