cAMP facilitates EDHF-type relaxations in conduit arteries by enhancing electrotonic conduction via gap junctions

Pathophysiology and Outcomes of Endothelium Function in Coronary Microvascular Diseases: A Systematic Review of Randomized Controlled Trials and Multicenter Study

BACKGROUND

Coronary macrovascular disease is a concept that has been well-studied within the literature and has long been the subject of debates surrounding coronary artery bypass grafting (CABG) vs. Percutaneous Coronary Intervention (PCI). ISCHEMIA trial reported no statistical difference in the primary clinical endpoint between initial invasive management and initial conservative management, while in the ORBITA trial PCI did not improve angina frequency score significantly more than placebo, albeit PCI resulted in more patient-reported freedom from angina than placebo. However, these results did not prove the superiority of the PCI against OMT, therefore do not indicate the benefit of PCI vs. the OMT. Please rephrase the sentence. We reviewed the role of different factors responsible for endothelial dysfunction from recent randomized clinical trials (RCTs) and multicentre studies.

METHODS

A detailed search strategy was performed using a dataset that has previously been published. Data of pooled analysis include research articles (human and animal models), CABG, and PCI randomized controlled trials (RCTs). Details of the search strategy and the methods used for data pooling have been published previously and registered with Open-Source Framework.

RESULTS

The roles of nitric oxide (NO), endothelium-derived contracting factors (EDCFs), and vasodilator prostaglandins (e.g., prostacyclin), as well as endothelium-dependent hyperpolarization (EDH) factors, are crucial for the maintenance of vasomotor tone within the coronary vasculature. These homeostatic mechanisms are affected by sheer forces and other several factors that are currently being studied, such as vaping. The role of intracoronary testing is crucial when determining the effects of therapeutic medications with further studies on the horizon.

CONCLUSION

The true impact of coronary microvascular dysfunction (CMD) is perhaps underappreciated, which supports the role of medical therapy in determining outcomes. Ongoing trials are underway to further investigate the role of therapeutic agents in secondary prevention.

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.

Endothelium-Derived Hyperpolarizing Factor and Myoendothelial Coupling: The in vivo Perspective

The endothelium controls vascular tone adopting blood flow to tissue needs. It releases chemical mediators [e.g., nitric oxide (NO), prostaglandins (PG)] and exerts appreciable dilation through smooth muscle hyperpolarization, thus termed endothelium-dependent hyperpolarization (EDH). Initially, EDH was attributed to release of a factor, but later it was suggested that smooth muscle hyperpolarization might be derived from radial spread of an initial endothelial hyperpolarization through heterocellular channels coupling these vascular cells. The channels are indeed present and formed by connexins that enrich in gap junctions (GJ). In vitro data suggest that myoendothelial coupling underlies EDH-type dilations as evidenced by blocking experiments as well as simultaneous, merely identical membrane potential changes in endothelial and smooth muscle cells (SMCs), which is indicative of coupling through ohmic resistors. However, connexin-deficient animals do not display any attenuation of EDH-type dilations in vivo, and endothelial and SMCs exhibit distinct and barely superimposable membrane potential changes exerted by different means in vivo. Even if studied in the exact same artery EDH-type dilation exhibits distinct features in vitro and in vivo: in isometrically mounted vessels, it is rather weak and depends on myoendothelial coupling through connexin40 (Cx40), whereas in vivo as well as in vitro under isobaric conditions it is powerful and independent of myoendothelial coupling through Cx40. It is concluded that EDH-type dilations are distinct and a significant dependence on myoendothelial coupling in vitro does not reflect the situation under physiologic conditions in vivo. Myoendothelial coupling may act as a backup mechanism that is uncovered in the absence of the powerful EDH-type response and possibly reflects a situation in a pathophysiologic environment.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

Adenosine kinase inhibition enhances microvascular dilator function and improves left ventricle diastolic dysfunction

OBJECTIVE

Inhibition of adenosine kinase (ADK), via augmenting endogenous adenosine levels exerts cardiovascular protection. We tested the hypothesis that ADK inhibition improves microvascular dilator and left ventricle (LV) contractile function under metabolic or hemodynamic stress.

METHODS AND RESULTS

In Obese diabetic Zucker fatty/spontaneously hypertensive heart failure F1 hybrid rats, treatment with the selective ADK inhibitor, ABT-702 (1.5 mg/kg, intraperitoneal injections for 8-week) restored acetylcholine-, sodium nitroprusside-, and adenosine-induced dilations in isolated coronary arterioles, an effect that was accompanied by normalized end-diastolic pressure (in mm Hg, Lean: 3.4 ± 0.6, Obese: 17.6 ± 4.2, Obese + ABT: 6.6 ± 1.4) and LV relaxation constant, Tau (in ms, Lean: 6.9 ± 1.5, Obese: 13.9 ± 1.7, Obese + ABT: 6.0 ± 1.1). Mice with vascular endothelium selective ADK deletion (ADK^VEC KO) exhibited an enhanced dilation to acetylcholine in isolated gracilis muscle (lgEC₅₀ WT: -8.2 ± 0.1, ADK^VEC KO: -8.8 ± 0.1, P < .05) and mesenteric arterioles (lgEC₅₀ WT: -7.4 ± 0.2, ADK^VEC KO: -8.1 ± 1.2, P < .05) when compared to wild-type (WT) mice, whereas relaxation of the femoral artery and aorta (lgEC₅₀ WT: -7.03 ± 0.6, ADK^VEC KO: -7.05 ± 0.8) was similar in the two groups. Wild-type mice progressively developed LV systolic and diastolic dysfunction when they underwent transverse aortic constriction surgery, whereas ADK^VEC -KO mice displayed a lesser degree in decline of LV function.

CONCLUSIONS

Our results indicate that ADK inhibition selectively enhances microvascular vasodilator function, whereby it improves LV perfusion and LV contractile function under metabolic and hemodynamic stress.

Regulation of vascular tone by transient receptor potential ankyrin 1 channels

The Ca²⁺-permeable, non-selective cation channel, TRPA1 (transient receptor potential ankyrin 1), is the sole member of the ankyrin TRP subfamily. TRPA1 channels are expressed on the plasma membrane of neurons as well as non-neuronal cell types, such as vascular endothelial cells. TRPA1 is activated by electrophilic compounds, including dietary molecules such as allyl isothiocyanate, a derivative of mustard. Endogenously, the channel is thought to be activated by reactive oxygen species and their metabolites, such as 4-hydroxynonenal (4-HNE). In the context of the vasculature, activation of TRPA1 channels results in a vasodilatory response mediated by two distinct mechanisms. In the first instance, TRPA1 is expressed in sensory nerves of the vasculature and, upon activation, mediates release of the potent dilator, calcitonin gene-related peptide (CGRP). In the second, work from our laboratory has demonstrated that TRPA1 is expressed in the endothelium of blood vessels exclusively in the cerebral vasculature, where its activation produces a localized Ca²⁺ signal that results in dilation of cerebral arteries. In this chapter, we provide an in-depth overview of the biophysical and pharmacological properties of TRPA1 channels and their importance in regulating vascular tone.

