Thromboxane receptor stimulation associated with loss of SKCa activity and reduced EDHF responses in the rat isolated mesenteric artery

Cooling-induced cutaneous vasodilatation is mediated by small-conductance, calcium-activated potassium channels in tail arteries from male mice

Cooling causes cutaneous dilatation to restrain cold-induced constriction and prevent tissue injury. Cooling increases communication through myoendothelial gap junctions (MEGJs), thereby increasing endothelium-derived hyperpolarization (EDH)-type dilatation. EDH is initiated by calcium-activated potassium channels (KCa ) activated by endothelial stimuli or muscle-derived mediators traversing MEGJs (myoendothelial feedback). The goal of this study was to determine the individual roles of KCa with small (SK3) and intermediate (IK1) conductance in cooling-induced dilatation. Vasomotor responses of mice isolated cutaneous tail arteries were analyzed by pressure myography at 37°C and 28°C. Cooling increased acetylcholine-induced EDH-type dilatation during inhibition of NO and prostacyclin production. IK1 inhibition did not affect dilatations to acetylcholine, whereas SK3 inhibition inhibited dilatation at both temperatures. Cooling uncovered myoendothelial feedback to inhibit constrictions in U46619. IK1 inhibition did not affect U46619 constrictions, whereas SK3 inhibition abolished the inhibitory effect of cooling without affecting U46619 constriction at 37°C. Immunoblots confirmed SK3 expression, which was localized (immunofluorescence) to holes in the internal elastic lamina consistent with myoendothelial projections. Immunoblots and Immunofluorescence did not detect IK1. Studies in non-cutaneous arteries have highlighted the predominant role of IK1 in EDH-type dilatation. Cutaneous arteries are distinctly reliant on SK3, which may enable EDH-type dilation to be amplified by cooling.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Endothelium-Dependent Hyperpolarization: The Evolution of Myoendothelial Microdomains

ABSTRACT

Endothelium-derived hyperpolarizing factor (EDHF) was envisaged as a chemical entity causing vasodilation by hyperpolarizing vascular smooth muscle (VSM) cells and distinct from nitric oxide (NO) ([aka endothelium-derived relaxing factor (EDRF)]) and prostacyclin. The search for an identity for EDHF unraveled the complexity of signaling within small arteries. Hyperpolarization originates within endothelial cells (ECs), spreading to the VSM by 2 branches, 1 chemical and 1 electrical, with the relative contribution varying with artery location, branch order, and prevailing profile of VSM activation. Chemical signals vary likewise and can involve potassium ion, lipid mediators, and hydrogen peroxide, whereas electrical signaling depends on physical contacts formed by homocellular and heterocellular (myoendothelial; MEJ) gap junctions, both able to conduct hyperpolarizing current. The discovery that chemical and electrical signals each arise within ECs resulted in an evolution of the single EDHF concept into the more inclusive, EDH signaling. Recognition of the importance of MEJs and particularly the fact they can support bidirectional signaling also informed the discovery that Ca2+ signals can pass from VSM to ECs during vasoconstriction. This signaling activates negative feedback mediated by NO and EDH forming a myoendothelial feedback circuit, which may also be responsible for basal or constitutive release of NO and EDH activity. The MEJs are housed in endothelial projections, and another spin-off from investigating EDH signaling was the discovery these fine structures contain clusters of signaling proteins to regulate both hyperpolarization and NO release. So, these tiny membrane bridges serve as a signaling superhighway or infobahn, which controls vasoreactivity by responding to signals flowing back and forth between the endothelium and VSM. By allowing bidirectional signaling, MEJs enable sinusoidal vasomotion, co-ordinated cycles of widespread vasoconstriction/vasodilation that optimize time-averaged blood flow. Cardiovascular disease disrupts EC signaling and as a result vasomotion changes to vasospasm.

Cyclopiazonic Acid-Induced Ca2+ Store Depletion Initiates Endothelium-Dependent Hyperpolarization-Mediated Vasorelaxation of Mesenteric Arteries in Healthy and Colitis Mice

Purposes: Since the role of store-operated calcium entry (SOCE) in endothelium-dependent hyperpolarization (EDH)-mediated vasorelaxation of mesenteric arteries in health and colitis is not fully understood, cyclopiazonic acid (CPA), a specific inhibitor of the sarco(endo) plasmic reticulum calcium-ATPases (SERCA), was used as a SOCE activator to investigate its role in normal mice and its alteration in colitis mice.

Methods: The changes in Ca²⁺ signaling in vascular endothelial cells (VEC) were examined by single cell Ca²⁺ imaging and tension of mesenteric arteries in response to CPA were examined using Danish DMT520A microvascular measuring system.

Results: CPA activated the SOCE through depletion of the endoplasmic reticulum (ER) Ca²⁺ in endothelial cells. CPA had a concentration-dependent vasorelaxing effect in endothelium-intact mesenteric arteries, which was lost after endothelial removal. Both nitric oxide (NO) and prostacyclin (PGI₂) inhibitors did not affect CPA-induced vasorelaxation; however, after both NO and PGI₂ were inhibited, K_Ca channel blocker [10 mM tetraethylammonium chloride (TEA)] inhibited CPA-induced vasorelaxation while K_Ca channel activator (0.3 μM SKA-31) promoted it. Two SOCE blockers [30 μM SKF96365 and 100 μM flufenamic acid (FFA)], and an Orai channel blocker (30 μM GSK-7975A) inhibited this vasorelaxation. The inhibition of both Na⁺/K⁺-ATPase (NKA) and Na⁺/Ca²⁺-exchange (NCX) also inhibited CPA-induced vasorelaxation. Finally, the CPA involved in EDH-induced vasorelaxation by the depletion of ER Ca²⁺ of mesenteric arteries was impaired in colitis mice.

Conclusion: Depletion of ER Ca²⁺ by CPA induces a vasorelaxation of mesenteric arteries that is mediated through EDH mechanism and invokes the activation of SOCE. The CPA-induced endothelium-dependent dilation is impaired in colitis which may limit blood perfusion to the intestinal mucosa.

Altered cyclooxygenase-1 and enhanced thromboxane receptor activities underlie attenuated endothelial dilatory capacity of omental arteries in obesity

AIMS

Obesity is a risk factor for endothelial dysfunction, the severity of which is likely to vary depending on extent and impact of adiposity on the vasculature. This study investigates the roles of cyclooxygenase isoforms and thromboxane receptor activities in the differential endothelial dilatory capacities of arteries derived from omental and subcutaneous adipose tissues in obesity.

MAIN METHODS

Small arteries were isolated from omental and subcutaneous adipose tissues obtained from consented morbidly obese patients (n = 65, BMI 45 ± 6 kg m^-2 [Mean ± SD]) undergoing bariatric surgery. Relaxation to acetylcholine was studied by wire myography in the absence or presence of indomethacin (10 μM, cyclooxygenase inhibitor), FR122047 (1 μM, cyclooxygenase-1 inhibitor), Celecoxib (4 μM, cyclooxygenase-2 inhibitor), Nω-Nitro-L-arginine methyl ester (L-NAME, 100 μM, nitric oxide synthase inhibitor) or combination of apamin (0.5 μM) and charybdotoxin (0.1 μM) that together inhibit endothelium-derived hyperpolarizing factor (EDHF). Contractions to U46619 (thromboxane A₂ mimetic) were also studied.

KEY FINDINGS

Acetylcholine relaxation was significantly attenuated in omental compared with subcutaneous arteries from same patients (p < 0.01). Indomethacin (p < 0.01) and FR122047 (p < 0.001) but not Celecoxib significantly improved the omental arteriolar relaxation. Cyclooxygenase-1 mRNA and U46619 contractions were both increased in omental compared with subcutaneous arteries (p < 0.05). L-NAME comparably inhibited acetylcholine relaxation in both arteries, while apamin+charybdotoxin were less effective in omental compared with subcutaneous arteries.

SIGNIFICANCE

The results show that the depot-specific reduction in endothelial dilatory capacity of omental compared with subcutaneous arteries in obesity is in large part due to altered cyclooxygenase-1 and enhanced thromboxane receptor activities, which cause EDHF deficiency.

