Nitric oxide (NO)-induced activation of large conductance Ca2+-dependent K+ channels (BK(Ca)) in smooth muscle cells isolated from the rat mesenteric artery

Ru(II) based dual nitric oxide donors: electrochemical and photochemical reactivities and vasorelaxant effect with no cytotoxicity

The synthesized complexes, cis-[Ru(NO)(NO2)(phen)2](PF6)2 (NONO2P) and cis-[Ru(NO)(NO2)(bpy)2](PF6)2 (NONO2B), were characterized by using elemental analysis, voltammetry and electronic and vibrational spectroscopy. Under electrochemical and photochemical stimulation in an aqueous medium, there are indications of the formation of complexes, which suggests that the nitro and nitrosyl groups are converted into nitric oxide. Both compounds do not show cytotoxic activity against human umbilical vein endothelial cells (HUVECs). The cis-[Ru(NO)(NO2)(phen)2](PF6)2 complex presented vasorelaxation activity in superior mesenteric arteries from Wistar rats: the biphasic concentration-response curve indicates two sites of action. In the presence of NO scavengers, we observed an impaired relaxing effect induced by NONO2P, suggesting that the vasorelaxant effect is due to NO production from this compound.

Ion Channel Disturbances in Migraine Headache: Exploring the Potential Role of the Kynurenine System in the Context of the Trigeminovascular System

Migraine is a primary headache disorder, which is an enormous burden to the healthcare system. While some aspects of the pathomechanism of migraines remain unknown, the most accepted theory is that activation and sensitization of the trigeminovascular system are essential during migraine attacks. In recent decades, it has been suggested that ion channels may be important participants in the pathogenesis of migraine. Numerous ion channels are expressed in the peripheral and central nervous systems, including the trigeminovascular system, affecting neuron excitability, synaptic energy homeostasis, inflammatory signaling, and pain sensation. Dysfunction of ion channels could result in neuronal excitability and peripheral or central sensitization. This narrative review covers the current understanding of the biological mechanisms leading to activation and sensitization of the trigeminovascular pain pathway, with a focus on recent findings on ion channel activation and modulation. Furthermore, we focus on the kynurenine pathway since this system contains kynurenic acid, which is an endogenous glutamate receptor antagonist substance, and it has a role in migraine pathophysiology.

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.

Sodium Nitroprusside-Induced Activation of Vascular Smooth Muscle BK Channels Is Mediated by PKG Rather Than by a Direct Interaction with NO

Nitric oxide (NO) is a powerful vasodilator in different vascular beds and NO-donors are widely used in clinical practice. Early data suggested that NO and NO-donors activate vascular smooth muscle high-conductance, calcium-activated potassium channels (BK channels). There exist two hypotheses explaining the effect of NO and NO-donors on BK channels—one stating that protein kinase G (PKG) mediates the effect of NO, and the other one stating that NO acts directly on the channel. Thus, the degree of contribution of PKG to the NO-induced activation of the BK channel is still not completely clear. This study tested the hypothesis that the sodium nitroprusside (SNP)-induced activation of vascular smooth muscle BK channels is fully mediated by PKG. This hypothesis was investigated using the patch-clamp technique and freshly isolated smooth muscle cells from rat tail artery. In whole-cell experiments, SNP considerably increased the outward current compared with the addition of the bath solution. SNP did not alter the current in the presence of iberiotoxin, the specific blocker of BK channels, during co-application with hydroxocobalamin, an NO-scavenger, and in the presence of Rp-8-Br-PET-cGMPS, the specific PKG-inhibitor. In inside-out patches, the activity of BK channels was increased by SNP, SNAP, and DEA-NO. However, these effects did not differ from the effect of the application of drug-free bath solution. Furthermore, a similar increase in single BK channel activity was induced by Rp-8-Br-PET-cGMPS, Rp-8-Br-PET-cGMPS together with SNP, hydroxocobalamin, and hydroxocobalamin together with SNP or DEA-NO. Finally, the activity of excised BK channels did not change in the absence of any application but was considerably increased by PKG compared with the addition of drug-free bath solution. These results suggest that NO released from NO-donors stimulates the BK current only through activation of PKG.

Cardiovascular characterization of the novel organic mononitrate NDIBP in rats

Organic nitrates are widely used to restore endogenous nitric oxide (NO) levels reduced by endothelial nitric oxide synthase dysfunction. However, these drugs are associated with undesirable side effects, including tolerance. This study aims to investigate the cardiovascular effects of the new organic nitrate 1,3-diisobutoxypropan-2-yl nitrate (NDIBP). Specifically, we assessed its effects on blood pressure, vascular reactivity, acute toxicity, and the ability to induce tolerance. In vitro and ex vivo techniques showed that NDIBP released NO both in a cell-free system and in isolated mesenteric arteries preparations through a process catalyzed by xanthine oxidoreductase. NDIBP also evoked endothelium-independent vasorelaxation, which was significantly attenuated by 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl 3-oxide (PTIO, 300 μM), a nitric oxide scavenger; 1-H-[1,2,4] oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 μM), a soluble guanylyl cyclase inhibitor; tetraethylammonium (TEA, 3 mM), a potassium channel blocker; febuxostat (500 nM), a xanthine oxidase inhibitor; and proadifen (10 μM), an inhibitor of cytochrome P450 enzyme. Furthermore, this organic nitrate did not induce tolerance in isolated vessels and presented low toxicity following acute oral administration. In vivo changes on cardiovascular parameters were assessed using normotensive and renovascular hypertensive rats. NDIBP evoked a reduction of blood pressure that was significantly higher in hypertensive animals. Our results suggest that NDIBP acts as a NO donor, inducing blood pressure reduction without having the undesirable effects of tolerance. Those effects seem to be mediated by activation of NO-sGC-cGMP pathway and positive modulation of K+ channels in vascular smooth muscle.

The Large-Conductance, Calcium-Activated Potassium Channel: A Big Key Regulator of Cell Physiology

Large-conductance Ca²⁺-activated K⁺ channels facilitate the efflux of K⁺ ions from a variety of cells and tissues following channel activation. It is now recognized that BK channels undergo a wide range of pre- and post-translational modifications that can dramatically alter their properties and function. This has downstream consequences in affecting cell and tissue excitability, and therefore, function. While finding the “silver bullet” in terms of clinical therapy has remained elusive, ongoing research is providing an impressive range of viable candidate proteins and mechanisms that associate with and modulate BK channel activity, respectively. Here, we provide the hallmarks of BK channel structure and function generally, and discuss important milestones in the efforts to further elucidate the diverse properties of BK channels in its many forms.

