Calcium-activated potassium channels in native endothelial cells from rabbit aorta: conductance, Ca2+ sensitivity and block

Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca²⁺-activated K⁺ (K_Ca) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. K_Ca channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by K_Ca channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of K_Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.

KATP channels in lymphatic function

KATP channels function as negative regulators of active lymphatic pumping and lymph transport. This review summarizes and critiques the evidence for the expression of specific KATP channel subunits in lymphatic smooth muscle and endothelium, the roles that they play in normal lymphatic function, and their possible involvement in multiple diseases, including metabolic syndrome, lymphedema, and Cantú syndrome. For each of these topics, suggestions are made for directions for future research.

Inhibition of BKCa negatively alters cardiovascular function

Large conductance calcium and voltage-activated potassium channels (BKCa ) are transmembrane proteins, ubiquitously expressed in the majority of organs, and play an active role in regulating cellular physiology. In the heart, BKCa channels are known to play a role in regulating the heart rate and protect it from ischemia-reperfusion injury. In vascular smooth muscle cells, the opening of BKCa channels results in membrane hyperpolarization which eventually results in vasodilation mediated by a reduction in Ca2+ influx due to the closure of voltage-dependent Ca2+ channels. Ex vivo studies have shown that BKCa channels play an active role in the regulation of the function of the majority of blood vessels. However, in vivo role of BKCa channels in cardiovascular function is not completely deciphered. Here, we have evaluated the rapid in vivo role of BKCa channels in regulating the cardiovascular function by using two well-established, rapid-acting, potent blockers, paxilline and iberiotoxin. Our results show that BKCa channels are actively involved in regulating the heart rate, the function of the left and right heart as well as major vessels. We also found that the effect on BKCa channels by blockers is completely reversible, and hence, BKCa channels can be exploited as potential targets for clinical applications for modulating heart rate and cardiac contractility.

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

KEY POINTS

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

ABSTRACT

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

Hydrogen sulfide-induced vasodilation mediated by endothelial TRPV4 channels

Hydrogen sulfide (H₂S) is a recently described gaseous vasodilator produced within the vasculature by the enzymes cystathionine γ-lyase and 3-mercaptopyruvate sulfurtransferase. Previous data demonstrate that endothelial cells (EC) are the source of endogenous H₂S production and are required for H₂S-induced dilation. However, the signal transduction pathway activated by H₂S within EC has not been elucidated. TRPV4 and large-conductance Ca²⁺-activated K channels (BK channels) are expressed in EC. H₂S-induced dilation is inhibited by luminal administration of iberiotoxin and disruption of the endothelium. Calcium influx through TRPV4 may activate these endothelial BK channels (eBK). We hypothesized that H₂S-mediated vasodilation involves activation of TRPV4 within the endothelium. In pressurized, phenylephrine-constricted mesenteric arteries, H₂S elicited a dose-dependent vasodilation blocked by inhibition of TRPV4 channels (GSK2193874A, 300 nM). H₂S (1 μM) increased TRPV4-dependent (1.8-fold) localized calcium events in EC of pressurized arteries loaded with fluo-4 and Oregon Green. In pressurized EC tubes, H₂S (1 μM) and the TRPV4 activator, GSK101679A (30 nM), increased calcium events 1.8- and 1.5-fold, respectively. H₂S-induced an iberiotoxin-sensitive outward current measured using whole cell patch-clamp techniques in freshly dispersed EC. H₂S increased K⁺ currents from 10 to 30 pA/pF at +150 mV. Treatment with Na₂S increased the level of sulfhydration of TRPV4 channels in aortic ECs. These results demonstrate that H₂S-mediated vasodilation involves activation of TRPV4-dependent Ca²⁺ influx and BK channel activation within EC. Activation of TRPV4 channels appears to cause calcium events that result in the opening of eBK channels, endothelial hyperpolarization, and subsequent vasodilation.

Inward Rectifier K+ Currents Are Regulated by CaMKII in Endothelial Cells of Primarily Cultured Bovine Pulmonary Arteries

Endothelium lines the interior surface of vascular walls and regulates vascular tones. The endothelial cells sense and respond to chemical and mechanical stimuli in the circulation, and couple the stimulus signals to vascular smooth muscles, in which inward rectifier K⁺ currents (Kir) play an important role. Here we applied several complementary strategies to determine the Kir subunit in primarily cultured pulmonary arterial endothelial cells (PAECs) that was regulated by the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII). In whole-cell voltage clamp, the Kir currents were sensitive to micromolar concentrations of extracellular Ba²⁺. In excised inside-out patches, an inward rectifier K⁺ current was observed with single-channel conductance 32.43 ± 0.45 pS and P_open 0.27 ± 0.04, which were consistent with known unitary conductance of Kir 2.1. RT-PCR and western blot results showed that expression of Kir 2.1 was significantly stronger than that of other subtypes in PAECs. Pharmacological analysis of the Kir currents demonstrated that insensitivity to intracellular ATP, pinacidil, glibenclamide, pH, GDP-β-S and choleratoxin suggested that currents weren’t determined by K_ATP, Kir2.3, Kir2.4 and Kir3.x. The currents were strongly suppressed by exposure to CaMKII inhibitor W-7 and KN-62. The expression of Kir2.1 was inhibited by knocking down CaMKII. Consistently, vasodilation was suppressed by Ba²⁺, W-7 and KN-62 in isolated and perfused pulmonary arterial rings. These results suggest that the PAECs express an inward rectifier K⁺ current that is carried dominantly by Kir2.1, and this K⁺ channel appears to be targeted by CaMKII-dependent intracellular signaling systems.

Inactivation of Endothelial Small/Intermediate Conductance of Calcium-Activated Potassium Channels Contributes to Coronary Arteriolar Dysfunction in Diabetic Patients

Background

Diabetes is associated with coronary arteriolar endothelial dysfunction. We investigated the role of the small/intermediate (SK_Ca/IK_Ca) conductance of calcium-activated potassium channels in diabetes-related endothelial dysfunction.

Methods and Results

Coronary arterioles (80 to 150 μm in diameter) were dissected from discarded right atrial tissues of diabetic (glycosylated hemoglobin = 9.6±0.25) and nondiabetic patients (glycosylated hemoglobin 5.4±0.12) during coronary artery bypass graft surgery (n=8/group). In-vitro relaxation response of precontracted arterioles was examined in the presence of the selective SK_Ca/IK_Ca activator NS309 and other vasodilatory agents. The channel density and membrane potential of diabetic and nondiabetic endothelial cells was measured by using the whole cell patch-clamp technique. The protein expression and distribution of the SK_Ca/IK_Ca in the human myocardium and coronary arterioles was examined by Western blotting and immunohistochemistry. Our results indicate that diabetes significantly reduced the coronary arteriolar response to the SK_Ca/IK_Ca activator NS309 compared to the respective responses of nondiabetic vessels (P<0.05 versus nondiabetes). The relaxation response of diabetic arterioles to NS309 was prevented by denudation of endothelium (P=0.001 versus endothelium-intact). Diabetes significantly decreased endothelial SK_Ca/IK_Ca currents and hyperpolarization induced by the SK_Ca/IK_Ca activator NS309 as compared with that of nondiabetics. There were no significant differences in the expression and distribution of SK_Ca/IK_Ca proteins in the coronary microvessels.

Conclusions

Diabetes is associated with inactivation of endothelial SK_Ca/IK_Ca channels, which may contribute to endothelial dysfunction in diabetic patients.