Mechanisms of vascular dysfunction evoked by ionizing radiation and possible targets for its pharmacological correction

Ionizing radiation (IR) leads to a variety of the cardiovascular diseases, including the arterial hypertension. A number of studies have demonstrated that blood vessels represent important target for IR, and the endothelium is one of the most vulnerable components of the vascular wall. IR causes an inhibition of nitric oxide (NO)-mediated endothelium-dependent vasodilatation and generation of reactive oxygen (ROS) and nitrogen (RNS) species trigger this process. Inhibition of NO-mediated vasodilatation could be due to endothelial NO synthase (eNOS) down-regulation, inactivation of endothelium-derived NO, and abnormalities in diffusion of NO from the endothelial cells (ECs) leading to a decrease in NO bioavailability. Beside this, IR suppresses endothelial large conductance Ca²⁺-activated K⁺ channels (BK_Ca) activity, which control NO synthesis. IR also leads to inhibition of the BK_Ca current in vascular smooth muscle cells (SMCs) which is mediated by protein kinase C (PKC). On the other hand, IR-evoked enhanced vascular contractility may result from PKC-mediated increase in SMCs myofilament Ca²⁺ sensitivity. Also, IR evokes vascular wall inflammation and atherosclerosis development. Vascular function damaged by IR can be effectively restored by quercetin-filled phosphatidylcholine liposomes and mesenchymal stem cells injection. Using RNA-interference technique targeted to different PKC isoforms can also be a perspective approach for pharmacological treatment of IR-induced vascular dysfunction.

Characterisation of the vasodilation effects of DHA and EPA, n-3 PUFAs (fish oils), in rat aorta and mesenteric resistance arteries

Background and purpose

Increasing evidence suggests that the omega-3 polyunsaturated acids (n-3 PUFA), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), are beneficial to cardiovascular health, promoting relaxation of vascular smooth muscle cells and vasodilation. Numerous studies have attempted to study these responses, but to date there has not been a systematic characterisation of both DHA and EPA mediated vasodilation in conduit and resistance arteries. Therefore, we aimed to fully characterise the n-3 PUFA-induced vasodilation pathways in rat aorta and mesenteric artery.

Methods

Wire myography was used to measure the vasomotor responses of freshly dissected rat mesenteric artery and aorta. Arteries were pre-constricted with U46619 and cumulative concentrations of either DHA or EPA (10 nM-30 μM) were added. The mechanisms by which n-3 PUFA relaxed arteries were investigated using inhibitors of vasodilator pathways, which include: nitric oxide synthase (NOS; L-NAME), cycloxygenase (COX; indomethacin), cytochrome P450 epoxygenase (CYP450; clotrimazole); and calcium-activated potassium channels (K_Ca), SK_Ca (apamin), IK_Ca (TRAM-34) and BK_Ca (paxilline).

Results

Both DHA- and EPA-induced relaxations were partially inhibited following endothelium removal in rat mesenteric arteries. Similarly, in aorta EPA-induced relaxation was partially suppressed due to endothelium removal. CYP450 also contributed to EPA-induced relaxation in mesenteric artery. Inhibition of IK_Ca partially attenuated DHA-induced relaxation in aorta and mesenteric artery along with EPA-induced relaxation in mesenteric artery. Furthermore, this inhibition of DHA- and EPA-induced relaxation was increased following the additional blockade of BK_Ca in these arteries.

Conclusions

This study provides evidence of heterogeneity in the vasodilation mechanisms of DHA and EPA in different vascular beds. Our data also demonstrates that endothelium removal has little effect on relaxations produced by either PUFA. We demonstrate IK_Ca and BK_Ca are involved in DHA-induced relaxation in rat aorta and mesenteric artery; and EPA-induced relaxation in rat mesenteric artery only. CYP450 derived metabolites of EPA may also be involved in BK_Ca dependent relaxation. To our knowledge this is the first study indicating the involvement of IK_Ca in n-3 PUFA mediated relaxation.

Peroxynitrite formed during a transient episode of brain ischaemia increases endothelium-derived hyperpolarization-type dilations in thromboxane/prostaglandin receptor-stimulated rat cerebral arteries

AIM

Increased thromboxane A₂ and peroxynitrite are hallmarks of cerebral ischaemia/reperfusion (I/R). Stimulation of thromboxane/prostaglandin receptors (TP) attenuates endothelium-derived hyperpolarization (EDH). We investigated whether EDH-type middle cerebral artery (MCA) relaxations following TP stimulation are altered after I/R and the influence of peroxynitrite.

METHODS

Vascular function was determined by wire myography after TP stimulation with the thromboxane A₂ mimetic 9,11-dideoxy-9α, 11α -methano-epoxy prostaglandin F2α (U46619) in MCA of Sprague Dawley rats subjected to MCA occlusion (90 min)/reperfusion (24 h) or sham operation, and in non-operated (control) rats. Some rats were treated with saline or the peroxynitrite decomposition catalyst 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III) (20 mg kg^-1 ). Protein expression was evaluated in MCA and in human microvascular endothelial cells submitted to hypoxia (overnight)/reoxygenation (24 h) (H/R) using immunofluorescence and immunoblotting.

RESULTS

In U46619-pre-constricted MCA, EDH-type relaxation by the proteinase-activated receptor 2 agonist serine-leucine-isoleucine-glycine-arginine-leucine-NH₂ (SLIGRL) was greater in I/R than sham rats due to an increased contribution of small-conductance calcium-activated potassium channels (SK_Ca ), which was confirmed by the enlarged relaxation to the SK_Ca activator N-cyclohexyl-N-2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-4-pyrimidinamine. I/R and H/R induced endothelial protein tyrosine nitration and filamentous-actin disruption. In control MCA, either cytochalasin D or peroxynitrite disrupted endothelial filamentous-actin and augmented EDH-type relaxation. Furthermore, peroxynitrite decomposition during I/R prevented the increase in EDH-type responses.

CONCLUSION

Following TP stimulation in MCA, EDH-type relaxation to SLIGRL is greater after I/R due to endothelial filamentous-actin disruption by peroxynitrite, which prevents TP-induced block of SK_Ca input to EDH. These results reveal a novel mechanism whereby peroxynitrite could promote post-ischaemic brain injury.

NFAT regulation of cystathionine γ-lyase expression in endothelial cells is impaired in rats exposed to intermittent hypoxia

Sleep apnea is a risk factor for cardiovascular disease, and intermittent hypoxia (IH, 20 episodes/h of 5% O2-5% CO2 for 7 h/day) to mimic sleep apnea increases blood pressure and impairs hydrogen sulfide (H2S)-induced vasodilation in rats. The enzyme that produces H2S, cystathionine γ-lyase (CSE), is decreased in rat mesenteric artery endothelial cells (EC) following in vivo IH exposure. In silico analysis identified putative nuclear factor of activated T cell (NFAT) binding sites in the CSE promoter. Therefore, we hypothesized that IH exposure reduces Ca2+ concentration ([Ca2+]) activation of calcineurin/NFAT to lower CSE expression and impair vasodilation. In cultured rat aortic EC, inhibiting calcineurin with cyclosporine A reduced CSE mRNA, CSE protein, and luciferase activity driven by a full-length but not a truncated CSE promoter. In male rats exposed to sham or IH conditions for 2 wk, [Ca2+] in EC in small mesenteric arteries from IH rats was lower than in EC from sham rat arteries (Δfura 2 ratio of fluorescence at 340 to 380 nm from Ca2+ free: IH = 0.05 ± 0.02, sham = 0.17 ± 0.03, P < 0.05), and fewer EC were NFATc3 nuclear positive in IH rat arteries than in sham rat arteries (IH = 13 ± 3, sham = 59 ± 11%, P < 0.05). H2S production was also lower in mesenteric tissue from IH rats vs. sham rats. Endothelium-dependent vasodilation to acetylcholine (ACh) was lower in mesenteric arteries from IH rats than in arteries from sham rats, and inhibiting CSE with β-cyanoalanine diminished ACh-induced vasodilation in arteries from sham but not IH rats but did not affect dilation to the H2S donor NaHS. Thus, IH lowers EC [Ca2+], NFAT activity, CSE expression and activity, and H2S production while inhibiting NFAT activation lowers CSE expression. The observations that IH exposure decreases NFATc3 activation and CSE-dependent vasodilation support a role for NFAT in regulating endothelial H2S production.NEW & NOTEWORTHY This study identifies the calcium-regulated transcription factor nuclear factor of activated T cells as a novel regulator of cystathionine γ-lyase (CSE). This pathway is basally active in mesenteric artery endothelial cells, but, after exposure to intermittent hypoxia to mimic sleep apnea, nuclear factor of activated T cells c3 nuclear translocation and CSE expression are decreased, concomitant with decreased CSE-dependent vasodilation.