Vasorelaxant-Mediated Antihypertensive Effect of the Leaf Aqueous Extract from Stephania abyssinica (Dillon & A. Rich) Walp (Menispermaceae) in Rat

Stephania abyssinica is a medicinal plant used in Cameroon alternative medicine to treat arterial hypertension (AHT). Previous in vitro studies demonstrated the endothelium nitric oxide-independent vasorelaxant property of the aqueous extract from Stephania abyssinica (AESA). But its effect on AHT is unknown. The present study was undertaken to explore other vasorelaxant mechanisms and to determine the antihypertensive effects of AESA in male Wistar rats. Phytochemical analysis of AESA was carried out using the liquid chromatography-mass spectrometry (LC-MS) method. The vasorelaxant effects of AESA (1-1000 μg/mL) were studied on rat isolated thoracic aorta rings, in the absence or presence of indomethacin (10 μM) or methylene blue (10 μM). The inhibitory effect of AESA on phenylephrine (PE, 10 μM) or KCl- (60 mM) induced contraction as well as the intracellular calcium release was also evaluated. The in vivo antihypertensive activity of AESA (43, 86, or 172 mg/kg/day) or captopril (20 mg/kg/day) administered orally was assessed in L-NAME- (40 mg/kg/day) treated rats. Blood pressure and heart rate (HR) were measured at the end of each week while serum or urinary nitric oxide (NO), creatinine, and glomerular filtration rate (GFR) were determined at the end of the 6 weeks of treatment, as well as histological analysis of the heart and the kidney. The LC-MS profiling of AESA identified 9 compounds including 7 alkaloids. AESA produced a concentration-dependent relaxation on contraction induced either by PE and KCl, which was significantly reduced in endothelium-denuded vessels, as well as in vessels pretreated with indomethacin and methylene blue. Moreover, AESA inhibited the intracellular Ca²⁺ release-induced contraction. In vivo, AESA reduced the AHT, heart rate (HR), and ventricular hypertrophy and increased serum NO, urine creatinine, and GFR. AESA also ameliorated heart and kidney lesions as compared to the L-NAME group. These findings supported the use of AESA as a potential antihypertensive drug.

Apelin Does Not Impair Coronary Artery Relaxation Mediated by Nitric Oxide-Induced Activation of BKCa Channels

Apelin-APJ receptor signaling regulates vascular tone in cerebral and peripheral arteries. We recently reported that apelin inhibits BK_Ca channel function in cerebral arteries, resulting in impaired endothelium-dependent relaxations. In contrast, apelin causes endothelium-dependent relaxation of coronary arteries. However, the effects of apelin on BK_Ca channel function in coronary arterial myocytes have not yet been explored. We hypothesized that apelin-APJ receptor signaling does not have an inhibitory effect on coronary arterial BK_Ca channels and hence does not alter nitric oxide (NO)-dependent relaxation of coronary arteries. Patch clamp recording was used to measure whole cell K⁺ currents in freshly isolated coronary smooth muscle cells. Apelin had no effect on the increases in current density in response to membrane depolarization or to NS1619 (a BK_Ca channel opener). Moreover, apelin did not inhibit NO/cGMP-dependent relaxations that required activation of BK_Ca channels in isolated coronary arteries. Apelin-APJ receptor signaling caused a marked increase in intracellular Ca²⁺ levels in coronary arterial smooth muscle cells, but failed to activate PI3-kinase to increase phosphorylation of Akt protein. Collectively, these data provide mechanistic evidence that apelin has no inhibitory effects on BK_Ca channel function in coronary arteries. The lack of inhibitory effect on BK_Ca channels makes it unlikely that activation of APJ receptors in coronary arteries would adversely affect coronary flow by creating a vasoconstrictive environment. It can be expected that apelin or other APJ receptor agonists in development will not interfere with the vasodilator effects of endogenous BK_Ca channel openers.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Homocysteine alters vasoreactivity of human internal mammary artery by affecting the KCa channel family

Background

Hyperhomocysteinemia is an independent risk factor for atherosclerotic heart disease. We previously demonstrated that disruption of calcium-activated potassium (K_Ca) channel activity is involved in homocysteine-induced dilatory dysfunction of porcine coronary arteries. Recently we reported that the K_Ca channel family, including large-, intermediate-, and small-conductance K_Ca (BK_Ca, IK_Ca, and SK_Ca) subtypes, are abundantly expressed in human internal mammary artery (IMA). In this study, we further investigated whether homocysteine affects the expression and functionality of the K_Ca channel family in this commonly used graft for coronary artery bypass surgery (CABG).

Methods

Residual IMA segments obtained from patients undergoing CABG were studied in a myograph for the role of K_Ca subtypes in both vasorelaxation and vasoconstriction. The expression and distribution of K_Ca subtypes were detected by Western blot and immunohistochemistry.

Results

Both the BK_Ca channel activator NS1619 and the IK_Ca/SK_Ca channel activator NS309 evoked significant IMA relaxation. Homocysteine exposure suppressed NS1619-induced relaxation whereas showed no influence on NS309-induced response. Blockade of BK_Ca but not IK_Ca and SK_Ca subtypes significantly suppressed acetylcholine-induced relaxation and enhanced U46619-induced contraction. Homocysteine compromised the vasodilating activity of the BK_Ca subtype in IMA, associated with a lowered protein level of the BK_Ca β1-subunit. Homocysteine potentiated the role of IK_Ca and SK_Ca subtypes in mediating endothelium-dependent relaxation without affecting the expression of these channels.

Conclusions

Homocysteine reduces the expression of BK_Ca β1-subunit and compromises the vasodilating activity of BK_Ca channels in IMA. Unlike BK_Ca, IK_Ca and SK_Ca subtypes are unessential for IMA vasoregulation, whereas the loss of BK_Ca functionality in hyperhomocysteinemia enhances the role of IK_Ca and SK_Ca subtypes in mediating endothelial dilator function. Targeting BK_Ca channels may form a strategy to improve the postoperative graft performance in CABG patients with hyperhomocysteinemia who receive IMA grafting.

Electrophysiological characterization of the activating action of a novel liposomal nitric oxide carrier on Maxi-K channels in pulmonary artery smooth muscle cells

The aim of this study was to establish the mechanisms of action of a novel liposomal nitric oxide (NO) carrier on large-conductance Ca²⁺-activated channels (BK_Ca or Maxi-K) expressed in vascular smooth muscle cells (VSMCs) isolated from the rat main pulmonary artery (MPA). Experimental design comprised of both whole-cell and cell-attached single-channel recordings using the patch-clamp techniques. The liposomal form of NO, Lip(NO), increased whole-cell outward K⁺ currents in a dose dependent manner while shifting the activation curve negatively by about 50 mV with respect to unstimulated cells with the EC₅₀ value of 0.55 ± 0.17 µM. At the single channel level, Lip(NO) increased the probability of the open state (Po) of Maxi-K channels from 0.0020 ± 0.0008 to 0.74 ± 0.02 with half-maximal activation occurring at 4.91 ± 0.01 μM, while sub-maximal activation was achieved at 10^-5 M Lip(NO). Channel activation was mainly due to significant decrease in the mean closed dwell time (about 500-fold), rather than an increase in the mean open dwell time, which was comparatively modest (about twofold). There was also a slight decrease in the amplitude of the elementary Maxi-K currents (approximately 15%) accompanied by an increase in current noise, which might indicate some non-specific effects of Lip(NO) on the plasma membrane itself and/or on the phospholipids environment of the channels. In conclusion, the activating action of Lip(NO) on the Maxi-K channel is due to the destabilization of the closed conformation of the channel protein, which causes its more frequent openings and, accordingly, increases the probability of channel transition to its open state.

Modeling of Hypervolemia in Pulmonary Circulation in Rats Changes the Structure of NO-Mediated Relaxation of Pulmonary Arteries

We analyzed the contribution of soluble guanylate cyclase-dependent pathway into NO-mediated relaxation of pulmonary arteries under conditions of high pulmonary blood flow modeled by creation of carotid artery-jugular vein shunt in rats. Inhibitor of soluble guanylate cyclase suppressed NO-donor induced relaxation was lower in rats with shunt, but dilatation in response to phosphodiesterase V inhibitor did not differ in the sham-operated and shunt groups. Thus, the structure of NO-mediated vasodilatation of pulmonary arteries under conditions of hypervolemia of pulmonary circulation was shifted to soluble guanylate cyclase-independent pathways, whereas intracellular soluble guanylate cyclase-dependent mechanisms of dilatation were in general unchanged.