Interactions between renal vascular resistance and endothelium-derived hyperpolarization in hypertensive rats in vivo

Endothelium derived signaling mechanisms play an important role in regulating vascular tone and endothelial dysfunction is often found in hypertension. Endothelium-derived hyperpolarization (EDH) plays a significant role in smaller renal arteries and arterioles, but its significance in vivo in hypertension is unresolved. The aim of this study was to characterize the EDH-induced renal vasodilation in normotensive and hypertensive rats during acute intrarenal infusion of ACh. Our hypothesis was that the increased renal vascular resistance (RVR) found early in hypertension would significantly correlate with reduced EDH-induced vasodilation. In isoflurane-anesthetized 12-week-old normo- and hypertensive rats blood pressure and renal blood flow (RBF) was measured continuously. RBF responses to acute intrarenal ACh infusions were measured before and after inhibition of NO and prostacyclin. Additionally, RVR was decreased or increased using inhibition or activation of adrenergic receptors or by use of papaverine and angiotensin II. Intrarenal infusion of ACh elicited a larger increase in RBF in hypertensive rats compared to normotensive rats suggesting that endothelial dysfunction is not present in 12-week-old hypertensive rats. The EDH-induced renal vasodilation (after inhibition of NO and prostacyclin) was similar between normo- and hypertensive rats. Reducing RVR by inhibition of α1 -adrenergic receptors significantly increased the renal EDH response in hypertensive rats, but a similar increase was found after activating α-adrenergic receptors using norepinephrine. The results show that renal EDH is present and functional in 12-week-old normo- and hypertensive rats. Interestingly, both activation and inactivation of α1 -adrenergic receptors elicited an increase in the renal EDH-induced vasodilation.

Endothelium-Dependent Hyperpolarization (EDH) in Diabetes: Mechanistic Insights and Therapeutic Implications

Diabetes mellitus is one of the major risk factors for cardiovascular disease and is an important health issue worldwide. Long-term diabetes causes endothelial dysfunction, which in turn leads to diabetic vascular complications. Endothelium-derived nitric oxide is a major vasodilator in large-size vessels, and the hyperpolarization of vascular smooth muscle cells mediated by the endothelium plays a central role in agonist-mediated and flow-mediated vasodilation in resistance-size vessels. Although the mechanisms underlying diabetic vascular complications are multifactorial and complex, impairment of endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells would contribute at least partly to the initiation and progression of microvascular complications of diabetes. In this review, we present the current knowledge about the pathophysiology and underlying mechanisms of impaired EDH in diabetes in animals and humans. We also discuss potential therapeutic approaches aimed at the prevention and restoration of EDH in diabetes.

Connexins in the control of vasomotor function

Vascular endothelial cells, as well as smooth muscle cells, show heterogeneity with regard to their receptor expression and reactivity. For the vascular wall to act as a functional unit, the various cells' responses require integration. Such an integration is not only required for a homogeneous response of the vascular wall, but also for the vasomotor behaviour of consecutive segments of the microvascular arteriolar tree. As flow resistances of individual sections are connected in series, sections require synchronization and coordination to allow effective changes of conductivity and blood flow. A prerequisite for the local coordination of individual vascular cells and different sections of an arteriolar tree is intercellular communication. Connexins are involved in a dual manner in this coordination. (i) By forming gap junctions between cells, they allow an intercellular exchange of signalling molecules and electrical currents. In particular, the spread of electrical currents allows for coordination of cell responses over longer distances. (ii) Connexins are able to interact with other proteins to form signalling complexes. In this way, they can modulate and integrate individual cells' responses also in a channel-independent manner. This review outlines mechanisms allowing the vascular connexins to exert their coordinating function and to regulate the vasomotor reactions of blood vessels both locally, and in vascular networks. Wherever possible, we focus on the vasomotor behaviour of small vessels and arterioles which are the main vessels determining vascular resistance, blood pressure and local blood flow.

Acid-base regulation and sensing: Accelerators and brakes in metabolic regulation of cerebrovascular tone

Metabolic regulation of cerebrovascular tone directs blood flow to areas of increased neuronal activity and during disease states partially compensates for insufficient perfusion by enhancing blood flow in collateral blood vessels. Acid-base disturbances frequently occur as result of enhanced metabolism or insufficient blood supply, but despite definitive evidence that acid-base disturbances alter arterial tone, effects of individual acid-base equivalents and the underlying signaling mechanisms are still being debated. H+ is an important intra- and extracellular messenger that modifies cerebrovascular tone. In addition, low extracellular [HCO3-] promotes cerebrovascular contraction through an endothelium-dependent mechanism. CO2 alters arterial tone development via changes in intra- and extracellular pH but it is still controversial whether CO2 also has direct vasomotor effects. Vasocontractile responses to low extracellular [HCO3-] and acute CO2-induced decreases in intracellular pH can counteract H+-mediated vasorelaxation during metabolic and respiratory acidosis, respectively, and may thereby reduce the risk of capillary damage and cerebral edema that could be consequences of unopposed vasodilation. In this review, the signaling mechanisms for acid-base equivalents in cerebral arteries and the mechanisms of intracellular pH control in the arterial wall are discussed in the context of metabolic regulation of cerebrovascular tone and local perfusion.

Phosphodiesterase-3 inhibitor cilostazol reverses endothelial dysfunction with ageing in rat mesenteric resistance arteries

Ageing impairs endothelial function, which is considered a hallmark of the development of cardiovascular diseases in elderly. Cilostazol, a phosphodiesterase-3 inhibitor, has antiplatelet, antithrombotic and protective effects on endothelial cells. Here, we hypothesized that cilostazol could improve endothelial function in mesenteric resistance arteries (MRA) from old rats. Using eight-week cilostazol-treated (100mg/kg/day) or untreated 72-week-old Wistar rats, we evaluate the relaxation to acetylcholine, sodium nitroprusside (SNP), forskolin and isoproterenol and the noradrenaline-induced contraction in MRA. Superoxide anion and nitric oxide (NO) was measured by dihydroethidium- and diaminofluorescein-2-emitted fluorescence, respectively. Normotensive old rats had impaired acetylcholine-induced NO- and EDHF-mediated relaxation and increased noradrenaline vasoconstriction than young rats. This age-associated endothelial dysfunction was restored by cilostazol treatment. Relaxation to SNP, forskolin or isoproterenol remained unmodified by cilostazol. Diaminofluorescein-2-emitted fluorescence was increased while dihydroethidium-emitted was decreased by cilostazol, indicating increased NO and reduced superoxide generation, respectively. Cilostazol improves endothelial function in old MRA without affecting blood pressure. This protective effect of cilostazol could be attributed to reduced oxidative stress, increased NO bioavailability and EDHF-type relaxation. Although these results are preliminary, we believe that should stimulate further interest in cilostazol as an alternative for the treatment of age-related vascular disorders.

