Kv channel subunits that contribute to voltage-gated K+ current in renal vascular smooth muscle

Cycling matters: Sex hormone regulation of vascular potassium channels

Sex hormones and the reproductive cycle (estrus in rodents and menstrual in humans) have a known impact on arterial function. In spite of this, sex hormones and the estrus/menstrual cycle are often neglected experimental factors in vascular basic preclinical scientific research. Recent research by our own laboratory indicates that cyclical changes in serum concentrations of sex -hormones across the rat estrus cycle, primary estradiol, have significant consequences for the subcellular trafficking and function of KV. Vascular potassium channels, including KV, are essential components of vascular reactivity. Our study represents a small part of a growing field of literature aimed at determining the role of sex hormones in regulating arterial ion channel function. This review covers key findings describing the current understanding of sex hormone regulation of vascular potassium channels, with a focus on KV channels. Further, we highlight areas of research where the estrus cycle should be considered in future studies to determine the consequences of physiological oscillations in concentrations of sex hormones on vascular potassium channel function.

Prostanoids contribute to regulation of inwardly rectifying K+ channels in intrarenal arterial smooth muscle cells

AIM

The inward rectifier K⁺ (Kir) channels and prostanoids are important factors in regulating vascular tone, but the relationship between them has not been well studied. We aimed to study the involvement of prostanoids in regulating Kir activity in the rat intrarenal arteries (RIRAs).

MAIN METHODS

The vascular tone of isolated RIRAs was recorded with a wire myograph. The intracellular Ca²⁺ concentrations ([Ca²⁺]_i) and Kir currents were measured with a Ca²⁺-sensitive fluorescence probe and patch clamp, respectively, in the arterial smooth muscle cell (ASMC) freshly isolated from RIRAs. Kir2.1 expression in RIRAs was assayed by Western blotting.

KEY FINDINGS

At 0.03-1.0 mM, BaCl₂ (a specific Kir blocker) concentration-dependently contracted RIRAs and elevated [Ca²⁺]_i levels. Mild stimulations with various vasoconstrictors at low concentrations significantly potentiated RIRA contraction induced by Kir closure with BaCl₂. In both the quiescent and the stimulated RIRAs, cyclooxygenase inhibition and thromboxane-prostanoid receptor (TPR) antagonism depressed BaCl₂-induced RIRA contraction, while nitric oxide (NO) synthetase inhibition and endothelium-denudation enhanced the contraction. Kir2.1 expression was significantly more abundant in smaller RIRAs. Ba²⁺-sensitive Kir currents were depressed by TPR agonist U46619 while increased by NO donor sodium nitroprusside.

SIGNIFICANCE

The present results reveal that vasoconstrictor stimulation augments RIRA contraction induced by Kir closure with Ba⁺ and indicate that prostanoid synthesis followed by TPR activation is involved in the modulation of the myocyte Kir activity. This study suggests that prostanoid synthesis and TPR may be potential targets for dysfunctions in renal blood circulation.

Kv channels and vascular smooth muscle cell proliferation

Kv channels are present in virtually all VSMCs and strongly influence contractile responses. However, they are also instrumental in the proliferative, migratory, and secretory functions of synthetic, dedifferentiated VSMCs upon PM. In fact, Kv channels not only contribute to all these processes but also are active players in the phenotypic switch itself. This review is focused on the role(s) of Kv channels in VSMC proliferation, which is one of the best characterized functions of dedifferentiated VSMCs. VSMC proliferation is a complex process requiring specific Kv channels at specific time and locations. Their identification is further complicated by their large diversity and the differences in expression across vascular beds. Of interest, both conserved changes in some Kv channels and vascular bed-specific regulation of others seem to coexist and participate in VSMC proliferation through complementary mechanisms. Such a system will add flexibility to the process while providing the required robustness to preserve this fundamental cellular response.

KV channels and the regulation of vascular smooth muscle tone

VSMCs in resistance arteries and arterioles express a diverse array of K_V channels with members of the K_V 1, K_V 2 and K_V 7 families being particularly important. Members of the K_V channel family: (i) are highly expressed in VSMCs; (ii) are active at the resting membrane potential of VSMCs in vivo (-45 to -30 mV); (iii) contribute to the negative feedback regulation of VSMC membrane potential and myogenic tone; (iv) are activated by cAMP-related vasodilators, hydrogen sulfide and hydrogen peroxide; (v) are inhibited by increases in intracellular Ca²⁺ and vasoconstrictors that signal through G_q -coupled receptors; (vi) are involved in the proliferative phenotype of VSMCs; and (vii) are modulated by diseases such as hypertension, obesity, the metabolic syndrome and diabetes. Thus, K_V channels participate in every aspect of the regulation of VSMC function in both health and disease.