4-Aminopyridine, a Voltage-Gated K+ Channel Inhibitor, Attenuates Nitric Oxide-Mediated Vasodilation of Retinal Arterioles in Rats

Nitric oxide (NO) is an important regulator of the retinal blood flow. The present study aimed to determine the role of voltage-gated K⁺ (K_V) channels and ATP-sensitive K⁺ (K_ATP) channels in NO-mediated vasodilation of retinal arterioles in rats. In vivo, the retinal vasodilator responses were assessed by measuring changes in the diameter of retinal arterioles from ocular fundus images. Intravitreal injection of 4-aminopyridine (a K_V channel inhibitor), but not glibenclamide (a K_ATP channel blocker), significantly attenuated the retinal vasodilator response to the NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Intravitreal injection of indomethacin (a non-selective cyclooxygenase inhibitor) also reduced the NOR3-induced retinal vasodilator response. The combination of 4-aminopyridine and indomethacin produced a greater reduction in the NOR3-induced response than either agent alone. 4-Aminopyridine had no significant effect on pinacidil (a K_ATP channel opener)-induced response. These results suggest that the vasodilatory effects of NO are mediated, at least in part, through the activation of 4-aminopyridine-sensitive K_V channels in the retinal arterioles of rats. NO exerts its dilatory effect on the retinal vasculature of rats through at least two mechanisms, activation of the K_V channels and enhancement of prostaglandin production.

Targeting BKCa Channels in Migraine: Rationale and Perspectives

Large (big)-conductance calcium-activated potassium (BK_Ca) channels are expressed in migraine-related structures such as the cranial arteries, trigeminal ganglion and trigeminal spinal nucleus, and they play a substantial role in vascular tonus and neuronal excitability. Using synthetic BK_Ca channels openers was associated with headache as a frequent adverse effect in healthy volunteers. Additionally, BK_Ca channels are downstream molecules in migraine signalling pathways that are activated by several compounds known to provoke migraine, including calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase-activating polypeptide (PACAP) and glyceryl trinitrate (GTN). Also, there is a high affinity and a close coupling between BK_Ca channels and ATP-sensitive potassium (K_ATP) channels, the role of which has recently been established in migraine pathophysiology. These observations raise the question as to whether direct BK_Ca channel activation can provoke migraine in migraine patients, and whether the BK_Ca channel could be a potential novel anti-migraine target. Hence, randomized and placebo-controlled clinical studies on BK_Ca channel openers or blockers in migraine patients are needed.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Downregulation of BK channel function and protein expression in coronary arteriolar smooth muscle cells of type 2 diabetic patients

Aims

Type 2 diabetes (T2D) is strongly associated with cardiovascular morbidity and mortality in patients. Vascular large conductance Ca2+-activated potassium (BK) channels, composed of four pore-forming α subunits (BK-α), and four regulatory β1 subunits (BK-β1), are densely expressed in coronary arterial smooth muscle cells (SMCs) and play an important role in regulating vascular tone and myocardial perfusion. However, the role of BK channels in coronary microvascular dysfunction of human subjects with diabetes is unclear. In this study, we examined BK channel function and protein expression, and BK channel-mediated vasodilation in freshly isolated coronary arterioles from T2D patients.

Methods and results

Atrial tissues were obtained from 16 patients with T2D and 25 matched non-diabetic subjects during cardiopulmonary bypass procedure. Microvessel videomicroscopy and immunoblot analysis were performed in freshly dissected coronary arterioles and inside-out single BK channel currents was recorded in enzymatically isolated coronary arteriolar SMCs. We found that BK channel sensitivity to physiological Ca2+ concentration and voltage was downregulated in the coronary arteriolar SMCs of diabetic patients, compared with non-diabetic controls. BK channel kinetics analysis revealed that there was significant shortening of the mean open time and prolongation of the mean closed time in diabetic patients, resulting in a remarkable reduction of the channel open probability. Functional studies showed that BK channel activation by dehydrosoyasaponin-1 was diminished and that BK channel-mediated vasodilation in response to shear stress was impaired in diabetic coronary arterioles. Immunoblot experiments confirmed that the protein expressions of BK-α and BK-β1 subunits were significantly downregulated, but the ratio of BK-α/BK-β1 was unchanged in the coronary arterioles of T2D patients.

Conclusions

Our results demonstrated for the first time that BK channel function and BK channel-mediated vasodilation were abnormal in the coronary microvasculature of diabetic patients, due to decreased protein expression and altered intrinsic properties of BK channels.

The Structure of NO-Mediated Dilatation of Pulmonary Arteries Depends on NO Availability

NO-mediated vasodilatation can be realized via two pathways: dependent and independent on soluble guanylate cyclase; the latter is implemented through NO interaction with ionic channels. We evaluated the contribution of these pathways into NO-induced relaxation of isolated pulmonary arteries in rats. In pulmonary arteries, in contrast to systemic vessels, soluble guanylate cyclase-independent mechanisms is more important, because it mediates relaxation in response to low concentrations of NO donor. The role of soluble guanylate cyclase-dependent mechanisms in the mechanisms of vasodilatation increases with increasing NO donor concentrations.

Protection of dilator function of coronary arteries from homocysteine by tetramethylpyrazine: Role of ER stress in modulation of BKCa channels

OBJECTIVES

We recently reported the involvement of ER stress-mediated BK_Ca channel inhibition in homocysteine-induced coronary dilator dysfunction. In another study, we demonstrated that tetramethylpyrazine (TMP), an active ingredient of the Chinese herb Chuanxiong, possesses potent anti-ER stress capacity. The present study investigated whether TMP protects BK_Ca channels from homocysteine-induced inhibition and whether suppression of ER stress is a mechanism contributing to the protection. Furthermore, we explored the signaling transduction involved in TMP-conferred protection on BK_Ca channels.

METHODS

BK_Ca channel-mediated relaxation was studied in porcine small coronary arteries. Expressions of BK_Ca channel subunits, ER stress molecules, and E3 ubiquitin ligases, as well as BK_Ca ubiquitination were determined in porcine coronary arterial smooth muscle cells (PCASMCs). Whole-cell BK_Ca currents were recorded.

RESULTS

Exposure of PCASMCs to homocysteine or the chemical ER stressor tunicamycin increased the expression of ER stress molecules, which was significantly inhibited by TMP. Suppression of ER stress by TMP preserved the BK_Ca β1 protein level and restored the BK_Ca current in PCASMCs, concomitant with an improved BK_Ca-mediated dilatation in coronary arteries. TMP attenuated homocysteine-induced BK_Ca β1 protein ubiquitination, in which inhibition of ER stress-mediated FoxO3a activation and FoxO3a-dependent atrogin-1 and Murf-1 was involved.

CONCLUSIONS

Reversal of BK_Ca channel inhibition via suppressing ER stress-mediated loss of β1 subunits contributes to the protective effect of TMP against homocysteine on coronary dilator function. Inhibition of FoxO3a-dependent ubiquitin ligases is involved in TMP-conferred normalization of BK_Ca β1 protein level. These results provide new mechanistic insights into the cardiovascular benefits of TMP.