Large conductance Ca(2+)-activated K(+) channel (BKCa) activating properties of a series of novel N-arylbenzamides: Channel subunit dependent effects

Large conductance calcium activated potassium channels (BKCa) are fundamental in the control of cellular excitability. Thus, compounds that activate BKCa channels could provide potential therapies in the treatment of pathologies of the cardiovascular and central nervous system. A series of novel N-arylbenzamide compounds, and the reference compound NS1619, were evaluated for BKCa channel opener properties in Human Embryonic Kidney (HEK293) cells expressing the human BKCa channel α-subunit alone or α+β1-subunit complex. Channel activity was determined using a non-radioactive Rb(+) efflux assay to construct concentration effect curves for each compound. All N-arylbenzamide compounds and NS1619 evoked significant (p <0.05) concentration related increases in Rb(+) efflux both in cells expressing α-subunit alone or α+β1-subunits. Co-expression of the β1-subunit modified the Rb(+) efflux responses, relative to that obtained in cells expressing the α-subunit alone, for most of the N-arylbenzamide compounds, in contrast to NS1619. The EC40 values of NS1619, BKMe1 and BKOEt1 were not significantly affected by the co-expression of the BKCa channel α+β1-subunits. In contrast, 5 other N-arylbenzamides (BKPr2, BKPr3, BKPr4, BKH1 and BKVV) showed a significant (p <0.05) 2- to 10-fold increase in EC40 values when tested on the BKCa α+β1-subunit expressing cells compared to BKCa α-subunit expressing cells. Further, the Emax values for BKPr4, BKVV and BKH1 were lower in the BKCa channel α+β1-subunit expressing cells. In conclusion, the N-arylbenzamides studied, like NS1619, were able to activate BKCa channels formed of the α-subunit only. The co-expression of the β1-subunit, however, modified the ability of certain compounds to active the channel leading to differentiated pharmacodynamic profiles.

Large-conductance Ca²⁺-activated potassium channel in mitochondria of endothelial EA.hy926 cells

In the present study, we describe the existence of a large-conductance Ca²⁺-activated potassium (BKCa) channel in the mitochondria of the human endothelial cell line EA.hy926. A single-channel current was recorded from endothelial mitoplasts (i.e., inner mitochondrial membrane) using the patch-clamp technique in the mitoplast-attached mode. A potassium-selective current was recorded with a mean conductance equal to 270 ± 10 pS in a symmetrical 150/150 mM KCl isotonic solution. The channel activity, which was determined as the open probability, increased with the addition of calcium ions and the potassium channel opener NS1619. Conversely, the activity of the channel was irreversibly blocked by paxilline and iberiotoxin, BKCa channel inhibitors. The open-state probability was found to be voltage dependent. The substances known to modulate BKCa channel activity influenced the bioenergetics of mitochondria isolated from human endothelial EA.hy926 cells. In isolated mitochondria, 100 μM Ca²⁺, 10 μM NS1619, and 0.5 μM NS11021 depolarized the mitochondrial membrane potential and stimulated nonphosphorylating respiration. These effects were blocked by iberiotoxin and paxilline in a potassium-dependent manner. Under phosphorylating conditions, NS1619-induced, iberiotoxin-sensitive uncoupling diverted energy from ATP synthesis during the phosphorylating respiration of the endothelial mitochondria. Immunological analysis with antibodies raised against proteins of the plasma membrane BKCa channel identified a pore-forming α-subunit and an auxiliary β₂-subunit of the channel in the endothelial mitochondrial inner membrane. In conclusion, we show for the first time that the inner mitochondrial membrane in human endothelial EA.hy926 cells contains a large-conductance calcium-dependent potassium channel with properties similar to those of the surface membrane BKCa channel.

BK Channels in Cardiovascular Diseases and Aging

Aging is a major risk factor for cardiovascular diseases, one of the main world-wide causes of death. Several structural and functional changes occur in the cardiovascular system during the aging process and the mechanisms involved in such alterations are yet to be completely described. BK channels are transmembrane proteins that play a key role in many physiological processes, including regulation of vascular tone. In vascular smooth muscle cells, BK opening and the consequent efflux of potassium (K(+)) leads to membrane hyperpolarization, which is followed by the closure of voltage-dependent Ca(2+) channels, reduction of Ca(2+) entry and vasodilatation. BK regulates nitric oxide-mediated vasodilatation and thus is crucial for normal endothelial function. Herein we will briefly review general structural properties of BK and focus on their function in the cardiovascular system emphasizing their role in cardiovascular aging and diseases.

Calcium-activated potassium channels in vasculature in response to ischemia-reperfusion

Based on the genetic relationship, single-channel conductance, and gating mechanisms, calcium-activated potassium (KCa) channels identified in vasculature can be divided into 3 groups including large-conductance KCa, small, and intermediate conductance KCa. KCa channels in smooth muscle and endothelial cells are essential for the regulation of vascular tone. Vascular dysfunction under ischemia-reperfusion (I-R) or hypoxia-reoxygenation (H-R) conditions is associated with modulations of KCa channels that are attributable to multiple mechanisms. Most studies in this regard relied on the change of relaxation components sensitive to certain channel blockers to indicate the alteration of KCa channels under I-R conditions, which however provided conflicting results for the effect of I-R. The possible mechanisms involved in KCa channel modulation under I-R/H-R include overproduction of reactive oxygen species such as superoxide anion, hydrogen peroxide, and peroxynitrite, increase of intracellular H ion, and lactate accumulation, etc. However, more studies are necessary to further understand the discrepancies in the sensitivity of KCa channels to I-R injury in different vascular beds.

Role of calcium-activated potassium channels in the genesis of 3,4-diaminopyridine-induced periodic contractions in isolated canine coronary artery smooth muscles

We found that 3,4-diaminopyridine (3,4-DAP), a voltage-gated potassium channel (K(V)) inhibitor, elicits pH-sensitive periodic contractions (PCs) of coronary smooth muscles. Underlying mechanisms of PCs, however, remained to be elucidated. The present study was performed to examine the roles of ion channels in the genesis of PCs. To determine the electromechanical changes of smooth muscles, isolated coronary arterial rings from beagles were suspended in organ chambers filled with Krebs-Henseleit solution, and 10(-2) M 3,4-DAP was added to elicit PCs. 3,4-DAP caused periodic spike-and-plateau depolarization accompanied by contraction. PCs were not produced when the CaCl(2) concentration in the chamber was ≤ 0.3 × 10(-3) or ≥ 10(-2) M. PCs were eliminated by a CaCl(2) concentration ≥ 5 × 10(-3) M or by lowering pH below 7.20 with HCl and recovered by the addition of iberiotoxin or charybdotoxin, which inhibit large-conductance calcium-activated potassium channels (K(Ca)), or by elevating pH above 7.35 with NaOH. PCs, as well as the spike-and-plateau depolarization, were eliminated by nifedipine, which inhibits L-type voltage-gated calcium channels (Ca(V)). Influx of Ca(2+) through L-type Ca(V), which was opened because closing of K(Ca), secondary to 3,4-DAP-induced closing of K(V), resulted in contraction; the intracellular Ca(2+) increased by this influx opened K(Ca), leading to closure of Ca(V) and consequent cessation of Ca(2+) influx with resultant relaxation. These processes were repeated spontaneously to cause PCs. H(+) and OH(-) were considered to act as the opener and closer of K(Ca), respectively.

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.

Pharmacology of mitochondrial potassium channels: dark side of the field

Mitochondrial potassium channels play an important role in cytoprotection. Potassium channels in the inner mitochondrial membrane are modulated by inhibitors and activators (potassium channel openers) previously described for plasma membrane potassium channels. The majority of mitochondrial potassium channel modulators exhibit a broad spectrum of off-target effects. These include uncoupling properties, inhibition of the respiratory chain and effects on cellular calcium homeostasis. Therefore, the rational application of channel inhibitors or activators is crucial to understanding the cellular consequences of mitochondrial channel inhibition or activation. Moreover, understanding their side-effects should facilitate the design of a specific mitochondrial channel opener with cytoprotective properties. In this review, we discuss the complex interactions of potassium channel inhibitors and activators with cellular structures.