Endothelium-Derived Hyperpolarization and Coronary Vasodilation: Diverse and Integrated Roles of Epoxyeicosatrienoic Acids, Hydrogen Peroxide, and Gap Junctions

Myocardial perfusion and coronary vascular resistance are regulated by signaling metabolites released from the local myocardium that act either directly on the VSMC or indirectly via stimulation of the endothelium. A prominent mechanism of vasodilation is EDH of the arteriolar smooth muscle, with EETs and H(2)O(2) playing important roles in EDH in the coronary microcirculation. In some cases, EETs and H(2)O(2) are released as transferable hyperpolarizing factors (EDHFs) that act directly on the VSMCs. By contrast, EETs and H(2)O(2) can also promote endothelial KCa activity secondary to the amplification of extracellular Ca(2+) influx and Ca(2+) mobilization from intracellular stores, respectively. The resulting endothelial hyperpolarization may subsequently conduct to the media via myoendothelial gap junctions or potentially lead to the release of a chemically distinct factor(s). Furthermore, in human isolated coronary arterioles dilator signaling involving EETs and H(2)O(2) may be integrated, being either complimentary or inhibitory depending on the stimulus. With an emphasis on the human coronary microcirculation, this review addresses the diverse and integrated mechanisms by which EETs and H(2)O(2) regulate vessel tone and also examines the hypothesis that myoendothelial microdomain signaling facilitates EDH activity in the human heart.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

P2Y₂ receptor activation decreases blood pressure via intermediate conductance potassium channels and connexin 37

AIMS

Nucleotides are important paracrine regulators of vascular tone. We previously demonstrated that activation of P2Y₂ receptors causes an acute, NO-independent decrease in blood pressure, indicating this signalling pathway requires an endothelial-derived hyperpolarization (EDH) response. To define the mechanisms by which activation of P2Y₂ receptors initiates EDH and vasodilation, we studied intermediate-conductance (KCa3.1, expressed in endothelial cells) and big-conductance potassium channels (KCa1.1, expressed in smooth muscle cells) as well as components of the myoendothelial gap junction, connexins 37 and 40 (Cx37, Cx40), all hypothesized to be part of the EDH response.

METHODS

We compared the effects of a P2Y₂/₄ receptor agonist in wild-type (WT) mice and in mice lacking KCa3.1, KCa1.1, Cx37 or Cx40 under anaesthesia, while monitoring intra-arterial blood pressure and heart rate.

RESULTS

Acute activation of P2Y₂/₄ receptors (0.01-3 mg kg(-1) body weight i.v.) caused a biphasic blood pressure response characterized by a dose-dependent and rapid decrease in blood pressure in WT (maximal response % of baseline at 3 mg kg(-1) : -38 ± 1%) followed by a consecutive increase in blood pressure (+44 ± 11%). The maximal responses in KCa3.1(-/-) and Cx37(-/-) were impaired (-13 ± 5, +17 ± 7 and -27 ± 1, +13 ± 3% respectively), whereas the maximal blood pressure decrease in response to acetylcholine at 3 μg kg(-1) was not significantly different (WT: -53 ± 3%; KCa3.1(-/-) : -52 ± 3; Cx37(-/-) : -53 ± 3%). KCa1.1(-/-) and Cx40(-/-) showed an identical biphasic response to P2Y2/4 receptor activation compared to WT.

CONCLUSIONS

The data suggest that the P2Y2/4 receptor activation elicits blood pressure responses via distinct mechanisms involving KCa3.1 and Cx37.

Traumatic brain injury in vivo and in vitro contributes to cerebral vascular dysfunction through impaired gap junction communication between vascular smooth muscle cells

Gap junctions (GJs) contribute to cerebral vasodilation, vasoconstriction, and, perhaps, to vascular compensatory mechanisms, such as autoregulation. To explore the effects of traumatic brain injury (TBI) on vascular GJ communication, we assessed GJ coupling in A7r5 vascular smooth muscle (VSM) cells subjected to rapid stretch injury (RSI) in vitro and VSM in middle cerebral arteries (MCAs) harvested from rats subjected to fluid percussion TBI in vivo. Intercellular communication was evaluated by measuring fluorescence recovery after photobleaching (FRAP). In VSM cells in vitro, FRAP increased significantly (p<0.05 vs. sham RSI) after mild RSI, but decreased significantly (p<0.05 vs. sham RSI) after moderate or severe RSI. FRAP decreased significantly (p<0.05 vs. sham RSI) 30 min and 2 h, but increased significantly (p<0.05 vs. sham RSI) 24 h after RSI. In MCAs harvested from rats 30 min after moderate TBI in vivo, FRAP was reduced significantly (p<0.05), compared to MCAs from rats after sham TBI. In VSM cells in vitro, pretreatment with the peroxynitrite (ONOO(-)) scavenger, 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron[III], prevented RSI-induced reductions in FRAP. In isolated MCAs from rats treated with the ONOO(-) scavenger, penicillamine, GJ coupling was not impaired by fluid percussion TBI. In addition, penicillamine treatment improved vasodilatory responses to reduced intravascular pressure in MCAs harvested from rats subjected to moderate fluid percussion TBI. These results indicate that TBI reduced GJ coupling in VSM cells in vitro and in vivo through mechanisms related to generation of the potent oxidant, ONOO(-).

Interactions between thromboxane A₂, thromboxane/prostaglandin (TP) receptors, and endothelium-derived hyperpolarization

Endothelium-dependent smooth muscle hyperpolarization (EDH) increasingly predominates over endothelium-derived nitric oxide (NO) as a participant in vasodilation as vessel size decreases. Its underlying nature is highly variable between vessel types, species, disease states, and exact experimental conditions, and is variably mediated by one or more transferable endothelium-derived hyperpolarizing factors and/or the electrotonic spread of endothelial hyperpolarization into the media via gap junctions. Although generally regarded (and studied) as a mechanism that is independent of NO and prostanoids, evidence has emerged that the endothelium-derived contracting factor and prostanoid thromboxane A2 can modulate several signalling components central to EDH, and therefore potentially curtail vasodilation through mechanisms that are distinct from those putatively involved in direct smooth muscle contraction. Notably, vascular production of thromboxane A2 is elevated in a number of cardiovascular disease states that promote endothelial dysfunction. This review will therefore discuss the mechanisms through which thromboxane A2 interacts with and modulates EDH, and will also consider the implications of such cross-talk in vasodilator control in health and disease.

Activation of KCa3.1 by SKA-31 induces arteriolar dilatation and lowers blood pressure in normo- and hypertensive connexin40-deficient mice

BACKGROUND AND PURPOSE

The calcium-activated potassium channel KCa3.1 is expressed in the vascular endothelium where its activation causes endothelial hyperpolarization and initiates endothelium-derived hyperpolarization (EDH)-dependent dilatation. Here, we investigated whether pharmacological activation of KCa3.1 dilates skeletal muscle arterioles and whether myoendothelial gap junctions formed by connexin40 (Cx40) are required for EDH-type dilatations and pressure depressor responses in vivo.

