Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

Inhibition of Na+ /K+ -ATPase and KIR channels abolishes hypoxic hyperaemia in resting but not contracting skeletal muscle of humans

KEY POINTS

Increasing blood flow (hyperaemia) to exercising muscle helps match oxygen delivery and metabolic demand. During exercise in hypoxia, there is a compensatory increase in muscle hyperaemia that maintains oxygen delivery and tissue oxygen consumption. Nitric oxide (NO) and prostaglandins (PGs) contribute to around half of the augmented hyperaemia during hypoxic exercise, although the contributors to the remaining response are unknown. In the present study, inhibiting NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels did not blunt augmented hyperaemia during hypoxic exercise beyond previous observations with NO/PG block alone. Furthermore, although inhibition of only Na⁺ /K⁺ -ATPase and K_IR channels abolished hyperaemia during hypoxia at rest, it had no effect on augmented hyperaemia during hypoxic exercise. This is the first study in humans to demonstrate that Na⁺ /K⁺ -ATPase and K_IR channel activation is required for augmented muscle hyperaemia during hypoxia at rest but not during hypoxic exercise, thus providing new insight into vascular control.

ABSTRACT

Exercise hyperaemia in hypoxia is augmented relative to the same exercise intensity in normoxia. During moderate-intensity handgrip exercise, endothelium-derived nitric oxide (NO) and vasodilating prostaglandins (PGs) contribute to ∼50% of the augmented forearm blood flow (FBF) response to hypoxic exercise (HypEx), although the mechanism(s) underlying the remaining response are unclear. We hypothesized that combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels would abolish the augmented hyperaemic response in HypEx. In healthy young adults, FBF responses were measured (Doppler ultrasound) and forearm vascular conductance was calculated during 5 min of rhythmic handgrip exercise at 20% maximum voluntary contraction under regional sympathoadrenal inhibition in normoxia and isocapnic HypEx (O₂ saturation ∼80%). Compared to control, combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and K_IR channels (l-NMMA + ketorolac + ouabain + BaCl_2; Protocol 1; n = 10) blunted the compensatory increase in FBF during HypEx by ∼50% (29 ± 6 mL min^-1 vs. 62 ± 8 mL min^-1 , respectively, P < 0.05). By contrast, ouabain + BaCl₂ alone (Protocol 2; n = 10) did not affect this augmented hyperaemic response (50 ± 11 mL min^-1 vs. 60 ± 13 mL min^-1 , respectively, P > 0.05). However, the blocked condition in both protocols abolished the hyperaemic response to hypoxia at rest (P < 0.05). We conclude that activation of Na⁺ /K⁺ -ATPase and K_IR channels is involved in the hyperaemic response to hypoxia at rest, although it does not contribute to the augmented exercise hyperaemia during hypoxia in humans.

Regulation of Coronary Blood Flow

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

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

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.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Reactive oxygen species facilitate the EDH response in arterioles by potentiating intracellular endothelial Ca(2+) release

There is abundant evidence that H2O2 can act as an endothelium-derived hyperpolarizing factor in the resistance vasculature. However, whilst scavenging H2O2 can abolish endothelial dependent hyperpolarization (EDH) and the associated vascular relaxation in some arteries, EDH-dependent vasorelaxation can often be mimicked only by using relatively high concentrations of H2O2. We have examined the role of H2O2 in EDH-dependent vasodilatation by simultaneously measuring vascular diameter and changes in endothelial cell (EC) [Ca(2+)]i during the application of H2O2 or carbachol, which triggers EDH. Carbachol (10µM) induced dilatation of phenylephrine-preconstricted rat cremaster arterioles was largely (73%) preserved in the presence of indomethacin (3µM) and l-NAME (300µM). This residual NO- and prostacyclin-independent dilatation was reduced by 89% upon addition of apamin (0.5µM) and TRAM-34 (10µM), and by 74% when an extracellular ROS scavenging mixture of SOD and catalase (S&C; 100Uml(-1) each) was present. S&C also reduced the carbachol-induced EC [Ca(2+)]i increase by 74%. When applied in Ca(2+)-free external medium, carbachol caused a transient increase in EC [Ca(2+)]i. This was reduced by catalase, and was enhanced when 1µM H2O2 was present in the bath. H2O2 -induced dilatation, which occurred only at concentrations ≥100µM, was reduced by a blocking antibody to TRPM2, which had no effect on carbachol-induced responses. Similarly, iberotoxin and Rp-8bromo cGMP reduced the vasodilatation induced by H2O2, but not by carbachol. Inhibiting PLC, PLA2 or CYP450 2C9 each greatly reduced the carbachol-induced increase in EC [Ca(2+)]i and vasodilatation, but adding 10µM H2O2 during PLA2 or CYP450 2C9 inhibition completely restored both responses. The nature of the effective ROS species was investigated by using Fe(2+) chelators to block the formation of ∙OH. A cell permeant chelator was able to inhibit EC Ca(2+) store release, but cell impermeant chelators reduced both the vasodilatation and EC Ca(2+) influx, implying that ∙OH is required for these responses. The results indicate that rather than mediating EDH by acting directly on smooth muscle, H2O2 promotes EDH by acting within EC to enhance Ca(2+) release.

Soluble epoxide hydrolase inhibitors and heart failure

Cardiovascular disease remains one of the leading causes of death in the Western societies. Heart failure (HF) is due primarily to progressive myocardial dysfunction accompanied by myocardial remodeling. Once HF develops, the condition is, in most cases, irreversible and is associated with a very high mortality rate. Soluble epoxide hydrolase (sEH) is an enzyme that catalyzes the hydrolysis of epoxyeicosatrienoic acids (EETs), which are lipid mediators derived from arachidonic acid through the cytochrome P450 epoxygenase pathway. EETs have been shown to have vasodilatory, antiinflammatory, and cardioprotective effects. When EETs are hydrolyzed by sEH to corresponding dihydroxyeicosatrienoic acids, their cardioprotective activities become less pronounced. In line with the recent genetic study that has identified sEH as a susceptibility gene for HF, the sEH enzyme has received considerable attention as an attractive therapeutic target for cardiovascular diseases. Indeed, sEH inhibition has been demonstrated to have antihypertensive and antiinflammatory actions, presumably due to the increased bioavailability of endogenous EETs and other epoxylipids, and several potent sEH inhibitors have been developed and tested in animal models of cardiovascular disease including hypertension, cardiac hypertrophy, and ischemia/reperfusion injury. sEH inhibitor treatment has been shown to effectively prevent pressure overload- and angiotensin II-induced cardiac hypertrophy and reverse the pre-established cardiac hypertrophy caused by chronic pressure overload. Application of sEH inhibitors in several cardiac ischemia/reperfusion injury models reduced infarct size and prevented the progressive cardiac remodeling. Moreover, the use of sEH inhibitors prevented the development of electrical remodeling and ventricular arrhythmias associated with cardiac hypertrophy and ischemia/reperfusion injury. The data published to date support the notion that sEH inhibitors may represent a promising therapeutic approach for combating detrimental cardiac remodeling and HF.

Pharmacological consequences of the coexpression of BK channel α and auxiliary β subunits

Coded by a single gene (Slo1, KCM) and activated by depolarizing potentials and by a rise in intracellular Ca(2+) concentration, the large conductance voltage- and Ca(2+)-activated K(+) channel (BK) is unique among the superfamily of K(+) channels. BK channels are tetramers characterized by a pore-forming α subunit containing seven transmembrane segments (instead of the six found in voltage-dependent K(+) channels) and a large C terminus composed of two regulators of K(+) conductance domains (RCK domains), where the Ca(2+)-binding sites reside. BK channels can be associated with accessory β subunits and, although different BK modulatory mechanisms have been described, greater interest has recently been placed on the role that the β subunits may play in the modulation of BK channel gating due to its physiological importance. Four β subunits have currently been identified (i.e., β1, β2, β3, and β4) and despite the fact that they all share the same topology, it has been shown that every β subunit has a specific tissue distribution and that they modify channel kinetics as well as their pharmacological properties and the apparent Ca(2+) sensitivity of the α subunit in different ways. Additionally, different studies have shown that natural, endogenous, and synthetic compounds can modulate BK channels through β subunits. Considering the importance of these channels in different pathological conditions, such as hypertension and neurological disorders, this review focuses on the mechanisms by which these compounds modulate the biophysical properties of BK channels through the regulation of β subunits, as well as their potential therapeutic uses for diseases such as those mentioned above.