Endothelin-1 shifts the mediator of bradykinin-induced relaxation from NO to H2 O2 in resistance arteries from patients with cardiovascular disease

BACKGROUND AND PURPOSE

We tested the hypothesis that in resistance arteries from cardiovascular disease (CVD) patients, effects of an endothelium-dependent vasodilator depend on the contractile stimulus.

EXPERIMENTAL APPROACH

Arteries dissected from parietal pericardium of cardiothoracic surgery patients were studied by myography and imaging techniques. Segments were sub-maximally contracted by K(+) , the TxA2 analogue U46619 or endothelin-1 (ET-1).

KEY RESULTS

Relaxing effects of Na-nitroprusside were comparable, but those of bradykinin (BK) were bigger in the presence of ET-1 compared with K(+) or U46619. BK-induced relaxation was (i) abolished by L-NAME in K(+) -contracted arteries, (ii) partly inhibited by L-NAME in the presence of U46619 and (iii) not altered by indomethacin, L-NAME plus inhibitors of small and intermediate conductance calcium-activated K(+) channels, but attenuated by catalase, in ET-1-contracted arteries. This catalase-sensitive relaxation was unaffected by inhibitors of NADPH oxidases or allopurinol. Exogenous H2 O2 caused a larger relaxation of ET-1-induced contractions than those evoked by K(+) or U46619 in the presence of inhibitors of other endothelium-derived relaxing factors. Catalase-sensitive staining of cellular ROS with CellROX Deep Red was significantly increased in the presence of both 1 μM BK and 2 nM ET-1 but not either peptide alone.

CONCLUSIONS AND IMPLICATIONS

In resistance arteries from patients with CVD, exogenous ET-1 shifts the mediator of relaxing responses to the endothelium-dependent vasodilator BK from NO to H2 O2 and neither NADPH oxidases, xanthine oxidase nor NOS appear to be involved in this effect. This might have consequences for endothelial dysfunction in conditions where intra-arterial levels of ET-1 are enhanced.

Constrictor prostanoids and uridine adenosine tetraphosphate: vascular mediators and therapeutic targets in hypertension and diabetes

Vascular dysfunction plays a pivotal role in the development of systemic complications associated with arterial hypertension and diabetes. The endothelium, or more specifically, various factors derived from endothelial cells tightly regulate vascular function, including vascular tone. In physiological conditions, there is a balance between endothelium-derived factors, that is, relaxing factors (endothelium-derived relaxing factors; EDRFs) and contracting factors (endothelium-derived contracting factors; EDCFs), which mediate vascular homeostasis. However, in disease states, such as diabetes and arterial hypertension, there is an imbalance between EDRF and EDCF, with a reduction of EDRF signalling and an increase of EDCF signalling. Among EDCFs, COX-derived vasoconstrictor prostanoids play an important role in the development of vascular dysfunction associated with hypertension and diabetes. Moreover, uridine adenosine tetraphosphate (Up4 A), identified as an EDCF in 2005, also modulates vascular function. However, the role of Up4 A in hypertension- and diabetes-associated vascular dysfunction is unclear. In the present review, we focused on experimental and clinical evidence that implicate these two EDCFs (vasoconstrictor prostanoids and Up4 A) in vascular dysfunction associated with hypertension and diabetes.

The effect of pH and ion channel modulators on human placental arteries

Chorionic plate arteries (CPA) are located at the maternofetal interface where they are able to respond to local metabolic changes. Unlike many other types of vasculature, the placenta lacks nervous control and requires autoregulation for controlling blood flow. The placental circulation, which is of low-resistance, may become hypoxic easily leading to fetal acidosis and fetal distress however the role of the ion channels in these circumstances is not well-understood. Active potassium channel conductances that are subject to local physicochemical modulation may serve as pathways through which such signals are transduced. The aim of this study was to investigate the modulation of CPA by pH and the channels implicated in these responses using wire myography. CPA were isolated from healthy placentae and pre-contracted with U46619 before testing the effects of extracellular pH using 1 M lactic acid over the pH range 7.4 - 6.4 in the presence of a variety of ion channel modulators. A change from pH 7.4 to 7.2 produced a 29±3% (n = 9) relaxation of CPA which increased to 61±4% at the lowest pH of 6.4. In vessels isolated from placentae of women with pre-eclampsia (n = 6), pH responses were attenuated. L-methionine increased the relaxation to 67±7% (n = 6; p<0.001) at pH 6.4. Similarly the TASK 1/3 blocker zinc chloride (1 mM) gave a maximum relaxation of 72±5% (n = 8; p<0.01) which compared with the relaxation produced by the TREK-1 opener riluzole (75±5%; n = 6). Several other modulators induced no significant changes in vascular responses. Our study confirmed expression of several ion channel subtypes in CPA with our results indicating that extracellular pH within the physiological range has an important role in controlling vasodilatation in the human term placenta.

Raised tone reveals ATP as a sympathetic neurotransmitter in the porcine mesenteric arterial bed

The relative importance of ATP as a functional sympathetic neurotransmitter in blood vessels has been shown to be increased when the level of preexisting vascular tone or pressure is increased, in studies carried out in rat mesenteric arteries. The aim of the present study was to determine whether tone influences the involvement of ATP as a sympathetic cotransmitter with noradrenaline in another species. We used the porcine perfused mesenteric arterial bed and porcine mesenteric large, medium and small arteries mounted for isometric tension recording, because purinergic cotransmission can vary depending on the size of the blood vessel. In the perfused mesenteric bed at basal tone, sympathetic neurogenic vasocontractile responses were abolished by prazosin, an α1-adrenoceptor antagonist, but there was no significant effect of α,β-methylene ATP, a P2X receptor-desensitizing agent. Submaximal precontraction of the mesenteric arterial bed with U46619, a thromboxane A2 mimetic, augmented the sympathetic neurogenic vasocontractile responses; under these conditions, both α,β-methylene ATP and prazosin attenuated the neurogenic responses. In the mesenteric large, medium and small arteries, prazosin attenuated the sympathetic neurogenic contractile responses under conditions of both basal and U46619-raised tone. α,β-Methylene ATP was effective in all of these arteries only under conditions of U46619-induced tone, causing a similar inhibition in all arteries, but had no significant effect on sympathetic neurogenic contractions at basal tone. These data show that ATP is a cotransmitter with noradrenaline in porcine mesenteric arteries; the purinergic component was revealed under conditions of partial precontraction, which is more relevant to physiological conditions.

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.

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.

Statins and selective inhibition of Rho kinase protect small conductance calcium-activated potassium channel function (K(Ca)2.3) in cerebral arteries

BACKGROUND

In rat middle cerebral and mesenteric arteries the K(Ca)2.3 component of endothelium-dependent hyperpolarization (EDH) is lost following stimulation of thromboxane (TP) receptors, an effect that may contribute to the endothelial dysfunction associated with cardiovascular disease. In cerebral arteries, K(Ca)2.3 loss is associated with NO synthase inhibition, but is restored if TP receptors are blocked. The Rho/Rho kinase pathway is central for TP signalling and statins indirectly inhibit this pathway. The possibility that Rho kinase inhibition and statins sustain K(Ca)2.3 hyperpolarization was investigated in rat middle cerebral arteries (MCA).

METHODS

MCAs were mounted in a wire myograph. The PAR2 agonist, SLIGRL was used to stimulate EDH responses, assessed by simultaneous measurement of smooth muscle membrane potential and tension. TP expression was assessed with rt-PCR and immunofluorescence.