EXPERIMENTAL APPROACH

We performed intravital microscopy in the cremaster muscle microcirculation and blood pressure telemetry in Cx40-deficient mice.

KEY RESULTS

In wild-type mice, the KCa3.1-activator SKA-31 induced pronounced concentration-dependent arteriolar EDH-type dilatations, amounting to ∼40% of maximal dilatation, and enhanced the effects of ACh. These responses were absent in mice devoid of KCa3.1 channels. In contrast, SKA-31-induced dilatations were not attenuated in mice with endothelial cells deficient in Cx40 (Cx40(fl/fl):Tie2-Cre). In isolated endothelial cell clusters, SKA-31 induced hyperpolarizations of similar magnitudes (by ∼38 mV) in Cx40(fl/fl):Tie2-Cre, ubiquitous Cx40-deficient mice (Cx40(-/-)) and controls (Cx40(fl/fl)), which were reversed by the specific KCa3.1-blocker TRAM-34. In normotensive wild-type and Cx40(fl/fl):Tie2-Cre as well as in hypertensive Cx40(-/-) animals, i.p. injections of SKA-31 (30 and 100 mg·kg(-1)) decreased arterial pressure by ∼32 mmHg in all genotypes. The depressor response to 100 mg·kg(-1) SKA-31 was associated with a decrease in heart rate.

CONCLUSIONS AND IMPLICATIONS

We conclude that endothelial hyperpolarization evoked by pharmacological activation of KCa3.1 channels induces EDH-type arteriolar dilatations that are independent of endothelial Cx40 and Cx40-containing myoendothelial gap junctions. As SKA-31 reduced blood pressure in hypertensive Cx40-deficient mice, KCa3.1 activators may be useful drugs for severe treatment-resistant hypertension.

Exchange protein activated by cAMP (Epac) induces vascular relaxation by activating Ca2+-sensitive K+ channels in rat mesenteric artery

Vasodilator-induced elevation of intracellular cyclic AMP (cAMP) is a central mechanism governing arterial relaxation but is incompletely understood due to the diversity of cAMP effectors. Here we investigate the role of the novel cAMP effector exchange protein directly activated by cAMP (Epac) in mediating vasorelaxation in rat mesenteric arteries. In myography experiments, the Epac-selective cAMP analogue 8-pCPT-2-O-Me-cAMP-AM (5 μM, subsequently referred to as 8-pCPT-AM) elicited a 77.6 ± 7.1% relaxation of phenylephrine-contracted arteries over a 5 min period (mean ± SEM; n = 6). 8-pCPT-AM induced only a 16.7 ± 2.4% relaxation in arteries pre-contracted with high extracellular K(+) over the same time period (n = 10), suggesting that some of Epac's relaxant effect relies upon vascular cell hyperpolarization. This involves Ca(2+)-sensitive, large-conductance K(+) (BK(Ca)) channel opening as iberiotoxin (100 nM) significantly reduced the ability of 8-pCPT-AM to reverse phenylephrine-induced contraction (arteries relaxed by only 35.0 ± 8.5% over a 5 min exposure to 8-pCPT-AM, n = 5; P < 0.05). 8-pCPT-AM increased Ca(2+) spark frequency in Fluo-4-AM-loaded mesenteric myocytes from 0.045 ± 0.008 to 0.103 ± 0.022 sparks s(-1) μm(-1) (P < 0.05) and reversibly increased both the frequency (0.94 ± 0.25 to 2.30 ± 0.72 s(-1)) and amplitude (23.9 ± 3.3 to 35.8 ± 7.7 pA) of spontaneous transient outward currents (STOCs) recorded in isolated mesenteric myocytes (n = 7; P < 0.05). 8-pCPT-AM-activated STOCs were sensitive to iberiotoxin (100 nM) and to ryanodine (30 μM). Current clamp recordings of isolated myocytes showed a 7.9 ± 1.0 mV (n = 10) hyperpolarization in response to 8-pCPT-AM that was sensitive to iberiotoxin (n = 5). Endothelial disruption suppressed 8-pCPT-AM-mediated relaxation in phenylephrine-contracted arteries (24.8 ± 4.9% relaxation after 5 min of exposure, n = 5; P < 0.05), as did apamin and TRAM-34, blockers of Ca(2+)-sensitive, small- and intermediate-conductance K(+) (SK(Ca) and IK(Ca)) channels, respectively, and N(G)-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase (NOS). In Fluo-4-AM-loaded mesenteric endothelial cells, 8-pCPT-AM induced a sustained increase in global Ca(2+). Our data suggest that Epac hyperpolarizes smooth muscle by (1) increasing localized Ca(2+) release from ryanodine receptors (Ca(2+) sparks) to activate BK(Ca) channels, and (2) endothelial-dependent mechanisms involving the activation of SK(Ca)/IK(Ca) channels and NOS. Epac-mediated smooth muscle hyperpolarization will limit Ca(2+) entry via voltage-sensitive Ca(2+) channels and represents a novel mechanism of arterial relaxation.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.

Endothelial alkalinisation inhibits gap junction communication and endothelium-derived hyperpolarisations in mouse mesenteric arteries

Abstract  Gap junctions mediate intercellular signalling in arteries and contribute to endothelium-dependent vasorelaxation, conducted vascular responses and vasomotion. Considering its putative role in vascular dysfunction, mechanistic insights regarding the control of gap junction conductivity are required. Here, we investigated the consequences of endothelial alkalinisation for gap junction communication and endothelium-dependent vasorelaxation in resistance arteries. We studied mesenteric arteries from NMRI mice by myography, confocal fluorescence microscopy and electrophysiological techniques. Removing CO2/HCO3(-), reducing extracellular [Cl(-)] or adding 4,4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited or reversed Cl(-)/HCO3(-) exchange, alkalinised the endothelium by 0.2-0.3 pH units and inhibited acetylcholine-induced vasorelaxation. NO-synthase-dependent vasorelaxation was unaffected by endothelial alkalinisation whereas vasorelaxation dependent on small- and intermediate-conductance Ca(2+)-activated K(+) channels was attenuated by ∼75%. The difference in vasorelaxation between arteries with normal and elevated endothelial intracellular pH (pHi) was abolished by the gap junction inhibitors 18β-glycyrrhetinic acid and carbenoxolone while other putative modulators of endothelium-derived hyperpolarisations - Ba(2+), ouabain, iberiotoxin, 8Br-cAMP and polyethylene glycol catalase - had no effect. In the absence of CO2/HCO3(-), addition of the Na(+)/H(+)-exchange inhibitor cariporide normalised endothelial pHi and restored vasorelaxation to acetylcholine. Endothelial hyperpolarisations and Ca(2+) responses to acetylcholine were unaffected by omission of CO2/HCO3(-). By contrast, dye transfer between endothelial cells and endothelium-derived hyperpolarisations of vascular smooth muscle cells stimulated by acetylcholine or the proteinase-activated receptor 2 agonist SLIGRL-amide were inhibited in the absence of CO2/HCO3(-). We conclude that intracellular alkalinisation of endothelial cells attenuates endothelium-derived hyperpolarisations in resistance arteries due to inhibition of gap junction communication. These findings highlight the role of pHi in modulating vascular function.