Pharmacological manipulation of arachidonic acid-epoxygenase results in divergent effects on renal damage

Kidney damage is markedly accelerated by high-salt (HS) intake in stroke-prone spontaneously hypertensive rats (SHRSP). Epoxyeicosatrienoic acids (EETs) are epoxygenase products of arachidonic acid which possess vasodepressor, natriuretic, and anti-inflammatory activities. We examined whether up-regulation (clofibrate) or inhibition [N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH)] of epoxygenase would alter systolic blood pressure (SBP) and/or renal pathology in SHRSP on HS intake (1% NaCl drinking solution). Three weeks of treatment with clofibrate induced renal cortical protein expression of CYP2C23 and increased urinary excretion of EETs compared with vehicle-treated SHRSP. SBP and urinary protein excretion (UPE) were significantly lowered with clofibrate treatment. Kidneys from vehicle-treated SHRSP, which were on HS intake for 3 weeks, demonstrated focal lesions of vascular fibrinoid degeneration, which were markedly attenuated with clofibrate treatment. In contrast, 2 weeks of treatment with the selective epoxygenase inhibitor, MS-PPOH, increased UPE without significantly altering neither urinary EET levels nor SBP. Kidneys from vehicle-treated SHRSP, which were on HS intake for 11 days, demonstrated occasional mild damage whereas kidneys from MS-PPOH-treated rats exhibited widespread malignant nephrosclerosis. These results suggest that pharmacological manipulation of epoxygenase results in divergent effects on renal damage and that interventions to increase EET levels may provide therapeutic strategies for treating salt-sensitive hypertension and renal damage.

Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE

The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified.

EXPERIMENTAL APPROACH

ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein.

KEY RESULTS

In endothelium-intact preparations, ACh (≤ 10⁻⁸ M) marginally increased the intracellular concentration of Ca²⁺ ([Ca²⁺](i)) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10⁻¹⁰ -10⁻⁸ M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10⁻⁸ -10⁻⁶ M) concentration-dependently increased endothelial [Ca²⁺](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca²⁺-activated K⁺ (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca²⁺ -activated K⁺ (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K⁺ (K(V) ) channel, K(V) 1].

CONCLUSIONS AND IMPLICATIONS

In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10⁻¹⁰ -10⁻⁸ M). At higher concentrations, ACh (10⁻⁸ -3 × 10⁻⁶ M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.

Use of metabolomic profiling in the study of arachidonic acid metabolism in cardiovascular disease

Arachidonic acid is one of the pivotal signaling molecules associated with inflammation, pain and homeostatic function. Drugs specifically targeting these signaling pathways represent more than 25% of annual pharmaceutical sales worldwide. However, chronic administration of nonsteroidal anti-inflammatory drugs (NSAIDs) and rofecoxib (Vioxx), a potent cyclooxygenase-2 inhibitor, have been associated with adverse cardiovascular events. Understanding the possible mechanisms underlying these adverse events is critical for evaluating the risks and benefits of this group of drugs and for development of safer drugs. Using a powerful metabolomics approach, 20-hydroxyeicosatetraenoic acid (20-HETE) was identified among many of arachidonic acid metabolic products as a likely culprit for adverse cardiovascular side effect associated with rofecoxib and NSAIDs. In addition, using a similar metabolomic approach, epoxyeicosatrienoic acids (EETs), which are lipid mediators derived from arachidonic acid through the cytochrome P450 epoxygenase pathway, have been shown to exhibit cardioprotective effects in a murine myocardial infarction (MI) model. Inhibitors of the soluble epoxide hydrolase increase titers of epoxy fatty acids and both block and reverse cardiac hypertrophy in rodent models. These highly potent, orally available compounds may be promising for treating heart failure and other cardiovascular disease. In this review, we will summarize some of the recent advances using metabolomic profiling to gain insights into the involvement of arachidonic acid pathways in cardiovascular disease.

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.

Role of potassium channels in coronary vasodilation

K+ channels in coronary arterial smooth muscle cells (CASMC) determine the resting membrane potential ( Em) and serve as targets of endogenous and therapeutic vasodilators. Em in CASMC is in the voltage range for activation of L-type Ca2+ channels; therefore, when K+ channel activity changes, Ca2+ influx and arterial tone change. This is why both Ca2+ channel blockers and K+ channel openers have such profound effects on coronary blood flow; the former directly inhibits Ca2+ influx through L-type Ca2+ channels, while the latter indirectly inhibits Ca2+ influx by hyperpolarizing Em and reducing Ca2+ channel activity. K+ channels in CASMC play important roles in vasodilation to endothelial, ischemic and metabolic stimuli. The purpose of this article is to review the types of K+ channels expressed in CASMC, discuss the regulation of their activity by physiological mechanisms and examine impairments related to cardiovascular disease.

Analysis of L-NAME-dependent and -resistant responses to acetylcholine in the rat

The mechanism by which acetylcholine (ACh) decreases systemic arterial pressure and hindlimb vascular resistance was investigated in the anesthetized rat. ACh injections caused dose-dependent decreases in systemic arterial pressure and hindlimb vascular resistance. N(omega)-nitro-L-arginine methyl ester (L-NAME) had little effect on the magnitude of depressor and vasodilator responses but decreased response duration when baseline parameters were corrected by a nitric oxide (NO) donor infusion. The decrease in the duration of the ACh depressor response was prevented by the administration of excess L-arginine. The L-NAME-resistant component of the depressor response to ACh was attenuated by ebselen, a glutathione peroxidase mimic. The calcium-activated potassium (K(Ca)) antagonists charybdotoxin (ChTX) and apamin decreased the magnitude but not the duration of the hindlimb vasodilator response to ACh. The combination of L-NAME, ChTX, and apamin reduced the magnitude and duration of the vasodilator response to ACh but not to sodium nitroprusside. Vasodepressor and hindlimb vasodilator responses to ACh were not modified by cytochrome P-450 and cyclooxygenase pathway inhibitors. These results suggest that the hindlimb vasodilator response to ACh has an initial L-NAME-resistant component mediated by the activation of K(Ca) channels and a sustained L-NAME-dependent component. The results with ebselen suggest that the L-NAME-resistant component of the depressor response involves a peroxide-sensitive mechanism. The present study suggests that vasodilator responses to ACh are not mediated by cytochrome P-450 products, since miconazole and 1-aminobentriazole alone or in combination did not affect either component of the response. The present data suggest that the hindlimb vasodilator response to ACh in the rat is mediated by two mechanisms with an initial ChTX- and apamin-sensitive, L-NAME-resistant phase not mediated by cytochrome P-450 products and a secondary sustained phase mediated by NO.

Epoxyeicosatrienoic Acid Relaxing Effects Involve Ca2+-Activated K+Channel Activation and CPI-17 Dephosphorylation in Human Bronchi

The aim of the present study was to provide a mechanistic insight into how 14,15-epoxyeicosatrienoic acid (EET) relaxes organ-cultured human bronchi. Tension measurements, performed on either fresh or 3-d-cultured bronchi, revealed that the contractile responses to 1 microM methacholine and 10 microM arachidonic acid were largely relaxed by the eicosanoid regioisomer in a concentration-dependent manner (0.01-10 microM). Pretreatments with 14,15-epoxyeicosa-5(Z)-enoic acid, a specific 14,15-EET antagonist, prevented the relaxing effect, whereas iberitoxin pretreatments (10 nM) partially abolished EET-induced relaxations. In contrast, pretreatments with 1 microM indomethacin amplified relaxations in explants and membrane hyperpolarizations triggered by 14,15-EET on airway smooth muscle cells. The relaxing responses induced by 14,15-EET were likely related to reduced Ca2+ sensitivity of the myofilaments, because free Ca2+ concentration-response curves performed on beta-escin-permeabilized cultured explants were shifted toward higher [Ca2+] (lower pCa2+ values). 14,15-EET also abolished the tonic responses induced by phorbol-ester-dybutyrate (PDBu) (a protein kinase C [PKC]-sensitizing agent), on both fresh (intact) and beta-escin-permeabilized explants. Western blot analyses, using two specific primary antibodies against CPI-17 and its PKC-dependent phosphorylated isoform (p-CPI-17), confirmed that the eicosanoid interferes with this intracellular process. These data indicate that 14,15-EET hyperpolarizes airway smooth muscle cells and relaxes precontracted human bronchi while reducing Ca2+ sensitivity of fresh and cultured explants. The intracellular effects are related to a PKC-dependent process involving a lower phosphorylation level of CPI-17.