Limits of isolation and culture: intact vascular endothelium and BKCa

The potential physiological role of plasmalemmal large-conductance calcium-activated potassium channels (BKCa) in vascular endothelial cells is controversial. Studies of freshly isolated and cultured vascular endothelial cells provide disparate results, both supporting and refuting a role for BKCa in endothelial function. Most studies using freshly isolated, intact, healthy arteries provide little support for a physiological role for BKCa in the endothelium, although recent work suggests that this may not be the case in diseased vessels. In isolated and cultured vascular endothelial cells, the autocrine action of growth factors, hormones, and vasoactive substances results in phenotypic drift. Such an induced heterogeneity is likely a primary factor accounting for the apparent differences, and often enhanced BKCa expression and function, in isolated and cultured vascular endothelial cells. In a similar manner, heterogeneity in endothelial BKCa expression and function in intact arteries may be representative of normal and disease states, BKCa being absent in normal intact artery endothelium and upregulated in disease where dysfunction induces signals that alter channel expression and function. Indeed, in some intact vessels, there is evidence for the presence of BKCa, such as mRNA and/or specific BK subunits, an observation that is consistent with the potential for rapid upregulation, as may occur in disease. This perspective proposes that the disparity in the results obtained for BKCa expression and function from freshly isolated and cultured vascular endothelial cells is largely due to variability in experimental conditions and, furthermore, that the expression of BKCa in intact artery endothelium is primarily associated with disease. Although answers to physiologically relevant questions may only be available in atypical physiological conditions, such as those of isolation and culture, the limitations of these methods require open and objective recognition.

Role of arachidonic acid lipoxygenase metabolites in the regulation of vascular tone

Stimulation of vascular endothelial cells with agonists such as acetylcholine (ACh) or bradykinin or with shear stress activates phospholipases and releases arachidonic acid (AA). AA is metabolized by cyclooxygenases, cytochrome P-450s, and lipoxygenases (LOs) to vasoactive products. In some arteries, a substantial component of the vasodilator response is dependent on LO metabolites of AA. Nitric oxide (NO)- and prostaglandin (PG)-independent vasodilatory responses to ACh and AA are reduced by inhibitors of LO and by antisense oligonucleotides specifically against 15-LO-1. Vasoactive 15-LO metabolites derived from the vascular endothelium include 15-hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-HEETA) that is hydrolyzed by soluble epoxide hydrolase to 11,12,15-trihydroxyeicosatrienoic acid (11,12,15-THETA). HEETA and THETA are endothelium-derived hyperpolarizing factors that induce vascular relaxations by activation of smooth muscle apamin-sensitive, calcium-activated, small-conductance K(+) channels causing hyperpolarization. In other arteries, the 12-LO metabolite 12-hydroxyeicosatetraenoic acid is synthesized by the vascular endothelium and relaxes smooth muscle by large-conductance, calcium-activated K(+) channel activation. Thus formation of vasodilator eicosanoids derived from LO pathways contributes to the regulation of vascular tone, local blood flow, and blood pressure.

Calcium-activated potassium channels and endothelial dysfunction: therapeutic options?

The three subtypes of calcium-activated potassium channels (K(Ca)) of large, intermediate and small conductance (BK(Ca), IK(Ca) and SK(Ca)) are present in the vascular wall. In healthy arteries, BK(Ca) channels are preferentially expressed in vascular smooth muscle cells, while IK(Ca) and SK(Ca) are preferentially located in endothelial cells. The activation of endothelial IK(Ca) and SK(Ca) contributes to nitric oxide (NO) generation and is required to elicit endothelium-dependent hyperpolarizations. In the latter responses, the hyperpolarization of the smooth muscle cells is evoked either via electrical coupling through myo-endothelial gap junctions or by potassium ions, which by accumulating in the intercellular space activate the inwardly rectifying potassium channel Kir2.1 and/or the Na(+)/K(+)-ATPase. Additionally, endothelium-derived factors such as cytochrome P450-derived epoxyeicosatrienoic acids and under some circumstances NO, prostacyclin, lipoxygenase products and hydrogen peroxide (H(2)O(2)) hyperpolarize and relax the underlying smooth muscle cells by activating BK(Ca). In contrast, cytochrome P450-derived 20-hydroxyeicosatetraenoic acid and various endothelium-derived contracting factors inhibit BK(Ca). Aging and cardiovascular diseases are associated with endothelial dysfunctions that can involve a decrease in NO bioavailability, alterations of EDHF-mediated responses and/or enhanced production of endothelium-derived contracting factors. Because potassium channels are involved in these endothelium-dependent responses, activation of endothelial and/or smooth muscle K(Ca) could prevent the occurrence of endothelial dysfunction. Therefore, direct activators of these potassium channels or compounds that regulate their activity or their expression may be of some therapeutic interest. Conversely, blockers of IK(Ca) may prevent restenosis and that of BK(Ca) channels sepsis-dependent hypotension.

Openers of SKCa and IKCa channels enhance agonist-evoked endothelial nitric oxide synthesis and arteriolar vasodilation

Recent data have led us to hypothesize that selective activation of endothelial small- and/or intermediate-conductance, calcium-activated potassium channels (SK(Ca) and IK(Ca) channels, respectively) by the opener compounds NS309 and DCEBIO would augment stimulated nitric oxide (NO) synthesis and vasodilation in resistance arteries. Experimentally, ATP-evoked changes in membrane potential, cytosolic Ca(2+), and NO synthesis were recorded by patch clamp and microfluorimetry in single human endothelial cells. Agonist-evoked inhibition of myogenic tone in isolated, pressurized arterioles from rat cremaster skeletal muscle was analyzed by video microscopy. NS309 and DCEBIO enhanced ATP-evoked membrane hyperpolarization and cytosolic Ca(2+) transients, along with acute NO synthesis in isolated endothelial cells. The acetylcholine-mediated inhibition of myogenic tone (IC(50)=237 nM) was left-shifted in the presence of NS309 and DCEBIO (10, 100, and 1000 nM) to IC(50) values of 101, 78, and 43 nM; endothelial denudation inhibited this drug effect. L-NAME attenuated the acetylcholine-induced inhibition of myogenic tone but did not interfere with NS309 and DCEBIO-evoked vasodilation. Collectively, our data demonstrate that drug-induced enhancement of endothelial SK(Ca) and IK(Ca) channel activities represents a novel cellular mechanism to increase vasodilation of small-resistance arterioles, thereby highlighting these channels as potential therapeutic targets in cardiovascular disease states associated with compromised NO signaling.

Mechanism of C-type natriuretic peptide-induced endothelial cell hyperpolarization

C-type natriuretic peptide (CNP) has a demonstrated hyperpolarizing effect on vascular smooth muscle cells. However, its autocrine function, including its electrophysiological effect on endothelial cells, is not known. Here, we report the effect of CNP on the membrane potential (E(m)) of pulmonary microvascular endothelial cells and describe its target receptors, second messengers, and ion channels. We measured changes in E(m) using fluorescence imaging and perforated patch-clamping techniques. In imaging experiments, samples were preincubated in the potentiometric dye DiBAC(4)(3), and subsequently exposed to CNP in the presence of selective inhibitors of ion channels or second messengers. CNP exposure induced a dose-dependent decrease in fluorescence, indicating that CNP induces endothelial cell hyperpolarization. CNP-induced hyperpolarization was inhibited by the K(+) channel blockers, tetraethylammonium or iberiotoxin, the nonspecific cation channel blocker, La(3+), or by depletion or repletion of extracellular Ca(2+) or K(+), respectively. CNP-induced hyperpolarization was also blocked by pharmacological inhibition of PKG or by small interfering RNA (siRNA)-mediated knockdown of natriuretic peptide receptor-B (NPR-B). CNP-induced hyperpolarization was mimicked by the PKG agonist, 8-bromo-cGMP, and attenuated by both the endothelial nitric oxide synthase (eNOS) inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME), and the soluble guanylyl cyclase (sGC) inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Presence of iberiotoxin-sensitive, CNP-induced outward current was confirmed by perforated patch-clamping experiments. We conclude that CNP hyperpolarizes pulmonary microvascular endothelial cells by activating large-conductance calcium-activated potassium channels mediated by the activation of NPR-B, PKG, eNOS, and sGC.

Hydrogen Peroxide Potentiates the EDHF Phenomenon by Promoting Endothelial Ca 2+ Mobilization

Objective— The purpose of this study was to test the hypothesis that H 2 O 2 contributes to the EDHF phenomenon by mobilizing endothelial Ca 2+ stores.