TRPA1 channels in the vasculature

This review is focused on the role of the ankyrin (A) transient receptor potential (TRP) channel TRPA1 in vascular regulation. TRPA1 is activated by environmental irritants, pungent compounds found in foods such as garlic, mustard and cinnamon, as well as metabolites produced during oxidative stress. The structure of the channel is distinguished by the ∼14-19 ankyrin repeat (AR) domains present in the intracellular amino terminus. TRPA1 has a large unitary conductance (98 pS) and slight selectivity for Ca(2+) versus Na(+) ions (P(Ca) /P(Na)  ≈ 7.9). TRPA1 is involved in numerous important physiological processes, including nociception, mechanotransduction, and thermal and oxygen sensing. TRPA1 agonists cause arterial dilation through two distinctive pathways. TRPA1 channels present in perivascular nerves mediate vasodilatation of peripheral arteries in response to chemical agonists through a mechanism requiring release of calcitonin gene-related peptide. In the cerebral circulation, TRPA1 channels are present in the endothelium, concentrated within myoendothelial junction sites. Activation of TRPA1 channels in this vascular bed causes endothelium-dependent smooth muscle cell hyperpolarization and vasodilatation that requires the activity of small and intermediate conductance Ca(2+) -activated K(+) channels. Systemic administration of TRPA1 agonists causes transient depressor responses, followed by sustained increases in heart rate and blood pressure that may result from elevated sympathetic nervous activity. These findings indicate that TRPA1 activity influences vascular function, but the precise role and significance of the channel in the cardiovascular system remains to be determined.

Myoendothelial contacts, gap junctions, and microdomains: anatomical links to function?

In several species and in many vascular beds, ultrastructural studies describe close contact sites between the endothelium and smooth muscle of <∼20nm. Such sites are thought to facilitate the local action of signaling molecules and/or the passage of current, as metabolic and electrical coupling conduits between the arterial endothelium and smooth muscle. These sites have the potential for bidirectional communication between the endothelium and smooth muscle, as a key pathway for coordinating vascular function. The aim of this brief review is to summarize the literature on the ultrastructural anatomy and distribution of key components of MECC sites in arteries. In addition to their traditional role of facilitating electrical coupling between the two cell layers, data on the role of MECC sites in arteries, as signaling microdomains involving a spatial localization of channels, receptors and calcium stores are highlighted. Diversity in the density and specific characteristics of MECC sites as signaling microdomains suggests considerable potential for functional diversity within and between arteries in health and disease.

Regulator of G protein signaling 2 deficiency causes endothelial dysfunction and impaired endothelium-derived hyperpolarizing factor-mediated relaxation by dysregulating Gi/o signaling

Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for G(q/11)α and G(i/o)α subunits. RGS2 deficiency is linked to hypertension in mice and humans, although causative mechanisms are not understood. Because endothelial dysfunction and increased peripheral resistance are hallmarks of hypertension, determining whether RGS2 regulates microvascular reactivity may reveal mechanisms relevant to cardiovascular disease. Here we have determined the effects of systemic versus endothelium- or vascular smooth muscle-specific deletion of RGS2 on microvascular contraction and relaxation. Contraction and relaxation of mesenteric resistance arteries were analyzed in response to phenylephrine, sodium nitroprusside, or acetylcholine with or without inhibitors of nitric oxide (NO) synthase or K(+) channels that mediate endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation. The results showed that deleting RGS2 in vascular smooth muscle had minor effects. Systemic or endothelium-specific deletion of RGS2 strikingly inhibited acetylcholine-evoked relaxation. Endothelium-specific deletion of RGS2 had little effect on NO-dependent relaxation but markedly impaired EDHF-dependent relaxation. Acute, inducible deletion of RGS2 in endothelium did not affect blood pressure significantly. Impaired EDHF-mediated vasodilatation was rescued by blocking G(i/o)α activation with pertussis toxin. These findings indicated that systemic or endothelium-specific RGS2 deficiency causes endothelial dysfunction resulting in impaired EDHF-dependent vasodilatation. RGS2 deficiency enables endothelial G(i/o) activity to inhibit EDHF-dependent relaxation, whereas RGS2 sufficiency facilitates EDHF-evoked relaxation by squelching endothelial G(i/o) activity. Mutation or down-regulation of RGS2 in hypertension patients therefore may contribute to endothelial dysfunction and defective EDHF-dependent relaxation. Blunting G(i/o) signaling might improve endothelial function in such patients.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

Effects of connexin-mimetic peptides on perfusion pressure in response to phenylephrine in isolated, perfused rat kidneys

BACKGROUND

Gap junction intercellular communication plays a fundamental role in various tissues and organs. Gap junctions transfer ions and molecules between adjacent cells and are formed by connexins (Cx). It is supposed that vascular conducted responses, which most likely spread through gap junctions in vascular beds, regulate microcirculatory blood flow and maintain vascular resistance. This study provides functional evidence supporting the critical role of gap junctions in a physiological setting and in phenylephrine (PE)-induced vasoconstriction using an ex vivo kidney perfusion technique.

METHODS

Using the isolated, perfused kidney model, infusion of gap junction inhibitors and PE, we examined the local effect of gap junction communication. Additionally, gap junction proteins Cx37, Cx40 and Cx43 were detected by immunofluorescence.

RESULTS

First, changes in the perfusion pressure were analyzed by infusing the nonselective gap junction uncoupler, 18α-glycyrrhetinic acid (18α-GA), and specific connexin-mimetic peptide inhibitors, (37,43)Gap27, (40)Gap27 and (43)Gap26. Administration of 18α-GA and (43)Gap26 significantly elevated perfusion pressure while infusion of (40)Gap27 and (37,43)Gap27 had no effect. Second, we examined the effect of infusing gap junction inhibitors on PE-induced vasoconstriction. Infusion of 18α-GA and (40)Gap27 significantly suppressed the increase in perfusion pressure induced by PE, while (43)Gap26 and (37,43)Gap27 had no effect. Third, we confirmed by immunofluorescence that Cx37, Cx40 and Cx43 were found in the endothelial cells of interstitial microvessels and that Cx40 was localized in glomerular mesangial cells as well as in smooth muscle cells of the juxtaglomerular area.

CONCLUSIONS

This study showed that Cx43 plays a pivotal role in regulating renal vascular resistance and that Cx40 attenuates PE-induced vasoconstriction. These results provide new evidence that gap junctions may control renal circulation and vascular responses.

