Role of potassium channels in the nitric oxide-independent vasodilator response to acetylcholine

Peptide Lv and Angiogenesis: A Newly Discovered Angiogenic Peptide

Peptide Lv is a small endogenous secretory peptide with ~40 amino acids and is highly conserved among certain several species. While it was first discovered that it augments L-type voltage-gated calcium channels (LTCCs) in neurons, thus it was named peptide "Lv", it can bind to vascular endothelial growth factor receptor 2 (VEGFR2) and has VEGF-like activities, including eliciting vasodilation and promoting angiogenesis. Not only does peptide Lv augment LTCCs in neurons and cardiomyocytes, but it also promotes the expression of intermediate-conductance KCa channels (KCa3.1) in vascular endothelial cells. Peptide Lv is upregulated in the retinas of patients with early proliferative diabetic retinopathy, a disease involving pathological angiogenesis. This review will provide an overview of peptide Lv, its known bioactivities in vitro and in vivo, and its clinical relevance, with a focus on its role in angiogenesis. As there is more about peptide Lv to be explored, this article serves as a foundation for possible future developments of peptide Lv-related therapeutics to treat or prevent diseases.

Peptide Lv Promotes Trafficking and Membrane Insertion of KCa3.1 through the MEK1-ERK and PI3K-Akt Signaling Pathways

Peptide Lv is a small endogenous secretory peptide that is proangiogenic through hyperpolarizing vascular endothelial cells (ECs) by enhancing the current densities of KCa3.1 channels. However, it is unclear how peptide Lv enhances these currents. One way to enhance the current densities of ion channels is to promote its trafficking and insertion into the plasma membrane. We hypothesized that peptide Lv-elicited KCa3.1 augmentation occurs through activating the mitogen-activated protein kinase kinase 1 (MEK1)-extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K)-protein kinase B (Akt) signaling pathways, which are known to mediate ion channel trafficking and membrane insertion in neurons. To test this hypothesis, we employed patch-clamp electrophysiological recordings and cell-surface biotinylation assays on ECs treated with peptide Lv and pharmaceutical inhibitors of ERK and Akt. Blocking ERK or Akt activation diminished peptide Lv-elicited EC hyperpolarization and increase in KCa3.1 current densities. Blocking PI3K or Akt activation decreased the level of plasma membrane-bound, but not the total amount of KCa3.1 protein in ECs. Therefore, the peptide Lv-elicited EC hyperpolarization and KCa3.1 augmentation occurred in part through channel trafficking and insertion mediated by MEK1-ERK and PI3K-Akt activation. These results demonstrate the molecular mechanisms of how peptide Lv promotes EC-mediated angiogenesis.

Peptide Lv augments intermediate-conductance calcium-dependent potassium channels (KCa3.1) in endothelial cells to promote angiogenesis

Peptide Lv is a small endogenous secretory peptide that is expressed in various tissues and conserved across different species. Patients with diabetic retinopathy, an ocular disease with pathological angiogenesis, have upregulated peptide Lv in their retinas. The pro-angiogenic activity of peptide Lv is in part through promoting vascular endothelial cell (EC) proliferation, migration, and sprouting, but its molecular mechanism is not completely understood. This study aimed to decipher how peptide Lv promotes EC-dependent angiogenesis by using patch-clamp electrophysiological recordings, Western immunoblotting, quantitative PCR, and cell proliferation assays in cultured ECs. Endothelial cells treated with peptide Lv became significantly hyperpolarized, an essential step for EC activation. Treatment with peptide Lv augmented the expression and current densities of the intermediate-conductance calcium-dependent potassium (KCa3.1) channels that contribute to EC hyperpolarization but did not augment other potassium channels. Blocking KCa3.1 attenuated peptide Lv-elicited EC proliferation. These results indicate that peptide Lv-stimulated increases of functional KCa3.1 in ECs contributes to EC activation and EC-dependent angiogenesis.