An Update on Hydrogen Sulfide and Nitric Oxide Interactions in the Cardiovascular System

Hydrogen sulfide (H₂S) and nitric oxide (NO) are now recognized as important regulators in the cardiovascular system, although they were historically considered as toxic gases. As gaseous transmitters, H₂S and NO share a wide range of physical properties and physiological functions: they penetrate into the membrane freely; they are endogenously produced by special enzymes, they stimulate endothelial cell angiogenesis, they regulate vascular tone, they protect against heart injury, and they regulate target protein activity via posttranslational modification. Growing evidence has determined that these two gases are not independent regulators but have substantial overlapping pathophysiological functions and signaling transduction pathways. H₂S and NO not only affect each other's biosynthesis but also produce novel species through chemical interaction. They play a regulatory role in the cardiovascular system involving similar signaling mechanisms or molecular targets. However, the natural precise mechanism of the interactions between H₂S and NO remains unclear. In this review, we discuss the current understanding of individual and interactive regulatory functions of H₂S and NO in biosynthesis, angiogenesis, vascular one, cardioprotection, and posttranslational modification, indicating the importance of their cross-talk in the cardiovascular system.

Activation of Large Conductance, Calcium-Activated Potassium Channels by Nitric Oxide Mediates Apelin-Induced Relaxation of Isolated Rat Coronary Arteries

Apelin increases coronary blood flow, cardiac contractility, and cardiac output. Based on these favorable hemodynamic effects, apelin and apelin-like analogs are being developed for treating heart failure and related disorders; however, the molecular mechanisms underlying apelin-induced coronary vasodilation are unknown. This study aimed to elucidate the signaling pathways by which apelin causes smooth muscle relaxation in coronary arteries. Receptors for apelin (APJ receptors) were expressed in coronary arteries, as determined by Western blot and polymerase chain reaction analyses. Immunofluorescence imaging studies identified APJ receptors on endothelial and smooth muscle cells. In isolated endothelial cells, apelin caused an increase in 4,5-diaminofluorescein fluorescence that was abolished by nitro-l-arginine (NLA) and F13A (H-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Ala-OH), an APJ receptor antagonist, consistent with increased nitric oxide (NO) production. In arterial rings, apelin caused endothelium-dependent relaxations that were abolished by NLA, F13A, and iberiotoxin. Neither oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) nor DT-2, a protein kinase G inhibitor, had any effect on apelin-induced relaxations, and apelin itself had no effect on intracellular cGMP accumulation in coronary arteries. Patch-clamp studies in isolated smooth muscle cells demonstrated that the NO donors, diethyl amine NONOate and sodium nitroprusside, caused increases in large conductance, calcium-activated potassium channel (BKCa) currents, which were inhibited by iberiotoxin but not ODQ. Thus, apelin causes endothelium-dependent relaxation of coronary arteries by stimulating endothelial APJ receptors and releasing NO, which acts in a cGMP-independent manner and increases BKCa activity in the underlying smooth muscle cells. These results provide a mechanistic basis for apelin-induced coronary vasodilation and may provide guidance for the future development of novel apelin-like therapeutic agents.

Chronic hypoxia upregulates DNA methyltransferase and represses large conductance Ca2+-activated K+ channel function in ovine uterine arteries

Chronic hypoxia during gestation suppresses large-conductance Ca2+-activated K+ (BKCa) channel function and impedes uterine arterial adaptation to pregnancy. This study tested the hypothesis that chronic hypoxia has a direct effect in upregulating DNA methyltransferase (DNMT) and epigenetically repressing BKCa channel beta-1 subunit (KCNMB1) expression in uterine arteries. Resistance-sized uterine arteries were isolated from near-term pregnant sheep maintained at ∼300 m above sea level or animals acclimatized to high-altitude (3,801 m) hypoxia for 110 days during gestation. For ex vivo hypoxia treatment, uterine arteries from normoxic animals were treated with 21.0% O2 or 10.5% O2 for 48 h. High-altitude hypoxia significantly upregulated DNMT3b expression and enzyme activity in uterine arteries. Similarly, ex vivo hypoxia treatment upregulated DNMT3b expression and enzyme activity that was blocked by a DNMT inhibitor 5-aza-2'-deoxycytidine (5-Aza). Of importance, 5-Aza inhibited hypoxia-induced hypermethylation of specificity protein (SP) 1 binding site at the KCNMB1 promoter and restored transcription factor binding to the KCNMB1 promoter, resulting in the recovery of KCNMB1 gene expression in uterine arteries. Furthermore, 5-Aza blocked the effect of hypoxia and rescued BKCa channel activity and reversed hypoxia-induced decrease in BKCa channel-mediated relaxations and increase in myogenic tone of uterine arteries. Collectively, these results suggest that chronic hypoxia during gestation upregulates DNMT expression and activity, resulting in hypermethylation and repression of KCNMB1 gene and BKCa channel function, impeding uterine arterial adaptation to pregnancy.

Calcium- and voltage-gated BK channels in vascular smooth muscle

Ion channels in vascular smooth muscle regulate myogenic tone and vessel contractility. In particular, activation of calcium- and voltage-gated potassium channels of large conductance (BK channels) results in outward current that shifts the membrane potential toward more negative values, triggering a negative feed-back loop on depolarization-induced calcium influx and SM contraction. In this short review, we first present the molecular basis of vascular smooth muscle BK channels and the role of subunit composition and trafficking in the regulation of myogenic tone and vascular contractility. BK channel modulation by endogenous signaling molecules, and paracrine and endocrine mediators follows. Lastly, we describe the functional changes in smooth muscle BK channels that contribute to, or are triggered by, common physiological conditions and pathologies, including obesity, diabetes, and systemic hypertension.

Vasoconstrictor stimulus determines the functional contribution of myoendothelial feedback to mesenteric arterial tone

KEY POINTS

In isolated resistance arteries, endothelial modulation of vasoconstrictor responses to α₁ -adrenoceptor agonists occurs via a process termed myoendothelial feedback: localized inositol trisphosphate (InsP₃ )-dependent Ca²⁺ transients activate intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels, hyperpolarizing the endothelial membrane potential to limit further reductions in vessel diameter. We demonstrate that IK_Ca channel-mediated myoendothelial feedback limits responses of isolated mesenteric arteries to noradrenaline and nerve stimulation, but not to the thromboxane A₂ mimetic U46619 or to increases in intravascular pressure. In contrast, in the intact mesenteric bed, although responses to exogenous noradrenaline were limited by IK_Ca channel-mediated myoendothelial feedback, release of NO and activation of endothelial small conductance Ca²⁺ -activated K⁺ (SK_Ca ) channels in response to increases in shear stress appeared to be the primary mediators of endothelial modulation of vasoconstriction. We propose that (1) the functional contribution of myoendothelial feedback to arterial tone is determined by the nature of the vasoconstrictor stimulus, and (2) although IK_Ca channel-mediated myoendothelial feedback may contribute to local control of arterial diameter, in the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

ABSTRACT

Constriction of isolated resistance arteries in response to α₁ -adrenoceptor agonists is limited by reciprocal engagement of inhibitory endothelial mechanisms via myoendothelial feedback. In the current model of feedback, agonist stimulation of smooth muscle cells results in localized InsP₃ -dependent Ca²⁺ transients that activate endothelial IK_Ca channels. The subsequent hyperpolarization of the endothelial membrane potential then feeds back to the smooth muscle to limit further reductions in vessel diameter. We hypothesized that the functional contribution of InsP₃ -IK_Ca channel-mediated myoendothelial feedback to limiting arterial diameter may be influenced by the nature of the vasoconstrictor stimulus. To test this hypothesis, we investigated the functional role of myoendothelial feedback in modulating responses of rat mesenteric resistance arteries to the adrenoceptor agonist noradrenaline, the thromboxane A₂ mimetic U46619, increases in intravascular pressure and stimulation of perivascular sympathetic nerves. In isolated arteries, responses to noradrenaline and stimulation of sympathetic nerves, but not to U46619 and increases in intravascular pressure, were modulated by IK_Ca channel-dependent myoendothelial feedback. In the intact mesenteric bed perfused under conditions of constant flow, responses to exogenous noradrenaline were modulated by myoendothelial feedback, but shear stress-induced release of NO and activation of endothelial SK_Ca channels appeared to be the primary mediators of endothelial modulation of vasoconstriction to agonists and nerve stimulation. Thus, we propose that myoendothelial feedback may contribute to local control of diameter within arterial segments, but at the level of the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

