Role of Calcium-Activated Potassium Channels with Small Conductance in Bradykinin-Induced Vasodilation of Porcine Retinal Arterioles

The vasoactive effects of bradykinin, vasoactive intestinal peptide, calcitonin gene-related peptide and neuropeptide Y depend on the perivascular tissue in porcine retinal arterioles in vitro

PURPOSE

The retina contains a number of vasoactive neuropeptides and corresponding receptors, but the role of these neuropeptides for tone regulation of retinal arterioles has not been studied in detail.

METHODS

Porcine arterioles with preserved perivascular retinal tissue were mounted in a wire myograph, and the tone was measured after the addition of increasing concentrations of bradykinin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP) and brain natriuretic peptide (BNP). The experiments were performed during inhibition of the synthesis of nitric oxide (NO), prostaglandins and dopamine and were repeated after removal of the perivascular retinal tissue.

RESULTS

Bradykinin, VIP and CGRP induced significant concentration-dependent dilatation and NPY significant concentration-dependent contraction of the arterioles in the presence of perivascular retinal tissue (p < 0.03 for all comparisons) but not on isolated arterioles. BNP and SP had no effect on vascular tone. The NOS inhibitor L-NAME reduced bradykinin- and VIP-induced relaxation (p < 0.001 for both comparisons), whereas none of the other inhibitors influenced the vasoactive effects of the studied neuropeptides.

CONCLUSION

The effects of neuropeptides on the tone of retinal arterioles depend on the perivascular retinal tissue and may involve effects other than those mediated by nitric oxide, prostaglandins and adrenergic compounds. Investigation of the mechanisms underlying the vasoactive effect of neuropeptides may be important for understanding and treating retinal diseases where disturbances in retinal flow regulation are involved in the disease pathogenesis.

Inhibition of mitochondrial reactive oxygen species improves coronary endothelial function after cardioplegic hypoxia/reoxygenation

OBJECTIVE

Cardioplegic ischemia-reperfusion and diabetes mellitus are correlated with coronary endothelial dysfunction and inactivation of small conductance calcium-activated potassium channels. Increased reactive oxidative species, such as mitochondrial reactive oxidative species, may contribute to oxidative injury. Thus, we hypothesized that inhibition of mitochondrial reactive oxidative species may protect coronary small conductance calcium-activated potassium channels and endothelial function against cardioplegic ischemia-reperfusion-induced injury.

METHODS

Small coronary arteries and endothelial cells from the hearts of mice with and without diabetes mellitus were isolated and examined by using a cardioplegic hypoxia and reoxygenation model to determine whether the mitochondria-targeted antioxidant Mito-Tempo could protect against coronary endothelial and small conductance calcium-activated potassium channel dysfunction. The microvessels or mouse heart endothelial cells were treated with or without Mito-Tempo (0-10 μM) 5 minutes before and during cardioplegic hypoxia and reoxygenation. Microvascular function was assessed in vitro by vessel myography. K+ currents of mouse heart endothelial cells were measured by whole-cell patch clamp. The levels of intracellular cytosolic free calcium (Ca2+) concentration, mitochondrial reactive oxidative species, and small conductance calcium-activated potassium protein expression of mouse heart endothelial cells were measured by Rhod-2 fluorescence staining, MitoSox, and Western blotting, respectively.

RESULTS

Cardioplegic hypoxia and reoxygenation significantly attenuated endothelial small conductance calcium-activated potassium channel activity, caused calcium overload, and increased mitochondrial reactive oxidative species of mouse heart endothelial cells in both the nondiabetic and diabetes mellitus groups. In addition, treating mouse heart endothelial cells with Mito-Tempo (10 μM) reduced cardioplegic hypoxia and reoxygenation-induced Ca2+ and mitochondrial reactive oxidative species overload in both the nondiabetic and diabetes mellitus groups, respectively (P < .05). Treatment with Mito-Tempo (10 μM) significantly enhanced coronary relaxation responses to adenosine 5'-diphosphate and NS309 (P < .05), and endothelial small conductance calcium-activated potassium channel currents in both the nondiabetic and diabetes mellitus groups (P < .05).

CONCLUSIONS

Administration of Mito-Tempo improves endothelial function and small conductance calcium-activated potassium channel activity, which may contribute to its enhancement of endothelium-dependent vasorelaxation after cardioplegic hypoxia and reoxygenation.

