Calcium-activated potassium channels in the endothelium of intact rat aorta

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.

Kv1.3 Channel Inhibition Limits Uremia-Induced Calcification in Mouse and Human Vascular Smooth Muscle

Chronic kidney disease (CKD) significantly increases cardiovascular risk. In advanced CKD stages, accumulation of toxic circulating metabolites and mineral metabolism alterations triggers vascular calcification, characterized by vascular smooth muscle cell (VSMC) transdifferentiation and loss of the contractile phenotype. Phenotypic modulation of VSMC occurs with significant changes in gene expression. Even though ion channels are an integral component of VSMC function, the effects of uremia on ion channel remodeling has not been explored. We used an in vitro model of uremia-induced calcification of human aorta smooth muscle cells (HASMCs) to study the expression of 92 ion channel subunit genes. Uremic serum-induced extensive remodeling of ion channel expression consistent with loss of excitability but different from the one previously associated with transition from contractile to proliferative phenotypes. Among the ion channels tested, we found increased abundance and activity of voltage-dependent K+ channel Kv1.3. Enhanced Kv1.3 expression was also detected in aorta from a mouse model of CKD. Pharmacological inhibition or genetic ablation of Kv1.3 decreased the amount of calcium phosphate deposition induced by uremia, supporting an important role for this channel on uremia-induced VSMC calcification.

BACE1 Translation: At the Crossroads Between Alzheimer's Disease Neurodegeneration and Memory Consolidation

 Human life unfolds not only in time and space, but also in the recollection and interweaving of memories. Therefore, individual human identity depends fully on a proper access to the autobiographical memory. Such access is hindered under pathological conditions such as Alzheimer’s disease, which affects millions of people worldwide. Unfortunately, no effective cure exists to prevent this disorder, the impact of which will rise alarmingly within the next decades. While Alzheimer’s disease is largely considered to be the outcome of amyloid-β (Aβ) peptide accumulation in the brain, conceiving this complex disorder strictly as the result of Aβ-neurotoxicity is perhaps a too straight-line simplification. Instead, complementary to this view, the tableau of molecular disarrangements in the Alzheimer’s disease brain may be reflecting, at least in part, a loss of function phenotype in memory processing. Here we take BACE1 translation and degradation as a gateway to study molecular mechanisms putatively involved in the transition between memory and neurodegeneration. BACE1 participates in the excision of Aβ-peptide from its precursor holoprotein, but plays a role in synaptic plasticity too. Its translation is governed by eIF2α phosphorylation: a hub integrating cellular responses to stress, but also a critical switch in memory consolidation. Paralleling these dualities, the eIF2α-kinase HRI has been shown to be a nitric oxide-dependent physiological activator of hippocampal BACE1 translation. Finally, beholding BACE1 as a representative protease active in the CNS, we venture a new perspective on the cellular basis of memory, which may incorporate neurodegeneration in itself as a drift in memory consolidating systems.

Ca2+-dependent potassium channels and cannabinoid signaling in the endothelium of apolipoprotein E knockout mice before plaque formation

Endothelial Ca2+-dependent K+ channels (KCa) regulate endothelial function. We also know that stimulation of type 2 cannabinoid (CB2) receptors ameliorates atherosclerosis. However, whether atherosclerosis is accompanied by altered endothelial KCa- and CB2 receptor-dependent signaling is unknown. By utilizing an in situ patch-clamp approach, we directly evaluated the KCa channel function and the CB2 receptor-dependent electrical responses in the endothelium of aortic strips from young ApoE-/- and C57Bl/6 mice. In the ApoE-/- group, the resting membrane potential (-30.1±1.1mV) was less negative (p<0.05) compared to WT (-38.9±1.4mV) and voltage ramps generated an overall KCa current of reduced amplitude. The peak hyperpolarization to 2μM Ach was not different between the groups. However, the sustained component was significantly reduced in ApoE-/- strips. In contrast, the peak hyperpolarization to 0.2μM Ach was increased in the ApoE-/- group, and SKA-31, a direct IKCa/SKCa channel opener, produced a hyperpolarization and whole-cell current of greater amplitude. The BKCa opener NS1619 produced hyperpolarization that was enhanced in ApoE-/- group. N-arachidonoyl glycine, a BKCa opener, produced a hyperpolarization of enhanced amplitude in ApoE-/- arteries. Selective CB2 receptor agonist AM1241 (5μM) had no effect on endothelial membrane potential in WT group; however, in ApoE-/- group, it elicited hyperpolarization that was inhibited by a selective CB2 receptor antagonist AM630. Conclusively, our data point to functional down-regulation of basal IKCa activity in unstimulated endothelium of ApoE-/- mice. Direct and indirect IKCa stimulation resulted in increased recruitment of the channels. In addition, our data point to up-regulation of endothelial BKCa channels and CB2 receptors in ApoE-/- arteries.

Overview of the Microenvironment of Vasculature in Vascular Tone Regulation

Hypertension is asymptomatic and a well-known “silent killer”, which can cause various concomitant diseases in human population after years of adherence. Although there are varieties of synthetic antihypertensive drugs available in current market, their relatively low efficacies and major application in only single drug therapy, as well as the undesired chronic adverse effects associated, has drawn the attention of worldwide scientists. According to the trend of antihypertensive drug evolution, the antihypertensive drugs used as primary treatment often change from time-to-time with the purpose of achieving the targeted blood pressure range. One of the major concerns that need to be accounted for here is that the signaling mechanism pathways involved in the vasculature during the vascular tone regulation should be clearly understood during the pharmacological research of antihypertensive drugs, either in vitro or in vivo. There are plenty of articles that discussed the signaling mechanism pathways mediated in vascular tone in isolated fragments instead of a whole comprehensive image. Therefore, the present review aims to summarize previous published vasculature-related studies and provide an overall depiction of each pathway including endothelium-derived relaxing factors, G-protein-coupled, enzyme-linked, and channel-linked receptors that occurred in the microenvironment of vasculature with a full schematic diagram on the ways their signals interact. Furthermore, the crucial vasodilative receptors that should be included in the mechanisms of actions study on vasodilatory effects of test compounds were suggested in the present review as well.

Comparisons between perivascular adipose tissue and the endothelium in their modulation of vascular tone

The endothelium is an established modulator of vascular tone; however, the recent discovery of the anti-contractile nature of perivascular adipose tissue (PVAT) suggests that the fat, which surrounds many blood vessels, can also modulate vascular tone. Both the endothelium and PVAT secrete vasoactive substances, which regulate vascular function. Many of these factors are common to both the endothelium and PVAT; therefore, this review will highlight the potential shared mechanisms in the modulation of vascular tone. Endothelial dysfunction is a hallmark of many vascular diseases, including hypertension and obesity. Moreover, PVAT dysfunction is now being reported in several cardio-metabolic disorders. Thus, this review will also discuss the mechanistic insights into endothelial and PVAT dysfunction in order to evaluate whether PVAT modulation of vascular contractility is similar to that of the endothelium in health and disease.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Overview of Antagonists Used for Determining the Mechanisms of Action Employed by Potential Vasodilators with Their Suggested Signaling Pathways

This paper is a review on the types of antagonists and the signaling mechanism pathways that have been used to determine the mechanisms of action employed for vasodilation by test compounds. Thus, we exhaustively reviewed and analyzed reports related to this topic published in PubMed between the years of 2010 till 2015. The aim of this paperis to suggest the most appropriate type of antagonists that correspond to receptors that would be involved during the mechanistic studies, as well as the latest signaling pathways trends that are being studied in order to determine the route(s) that atest compound employs for inducing vasodilation. The methods to perform the mechanism studies were included. Fundamentally, the affinity, specificity and selectivity of the antagonists to their receptors or enzymes were clearly elaborated as well as the solubility and reversibility. All the signaling pathways on the mechanisms of action involved in the vascular tone regulation have been well described in previous review articles. However, the most appropriate antagonists that should be utilized have never been suggested and elaborated before, hence the reason for this review.