Nitric Oxide-Dependent Feedback Loop Regulates Transient Receptor Potential Vanilloid 4 (TRPV4) Channel Cooperativity and Endothelial Function in Small Pulmonary Arteries

BACKGROUND

Recent studies demonstrate that spatially restricted, local Ca²⁺ signals are key regulators of endothelium-dependent vasodilation in systemic circulation. There are drastic functional differences between pulmonary arteries (PAs) and systemic arteries, but the local Ca²⁺ signals that control endothelium-dependent vasodilation of PAs are not known. Localized, unitary Ca²⁺ influx events through transient receptor potential vanilloid 4 (TRPV4) channels, termed TRPV4 sparklets, regulate endothelium-dependent vasodilation in resistance-sized mesenteric arteries via activation of Ca²⁺-dependent K⁺ channels. The objective of this study was to determine the unique functional roles, signaling targets, and endogenous regulators of TRPV4 sparklets in resistance-sized PAs.

METHODS AND RESULTS

Using confocal imaging, custom image analysis, and pressure myography in fourth-order PAs in conjunction with knockout mouse models, we report a novel Ca²⁺ signaling mechanism that regulates endothelium-dependent vasodilation in resistance-sized PAs. TRPV4 sparklets exhibit distinct spatial localization in PAs when compared with mesenteric arteries, and preferentially activate endothelial nitric oxide synthase (eNOS). Nitric oxide released by TRPV4-endothelial nitric oxide synthase signaling not only promotes vasodilation, but also initiates a guanylyl cyclase-protein kinase G-dependent negative feedback loop that inhibits cooperative openings of TRPV4 channels, thus limiting sparklet activity. Moreover, we discovered that adenosine triphosphate dilates PAs through a P2 purinergic receptor-dependent activation of TRPV4 sparklets.

CONCLUSIONS

Our results reveal a spatially distinct TRPV4-endothelial nitric oxide synthase signaling mechanism and its novel endogenous regulators in resistance-sized PAs.

Aluminum exposure at human dietary levels promotes vascular dysfunction and increases blood pressure in rats: A concerted action of NAD(P)H oxidase and COX-2

Aluminum (Al) is a non-essential metal and a significant environmental contaminant and is associated with a number of human diseases including cardiovascular disease. We investigated the effects of Al exposure at doses similar to human dietary levels on the cardiovascular system over a 60day period. Wistar male rats were divided into two major groups and received orally: 1) Low aluminum level - rats were subdivided and treated for 60days as follows: a) Untreated - ultrapure water; b) AlCl₃ at a dose of 8.3mg/kg bw for 60days, representing human Al exposure by diet; and 2) High aluminum level - rats were subdivided and treated for 42days as follows: C) Untreated - ultrapure water; d) AlCl₃ at 100mg/kg bw for 42days, representing a high level of human exposure to Al. Effects on systolic blood pressure (SBP) and vascular function of aortic and mesenteric resistance arteries (MRA) were studied. Endothelium and smooth muscle integrity were evaluated by concentration-response curves to acetylcholine (ACh) and sodium nitroprusside. Vasoconstrictor responses to phenylephrine (Phe) in the presence and absence of endothelium and in the presence of the NOS inhibitor L-NAME, the potassium channels blocker TEA, the NAD(P)H oxidase inhibitor apocynin, superoxide dismutase (SOD), the non-selective COX inhibitor indomethacin and the selective COX-2 inhibitor NS 398 were analyzed. Vascular reactive oxygen species (ROS), lipid peroxidation and total antioxidant capacity, were measured. The mRNA expressions of eNOS, NAD(P)H oxidase 1 and 2, SOD1, COX-2 and thromboxane A2 receptor (TXA-2 R) were also investigated. Al exposure at human dietary levels impaired the cardiovascular system and these effects were almost the same as Al exposure at much higher levels. Al increased SBP, decreased ACh-induced relaxation, increased response to Phe, decreased endothelial modulation of vasoconstrictor responses, the bioavailability of nitric oxide (NO), the involvement of potassium channels on vascular responses, as well as increased ROS production from NAD(P)H oxidase and contractile prostanoids mainly from COX-2 in both aorta and mesenteric arteries. Al exposure increased vascular ROS production and lipid peroxidation as well as altered the antioxidant status in aorta and MRA. Al decreased vascular eNOS and SOD1 mRNA levels and increased the NAD(P)H oxidase 1, COX-2 and TXA-2 R mRNA levels. Our results point to an excess of ROS mainly from NAD(P)H oxidase after Al exposure and the increased vascular prostanoids from COX-2 acting in concert to decrease NO bioavailability, thus inducing vascular dysfunction and increasing blood pressure. Therefore, 60-day chronic exposure to Al, which reflects common human dietary Al intake, appears to pose a risk for the cardiovascular system.

The triterpenoid alpha, beta-amyrin prevents the impaired aortic vascular reactivity in high-fat diet-induced obese mice

To characterize the protective effects of the triterpenoid mixture alpha, beta-amyrin (AMY, 20 mg/kg, during 15 days) on the reactivity of isolated aorta of high-fat diet (HFD)-induced obese mice. Male Swiss mice were fed with HFD or normal diet (ND) for 15 weeks. Contractions of thoracic aorta in response to KCl or phenylephrine (PHE) and relaxation by acetylcholine (ACh) or sodium nitroprusside (SNP) were analyzed. HFD-fed mice developed hyperglycemia, hyperlipidemia, and significant body weight gain, parameters prevented by AMY treatment. Whereas aortic contractility did not differ in response to KCl, contractions induced by PHE (1 μM) as well as relaxation induced by ACh (1-30 μM) or SNP (1 nM-0.1 mM) on PHE-contracted aorta were decreased (p < 0.05) in tissues of HFD compared to ND mice, phenomenon significantly (p < 0.05) diminished in HFD mice treated with AMY. The relaxant actions of ACh and SNP were inhibited (p < 0.05) by tetraethylammonium (TEA, 5 mM), apamin (0.1 μM), and 4-aminopyridine (4-AP; 3 mM) in aortae from ND group, but not from HFD. Treatment of HFD mice with AMY rescued the inhibitory effect of TEA (p < 0.05) on vasorelaxant actions of ACh and SNP. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) inhibited similarly the relaxant effects of SNP in all groups. 8-Br-cGMP relaxed with similar profile aortae of all groups. By preventing HFD-induced obesity in mice, AMY rescued the blunted contractile response to PHE, and the attenuated vasorelaxation and K⁺ channel activation (opening) induced by ACh and SNP in isolated aorta.

Erectile Dysfunction in Heme-Deficient Nitric Oxide-Unresponsive Soluble Guanylate Cyclase Knock-In Mice

INTRODUCTION

The nitric oxide (NO), soluble guanylate cyclase (sGC), and cyclic guanosine monophosphate (cGMP) pathway is the leading pathway in penile erection.

AIM

To assess erectile function in a mouse model in which sGC is deficient in heme (apo-sGC) and unresponsive to NO.