Methods and Results— Myograph studies with rabbit iliac arteries demonstrated that EDHF-type relaxations evoked by the SERCA inhibitor cyclopiazonic acid (CPA) required activation of K Ca channels and were potentiated by exogenous H 2 O 2 and the thiol oxidant thimerosal. Preincubation with a submaximal concentration of CPA unmasked an ability of exogenous H 2 O 2 to stimulate an EDHF-type response that was sensitive to K Ca channel blockade. Imaging of cytosolic and endoplasmic reticulum [Ca 2+ ] in rabbit aortic valve endothelial cells with Fura-2 and Mag-fluo-4 demonstrated that H 2 O 2 and thimerosal, which sensitizes the InsP 3 receptor, both enhanced CPA-evoked Ca 2+ release from stores, and that the potentiating effect of H 2 O 2 was suppressed by the cell-permeant thiol reductant glutathione monoethylester. CPA-evoked relaxations were attenuated by exogenous catalase and potentiated by the catalase inhibitor 3-aminotriazole, and were abolished by the connexin-mimetic peptide 43 Gap26, which interrupts intercellular communication via gap junctions constructed from connexin 43.

Conclusions— H 2 O 2 can enhance EDHF-type relaxations by potentiating Ca 2+ release from endothelial stores, probably via redox modification of the InsP 3 receptor, leading to the opening of hyperpolarizing endothelial K Ca channels and an electrotonically-mediated relaxant response.

Ca2+-activated K+ channels in murine endothelial cells: block by intracellular calcium and magnesium

The intermediate (IK(Ca)) and small (SK(Ca)) conductance Ca(2+)-sensitive K(+) channels in endothelial cells (ECs) modulate vascular diameter through regulation of EC membrane potential. However, contribution of IK(Ca) and SK(Ca) channels to membrane current and potential in native endothelial cells remains unclear. In freshly isolated endothelial cells from mouse aorta dialyzed with 3 microM free [Ca(2+)](i) and 1 mM free [Mg(2+)](i), membrane currents reversed at the potassium equilibrium potential and exhibited an inward rectification at positive membrane potentials. Blockers of large-conductance, Ca(2+)-sensitive potassium (BK(Ca)) and strong inward rectifier potassium (K(ir)) channels did not affect the membrane current. However, blockers of IK(Ca) channels, charybdotoxin (ChTX), and of SK(Ca) channels, apamin (Ap), significantly reduced the whole-cell current. Although IK(Ca) and SK(Ca) channels are intrinsically voltage independent, ChTX- and Ap-sensitive currents decreased steeply with membrane potential depolarization. Removal of intracellular Mg(2+) significantly increased these currents. Moreover, concomitant reduction of the [Ca(2+)](i) to 1 microM caused an additional increase in ChTX- and Ap-sensitive currents so that the currents exhibited theoretical outward rectification. Block of IK(Ca) and SK(Ca) channels caused a significant endothelial membrane potential depolarization (approximately 11 mV) and decrease in [Ca(2+)](i) in mesenteric arteries in the absence of an agonist. These results indicate that [Ca(2+)](i) can both activate and block IK(Ca) and SK(Ca) channels in endothelial cells, and that these channels regulate the resting membrane potential and intracellular calcium in native endothelium.

Vascular large conductance calcium-activated potassium channels: functional role and therapeutic potential

Large-conductance Ca2+-activated K+ channels (BK Ca or maxiK channels) are expressed in different cell types. They play an essential role in the regulation of various cell functions. In particular, BK Ca channels have been extensively studied in vascular smooth muscle cells, where they contribute to the control of vascular tone. They facilitate the feedback regulation against the rise of intracellular Ca2+, membrane depolarization and vasoconstriction. BK Ca channels promote a K+ outward current and lead to membrane hyperpolarization. In endothelial cells expression and function of BK Ca channels play an important role in the regulation of the vascular smooth muscle activity. Endothelial BK Ca channels modulate the biosyntheses and release of various vasoactive modulators and regulate the membrane potential. Because of their regulatory role in vascular tone, endothelial BK Ca channels have been suggested as therapeutic targets for the treatment of cardiovascular diseases. Hypertension, atherosclerosis, and diabetes are associated with altered current amplitude, open probability, and Ca2+-sensing of BK Ca channels. The properties of BK Ca channels and their role in endothelial and vascular smooth muscle cells would address them as potential therapeutic targets. Further studies are necessary to identify the detailed molecular mechanisms of action and to investigate selective BK Ca channels openers as possible therapeutic agents for clinical use.

Carbon monoxide stimulates the Ca2(+)-activated big conductance k channels in cultured human endothelial cells

We used the whole-cell patch-clamp technique to study K channels in the human umbilical vein endothelial cells and identified a 201 pS K channel, which was blocked by tetraethylammonium and iberiotoxin but not by TRAM34 and apamin. This suggests that the Ca(2+)-activated big-conductance K channel (BK) is expressed in endothelial cells. Application of carbon monoxide (CO) or tricarbonylchloro(glycinato)ruthenium(II), a water soluble CO donor, stimulated the BK channels. Moreover, application of hemin, a substrate of heme oxygenase, mimicked the effect of CO and increased the BK channel activity. The stimulatory effect of hemin was significantly diminished by tin mesoporphyrin, an inhibitor of heme oxygenase. To determine whether the stimulatory effect of CO on the BK channel was mediated by NO and the cGMP-dependent pathway, we examined the effect of CO on BK channels in cells treated with, N(G)-nitro-l-arginine methyl ester, 1H(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one, an inhibitor of soluble guanylate cyclase, or KT5823, an inhibitor of protein kinase G. Addition of either diethylamine NONOate or sodium nitroprusside significantly increased BK channel activity. Inhibition of endogenous NO synthesis with N(G)-nitro-l-arginine methyl ester, blocking soluble guanylate cyclase or protein kinase G, delayed but did not prevent the CO-induced activation of BK channels. Finally, application of an antioxidant agent, ebselen, had no effect on CO-mediated stimulation of BK channels in human umbilical vein endothelial cells. We conclude that BK channels are expressed in human umbilical vein endothelial cells and that they are activated by both CO and NO. CO activates BK channels directly, as well as via a mechanism involving NO or the cGMP-dependent pathway.

Small- and intermediate-conductance Ca2+-activated K+ channels directly control agonist-evoked nitric oxide synthesis in human vascular endothelial cells

The contribution of small-conductance (SK(Ca)) and intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels to the generation of nitric oxide (NO) by Ca(2+)-mobilizing stimuli was investigated in human umbilical vein endothelial cells (HUVECs) by combining single-cell microfluorimetry with perforated patch-clamp recordings to monitor agonist-evoked NO synthesis, cytosolic Ca(2+) transients, and membrane hyperpolarization in real time. ATP or histamine evoked reproducible elevations in NO synthesis and cytosolic Ca(2+), as judged by 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and fluo-3 fluorescence, respectively, that were tightly associated with membrane hyperpolarizations. Whereas evoked NO synthesis was unaffected by either tetraethylammonium (10 mmol/l) or BaCl(2) (50 micromol/l) + ouabain (100 micromol/l), depleting intracellular Ca(2+) stores by thapsigargin or removing external Ca(2+) inhibited NO production, as did exposure to high (80 mmol/l) external KCl. Importantly, apamin and charybdotoxin (ChTx)/ triarylmethane (TRAM)-34, selective blockers SK(Ca) and IK(Ca) channels, respectively, abolished both stimulated NO synthesis and membrane hyperpolarization and decreased evoked Ca(2+) transients. Apamin and TRAM-34 also inhibited an agonist-induced outwardly rectifying current characteristic of SK(Ca) and IK(Ca) channels. Under voltage-clamp control, we further observed that the magnitude of agonist-induced NO production varied directly with the degree of membrane hyperpolarization. Mechanistically, our data indicate that SK(Ca) and IK(Ca) channel-mediated hyperpolarization represents a critical early event in agonist-evoked NO production by regulating the influx of Ca(2+) responsible for endothelial NO synthase activation. Moreover, it appears that the primary role of agonist-induced release of intracellular Ca(2+) stores is to trigger the opening of both K(Ca) channels along with Ca(2+) entry channels at the plasma membrane. Finally, the observed inhibition of stimulated NO synthesis by apamin and ChTx/TRAM-34 demonstrates that SK(Ca) and IK(Ca) channels are essential for NO-mediated vasorelaxation.