4-Aminopyridine: a pan voltage-gated potassium channel inhibitor that enhances Kv 7.4 currents and inhibits noradrenaline-mediated contraction of rat mesenteric small arteries

BACKGROUND AND PURPOSE

K_v 7.4 and K_v 7.5 channels are regulators of vascular tone. 4-Aminopyridine (4-AP) is considered a broad inhibitor of voltage-gated potassium (K_V ) channels, with little inhibitory effect on K_v 7 family members at mmol concentrations. However, the effect of 4-AP on K_v 7 channels has not been systematically studied. The aim of this study was to investigate the pharmacological activity of 4-AP on K_v 7.4 and K_v 7.5 channels and characterize the effect of 4-AP on rat resistance arteries.

EXPERIMENTAL APPROACH

Voltage clamp experiments were performed on Xenopus laevis oocytes injected with cRNA encoding KCNQ4 or KCNQ5, HEK cells expressing K_v 7.4 channels and on rat, freshly isolated mesenteric artery smooth muscle cells. The effect of 4-AP on tension, membrane potential, intracellular calcium and pH was assessed in rat mesenteric artery segments.

KEY RESULTS

4-AP increased the K_v 7.4-mediated current in oocytes and HEK cells but did not affect K_v 7.5 current. 4-AP also enhanced native mesenteric artery myocyte K⁺ current at sub-mmol concentrations. When applied to NA-preconstricted mesenteric artery segments, 4-AP hyperpolarized the membrane, decreased [Ca²⁺ ]_i and caused concentration-dependent relaxations that were independent of 4-AP-mediated changes in intracellular pH. Application of the K_v 7 channel blocker XE991 and BK_Ca channel blocker iberiotoxin attenuated 4-AP-mediated relaxation. 4-AP also inhibited the NA-mediated signal transduction to elicit a relaxation.

CONCLUSIONS AND IMPLICATIONS

These data show that 4-AP is able to relax NA-preconstricted rat mesenteric arteries by enhancing the activity of K_v 7.4 and BK_Ca channels and attenuating NA-mediated signalling.

Role of renal vascular potassium channels in physiology and pathophysiology

The control of renal vascular tone is important for the regulation of salt and water balance, blood pressure and the protection against damaging elevated glomerular pressure. The K⁺ conductance is a major factor in the regulation of the membrane potential (V_m ) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V_m via its effect on the opening probability of voltage-operated Ca²⁺ channels (VOCC) in VSMC. When K⁺ conductance increases V_m becomes more negative and vasodilation follows, while deactivation of K⁺ channels leads to depolarization and vasoconstriction. K⁺ channels in EC indirectly participate in the control of vascular tone by endothelium-derived vasodilation. Therefore, by regulating the tone of renal resistance vessels, K⁺ channels have a potential role in the control of fluid homoeostasis and blood pressure as well as in the protection of the renal parenchyma. The main classes of K⁺ channels (calcium activated (K_Ca ), inward rectifier (K_ir ), voltage activated (K_v ) and ATP sensitive (K_ATP )) have been found in the renal vessels. In this review, we summarize results available in the literature and our own studies in the field. We compare the ambiguous in vitro and in vivo results. We discuss the role of single types of K⁺ channels and the integrated function of several classes. We also deal with the possible role of renal vascular K⁺ channels in the pathophysiology of hypertension, diabetes mellitus and sepsis.

A novel KCNA1 mutation in a patient with paroxysmal ataxia, myokymia, painful contractures and metabolic dysfunctions

Episodic ataxia type 1 (EA1) is a human dominant neurological syndrome characterized by continuous myokymia, episodic attacks of ataxic gait and spastic contractions of skeletal muscles that can be triggered by emotional stress and fatigue. This rare disease is caused by missense mutations in the KCNA1 gene coding for the neuronal voltage gated potassium channel Kv1.1, which contributes to nerve cell excitability in the cerebellum, hippocampus, cortex and peripheral nervous system. We identified a novel KCNA1 mutation, E283K, in an Italian proband presenting with paroxysmal ataxia and myokymia aggravated by painful contractures and metabolic dysfunctions. The E283K mutation is located in the S3-S4 extracellular linker belonging to the voltage sensor domain of Kv channels. In order to test whether the E283K mutation affects Kv1.1 biophysical properties we transfected HEK293 cells with WT or mutant cDNAs alone or in a 1:1 combination, and recorded relative potassium currents in the whole-cell configuration of patch-clamp. Mutant E283K channels display voltage-dependent activation shifted by 10mV toward positive potentials and kinetics of activation slowed by ~2 fold compared to WT channels. Potassium currents resulting from heteromeric WT/E283K channels show voltage-dependent gating and kinetics of activation intermediate between WT and mutant homomeric channels. Based on homology modeling studies of the mutant E283K, we propose a molecular explanation for the reduced voltage sensitivity and slow channel opening. Overall, our results suggest that the replacement of a negatively charged residue with a positively charged lysine at position 283 in Kv1.1 causes a drop of potassium current that likely accounts for EA-1 symptoms in the heterozygous carrier.