NO-induced vasodilation correlates directly with BP in smooth muscle-Na/Ca exchanger-1-engineered mice: elevated BP does not attenuate endothelial function

Arterial smooth muscle Na+/Ca2+ exchanger-1 (SM-NCX1) promotes vasoconstriction or vasodilation by mediating, respectively, Ca2+ influx or efflux. In vivo, SM-NCX1 mediates net Ca2+ influx to help maintain myogenic tone (MT) and neuronally activated constriction. SM-NCX1-TG (overexpressing transgenic) mice have increased MT and mean blood pressure (MBP; +13.5 mmHg); SM-NCX1-KO (knockout) mice have reduced MT and MBP (-11.1 mmHg). Endothelium-dependent vasodilation (EDV) is often impaired in hypertension. We tested whether genetically engineered SM-NCX1 expression and consequent BP changes similarly alter EDV. Isolated, pressurized mesenteric resistance arteries with MT from SM-NCX1-TG and conditional SM-NCX1-KO mice, and femoral arteries in vivo from TG mice were studied. Acetylcholine (ACh)-dilated TG arteries with MT slightly more than control or KO arteries, implying that SM-NCX1 overexpression does not impair EDV. In preconstricted KO, but not TG mouse arteries, however, ACh- and bradykinin-triggered vasodilation was markedly attenuated. To circumvent the endothelium, phenylephrine-constricted resistance arteries were tested with Na-nitroprusside [SNP; nitric oxide (NO) donor] and cGMP. This endothelium-independent vasodilation was augmented in TG but attenuated in KO arteries that lack NCX1-mediated Ca2+ clearance. Baseline cytosolic Ca2+ ([Ca2+]cyt) was elevated in TG femoral arteries in vivo, supporting the high BP; furthermore, SNP-triggered [Ca2+]cyt decline and vasodilation were augmented as NO and cGMP promote myocyte polarization thereby enhancing NCX1-mediated Ca2+ efflux. The TG mouse data indicate that BP elevation does not attenuate endothelium-dependent vasodilation. Thus, in essential hypertension and many models the endothelial impairment that supports the hypertension apparently is not triggered by BP elevation but by extravascular mechanisms.NEW & NOTEWORTHY Endothelium-dependent, ACh-induced vasodilation (EDV) is attenuated, and arterial myocyte Na+/Ca2+ exchangers (NCX1) are upregulated in many forms of hypertension. Surprisingly, mildly hypertensive smooth muscle-specific (SM)-NCX1 transgenic mice exhibited modestly enhanced EDV and augmented endothelium-independent vasodilation (EIV). Conversely, mildly hypotensive SM-NCX1-knockout mice had greatly attenuated EIV. These adaptations help compensate for NCX1 expression-induced alterations in cytosolic Ca2+ and blood pressure (BP) and belie the view that elevated BP, itself, causes the endothelial dysregulation in hypertension.

Amplification of endothelium-dependent vasodilatation in contracting human skeletal muscle: role of KIR channels

KEY POINTS

In humans, the vasodilatory response to skeletal muscle contraction is mediated in part by activation of inwardly rectifying potassium (K_IR ) channels. Evidence from animal models suggest that K_IR channels serve as electrical amplifiers of endothelium-dependent hyperpolarization (EDH). We found that skeletal muscle contraction amplifies vasodilatation to the endothelium-dependent agonist ACh, whereas there was no change in the vasodilatory response to sodium nitroprusside, an endothelium-independent nitric oxide donor. Blockade of K_IR channels reduced the exercise-induced amplification of ACh-mediated vasodilatation. Conversely, pharmacological activation of K_IR channels in quiescent muscle via intra-arterial infusion of KCl independently amplified the vasodilatory response to ACh. This study is the first in humans to demonstrate that specific endothelium-dependent vasodilatory signalling is amplified in the vasculature of contracting skeletal muscle and that K_IR channels may serve as amplifiers of EDH-like vasodilatory signalling in humans.

ABSTRACT

The local vasodilatory response to muscle contraction is due in part to the activation of inwardly rectifying potassium (K_IR ) channels. Evidence from animal models suggest that K_IR channels function as 'amplifiers' of endothelium-dependent vasodilators. We tested the hypothesis that contracting muscle selectively amplifies endothelium-dependent vasodilatation via activation of K_IR channels. We measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to local intra-arterial infusion of ACh (endothelium-dependent dilator) during resting conditions, handgrip exercise (5% maximum voluntary contraction) or sodium nitroprusside (SNP; endothelium-independent dilator) which served as a high-flow control condition (n = 7, young healthy men and women). Trials were performed before and after blockade of K_IR channels via infusion of barium chloride. Exercise augmented peak ACh-mediated vasodilatation (ΔFVC saline: 117 ± 14; exercise: 236 ± 21 ml min^-1 (100 mmHg)^-1 ; P < 0.05), whereas SNP did not impact ACh-mediated vasodilatation. Blockade of K_IR channels attenuated the exercise-induced augmentation of ACh. In eight additional subjects, SNP was administered as the experimental dilator. In contrast to ACh, exercise did not alter SNP-mediated vasodilatation (ΔFVC saline: 158 ± 35; exercise: 121 ± 22 ml min^-1 (100 mmHg)^-1 ; n.s.). Finally, in a subset of six subjects, direct pharmacological activation of K_IR channels in quiescent muscle via infusion of KCl amplified peak ACh-mediated vasodilatation (ΔFVC saline: 97 ± 15, KCl: 142 ± 16 ml min^-1  (100 mmHg)^-1 ; respectively; P < 0.05). These findings indicate that skeletal muscle contractions selectively amplify endothelium-dependent vasodilatory signalling via activation of K_IR channels, and this may be an important mechanism contributing to the normal vasodilatory response to exercise in humans.