METHODS

Mutant mice (sGCβ₁^ki/ki) that express an sGC enzyme that retains basal activity but fails to respond to NO because of heme deficiency (apo-sGC) were used. Isolated corpora cavernosa from sGCβ₁^ki/ki and wild-type mice were mounted in vitro for isometric tension recordings in response to sGC-dependent and -independent vasorelaxant agents. In addition, the erectile effects of some of these agents were tested in vivo at intracavernosal injection.

MAIN OUTCOME MEASURES

In vitro and in vivo recordings of erectile responses in sGCβ₁^ki/ki and wild-type mice after stimulation with sGC-dependent and -independent vasorelaxant agents.

RESULTS

NO-induced responses were abolished in sGCβ₁^ki/ki mice in vitro and in vivo. The ability of the heme-dependent, NO-independent sGC stimulator BAY 41-2272 to relax the corpora cavernosa was markedly attenuated in sGCβ₁^ki/ki mice. In contrast, the relaxation response to the heme- and NO-independent sGC activator BAY 58-2667 was significantly enhanced in sGCβ₁^ki/ki mice. The relaxing effect of sGC-independent vasorelaxant agents was similar in wild-type and sGCβ₁^ki/ki mice, illustrating that the observed alterations in vasorelaxation are limited to NO-sGC-cGMP-mediated processes.

CONCLUSION

Our results suggest that sGC is the sole target of NO in erectile physiology. Furthermore, this study provides indirect evidence that, in addition to sGCα₁β₁, sGCα₂β₁ is important for erectile function. In addition, the significant relaxation observed in sGCβ₁^ki/ki mice with the cumulative addition of the sGC activator BAY 58-2667 indicates that sGC activators might offer value in treating erectile dysfunction.

Activation of PERK branch of ER stress mediates homocysteine-induced BKCa channel dysfunction in coronary artery via FoxO3a-dependent regulation of atrogin-1

The molecular mechanism of endoplasmic reticulum (ER) stress in vascular pathophysiology remains inadequately understood. We studied the role of ER stress in homocysteine-induced impairment of coronary dilator function, with uncovering the molecular basis of the effect of ER stress on smooth muscle large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. The vasodilatory function of BK_Ca channels was studied in a myograph using endothelium-denuded porcine small coronary arteries. Primary cultured porcine coronary artery smooth muscle cells were used for mRNA and protein measurements and current recording of BK_Ca channels. Homocysteine inhibited vasorelaxant response to the BK_Cachannel opener NS1619, lowered BK_Ca β1 subunit protein level and suppressed BK_Ca current. Inhibition of ER stress restored BK_Ca β1 protein level and NS1619-evoked vasorelaxation. Selective blockade of the PKR-like ER kinase (PERK) yielded similarly efficient restoration of BK_Ca β1, preserving BK_Ca current and BK_Ca-mediated vasorelaxation. The restoration of BK_Ca β1 by PERK inhibition was associated with reduced atrogin-1 expression and decreased nuclear localization of forkhead box O transcription factor 3a (FoxO3a). Silencing of atrogin-1 prevented homocysteine-induced BK_Ca β1 loss and silencing of FoxO3a prevented atrogin-1 upregulation induced by homocysteine, accompanied by preservation of BK_Ca β1 protein level and BK_Ca current. ER stress mediates homocysteine-induced BK_Ca channel inhibition in coronary arteries. Activation of FoxO3a by PERK branch underlies the ER stress-mediated BK_Ca inhibition through a mechanism involving ubiquitin ligase-enhanced degradation of the channel β1 subunit.

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

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

EDH: endothelium-dependent hyperpolarization and microvascular signalling

Endothelium-dependent hyperpolarizing factor (EDHF) is a powerful vasodilator influence in small resistance arteries and thus an important modulator of blood pressure and flow. As the name suggests, EDHF was thought to describe a diffusible factor stimulating smooth muscle hyperpolarization (and thus vasodilatation). However, this idea has evolved with the recognition that a factor can operate alongside the spread of hyperpolarizing current from the endothelium to the vascular smooth muscle (VSM). As such, the pathway is now termed endothelium-dependent hyperpolarization (EDH). EDH is activated by an increase in endothelial [Ca²⁺ ]_i , which stimulates two Ca²⁺ -sensitive K channels, SK_Ca and IK_Ca . This was discovered because apamin and charybdotoxin applied in combination blocked EDHF responses, but iberiotoxin - a blocker of BK_Ca - was not able to substitute for charybdotoxin. SK_Ca and IK_Ca channels are arranged in endothelial microdomains, particularly within projections towards the adjacent smooth muscle, which are rich in IK_Ca channels and close to interendothelial gap junctions where SK_Ca channels, are prevalent. K_Ca activation hyperpolarizes endothelial cells, and K⁺ efflux through them can act as a diffusible 'EDHF' by stimulating VSM Na⁺ ,K⁺ -ATPase and inwardly rectifying K channels (K_IR ). In parallel, hyperpolarizing current spreads from the endothelium to the smooth muscle through myoendothelial gap junctions located on endothelial projections. The resulting radial EDH is complemented by the spread of 'conducted' hyperpolarization along the endothelium of arteries and arterioles to affect conducted vasodilatation (CVD). Retrograde CVD effectively integrates blood flow within the microcirculation, but how the underlying hyperpolarization is sustained is unclear.

The Beta-1-Receptor Blocker Nebivolol Elicits Dilation of Cerebral Arteries by Reducing Smooth Muscle [Ca2+]i

Rationale

Nebivolol is known to have beta-1 blocker activity, but it was also suggested that it elicits relaxation of the peripheral arteries in part via release of nitric oxide (NO). However, the effect of nebivolol on the vasomotor tone of cerebral arteries is still unclear.

Objective

To assess the effects of nebivolol on the diameter of isolated rat basilar arteries (BA) in control, in the presence of inhibitors of vasomotor signaling pathways of know action and hemolysed blood.

Methods and Results

Vasomotor responses were measured by videomicroscopy and the intracellular Ca²⁺ by the Fura-2 AM ratiometric method. Under control conditions, nebivolol elicited a substantial dilation of the BA (from 216±22 to 394±20 μm; p<0.05) in a concentration-dependent manner (10⁻⁷ to 10⁻⁴ M). The dilatation was significantly reduced by endothelium denudation or by L-NAME (inhibitor of NO synthase) or by SQ22536 (adenylyl cyclase blocker). Dilatation of BA was also affected by beta-2 receptor blockade with butoxamine, but not by the guanylate cyclase blocker ODQ. Interestingly, beta-1 blockade by atenolol inhibited nebivolol-induced dilation. Also, the BK_Ca channel blocker iberiotoxin and K_Ca channel inhibitor TEA significantly reduced nebivolol-induced dilation. Nebivolol significantly reduced smooth muscle Ca²⁺ level, which correlated with the increases in diameters and moreover it reversed the hemolysed blood-induced constriction of BA.