Unitary TRPV3 channel Ca2+ influx events elicit endothelium-dependent dilation of cerebral parenchymal arterioles

Cerebral parenchymal arterioles (PA) regulate blood flow between pial arteries on the surface of the brain and the deeper microcirculation. Regulation of PA contractility differs from that of pial arteries and is not completely understood. Here, we investigated the hypothesis that the Ca(2+) permeable vanilloid transient receptor potential (TRPV) channel TRPV3 can mediate endothelium-dependent dilation of cerebral PA. Using total internal reflection fluorescence microscopy (TIRFM), we found that carvacrol, a monoterpenoid compound derived from oregano, increased the frequency of unitary Ca(2+) influx events through TRPV3 channels (TRPV3 sparklets) in endothelial cells from pial arteries and PAs. Carvacrol-induced TRPV3 sparklets were inhibited by the selective TRPV3 blocker isopentenyl pyrophosphate (IPP). TRPV3 sparklets have a greater unitary amplitude (ΔF/F0 = 0.20) than previously characterized TRPV4 (ΔF/F0 = 0.06) or TRPA1 (ΔF/F0 = 0.13) sparklets, suggesting that TRPV3-mediated Ca(2+) influx could have a robust influence on cerebrovascular tone. In pressure myography experiments, carvacrol caused dilation of cerebral PA that was blocked by IPP. Carvacrol-induced dilation was nearly abolished by removal of the endothelium and block of intermediate (IK) and small-conductance Ca(2+)-activated K(+) (SK) channels. Together, these data suggest that TRPV3 sparklets cause dilation of cerebral parenchymal arterioles by activating IK and SK channels in the endothelium.

Endothelial atypical cannabinoid receptor: do we have enough evidence?

Cannabinoids and their synthetic analogues affect a broad range of physiological functions, including cardiovascular variables. Although direct evidence is still missing, the relaxation of a vast range of vascular beds induced by cannabinoids is believed to involve a still unidentified non-CB1 , non-CB2 Gi/o protein-coupled receptor located on endothelial cells, the so called endothelial cannabinoid receptor (eCB receptor). Evidence for the presence of an eCB receptor comes mainly from vascular relaxation studies, which commonly employ pertussis toxin as an indicator for GPCR-mediated signalling. In addition, a pharmacological approach is widely used to attribute the relaxation to eCB receptors. Recent findings have indicated a number of GPCR-independent targets for both agonists and antagonists of the presumed eCB receptor, warranting further investigations and cautious interpretation of the vascular relaxation studies. This review will provide a brief historical overview on the proposed novel eCB receptor, drawing attention to the discrepancies between the studies on the pharmacological profile of the eCB receptor and highlighting the Gi/o protein-independent actions of the eCB receptor inhibitors widely used as selective compounds. As the eCB receptor represents an attractive pharmacological target for a number of cardiovascular abnormalities, defining its molecular identity and the extent of its regulation of vascular function will have important implications for drug discovery. This review highlights the need to re-evaluate this subject in a thoughtful and rigorous fashion. More studies are needed to differentiate Gi/o protein-dependent endothelial cannabinoid signalling from that involving the classical CB1 and CB2 receptors as well as its relevance for pathophysiological conditions.

Protection of coronary endothelial function during cardiac surgery: potential of targeting endothelial ion channels in cardioprotection

Vascular endothelium plays a critical role in the control of blood flow by producing vasoactive factors to regulate vascular tone. Ion channels, in particular, K(+) channels and Ca(2+)-permeable channels in endothelial cells, are essential to the production and function of endothelium-derived vasoactive factors. Impairment of coronary endothelial function occurs in open heart surgery that may result in reduction of coronary blood flow and thus in an inadequate myocardial perfusion. Hyperkalemic exposure and concurrent ischemia-reperfusion during cardioplegic intervention compromise NO and EDHF-mediated function and the impairment involves alterations of K(+) channels, that is, KATP and KCa, and Ca(2+)-permeable TRP channels in endothelial cells. Pharmacological modulation of these channels during ischemia-reperfusion and hyperkalemic exposure show promising results on the preservation of NO and EDHF-mediated endothelial function, which suggests the potential of targeting endothelial K(+) and TRP channels for myocardial protection during cardiac surgery.

Diversity of potassium channels in human umbilical artery smooth muscle cells: a review of their roles in human umbilical artery contraction

Through their control of cell membrane potential, potassium (K(+)) channels are among the best known regulators of vascular tone. This article discusses the expression and function of K(+) channels in human umbilical artery smooth muscle cells (HUASMCs). We review the bibliographic reports and also present single-channel data recorded in freshly isolated cells. Electrophysiological properties of big conductance, voltage- and Ca(2+)-sensitive K(+) channel and voltage-dependent K(+) channels are clearly established in this vessel, where they are involved in contractile state regulation. Their role in the maintenance of membrane potential is an important control mechanism in the determination of the vessel diameter. Additionally, small conductance Ca(2+)-sensitive K(+) channels, 2-pore domains K(+) channels and inward rectifier K(+) channels also appear to be present in HUASMCs, while intermediate conductance Ca(2+)-sensitive K(+) channels and ATP-sensitive K(+) channels could not be identified. In both cases, additional investigation is necessary to reach conclusive evidence of their expression and/or functional role in HUASMCs. Finally, we discuss the role of K(+) channels in pregnancy-related pathologies like gestational diabetes and preeclampsia.

Reduced hyperpolarization of endothelial cells following high dietary Na+: effects of enalapril and tempol

1. High dietary Na(+) is associated with impaired vascular endothelial function. However, the underlying mechanisms are not completely understood. In the present study, we investigated whether the endothelial hyperpolarization response to acetylcholine (ACh) exhibited any abnormalities in Wistar rats fed a high-salt diet (HSD) for 1 month and, if so, whether chronic treatment with the angiotensin-converting enzyme inhibitor enalapril or the anti-oxidant tempol could normalize the response. Membrane potential was recorded using the perforated patch-clamp technique on the endothelium of rat aorta. 2. Acetylcholine (2 μmol/L) produced a hyperpolarization sensitive to TRAM-34, a blocker of intermediate-conductance Ca(2+) -sensitive K(+) channels (IK(Ca)), but not to apamin, a blocker of small-conductance Ca(2+)-sensitive K(+) channels (SK(Ca)). NS309 (3 μmol/L), an activator of SK(Ca) and IK(Ca) channels, produced a hyperpolarization of similar magnitude as ACh. 3. In the HSD group, the ACh-evoked hyperpolarization was significantly attenuated compared with that in the control group, which was fed normal chow rather than an HSD. Similarly, the hyperpolarization produced by NS309 was weaker in tissues from HSD-fed rats. 4. Combination of HSD with chronic enalapril treatment (20 mg/kg per day for 1 month) normalized endothelial hyperpolarizing responses to ACh. Chronic tempol treatment (1 mmol/L in tap water for 1 month) prevented the reduced hyperpolarization to ACh. 5. The results of the present study indicate that excess in dietary Na(+) results in a failure of endothelial cells to generate normal IK(Ca) channel-mediated hyperpolarizing responses. Our observations implicate oxidative stress mediated by increased angiotensin II signalling as a mechanism underlying altered endothelial hyperpolarization during dietary salt loading.