Effect of Rho Kinase Inhibitor Ripasudil (K-115) on Isolated Porcine Retinal Arterioles

Purpose: To investigate the vasorelaxation effect of ripasudil (K-115), a novel Rho-associated coiled-coil-containing protein kinase (ROCK) inhibitor, on isolated retinal arterioles. We determined whether the actions of ripasudil on the retinal microvascular diameter were dependent on the endothelium and/or potassium channels in the smooth muscle, with the goals of uncovering the signaling mechanisms required for this vasomotor activity and inhibiting the action of endothelin-1 (ET-1). Methods: In this in vitro study, we isolated porcine retinal arterioles, which were cannulated and pressurized without flow. We recorded diametric changes using videomicroscopic techniques. Results: In a dose-dependent (10 nM-30 μM) manner, retinal arterioles were relaxed in response to ripasudil [maximum % resting diameter, 160.3% ± 7.7% (mean ± standard error of the mean)]. The ripasudil-induced vasorelaxation was unaffected by endothelium removal, using nonselective potassium channel blocker tetraethylammonium, Ca²⁺-activated large-conductance potassium channel blocker iberiotoxin, voltage-gated potassium channel blocker 4-AP, ATP-sensitive potassium channel blocker glibenclamide, and inward rectifier potassium channel blocker BaCl₂. Ripasudil prevented ET-1-caused vasoconstriction of the retinal arterioles regardless of the presence of endothelium to a similar extent. Conclusion: The ROCK inhibitor ripasudil elicits endothelium-independent relaxation and inhibits the action of ET-1 on the retinal arterioles. Determining the relaxation properties of ripasudil on the retinal microvasculature will likely support the development of potential therapies for glaucoma.

Investigating the effect of some fluoroquinolones on C-reactive protein levels and ACh-Induced blood pressure reduction deviations after aging of diabetes in STZ-Induced diabetic wistar rats

The treatment of infections in diabetic patients by fluoroquinolone antibiotics is associated with a reduced risk of coronary artery disease, and may improve endothelium-derived hyperpolarizing factor (EDHF) efficacy. The inflammatory marker C-reactive protein (CRP) is an important predictor of cardiovascular events, and vascular endothelium dysfunction, which makes this marker a target for drug-based treatment. This study aims to investigate the relation between the treatment by fluoroquinolones with CRP plasma levels, as well as acetylecholine (ACh)-induced small conductance calcium-activated potassium channels (SK_Ca)-dependent blood pressure (BP) reduction deviations in wistar rats after inducing a type 2-like diabetes with aging state after four months of streptozotocin (STZ) injection. Experimental animals were divided into four groups, group 1: diabetic animals were treated with moxifloxacin (n = 15), group 2: diabetic animals were treated with levofloxacin (n = 15), group 3: diabetic control animals (n = 15), and group 4: non-diabetic control animals (n = 6). The levels of plasma CRP, as well as ACh-induced SK_Ca-dependent BP reduction deviations were compared four months after the development of diabetes, after that; two groups were treated with fluoroquinolones, four months after the treatment; CRP-plasma levels, as well as ACh-induced SK_Ca-dependent BP reduction deviations were also evaluated and compared for all groups. Sustained hyperglycemia after the induction of diabetes elevated CRP plasma levels, and reduced ACh-induced SK_Ca-dependent BP reduction, observed diabetes-induced variations were minimal in fluoroquinolones treated diabetic groups compared with diabetic control group, In conclusion, the treatment with fluoroquinolone antibiotics in diabetic wistars may be associated with a lowering in CRP levels progression, and improvement in SK_Ca vitality, which indicates the importance of treating infections in diabetics by fluoroquinolones to mitigate some vascular complications signs that lead to morbidity and mortality in diabetes.

Short-Time Ocular Ischemia Induces Vascular Endothelial Dysfunction and Ganglion Cell Loss in the Pig Retina

Visual impairment and blindness are often caused by retinal ischemia-reperfusion (I/R) injury. We aimed to characterize a new model of I/R in pigs, in which the intraocular pathways were not manipulated by invasive methods on the ocular system. After 12 min of ischemia followed by 20 h of reperfusion, reactivity of retinal arterioles was measured in vitro by video microscopy. Dihydroethidium (DHE) staining, qPCR, immunohistochemistry, quantification of neurons in the retinal ganglion cell layer, and histological examination was performed. Retinal arterioles of I/R-treated pigs displayed marked attenuation in response to the endothelium-dependent vasodilator, bradykinin, compared to sham-treated pigs. DHE staining intensity and messenger RNA levels for HIF-1α, VEGF-A, NOX2, and iNOS were elevated in retinal arterioles following I/R. Immunoreactivity to HIF-1α, VEGF-A, NOX2, and iNOS was enhanced in retinal arteriole endothelium after I/R. Moreover, I/R evoked a substantial decrease in Brn3a-positive retinal ganglion cells and noticeable retinal thickening. In conclusion, the results of the present study demonstrate that short-time ocular ischemia impairs endothelial function and integrity of retinal blood vessels and induces structural changes in the retina. HIF-1α, VEGF-A, iNOS, and NOX2-derived reactive oxygen species appear to be involved in the pathophysiology.