Functional contribution of the endothelial component to the vasorelaxing effect of resveratrol and NS 1619, activators of the large-conductance calcium-activated potassium channels

Large-conductance calcium-activated potassium channels (BK) of smooth muscle play a role in the relevant modulation of vascular tone, due to their calcium- and voltage-dependent mechanisms of activation. A potential role of endothelial BK channels has also been suggested by approaches on endothelial cell cultures. However, no functional study, aimed at evaluating the contribution of endothelial BK channels to the effect of BK-openers, has been reported. Resveratrol and NS 1619, BK-openers, have been tested on endothelium-intact and -denuded aortic rings. Furthermore, the effects of high depolarisation of potassium channel blockers TEA (Tetraethylammonium), 4-AP ( 4-Aminopyridine) and IbTX (Iberiotoxin) and of inhibitors of NO-pathway (L-NAME and ODQ) have been evaluated. The presence of endothelium increased the vasorelaxing potency of BK-openers. This potentiation was eliminated by L-NAME and ODQ. TEA, 4-AP, IbTX and high depolarisation had modest or no antagonist influence on resveratrol in endothelium-denuded aortic rings. The effects of NS 1619 on endothelium-denuded aortic rings were not affected by IbTX, and were modestly antagonised by TEA, 4-AP and high depolarisation. In intact endothelium vessels, TEA, IbTX and 4-AP antagonised the vasorelaxing effect of the two BK-activators. A BK-mediated release of endothelial NO seems a very important factor, determining a strong influence on vasodilator profile of BK-openers. Therefore, an eventual therapy with a BK-opener could promote a series of cardiovascular impacts not confined to the only direct vasorelaxing effects, but also due to a significant contribution of endothelial NO.

Cilostazol Suppresses Superoxide Production and Expression of Adhesion Molecules in Human Endothelial Cells via Mediation of cAMP-Dependent Protein Kinase-Mediated Maxi-K Channel Activation

This study shows whether increased intracellular cAMP level by cilostazol is directly coupled to its maxi-K channel activation in human endothelial cells. Cilostazol (1 microM) increased the K+ currents in the human endothelial cells by activating maxi-K channels, which was abolished by iberiotoxin (100 nM), a maxi-K channel blocker. On incubation of human coronary artery endothelial cells with tumor necrosis factor-alpha (TNF-alpha) (50 ng/ml), monocyte adhesion significantly increased with increased superoxide generation and expression of vascular cell adhesion molecule-1 (VCAM-1) and monocyte chemoattractant protein-1 (MCP-1) accompanied by increased degradation of inhibitory kappaBalpha in cytoplasm and activation of nuclear factor-kappaB p65 in nucleus. All these variables were significantly suppressed by cilostazol (10 microM), which was antagonized by iberiotoxin (1 microM) and (9R,10S,12S)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-l] [1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT 5720) (300 nM, cAMP-dependent protein kinase inhibitor), but not by (9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindo-lo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-I][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT 5823) (300 nM, cGMP-dependent protein kinase inhibitor). In the human endothelial cells transfected with siRNA-targeting maxi-K channels, cilostazol did not suppress the superoxide generation, VCAM-1 and MCP-1 expressions, and monocyte adhesion as contrasted with the wild-type cells. These findings were similarly evident with (3S)-(+)-(5-chloro-2-methoxyphenyl)-1,3-dihydro-3-fluoro-6-(trifluoromethyl)-2H-indole-2-one (BMS-204352), a maxi-K channel opener, and forskolin and dibutyryl cAMP. In conclusion, increased cAMP level by cilostazol is directly coupled to its maxi-K channel opening action via protein kinase activation in human endothelial cells, thereby suppressing TNF-alpha-stimulated superoxide production and expression of adhesion molecules.

Sphingosine-1-phosphate activates BKCa channels independently of G protein-coupled receptor in human endothelial cells

The effect of sphingosine-1-phosphate (S1P) on large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels was examined in primary cultured human umbilical vein endothelial cells by measuring intracellular Ca(2+) concentration ([Ca(2+)](i)), whole cell membrane currents, and single-channel activity. In nystatin-perforated current-clamped cells, S1P hyperpolarized the membrane and simultaneously increased [Ca(2+)](i). [Ca(2+)](i) and membrane potentials were strongly correlated. In whole cell clamped cells, BK(Ca) currents were activated by increasing [Ca(2+)](i) via cell dialysis with pipette solution, and the activated BK(Ca) currents were further enhanced by S1P. When [Ca(2+)](i) was buffered at 1 microM, the S1P concentration required to evoke half-maximal activation was 403 +/- 13 nM. In inside-out patches, when S1P was included in the bath solution, S1P enhanced BK(Ca) channel activity in a reversible manner and shifted the relationship between Ca(2+) concentration in the bath solution and the mean open probability to the left. In whole cell clamped cells or inside-out patches loaded with guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS; 1 mM) using a patch pipette, GDPbetaS application or pretreatment of cells with pertussis toxin (100 ng/ml) for 15 h did not affect S1P-induced BK(Ca) current and channel activation. These results suggest that S1P enhances BK(Ca) channel activity by increasing Ca(2+) sensitivity. This channel activation hyperpolarizes the membrane and thereby increases Ca(2+) influx through Ca(2+) entry channels. Inasmuch as S1P activates BK(Ca) channels via a mechanism independent of G protein-coupled receptors, S1P may be a component of the intracellular second messenger that is involved in Ca(2+) mobilization in human endothelial cells.

Functional expression of Kir2.x in human aortic endothelial cells: the dominant role of Kir2.2

Inward rectifier K+channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K+current ( IK) demonstrated that 1) IKcurrent sensitivity to Ba2+and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of ∼4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced ∼50% by dnKir2.1 and ∼85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K+conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells.

The cellular mechanisms by which adenosine evokes release of nitric oxide from rat aortic endothelium

Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor-mediated NO release from the endothelium. In studies on freshly excised rat aorta, second-messenger systems were pharmacologically modulated by appropriate antagonists while a NO-sensitive electrode was used to measure adenosine-evoked NO release from the endothelium. We showed that A1-mediated NO release requires extracellular Ca2+, phospholipase A2 (PLA2) and ATP-sensitive K+ (KATP) channel activation whereas A2A-mediated NO release requires extracellular Ca2+ and Ca2+-activated K+ (KCa) channels. Since our previous study showed that A1- and A2A-receptor-mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K+ efflux resulting from A1-receptor-coupled KATP-channel activation facilitates Ca2+ influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca2+]i also stimulates PLA2 to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI2). PGI2 acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K+ efflux resulting from A2A-coupled KCa channels facilitates Ca2+ influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A2A-receptor-mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium- and NO-dependent manner has been shown to be important.

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.