Effects of 5-oxo-ETE and 14,15-EET on reactivity and Ca2+ sensitivity in guinea pig bronchi

The reactivity and Ca2+ sensitivity of fresh as well as organ-cultured guinea pig bronchi challenged with 5-oxo-ETE and 14,15-EET were compared. Tension measurements, performed on fresh and 3-day cultured bronchi, revealed that the contractile responses to 5-oxo-ETE were largely increased in cultured explants, while 14,15-EET induced larger relaxations on Carbamylcholine (CCh) pre-contracted explants. In fresh bronchi, the contractile responses to 5-oxo-ETE were inhibited by 10 microM indomethacin whereas the relaxing responses induced by 14,15-EET were amplified in the presence of COX inhibitors. COX down expression resulted in a lack of indomethacin effect in cultured explants. One micromolar 5-oxo-ETE increased Ca2+ sensitivity in beta-escin-permeabilized cultured explants, while 1 microM Y-27632 abolished this hypersensitivity. In contrast, 1 microM 14,15-EET significantly reduced the Ca2+ hypersensitivity developed by cultured bronchi. In conclusion, pre-treatment of cultured guinea pig bronchi for 48 h with these eicosanoids modifies the pharmacological responsiveness and Ca2+ sensitivity of these cultured explants.

Rat mesenteric arterial dilator response to 11,12-epoxyeicosatrienoic acid is mediated by activating heme oxygenase

11,12-Epoxyeicosatrienoic acid (11,12-EET), a potent vasodilator produced by the endothelium, acts on calcium-activated potassium channels and shares biological activities with the heme oxygenase/carbon monoxide (HO/CO) system. We examined whether activation of HO mediates the dilator action of 11,12-EET, and that of the other EETs, on rat mesenteric arteries. Dose-response curves (10(-9) to 10(-6) M) to 5,6-EET, 8,9-EET, 11,12-EET, 14,15-EET, and ACh (10(-9) to 10(-4) M) were evaluated in preconstricted (10(-6) mol/l phenylephrine) mesenteric arteries (<350 microm diameter) in the presence or absence of 1) the cyclooxygenase inhibitor indomethacin (2.8 microM), 2) the HO inhibitor chromium mesoporphyrin (CrMP) (15 microM), 3) the soluble guanylyl cyclase (GC) inhibitor ODQ (10 microM), and 4) the calcium-activated potassium channel inhibitor iberiotoxin (25 nM). The vasodilator response to 11,12-EET was abolished by CrMP and iberiotoxin, whereas indomethacin and ODQ had no effect. In contrast, the effect of ACh was attenuated by ODQ but not by CrMP. The vasodilator effect of 8,9-EET, like that of 11,12-EET, was greatly attenuated by HO inhibition. In contrast, the mesenteric vasodilator response to 5,6-EET was independent of both HO and GC, whereas that to 14,15-EET demonstrated two components, an HO and a GC, of equal magnitude. Incubation of mesenteric microvessels with 11,12-EET caused a 30% increase in CO release, an effect abolished by inhibition of HO. We conclude that the rat mesenteric vasodilator action of 11,12-EET is mediated via an increase in HO activity and an activation of calcium-activated potassium channels.

Emerging role of epoxyeicosatrienoic acids in coronary vascular function

The importance of endothelium-derived nitric oxide in coronary vascular regulation is well-established and the loss of this vasodilator compound is associated with endothelial dysfunction, tissue hypoperfusion and atherosclerosis. Numerous studies indicate that the endothelium produces another class of compounds, the epoxyeicosatrienoic acids (EETs), which may partially compensate for the loss of nitric oxide in cardiovascular disease. The EETs are endogenous lipids which are derived through the metabolism of arachidonic acid by cytochrome P450 epoxygenase enzymes. Also, EETs hyperpolarize vascular smooth muscle and induce dilation of coronary arteries and arterioles, and therefore may be endogenous mediators of coronary vasomotor tone and myocardial perfusion. In addition, EETs have been shown to inhibit vascular smooth muscle migration, decrease inflammation, inhibit platelet aggregation and decrease adhesion molecule expression, therefore representing an endogenous protective mechanism against atherosclerosis. Endogenous EETs are degraded to less active dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. Pharmacological inhibition of soluble epoxide hydrolase has received considerable attention as a potential approach to enhance EET-mediated vascular protection, and several compounds have appeared promising in recent animal studies. The present review discusses the emerging role of EETs in coronary vascular function, as well as recent advancements in the development of pharmacological agents to enhance EET bioavailability.

Effects of Endothelium-Derived Hyperpolarizing Factor and Nitric Oxide on Endothelial Function in Femoral Resistance Arteries of Spontaneously Hypertensive Rats

In hypertension, endothelium-dependent relaxation is attenuated and this attenuation contributes to the increased peripheral resistance. However, the role of endothelium-derived hyperpolarizing factor (EDHF) in the arteries of hypertensive rats remains unclear. Therefore, the aim of this study was to evaluate the role of EDHF in the femoral resistance arteries of hypertensive rats. The femoral resistance arteries were isolated from 5-, 15- and 25-week-old spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). Changes in internal diameter were examined with videomicroscopy. EDHF-mediated dilatation was determined by differences between the degree of acetylcholine (ACh)-induced dilatation in the presence of NG-monomethy-L-arginine (L-NMMA) plus a prostaglandin I2 inhibitor (indomethacin) and the degree of such dilatation in the presence of L-NMMA, indomethacin and KCl. Charybdotoxin (CTx) and apamin (a Ca2+-activated K+ channel [KCa] inhibitor)-sensitive EDHF dilatation was also compared between in 5-, 15- and 25-week-old SHR and WKY. ACh-induced vasodilatation was not different between 5-week-old SHR and WKY. There was no difference between NO- and EDHF-mediated vasodilatation in 5-week-old rats. ACh-induced vasodilatation was weaker in 15-week-old SHR than in WKY. NO-mediated vasodilatation did not differ between the two groups. EDHF-mediated dilatation was attenuated in SHR but not in WKY. ACh-induced dilatation was weaker in 25-week-old SHR than in WKY. NO- and EDHF-mediated vasodilatation were attenuated in SHR but not WKY. EDHF-mediated vasodilatation was attenuated before the loss of NO-mediated vasodilatation in the femoral resistance arteries of SHR. The attenuation of this vasodilatation was mediated by the CTx plus apamin-sensitive EDHF.

Role of 20-hydroxyeicosatetraenoic acid (20-HETE) in vascular system

Cytochrome P450s (P450) metabolize arachidonic acid (AA) to hydroxyeicosatetraenoic acids (HETEs) and epoxyeicosatrienoic acids (EETs). Among these eicosanoids, 20-HETE is formed in a tissue and cell-specific fashion and plays an important role in the regulation of vascular tone in the brain, kidney, heart and splanchnic beds. 20-HETE is a potent vasoconstrictor produced in vascular smooth muscle (VSM) cells. It depolarizes VSM by blocking the open-state probability of Ca2+-activated K+-channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal and cerebral arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. The formation of 20-HETE in vascular smooth muscle is stimulated by angiotensin II, endothelin and norepinephrine and is inhibited by nitric oxide (NO). 20-HETE also stimulates mitogenic and angiogenic responses in vitro and in vivo. Changes in the production of 20-HETE have been observed in ischemic cerebrovascular diseases, cardiac ischemia-reperfusion injury, kidney diseases, hypertension, diabetes, uremia, toxemia of pregnancy. The physiological and pathophysiological role of 20-HETE in the regulation of vascular tone are being revealed by the use of newly developed inhibitors of the synthesis of 20-HETE and 20-HETE analogs. The present review summarizes recent findings implicating a critical role for 20-HETE in altering cardiovascular function in a variety of pathological conditions.