Translational research in retinal vascular disease. An approach

The clinical presentation of the most frequent vision threatening retinal diseases is dominated by lesions indicating that disturbances in retinal blood flow are involved in the pathogenesis of these diseases. The present review describes the experience from a translational strategy pursued to investigate retinal vascular diseases with diabetic retinopathy as the main object. The normal regulation of retinal blood flow is investigated in porcine retinal vessels in vitro and ex vivo. Subsequently, the in vitro findings are translated to clinical studies in normal persons in vivo, and it is investigated whether the mechanisms are disturbed in retinal vascular disease. This is followed by clinical intervention studies on these diseases. The approach has been used to investigate pressure autoregulation, metabolic autoregulation and vasomotion in retinal vessels. The investigations have shown that retinal vascular tone can be regulated by receptor-specific agonists and antagonists to vasoactive compounds such as purines, prostaglandins and nitric oxide synthesis and that the vasoactive effects can be modulated by the concentration and the mode of administration of these compounds. Additionally, it has been shown that retinal precapillary arterioles and capillaries not visible by ophthalmoscopy may play an important role for the pathophysiology of retinal vascular disease and its treatment. Future studies should focus on investigating normal and pathological regulation of retinal blood flow in these smaller vessels.

Extracellular l-arginine Enhances Relaxations Induced by Opening of Calcium-Activated SKCa Channels in Porcine Retinal Arteriole

We investigated whether the substrate for nitric oxide (NO) production, extracellular l-arginine, contributes to relaxations induced by activating small (SKCa) conductance Ca²⁺-activated potassium channels. In endothelial cells, acetylcholine increased ³H-l-arginine uptake, while blocking the SKCa and the intermediate (IKCa) conductance Ca²⁺-activated potassium channels reduced l-arginine uptake. A blocker of the y+ transporter system, l-lysine also blocked ³H-l-arginine uptake. Immunostaining showed co-localization of endothelial NO synthase (eNOS), SKCa3, and the cationic amino acid transporter (CAT-1) protein of the y+ transporter system in the endothelium. An opener of SKCa channels, cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA) induced large currents in endothelial cells, and concentration-dependently relaxed porcine retinal arterioles. In the presence of l-arginine, concentration-response curves for CyPPA were leftward shifted, an effect unaltered in the presence of low sodium, but blocked by l-lysine in the retinal arterioles. Our findings suggest that SKCa channel activity regulates l-arginine uptake through the y+ transporter system, and we propose that in vasculature affected by endothelial dysfunction, l-arginine administration requires the targeting of additional mechanisms such as SKCa channels to restore endothelium-dependent vasodilatation.

Calcium-Activated Potassium Channels: Potential Target for Cardiovascular Diseases

Ca(2+)-activated K(+) channels (KCa) are classified into three subtypes: big conductance (BKCa), intermediate conductance (IKCa), and small conductance (SKCa) KCa channels. The three types of KCa channels have distinct physiological or pathological functions in cardiovascular system. BKCa channels are mainly expressed in vascular smooth muscle cells (VSMCs) and inner mitochondrial membrane of cardiomyocytes, activation of BKCa channels in these locations results in vasodilation and cardioprotection against cardiac ischemia. IKCa channels are expressed in VSMCs, endothelial cells, and cardiac fibroblasts and involved in vascular smooth muscle proliferation, migration, vessel dilation, and cardiac fibrosis. SKCa channels are widely expressed in nervous and cardiovascular system, and activation of SKCa channels mainly contributes membrane hyperpolarization. In this chapter, we summarize the physiological and pathological roles of the three types of KCa channels in cardiovascular system and put forward the possibility of KCa channels as potential target for cardiovascular diseases.

Hypoxia-induced relaxation of porcine retinal arterioles in vitro depends on inducible NO synthase and EP4 receptor stimulation in the perivascular retina

PURPOSE

Hypoxia-induced relaxation of porcine retinal arterioles has been shown to be reduced during inhibition of prostaglandin synthesis and nitric oxide synthase (NOS). The purpose of this study was to identity the specific prostaglandin receptor(s) and source(s) of NO mediating this effect.

METHODS

Porcine retinal arterioles with preserved perivascular retinal tissue were mounted in a myograph and were exposed to hypoxia in the presence of one of the following: the general NO synthase inhibitor L-NAME, the selective iNOS inhibitor 1400W, the selective nNOS inhibitor 7-nitroindazole, the general cyclooxygenase (COX) inhibitor ibuprofen or an antagonist to the FP- (AL 8810), DP- (BWA868C), EP1 - (SC-19220), EP2 - (PF-044189) or EP4 receptors (GW627368X). The experiments were repeated after removal of the perivascular retinal tissue.

RESULTS

Hypoxia induced relaxation of retinal arterioles with preserved perivascular retinal tissue. This relaxation was significantly reduced in the presence of L-NAME, 1400W, ibuprofen and the EP4 receptor antagonist GW627368X. The simultaneous addition of L-NAME or 1400W in combination with ibuprofen, but not GW627368X, reduced hypoxia-induced vasorelaxation additively as compared to the effect of the compounds individually.

CONCLUSION

Hypoxia-induced vasorelaxation of porcine retinal arterioles is mediated by inducible NOS and stimulation of EP4 receptors acting through separate pathways, but mechanisms unrelated to the studied prostaglandin receptors and NOS products are also involved.