RESULTS

Immunofluorescence detected TP in the endothelial cell layer of MCA. Vasoconstriction to the TP agonist, U46619 was reduced by Rho kinase inhibition. TP receptor stimulation lead to loss of K(Ca)2.3 mediated hyperpolarization, an effect that was reversed by Rho kinase inhibitors or simvastatin. K(Ca)2.3 activity was lost in L-NAME-treated arteries, but was restored by Rho kinase inhibition or statin treatment. The restorative effect of simvastatin was blocked after incubation with geranylgeranyl-pyrophosphate to circumvent loss of isoprenylation.

CONCLUSIONS

Rho/Rho kinase signalling following TP stimulation and L-NAME regulates endothelial cell K(Ca)2.3 function. The ability of statins to prevent isoprenylation and perhaps inhibit of Rho restores/protects the input of K(Ca)2.3 to EDH in the MCA, and represents a beneficial pleiotropic effect of statin treatment.

Endothelium-mediated control of vascular tone: COX-1 and COX-2 products

Endothelium-dependent contractions contribute to endothelial dysfunction in various animal models of aging, diabetes and cardiovascular diseases. In the spontaneously hypertensive rat, the archetypal model for endothelium-dependent contractions, the production of the endothelium-derived contractile factors (EDCF) involves an increase in endothelial intracellular calcium concentration, the production of reactive oxygen species, the predominant activation of cyclooxygenase-1 (COX-1) and to a lesser extent that of COX-2, the diffusion of EDCF towards the smooth muscle cells and the subsequent stimulation of their thromboxane A2-endoperoxide TP receptors. Endothelium-dependent contractions are also observed in various models of hypertension, aging and diabetes. They generally also involve the generation of COX-1- and/or COX-2-derived products and the activation of smooth muscle TP receptors. Depending on the model, thromboxane A(2), PGH(2), PGF(2α), PGE(2) and paradoxically PGI(2) can all act as EDCFs. In human, the production of COX-derived EDCF is a characteristic of the aging and diseased blood vessels, with essential hypertension causing an earlier onset and an acceleration of this endothelial dysfunction. As it has been observed in animal models, COX-1, COX-2 or both isoforms can contribute to these endothelial dysfunctions. Since in most cases, the activation of TP receptors is the common downstream effector, selective antagonists of this receptor should curtail endothelial dysfunction and be of therapeutic interest in the treatment of cardiovascular disorders.

Shear stress-dependent effects of lysophosphatidic acid on agonist-induced vasomotor responses in rat mesenteric artery

We have previously shown that lysophosphatidic acid (LPA), a bioactive plasma lysophospholipid, markedly accelerates shear stress-induced Ca2+ responses in cultured vascular endothelial cells (ECs). This study aimed to demonstrate the impact of LPA and luminal shear stress on vasomotor regulation in the isolated rat mesenteric artery (MA) using a videomicroscopic technique. Although the addition of LPA to the perfusate in a concentration range of 0.03-0.3 μM had no significant effect on the basal MA tone, LPA in a similar concentration range led to increased phenylephrine-induced MA contraction and reduced acetylcholine-induced MA relaxation under physiological shear conditions. These vasomodulatory actions of LPA, which vanished upon removal of ECs, were positively dependent on luminal shear stress levels and were markedly inhibited by the LPA receptor antagonist Ki16425, the cyclooxygenase inhibitor indomethacin, and the thromboxane A2 receptor antagonist SQ29548. These data thus suggest that LPA can modify the agonist-induced vasomotor responses in MAs in a shear stress-dependent manner. This effect of LPA was mediated through ECs, the LPA receptor, and cyclooxygenase/thromboxane A2 signaling.

Openers of small conductance calcium-activated potassium channels selectively enhance NO-mediated bradykinin vasodilatation in porcine retinal arterioles

BACKGROUND AND PURPOSE

Small (SK(Ca) or K(Ca)2) and intermediate (IK(Ca) or K(Ca)3.1) conductance calcium-activated potassium channels are involved in regulation of vascular tone and blood pressure. The present study investigated whether NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime) and CyPPA (cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine), which are selective openers of SK(Ca) and IK(Ca) channels and of SK(Ca)2 and SK(Ca)3 channels, respectively, enhance endothelium-dependent vasodilatation in porcine retinal arterioles.

EXPERIMENTAL APPROACH

In porcine retinal arterioles, SK(Ca)3 and IK(Ca) protein localization was examined by immunolabelling. Endothelial cell calcium was measured by fluorescence imaging. For functional studies, arterioles with internal diameters of 116 +/- 2 microm (n = 276) were mounted in microvascular myographs for isometric tension recordings.

KEY RESULTS

SK(Ca)3 and IK(Ca) protein was localized in the endothelium. Bradykinin, but not NS309 or CyPPA increased endothelial cell calcium. Pre-incubation with NS309 or CyPPA enhanced bradykinin relaxation without changing endothelial cell calcium. This enhanced relaxation was abolished by blocking SK(Ca) channels with apamin. In the presence of NS309 or CyPPA, mainly inhibition of NO synthase with asymmetric dimethylarginine, but also inhibition of cyclooxygenase with indomethacin, reduced bradykinin relaxation. Bradykinin relaxation was completely abolished by NO synthase and cyclooxygenase inhibition together with a NO scavenger, oxyhaemoglobin.

CONCLUSIONS AND IMPLICATIONS

In porcine retinal arterioles, bradykinin increases endothelial cell calcium leading to activation of SK(Ca) and IK(Ca) channels. Without altering endothelial cell calcium, NS309 and CyPPA open SK(Ca) channels that enhance NO-mediated bradykinin relaxations. These results imply that opening SK(Ca) channels improves endothelium-dependent relaxation and makes this channel a potential target for treatments aimed at restoring retinal blood flow.

Recycling of the Ca2+-activated K+ channel, KCa2.3, is dependent upon RME-1, Rab35/EPI64C, and an N-terminal domain

Regulation of the number of Ca(2+)-activated K(+) channels at the endothelial cell surface contributes to control of the endothelium-derived hyperpolarizing factor response, although this process is poorly understood. To address the fate of plasma membrane-localized KCa2.3, we utilized an extracellular epitope-tagged channel in combination with fluorescence and biotinylation techniques in both human embryonic kidney cells and the human microvascular endothelial cell line, HMEC-1. KCa2.3 was internalized from the plasma membrane and degraded with a time constant of 18 h. Cell surface biotinylation demonstrated that KCa2.3 was rapidly endocytosed and recycled back to the plasma membrane. Consistent with recycling, expression of a dominant negative (DN) RME-1 or Rab35 as well as wild type EPI64C, the Rab35 GTPase-activating protein, resulted in accumulation of KCa2.3 in an intracellular compartment. Expression of DN RME-1, DN Rab35, or wild type EPI64C resulted in a decrease in steady-state plasma membrane expression. Knockdown of EPI64C increased cell surface expression of KCa2.3. Furthermore, the effect of EPI64C was dependent upon its GTPase-activating proteins activity. Co-immunoprecipitation studies confirmed an association between KCa2.3 and both Rab35 and RME-1. In contrast to KCa2.3, KCa3.1 was rapidly endocytosed and degraded in an RME-1 and Rab35-independent manner. A series of N-terminal deletions identified a 12-amino acid region, Gly(206)-Pro(217), as being required for the rapid recycling of KCa2.3. Deletion of Gly(206)-Pro(217) had no effect on the association of KCa2.3 with Rab35 but significantly decreased the association with RME-1. These represent the first studies elucidating the mechanisms by which KCa2.3 is maintained at the plasma membrane.

A mathematical model of vasoreactivity in rat mesenteric arterioles: I. Myoendothelial communication

To study the effect of myoendothelial communication on vascular reactivity, we integrated detailed mathematical models of Ca(2+) dynamics and membrane electrophysiology in arteriolar smooth muscle (SMC) and endothelial (EC) cells. Cells are coupled through the exchange of Ca(2+), Cl(-), K(+), and Na(+) ions, inositol 1,4,5-triphosphate (IP(3)), and the paracrine diffusion of nitric oxide (NO). EC stimulation reduces intracellular Ca(2+) ([Ca(2+)](i)) in the SMC by transmitting a hyperpolarizing current carried primarily by K(+). The NO-independent endothelium-derived hyperpolarization was abolished in a synergistic-like manner by inhibition of EC SK(Ca) and IK(Ca) channels. During NE stimulation, IP(3) diffusing from the SMC induces EC Ca(2+) release, which, in turn, moderates SMC depolarization and [Ca(2+)](i) elevation. On the contrary, SMC [Ca(2+)](i) was not affected by EC-derived IP(3). Myoendothelial Ca(2+) fluxes had no effect in either cell. The EC exerts a stabilizing effect on calcium-induced calcium release-dependent SMC Ca(2+) oscillations by increasing the norepinephrine concentration window for oscillations. We conclude that a model based on independent data for subcellular components can capture major features of the integrated vessel behavior. This study provides a tissue-specific approach for analyzing complex signaling mechanisms in the vasculature.