Blocking 5-HT2 receptor restores cardiovascular disorders in type 1 experimental diabetes

This study aimed to determine whether the serotonergic modulation, through selective 5-HT₂ receptor blockade, restores cardiovascular disturbances in type 1 diabetic rats. Diabetes was induced by alloxan (150 mg/kg, s.c.) and maintained for 4 weeks. 5-HT₂ receptor was blocked by sarpogrelate (30 mg/kg.day; 14 days; p.o.). Systolic blood pressure (SBP), heart rate (HR), glycaemia and body weight (BW) were monitored periodically. Animals were sacrificed at the end of the study and the heart, right kidney and thoracic aorta were removed; plasma samples were also obtained. Left ventricular hypertrophy index (LVH) and renal hypertrophy index (RH) were determined. Vascular function was studied in aorta rings; additionally, superoxide anion (O₂•⁻) production (by lucigenin-enhanced chemiluminescence) and lipid peroxidation (by thiobarbituric acid reactive substances assay) were measured. Neither alloxan nor sarpogrelate treatments altered HR, LVH or endothelium-independent relaxation. SBP, glycaemia, BW, RH, O₂•⁻ production and lipid peroxidation were significantly altered in diabetic animals compared with controls. Sarpogrelate treatment considerably decreased SBP, RH, O₂•⁻ production and lipid peroxidation. Endothelium-dependent relaxation was severely reduced in diabetic animal aortas compared to controls; sarpogrelate treatment markedly improved it. Our outcomes show that selectively blocking 5-HT₂ receptors has beneficial effects on impaired cardiovascular parameters in diabetes.

Alteration of endothelium-mediated vasodilator response in the rat hindlimb vasculature consecutive to chronic hypoxic stress: NO and EDHF involvement

The previously documented impairment of hindlimb blood flow consecutive to chronic hypoxia might be related to endothelial vasomotor dysfunction. The aim of this study was to assess in-vivo the effect of chronic hypoxic stress on endothelium-mediated vasodilator response of hindlimb vascular bed, especially as regards to endothelium-derived hyperpolarizing factor (EDHF) and nitric oxide (NO) pathway contribution. Dark Agouti rats were randomly assigned to live at barometric pressure approximately 760 mmHg (N rats) or approximately 550 mmHg (CH rats). Under anesthesia, catheters were placed in the carotid artery for arterial pressure measurement, and in the saphenous vein and iliac artery for drug delivery. Hindlimb blood flow (HBF) was measured by transit-time ultrasound flowmetry, at baseline and during endothelium-dependent vasodilator response induced by intra-arterial injection of acetylcholine (0.75 ng and 7.5 ng) with and without specific blockers of NOS (L-NAME) and EDHF (Charybdotoxin+Apamin). HBF and hindlimb vascular conductance changes in response to ACh infusion were significantly lower in CH than in N rats. The mechanisms responsible for this blunted response involved impairment in both NO pathway and EDHF. The chronic hypoxia-induced alteration of NO pathway was mainly related to the bioavailability of its substrate l-Arginine, since the infusion of l-Arginine restored the endothelial response to ACh in CH rats to the level of N rats. These results demonstrate that the impairment in endothelium-mediated vasodilator response of the hindlimb vascular tree induced by chronic hypoxic stress involves both NO and EDHF.