Sex differences in endothelial function in porcine coronary arteries: a role for H2O2 and gap junctions?

BACKGROUND AND PURPOSE

Cardiovascular risk is higher in men and postmenopausal women compared with premenopausal women. This may be due to sex differences in endothelial function. Here, sex differences in endothelial function of porcine coronary arteries (PCAs) were investigated.

EXPERIMENTAL APPROACH

Distal PCAs were studied under myographic conditions and after precontraction with U46619. Concentration-response curves to bradykinin were constructed in the presence of a range of inhibitors.

KEY RESULTS

In male and female PCAs, bradykinin produced comparable vasorelaxant responses. Inhibition of NO and prostanoid synthesis produced greater inhibition in males compared with females. Removing H2 O2 with PEG-catalase reduced the maximum relaxation in the absence, but not the presence of L-NAME and indomethacin in females, and had no effect in males. Blocking gap junctions with 100 µM carbenoxolone or 18α-glycyrrhetinic acid further inhibited the endothelium-derived hyperpolarization (EDH)-mediated response in females but not in males. In female PCAs, the maximum EDH-mediated response was reduced by inhibiting SKCa with apamin and by inhibiting IKCa with TRAM-34, or with both. In male PCAs, at maximum bradykinin concentration, the EDH-mediated response was reduced in the presence of apamin but not TRAM-34. Western blot did not detect any differences in connexins 40 or 43 or in IKCa expression between male and female PCAs.

CONCLUSIONS AND IMPLICATIONS

H2 O2 mediated some part of endothelium-dependent vasorelaxation in female PCAs and EDH was more important in females, with differences in the contribution of gap junctions and IKCa channels. These findings may contribute to understanding vascular protection in premenopausal women.

Activation of endothelial transient receptor potential C3 channel is required for small conductance calcium-activated potassium channel activation and sustained endothelial hyperpolarization and vasodilation of cerebral artery

Background

Transient receptor potential C3 (TRPC3) has been demonstrated to be involved in the regulation of vascular tone through endothelial cell (EC) hyperpolarization and endothelium‐dependent hyperpolarization–mediated vasodilation. However, the mechanism by which TRPC3 regulates these processes remains unresolved. We tested the hypothesis that endothelial receptor stimulation triggers rapid TRPC3 trafficking to the plasma membrane, where it provides the source of Ca²⁺ influx for small conductance calcium‐activated K⁺ (SK_Ca) channel activation and sustained EC hyperpolarization.

Methods and Results

Pressurized artery studies were performed with isolated mouse posterior cerebral artery. Treatment with a selective TRPC3 blocker (Pyr3) produced significant attenuation of endothelium‐dependent hyperpolarization–mediated vasodilation and endothelial Ca²⁺ response (EC‐specific Ca²⁺ biosensor) to intraluminal ATP. Pyr3 treatment also resulted in a reduced ATP‐stimulated global Ca²⁺ and Ca²⁺ influx in primary cultures of cerebral endothelial cells. Patch‐clamp studies with freshly isolated cerebral ECs demonstrated 2 components of EC hyperpolarization and K⁺ current activation in response to ATP. The early phase was dependent on intermediate conductance calcium‐activated K⁺ channel activation, whereas the later sustained phase relied on SK_Ca channel activation. The SK_Ca channel–dependent phase was completely blocked with TRPC3 channel inhibition or in ECs of TRPC3 knockout mice and correlated with increased trafficking of TRPC3 (but not SK_Ca channel) to the plasma membrane.

Conclusions

We propose that TRPC3 dynamically regulates SK_Ca channel activation through receptor‐dependent trafficking to the plasma membrane, where it provides the source of Ca²⁺ influx for sustained SK_Ca channel activation, EC hyperpolarization, and endothelium‐dependent hyperpolarization–mediated vasodilation.

Pulmonary hypertension in wild type mice and animals with genetic deficit in KCa2.3 and KCa3.1 channels

Objective

In vascular biology, endothelial K_Ca2.3 and K_Ca3.1 channels contribute to arterial blood pressure regulation by producing membrane hyperpolarization and smooth muscle relaxation. The role of K_Ca2.3 and K_Ca3.1 channels in the pulmonary circulation is not fully established. Using mice with genetically encoded deficit of K_Ca2.3 and K_Ca3.1 channels, this study investigated the effect of loss of the channels in hypoxia-induced pulmonary hypertension.