The contribution of d-tubocurarine-sensitive and apamin-sensitive K-channels to EDHF-mediated relaxation of mesenteric arteries from eNOS-/- mice

The nature of the potassium channels involved in determining endothelium-derived hyperpolarizing factor-mediated relaxation was investigated in first-order small mesenteric arteries from male endothelial nitric oxide synthase (eNOS-/-)-knockout and control (+/+) mice. Acetylcholine-induced endothelium-dependent relaxation of small mesenteric arteries of eNOS-/- was resistant to N-nitro-L-arginine and indomethacin and the guanylyl cyclase inhibitor, 1H-(1,2,4) oxadiazolo (4,3-a) quinoxalin-1-one. Apamin and the combination of apamin and iberiotoxin or apamin and charybdotoxin induced a transient endothelium-dependent contraction of small mesenteric arteries from both eNOS-/- and +/+ mice. Acetylcholine-induced relaxation in eNOS-/- mice was unaffected by charybdotoxin or apamin alone but significantly inhibited by the combination of these agents. However, the combination of scyllatoxin and iberiotoxin did not mimic the inhibitory effect of the apamin/charybdotoxin combination. Tubocurarine alone completely blocked acetylcholine-induced relaxation in eNOS-/- mice. Single channel analysis of myocytes from small mesenteric arterioles revealed a large conductance calcium-activated potassium channel that was sensitive to iberiotoxin, charybdotoxin, and tetraethylammonium. Tubocurarine blocked this channel from the cytosolic side but not when applied extracellularly. Solutions of nitric oxide (NO) gas also relaxed small mesenteric arteries that had been contracted with cirazoline in a concentration-dependent manner, and the sensitivity to NO was reduced by iberiotoxin and the combination of apamin, scyllatoxin, or tubocurarine with charybdotoxin but not by apamin, charybdotoxin, scyllatoxin, or tubocurarine alone. These data indicate that acetylcholine-induced endothelium-derived hyperpolarizing factor-mediated relaxation in small mesenteric arteries from eNOS-/- involved the activation of tubocurarine and apamin-/charybdotoxin-sensitive K-channels. In eNOS+/+ mice, the acetylcholine-induced response was primarily mediated by NO and was sensitive to iberiotoxin and the combination of apamin and charybdotoxin.

The gastrointestinal peptide obestatin induces vascular relaxation via specific activation of endothelium-dependent NO signalling

BACKGROUND AND PURPOSE

Obestatin is a recently discovered gastrointestinal peptide with established metabolic actions, which is linked to diabetes and may exert cardiovascular benefits. Here we aimed to investigate the specific effects of obestatin on vascular relaxation.

EXPERIMENTAL APPROACH

Cumulative relaxation responses to obestatin peptides were assessed in rat isolated aorta and mesenteric artery (n≥ 8) in the presence and absence of selective inhibitors. Complementary studies were performed in cultured bovine aortic endothelial cells (BAEC).

KEY RESULTS

Obestatin peptides elicited concentration-dependent relaxation in both aorta and mesenteric artery. Responses to full-length obestatin(1-23) were greater than those to obestatin(1-10) and obestatin(11-23). Obestatin(1-23)-induced relaxation was attenuated by endothelial denudation, l-NAME (NOS inhibitor), high extracellular K(+) , GDP-β-S (G-protein inhibitor), MDL-12,330A (adenylate cyclase inhibitor), wortmannin (PI3K inhibitor), KN-93 (CaMKII inhibitor), ODQ (guanylate cyclase inhibitor) and iberiotoxin (BK(Ca) blocker), suggesting that it is mediated by an endothelium-dependent NO signalling cascade involving an adenylate cyclase-linked GPCR, PI3K/PKB, Ca(2+) -dependent eNOS activation, soluble guanylate cyclase and modulation of vascular smooth muscle K(+) . Supporting data from BAEC indicated that nitrite production, intracellular Ca(2+) and PKB phosphorylation were increased after exposure to obestatin(1-23). Relaxations to obestatin(1-23) were unaltered by inhibitors of candidate endothelium-derived hyperpolarizing factors (EDHFs) and combined SK(Ca) /IK(Ca) blockade, suggesting that EDHF-mediated pathways were not involved.

CONCLUSIONS AND IMPLICATIONS

Obestatin produces significant vascular relaxation via specific activation of endothelium-dependent NO signalling. These actions may be important in normal regulation of vascular function and are clearly relevant to diabetes, a condition characterized by endothelial dysfunction and cardiovascular complications.

Endothelial calcium-activated potassium channels as therapeutic targets to enhance availability of nitric oxide

The vascular endothelium plays a critical role in vascular health by controlling arterial diameter, regulating local cell growth, and protecting blood vessels from the deleterious consequences of platelet aggregation and activation of inflammatory responses. Circulating chemical mediators and physical forces act directly on the endothelium to release diffusible relaxing factors, such as nitric oxide (NO), and to elicit hyperpolarization of the endothelial cell membrane potential, which can spread to the surrounding smooth muscle cells via gap junctions. Endothelial hyperpolarization, mediated by activation of calcium-activated potassium (K(Ca)) channels, has generally been regarded as a distinct pathway for smooth muscle relaxation. However, recent evidence supports a role for endothelial K(Ca) channels in production of endothelium-derived NO, and indicates that pharmacological activation of these channels can enhance NO-mediated responses. In this review we summarize the current data on the functional role of endothelial K(Ca) channels in regulating NO-mediated changes in arterial diameter and NO production, and explore the tempting possibility that these channels may represent a novel avenue for therapeutic intervention in conditions associated with reduced NO availability such as hypertension, hypercholesterolemia, smoking, and diabetes mellitus.

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.