Endothelium-derived hyperpolarizing factor and diabetes

In addition to its role as a barrier between blood and tissues, the vascular endothelium is responsible for the synthesis and released of a number of vasodilators including prostaglandins, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). As one of these vasodilators, the specific nature of EDHF has not been fully elucidated, although a number of roles have been proposed. Importantly, many conditions, such as hypertension, hyperlipidemia, heart failure, ischemia-reperfusion and diabetes mellitus comprise vascular endothelial dysfunction with EDHF dysregulation. This article reviews reports on the role of EDHF in diabetes-related endothelial dysfunction.

Control of muscle blood flow during exercise: local factors and integrative mechanisms

Understanding the control mechanisms of blood flow within the vasculature of skeletal muscle is clearly fascinating from a theoretical point of view due to the extremely tight coupling of tissue oxygen demands and blood flow. It also has practical implications as impairment of muscle blood flow and its prevention/reversal by exercise training has a major impact on widespread diseases such as hypertension and diabetes. Here we analyse the role of mediators generated by skeletal muscle activity on smooth muscle relaxation in resistance vessels in vitro and in vivo. We summarize their cellular mechanisms of action and their relative roles in exercise hyperaemia with regard to early and late responses. We also discuss the consequences of interactions among mediators with regard to identifying their functional significance. We focus on (potential) mechanisms integrating the action of the mediators and their effects among the cells of the intact arteriolar wall. This integration occurs both locally, partly due to myoendothelial communication, and axially along the vascular tree, thus enabling the local responses to be manifest along an entire functional vessel path. Though the concept of signal integration is intriguing, its specific role on the control of exercise hyperaemia and the consequences of its modulation under physiological and pathophysiological conditions still await additional analysis.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Hydrogen peroxide as an endothelium-derived hyperpolarizing factor

The endothelium plays an important role in maintaining cardiovascular homeostasis by synthesizing and releasing several vasodilating substances, including vasodilator prostaglandins, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). Since the first report on the existence of EDHF, several substances/mechanisms have been proposed for the nature of EDHF, including epoxyeicosatrienoic acids (metabolites of arachidonic P450 epoxygenase pathway), K ions, and electrical communications through myoendothelial gap junctions. We have demonstrated that endothelium-derived hydrogen peroxide (H(2)O(2)) is an EDHF in animals and humans. For the synthesis of H(2)O(2)/EDHF, endothelial NO synthase system that is functionally coupled with Cu,Zn-superoxide dismutase plays a crucial role. Importantly, endothelium-derived H(2)O(2) plays important protective roles in the coronary circulation, including coronary autoregulation, protection against myocardial ischemia/reperfusion injury, and metabolic coronary vasodilatation. Indeed, our H(2)O(2)/EDHF theory demonstrates that endothelium-derived H(2)O(2), another reactive oxygen species in addition to NO, plays important roles as a redox-signaling molecule to cause vasodilatation as well as cardioprotection. In this review, we summarize our current knowledge on H(2)O(2)/EDHF regarding its identification and mechanisms of synthesis and actions.

Vascular smooth muscle modulates endothelial control of vasoreactivity via reactive oxygen species production through myoendothelial communications

Background

Endothelial control of vascular smooth muscle plays a major role in the resulting vasoreactivity implicated in physiological or pathological circulatory processes. However, a comprehensive understanding of endothelial (EC)/smooth muscle cells (SMC) crosstalk is far from complete. Here, we have examined the role of gap junctions and reactive oxygen species (ROS) in this crosstalk and we demonstrate an active contribution of SMC to endothelial control of vasomotor tone.

Methodology/Principal Findings

In small intrapulmonary arteries, quantitative RT-PCR, Western Blot analyses and immunofluorescent labeling evidenced connexin (Cx) 37, 40 and 43 in EC and/or SMC. Functional experiments showed that the Cx-mimetic peptide targeted against Cx 37 and Cx 43 (^37,43Gap27) (1) reduced contractile and calcium responses to serotonin (5-HT) simultaneously recorded in pulmonary arteries and (2) abolished the diffusion in SMC of carboxyfluorescein-AM loaded in EC. Similarly, contractile and calcium responses to 5-HT were decreased by superoxide dismutase and catalase which, catabolise superoxide anion and H₂O₂, respectively. Both Cx- and ROS-mediated effects on the responses to 5-HT were reversed by L-NAME, a NO synthase inhibitor or endothelium removal. Electronic paramagnetic resonance directly demonstrated that 5-HT-induced superoxide anion production originated from the SMC. Finally, whereas 5-HT increased NO production, it also decreased cyclic GMP content in isolated intact arteries.

Conclusions/Significance

These data demonstrate that agonist-induced ROS production in SMC targeting EC via myoendothelial gap junctions reduces endothelial NO-dependent control of pulmonary vasoreactivity. Such SMC modulation of endothelial control may represent a signaling pathway controlling vasoreactivity under not only physiological but also pathological conditions that often implicate excessive ROS production.

Role of cyclic nucleotides in the control of cytosolic Ca2+ levels in vascular endothelial cells

1. Endothelial cells have a key role in the cardiovascular system. Most endothelial cell functions depend on changes in cytosolic Ca(2+) concentrations ([Ca(2+)](i)) to some extent and Ca2+ signalling acts to link external stimuli with the synthesis and release of regulatory factors in endothelial cells. The [Ca(2+)](i) is maintained by a well-balanced Ca(2+) flux across the endoplasmic reticulum and plasma membrane. 2. Cyclic nucleotides, such as cAMP and cGMP, are very important second messengers. The cyclic nucleotides can affect [Ca(2+)](i) directly or indirectly (via the actions of protein kinase (PK) A or PKG-mediated phosphorylation) by regulating Ca(2+) mobilization and Ca(2+) influx. Fine-tuning of [Ca(2+)](i) is also fundamental to protect endothelial cells against damaged caused by the excessive accumulation of Ca(2+). 3. Therapeutic agents that control cAMP and cGMP levels have been used to treat various cardiovascular diseases. 4. The aim of the present review is to discuss: (i) the functions of endothelial cells; (ii) the importance of [Ca(2+)](i) in endothelial cells; (iii) the impact of excessive [Ca(2+)](i) in endothelial cells; and (iv) the balanced control of [Ca(2+)](i) in endothelial cells via involvement of cyclic nucleotides (cAMP and cGMP) and their general effectors.

Prominent role of KCa3.1 in endothelium-derived hyperpolarizing factor-type dilations and conducted responses in the microcirculation in vivo

AIMS

The activation of endothelial Ca2+-dependent K+-channels, KCa3.1 (IKCa), and KCa2.3 (SKCa) has been proposed to be a prerequisite for endothelial hyperpolarization, which subsequently hyperpolarizes and relaxes smooth muscle [endothelium-derived hyperpolarizing factor (EDHF)-type dilation] and initiates conducted dilations. Although EDHF is the main mediator of acetylcholine (ACh)-induced dilation in the murine skeletal microcirculation, the differential contribution of KCa3.1 and KCa2.3 is not known.