Endothelial Ca+-activated K+ channels in normal and impaired EDHF-dilator responses--relevance to cardiovascular pathologies and drug discovery

The arterial endothelium critically contributes to blood pressure control by releasing vasodilating autacoids such as nitric oxide, prostacyclin and a third factor or pathway termed 'endothelium-derived hyperpolarizing factor' (EDHF). The nature of EDHF and EDHF-signalling pathways is not fully understood yet. However, endothelial hyperpolarization mediated by the Ca(2+)-activated K(+) channels (K(Ca)) has been suggested to play a critical role in initializing EDHF-dilator responses in conduit and resistance-sized arteries of many species including humans. Endothelial K(Ca) currents are mediated by the two K(Ca) subtypes, intermediate-conductance K(Ca) (KCa3.1) (also known as, a.k.a. IK(Ca)) and small-conductance K(Ca) type 3 (KCa2.3) (a.k.a. SK(Ca)). In this review, we summarize current knowledge about endothelial KCa3.1 and KCa2.3 channels, their molecular and pharmacological properties and their specific roles in endothelial function and, particularly, in the EDHF-dilator response. In addition we focus on recent experimental evidences derived from KCa3.1- and/or KCa2.3-deficient mice that exhibit severe defects in EDHF signalling and elevated blood pressures, thus highlighting the importance of the KCa3.1/KCa2.3-EDHF-dilator system for blood pressure control. Moreover, we outline differential and overlapping roles of KCa3.1 and KCa2.3 for EDHF signalling as well as for nitric oxide synthesis and discuss recent evidence for a heterogeneous (sub) cellular distribution of KCa3.1 (at endothelial projections towards the smooth muscle) and KCa2.3 (at inter-endothelial borders and caveolae), which may explain their distinct roles for endothelial function. Finally, we summarize the interrelations of altered KCa3.1/KCa2.3 and EDHF system impairments with cardiovascular disease states such as hypertension, diabetes, dyslipidemia and atherosclerosis and discuss the therapeutic potential of KCa3.1/KCa2.3 openers as novel types of blood pressure-lowering drugs.

Eicosapentaenoic acid improves imbalance between vasodilator and vasoconstrictor actions of endothelium-derived factors in mesenteric arteries from rats at chronic stage of type 2 diabetes

Accumulating evidence demonstrates that dietary intake of n-3 polyunsaturated fatty acids (PUFAs) is associated with a reduced incidence of several cardiovascular diseases that involve endothelial dysfunction. However, the molecular mechanism remains unclear. We previously reported that mesenteric arteries from type 2 diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats exhibit endothelial dysfunction, leading to an imbalance between endothelium-derived vasodilators [namely, nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)] and vasoconstrictors [endothelium-derived contracting factors (EDCFs)] [namely cyclooxygenase (COX)-derived prostanoids] (Am J Physiol Heart Circ Physiol 293:H1480-H1490, 2007). We hypothesized that treating OLETF rats with eicosapentaenoic acid (EPA), a major n-3 PUFA, may improve endothelial dysfunction by correcting this imbalance. In OLETF rats [compared with age-matched control Long-Evans Tokushima Otsuka (LETO) rats]: 1) acetylcholine (ACh)-induced (endothelium-dependent) relaxation was impaired, 2) NO- and EDHF-mediated relaxations and nitrite production were reduced, and 3) ACh-induced EDCF-mediated contraction, production of prostanoids, and the protein expressions of COX-1 and COX-2 were all increased. When OLETF rats received chronic EPA treatment long-term (300 mg/kg/day p.o. for 4 weeks), their isolated mesenteric arteries exhibited: 1) improvements in ACh-induced NO- and EDHF-mediated relaxations and COX-mediated contraction, 2) reduced EDCF- and arachidonic acid-induced contractions, 3) normalized NO metabolism, 4) suppressed production of prostanoids, 5) reduced COX-2 expression, and 6) reduced phosphoextracellular signal-regulated kinase (ERK) expression. Moreover, EPA treatment reduced both ERK2 and nuclear factor (NF)-kappaB activities in isolated OLETF aortas. We propose that EPA ameliorates endothelial dysfunction in OLETF rats by correcting the imbalance between endothelium-derived factors, at least partly, by inhibiting ERK, decreasing NF-kappaB activation, and reducing COX-2 expression.

Metformin normalizes endothelial function by suppressing vasoconstrictor prostanoids in mesenteric arteries from OLETF rats, a model of type 2 diabetes

We previously reported that in mesenteric arteries from aged Otsuka Long-Evans Tokushima fatty (OLETF) rats (a type 2 diabetes model) endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired while endothelium-derived contracting factor (EDCF)-mediated contraction is enhanced (Matsumoto T, Kakami M, Noguchi E, Kobayashi T, Kamata K. Am J Physiol Heart Circ Physiol 293: H1480-H1490, 2007). Here we investigated whether acute and/or chronic treatment with metformin might improve this imbalance between the effects of the above endothelium-derived factors in mesenteric arteries isolated from OLETF rats. In acute studies on OLETF mesenteric arteries, ACh-induced relaxation was impaired and the relaxation became weaker at high ACh concentrations. Both metformin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside [AICAR, an AMP-activated protein kinase (AMPK) activator that is also activated by metformin] 1) diminished the tendency for the relaxation to reverse at high ACh concentrations and 2) suppressed both ACh-induced EDCF-mediated contraction and ACh-stimulated production of prostanoids (thromboxane A2 and PGE2). In studies on OLETF arteries from chronically treated animals, metformin treatment (300 mg.kg(-1).day(-1) for 4 wk) 1) improved ACh-induced nitric oxide- or EDHF-mediated relaxation and cyclooxygenase (COX)-mediated contraction, 2) reduced EDCF-mediated contraction, 3) suppressed production of prostanoids, and 4) reduced superoxide generation. Metformin did not alter the protein expressions of endothelial nitric oxide synthase (eNOS), phospho-eNOS (Ser1177), or COX-1, but it increased COX-2 protein. These results suggest that metformin improves endothelial functions in OLETF mesenteric arteries by suppressing vasoconstrictor prostanoids and by reducing oxidative stress. Our data suggest that within the timescale studied here, metformin improves endothelial function through this direct mechanism, rather than by improving metabolic abnormalities.

Evidence against C-type natriuretic peptide as an arterial 'EDHF'

C-type natriuretic peptide (CNP) is found in and released from vascular endothelial cells. Recently, a novel role has been suggested for this peptide, that of an endothelium-derived hyperpolarizing factor or EDHF. Implicit in this proposal is a widespread role for CNP as a key mediator of vascular dilatation. In this issue of the British Journal of Pharmacology, Leuranguer et al. compare the profile of membrane potential changes evoked with this putative EDHF or with endogenous EDHF (activated with ACh) in small carotid arteries. Marked differences between the two profiles lead them to discount a possible role for CNP as an EDHF.