Approach and Result

Male wild type and K_Ca3.1^−/−/K_Ca2.3^T/T(+DOX) mice were exposed to chronic hypoxia for four weeks to induce pulmonary hypertension. The degree of pulmonary hypertension was evaluated by right ventricular pressure and assessment of right ventricular hypertrophy. Segments of pulmonary arteries were mounted in a wire myograph for functional studies and morphometric studies were performed on lung sections. Chronic hypoxia induced pulmonary hypertension, right ventricular hypertrophy, increased lung weight, and increased hematocrit levels in either genotype. The K_Ca3.1^−/−/K_Ca2.3^T/T(+DOX) mice developed structural alterations in the heart with increased right ventricular wall thickness as well as in pulmonary vessels with increased lumen size in partially- and fully-muscularized vessels and decreased wall area, not seen in wild type mice. Exposure to chronic hypoxia up-regulated the gene expression of the K_Ca2.3 channel by twofold in wild type mice and increased by 2.5-fold the relaxation evoked by the K_Ca2.3 and K_Ca3.1 channel activator NS309, whereas the acetylcholine-induced relaxation - sensitive to the combination of K_Ca2.3 and K_Ca3.1 channel blockers, apamin and charybdotoxin - was reduced by 2.5-fold in chronic hypoxic mice of either genotype.

Conclusion

Despite the deficits of the K_Ca2.3 and K_Ca3.1 channels failed to change hypoxia-induced pulmonary hypertension, the up-regulation of K_Ca2.3-gene expression and increased NS309-induced relaxation in wild-type mice point to a novel mechanism to counteract pulmonary hypertension and to a potential therapeutic utility of K_Ca2.3/K_Ca3.1 activators for the treatment of pulmonary hypertension.

Activation of endothelial and epithelial K(Ca) 2.3 calcium-activated potassium channels by NS309 relaxes human small pulmonary arteries and bronchioles

BACKGROUND AND PURPOSE

Small (K(Ca) 2) and intermediate (K(Ca) 3.1) conductance calcium-activated potassium channels (K(Ca) ) may contribute to both epithelium- and endothelium-dependent relaxations, but this has not been established in human pulmonary arteries and bronchioles. Therefore, we investigated the expression of K(Ca) 2.3 and K(Ca) 3.1 channels, and hypothesized that activation of these channels would produce relaxation of human bronchioles and pulmonary arteries.

EXPERIMENTAL APPROACH

Channel expression and functional studies were conducted in human isolated small pulmonary arteries and bronchioles. K(Ca) 2 and K(Ca) 3.1 currents were examined in human small airways epithelial (HSAEpi) cells by whole-cell patch clamp techniques.

RESULTS

While K(Ca) 2.3 expression was similar, K(Ca) 3.1 protein was more highly expressed in pulmonary arteries than bronchioles. Immunoreactive K(Ca) 2.3 and K(Ca) 3.1 proteins were found in both endothelium and epithelium. K(Ca) currents were present in HSAEpi cells and sensitive to the K(Ca) 2.3 blocker UCL1684 and the K(Ca) 3.1 blocker TRAM-34. In pulmonary arteries contracted by U46619 and in bronchioles contracted by histamine, the K(Ca) 2.3/ K(Ca) 3.1 activator, NS309, induced concentration-dependent relaxations. NS309 was equally potent in relaxing pulmonary arteries, but less potent in bronchioles, than salbutamol. NS309 relaxations were blocked by the K(Ca) 2 channel blocker apamin, while the K(Ca) 3.1 channel blocker, charybdotoxin failed to reduce relaxation to NS309 (0.01-1 µM).

CONCLUSIONS AND IMPLICATIONS

K(Ca) 2.3 and K(Ca) 3.1 channels are expressed in the endothelium of human pulmonary arteries and epithelium of bronchioles. K(Ca) 2.3 channels contributed to endo- and epithelium-dependent relaxations suggesting that these channels are potential targets for treatment of pulmonary hypertension and chronic obstructive pulmonary disease.

Dorzolamide-induced relaxation of intraocular porcine ciliary arteries in vitro depends on nitric oxide and the vascular endothelium

PURPOSE

Carbonic anhydrase inhibitors (CAIs) are used to reduce aqueous production in glaucoma, which includes a direct effect on the ciliary body. However, CAIs also affect ciliary blood flow, but the mechanisms of action of CAIs on the tone of intraocular ciliary arteries supplying the ciliary body have not been studied in detail.

MATERIALS AND METHODS

The intraocular part of porcine ciliary arteries was isolated and mounted in a wire myograph system for isometric tension recordings. After contraction with the thromboxane analogue U46619, the vasorelaxing effect of the CAIs acetazolamide, brinzolamide and dorzolamide was studied. Subsequently, the involvement of the carbonic anhydrase reaction and nitric oxide (NO) in dorzolamide-induced vasorelaxation was characterized.

RESULTS

All CAIs induced relaxation of contracted ciliary arteries, but the effect of dorzolamide was most pronounced. Dorzolamide-induced relaxation was unaffected by changes in pH and CO(2), and by removal of substrates to the carbonic anhydrase enzyme, but was abolished after inhibition of NO synthase and guanylyl cyclase and after removal of the vascular endothelium.

CONCLUSIONS

Dorzolamide-induced vasorelaxation of ciliary arteries is independent of changes in the substances involved in the carbonic anhydrase reaction, but depends on NO and the vascular endothelium. The mechanism of action of dorzolamide in ocular disease may involve an effect on vascular tone mediated by NO.