Conclusions

Nebivolol seems to have an important dilator effect in cerebral arteries, which is mediated via several vasomotor mechanisms, converging on the reduction of smooth muscle Ca²⁺ levels. As such, nebivolol may be effective to improve cerebral circulation in various diseased conditions, such as hemorrhage.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

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

High-fat diet-induced obesity alters nitric oxide-mediated neuromuscular transmission and smooth muscle excitability in the mouse distal colon

We tested the hypothesis that colonic enteric neurotransmission and smooth muscle cell (SMC) function are altered in mice fed a high-fat diet (HFD). We used wild-type (WT) mice and mice lacking the β1-subunit of the BK channel (BKβ1 (-/-)). WT mice fed a HFD had increased myenteric plexus oxidative stress, a 28% decrease in nitrergic neurons, and a 20% decrease in basal nitric oxide (NO) levels. Circular muscle inhibitory junction potentials (IJPs) were reduced in HFD WT mice. The NO synthase inhibitor nitro-l-arginine (NLA) was less effective at inhibiting relaxations in HFD compared with control diet (CD) WT mice (11 vs. 37%, P < 0.05). SMCs from HFD WT mice had depolarized membrane potentials (-47 ± 2 mV) and continuous action potential firing compared with CD WT mice (-53 ± 2 mV, P < 0.05), which showed rhythmic firing. SMCs from HFD or CD fed BKβ1 (-/-) mice fired action potentials continuously. NLA depolarized membrane potential and caused continuous firing only in SMCs from CD WT mice. Sodium nitroprusside (NO donor) hyperpolarized membrane potential and changed continuous to rhythmic action potential firing in SMCs from HFD WT and BKβ1 (-/-) mice. Migrating motor complexes were disrupted in colons from BKβ1 (-/-) mice and HFD WT mice. BK channel α-subunit protein and β1-subunit mRNA expression were similar in CD and HFD WT mice. We conclude that HFD-induced obesity disrupts inhibitory neuromuscular transmission, SMC excitability, and colonic motility by promoting oxidative stress, loss of nitrergic neurons, and SMC BK channel dysfunction.

SERCA, complex I of the respiratory chain and ATP-synthase inhibition are involved in pleiotropic effects of NS1619 on endothelial cells

A large conductance potassium (BKCa) channel opener, NS1619 (1,3-dihydro-1- [2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazole-2-one), is well known for its protective effects against ischemia-reperfusion injury; however, the exact mode of its action remains unclear. The aim of this study was to characterize the effect of NS1619 on endothelial cells. The endothelial cell line EA.hy926, guinea pig hearts and submitochondrial particles isolated from the heart were used. In the isolated guinea pig hearts, which were perfused using the Langendorff technique, NS1619 caused a dose-dependent increase in coronary flow that was inhibited by L-NAME. In EA.hy926 cells, NS1619 also caused a dose-dependent increase in the intracellular calcium ion concentration [Ca(2+)]i, as measured using the FURA-2 fluorescent probe. Moreover, NS1619 decreased the oxygen consumption rate in EA.hy926 cells, as assessed using a Clark-type oxygen electrode. However, when NS1619 was applied in the presence of oligomycin, the oxygen consumption increased. NS1619 also decreased the mitochondrial membrane potential, as measured using a JC-1 fluorescent probe in the presence and absence of oligomycin. Additionally, the application of NS1619 to submitochondrial particles inhibited ATP synthase. In summary, NS1619 has pleiotropic actions on EA.hy926 cells and acts not only as an opener of the BKCa channel in EA.hy926 cells but also as an inhibitor of the respiratory chain component, sarcoplasmic reticulum ATPase, which leads to the release of Ca(2+) from the endoplasmic reticulum. Furthermore, NS1619 has the oligomycin-like property of inhibiting mitochondrial ATP synthase.

Stimulation of calcium-sensing receptors induces endothelium-dependent vasorelaxations via nitric oxide production and activation of IKCa channels

Stimulation of vascular calcium-sensing receptors (CaSRs) is reported to induce both constrictions and relaxations. However, cellular mechanisms involved in these responses remain unclear. The present study investigates the effect of stimulating CaSRs on vascular contractility and focuses on the role of the endothelium, nitric oxide (NO) and K(+) channels in these responses. In wire myography studies, increasing [Ca(2+)]o from 1mM to 6mM induced concentration-dependent relaxations of methoxamine pre-contracted rabbit mesenteric arteries. [Ca(2+)]o-induced relaxations were dependent on a functional endothelium, and were inhibited by the negative allosteric CaSR modulator Calhex-231. [Ca(2+)]o-induced relaxations were reduced by inhibitors of endothelial NO synthase, guanylate cyclase, and protein kinase G. CaSR activation also induced NO production in freshly isolated endothelial cells (ECs) in experiments using the fluorescent NO indicator DAF-FM. Pre-treatment with inhibitors of large (BKCa) and intermediate (IKCa) Ca(2+)-activated K(+) channels (iberiotoxin and charybdotoxin), and Kv7 channels (linopirdine) also reduced [Ca(2+)]o-induced vasorelaxations. Increasing [Ca(2+)]o also activated IKCa currents in perforated-patch recordings of isolated mesenteric artery ECs. These findings indicate that stimulation of CaSRs induces endothelium-dependent vasorelaxations which are mediated by two separate pathways involving production of NO and activation of IKCa channels. NO stimulates PKG leading to BKCa activation in vascular smooth muscle cells, whereas IKCa activity contributes to endothelium-derived hyperpolarisations.

Nitroxyl (HNO): A Reduced Form of Nitric Oxide with Distinct Chemical, Pharmacological, and Therapeutic Properties

Nitroxyl (HNO), the one-electron reduced form of nitric oxide (NO), shows a distinct chemical and biological profile from that of NO. HNO is currently being viewed as a vasodilator and positive inotropic agent that can be used as a potential treatment for heart failure. The ability of HNO to react with thiols and thiol containing proteins is largely used to explain the possible biological actions of HNO. Herein, we summarize different aspects related to HNO including HNO donors, chemistry, biology, and methods used for its detection.

Interaction of Hydrogen Sulfide with Nitric Oxide in the Cardiovascular System

Historically acknowledged as toxic gases, hydrogen sulfide (H₂S) and nitric oxide (NO) are now recognized as the predominant members of a new family of signaling molecules, “gasotransmitters” in mammals. While H₂S is biosynthesized by three constitutively expressed enzymes (CBS, CSE, and 3-MST) from L-cysteine and homocysteine, NO is generated endogenously from L-arginine by the action of various isoforms of NOS. Both gases have been transpired as the key and independent regulators of many physiological functions in mammalian cardiovascular, nervous, gastrointestinal, respiratory, and immune systems. The analogy between these two gasotransmitters is evident not only from their paracrine mode of signaling, but also from the identical and/or shared signaling transduction pathways. With the plethora of research in the pathophysiological role of gasotransmitters in various systems, the existence of interplay between these gases is being widely accepted. Chemical interaction between NO and H₂S may generate nitroxyl (HNO), which plays a specific effective role within the cardiovascular system. In this review article, we have attempted to provide current understanding of the individual and interactive roles of H₂S and NO signaling in mammalian cardiovascular system, focusing particularly on heart contractility, cardioprotection, vascular tone, angiogenesis, and oxidative stress.

Anthelmintics: The best way to predict the future is to create it

'The best way to predict the future is to create it.' When we look at drugs that are used to control parasites, we see that new knowledge has been created (discovered) about their modes of action. This knowledge will allow us to predict combinations of drugs which can be used together rationally to increase the spectrum of action and to slow the development of anthelmintic resistance. In this paper we comment on some recent observations of ours on the modes of action of emodepside, diethylcarbamazine and tribendimidine. Emodepside increases the activation of a SLO-1 K(+) current inhibiting movement, and diethylcarbamazine has a synergistic effect on the effect of emodepside on the SLO-1 K(+) current, increasing the size of the response. The combination may be considered for further testing for therapeutic use. Tribendimidine is a selective cholinergic nematode B-subtype nAChR agonist, producing muscle depolarization and contraction. It has different subtype selectivity to levamisole and may be effective in the presence of some types of levamisole resistance. The new information about the modes of action may aid the design of rational drug combinations designed to slow the development of resistance or increase the spectrum of action.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Hydrogen sulfide regulates the colonic motility by inhibiting both L-type calcium channels and BKCa channels in smooth muscle cells of rat colon

Objective

To examine the hypothesis that hydrogen sulfide (H₂S) regulates the colonic motility by modulating both L-type voltage-dependent calcium channels and large conductance Ca²⁺-activated K⁺ (BK_Ca) channels.