Regulation of endothelial-dependent relaxation in human systemic arteries by SKCa and IKCa channels

Blockade of small-conductance Ca (2)(+)-activated K(+) channels (SK(Ca)) and intermediate conductance Ca(2)(+)-activated K(+) channels (IK(Ca)) can cause inhibition of endothelium-dependent hyperpolarizing factor (EDHF) in many vascular beds from animals, but there is a relative paucity of data in human vessels. Systemic arteries, isolated from women with healthy pregnancies, relax to the endothelial-dependent agonist bradykinin via a nonprostacyclin and non-nitric oxide pathway attributable to EDHF. Therefore, in this study, the authors investigated the effect of pharmacological blockade of SK(Ca) and IK(Ca) on EDHF-mediated relaxation of human omental and myometrial arteries preconstricted with either arginine vasopressin or U46619. Human arteries were isolated from omental and myometrial biopsies taken from healthy women undergoing planned cesarean section at term. Endothelial function was assessed using wire myography. In all vessels examined, nonspecific blockade of IK(Ca) with charybdotoxin attenuated EDHF-attributed relaxation. However, when Tram 34 was used to block IK(Ca), the attenuation of relaxation was evident only with U46619 preconstriction. In arteries from both vascular beds, and with either preconstrictor, a combination of either apamin and charybdotoxin or apamin plus Tram 34 almost ablated EDHF-attributable relaxation. These data support the notion that in human systemic arteries, activation of, primarily, SK(Ca) and IK(Ca)K(+) channel subtypes underlies EDHF-mediated relaxation. These results have important implications for future studies ascertaining the molecular mechanisms of hypertensive disorders (eg, preeclampsia, in which EDHF is thought to be aberrant).

Imbalance between endothelium-derived relaxing and contracting factors in mesenteric arteries from aged OLETF rats, a model of Type 2 diabetes

We investigated whether the balance between endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs) might be altered in mesenteric arteries from aged Otsuka Long-Evans Tokushima Fatty (OLETF) rats (a Type 2 diabetic model) [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats]. ACh-induced relaxation was impaired in the OLETF group, and a tendency for the relaxation to reverse at high ACh concentrations was observed in both groups. This tendency was abolished by indomethacin. Nitric oxide- and/or endothelium-derived hypolarizing factor-mediated relaxation and the protein expressions of phospho-endothelial nitric oxide synthase (Ser1177) and extracellular superoxide dismutase were also reduced in OLETF. An ACh-induced contraction was observed at higher ACh concentrations in the presence of NG-nitro-l-arginine (l-NNA) but was greater in OLETF rats. This contraction in OLETF rats was reduced by cyclooxygenase (COX) inhibitors and by prostanoid-receptor antagonists. The ACh-induced productions of thromboxane A2 and PGE2 were greater in OLETF than LETO rats, as were the mesenteric artery COX-1 and COX-2 protein expressions. Moreover, tert-butyl hydroperoxide ( t-BOOH) (membrane-permeant oxidant) induced a concentration-dependent contraction that was greater in OLETF rats. The t-BOOH-mediated contraction was increased both by l-NNA and by endothelium removal in LETO but not OLETF rats, suggesting that a negative modulatory role of the endothelium was lost in OLETF rats. These results suggest that an imbalance between EDRFs and EDCFs may be implicated in the endothelial dysfunction seen in aged OLETF mesenteric arteries, and may be attributable to increased oxidative stress.

Reduced expression of SKCa and IKCa channel proteins in rat small mesenteric arteries during angiotensin II-induced hypertension

Ca2+-activated K+ channels (KCa), in particular, the small and intermediate KCa (SKCa and IKCa, respectively) channels, are key players in endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation in small arteries. Hypertension is characterized by an endothelial dysfunction, possibly via reduced EDHF release and/or function. We hypothesize that during angiotensin II (14 days)-induced hypertension (ANG II-14d), the contribution of SKCa and IKCa channels in ACh-induced relaxations is reduced due to decreased expression of SKCa and IKCa channel proteins in rat small mesenteric arteries (MAs). Nitric oxide- and prostacyclin-independent vasorelaxation to ACh was similar in small MAs of sham-operated and ANG II-14d rats. Catalase had no inhibitory effects on these relaxations. The highly selective SKCa channel blocker UCL-1684 almost completely blocked these responses in MAs of sham-operated rats but partially in MAs of ANG II-14d rats. These changes were pressure dependent since UCL-1684 caused a greater inhibition in MAs of 1-day ANG II-treated normotensive rats compared with ANG II-14d rats. Expression levels of both mRNA and protein SK3 were significantly reduced in MAs of ANG II-14d rats. The IKCa channel blocker 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) resulted in comparable reductions in the relaxation responses to ACh in MAs of sham-operated and ANG II-14d rats. Relative mRNA expression levels of IK1 were significantly reduced in MAs of ANG II-14d rats, whereas protein levels of IK1 were not but tended to be lower in MAs of ANG II-14d rats. The findings demonstrate that EDHF-like responses are not compromised in a situation of reduced functional activity and expression of SK3 channels in small MAs of ANG II-induced hypertensive rats. The role of IK1 channels is less clear but might compensate for reduced SK3 activity.

Thromboxane A2 inhibition of SKCa after NO synthase block in rat middle cerebral artery

BACKGROUND AND PURPOSE

NO/prostanoid independent, EDHF-mediated hyperpolarization and dilation in rat middle cerebral arteries is mediated solely by endothelial cell IK(Ca). However, when the NO-pathway is also active, both SK(Ca) and IK(Ca) contribute to EDHF responses. As the SK(Ca) component can be inhibited by stimulation of thromboxane A(2) (TxA(2)) TP receptors and NO has the potential ability to inhibit thromboxane synthesis, we investigated whether TxA(2) might explain loss of functional input from SK(Ca) during NOS inhibition in cerebral arteries.

EXPERIMENTAL APPROACH

Rat middle cerebral arteries were mounted in a wire myograph. Endothelium-dependent responses to the PAR2 agonist, SLIGRL were assessed as simultaneous changes in smooth muscle membrane potential and tension.

KEY RESULTS

Responses were obtained in the presence of L-NAME as appropriate. Inhibition of TP receptors with either ICI 192,605 or SQ 29,548, did not affect EDHF mediated hyperpolarization and relaxation, but in their presence neither TRAM-34 nor apamin (to block IK(Ca) and SK(Ca) respectively) individually affected the EDHF response. However, in combination they virtually abolished it. Similar effects were obtained in the presence of the thromboxane synthase inhibitor, furegrelate, which additionally revealed an iberiotoxin-sensitive residual EDHF hyperpolarization and relaxation in the combined presence of TRAM-34 and apamin.

CONCLUSIONS AND IMPLICATIONS

In the rat middle cerebral artery, inhibition of NOS leads to a loss of the SK(Ca) component of EDHF responses. Either antagonism of TP receptors or block of thromboxane synthase restores an input through SK(Ca). These data indicate that NO normally enables SK(Ca) activity in rat middle cerebral arteries.

Raised tone reveals purinergic-mediated responses to sympathetic nerve stimulation in the rat perfused mesenteric vascular bed

Noradrenaline and ATP are sympathetic co-transmitters. In rat isolated mesenteric small arteries, activation of sympathetic nerves can produce a vasoconstrictor response mediated by ATP. In contrast, the rat perfused mesenteric bed displays vasoconstrictor responses that are blocked solely by alpha1-adrenoceptor antagonists. This study assessed the effect of raising tone with a vasoconstrictor on purinergic and noradrenergic responses to sympathetic nerve stimulation in the rat perfused mesentery. Rat mesenteric vascular beds were perfused with physiological salt solution and responses to nerve stimulation, or P2X-receptor agonists, were determined under basal conditions and after raising tone with endothelin-1. The contribution of noradrenaline and ATP to sympathetic nerve-mediated responses was assessed using the alpha1-adrenoceptor antagonist, prazosin and the P2X-receptor desensitizing agent, alpha,beta-methyleneATP. The effect of endothelin-1 on excitatory junction potentials generated in response to nerve stimulation in isolated mesenteric arteries was also assessed. Under baseline conditions, responses to nerve stimulation were mediated solely by activation of alpha1-adrenoceptors. After raising perfusion pressure with endothelin-1 or the thromboxane mimetic 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha (U44619), sympathetic nerve-mediated responses were larger than under basal conditions and the response was partly sensitive to P2X-receptor desensitization. Responses to exogenous P2X-receptor agonists were enhanced after treatment with endothelin-1, while endothelin-1 decreased the amplitude of excitatory junction potentials. These results indicate that ATP acts as an important, functional, sympathetic neurotransmitter in the perfused mesentery under raised tone conditions, where the perfusion pressure is closer to that found in vivo. This effect is due to a postjunctional enhancement of purinergic function.