METHODS AND RESULTS

We assessed agonist-induced and conducted dilations as well as endothelial hyperpolarization in the cremaster microcirculation of KCa3.1-deficient (KCa3.1-/-) and wild-type mice (wt) in vivo after blockade of NO and prostaglandins. Compared with wt, resting tone was enhanced by approximately 25% in arterioles of KCa3.1-/- mice. ACh-induced dilations in KCa3.1-/- mice were virtually abolished at low and intermediate concentrations and a remaining dilation at 10 micromol/L ACh was abrogated by blockade of KCa2.3 with UCL1684. Sodium nitroprusside- and adenosine-induced dilations were similar in wt and KCa3.1-/-. Focal application of ACh induced dilations at the local site in both genotypes, which conducted along the vessel. However, the amplitude of the dilation decreased with distance only in KCa3.1-/-. Blockade of KCa2.3 in wt did not affect conducted dilations. A KCa3.1 opener induced a conducting dilation in wt but not in KCa3.1-/-. Membrane potential recordings in vivo demonstrated endothelial hyperpolarization in response to ACh in both genotypes; however, the hyperpolarization was severely impaired in KCa3.1-/- (Delta membrane potential: -3 +/- 1 vs. -14 +/- 2 mV).

CONCLUSION

We conclude that KCa3.1 is of major importance for endothelial hyperpolarization and EDHF-type responses in skeletal muscle arterioles, and its deficiency is not compensated by KCa2.3. Sole activation of KCa3.1 is capable of initiating conducted responses, and KCa3.1 may contribute to the propagation of the signal, although its presence is not mandatory.

Potassium softens vascular endothelium and increases nitric oxide release

In the presence of aldosterone, plasma sodium in the high physiological range stiffens endothelial cells and reduces the release of nitric oxide. We now demonstrate effects of extracellular potassium on stiffness of individual cultured bovine aortic endothelial cells by using the tip of an atomic force microscope as a mechanical nanosensor. An acute increase of potassium in the physiological range swells and softens the endothelial cell and increases the release of nitric oxide. A high physiological sodium concentration, in the presence of aldosterone, prevents these changes. We propose that the potassium effects are caused by submembranous cortical fluidization because cortical actin depolymerization induced by cytochalasin D mimics the effect of high potassium. In contrast, a low dose of trypsin, known to activate sodium influx through epithelial sodium channels, stiffens the submembranous cell cortex. Obviously, the cortical actin cytoskeleton switches from gelation to solation depending on the ambient sodium and potassium concentrations, whereas the center of the cell is not involved. Such a mechanism would control endothelial deformability and nitric oxide release, and thus influence systemic blood pressure.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Cilostazol improves endothelial dysfunction by increasing endothelium-derived hyperpolarizing factor response in mesenteric arteries from Type 2 diabetic rats

Diabetes mellitus impairs endothelial function, an effect that can be considered a hallmark of the development of cardiovascular diseases in diabetics. Cilostazol, a selective phosphodiesterase 3 inhibitor, is currently used to treat patients with diabetic vascular complications. However, the effects of cilostazol on responses mediated by endothelium-derived relaxing [in particular, nitric oxide (NO) and hyperpolarizing factors (EDHF)] and contracting factors remain unclear. Here, we hypothesized that cilostazol could improve endothelial dysfunctions in mesenteric arteries isolated from type 2 diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Using cilostazol-treated (100 mg/kg/day for 4 weeks) or -untreated OLETF and control (Long Evans Tokushima Otsuka) rats, we examined the acetylcholine-induced endothelium-dependent responses and the cell-permeant cyclic adenosine monophosphate (cAMP) analog-induced relaxations in the superior mesenteric artery. We also determined blood parameters in these animals. In OLETF rats, chronic treatment with cilostazol reduced the blood levels of triglyceride, non-esterified fatty acids, and leptin, and increased antioxidant capacity, but did not alter the blood glucose or insulin levels. In studies on mesenteric arteries from cilostazol-treated OLETF animals, the cilostazol treatment improved: (a) the acetylcholine-induced EDHF-mediated relaxation and (b) the cAMP-mediated relaxation. However, cilostazol did not alter the NO-mediated relaxation or the endothelium-derived contracting factor-mediated contraction. These results suggest that cilostazol improves endothelial functions in OLETF mesenteric arteries by increasing EDHF signaling, and that it normalizes some metabolic abnormalities in OLETF rats. On that basis, cilostazol may prove to be a potent drug for the clinical treatment of diabetic vasculopathy.

Connexin mimetic peptides fail to inhibit vascular conducted calcium responses in renal arterioles

Vascular conducted responses are believed to play a central role in controlling the microcirculatory blood flow. The responses most likely spread through gap junctions in the vascular wall. At present, four different connexins (Cx) have been detected in the renal vasculature, but their role in transmission of conducted vasoconstrictor signals in the preglomerular arterioles is unknown. Connexin mimetic peptides were previously reported to target and inhibit specific connexins. We, therefore, investigated whether conducted vasoconstriction in isolated renal arterioles could be blocked by the use of mimetic peptides directed against one or more connexins. Preglomerular resistance vessels were microdissected from kidneys of Sprague-Dawley rats and loaded with fura 2. The vessels were stimulated locally by applying electrical current through a micropipette, and the conducted calcium response was measured 500 mum from the site of stimulation. Application of connexin mimetic peptides directed against Cx40, 37/43, 45, or a cocktail with equimolar amounts of each, did not inhibit the propagated response, whereas the nonselective gap junction uncoupler carbenoxolone completely abolished the propagated response. However, the connexin mimetic peptides were able to reduce dye coupling between rat aorta endothelial cells shown to express primarily Cx40. In conclusion, we did not observe any attenuating effects on conducted calcium responses in isolated rat interlobular arteries when exposed to connexin mimetic peptides directed against Cx40, 37/43, or 45. Further studies are needed to determine whether conducted vasoconstriction is mediated via previously undescribed pathways.

Extracellular-loop peptide antibodies reveal a predominant hemichannel organization of connexins in polarized intestinal cells

Shigella, the causative agent of bacillary dysentery, invades colonic epithelial cells to elicit an intense inflammatory reaction leading to destruction of the mucosa. ATP-dependent paracrine signalling induced by connexin (Cx) hemichannel opening was previously shown to favor Shigella flexneri invasion and dissemination in transfectants of HeLa cells [G. Tran Van Nhieu, C. Clair, R. Bruzzone, M. Mesnil, P. Sansonetti and L. Combettes. (2003). Connexin-dependent intercellular communication increases invasion and dissemination of Shigella in epithelial cells. Nat. Cell Biol. 5, 720-726.]. However, although Cxs have been described in polarized epithelial cells, little is known about their structural organization and the role of hemichannels during S. flexneri invasion. We show here that polarized Caco-2/TC7 cells express significant amounts of Cx26, Cx32 and Cx43, but that unexpectedly, cell-cell coupling assessed by dye-transfer experiments is inefficient. Consistent with a predominant Cx organization in hemichannels, dye loading induced by low calcium was readily observed, with preferential loading at the basolateral side. Antibodies (Abs) against connexin extracellular loop peptides (CELAbs) demonstrated the importance of hemichannel signalling since they inhibited dye uptake at low calcium and at physiological calcium concentrations during S. flexneri invasion. Importantly, CELAbs allowed the visualization of hemichannels at the surface of epithelial cells, as structures distinct from gap intercellular junctions.