The GPR55 agonist lysophosphatidylinositol directly activates intermediate-conductance Ca2+ -activated K+ channels

Lysophosphatidylinositol (LPI) was recently shown to act both as an extracellular mediator binding to G protein-coupled receptor 55 (GPR55) and as an intracellular messenger directly affecting a number of ion channels including large-conductance Ca(2+) and voltage-gated potassium (BK(Ca)) channels. Here, we explored the effect of LPI on intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels using excised inside-out patches from endothelial cells. The functional expression of IK(Ca) was confirmed by the charybdotoxin- and TRAM-34-sensitive hyperpolarization to histamine and ATP. Moreover, the presence of single IK(Ca) channels with a slope conductance of 39 pS in symmetric K(+) gradient was directly confirmed in inside-out patches. When cytosolically applied in the range of concentrations of 0.3-10 μM, which are well below the herein determined critical micelle concentration of approximately 30 μM, LPI potentiated the IK(Ca) single-channel activity in a concentration-dependent manner, while single-channel current amplitude was not affected. In the whole-cell configuration, LPI in the pipette was found to facilitate membrane hyperpolarization in response to low (0.5 μM) histamine concentrations in a TRAM-34-sensitive manner. These results demonstrate a so far not-described receptor-independent effect of LPI on the IK(Ca) single-channel activity of endothelial cells, thus, highlighting LPI as a potent intracellular messenger capable of modulating electrical responses in the vasculature.

Single-nucleotide polymorphisms in vascular Ca2+-activated K+-channel genes and cardiovascular disease

In the cardiovascular system, Ca2+-activated K+-channels (KCa) are considered crucial mediators in the control of vascular tone and blood pressure by modulating the membrane potential and shaping Ca2+-dependent contraction. Vascular smooth muscle cells express the BKCa channel which fine-tunes contractility by providing a negative feedback on Ca2+-elevations. BKCa channel's ion-conducting alpha-subunit is encoded by the KCa1.1 gene, and the accessory and Ca2+-sensitivity modulating beta1-subunit is encoded by the KCNMB1 gene. Vascular endothelial cells express the calmodulin-gated KCa channels IKCa (encoded by the KCa3.1 gene) and SKCa (encoded by the KCa2.3 gene). These two channels mediate endothelial hyperpolarization and initiate the endothelium-derived hyperpolarizing factor-dilator response. Considering these essential roles of KCa in arterial function, mutations in KCa genes have been suspected to contribute to cardiovascular disease in humans. So far, DNA sequence analysis in the population and patient cohorts has identified single-nucleotide polymorphisms (SNPs) in the BKCa beta1-subunit gene as well as in the alpha-subunit gene (KCa1.1). Some of these SNPs produce amino acid exchanges and evoke alterations of channel functions ("gain-of-function" as well as "loss-of-function"). Moreover, the epidemiological studies showed that the presence of the E65K polymorphism in, e.g., BKCa beta1-subunit gene (producing a "gain-of-function") lowers the prevalence for severe hypertension and myocardial infarction. Other SNPs in the BKCa alpha-subunit gene and also in the KCa3.1 gene expressed in the endothelium have been suggested to increase the risk of cardiovascular disease. These findings from sequence analysis of human KCa genes, and epidemiological studies thus provide evidence that genetic variations and mutations in KCa channel genes contribute to human cardiovascular disease.

Characteristic changes in coronary artery at the early hyperglycaemic stage in a rat type 2 diabetes model and the effects of pravastatin

BACKGROUND AND PURPOSE

Diabetes is a risk factor for the development of coronary artery disease but it is not known whether the functions of endothelium-derived nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in coronary arteries are altered in the early stage of diabetes. Such alterations and the effects of pravastatin were examined in left anterior descending coronary arteries (LAD) from Otsuka Long-Evans Tokushima Fatty (OLETF) rats (type 2 diabetes model) at the early hyperglycaemic stage [vs. non-diabetic Long-Evans Tokushima Otsuka (LETO) rats].

EXPERIMENTAL APPROACH

Isometric tension, membrane potential and superoxide production were measured, as were protein expression of NAD(P)H oxidase components and endothelial NO synthase (eNOS).

KEY RESULTS

Superoxide production and the protein expressions of both the nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] oxidase components and eNOS were increased in OLETF rats. These changes were normalized by pravastatin administration. Not only acetylcholine (ACh)-induced endothelial NO production but also functions of endothelium-derived NO [from (i) the absolute tension induced by epithio-thromboxane A(2) (STA(2)) or high K(+); (ii) enhancement of the STA(2)-contraction by a nitric oxide synthase (NOS) inhibitor; and (iii) the ACh-induced endothelium-dependent relaxation of high K(+)-induced contraction] or EDHF [from (iv) ACh-induced endothelium-dependent smooth muscle cell hyperpolarization and relaxation in the presence of a NOS inhibitor] were similar between LETO and OLETF rats [whether or not the latter were pravastatin-treated or -untreated].

CONCLUSIONS AND IMPLICATIONS

Under conditions of increased vascular superoxide production, endothelial function is retained in LAD in OLETF rats at the early hyperglycaemic stage, partly due to enhanced endothelial NOS protein expression. Inhibition of superoxide production may contribute to the beneficial vascular effects of pravastatin.

EDHF: an update

The endothelium controls vascular tone not only by releasing NO and prostacyclin, but also by other pathways causing hyperpolarization of the underlying smooth muscle cells. This characteristic was at the origin of the term 'endothelium-derived hyperpolarizing factor' (EDHF). However, this acronym includes different mechanisms. Arachidonic acid metabolites derived from the cyclo-oxygenases, lipoxygenases and cytochrome P450 pathways, H(2)O(2), CO, H(2)S and various peptides can be released by endothelial cells. These factors activate different families of K(+) channels and hyperpolarization of the vascular smooth muscle cells contribute to the mechanisms leading to their relaxation. Additionally, another pathway associated with the hyperpolarization of both endothelial and vascular smooth muscle cells contributes also to endothelium-dependent relaxations (EDHF-mediated responses). These responses involve an increase in the intracellular Ca(2+) concentration of the endothelial cells, followed by the opening of SK(Ca) and IK(Ca) channels (small and intermediate conductance Ca(2+)-activated K(+) channels respectively). These channels have a distinct subcellular distribution: SK(Ca) are widely distributed over the plasma membrane, whereas IK(Ca) are preferentially expressed in the endothelial projections toward the smooth muscle cells. Following SK(Ca) activation, smooth muscle hyperpolarization is preferentially evoked by electrical coupling through myoendothelial gap junctions, whereas, following IK(Ca) activation, K(+) efflux can activate smooth muscle Kir2.1 and/or Na(+)/K(+)-ATPase. EDHF-mediated responses are altered by aging and various pathologies. Therapeutic interventions can restore these responses, suggesting that the improvement in the EDHF pathway contributes to their beneficial effect. A better characterization of EDHF-mediated responses should allow the determination of whether or not new drugable targets can be identified for the treatment of cardiovascular diseases.

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.

Novel approaches to improving endothelium-dependent nitric oxide-mediated vasodilatation

Endothelial dysfunction, which is defined by decreased endothelium-dependent vasodilatation, is associated with an increased number of cardiovascular events. Nitric oxide (NO) bioavailability is reduced by altered endothelial signal transduction or increased formation of radical oxygen species reacting with NO. Endothelial dysfunction is therapeutically reversible and physical exercise, calcium channel blockers, angiotensin converting enzyme inhibitors, and angiotensin receptor antagonists improve flow-evoked endothelium-dependent vasodilation in patients with hypertension and diabetes. We have investigated three different approaches, with the aim of correcting endothelial dysfunction in cardiovascular disease. Thus, (1) we evaluated the effect of a cell permeable superoxide dismutase mimetic, tempol, on endothelial dysfunction in small arteries exposed to high pressure, (2) investigated the endothelial signal transduction pathways involved in vasorelaxation and NO release induced by an olive oil component, oleanolic acid, and (3) investigated the role of calcium-activated K channels in the release of NO induced by receptor activation. Tempol increases endothelium-dependent vasodilatation in arteries from hypertensive animals most likely through the lowering of radical oxygen species, but other mechanisms also appear to contribute to the effect. While oleanolic acid leads to the release of NO by calcium-independent phosphorylation of endothelial NO synthase, endothelial calcium-activated K channels and an influx of calcium play an important role in G-protein coupled receptor-evoked release of NO. Thus, all three approaches increase bioavailability of NO in the vascular wall, but it remains to be addressed whether these actions have any direct benefit at a clinical level.