Bradykinin-induced, endothelium-dependent responses in porcine coronary arteries: involvement of potassium channel activation and epoxyeicosatrienoic acids

In coronary arteries, bradykinin opens endothelial intermediate- and small-conductance Ca2+-sensitive K+ channels (IK(Ca) and SK(Ca)) and, additionally, releases epoxyeicosatrienoic acids (EETs) from the endothelium. To clarify the involvement of these pathways in endothelium-dependent myocyte hyperpolarization, bradykinin-induced electrical changes in endothelial cells and myocytes of porcine coronary arteries (following nitric oxide (NO) synthase and cyclooxygenase inhibition) were measured using sharp microelectrodes. Hyperpolarization of endothelial cells by bradykinin (27.0 +/- 0.9 mV, n = 4) was partially inhibited (74%) by blockade of IK(Ca) and SK(Ca) channels using 10 microM TRAM-39 (2-(2-chlorophenyl)-2,2-diphenylacetonitrile) plus 100 nM apamin (leaving an iberiotoxin-sensitive component), whereas the response to substance P was abolished. After gap junction blockade with HEPES, (N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulphonic acid)) hyperpolarization of the endothelium by 100 nM bradykinin was abolished by TRAM-39 plus apamin, whereas myocyte hyperpolarization still occurred (12.9 +/- 1.0 mV, n=4). The residual hyperpolarizations to 100 nM bradykinin were antagonized by the EET antagonist, 14,15-EEZE (14,15-epoxyeicosa-5(Z)-enoic acid) (10 microM), and abolished by iberiotoxin. Bradykinin-induced myocyte hyperpolarizations were also reduced by 14,15-EEZE-mSI (14,15-EEZE-methylsulfonylimide) (5,6- and 14,15-EET antagonist), whereas those to exogenous 11,12-EET were unaffected. These data show that bradykinin-induced hyperpolarization of endothelial cells (due to the opening of IK(Ca) and SK(Ca) channels) is electrotonically transferred to the myocytes via gap junctions. Bradykinin (but not substance P) also hyperpolarizes myocytes by a mechanism (independent of endothelial cell hyperpolarization) which involves endothelial cell production of EETs (most likely 14,15- and/or 11,12-EET). These open endothelial IK(Ca) and SK(Ca) channels and also activate large-conductance calcium-sensitive K+ channels (BK(Ca)) on the surrounding myocytes.

5,6-EET-induced contraction of intralobar pulmonary arteries depends on the activation of Rho-kinase

The mechanism mediating epoxyeicosatrienoic acid (EET)-induced contraction of intralobar pulmonary arteries (PA) is currently unknown. EET-induced contraction of PA has been reported to require intact endothelium and activation of the thromboxane/endoperoxide (TP) receptor. Because TP receptor occupation with the thromboxane mimetic U-46619 contracts pulmonary artery via Rho-kinase activation, we examined the hypothesis that 5,6-EET-induced contraction of intralobar rabbit pulmonary arteries is mediated by a Rho-kinase-dependent signaling pathway. In isolated rings of second-order intralobar PA (1–2 mm OD) at basal tension, 5,6-EET (0.3–10 μM) induced increases in active tension that were inhibited by Y-27632 (1 μM) and HA-1077 (10 μM), selective inhibitors of Rho-kinase activity. In PA in which smooth muscle intracellular Ca2+ concentration ([Ca2+]i) was increased with KCl (25 mM) to produce a submaximal contraction, 5,6-EET (1 μM) induced a contraction that was 7.0 ± 1.6 times greater than without KCl. 5,6-EET (10 μM) also contracted β-escin permeabilized PA in which [Ca2+]i was clamped at a concentration resulting in a submaximal contraction. Y-27632 inhibited the 5,6-EET-induced contraction in permeabilized PA. 5,6-EET (10 μM) increased phosphorylation of myosin light chain (MLC), increasing the ratio of phosphorylated MLC/total MLC from 0.10 ± 0.03 to 0.30 ± 0.02. Y-27632 prevented this increase in MLC phosphorylation. These data suggest that 5,6-EET induces contraction in intralobar PA by increasing Rho-kinase activity, phosphorylating MLC, and increasing the Ca2+ sensitivity of the contractile apparatus.

Red blood cells: reservoirs of cis- and trans-epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are candidate endothelium-derived hyperpolarizing factors that demonstrate a wide range of biological effects. The presence of both cis- and trans-EETs in rat plasma was identified with HPLC-electrospray ionization tandem mass spectrometry in this study. The total EETs in plasma are 38.2 ng/ml with cis-EETs representing 21.4 +/- 0.4 ng/ml and trans-EETs 16.8 +/- 0.4 ng/ml. EETs in RBCs were estimated to be 20.2 ng/10(9) RBCs, which corresponds to 200 ng in RBCs contained in 1 ml blood. RBC incubation with 10 mM tert-butyl hydroperoxide resulted in 4.4-fold increase of total cis-EETs (from 9.2 to 40.2 ng/10(9) RBCs) and 5.5-fold increase of total trans-EETs (from 11.0 to 60.8 ng/10(9) RBCs). EETs were released (2 ng/ml) from RBCs after incubation at 37 degrees C for 10 min even after being washed 3 times, indicating that RBCs are reservoirs of plasma EETs. The identification of cis- and trans-EETs in RBCs and in plasma as well as their release from RBCs suggest a vasoregulatory role of RBCs in view of their potent vasoactivity.

MaxiK channel roles in blood vessel relaxations induced by endothelium-derived relaxing factors and their molecular mechanisms

The endothelium of blood vessels plays a crucial role in the regulation of blood flow by controlling mechanical functions of underlying vascular smooth muscle. The regulation by the endothelium of vascular smooth muscle relaxation and contraction is mainly achieved via the release of vasoactive substances upon stimulation with neurohumoural substances and physical stimuli. Nitric oxide (NO) and prostaglandin I2 (prostacyclin, PGI2) are representative endothelium-derived chemicals that exhibit powerful blood vessel relaxation. NO action involves activation of soluble guanylyl cyclase and PGI2 action is initiated by the stimulation of a cell-surface receptor (IP receptor, IPR) that is coupled with Gs-protein-adenylyl cyclase cascade. Many studies on the mechanisms by which NO and PGI2 elicit blood vessel relaxation have highlighted a role of the large conductance, Ca2+-activated K+ (MaxiK, BKCa) channel in smooth muscle as their common downstream effector. Furthermore, their molecular mechanisms have been unravelled to include new routes different from the conventionally approved intracellular pathways. MaxiK channel might also serve as a target for endothelium-derived hyperpolarizing factor (EDHF), the non-NO, non-PGI2 endothelium-derived relaxing factor in some blood vessels. In this brief article, we review how MaxiK channel serves as an endothelium-vascular smooth muscle transducer to communicate the chemical signals generated in the endothelium to control blood vessel mechanical functions and discuss their molecular mechanisms.