Elevated pressure selectively blunts flow-evoked vasodilatation in rat mesenteric small arteries

BACKGROUND AND PURPOSE

The present study investigated mechanisms underlying impaired endothelium-dependent vasodilatation elicited by elevating the intraluminal pressure in rat mesenteric small arteries.

EXPERIMENTAL APPROACH

Arterial segments (internal diameter 316+/-2 microm, n=86) were mounted in a pressure myograph. The effect of elevating pressure from 50 to 120 mmHg for 1 h before resetting it to 50 mmHg was studied on endothelium-dependent vasodilatation.

KEY RESULTS

In arteries constricted with U46619 in the presence of indomethacin, shear stress generated by flow, evoked vasodilatation that was abolished by an inhibitor of nitric oxide (NO) synthase, asymmetric dimethylarginine (1 mM), whereas acetylcholine-induced vasodilatation was unchanged. After elevation of intraluminal pressure for 1 h and then resetting it to 50 mmHg, vasodilatation induced by shear stress and the NO donor, S-nitrosopenicillamine was inhibited, while vasodilatation induced by a guanylyl cyclase activator, BAY 412272, and acetylcholine was unaltered. Superoxide levels sensitive to polyethylene glycol superoxide dismutase were increased in segments exposed to elevated pressure. A superoxide scavenger, tempol (300 microM), a general endothelin receptor antagonist, SB 217242 and the selective ET(A) receptor antagonist, BQ 123 preserved shear stress-evoked vasodilatation.

CONCLUSIONS AND IMPLICATIONS

The present study shows that transient exposure to an elevated intraluminal pressure selectively inhibits flow-evoked NO-mediated vasodilatation, probably through activation of endothelin receptors and increased formation of superoxide. In contrast, elevation of pressure did not affect the acetylcholine-evoked endothelium-derived hyperpolarizing factor type vasodilatation in mesenteric small arteries.

Impaired endothelium-derived hyperpolarizing factor-mediated dilations and increased blood pressure in mice deficient of the intermediate-conductance Ca2+-activated K+ channel

The endothelium plays a key role in the control of vascular tone and alteration in endothelial cell function contributes to several cardiovascular disease states. Endothelium-dependent dilation is mediated by NO, prostacyclin, and an endothelium-derived hyperpolarizing factor (EDHF). EDHF signaling is thought to be initiated by activation of endothelial Ca(2+)-activated K(+) channels (K(Ca)), leading to hyperpolarization of the endothelium and subsequently to hyperpolarization and relaxation of vascular smooth muscle. In the present study, we tested the functional role of the endothelial intermediate-conductance K(Ca) (IK(Ca)/K(Ca)3.1) in endothelial hyperpolarization, in EDHF-mediated dilation, and in the control of arterial pressure by targeted deletion of K(Ca)3.1. K(Ca)3.1-deficient mice (K(Ca)3.1(-/-)) were generated by conventional gene-targeting strategies. Endothelial K(Ca) currents and EDHF-mediated dilations were characterized by patch-clamp analysis, myography and intravital microscopy. Disruption of the K(Ca)3.1 gene abolished endothelial K(Ca)3.1 currents and significantly diminished overall current through K(Ca) channels. As a consequence, endothelial and smooth muscle hyperpolarization in response to acetylcholine was reduced in K(Ca)3.1(-/-) mice. Acetylcholine-induced dilations were impaired in the carotid artery and in resistance vessels because of a substantial reduction of EDHF-mediated dilation in K(Ca)3.1(-/-) mice. Moreover, the loss of K(Ca)3.1 led to a significant increase in arterial blood pressure and to mild left ventricular hypertrophy. These results indicate that the endothelial K(Ca)3.1 is a fundamental determinant of endothelial hyperpolarization and EDHF signaling and, thereby, a crucial determinant in the control of vascular tone and overall circulatory regulation.

Mechanisms underlying the impaired EDHF-type relaxation response in mesenteric arteries from Otsuka Long-Evans Tokushima Fatty (OLETF) rats

We previously reported that in mesenteric arteries from streptozotocin-induced diabetic rats, the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired, possibly due to a reduced action of cAMP. Here, we observed an impairment of acetylcholine-induced EDHF-type relaxation in mesenteric arteries from a type 2 diabetic model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats], and we investigated the mechanism underlying this impairment. In the LETO group, this EDHF-type relaxation was attenuated by 18alpha-glycyrrhetinic acid (a gap-junction inhibitor) and by a protein kinase A (PKA) inhibitor. In both groups (OLETF and LETO), it was enhanced by 3-isobutyl-1-methylxanthine, a cAMP-phosphodiesterase (PDE) inhibitor, but following these enhancements it was still weaker in OLETF rats than in LETO rats. The relaxations induced by cilostamide (a selective PDE3 inhibitor) and 8-bromo-cAMP (a cell-permeant cAMP analog) were reduced in OLETF rats, as was PKA activity. The relaxations induced by two activators of Ca(2+)-activated K(+) channels (K(Ca)) [1-ethyl-2-benzimidazolinone (1-EBIO), intermediate-conductance K(Ca) channel (IK(Ca)) activator, and riluzole, small-conductance K(Ca) channel (SK(Ca)) activator] were also impaired in OLETF rats. We conclude that the impairment of EDHF-type relaxation seen in OLETF rats may be attributable not only to a reduction in cAMP/PKA signaling, but also to reduced endothelial K(Ca) channel activities.

Endothelial dysfunction in the streptozotocin-induced diabetic apoE-deficient mouse

Endothelial dysfunction plays a role in the development of atherosclerosis and diabetes-associated vascular disease and, in the streptozotocin (STZ)-induced apoE-deficient diabetic mouse, we report that, when compared to the citrate (CIT)-treated nondiabetic apoE-deficient control, acetylcholine (Ach)-mediated endothelium-dependent relaxation was reduced in the small mesenteric arteries (SMA) and the plaque-prone regions of the aorta from the STZ-diabetic mouse. In the SMA the component of Ach-mediated relaxation that was attributed to nitric oxide (NO) from STZ-treated diabetic apoE-deficient mice was enhanced; however, the endothelium-derived hyperpolarizing factor (EDHF)-mediated component was reduced. The EDHF component was assessed by determining the component of the Ach-mediated response that was resistant to the combination of the NO synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester, cyclooxygenase inhibitor, indomethacin, and soluble guanylate cyclase inhibitor, ODQ, and inhibited by the combination of the intermediate conductance KCa (IKCa) inhibitor TRAM-34 and the small-conductance KCa (SKCa) inhibitor apamin. Endothelial NOS was increased but SK2, SK3 and connexin (Cx) 37 mRNA expressions were significantly (P<0.05) decreased in the SMA from STZ-treated apoE-deficient mice compared to the CIT-treated controls. There was no difference in the IKCa expression or in Cx 40, 43 and 45 mRNA levels between STZ- and CIT-treated mice. The microvasculature of STZ-induced apoE-deficient mice developed endothelial dysfunction, which may be linked to a decrease in the contribution of the EDHF component due to a decrease in SK2 and 3 and Cx 37 expression.

Nitric oxide modulates angiotensin II-induced endothelial vasoconstrictor prostanoid release

This study investigated the modulation of angiotensin II-induced endothelial prostanoid release in rabbit aortic rings. Two cumulative dose response curves with 90-min washing interval were performed. Incubation with L-N(G)-nitroarginine methyl ester (L-NAME) 10(-4) M increased angiotensin II maximal contractile response (E(max)). This effect was reversed by indomethacin 10(-5) M, diphenyliodinum 10(-5) M, Tempol 10(-5) M or ascorbic acid 10(-4) M in both cumulative dose response curves and by SQ 29548 10(-6) M in the second cumulative dose response curve. When segments were treated with tetraethylamonium 10(-3) M but not with glibenclamide 10(-5) M during the washing period, L-NAME recovered its ability to enhance the E(max) in arteries incubated with SQ 29548.

CONCLUSIONS

nitric oxide modulates angiotensin II-induced endothelial release of cyclooxygenase-dependent eicosanoids, one of which acts through thromboxane A(2)/prostaglandin H(2) receptors and would decrease K(Ca) channel activity. An increase in free radical production may account for the enhancement of such prostanoid release. Furthermore, it was found that in the present conditions, the release of the hyperpolarizing factor would improve in order to maintain the vascular tone.