K(Ca)2 and k(ca)3 channels in learning and memory processes, and neurodegeneration

Calcium-activated potassium (K_Ca) channels are present throughout the central nervous system as well as many peripheral tissues. Activation of K_Ca channels contribute to maintenance of the neuronal membrane potential and was shown to underlie the afterhyperpolarization (AHP) that regulates action potential firing and limits the firing frequency of repetitive action potentials. Different subtypes of K_Ca channels were anticipated on the basis of their physiological and pharmacological profiles, and cloning revealed two well defined but phylogenetic distantly related groups of channels. The group subject of this review includes both the small conductance K_Ca2 channels (K_Ca2.1, K_Ca2.2, and K_Ca2.3) and the intermediate-conductance (K_Ca3.1) channel. These channels are activated by submicromolar intracellular Ca²⁺ concentrations and are voltage independent. Of all K_Ca channels only the K_Ca2 channels can be potently but differentially blocked by the bee-venom apamin. In the past few years modulation of K_Ca channel activation revealed new roles for K_Ca2 channels in controlling dendritic excitability, synaptic functioning, and synaptic plasticity. Furthermore, K_Ca2 channels appeared to be involved in neurodegeneration, and learning and memory processes. In this review, we focus on the role of K_Ca2 and K_Ca3 channels in these latter mechanisms with emphasis on learning and memory, Alzheimer’s disease and on the interplay between neuroinflammation and different neurotransmitters/neuromodulators, their signaling components and K_Ca channel activation.

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.

Opening of small and intermediate calcium-activated potassium channels induces relaxation mainly mediated by nitric-oxide release in large arteries and endothelium-derived hyperpolarizing factor in small arteries from rat

This study was designed to investigate whether calcium-activated potassium channels of small (SK(Ca) or K(Ca)2) and intermediate (IK(Ca) or K(Ca)3.1) conductance activated by 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) are involved in both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in large and small rat mesenteric arteries. Segments of rat superior and small mesenteric arteries were mounted in myographs for functional studies. NO was recorded using NO microsensors. SK(Ca) and IK(Ca) channel currents and mRNA expression were investigated in human umbilical vein endothelial cells (HUVECs), and calcium concentrations were investigated in both HUVECs and mesenteric arterial endothelial cells. In both superior (∼1093 μm) and small mesenteric (∼300 μm) arteries, NS309 evoked endothelium- and concentration-dependent relaxations. In superior mesenteric arteries, NS309 relaxations and NO release were inhibited by both N(G),N(G)-asymmetric dimethyl-l-arginine (ADMA) (300 μM), an inhibitor of NO synthase, and apamin (0.5 μM) plus 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) (1 μM), blockers of SK(Ca) and IK(Ca) channels, respectively. In small mesenteric arteries, NS309 relaxations were reduced slightly by ADMA, whereas apamin plus an IK(Ca) channel blocker almost abolished relaxation. Iberiotoxin did not change NS309 relaxation. HUVECs expressed mRNA for SK(Ca) and IK(Ca) channels, and NS309 induced increases in calcium, outward current, and NO release that were blocked by apamin and TRAM-34 or charybdotoxin. These findings suggest that opening of SK(Ca) and IK(Ca) channels leads to endothelium-dependent relaxation that is mediated mainly by NO in large mesenteric arteries and by EDHF-type relaxation in small mesenteric arteries. NS309-induced calcium influx appears to contribute to the formation of NO.

Retina evokes biphasic relaxations in retinal artery unrelated to endothelium, K(V), K(ATP), K(Ca) channels and methyl palmitate

Retinal relaxing factor (RRF) is suggested to be released from the retina and to contribute in the maintenance of retinal arterial tone. Herein, we aimed to clarify the effects of retinal tissue in isolated bovine retinal arteries in comparison with choroidal tissue and to evaluate the possible role of endothelium and potassium channels. In parallel, the effects of palmitic acid methyl ester (PAME), a putative vasodilator proposed to be released from the retina, was also examined. A piece of bovine retinal or choroidal tissue was placed within a close proximity on top of retinal arteries mounted in a wire myograph and precontracted with noradrenaline, prostaglandin F(2α), endothelin-1, thromboxane A(2) mimetic, U46619 or potassium (K(+)). To elucidate possible mechanisms in the effects of retinal tissue, retinal arteries were either deendothelized or incubated with inhibitors of endothelial vasodilators, i.e. nitric oxide (NO) and prostaglandins, or K(+) channels. Unlike the choroid, retinal tissue produced rapid, biphasic and complete relaxations in isolated bovine retinal arteries precontracted with various spasmogens acting on distinct receptors. Endothelium removal or preincubation of retinal arteries with inhibitors of NO synthase; L-NOARG (10(-4)M), guanylate cyclase; ODQ (10(-5)M) and cyclooxygenase; indomethacin (10(-5)M), did not cause a significant difference in the relaxation profile. Additionally, retinal relaxations remained unchanged in the presence of respective inhibitors of ATP-sensitive (K(ATP)) (glibenclamide, 10(-5)M), voltage-dependent (K(V)) (4-aminopyridine, 2×10(-3)M), and calcium-activated (K(Ca)) (tetraethylammonium 10mM; charybdotoxin, 10(-7)M; and apamin, 5×10(-7)M) K(+) channels. Thus, our results provide novel evidence regarding the biphasic relaxing profile of RRF in the retinal artery which was unrelated to endothelium and K(+) channels (K(ATP), K(V) and K(Ca)). Interestingly, PAME (10(-14)-10(-5)M) did not provoke a relaxation in bovine retinal artery suggesting no association with RRF.