Role of SK(Ca) and IK(Ca) in endothelium-dependent hyperpolarizations of the guinea-pig isolated carotid artery

1. This study was designed to determine whether the endothelium-dependent hyperpolarizations evoked by acetylcholine in guinea-pig carotid artery involve a cytochrome P450 metabolite and whether they are linked to the activation of two distinct populations of endothelial K(Ca) channels, SK(Ca) and IK(Ca.) 2. The membrane potential was recorded in the vascular smooth muscle cells of the guinea-pig isolated carotid artery. All the experiments were performed in the presence of N(omega)-L-nitro arginine (100 microM) and indomethacin (5 microM). 3. Under control conditions (Ca(2+): 2.5 mM), acetylcholine (10 nM to 10 muM) induced a concentration- and endothelium-dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SK(Ca), apamin (0.5 microM) or UCL 1684 (10 microM), produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1 microM) and TRAM-34 (10 microM), a nonpeptidic and specific blocker of IK(Ca), were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM-34 (10 microM) or UCL 1684 (10 microM) plus TRAM-34 (10 microM) virtually abolished the acetylcholine-induced hyperpolarization. 4. In the presence of a combination of apamin and a subeffective dose of TRAM-34 (5 microM), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15-EEZE (10 microM) or the specific blocker of BK(Ca), iberiotoxin (0.1 microM). 5. In presence of 0.5 mM Ca(2+), the hyperpolarization in response to acetylcholine (1 microM) was significantly lower than in 2.5 mM Ca(2+). The EDHF-mediated responses became predominantly sensitive to charybdotoxin or TRAM-34 but resistant to apamin. 6. This investigation shows that the production of a cytochrome P450 metabolite, and the subsequent activation of BK(Ca), is unlikely to contribute to the EDHF-mediated responses in the guinea-pig carotid artery. Furthermore, the EDHF-mediated response involves the activation of both endothelial IK(Ca) and SK(Ca) channels, the activation of either one being able to produce a true hyperpolarization.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

14,15-Epoxyeicosatrienoic Acid Represents a Transferable Endothelium-Dependent Relaxing Factor in Bovine Coronary Arteries

Bradykinin causes arterial relaxation and hyperpolarization, which is mediated by a transferable endothelium-derived hyperpolarizing factor (EDHF). In coronary arteries, epoxyeicosatrienoic acids (EETs) are involved in the EDHF response. However, the role of EETs as transferable mediators of EDHF-dependent relaxation remains poorly defined. Two small bovine coronary arteries were cannulated and perfused in tandem in the presence of the nitric oxide synthase inhibitor, nitro- l -arginine (30 μmol/L), and the cyclooxygenase inhibitor, indomethacin (10 μmol/L). Luminal perfusate from donor arteries with intact endothelium perfused endothelium-denuded detector arteries. Detector arteries were constricted with U46619 and diameters were monitored. Bradykinin (10 nmol/L) added to detector arteries did not induce dilation (5±2%), whereas bradykinin addition to donor arteries dilated detector arteries by 26.5±7% ( P <0.05). These dilations were blocked by donor artery endothelium removal and detector artery treatment with the EET-selective antagonist, 14,15-epoxyeicosa-5(Z)-monoenoic acid (14,15-EEZE; 10 μmol/L, −5±6%) but not 14,15-EEZE treatment of donor arteries (20±5%). 14,15-EET (0.1 to 10 μmol/L) added to detector arteries induced maximal dilations of 82±5% that were inhibited 50% by detector artery treatment with 14,15-EEZE (32±12%) but not donor artery treatment with 14,15-EEZE. Liquid chromatography–electrospray ionization mass spectrometry analysis verified the presence of 14,15-EET in the perfusate from an endothelium-intact but not denuded artery. These results show that bradykinin stimulates donor artery 14,15-EET release that dilates detector arteries. 14,15-EEZE blocked the donor artery, endothelium-dependent, bradykinin-induced relaxations, and attenuated relaxations to 14,15-EET. These results suggest that EETs are transferable EDHFs in coronary arteries.

Arachidonic acid relaxes human pulmonary arteries through K+ channels and nitric oxide pathways

We aimed to investigate the role of K(+) channels and nitric oxide (NO) on the relaxant effects of arachidonic acid in the human intralobar pulmonary arteries. Arachidonic acid produced a concentration-dependent relaxation (E(max)=93+/-3% of maximal relaxation induced by papaverine 0.1 mM;-log EC(30)=7.03+/-0.09) that was antagonized by the cyclooxygenase inhibitor indomethacin (1 microM), by the combination of cyclooxygenase blockade and cytochrome P450 (CYP) blockade with 17-octadecynoic acid (17-ODYA, 10 microM), by the combination of cyclooxygenase inhibition and NO synthase (NOS) inhibition with N(omega)-nitro-l-arginine (l-NOARG, 100 microM), by the simultaneous inhibition of CYP and NOS and by the simultaneous blockade of cyclooxygenase, CYP and NOS. Arachidonic acid-induced relaxation was significantly inhibited by glibenclamide (1 microM, ATP-dependent K(+) channel (K(ATP)) blocker), apamin and charybdotoxin (0.3 microM small (SK(Ca)) and 0.1 microM big (BK(Ca)) conductance Ca(2+)-sensitive K(+) channel blocker, respectively), and 4-aminopyridine (1 mM, voltage-dependent K(+) channel (K(V)) blocker). Indomethacin and ketoconazole suppressed the antagonistic effects of glibenclamide and apamin and 17-ODYA those of all the K(+) channel blockers tested. l-NOARG suppressed only the antagonistic effect of glibenclamide. We suggest that K(ATP), SK(Ca), BK(Ca) and K(V) are involved in the arachidonic acid-induced relaxation of human pulmonary arteries. Cyclooxygenase metabolites are the main relaxing agents of arachidonic acid, involving K(ATP) and SK(Ca) channels. CYP-dependent metabolites modulate arachidonic acid-induced relaxation through a pathway involving K(+) channels. K(ATP) channels are involved through a NOS-dependent pathway.

Endothelial potassium channels, endothelium-dependent hyperpolarization and the regulation of vascular tone in health and disease

1. The elusive nature of endothelium-derived hyperpolarizing factor (EDHF) has hampered detailed study of the ionic mechanisms that underlie the EDHF hyperpolarization and relaxation. Most studies have relied on a pharmacological approach in which interpretations of results can be confounded by limited specificity of action of the drugs used. Nevertheless, small-, intermediate- and large-conductance Ca2+-activated K+ channels (SKCa, IKCa and BKCa, respectively) have been implicated, with inward rectifier K+ channels (KIR) and Na+/K+-ATPase also suggested by some studies. 2. Endothelium-dependent membrane currents recorded using single-electrode voltage-clamp from electrically short lengths of arterioles in which the smooth muscle and endothelial cells remained in their normal functional relationship have provided useful insights into the mechanisms mediating EDHF. Charybdotoxin (ChTx) or apamin reduced, whereas apamin plus ChTx abolished, the EDHF current. The ChTx- and apamin-sensitive currents both reversed near the expected K+ equilibrium potential, were weakly outwardly rectifying and displayed little, if any, time- or voltage-dependent gating, thus having the biophysical and pharmacological characteristics of IKCa and SKCa channels, respectively. 3. The IKCa and SKCa channels occur in abundance in endothelial cells and their activation results in EDHF-like hyperpolarization of these cells. There is little evidence for a significant number of these channels in healthy, contractile vascular smooth muscle cells. 4. In a number of blood vessels in which EDHF occurs, the endothelial and smooth muscle cells are coupled electrically via myoendothelial gap junctions. In contrast, in the adult rat femoral artery, in which the smooth muscle and endothelial layers are not coupled electrically, EDHF does not occur, even though acetylcholine evokes hyperpolarization in the endothelial cells. 5. In vivo studies indicate that EDHF contributes little to basal conductance of the vasculature, but it contributes appreciably to evoked increases in conductance. 6. Endothelium-derived hyperpolarizing factor responses are diminished in some diseases, including hypertension, pre-eclampsia and some models of diabetes. 7. The most economical explanation for EDHF in vitro and in vivo in small vessels is that it arises from the activation of IKCa and SKCa channels in endothelial cells. The resulting endothelial hyperpolarization spreads via myoendothelial gap junctions to result in the EDHF-attributed hyperpolarization and relaxation of the smooth muscle.