Caveolae Targeting and Regulation of Large Conductance Ca2+-activated K+ Channels in Vascular Endothelial Cells

The vascular endothelium is richly endowed with caveolae, which are specialized membrane microdomains that facilitate the integration of specific cellular signal transduction processes. We found that the large conductance Ca(2+)-activated K+ (BK) channels are associated with caveolin-1 in bovine aortic endothelial cells (BAECs). OptiPrep gradient cell fractionation demonstrated that BK channels were concentrated in the caveolae-rich fraction in BAECs. Immunofluorescence imaging showed co-localization of caveolin-1 and BK channels in the BAEC membrane. Immunoprecipitation and glutathione S-transferase pull-down assay results indicated that caveolin-1 and BK channels are physically associated. However, whole cell patch clamp recordings could not detect BK (iberiotoxin-sensitive) currents in cultured BAECs under baseline conditions, even though the presence of BK mRNA and protein expression was confirmed by reverse transcription-PCR and Western blots. Cholesterol depletion redistributed the BK channels to non-caveolar fractions of BAECs, resulting in BK channel activation (7.3 +/- 1.6 pA/picofarad (pF), n = 5). BK currents were also activated by isoproterenol (ISO, 1 microM, 6.9 +/- 2.4 pA/pF, n = 6). Inclusion of a caveolin-1 scaffolding domain peptide (10 microM) in the pipette solution completely abrogated the effects of ISO on BK channel activation, whereas inclusion of the scrambled control peptide (10 microM) did not inhibit the ISO effects. We have also found that caveolin-1 knockdown by small interference RNA activated BK currents (5.3 +/- 1.4 pA/pF, n = 6). We conclude that: 1) BK channels are targeted to caveolae microdomains in vascular endothelial cells; 2) caveolin-1 interacts with BK channels and exerts a negative regulatory effect on channel functions; and 3) BK channels are inactive under control conditions but can be activated by cholesterol depletion, knockdown of caveolin-1 expression, or ISO stimulation. These novel findings may have important implications for the role of BK channels in the regulation of endothelial function.

Endothelin-1-induced proliferation of human endothelial cells depends on activation of K+ channels and Ca2+ influx

AIMS

Endothelin-1 (ET-1) promotes endothelial cell growth. Endothelial cell proliferation involves the activation of Ca2+-activated K+ channels. In this study, we investigated whether Ca2+-activated K+ channels with big conductance (BK(Ca)) contribute to endothelial cell proliferation induced by ET-1.

METHODS

The patch-clamp technique was used to analyse BK(Ca) activity in endothelial cells derived from human umbilical cord veins (HUVEC). Endothelial proliferation was examined using cell counts and measuring [3H]-thymidine incorporation. Changes of intracellular Ca2+ levels were examined using fura-2 fluorescence imaging.

RESULTS

Characteristic BK(Ca) were identified in cultured HUVEC. Continuous perfusion of HUVEC with 10 nmol L(-1) ET-1 caused a significant increase of BK(Ca) open-state probability (n = 14; P < 0.05; cell-attached patches). The ET(B)-receptor antagonist (BQ-788, 1 micromol L(-1)) blocked this effect. Stimulation with Et-1 (10 nmol L(-1)) significantly increased cell growth by 69% (n = 12; P < 0.05). In contrast, the combination of ET-1 (10 nmol L(-1)) and the highly specific BK(Ca) blocker iberiotoxin (IBX; 100 nmol L(-1)) did not cause a significant increase in endothelial cell growth. Ca2+ dependency of ET-1-induced proliferation was tested using the intracellular Ca2+-chelator BAPTA (10 micromol L(-1)). BAPTA abolished ET-1 induced proliferation (n = 12; P < 0.01). In addition, ET-1-induced HUVEC growth was significantly reduced, if cells were kept in a Ca2+-reduced solution (0.3 mmol L(-1)), or by the application of 2 aminoethoxdiphenyl borate (100 micromol L(-1)) which blocks hyperpolarization-induced Ca2+ entry (n = 12; P < 0.05).

CONCLUSION

Activation of BK(Ca) by ET-1 requires ET(B)-receptor activation and induces a capacitative Ca2+ influx which plays an important role in ET-1-mediated endothelial cell proliferation.

Sodium-calcium exchanger contributes to membrane hyperpolarization of intact endothelial cells from rat aorta during acetylcholine stimulation

1. The role of sodium-calcium exchanger in acetylcholine (Ach)-induced hyperpolarization of intact endothelial cells was studied in excised rat aorta. The membrane potential was recorded using perforated patch-clamp technique. 2. The mean resting potential of endothelial cells was -44.1+/-1.4 mV. A selective inhibitor of sodium-calcium exchanger benzamil (100 microm) had no significant effect on resting membrane potential, but reversibly decreased the amplitude of sustained Ach-induced endothelial hyperpolarization from 20.9+/-1.4 to 5.7+/-1.1 mV when applied during the plateau phase. 3. The blocker of reversed mode of the exchanger KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate, 20 microm) reversibly decreased the amplitude of sustained Ach-induced hyperpolarization from 20.5+/-2.9 to 7.5+/-1.8 mV. 4. Introduction of tetraethylammonium (10 mm) in the continuous presence of Ach decreased the sustained phase of hyperpolarization from 17.9+/-1.5 by 12.9+/-0.9 mV. Subsequent addition of 20 microm KB-R7943 further depolarized endothelial cells by 4.8+/-1.1 mV. 5. Substituting external sodium with N-methyl d-glucamine during the plateau phase of Ach-evoked hyperpolarization reversibly decreased the hyperpolarization from -61.8+/-2.7 to -54.2+/-1.9 mV. In the majority of preparations, the initial response to removal of external sodium was a transient further rise in the membrane potential of several mV. Sodium ionophore monensin hyperpolarized endothelium by 10.3+/-0.7 mV. 6. The inhibitory effect of benzamil on Ach-induced endothelial sustained hyperpolarization was observed in endothelium mechanically isolated from smooth muscle. 7. These results suggest that the sodium-calcium exchanger of intact endothelial cells is able to operate in reverse following stimulation by Ach, contributing to sustained hyperpolarization. Myoendothelial electrical communications do not mediate the effect of blockers of sodium-calcium exchanger.

Basic fibroblast growth factor-induced endothelial proliferation and NO synthesis involves inward rectifier K+ current

OBJECTIVE

Inward rectifier K+ currents (K(ir)) determine the resting membrane potential and thereby modulate essential Ca2+-dependent pathways, like cell growth and synthesis of vasoactive agents in endothelial cells. Basic fibroblast growth factor (bFGF) acts as a vasodilatator and angiogenic factor. Therefore, we investigated the effect of bFGF on K(ir) and assessed the role in proliferation and nitric oxide (NO) formation of endothelial cells.

METHODS AND RESULTS

Using the patch-clamp technique, we found characteristic K(ir) in human umbilical cord vein endothelial cells (HUVEC), which were dose-dependently blocked by barium (10 to 100 micromol/L). Perfusion with bFGF (50 ng/mL) caused a significant increase of K(ir), which was blocked by 100 micromol/L barium (n=18, P<0.01). The bFGF-induced HUVEC proliferation was significantly inhibited when using 50 to 100 micromol/L barium (n=6; P<0.01). NO production was examined using a cGMP radioimmunoassay. bFGF caused a significant increase of cGMP levels (n=10; P<0.05), which were blocked by barium.

CONCLUSIONS

Modulation of K(ir) plays an important role in bFGF-mediated endothelial cell growth and NO formation.

Potassium Channels and Membrane Potential in the Modulation of Intracellular Calcium in Vascular Endothelial Cells

The endothelium plays a vital role in the control of vascular functions, including modulation of tone; permeability and barrier properties; platelet adhesion and aggregation; and secretion of paracrine factors. Critical signaling events in many of these functions involve an increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)). This rise in [Ca(2+)](i) occurs via an interplay between several mechanisms, including release from intracellular stores, entry from the extracellular space through store depletion and second messenger-mediated processes, and the establishment of a favorable electrochemical gradient. The focus of this review centers on the role of potassium channels and membrane potential in the creation of a favorable electrochemical gradient for Ca(2+) entry. In addition, evidence is examined for the existence of various classes of potassium channels and the possible influence of regional variation in expression and experimental conditions.

Mechanisms underlying endothelium-dependent flow increase in perfused rat mesenteric vascular bed

The isolated rat mesenteric vasculature was perfused at constant pressures of 40, 80 or 120 mm Hg and the change in flow rate was measured. In the presence of phenylephrine, treatment with 3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonate (CHAPS) or N(G)-nitro-L-arginine (L-NA) significantly inhibited the pressure-dependent flow rate increase, but treatment with indomethacin or charybdotoxin plus apamin did not. Acetylcholine, bradykinin and ADP increased the flow rate, which had been markedly suppressed by CHAPS. At 80 mm Hg, the flow rate increase induced by these agonists was not affected by indomethacin plus L-NA, but was suppressed by subsequent treatment with charybdotoxin plus apamin. Changes in the perfusion pressure did not significantly affect the flow rate increases induced by the agonists. In conclusion, the opening of charybdotoxin plus apamin-sensitive Ca(2+)-dependent K(+) channels may be mainly involved in the endothelium-dependent flow rate increase induced by the agonists, whereas nitric oxide (NO) may be responsible for the endothelium-dependent, pressure-induced flow rate increase.