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.

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.

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.

Effects of increased intracellular Cl- concentration on membrane responses to acetylcholine in the isolated endothelium of guinea pig mesenteric arteries

ACh-induced membrane responses in vascular endothelial cells that have been reported vary between preparations from a sustained hyperpolarization to a transient hyperpolarization followed by a depolarization; the reason for this variation is unknown. Using the perforated whole-cell clamp technique, we investigated ACh-induced membrane currents in freshly isolated endothelial layers having a resting membrane potential of less negative than -10 mV. A group of cells was electrically isolated using a wide-bore micropipette, and their membrane potential was well controlled. ACh activated K(+) and Cl(-) currents simultaneously. The K(+) current was blocked by a combination of charybdotoxin and apamin and appears to result from the opening of IK(Ca) and SK(Ca) channels. The Cl(-) current was partially blocked by tamoxifen, niflumic acid, or DIDS and appears to be produced by Ca(2+)-activated Cl(-) channels. When the pipettes contained 20 mM Cl(-), the ACh-induced K(+) conductance started decreasing during a 1-min application of ACh while the Cl(-) conductance continued, making the ACh-induced hyperpolarization sustained. When the pipettes contained 150 mM Cl(-), both conductances started decreasing during a 1-min application of ACh, making the ACh-induced hyperpolarization small and transient. [Cl(-)](i) is very likely modified by experimental procedures such as the cell isolation and the intracellular dialysis with the pipette solution. Such a variability in [Cl(-)](i) may be one of the reasons for the variations in the ACh-induced membrane response.

Reduced hyperpolarization in endothelial cells of rabbit aortic valve following chronic nitroglycerine administration

This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the hyperpolarization induced by acetylcholine (ACh) in aortic valve endothelial cells (AVECs) of the rabbit and, if so, whether antioxidant agents can normalize this downregulated hyperpolarization. ACh (0.03-3 microM) induced a hyperpolarization through activations of both apamin- and charybdotoxin-sensitive Ca2+-activated K+ channels (K(Ca)) in rabbit AVECs. The intermediate-conductance K(Ca) channel (IK(Ca)) activator 1-ethyl-2-benzimidazolinone (1-EBIO, 0.3 mM) induced a hyperpolarization of the same magnitude as ACh (3 microM). The ACh-induced hyperpolarization was significantly weaker, although the ACh-induced [Ca2+]i increase was unchanged, in NTG-treated rabbits (versus NTG-untreated control rabbits). The hyperpolarization induced by 1-EBIO was also weaker in NTG-treated rabbits. The reduced ACh-induced hyperpolarization seen in NTG-treated rabbits was not modified by in vitro application of the superoxide scavengers Mn-TBAP, tiron or ascorbate, but it was normalized when ascorbate was coadministered with NTG in vivo. Superoxide production within the endothelial cell (estimated by ethidium fluorescence) was increased in NTG-treated rabbits and this increased production was normalized by in vivo coadministration of ascorbate with the NTG. It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced hyperpolarization in rabbit AVECs, possibly through chronic actions mediated by superoxide.

Combination of Ca2+ -activated K+ channel blockers inhibits acetylcholine-evoked nitric oxide release in rat superior mesenteric artery

BACKGROUND AND PURPOSE

The present study investigated whether calcium-activated K+ channels are involved in acetylcholine-evoked nitric oxide (NO) release and relaxation.

EXPERIMENTAL APPROACH

Simultaneous measurements of NO concentration and relaxation were performed in rat superior mesenteric artery and endothelial cell membrane potential and intracellular calcium ([Ca2+]i) were measured.

KEY RESULTS

A combination of apamin plus charybotoxin, which are, respectively, blockers of small-conductance and of intermediate- and large-conductance Ca2+ -activated K channels abolished acetylcholine (10 microM)-evoked hyperpolarization of endothelial cell membrane potential. Acetylcholine-evoked NO release was reduced by 68% in high K+ (80 mM) and by 85% in the presence of apamin plus charybdotoxin. In noradrenaline-contracted arteries, asymmetric dimethylarginine (ADMA), an inhibitor of NO synthase inhibited acetylcholine-evoked NO release and relaxation. However, only further addition of oxyhaemoglobin or apamin plus charybdotoxin eliminated the residual acetylcholine-evoked NO release and relaxation. Removal of extracellular calcium or an inhibitor of calcium influx channels, SKF96365, abolished acetylcholine-evoked increase in NO concentration and [Ca2+]i. Cyclopiazonic acid (CPA, 30 microM), an inhibitor of sarcoplasmic Ca2+ -ATPase, caused a sustained NO release in the presence, but only a transient increase in the absence, of extracellular calcium. Incubation with apamin and charybdotoxin did not change acetylcholine or CPA-induced increases in [Ca2+]i, but inhibited the sustained NO release induced by CPA.

CONCLUSIONS AND IMPLICATIONS

Acetylcholine increases endothelial cell [Ca2+]i by release of stored calcium and calcium influx resulting in activation of apamin and charybdotoxin-sensitive K channels, hyperpolarization and release of NO in the rat superior mesenteric artery.

Impaired small-conductance Ca2+-activated K+ channel-dependent EDHF responses in Type II diabetic ZDF rats

We have examined the relative contributions of small- and intermediate-conductance Ca(2+)-activated K(+) channels (SK(Ca) and IK(Ca)) to the endothelium-derived hyperpolarizing factor (EDHF) pathway response in small mesenteric arteries of Zucker Diabetic Fatty (ZDF) rats, before and after the development of Type II diabetes, together with Lean controls. Smooth muscle membrane potential was recorded using sharp microelectrodes in the presence of 10 microM indomethacin plus 100 microM N(omega)-nitro-L-arginine. SK(Ca) was selectively inhibited with 100 nM apamin, whereas IK(Ca) was blocked with 10 microM TRAM-39 (2-(2-chlorophenyl)-2,2-diphenylacetonitrile). Resting membrane potentials were similar in arteries from 17- to 20-week-old control and diabetic rats (approximately -54 mV). Responses elicited by 1 and 10 microM acetylcholine (ACh) were significantly smaller in the diabetic group (e.g. hyperpolarizations to -69.5 +/- 0.8 mV (ZDF; n = 12) and -73.2 +/- 0.6 mV (Lean; n = 12; P < 0.05) evoked by 10 microM ACh). The IK(Ca)-mediated components of the ACh responses were comparable between groups (hyperpolarizations to approximately -65 mV on exposure to 10 microM ACh). However, SK(Ca)-mediated responses were significantly reduced in the diabetic group (hyperpolarizations to -63.1 +/- 1.0 mV (ZDF; n = 6) and -71.5 +/- 1.2 mV (Lean; n = 6; P < 0.05) on exposure to 10 microM ACh. Impaired ACh responses were not observed in arteries from 5- to 6-week-old (pre-diabetic) animals. SK(Ca) subunit mRNA expression was increased in the diabetic group. The EDHF pathway, especially the SK(Ca)-mediated response, is impaired in Type II diabetic ZDF rats without a reduction in channel gene expression. These results may be particularly relevant to the microvascular complications of diabetes. The functional separation of SK(Ca) and IK(Ca) pathways is discussed.