Protective effect of N-acetylcysteine against oxygen radical-mediated coronary artery injury

The present study investigated the protective effect of N-acetylcysteine (NAC) against oxygen radical-mediated coronary artery injury. Vascular contraction and relaxation were determined in canine coronary arteries immersed in Kreb's solution (95% O2-5% CO2), incubated or not with NAC (10 mM), and exposed to free radicals (FR) generated by xanthine oxidase (100 mU/ml) plus xanthine (0.1 mM). Rings not exposed to FR or NAC were used as controls. The arteries were contracted with 2.5 microM prostaglandin F2alpha. Subsequently, concentration-response curves for acetylcholine, calcium ionophore and sodium fluoride were obtained in the presence of 20 microM indomethacin. Concentration-response curves for bradykinin, calcium ionophore, sodium nitroprusside, and pinacidil were obtained in the presence of indomethacin plus Nomega-nitro-L-arginine (0.2 mM). The oxidative stress reduced the vascular contraction of arteries not exposed to NAC (3.93 +/- 3.42 g), compared to control (8.56 +/- 3.16 g) and to NAC group (9.07 +/- 4.0 g). Additionally, in arteries not exposed to NAC the endothelium-dependent nitric oxide (NO)-dependent relaxation promoted by acetylcholine (1 nM to 10 microM) was also reduced (maximal relaxation of 52.1 +/- 43.2%), compared to control (100%) and NAC group (97.0 +/- 4.3%), as well as the NO/cyclooxygenase-independent receptor-dependent relaxation provoked by bradykinin (1 nM to 10 microM; maximal relaxation of 20.0 +/- 21.2%), compared to control (100%) and NAC group (70.8 +/- 20.0%). The endothelium-independent relaxation elicited by sodium nitroprusside (1 nM to 1 microM) and pinacidil (1 nM to 10 microM) was not affected. In conclusion, the vascular dysfunction caused by the oxidative stress, expressed as reduction of the endothelium-dependent relaxation and of the vascular smooth muscle contraction, was prevented by NAC.

Hyperosmolar solution effects in guinea pig airways. III. Studies on the identity of epithelium-derived relaxing factor in isolated perfused trachea using pharmacological agents

Hyperosmolar challenge of airway epithelium stimulates the release of epithelium-derived relaxing factor (EpDRF), but the identity of EpDRF is not known. We examined the effects of pharmacological agents on relaxant responses of methacholine (3 x 10(-7) M)-contracted guinea pig perfused trachea to mucosal hyperosmolar challenge using D-mannitol. Responses were inhibited by gossypol (5 x 10(-6) M), an agent with diverse actions, by the carbon monoxide (CO) scavenger hemoglobin (10(-6) M), and by the heme oxygenase (HO) inhibitor zinc (II) protoporphyrin IX (10(-4) M). The HO inhibitor chromium (III) mesoporphyrin IX (10(-4) M) was not inhibitory, and the HO activator heme-L-lysinate (3 x 10(-4) M) did not evoke relaxant responses. The CO donor tricarbonyldichlororuthenium (II) dimer (2.2 x 10(-4) M) elicited small relaxation responses. Other agents without an effect on responses included: apyrase, adenosine, 6-anilino-5,8-quinolinequinone (LY83583), proadifen, (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK 571), diphenhydramine, glibenclamide, HgCl2, tetrodotoxin, nystatin, alpha-hemolysin, 8-bromoguanosine 3',5'-cyclic monophosphothioate, Rp-isomer, 12-O-tetradecanoylphorbol-13-acetate, cholera toxin, pertussis toxin, thapsigargin, nifedipine, Ca(2+)-free mucosal solution, hydrocortisone, and epidermal growth factor. Cytoskeleton inhibitors, includingerythro-9-(2-hydroxyl-3-nonyl)adenine, colchicine, nocodazole, latrunculin B, and cytochalasins B and D, had no effect on relaxation responses. The results suggest provisionally that a portion of EpDRF activity may be due to CO and that the release of EpDRF does not involve cytoskeletal reorganization.

Mediation of arachidonic acid metabolite(s) produced by endothelial cytochrome P-450 3A4 in monkey arterial relaxation

We investigated mechanisms of endothelium-dependent relaxation by acetylcholine resistant to indomethacin and N(G)-nitro-L-arginine and sensitive to cytochrome P-450 (CYP) inhibitors or charybdotoxin + apamin in the monkey lingual artery. Treatment with quinacrine, an inhibitor of phospholipase A2, abolished the relaxation by acetylcholine. However, treatment with alpha-glycyrrhetinic acid, an inhibitor of gap junctions, or catalase, an enzyme which dismutates hydrogen peroxide to form water and oxygen, did not affect the relaxation by acetylcholine. Immunohistochemistry demonstrated the presence of CYP3A4 in endothelial cells of the artery. Anti-CYP3A4 antibody inhibited relaxations by products of arachidonic acid incubated with human liver microsomes rich in CYPs in the endothelium-denuded artery. Purified CYP3A4 produced epoxyeicosatrienoic acids (EETs) from arachidonic acid, and the production was abolished by a selective CYP3A inhibitor, ketoconazole. It may be concluded that endothelium-derived relaxing substance(s) other than nitric oxide and prostanoids in the monkey lingual artery opens charybdotoxin + apamin-sensitive K+ channels in smooth muscle cells, and arachidonic acid metabolite(s) produced by endothelial CYP3A4 is likely to be the major substance.

Inhibition of EDHF by two new combinations of K+-channel inhibitors in rat isolated mesenteric arteries

It is widely established that in rat mesenteric arteries, endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation evoked by acetylcholine is abolished by a combination of charybdotoxin plus apamin. 4-Aminopyridine, an inhibitor of voltage-gated (Kv) K(+)-channels, in combination with apamin had moderate effects on the EDHF-mediated relaxation. Maurotoxin (MTX), an inhibitor of Kv and intermediate-conductance Ca(2+)-activated K(+)-channels (IK), had no effect on EDHF-mediated relaxation. However, MTX in combination with apamin completely abolished EDHF-mediated relaxation and endothelial cell hyperpolarization. The selective IK inhibitor 2-(2-chlorophenyl)-2,2-diphenyl acetonitrile (TRAM-39) had no significant effect on EDHF-mediated relaxation. EDHF-mediated vasorelaxation and hyperpolarization was abolished by a combination of TRAM-39 and apamin. These data demonstrate two new combinations of K(+)-channel inhibitors for the investigation of EDHF. Furthermore, by using TRAM-39, a potent selective inhibitor of IK channels, we provide the first direct evidence that abolition of EDHF requires the simultaneous presence of intermediate- and small-conductance Ca(2+)-activated K(+)-channel inhibitors.

Mechanisms of bradykinin-mediated dilation in newborn piglet pulmonary conducting and resistance vessels

Bradykinin (BK) is a potent dilator of the perinatal pulmonary circulation. We investigated segmental differences in BK-induced dilation in newborn pig large conducting pulmonary artery and vein rings and in pressurized pulmonary resistance arteries (PRA). In conducting pulmonary arteries and veins, BK-induced relaxation is abolished by endothelial disruption and by inhibition of nitric oxide (NO) synthase with nitro-L-arginine (L-NA). In PRA, two-thirds of the dilation response is L-NA insensitive. Charybdotoxin plus apamin and depolarization with KCl abolish the L-NA-insensitive dilations, findings that implicate the release of endothelium-derived hyperpolarizing factor (EDHF). However, endothelium-disrupted PRA retain the ability to dilate to BK but not to ACh or A-23187. In endothelium-disrupted PRA, dilation was inhibited by charybdotoxin. Thus in PRA, BK elicits dilation by multiple and duplicative signaling pathways. Release of NO and EDHF contributes to the response in endothelium-intact PRA; in endothelium-disrupted PRA, dilation occurs by direct activation of vascular smooth muscle calcium-dependent potassium channels. Redundant signaling pathways mediating pulmonary dilation to BK may be required to assure a smooth transition to extrauterine life.