Calcium-activated potassium channels - a therapeutic target for modulating nitric oxide in cardiovascular disease?

IMPORTANCE OF THE FIELD

Cardiovascular risk factors are often associated with endothelial dysfunction, which is also prognostic for occurrence of cardiovascular events. Endothelial dysfunction is reflected by blunted vasodilatation and reduced nitric oxide (NO) bioavailability. Endothelium-dependent vasodilatation is mediated by NO, prostacyclin, and an endothelium-derived hyperpolarising factor (EDHF), and involves small (SK) and intermediate (IK) conductance Ca(2+)-activated K(+) channels. Therefore, SK and IK channels may be drug targets for the treatment of endothelial dysfunction in cardiovascular disease.

AREAS COVERED IN THIS REVIEW

SK and IK channels are involved in EDHF-type vasodilatation, but recent studies suggest that these channels are also involved in the regulation of NO bioavailability. Here we review how SK and IK channels may regulate NO bioavailability.

WHAT THE READER WILL GAIN

Opening of SK and IK channels is associated with EDHF-type vasodilatation, but, through increased endothelial cell Ca(2+) influx, L-arginine uptake, and decreased ROS production, it may also lead to increased NO bioavailability and endothelium-dependent vasodilatation.

TAKE HOME MESSAGE

Opening of SK and IK channels can increase both EDHF and NO-mediated vasodilatation. Therefore, openers of SK and IK channels may have the potential of improving endothelial cell function in cardiovascular disease.

Role of calcium-activated potassium channels in acetylcholine-induced vasodilation of rat retinal arterioles in vivo

The vascular endothelium plays an important role in regulating retinal blood flow via actions of several vasodilators, including nitric oxide (NO), prostaglandin I₂, and an endothelium-derived hyperpolarizing factor (EDHF). Our previous in vivo studies demonstrated that acetylcholine (ACh) dilates the rat retinal arteriole partly through NO- and prostaglandin-independent pathway, possibly the EDHF-mediated pathway, but the underlying mechanism(s) remains to be elucidated. It has been suggested that activation of Ca²+-activated K+ (K(Ca)) channels contributes to the EDHF-mediated responses; therefore, the roles of K(Ca) channels in ACh-induced vasodilation of retinal arterioles were examined in rats. The retinal vascular responses were assessed by determining changes in diameters of retinal arterioles in ocular fundus images that were captured with an original fundus camera system. Intravitreal injection of charybdotoxin, an inhibitor of intermediate- and large-conductance K(Ca) (I/BK(Ca)) channels, or iberiotoxin, an inhibitor of large-conductance K(Ca) (BK(Ca)) channels, significantly reduced the ACh-induced vasodilation of retinal arterioles, whereas neither apamin, an inhibitor of small-conductance K(Ca) (SK(Ca)) channels, nor TRAM-34, an inhibitor of intermediate-conductance K(Ca) (IK(Ca)) channels, altered the response. The vasodilator response to ACh observed under the combined blockade of NO synthase and cyclooxygenase with N(G)-nitro-L-arginine methyl ester plus indomethacin was also diminished by iberiotoxin. Iberiotoxin did not affect the NO donor NOR3-induced vasodilation of retinal arterioles, whereas it significantly reduced the BK(Ca) channel opener BMS-191011-induced responses. These results suggest that activation of BK(Ca) channels is involved in the EDHF-mediated component of the vasodilator response to ACh in the rat retinal arterioles in vivo.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Mechanisms underlying epithelium-dependent relaxation in rat bronchioles: analogy to EDHF-type relaxation in rat pulmonary arteries