Endothelium-derived hyperpolarizing factor: where are we now?

The endothelium controls vascular tone not only by releasing nitric oxide (NO) and prostacyclin but also by other pathways causing hyperpolarization of the underlying smooth muscle cells. This characteristic was at the origin of the denomination endothelium-derived hyperpolarizing factor (EDHF). We know now that this acronym includes different mechanisms. In general, EDHF-mediated responses involve an increase in the intracellular calcium concentration, the opening of calcium-activated potassium channels of small and intermediate conductance and the hyperpolarization of the endothelial cells. This results in an endothelium-dependent hyperpolarization of the smooth muscle cells, which can be evoked by direct electrical coupling through myo-endothelial junctions and/or the accumulation of potassium ions in the intercellular space. Potassium ions hyperpolarize the smooth muscle cells by activating inward rectifying potassium channels and/or Na+/K(+)-ATPase. In some blood vessels, including large and small coronary arteries, the endothelium releases arachidonic acid metabolites derived from cytochrome P450 monooxygenases. The epoxyeicosatrienoic acids (EET) generated are not only intracellular messengers but also can diffuse and hyperpolarize the smooth muscle cells by activating large conductance calcium-activated potassium channels. Additionally, the endothelium can produce other factors such as lipoxygenases derivatives or hydrogen peroxide (H2O2). These different mechanisms are not necessarily exclusive and can occur simultaneously.

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.

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.

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

1. The possibility that thromboxane (TXA(2)) receptor stimulation causes differential block of the SK(Ca) and IK(Ca) channels which underlie EDHF-mediated vascular smooth muscle hyperpolarization and relaxation was investigated in the rat isolated mesenteric artery. 2. Acetylcholine (30 nm-3 microm ACh) or cyclopiazonic acid (10 microm CPA, SERCA inhibitor) were used to stimulate EDHF-evoked smooth muscle hyperpolarization. In each case, this led to maximal hyperpolarization of around 20 mV, which was sensitive to block with 50 nm apamin and abolished by repeated stimulation of mesenteric arteries with the thromboxane mimetic, U46619 (30 nm-0.1 microm), but not the alpha(1)-adrenoceptor agonist phenylephrine (PE). 3. The ability of U46619 to abolish EDHF-evoked smooth muscle hyperpolarization was prevented by prior exposure of mesenteric arteries to the TXA(2) receptor antagonist 1 microm SQ29548. 4. Similar-sized smooth muscle hyperpolarization evoked with the SK(Ca) activator 100 microm riluzole was also abolished by prior stimulation with U46619, while direct muscle hyperpolarization in response to either levcromakalim (1 microm, K(ATP) activator) or NS1619 (40 microm, BK(Ca) activator) was unaffected. 5. During smooth muscle contraction and depolarization to either PE or U46619, ACh evoked concentration-dependent hyperpolarization (to -67 mV) and complete relaxation. These responses were well maintained during repeated stimulation with PE, but with U46619 there was a progressive decline, so that during a third exposure to U46619 maximum hyperpolarization only reached -52 mV and relaxation was reduced by 20%. This relaxation could now be blocked with charybdotoxin alone. The latter responses could be mimicked with 300 microm 1-EBIO (IK(Ca) activator), an action not modified by exposure to U46619. 6. An early consequence of TXA(2) receptor stimulation is a reduction in the arterial hyperpolarization and relaxation attributed to EDHF. This effect appears to reflect a loss of SK(Ca) activity.

Chronic treatment of male rats with daidzein and 17 beta-oestradiol induces the contribution of EDHF to endothelium-dependent relaxation

1. We investigated the effect of chronic (7 days) treatment of male rats with the isoflavone daidzein (0.2 mg kg(-1) sc per day) or 17beta-oestradiol (0.1 mg kg(-1) sc per day) on the contribution of nitric oxide (NO), prostaglandins and endothelium-derived hyperpolarising factor (EDHF) to endothelium-dependent relaxation of isolated aortic rings. 2. The sensitivity and maximum relaxation to acetylcholine (ACh) were significantly greater in aortic rings from rats treated with daidzein or 17beta-oestradiol, in comparison to vehicle-treated rats. Inhibition of nitric oxide synthase with N-nitro-l-arginine (l-NOARG) abolished ACh-induced relaxation in the aortae from vehicle-treated rats, but only attenuated relaxation in aortae from daidzein or 17beta-oestradiol-treated rats. The presence of haemoglobin in addition to l-NOARG did not cause any further inhibition of relaxation. 3. The cyclooxygenase inhibitor indomethacin had no effect on endothelium-dependent relaxation in aortae from any treatment group. Charybdotoxin (ChTX), which blocks large-conductance calcium-activated potassium channels (BK(Ca)) and intermediate-conductance calcium-activated potassium channels (IK(Ca)), plus apamin, which blocks small-conductance calcium-activated potassium channels (SK(Ca)), but not iberiotoxin, which only blocks BK(Ca), attenuated endothelium-dependent relaxation of aortae from daidzein or 17beta-oestradiol-treated rats. Blockade of K(Ca) channels had no effect on the responses to ACh in aortae from vehicle-treated rats. In aortae from daidzein- or 17beta-oestradiol-treated rats, endothelium-dependent relaxation was also attenuated by inhibition of cytochrome P450 (CYP450) epoxygenase with 6-(2-propargylloxyphenyl)hexanoic acid (PPOH) or inhibition of K(IR) channels and Na(+)/K(+)-ATPase with barium and oubain, respectively. 4. This study demonstrates that endothelium-dependent relaxation of male rat aorta is normally entirely mediated by NO, whereas treatment with daidzein or 17beta-oestradiol stimulates a contribution from a non-NO, nonprostaglandin factor acting through the opening of SK(Ca) and IK(Ca) channels, and involving activation of Na/K-ATPase, K(IR) and CYP450 epoxygenase. This pattern of sensitivity to the tested inhibitors is consistent with the contribution of EDHF to relaxation. Thus, EDHF contributes to the enhanced endothelium-dependent relaxation that is observed after chronic treatment with the phytoestrogen daidzein or with 17beta-oestradiol.