Role for endothelium-derived hyperpolarizing factor in vascular tone in rat mesenteric and hindlimb circulations in vivo

The role of endothelium-derived hyperpolarizing factor (EDHF) in the regulation of blood flow in vivo was examined in the mesenteric and hindlimb circulations of anaesthetized rats. Basal mesenteric conductance decreased from 57 +/- 5 to 20 +/- 6 microl min(-1) mmHg(-1) when nitric oxide (NO) production was inhibited, and combined blockade of intermediate- and small-conductance Ca(2+)-activated K(+) (K(Ca)) channels with charybdotoxin (ChTx) and apamin had no further effect. Basal hindlimb conductance was reduced from 39 +/- 3 to 22 +/- 2 microl min(-1) mmHg(-1) by NO synthesis inhibition, with no effect of the K(Ca) channel blockers. Endothelial stimulation with acetylcholine (ACh) infusion directly into the mesenteric bed increased conductance by 20 +/- 2 microl min(-1) mmHg(-1). Blockade of NO synthesis decreased this conductance to 15 +/- 1 microl min(-1) mmHg(-1), leaving the response attributable to EDHF. This was reduced to 2 +/- 1 microl min(-1) mmHg(-1) by ChTx plus apamin but not by iberiotoxin, which selectively blocks large-conductance K(Ca) channels. Similar results were obtained when bradykinin (BK) was used to stimulate the endothelium. Nitroprusside, which directly relaxes smooth muscle, evoked an increase in conductance that was resistant to all blockers tested. ACh-induced increases in hindlimb conductance were reduced from 19 +/- 1 to 12 +/- 1 microl min(-1) mmHg(-1) by NO synthesis inhibition and further reduced to 2 +/- 2 microl min(-1) mmHg(-1) by ChTx plus apamin. In contrast to NO, ChTx- and apamin-sensitive EDHF appears to contribute little to basal conductance in rat mesenteric and hindlimb beds. However, EDHF accounts for a significant component of the conductance increase during endothelial stimulation by ACh and BK. In these beds, intermediate- and small-conductance K(Ca) channels underpin EDHF-mediated vasodilatation.

Myoendothelial electrical coupling in arteries and arterioles and its implications for endothelium-derived hyperpolarizing factor

1. Considerable progress has been made over the past decade in evaluating the presence of electrical coupling between the endothelial and smooth muscle layers of blood vessels, prompted, in part, by ultrastructural evidence for the presence of myoendothelial junctions. 2. In a variety of vessels ranging in size from conduit arteries down to small arterioles, action potentials have been recorded from endothelial cells that were associated with constriction of the vessels and/or occurred in synchrony with and were indistinguishable from action potentials recorded from the smooth muscle. From these results, it is now firmly established that myoendothelial electrical coupling occurs in at least some blood vessels. 3. Spread of hyperpolarizing current from the endothelium to the smooth muscle is the most likely explanation of the smooth muscle hyperpolarization attributed to endothelium-derived hyperpolarizing factor. Because this hyperpolarization can evoke considerable relaxation of the smooth muscle, myoendothelial electrical coupling has important implications for endothelial regulation of the contractile activity of blood vessels.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

Cellular target of voltage and calcium-dependent K(+) channel blockers involved in EDHF-mediated responses in rat superior mesenteric artery

1. We have investigated the cellular target of K(+) channel blockers responsible for the inhibition of the EDHF-mediated relaxation in the rat mesenteric artery by studying their effects on tension, smooth muscle cell (SMC) membrane potential and endothelial cell Ca(2+) signal ([Ca(2+)](endo)). 2. In arteries contracted with prostaglandin F(2 alpha) (2.5 - 10 microM), relaxation evoked by ACh (0.01 - 3 microM) was abolished by a combination of charybdotoxin (ChTX, 0.1 microM) plus apamin (Apa, 0.1 microM) and was inhibited by 68+/-6% (n=6) by 4-aminopyridine (4-AP, 5 mM). 3. ACh(0.001 - 3 microM) increased [Ca(2+)](endo) and hyperpolarized SMCs with the same potency, the pD(2) values were equal to 7.2+/-0.08 (n=4) and 7.2+/-0.07 (n=9), respectively. SMCs hyperpolarization to ACh (1 microM) was abolished by high K(+) solution or by ChTX/Apa. It was decreased by 66+/-5% (n=6) by 4-AP. 4. The increase in [Ca(2+)](endo) evoked by ACh (1 microM) was insensitive to ChTX/Apa but was depressed by 58+/-16% (n=6) and 27+/-4% (n=7) by raising external K(+) concentration and by 4-AP, respectively. 5. The effect of 4-AP on [Ca(2+)](endo) was not affected by increasing external K(+) concentration. In Ca-free/EGTA solution, the transient increase in [Ca(2+)](endo) evoked by ACh (1 microM) was abolished by thapsigargin (1 microM) and was decreased by 75+/-7% (n=5) by 4-AP. 6. These results show that inhibition of EDHF-evoked responses by 4-AP may be attributed to a decrease in the Ca(2+) release activated by ACh in endothelial cells. The abolition of SMCs hyperpolarization to ACh by ChTX/Apa is not related to an interaction with the [Ca(2+)](endo).

Further investigations into the endothelium-dependent hyperpolarizing effects of bradykinin and substance P in porcine coronary artery

In porcine coronary arteries, smooth muscle hyperpolarizations produced by the nitric oxide donor, NOR-1, and the prostacyclin analogue, iloprost, were compared with those induced by substance P and bradykinin and attributed to the endothelium-derived hyperpolarizing factor (EDHF). In the presence of 300 microM L-nitroarginine and 10 microM indomethacin, iloprost-induced hyperpolarizations were partially inhibited by 10 microM glibenclamide whereas those to NOR-1, substance P and bradykinin were unaffected. Hyperpolarizations produced by maximally-effective concentrations of NOR-1 and NS1619 were identical (to -65 mV). They were significantly less than those generated by either substance P or bradykinin (to approximately -80 mV) and were abolished by iberiotoxin 100 nM, a concentration which had essentially no effect on responses to substance P or bradykinin. Incubation of segments of intact arteries for 16 - 22 h in bicarbonate-buffered Krebs solution had little effect on EDHF responses to substance P or bradykinin. In contrast, after incubation for this period of time in HEPES-buffered Tyrode solution or Krebs containing 10 mM HEPES the EDHF response to substance P was abolished and that to bradykinin was markedly reduced. The residual bradykinin-induced hyperpolarization following incubation in Tyrode solution was inhibited by iberiotoxin and by 10 microM 17-octadecynoic acid. We conclude that substance P activates only the EDHF pathway in the presence of nitric oxide synthase and cyclo-oxygenase inhibitors. Incubation in HEPES-buffered Tyrode solution abolishes the EDHF responses to substance P and bradykinin to reveal an additional hyperpolarizing mechanism, associated with the opening of K(+) channels, activated only by bradykinin.

EDHF is not K+ but may be due to spread of current from the endothelium in guinea pig arterioles

Endothelium-derived hyperpolarizing factor (EDHF)-attributed hyperpolarizations and relaxations were recorded simultaneously from submucosal arterioles of guinea pigs with the use of intracellular microelectrodes and a video-based system, respectively. Membrane currents were recorded from electrically short segments of arterioles under single-electrode voltage clamp. Substance P evoked an outward current with a current-voltage relationship that was well described by the Goldman-Hodgkin-Katz equation for a K+ current, consistent with the involvement of intermediate- and small-conductance Ca2+-activated K+ channels. 1-Ethyl-2-benzimidazolinone relaxed the arterioles and evoked hyperpolarizations that were blocked by charybdotoxin, but not by iberiotoxin. Application of K+induced depolarization under conditions in which EDHF evoked hyperpolarization. The Ba2+-sensitive component of the K+-induced current was inwardly rectifying, in contrast to the outwardly rectifying current evoked by substance P. EDHF-attributed hyperpolarizations in dye-identified smooth muscle cells were indistinguishable from those recorded from dye-identified endothelial cells in the same arterioles. These results provide evidence that EDHF is not K+ but may involve electrotonic spread of hyperpolarization from the endothelial cells to the smooth muscle cells.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

EETs relax airway smooth muscle via an EpDHF effect: BK(Ca) channel activation and hyperpolarization