Methods

Immunohistochemistry was performed on rat colonic samples to investigate the localization of the H₂S-producing enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE). The contractions of proximal colonic smooth muscle were studied in an organ bath system. The whole-cell patch-clamp technique was used to record both L-type calcium currents (I_Ca,L) and BK_Ca currents in colonic smooth muscle cells (SMCs) isolated from male Wistar rats.

Results

Immunohistochemistry revealed the presence of CBS and CSE in mucosa, smooth muscle cells and myenteric neurons. The H₂S donor NaHS inhibited spontaneous contractions of the longitudinal muscle and circular muscle strips in a dose-dependent manner, and the inhibitory effects were not blocked by tetrodotoxin. NaHS inhibited the peak I_Ca,L in colonic SMCs at a membrane potential of 0 mV. The current-voltage (I-V) relationship of L-type calcium channels was modified by NaHS, and the peak of the I-V curve was shifted to the right. NaHS (200μΜ) evoked a significant rightward shift of the steady-state activation curve and inhibited the inactivation of L-type calcium channels. Furthermore, NaHS reversibly decreased the peak I_Ca,L in a dose-dependent manner. Likewise, BK_Ca channels were significantly inhibited by NaHS, and the addition of NaHS caused a time- and dose-dependent reduction in the BK_Ca current.

Conclusion

The relaxant effect of H₂S on colonic muscle strips may be associated with the direct inhibition of H₂S on L-type calcium channels. H₂S may be involved in the regulation of calcium homeostasis in colonic SMCs of rat colon.

Nicorandil directly and cyclic GMP-dependently opens K+ channels in human bypass grafts

As we previously demonstrated the role of different K(+) channels in the action of nicorandil on human saphenous vein (HSV) and human internal mammary artery (HIMA), this study aimed to analyse the contribution of the cGMP pathway in nicorandil-induced vasorelaxation and to determine the involvement of cGMP in the K(+) channel-activating effect of nicorandil. An inhibitor of soluble guanylate cyclase (GC), ODQ, significantly inhibited nicorandil-induced relaxation, while ODQ plus glibenclamide, a selective ATP-sensitive K(+) (KATP) channel inhibitor, produced a further inhibition of both vessels. In HSV, ODQ in combination with 4-aminopyridine, a blocker of voltage-gated K(+) (KV) channels, did not modify the concentration-response to nicorandil compared with ODQ, whereas in HIMA, ODQ plus iberiotoxin, a selective blocker of large-conductance Ca(2+)-activated K(+) (BKCa) channels, produced greater inhibition than ODQ alone. We showed that the cGMP pathway plays a significant role in the vasorelaxant effect of nicorandil on HSV and HIMA. It seems that nicorandil directly opens KATP channels in both vessels and BKCa channels in HIMA, although it is possible that stimulation of GC contributes to KATP channels activation in HIMA. Contrary, the activation of KV channels in HSV is probably due to GC activation and increased levels of cGMP.

4-Hydroxy-2-nonenal attenuates β2-adrenoceptor-mediated vasodilation of rat retinal arterioles

4-Hydroxy-2-nonenal (4-HNE) is a major reactive aldehyde formed by lipid peroxidation, and it plays an important role in the pathogenesis of several vascular diseases, including diabetes mellitus. In this study, we examined the effects of 4-HNE on the vasodilatory mechanisms of rat retinal arterioles. The retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles in the fundus images. Intravitreal injection of 4-HNE significantly prevented the vasodilation of retinal arterioles induced by the β2-adrenoceptor agonist salbutamol but not the nitric oxide donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Iberiotoxin, an inhibitor of large-conductance KCa (BKCa) channels, significantly reduced the salbutamol-induced vasodilation of retinal arterioles. The vasodilator effect of BMS-191011, a BKCa channel opener, on retinal arterioles was significantly attenuated by 4-HNE. These results suggest that 4-HNE attenuates retinal vasodilator responses to β2-adrenoceptor agonists through the impairment of the BKCa pathway. The direct effect of 4-HNE on retinal blood vessels may, therefore, contribute to the retinal vascular dysfunction observed in patients with diabetes mellitus.

Contribution of Kv7 channels to natriuretic peptide mediated vasodilation in normal and hypertensive rats

The Kv7 family of voltage-gated potassium channels are expressed within the vasculature where they are key regulators of vascular tone and mediate cAMP-linked endogenous vasodilator responses, a pathway that is compromised in hypertension. However, the role of Kv7 channels in non-cAMP-linked vasodilator pathways has not been investigated. Natriuretic peptides are potent vasodilators, which operate primarily through the activation of a cGMP-dependent signaling pathway. This study investigated the putative role of Kv7 channels in natriuretic peptide-dependent relaxations in the vasculature of normal and hypertensive animals. Relaxant responses of rat aorta to both atrial and C-type natriuretic peptides and the nitric oxide donor sodium nitroprusside were impaired by the Kv7 blocker linopirdine (10 μmol/L) but not by the Kv7.1-specific blocker HMR1556 (10 μmol/L) and other K(+) channel blockers. In contrast, only the atrial natriuretic peptide response was sensitive to linopirdine in the renal artery. These Kv7-mediated responses were attenuated in arteries from hypertensive rats. Quantitative polymerase chain reaction showed that A- and B-type natriuretic peptide receptors were expressed at high levels in the aorta and renal artery from normal and spontaneously hypertensive rats. This study provides the first evidence that natriuretic peptide responses are impaired in hypertension and that recruitment of Kv7 channels is a key component of natriuretic peptide-dependent vasodilations.

Lisinopril alters contribution of nitric oxide and K(Ca) channels to vasodilatation in small mesenteric arteries of spontaneously hypertensive rats

To investigate lisinopril effect on the contribution of nitric oxide (NO) and K(Ca) channels to acetylcholine (ACh)-induced relaxation in isolated mesenteric arteries of spontaneously hypertensive rats (SHRs). Third branch mesenteric arteries isolated from lisinopril treated SHR rats (20 mg/kg/day for ten weeks, SHR-T) or untreated (SHR-UT) or normotensive WKY rats were mounted on tension myograph and ACh concentration-response curves were obtained. Westernblotting of eNOS and K(Ca) channels was performed. ACh-induced relaxations were similar in all groups while L-NMMA and indomethacin caused significant rightward shift only in SHR-T group. Apamin and TRAM-34 (SK(Ca) and IK(Ca) channels blockers, respectively) significantly attenuated ACh-induced maximal relaxation by similar magnitude in vessels from all three groups. In the presence of L-NMMA, indomethacin, apamin and TRAM-34 further attenuated ACh-induced relaxation only in SHR-T. Furthermore, lisinopril treatment increased expression of eNOS, SK(Ca) and BK(Ca) proteins. Lisinopril treatment increased expression of eNOS, SK(Ca), BK(Ca) channel proteins and increased the contribution of NO to ACh-mediated relaxation. This increased role of NO was apparent only when EDHF component was blocked by inhibiting SK(Ca) and IK(Ca) channels. Such may suggest that in mesenteric arteries, non-EDHF component functions act as a reserve system to provide compensatory vasodilatation if (and when) hyperpolarization that is mediated by SK(Ca) and IK(Ca) channels is reduced.