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

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

Hyperthermia-induced vasoconstriction of the carotid artery and the role of potassium channels

Clinical experience and experimental studies have shown that hyperthermia can cause cerebral ischaemia and brain damage. By in vitro experiments with heating, we previously were able to induce carotid artery constriction. The objective of the present study was to clarify the mechanism of this thermal response. Isometric tension was recorded in rabbit carotid artery specimens using organ baths during stepwise temperature elevation. The heating responses were investigated at basal tone, in precontracted vessels, after blocking of adrenergic responses and administration of potassium (K)-channel activators and inhibitors. Stepwise heating of carotid artery strips from 37 degrees C to 47 degrees C induced reproducible graded contraction. The hyperthermic responses were not due to adrenergic stimulation, which were reduced and resistant to neurogenic blockade by tetrodotoxin. Heating-induced contractions were potentiated by the K-channel inhibitors tetraethylammonium, BaCl2, charybdotoxin, and the Na+/K+ ATPase inhibitor ouabain. Levcromakalim (BRL), a K+-channel activator, reduced heating induced contractions. Heating of carotid artery preparations induced reversible graded vasoconstriction proportional to temperature. The heating-induced contractions were not mediated by an adrenergenic process, but rather were due to inhibition of K+ channels, which increases Ca2+ entry. In vivo, this reaction may lead to a disturbance of autoregulation of cerebral blood flow and ischemia with brain damage.

An ethyl acetate fraction obtained from a Southern Brazilian red wine relaxes rat mesenteric arterial bed through hyperpolarization and NO-cGMP pathway

A number of studies suggest that moderate consumption of red wine may be more effective than other alcoholic beverages in decreasing the risk of coronary heart disease (CAD). In this study, we investigated the effect of a crude extract (CE), as well as an ethyl acetate fraction (EAF) obtained from a Brazilian red wine in the mesenteric arterial bed (MAB) from rats. Our results showed that after the tonus of MAB was increased with phenylephrine (PE), increasing concentrations of CE induced a concentration-dependent relaxation; moreover, EAF was more potent in relaxing the MAB when compared with CE. In vessels depolarized with KCl (80 mM) or treated with the Na(+)/K(+)-ATPase pump inhibitor, ouabain (OUA; 100 microM), or with the K(+) channel blockers: barium (BaCl(2), 100 microM) and tetraethylammonium (TEA; 500 microM), the effect of EAF was significantly reduced. However, this effect was not altered by the ATP-dependent K(+) (K(ATP)) channel blocker, glibenclamide (GLI; 100 microM) as well as Charybdotoxin (ChTx 10 nM), a nonselective inhibitor of K(Ca) channels of large and intermediate conductance plus Apamin (Apamin 100 nM), a specific inhibitor of K(Ca) channels of small conductance. The residual vasodilator effect of EAF observed in vessels pretreated with L-NOARG (100 microM), 1H-[1,2,4,] oxadiazolo[4,3-alfa]quinoxalin, ODQ (10 microM) or KCl (80 mM), given separately, was reduced by the administration of KCl (40 mM) plus L-NOARG (100 microM). The present study demonstrates that the vasodilator effect of EAF is partially dependent upon membrane hyperpolarization in combination with nitric oxide (NO) release.

Effect of binge cocaine treatment on hindlimb vascular function

Chronic cocaine abuse is known to cause endothelial dysfunction and atherosclerosis. The present study investigated the effect of binge cocaine treatment, a model for chronic cocaine abuse, on the blood flow responses to the adrenergic agonists norepinephrine, phenylephrine and isoproterenol, the endothelium-dependent vasodilator acetylcholine, and the endothelium independent vasodilator sodium nitroprusside (SNP) in the hindlimb vascular bed of male Sprague Dawley rats. Rats received either single binge or double binge treatment. Each binge treatment consisted of three doses of cocaine (30 mg kg(-1) i.p.) for 3 days. For double binge treatment, there was a 4 day recovery period between the binges. At the end of the treatment the rats were anesthetized and agonists were administered into the right hindlimb circulation through a catheter in the left iliac artery and blood flow responses were measured with a flow probe around the right iliac artery. Rats receiving double cocaine binges showed a significant decrease in the magnitude and duration of the blood flow response to norepinephrine and a decrease in the duration of the blood flow response to phenylephrine, isoproterenol and acetylcholine when compared with saline controls. The blood flow response to SNP was not changed. Total plasma nitrate-nitrite levels were significantly reduced and big endothelin levels were significantly increased in rats receiving double cocaine binges. This study demonstrates that binge cocaine treatment can alter endothelial function, while not changing smooth muscle function, and impairs the adrenergic pathway.