This study investigated the mechanisms underlying epithelium-derived hyperpolarizing factor (EpDHF)-type relaxation in rat bronchioles. Immunohistochemistry was performed, and rat bronchioles and pulmonary arteries were mounted in microvascular myographs for functional studies. An opener of small (SKCa) and intermediate (IKCa)-conductance calcium-activated potassium channels, NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime) was used to induce EpDHF-type relaxation. IKCa and SKCa3 positive immunoreactions were observed mainly in the epithelium and endothelium of bronchioles and arteries, respectively. In 5-hydroxytryptamine (1 μM)-contracted bronchioles (828 ± 20 μm, n = 84) and U46619 (0.03 μM)-contracted arteries (720 ± 24 μm, n = 68), NS309 (0.001–10 μM) induced concentration-dependent relaxations that were reduced by epithelium/endothelium removal and by blocking IKCa channels with charybdotoxin and in bronchioles also by blocking SKCa channels with apamin. Inhibition of cyclooxygenase, nitric oxide synthase, and cytochrome 2C isoenzymes, or blockade of large (BKCa)-conductance calcium-activated potassium channels with iberiotoxin, failed to reduce NS309 relaxation. In contrast to the pulmonary arteries, relaxations to a β2-adrenoceptor agonist, salbutamol, were reduced in bronchioles by removing the epithelium or blocking IKCa and/or SKCa channels. Extracellular K+ (2–20 mM) induced relaxation in both bronchioles and arteries. An inhibitor of Na+-K+-ATPase, ouabain, abolished relaxations to NS309, salbutamol, and K+. These results suggest that IKCa and SKCa3 channels are located in the epithelium of bronchioles and endothelium of pulmonary arteries. Analog to the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in pulmonary arteries, these channels may be involved in EpDHF-type relaxation of bronchioles caused by epithelial K+ efflux followed by activation of Na+-K+-ATPase in the underlying smooth muscle layer.

Vascular KCa-channels as therapeutic targets in hypertension and restenosis disease

IMPORTANCE OF THE FIELD

Cardiovascular disease is a leading cause of death in modern societies. Hyperpolarizing Ca(2+)-activated K(+) channels (K(Ca)) are important membrane proteins in the control of arterial tone and pathological vascular remodelling and thus could serve as new drug targets.

AREAS COVERED IN THIS REVIEW

We summarize recent advances in the field of vascular K(Ca) and their roles in cardiovascular pathologies such as hypertension and restenosis disease and draw attention to novel small-molecule channel modulators and their possible therapeutic utility. This review focuses on literature from the last four to five years.

WHAT THE READER WILL GAIN

Pharmacological opening of endothelial KCa3.1/KCa2.3 channels stimulates endothelium-derived-hyperpolarizing-factor-mediated arteriolar dilation and lowers blood pressure. Inhibition of smooth muscle KCa3.1 channels has beneficial effects in restenosis disease and atherosclerosis. We consider the therapeutic potential of KCa3.1/KCa2.3 openers as novel endothelium-specific antihypertensive drugs as well as of KCa3.1-blockers for the treatment of pathological vascular remodelling and discuss advantages and disadvantages of the pharmacotherapeutic approaches.

TAKE HOME MESSAGE

Pharmacological manipulation of vascular K(Ca) channels by novel small-molecule modulators offers new venues for alternative treatments of hypertension, restenosis and atherosclerosis. Additional efforts are required to optimize these compounds and to validate them as cardiovascular-protective drugs.

NS309 restores EDHF-type relaxation in mesenteric small arteries from type 2 diabetic ZDF rats

BACKGROUND AND PURPOSE

The endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in mesenteric small arteries from 21 week old Zucker lean (ZL) and Zucker diabetic fatty (ZDF) rats was investigated using (6,7-dichloro-1H-indole-2,3-dione 3-oxime) (NS309), a potent activator of small-conductance, calcium-activated potassium channel (SK(Ca)) and intermediate-conductance, calcium-activated potassium channel (IK(Ca)).

EXPERIMENTAL APPROACH

In the presence of inhibitors of cyclooxygenase and nitric oxide synthase [indomethacin and N(omega)-nitro-L-arginine methyl ester (l-NAME), respectively], acetylcholine (ACh)-induced hyperpolarization and EDHF-type relaxation were investigated under isometric conditions in the wire myograph using 0.5 and 1 microM NS309 and/or selective blockers of SK(Ca) and IK(Ca) channels. Membrane potential was recorded with glass microelectrodes, and changes in the intracellular calcium concentration of endothelial cells were visualized by confocal microscopy. SK(Ca) expression was assessed by Western blotting.

KEY RESULTS

In arteries from ZDF rats, ACh-induced relaxation and membrane hyperpolarization were attenuated and, compared with arteries from ZL rats, NS309 was less potent at causing relaxation. Incubation with 0.5 microM NS309 did not increase ACh-induced relaxation in arteries from ZDF rats significantly. However, 1 microM NS309 restored it (both in the absence and in the presence of indomethacin and l-NAME) without changing endothelial intracellular calcium concentration. The restored EDHF-type relaxation was more sensitive to TRAM-34 (1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole) (1 microM) than to apamin. Expression of the SK(Ca) channel was unaltered.

CONCLUSIONS AND IMPLICATIONS

The attenuated EDHF-type relaxation in mesenteric small arteries from ZDF rats can be restored by NS309 without changes in the intracellular calcium concentration of endothelial cells. These results may have clinical implications for the treatment of endothelial dysfunction in overweight type 2 diabetic patients.