Characterization of a charybdotoxin-sensitive intermediate conductance Ca2+-activated K+ channel in porcine coronary endothelium: relevance to EDHF

1. This study characterizes the K(+) channel(s) underlying charybdotoxin-sensitive hyperpolarization of porcine coronary artery endothelium. 2. Two forms of current-voltage (I/V) relationship were evident in whole-cell patch-clamp recordings of freshly-isolated endothelial cells. In both cell types, iberiotoxin (100 nM) inhibited a current active only at potentials over +50 mV. In the presence of iberiotoxin, charybdotoxin (100 nM) produced a large inhibition in 38% of cells and altered the form of the I/V relationship. In the remaining cells, charybdotoxin also inhibited a current but did not alter the form. 3. Single-channel, outside-out patch recordings revealed a 17.1+/-0.4 pS conductance. Pipette solutions containing 100, 250 and 500 nM free Ca(2+) demonstrated that the open probability was increased by Ca(2+). This channel was blocked by charybdotoxin but not by iberiotoxin or apamin. 4. Hyperpolarizations of intact endothelium elicited by substance P (100 nM; 26.1+/-0.7 mV) were reduced by apamin (100 nM; 17.0+/-1.8 mV) whereas those to 1-ethyl-2-benzimidazolinone (1-EBIO, 600 microM, 21.0+/-0.3 mV) were unaffected (21.7+/-0.8 mV). Substance P, bradykinin (100 nM) and 1-EBIO evoked charybdotoxin-sensitive, iberiotoxin-insensitive whole-cell perforated-patch currents. 5 A porcine homologue of the intermediate-conductance Ca(2+)-activated K(+) channel (IK1) was identified in endothelial cells. 6. In conclusion, porcine coronary artery endothelial cells express an intermediate-conductance Ca(2+)-activated K(+) channel and the IK1 gene product. This channel is opened by activation of the EDHF pathway and likely mediates the charybdotoxin-sensitive component of the EDHF response.

Decreased 4-aminopyridine sensitive K+ currents in endothelial cells from hypertensive rats

Endothelial cell function is altered in hypertension. The present study was performed to evaluate the alterations in K+ channels in endothelial cells from hypertensive rats. Currents and membrane potentials were recorded in endothelial cells freshly dissociated from the aorta of stroke-prone spontaneously hypertensive rats (SHR-SP) and Wistar-Kyoto rats (WKY). Ca2+-dependent K+ channel blockers, charybdotoxin and apamin, a voltage-dependent K+ channel blocker, 4-aminopyridine, and a non-selective K+ channel blocker, tetrabutylammonium, were used to characterize K+ currents. Depolarizing command steps evoked delayed K+ outward currents in cells from both strains. The current density of 4-aminopyridine sensitive K+ currents was significantly smaller in SHR-SP than in WKY (1.5 +/- 0.4 vs. 4.9 +/- 0.6 pA/pF, at 36 mV, n = 13, p < 0.01), whereas that of other K+ current components did not differ between strains. The resting membrane potential of cells was significantly less negative in SHR-SP than in WKY (-25.0 +/- 1.7, n = 54 vs. -33.5 +/- 1.4 mV, n = 50, p < 0.01). Depolarization by 4-aminopyridine, but not that by charybdotoxin+apamin, abolished the difference in membrane potentials between SHR-SP and WKY (n=7-10 in each strain). Immunostaining of endothelial cells by anti-Kv1.5 antibody was decreased in SHR-SP compared to WKY. In summary, the 4-aminopyridine sensitive K+ currents in aortic endothelial cells were decreased in SHR-SP, which could contribute to the membrane depolarization. Decreased expression of Kv1.5 in SHR-SP might be associated with this alteration.

Endothelium-dependent relaxation in isolated renal arteries of diabetic rabbits

1 In this study, we have investigated the vasodilator response to acetylcholine under diabetes conditions in isolated renal arteries of rabbits. We have also examined the contribution of endothelium-derived nitric oxide (EDNO) and endothelium-derived hyperpolarizing factor (EDHF) to the endothelium-dependent relaxation caused by acetylcholine in the renal arteries of alloxan-induced diabetic rabbits. 2 Acetylcholine (10(-10) - 10(-4) M) produced cumulative concentration-response curve in the renal arteries of both control and diabetic rabbits. The EC50 values and maximal responses to acetylcholine were not significantly different relative to diabetic conditions. In order to isolate the EDHF component of acetylcholine-induced vasodilator response, L-nitro-methyl arginine ester (L-NAME, 10(-4) M) and indomethacin (10(-6) M) were added to the Krebs' solution throughout the experiment. Under these conditions, acetylcholine induced vasodilatation in the isolated renal arteries from both control and diabetic rabbits. The vasodilator response to acetylcholine was not affected under diabetic conditions. 3 Sodium nitroprusside (SNP)-induced relaxation was increased in the diabetic rabbits compared with the control animals. 4 Tetrabutyl ammonium (TBA, 0.5 mM) produced a significant reduction in acetylcholine-induced vasodilatation in both preparations from control and diabetic animals, consistent with involvement of K+ channels in mediating this response. Glibenclamide (1 microM) attenuated acetylcholine-induced vasodilatation in preparations from control animals only, while iberiotoxin (0.05 microM) significantly reduced the vasodilator response to acetylcholine in preparations from both control and diabetic animals. 5 The role of EDNO in mediating acetylcholine-induced vasodilatation was examined. The vascular preparations were incubated with 20 mM K(+)-Krebs' solution to inhibit the EDHF contribution to acetylcholine-induced vasodilatation. Under this condition, acetylcholine induced a vasodilator response in both preparations from control and diabetic rats. Pretreatment with L-NAME (10(-4) M) attenuated acetylcholine-induced vasodilatation in both preparations, indicating an nitric oxide-mediated vasodilator response. 6 Our results indicated that acetylcholine-induced vasodilatation in the isolated renal arteries of alloxan-induced diabetic rabbits was not affected under diabetic conditions. Acetylcholine-induced vasodilatation is mediated by two vasodilator components; namely, EDHF and EDNO. The contribution of EDHF and EDNO to acetylcholine-induced vasodilatation was not affected under diabetic conditions and there was no indication of endothelial dysfunction associated with diabetes. EDHF component was found to act mainly through high conductance Ca(2+)-activated K+ channels under normal and diabetic conditions, while the adenosine triphosphate-dependent K+ channels were involved in mediating acetylcholine vasodilator response in the control preparations only.

Subplasmalemmal endoplasmic reticulum controls K(Ca) channel activity upon stimulation with a moderate histamine concentration in a human umbilical vein endothelial cell line

This study was designed to elucidate the role of the subplasmalemmal endoplasmic reticulum (sER) in autacoid-induced stimulation of Ca(2+)-dependent K(+) channels in the umbilical vein endothelial cell-derived cell line EA.hy926. Cells were transfected with the Ca(2+) probe cameleon targeted to the ER for visualization of the ER network. A patch pipette was then placed close to or far (> 5 microm away) from the sER, single channel recordings (patch clamp technique) were monitored simultaneously with measurements of either ER Ca(2+) concentration (using the Ca(2+) probe Cam4-ER) or cytosolic free Ca(2+) concentration ([Ca(2+)](i); using fura-2) using a deconvolution imaging device. A voltage-dependent, large conductance Ca(2+)-dependent K(+) channel (BK(Ca); single channel conductance (gamma), 250 pS) was found. At membrane potentials of +40 and -40 mV, the EC(50) for Ca(2+) was 2.7 and 49.7 microM, respectively. In the vicinity of the sER, the BK(Ca) channel activity induced by 10 microM histamine was 32 times higher (open probability (P(o)) = 0.083 +/- 0.026) than in areas away from the sER (P(o) = 0.0026 +/- 0.002). However, at supramaximal histamine stimulation (100 microM), BK(Ca) channel activation was similar in patches in the vicinity of or away from the sER (P(o) = 0.18 +/- 0.09 and 0.25 +/- 0.07, respectively). In contrast to BK(Ca) channel activity, ER Ca(2+) depletion (Cam4-ER) and elevation of [Ca(2+)](i) in response to 10 and 100 microM histamine were not influenced by the pipette position. We conclude that in endothelial cells, the activation of BK(Ca) channels in response to moderate histamine concentration essentially depends on the proximity of the sER domains to the mouth of this K(+) channel. These findings further support our concept of the subplasmalemmal Ca(2+) control unit (SCCU) and add the local activation of Ca(2+)-activated K(+)-channels to the function of the SCCU.