Role of caveolar compartmentation in endothelium-derived hyperpolarizing factor-mediated relaxation: Ca2+ signals and gap junction function are regulated by caveolin in endothelial cells

BACKGROUND

In endothelial cells, caveolin-1, the structural protein of caveolae, acts as a scaffolding protein to cluster lipids and signaling molecules within caveolae and, in some instances, regulates the activity of proteins targeted to caveolae. Specifically, different putative mediators of the endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation are located in caveolae and/or regulated by the structural protein caveolin-1, such as potassium channels, calcium regulatory proteins, and connexin 43, a molecular component of gap junctions.

METHODS AND RESULTS

Comparing relaxation in vessels from caveolin-1 knockout mice and their wild-type littermates, we observed a complete absence of EDHF-mediated vasodilation in isolated mesenteric arteries from caveolin-1 knockout mice. The absence of caveolin-1 is associated with an impairment of calcium homeostasis in endothelial cells, notably, a decreased activity of Ca2+-permeable TRPV4 cation channels that participate in nitric oxide- and EDHF-mediated relaxation. Moreover, morphological characterization of caveolin-1 knockout and wild-type arteries showed fewer gap junctions in vessels from knockout animals associated with a lower expression of connexins 37, 40, and 43 and altered myoendothelial communication. Finally, we showed that TRPV4 channels and connexins colocalize with caveolin-1 in the caveolar compartment of the plasma membrane.

CONCLUSIONS

We demonstrated that expression of caveolin-1 is required for EDHF-related relaxation by modulating membrane location and activity of TRPV4 channels and connexins, which are both implicated at different steps in the EDHF-signaling pathway.

Enhanced spontaneous Ca2+ events in endothelial cells reflect signalling through myoendothelial gap junctions in pressurized mesenteric arteries

Increases in global Ca(2+) in the endothelium are a crucial step in releasing relaxing factors to modulate arterial tone. In the present study we investigated spontaneous Ca(2+) events in endothelial cells, and the contribution of smooth muscle cells to these Ca(2+) events, in pressurized rat mesenteric resistance arteries. Spontaneous Ca(2+) events were observed under resting conditions in 34% of cells. These Ca(2+) events were absent in arteries preincubated with either cyclopiazonic acid or U-73122, but were unaffected by ryanodine or nicotinamide. Stimulation of smooth muscle cell depolarization and contraction with either phenylephrine or high concentrations of KCl significantly increased the frequency of endothelial cell Ca(2+) events. The putative gap junction uncouplers carbenoxolone and 18alpha-glycyrrhetinic acid each inhibited spontaneous and evoked Ca(2+) events, and the movement of calcein from endothelial to smooth muscle cells. In addition, spontaneous Ca(2+) events were diminished by nifedipine, lowering extracellular Ca(2+) levels, or by blockers of non-selective Ca(2+) influx pathways. These findings suggest that in pressurized rat mesenteric arteries, spontaneous Ca(2+) events in the endothelial cells appear to originate from endoplasmic reticulum IP(3) receptors, and are subject to regulation by surrounding smooth muscle cells via myoendothelial gap junctions, even under basal conditions.

Endothelium-dependent relaxation is resistant to inhibition of nitric oxide synthesis, but sensitive to blockade of calcium-activated potassium channels in essential hypertension

In human essential hypertension (EH), endothelium-dependent relaxation can occur independent of nitric oxide (NO) and prostacyclin (PGI(2)). Recent in vivo data suggest that rapid compensatory upregulation of endothelial cytochrome P450 epoxygenase 2C9 occurs to preserve vasorelaxation under conditions of decreased NO bioavailability. As one of the vascular actions of CYP2C9 is to modulate small and intermediate conductance endothelial calcium-activated potassium channels (SK(Ca) and IK(Ca)), we examined whether endothelium-dependent relaxation is sensitive to inhibitors of these channels (apamin and charybdotoxin) in resistance-sized vessels from human with EH. Subcutaneous gluteal biopsies were performed on 12 humans with EH and 12 matched control subjects. Resistance arteries were dissected and relaxation responses to carbachol were assessed ex vivo using wire myography in the presence of: (i) N(G)-nitro-L-arginine (L-NOARG)/indomethacin; and (ii) apamin/charybdotoxin. Maximal carbachol relaxation was impaired in EH vs control subjects. No differences in responses were observed with the endothelium-independent agonist, S-nitroso-N-acetyl-penicillamine. Relaxation to carbachol was attenuated following incubation with L-NOARG/indomethacin in vessels from control subjects (P<0.01 analysis of variance (ANOVA)), but not in vessels from patients with EH. The reverse pattern was seen following apamin/charybdotoxin with carbachol relaxation attenuated only in EH vessels (P<0.001 ANOVA). Endothelium-dependent relaxation is resistant to endothelial nitric oxide synthase inhibition but sensitive to blockade of calcium-activated potassium channels in human EH. Studies with more specific inhibitors are required to determine whether this response is mediated by endothelial potassium channel subtypes (SK(Ca) and IK(Ca)).

Possible role of exogenous cAMP to improve vascular endothelial dysfunction in hypertensive rats

The study has been designed to investigate the effect of 8-Br-cAMP, an activator of protein kinase A, in hypertension-induced vascular endothelial dysfunction. Rats were uninephroctomized and desoxycortisone acetate (DOCA) (40 mg/kg, s.c.) was administered to rats to produce hypertension (mean arterial blood pressure > 140 mmHg). Vascular endothelial dysfunction was assessed using isolated aortic ring preparation, electron microscopy of thoracic aorta and serum concentration of nitrite/nitrate. The expression of mRNA for p22phox and eNOS was assessed by using reverse transcriptase-polymerase chain reaction. Serum thiobarbituric acid reactive substances concentration and aortic superoxide anion concentration were estimated to assess oxidative stress. 8-Br-cAMP (5 mg/kg, i.p.) or atorvastatin (30 mg/kg, p.o.) prevented hypertension-induced attenuation of acetylcholine-induced endothelium-dependent relaxation, impairment of vascular endothelial lining, decrease in expression of mRNA for endothelial nitric oxide synthase (eNOS), serum nitrite/nitrate concentration and increase in expression of mRNA for p22phox, superoxide anion and serum TBARS. The ameliorative effect of 8-Br-cAMP was prevented by N-nitro-L-arginine methyl ester (25 mg/kg, i.p.) and glibenclamide (30 mg/kg, i.p.). It may be concluded that 8-Br-cAMP may stimulate expression and activity of eNOS and suppress expression of p22phox subunit of NADPH oxidase to reduce oxidative stress and subsequently improve vascular endothelial dysfunction.