Implications for repetitive application of acetylcholine in the determination of the mechanisms of endothelium-dependent relaxation

The influence of repetitive vasodilator concentration response curve determination was investigated in rat mesenteric and femoral small arteries. Arteries were precontracted with noradrenaline and relaxed with acetylcholine (ACh) or K+. Mesenteric arteries exhibited attenuation of ACh-stimulated relaxations during repetitive precontraction/relaxation cycles that was not prevented by SQ29548. Apamin, but not charybdotoxin, prevented the attenuation of this relaxation response. Borderline (p=0.064) statistical differences in the relaxations of mesenteric arteries in response to ACh remained in the presence of the nitric oxide synthase inhibitor L-NNA. In contrast, femoral arterial relaxations increased with repeated acetylcholine challenges. The enhanced responsiveness was prevented with L-NNA or Ba2+. In one experimental series, Ba2+ appeared to be without influence upon K+-stimulated relaxation of femoral arteries but a significant inhibitory effect was revealed when appropriate time control experiments were considered. These experiments reveal that impairment of SK(Ca) function and, to a lesser extent, an impaired NO signalling account for the attenuation of the relaxation responses of rat mesenteric arteries. In contrast, alterations of nitric oxide signalling and inward rectifier K channel activity contribute to enhanced relaxation responses in rat femoral arteries. These experiments highlight the importance of appropriate time control experiments for the proper interpretation of results derived from pharmacological experiments directed toward the elucidation of mechanisms of arterial vasorelaxation.

Testosterone treatment increases thromboxane function in rat cerebral arteries

We previously showed that testosterone, administered in vivo, increases the tone of cerebral arteries. A possible underlying mechanism is increased vasoconstriction through the thromboxane A2 (TxA2) pathway. Therefore, we investigated the effect of chronic testosterone treatment (4 wk) on TxA2 synthase levels and the contribution of TxA2 to vascular tone in rat middle cerebral arteries (MCAs). Using immunofluorescence and confocal microscopy, we demonstrated that TxA2 synthase is present in MCA segments in both smooth muscle and endothelial layers. Using Western blot analysis, we found that TxA2 synthase protein levels are higher in cerebral vessel homogenates from testosterone-treated orchiectomized (ORX+T) rats compared with orchiectomized (ORX) control animals. Functional consequences of changes in cerebrovascular TxA2 synthase were determined using cannulated, pressurized MCA segments in vitro. Constrictor responses to the TxA2 mimetic U-46619 were not different between the ORX+T and ORX groups. However, dilator responses to either the selective TxA2 synthase inhibitor furegrelate or the TxA2-endoperoxide receptor (TP) antagonist SQ-29548 were greater in the ORX+T compared with ORX group. In endothelium-denuded arteries, the dilation to furegrelate was attenuated in both the ORX and ORX+T groups, and the difference between the groups was abolished. These data suggest that chronic testosterone treatment enhances TxA2-mediated tone in rat cerebral arteries by increasing endothelial TxA2 synthesis without altering the TP receptors mediating constriction. The effect of in vivo testosterone on cerebrovascular TxA2 synthase, observed here after chronic hormone administration, may contribute to the risk of vasospasm and thrombosis related to cerebrovascular disease.

Mechanism of Contraction of Rat Isolated Tail Arteries by Hyposmotic Solutions

Contraction induced by hyposmotic swelling was examined in rat tail arteries mounted on a myograph containing a modified Krebs physiological saline solution (PSS) containing 50 mM mannitol (300 mosm/l). Hyposmotic swelling was induced by removing mannitol. In arteries having basal tone or arteries precontracted with K(+) or the thromboxane mimetic U-46619, removal of mannitol caused a concentration dependent contraction of rat tail arteries. Concurrent measurement of tension and intracellular calcium [Ca(2+)](i )in arteries loaded with fura-2 showed that both tension and [Ca(2+)](i) increased on exposure to a hyposmotic solution. Removal of endothelium or inhibition of nitric oxide and cyclooxygenase together did not affect contractile responses. Removal of extracellular Ca(2+) abolished the contractile response to hyposmotic solution and NiCl(2), a nonspecific inhibitor of Ca(2+) influx pathways, blocked the rise in [Ca(2+)](i) and tension in response to a hyposmotic solution. Verapamil and nisoldipine, inhibitors of Ca(v)1.2 (L-type) calcium channels significantly reduced the contractile response to a hyposmotic solution. Addition of NiCl(2) to nisoldipine caused an additional inhibition of the response to a hyposmotic solution. Inhibition of calcium release from the sarcoplasmic reticulum by ryanodine or cyclopiazonic acid (CPA) did not cause any change in the tension response to a hyposmotic solution. CPA did not significantly inhibit the response to a hyposmotic solution in the presence of N(G)-methyl-L-arginine, oxyhaemoglobin and indomethacin. We conclude that contraction induced by a hyposmotic solution is largely due to Ca(v)1.2 calcium channels although other Ca(2+) influx pathways also contribute.

Redox Modulation of Vascular Tone

Opening of potassium channels on vascular smooth muscle cells with resultant hyperpolarization plays a central role in several mechanisms of vasodilation. For example, in the arteriolar circulation where tissue perfusion is regulated, there is an endothelial derived hyperpolarizing factor that opens vascular smooth muscle calcium-activated potassium channels, eliciting dilation. Metabolic vasodilation involves the opening of sarcolemmal ATP-sensitive potassium channels. Adrenergic dilation as well as basal vasomotor tone in several vascular beds depend upon voltage-dependent potassium channels in smooth muscle. Thus hyperpolarization through potassium channel opening is a fundamental mechanism for vasodilation. Disease states such as coronary atherosclerosis and its risk factors are associated with elevated levels of reactive oxygen (ROS) and nitrogen species that have well-defined inhibitory effects on nitric oxide-mediated vasodilation. Effects of ROS on hyperpolarization mechanisms of dilation involving opening of potassium channels are less well understood but are very important because hyperpolarization-mediated dilation often compensates for loss of other dilator mechanisms. We review the effect of ROS on potassium channel function in the vasculature. Depending on the oxidative species, ROS can activate, inhibit, or leave unaltered potassium channel function in blood vessels. Therefore, discerning the activity of enzymes regulating production or degradation of ROS is important when assessing tissue perfusion in health and disease.

Factors, fiction and endothelium-derived hyperpolarizing factor

1. The principal mediators of vascular tone are neural, endothelial and physical stimuli that result in the initiation of dilator and constrictor responses to facilitate the control of blood pressure. Two primary vasodilatory stimuli produced by the endothelium are nitric oxide (NO) and prostaglandins. An additional endothelium-dependent vasodilatory mechanism is characterized as the hyperpolarization-mediated relaxation that remains after the inhibition of the synthesis of NO and prostaglandins. This mechanism is due to the action of a so-called endothelium-derived hyperpolarizing factor (EDHF) and is dependent on either the release of diffusible factor(s) and/or to a direct contact-mediated mechanism. 2. Most evidence supports the concept that 'EDHF' activity is dependent on contact-mediated mechanisms. This involves the transfer of an endothelium-derived electrical current, as an endothelium-derived hyperpolarization (EDH), through direct heterocellular coupling of endothelial cells and smooth muscle cells via myoendothelial gap junctions (MEGJ). However, there is a lack of consensus with regard to the nature and mechanism of action of EDHF/EDH (EDH(F)), which has been shown to vary within and between vascular beds, as well as among species, strains, sex and during development, ageing and disease. 3. In addition to actual heterogeneity in EDH(F), further heterogeneity has resulted from the less-than-optimal design, analysis and interpretation of data in some key papers in the EDHF literature; with such views being perpetuated in the subsequent literature. 4. The focus of the present brief review is to examine what factors are proposed as EDH(F) and highlight the correlative structural and functional studies from our laboratory that demonstrate an integral role for MEGJ in the conduction of EDH, which account for the heterogeneity in EDH(F), while incorporating the reported diffusible mechanisms in the regulation of this activity. Furthermore, in addition to the reported heterogeneity in the nature and mechanism of action of EDH(F), the contribution of experimental design and technique to this heterogeneity will be examined.