Prevention of lipopolysaccharide-induced apoptosis by (2S,3S,4R)-N"-cyano-N-(6-amino-3,4-dihydro-3-hydroxy-2-methyl-2-dimethoxymethyl-2H-benzopyran-4-yl)-N'-benzylguanidine, a benzopyran analog, in endothelial cells

This study describes the antiapoptotic action of (2S,3S,4R)-N"-cyano-N-(6-amino-3,4-dihydro-3-hydroxy-2-methyl-2-dimethoxymethyl-2H-benzopyran-4-yl)-N'-benzylguanidine (KR-31378), a novel benzopyran analog, in human umbilical vein endothelial cells (HUVECs) in comparison with its acetylated metabolite, (2S,3S,4R)-N"-cyano-N-(6-acetylamino-3,4-dihydro-3-hydroxy-2-methyl-2-dimethoxymethyl-2H-benzopyran-4-yl)-N'-benzylguanidine (KR-31612), and with alpha-tocopherol. Exposure of HUVECs to lipopolysaccharide (LPS) (1 microg/ml) induced time- and concentration-dependent cytotoxicity and oligonucleosomal DNA fragmentation. KR-31378, KR-31612, and alpha-tocopherol potently suppressed LPS-induced cell death in association with significant reduction in the intracellular reactive oxygen species (ROS) and tumor necrosis factor-alpha (TNF-alpha) that are stimulated by LPS. KR-31378 more effectively protected HUVECs from LPS-induced DNA fragmentation and was more effective in peroxyl radical scavenging than alpha-tocopherol. Incubation with LPS markedly decreased the Bcl-2 level, which was totally reversed by KR-31378 and to a lesser degree by KR-31612 and by alpha-tocopherol. In contrast, the greatly increased Bax protein and cytochrome c release stimulated by LPS were markedly suppressed by KR-31378 and by KR-31612, and to a lesser degree by alpha-tocopherol. Taken together, KR-31378 strongly inhibited cell death in HUVECs in association with antiapoptotic effects, which were accompanied by up-regulation of Bcl-2 protein expression and down-regulation of Bax protein and suppression of cytochrome c release. KR-31378 also showed the properties to scavenge the intracellular ROS and peroxyl radicals, and to reduce the TNF-alpha production induced by LPS.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

K(Ca) channel-opening activity of Ginkgo Biloba extracts and ginsenosides in cultured endothelial cells

1. Extracts of Ginkgo biloba (EGb) and ginsenosides (GS) have been reported to induce vasorelaxation. In the present study, the role of K+ channels in the action of EGb and GS to activate nitric oxide synthase (NOS) activity was investigated in cultured endothelial cells. 2. Nitric oxide synthase activity of cultured endothelial cells detected by the reduced nicotinamide adenine dinucleotide phosphate (NADPH) histochemistry method was significantly increased after treatment with 20 microg/mL EGb or 40 microg/mL GS plus 10 mmol/L L-arginine. The effect was completely abolished by the addition of 0.5 micromol/L Nomega-nitro-L-arginine, an inhibitor of NOS, to the incubation medium and partially inhibited by 10 micromol/L tetraethylammonium (TEA), an inhibitor of Ca2+-activated K+ (KCa) channels. 3. Application of EGb to the intracellular surface of excised inside-out patches activated K+ channels in a concentration-dependent manner in the concentration range 1-100 microg/mL. Channel activity was also activated by application of GS at concentrations ranging from 1 to 300 microg/mL. The modulation of channel activity was inhibited by 0.5 mmol/L TEA but not by 0.5 mmol/L glibenclamide, an inhibitor of ATP-sensitive K+ channels. 4. Thus, in cultured endothelial cells, the increase in NOS activity induced by EGb or GS depends on the activity of KCa channels. These compounds may regulate nitric oxide release by changing the cell membrane potential in vascular endothelial cells.

Endothelial K(ca) channels mediate flow-dependent dilation of arterioles of skeletal muscle and mesentery

The role of Ca(2+)-activated potassium channels (K(Ca)) in flow-initiated intracellular events in microvessels is not known. We hypothesized that K(Ca) channels in the arteriolar endothelium are responsible for the mechanotransduction of flow/shear stress-induced arteriolar dilation in skeletal muscle and mesentery of rats. The active diameter of arterioles isolated from gracilis (80 mm Hg) and cremaster (60 mm Hg) muscles and mesentery (80 mm Hg) at a constant intraluminal pressure was 53 +/- 3, 77 +/- 5, and 72 +/- 6 microm, respectively. Their passive diameter (in Ca(2+)-free solution) was 113 +/- 3, 152 +/- 12, and 121 +/- 7 microm, respectively. At a constant intraluminal pressure stepwise increases in perfusate flow (25, 40, and 14 microL/min in 5, 10, and 2 microL/min steps) elicited a gradual increase in diameter of all three groups of arterioles up to 93 +/- 5, 137 +/- 11, and 102 +/- 7 microm, respectively. Flow-induced dilations of arterioles were eliminated by intraluminal administration of iberiotoxin (ibTX 10(-9) M), an inhibitor of high conductance K(Ca) channels (BK(Ca)). In contrast, arteriolar dilations to acetylcholine and sodium nitroprusside were not altered by this agent, indicating that BK(Ca) channels are not involved in the receptor-mediated endothelial synthesis of nitric oxide (NO) and that the inhibitor did not affect the action of NO on smooth muscle. Abluminal application of ibTX (10(-8) M) did not affect flow-dependent dilation. We conclude that in arterioles of several tissues activation of endothelial BK(Ca) channels is an obligatory step in the transduction of the signal initiated by changes in intraluminal flow/shear stress, leading to the release of endothelial factors evoking dilation.

Background nonselective cationic current and the resting membrane potential in rabbit aorta endothelial cells

The ion channel conductances that regulate the membrane potential was investigated by using a perforated patch-clamp technique in rabbit aorta endothelial cells (RAECs). The whole-cell current/voltage (I-V) relation showed a slight outward rectification under physiological ionic conditions. The resting membrane potential was -23.3 +/- 1.1 mV (mean +/- SEM, n = 19). The slope conductances at the potentials of -80 and 50 mV were 31.0 +/- 4.0 and 62.8 +/- 7.1 pS pF(-1), respectively (n = 15). Changes in the extracellular and intracellular Cl(-) concentrations did not affect the reversal potential on I-V curves. The background nonselective cationic (NSC) current was isolated after the K(+) current was suppressed. The relative permeabilities calculated from the changes in reversal potentials using the constant-field theory were P(K):P(Cs):P(Na):P(Li) = 1:0.87:0.40:0.27 and P(Cs):P(Ca) = 1:0.21. Increases in the external Ca(2+) decreased the background NSC current in a dose-dependent manner. The concentration for half block by Ca(2+) was 1.1 +/- 0.3 mM (n = 7). Through the continuous recording of the membrane potential in a current-clamp mode, it was found that the background NSC conductance is the major determinant of resting membrane potential. Taken together, it could be concluded that the background NSC channels function as the major determinant for the resting membrane potential and can be responsible for the background Ca(2+) entry pathway in freshly isolated RAECs.