Epoxyeicosatrienoic acids (EETs) are produced from arachidonic acid via the cytochrome P-450 epoxygenase pathway. EETs are able to modulate smooth muscle tone by increasing K(+) conductance, hence generating hyperpolarization of the tissues. However, the molecular mechanisms by which EETs induce smooth muscle relaxation are not fully understood. In the present study, the effects of EETs on airway smooth muscle (ASM) were investigated using three electrophysiological techniques. 8,9-EET and 14,15-EET induced concentration-dependent relaxations of the ASM precontracted with a muscarinc agonist (carbamylcholine chloride), and these relaxations were partly inhibited by 10 nM iberiotoxin (IbTX), a specific large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel blocker. Moreover, 3 microM 8,9- or 14,15-EET induced hyperpolarizations of -12 +/- 3.5 and -16 +/- 3 mV, with EC(50) values of 0.13 and 0.14 microM, respectively, which were either reversed or blocked on addition of 10 nM IbTX. These results indicate that BK(Ca) channels are involved in hyperpolarization and participate in the relaxation of ASM. In addition, complementary experiments demonstrated that 8,9- and 14,15-EET activate reconstituted BK(Ca) channels at low free Ca(2+) concentrations without affecting their unitary conductance. These increases in channel activity were IbTX sensitive and correlated well with the IbTX-sensitive hyperpolarization and relaxation of ASM. Together these results support the view that, in ASM, the EETs act through an epithelium-derived hyperpolarizing factorlike effect.

Endothelial dysfunction in cirrhosis and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome associated with chronic liver diseases; it is characterized by a pathological increase in portal pressure. Pharmacotherapy for PHT is aimed at reducing both intrahepatic vascular tone and elevated splanchnic blood flow. Due to the altered hemodynamic profile in PHT, dramatic changes in mechanical forces, both pressure and flow, may play a pivotal role in controlling endothelial and vascular smooth muscle cell signaling, structure, and function in cirrhotics. Nitric oxide, prostacyclin, endothelial-derived contracting factors, and endothelial-derived hyperpolarizing factor are powerful vasoactive substances released from the endothelium in response to both humoral and mechanical stimuli that can profoundly affect both the function and structure of the underlying vascular smooth muscle. This review will examine the contributory role of hormonal- and mechanical force-induced changes in endothelial function and signaling and the consequence of these changes on the structural and functional response of the underlying vascular smooth muscle. It will focus on the pivotal role of hormonal and mechanical force-induced endothelial release of vasoactive substances in dictating the reactivity of the underlying vascular smooth muscle, i.e., whether hyporeactive or hyperreactive, and will examine the extent to which these substances may exert a protective and/or detrimental influence on the structure of the underlying vascular smooth muscle in both a normal hemodynamic environment and following hemodynamic perturbations typical of PHT and cirrhosis. Finally, it will discuss the intracellular processes that regulate the release/expression of these vasoactive substances and that control the transformation of this normally protective cell to one that may promote the development of vasculopathy in PHT.

Bradykinin attenuates the [Ca(2+)](i) response to angiotensin II of renal juxtamedullary efferent arterioles via an EDHF

1. Bradykinin (BK) effect on the [Ca(2+)](i) response to 1 nM angiotensin II was examined in muscular juxtamedullary efferent arterioles (EA) of rat kidney. 2. BK (10 nM) applied during the angiotensin II-stimulated [Ca(2+)](i) increase, induced a [Ca(2+)](i) drop (73+/-2%). This drop was prevented by de-endothelialization and suppressed by HOE 140, a B2 receptor antagonist. It was neither affected by L-NAME or indomethacin, nor mimicked by sodium nitroprusside, 8-bromo-cyclic GMP or PGI(2). The BK effect did not occur when the [Ca(2+)](i) increase was caused by 100 mM KCl-induced membrane depolarization and was abolished by 0.1 microM charybdotoxin, a K(+) channel blocker. 3. Although proadifen prevented the BK-caused [Ca(2+)](i) fall, more selective cytochrome P450 inhibitors, 17-octadecynoic acid (50 microM) and 7-ethoxyresorufin (10 microM) were without effect. 4. Increasing extracellular potassium from 5 to 15 mM during angiotensin II stimulation caused a [Ca(2+)](i) decrease (26+/-4%) smaller than BK which was charybdotoxin-insensitive. Inhibition of inward rectifying K(+) channels by 30 microM BaCl(2) and/or of Na(+)/K(+) ATPase by 1 mM ouabain abolished the [Ca(2+)](i) decrease elicited by potassium but not by BK. 5. A voltage-operated calcium channel blocker, nifedipine (1 microM) did not prevent the BK effect but reduced the [Ca(2+)](i) drop. 6. These results indicate that the BK-induced [Ca(2+)](i) decrease in angiotensin II-stimulated muscular EA is mediated by an EDHF which activates charybdotoxin-sensitive K(+) channels. In these vessels, EDHF seems to be neither a cytochrome P450-derived arachidonic acid metabolite nor K(+) itself. The closure of voltage-operated calcium channels is not the only cellular mechanism involved in this EDHF-mediated [Ca(2+)](i) decrease.

Novel endothelium-derived relaxing factors. Identification of factors and cellular targets

Nitric oxide (NO), together with prostacyclin (PGI2), mediates shear stress and endothelium-dependent vasodilator-mediated vasorelaxation. In the presence of inhibition of NO synthase (NOS) with nitroarginine analogues, such as of N(w)-nitro-L-arginine methyl ester (L-NAME) and N(w)-nitro-L-arginine (L-NNA), and indomethacin, to inhibit cyclooxygenase (COX) and the synthesis of PGI2, many blood vessels still respond with an endothelium-dependent relaxation to either chemical [i.e. acetylcholine (ACh)] or mechanical (shear stress) activation. This non-NO and non-PGI2 vasorelaxation appears to be mediated by hyperpolarization of the vascular smooth muscle cell (VSMC). Although NO can hyperpolarize VSMC, a novel mediator, the endothelium-derived hyperpolarizing factor (EDHF), which opens a VSMC K(+) channel(s) notably in resistance vessels, has been proposed. Little agreement exists as to the nature of this putative factor, but several candidate molecules have been proposed and evidence, notably from the microcirculation, suggests that endothelium-dependent hyperpolarization (EDH) may be mediated via low electrical resistance coupling via myoendothelial gap junctions. We describe a number of techniques that are being used to identify EDHF and present data that address the contribution of a small increase in extracellular K(+) as an EDHF.

Role of endothelial cell hyperpolarization in EDHF-mediated responses in the guinea-pig carotid artery

1. Experiments were performed to identify the potassium channels involved in the acetylcholine-induced endothelium-dependent hyperpolarization of the guinea-pig internal carotid artery. Smooth muscle and endothelial cell membrane potentials were recorded in isolated arteries with intracellular microelectrodes. Potassium currents were recorded in freshly-dissociated smooth muscle cells using patch clamp techniques. 2. In single myocytes, iberiotoxin (0.1 microM)-, charybdotoxin (0.1 microM)-, apamin (0.5 microM)- and 4-aminopyridine (5 mM)-sensitive potassium currents were identified indicating the presence of large- and small-conductance calcium-sensitive potassium channels (BK(Ca) and SK(Ca)) as well as voltage-dependent potassium channels (K(V)). Charybdotoxin and iberiotoxin inhibited the same population of BK(Ca) but a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected. 4-aminopyridine (0. 1 - 25 mM) induced a concentration-dependent inhibition of K(V) without affecting the iberiotoxin- or the apamin-sensitive currents. 3. In isolated arteries, both the endothelium-dependent hyperpolarization of smooth muscle and the hyperpolarization of endothelial cells induced by acetylcholine or by substance P were inhibited by 5 mM 4-aminopyridine. 4. These results indicate that in the vascular smooth muscle cells of the guinea-pig carotid artery, a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected, comforting the hypothesis that the combination of these two toxins should act on the endothelial cells. Furthermore, the inhibition by 4-aminopyridine of both smooth muscle and endothelial hyperpolarizations, suggests that in order to observe an endothelium-dependent hyperpolarization of the vascular smooth muscle cells, the activation of endothelial potassium channels is likely to be required.