Different modulation by Ca2+-activated K+ channel blockers and herbimycin of acetylcholine- and flow-evoked vasodilatation in rat mesenteric small arteries

1. The present study addressed whether endothelium-dependent vasodilatation evoked by acetylcholine and flow are mediated by the same mechanisms in isolated rat mesenteric small arteries, suspended in a pressure myograph for the measurement of internal diameter. 2. In pressurized arterial segments contracted with U46619 in the presence of indomethacin, shear stress generated by the flow evoked relaxation. Thus, in endothelium-intact segments low (5.1+/-0.6 dyn cm(-2)) and high (19+/-2 dyn cm(-2)) shear stress evoked vasodilatations that were reduced by, respectively, 68+/-11 and 68+/-8% (P<0.05, n=7) by endothelial cell removal. Acetylcholine (0.01-1 microM) evoked concentration-dependent vasodilatation that was abolished by endothelial cell removal. 3. Incubation with indomethacin alone did not change acetylcholine and shear stress-evoked vasodilatation, while the combination of indomethacin with the nitric oxide (NO) synthase inhibitor, N(G),N(G)-asymmetric dimethyl-L-arginine (ADMA 1 mM), reduced low and high shear stress-evoked vasodilatation with, respectively, 52+/-15 and 58+/-10% (P<0.05, n=9), but it did not change acetylcholine-evoked vasodilatation. 4. Inhibition of Ca(2+)-activated K(+) channels with a combination of apamin (0.5 microM) and charybdotoxin (ChTX) (0.1 microM) did not change shear stress- and acetylcholine-evoked vasodilatation. In the presence of indomethacin and ADMA, the combination of apamin (0.5 microM) and ChTx (0.1 microM) increased contraction induced by U46619, but these blockers did not change the vasodilatation evoked by shear stress. In contrast, acetylcholine-evoked vasodilatation was abolished by the combination of apamin and charybdotoxin. 5. In the presence of indomethacin, the tyrosine kinase inhibitor, herbimycin A (1 microM), inhibited low and high shear stress-evoked vasodilatation with, respectively, 32+/-12 and 68+/-14% (P<0.05, n=8), but it did not change vasodilatation induced by acetylcholine. In the presence of indomethacin and ADMA, herbimycin A neither changed shear stress nor acetylcholine-evoked vasodilatation. 6. The present study suggests that Ca(2+)-activated K(+) channels sensitive for the combination of apamin and ChTx are involved in acetylcholine-evoked, mainly non-NO nonprostanoid factor-mediated, vasodilatation, while an Src tyrosine kinase plays a role for flow-evoked NO-mediated vasodilatation in rat mesenteric small arteries.

[Recent advances in the study of endothelium-dependent hyperpolarizing factor (EDHF)]

Recent advances in the properties and physiological functions of endothelium-dependent hyperpolarizing factor (EDHF) in vascular tissues were reviewed briefly. The EDHF-induced hyperpolarization is inhibited by charybdotoxin, indicating that the potential is produced mainly by activation of intermediate conductance Ca-sensitive K-channels. During generation of EDHF responses, endothelial Ca2+ concentration was elevated, suggesting that the activated K-channels were distributed on the endothelial membrane. This was confirmed by direct recording of membrane potentials from endothelial and smooth muscle cells using double patch electrodes. Measurement of the propagation of potentials applied to endothelial or smooth muscle cells to surrounding cells revealed that there were tight electrical connections between endothelial cells much more than between endothelial and smooth muscle cells or between smooth muscle cells, and these observations yielded a possible spread of electrical signal along the endothelial layer first, and then the signals would be conducted to smooth muscle cell layers. These properties of vascular tissues allow speculating that EDHF is an electrical signal propagated from endothelial cells electrotonically through myoendothelial gap junctions. Several candidates have been proposed as EDHF, and possibilities of individual substances for EDHF were discussed. The cellular mechanism of the hyperpolarization-induced vasodilatation remains unclear, and this should be clarified in the future for further understanding of the EDHF-induced vasodilatation.

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.

Reduced function of endothelial prostacyclin in human omental resistance arteries in pre-eclampsia

It remains unclear in pre-eclampsia whether or not a functional change occurs in the role played by prostacyclin in endothelium-dependent relaxation in resistance arteries. We examined this using human omental resistance arteries (obtained from pre-eclamptic or normotensive pregnant women) in the presence of N(G)-nitro-L-arginine (L-NNA, an inhibitor of nitric oxide synthase). In endothelium-intact strips from both groups, 9,11-epithio-11,12-methano-thromboxane A(2) (STA(2), a thromboxane A(2) mimetic) produced a contraction. Diclofenac (an inhibitor of cyclooxygenase) enhanced the STA(2) contraction only in the normotensive pregnant group (1.4 times control, P < 0.01). In the presence of STA(2), bradykinin (0.1 microM) produced an endothelium-dependent relaxation in both groups, the relaxation being significantly smaller for the pre-eclamptic group (P < 0.002). Diclofenac significantly attenuated the bradykinin-induced relaxation only for the normotensive pregnant group (31 % inhibition, P < 0.001). The bradykinin-induced membrane hyperpolarization consisted of diclofenac-sensitive and -insensitive components. The former, but not the latter, was significantly smaller in pre-eclampsia (-4.3 vs. -2.6 mV, P < 0.05). The concentrations of 6-keto-PGF(1alpha) (a stable metabolite of prostacyclin) in these arteries were significantly lower in pre-eclampsia in both the absence and presence of bradykinin (about 0.2-0.4 times the normotensive pregnant value in each case, P < 0.01). By contrast, both the relaxation and the membrane hyperpolarization in response to beraprost (10 nM, a stable analogue of prostacyclin) were similar between the two groups. We conclude that, in pre-eclampsia, a reduced part is played by prostaglandins in the endothelium-dependent relaxation seen in resistance arteries and that this may be due to a reduced production of prostacyclin by the endothelium.

Endothelium-dependent hyperpolarization to acetylcholine in carotid artery of guinea pig: role of lipoxygenase

This study was designed to determine whether lipoxygenase-dependent metabolites of arachidonic acid are involved in the endothelium-dependent hyperpolarization of the guinea pig carotid artery. The membrane potential of vascular smooth muscle cells was measured with intracellular microelectrodes and potassium channels were studied on freshly isolated cells with the patch-clamp technique. Acetylcholine-induced hyperpolarizations were not affected by arachidonyl trifluoromethyl ketone (AACOCF3), quinacrine (phospholipase A inhibitors), or eicosatetraenoic acid (nonspecific inhibitor of lipoxygenase, cytochrome P450, and cyclooxygenase). In contrast, cinnamyl-3,4 dihydroxy-alpha-cyanocinnamate (CDC) and AA861 (lipoxygenase inhibitors) as well as 1-(6-(17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino) hexyl)-1H-pyrrole-2,5-dione (U-73122) (phospholipase C inhibitor) produced a significant inhibition of the hyperpolarization. An opener of intermediate conductance calcium-activated potassium channels, 1-ethyl-2-benzamidazolinone (1-EBIO), induced a hyperpolarization that was unaffected by AACOCF3, CDC, AA861, or U-73122 but was inhibited by charybdotoxin. (+/-)12-hydroxy-eicosatetraenoic acid (12-HETE) and 12(S)-hydroperoxy-eicosatetraenoic acid (12(S)-HpETE) did not induce any significant changes in membrane potential. CDC inhibited the voltage-gated potassium current and increased the large conductance calcium-activated potassium current whereas AA861 inhibited both potassium currents. These results confirm that, in the isolated carotid artery of the guinea pig, stimulation of endothelial muscarinic receptors involves phospholipase C activation and indicate that the activation of phospholipase A2 and the release of lipoxygenase metabolites is unlikely to explain endothelium-dependent hyperpolarization.

Beta 3-adrenoceptor in rat aorta: molecular and biochemical characterization and signalling pathway

1. We have previously demonstrated that beta(3)-adrenoceptor (beta(3)-AR) stimulation induces endothelium-dependent vasorelaxation in rat aorta through the activation of an endothelial NO synthase associated with an increase in intracellular cGMP. The aim of the present study was to localise beta(3)-AR to confirm our functional study and to complete the signalling pathway of beta(3)-AR in rat aorta. 2. By RT-PCR, we have detected beta(3)-AR transcripts both in aorta and in freshly isolated endothelial cells. The absence of markers for adipsin or hormone-sensitive lipase in endothelial cells excluded the presence of beta(3)-AR from adipocytes. The localization of beta(3)-AR in aortic endothelial cells was confirmed by immunohistochemistry using a rat beta(3)-AR antibody. 3. To identify the G protein linked to beta(3)-AR, experiments were performed in rat pre-treated with PTX (10 microg kg(-1)), a G(i/0) protein inhibitor. The blockage of G(i/0) protein by PTX was confirmed by the reduction of vasorelaxation induced by UK 14304, a selective alpha(2)-AR agonist. The cumulative concentration-response curve for SR 58611A, a beta(3)-AR agonist, was not significantly modified on aorta rings from PTX pre-treated rats. 4. At the same level of contraction, the relaxations induced by 10 microM SR 58611A were significantly reduced in 30 mM-KCl pre-constricted rings (E(max)=16.7+/-8.4%, n=5), in comparison to phenylephrine (0.3 microM) pre-constricted rings (E(max)=49.11+/-11.0%, n=5, P<0.05). In addition, iberotoxin (0.1 microM), glibenclamide (1 microM) and 4-aminopyridine (1 mM), selective potassium channels blockers of K(Ca), K(ATP), and K(v) respectively, decreased the SR 58611A-mediated relaxation. 5. We conclude that beta(3)-AR is preferentially expressed in rat aortic endothelial cells. Beta(3)-AR-mediated aortic relaxation is independent of G(i/0) proteins stimulation, but results from the activation of several potassium channels, K(Ca), K(ATP), and K(v).

Vasorelaxing effect of mesaconitine, an alkaloid from Aconitum japonicum, on rat small gastric artery: possible involvement of endothelium-derived hyperpolarizing factor

Aconiti tuber, roots of aconite (Aconitum japonicum), has been used for centuries in Japan and China to increase peripheral body temperature. We previously reported that mesaconitine, an alkaloid from Aconitum japonicum, elicits endothelium-dependent and nitric oxide-mediated relaxation in isolated rat aorta. In the present study, we investigated the effect of mesaconitine on isolated rat small gastric arteries. Mesaconitine elicited a concentration-dependent (10, 30, 100 microM) vasorelaxation in isolated rat gastric artery precontracted with norepinephrine, which was resistant to N(omega)-nitro-L-arginine (L-NNA) (an inhibitor of nitric oxide synthase) and indomethacin (an inhibitor of cyclooxygenase). The L-NNA- and indomethacin-resistant relaxation by mesaconitine was mainly endothelium-dependent, inhibited by high K+ (30 mM), and inhibited by a combination of Ca2+-dependent K channel blockers, charybdotoxin and apamin. The relaxation by mesaconitine was proportional to the external Ca2+ concentration. These results suggest that mesaconitine elicits vasorelaxation of isolated rat small gastric artery mainly via release of endothelium-derived hyperpolarizing factor.

The effects of melatonin on Ca(2+) homeostasis in endothelial cells

The effect of melatonin on the Ca(2+) signaling process in bovine aortic endothelial cells (BAE) and in primary cultured vascular endothelial cells from normotensive Sprague Dawley (SDR) and genetically hypertensive (SHR) rats was investigated using the Ca(2+) indicator Fura-2. Acute applications of melatonin failed to initiate a Ca(2+) response in the three cell types considered. However, preincubating SHR aortic endothelial cells with exposure to melatonin increased the internal Ca(2+) release triggered by bradykinin (BK) and ATP while stimulating the related agonist-evoked Ca(2+) entry. This effect appeared specific for SHR cells, as a similar incubation period failed to alter the Ca(2+) responses in BAE and SDR cells. Because of the known overproduction of free radicals in SHR cells, the effect of melatonin on Ca(2+) signaling was also tested in SDR and BAE cells exposed to the superoxide anion radical. Melatonin reversed the deleterious action of free radicals on Ca(2+) signaling in both cases, suggesting that its stimulatory effect in SHR was linked to its antioxidative properties. Finally, experiments where melatonin was applied between successive BK stimulation periods showed an enhancement of the agonist-evoked Ca(2+) entry in BAE and SDR cells. This effect appeared to be independent of the production of second messengers as no specific binding sites for melatonin, including MT1, MT2 and MT3 receptors, could be detected in BAE cells. We conclude that melatonin improves Ca(2+) signaling in dysfunctional endothelial cells characterized by an overproduction of free radicals while stimulating the agonist-evoked Ca(2+) entry in normal endothelial cells through a mechanism not related to its antioxidative properties.

Charybdotoxin-sensitive small conductance K(Ca) channel activated by bradykinin and substance P in endothelial cells

1 In cultured porcine coronary artery endothelial cells, we have recently shown that substance P and bradykinin stimulated different types of Ca(2+)-dependent K(+) (K(Ca)) current. A large part of this current was insensitive to iberiotoxin and apamin. The aim of the present study was to characterize the K(Ca) channel responsible for this current. 2 In cell-attached configuration and asymmetrical K(+) concentration, 100 nM bradykinin or substance P activated a 10 pS K(+) channel. In inside-out configuration, the channel was half-maximally activated by 795 nM free Ca(2+). 3 Apamin (1 micro M) added to the pipette solution failed to inhibit the channel activity while charybdotoxin (50 nM), completely blocked it. Perfusion at the intracellular face of the cell, of an opener of intermediate conductance K(Ca) channel, 500 micro M 1-ethyl-benzimidazolinone (1-EBIO) increased the channel activity by about 4.5 fold. 4 In whole-cell mode, bradykinin and substance P stimulated an outward K(+) current of similar amplitude. Charybdotoxin inhibited by 75% the bradykinin-induced current and by 80% the substance P-induced current. Charybdotoxin plus iberiotoxin (50 nM each) inhibited by 97% the bradykinin-response. Charybdotoxin plus apamin did not increase the inhibition of the substance P-response obtained in the presence of charybdotoxin alone. 5 1-EBIO activated a transient outward K(+) current and hyperpolarized the membrane potential by about 13 mV. Charybdotoxin reduced the hyperpolarization to about 3 mV. 6 Taken together these results show that bradykinin and substance P activate a 10 pS K(Ca) channel, which largely contributes to the total K(+) current activated by these agonists. Despite its small conductance, this channel shares pharmacological characteristics with intermediate conductance K(Ca) channels.