Unravelling the complexities of vascular smooth muscle ion channels: Fine tuning of activity by ancillary subunits

Which ion channel is the most important for regulating vascular tone? Which one is responsible for controlling the resting membrane potential or repolarization? Which channels are recruited by different intracellular signalling pathways or change in certain vascular diseases? Many different ion channels have been identified in the vasculature over the years and claimed as future therapeutic targets. Unfortunately, several of these ion channels are not just found in the vasculature, with many of them also found to have prominent functional roles in different organs of the body, which then leads to off-target effects. As cardiovascular diseases are expected to increase worldwide to epidemic proportions, ion channel research and the hunt for the next major therapeutic target to treat different vascular diseases has never been more important. However, I believe that the question we should now be asking is: which ancillary subunits are involved in regulating specific ion channels in the vasculature and do they have the potential to be new therapeutic targets?

Functional Expression Profile of Voltage-Gated K(+) Channel Subunits in Rat Small Mesenteric Arteries

Multiple K v channel complexes contribute to total K v current in numerous cell types and usually subserve different physiological functions. Identifying the complete compliment of functional K v channel subunits in cells is a prerequisite to understanding regulatory function. It was the goal of this work to determine the complete K v subunit compliment that contribute to functional K v currents in rat small mesenteric artery (SMA) myocytes as a prelude to studying channel regulation. Using RNA prepared from freshly dispersed myocytes, high levels of K v 1.2, 1.5, and 2.1 and lower levels of K v 7.4 α-subunit expressions were demonstrated by quantitative PCR and confirmed by Western blotting. Selective inhibitors correolide (K v 1; COR), stromatoxin (K v 2.1; ScTx), and linopirdine (K v 7.4; LINO) decreased K v current at +40 mV in SMA by 46 ± 4, 48 ± 4, and 6.5 ± 2 %, respectively, and K v current in SMA was insensitive to α-dendrotoxin. Contractions of SMA segments pretreated with 100 nmol/L phenylephrine were enhanced by 27 ± 3, 30 ± 8, and 7 ± 3 % of the response to 120 mmol/L KCl by COR, ScTX, and LINO, respectively. The presence of K v 6.1, 9.3, β1.1, and β1.2 was demonstrated by RT-PCR using myocyte RNA with expressions of K vβ1.2 and K v 9.3 about tenfold higher than K vβ1.1 and K v 6.1, respectively. Selective inhibitors of K v 1.3, 3.4, 4.1, and 4.3 channels also found at the RNA and/or protein level had no significant effect on K v current or contraction. These results suggest that K v current in rat SMA myocytes are dominated equally by two major components consisting of K v 1.2-1.5-β1.2 and K v 2.1-9.3 channels along with a smaller contribution from K v 7.4 channels but differences in voltage dependence of activation allows all three to provide significant contributions to SMA function at physiological voltages.

Selective Kv1.3 channel blocker as therapeutic for obesity and insulin resistance

Obesity is an epidemic, calling for innovative and reliable pharmacological strategies. Here, we show that ShK-186, a selective and potent blocker of the voltage-gated Kv1.3 channel, counteracts the negative effects of increased caloric intake in mice fed a diet rich in fat and fructose. ShK-186 reduced weight gain, adiposity, and fatty liver; decreased blood levels of cholesterol, sugar, HbA1c, insulin, and leptin; and enhanced peripheral insulin sensitivity. These changes mimic the effects of Kv1.3 gene deletion. ShK-186 did not alter weight gain in mice on a chow diet, suggesting that the obesity-inducing diet enhances sensitivity to Kv1.3 blockade. Several mechanisms may contribute to the therapeutic benefits of ShK-186. ShK-186 therapy activated brown adipose tissue as evidenced by a doubling of glucose uptake, and increased β-oxidation of fatty acids, glycolysis, fatty acid synthesis, and uncoupling protein 1 expression. Activation of brown adipose tissue manifested as augmented oxygen consumption and energy expenditure, with no change in caloric intake, locomotor activity, or thyroid hormone levels. The obesity diet induced Kv1.3 expression in the liver, and ShK-186 caused profound alterations in energy and lipid metabolism in the liver. This action on the liver may underlie the differential effectiveness of ShK-186 in mice fed a chow vs. an obesity diet. Our results highlight the potential use of Kv1.3 blockers for the treatment of obesity and insulin resistance.

Ion channel remodeling in vascular smooth muscle during hypertension: Implications for novel therapeutic approaches

Ion channels are multimeric, transmembrane proteins that selectively mediate ion flux across the plasma membrane in a variety of cells including vascular smooth muscle cells (VSMCs). The dynamic interplay of Ca(2+) and K(+) channels on the plasma membrane of VSMCs plays a pivotal role in modulating the vascular tone of small arteries and arterioles. The abnormally-elevated arterial tone observed in hypertension thus points to an aberrant expression and function of Ca(2+) and K(+) channels in the VSMCs. In this short review, we focus on the three well-studied ion channels in VSMCs, namely the L-type Ca(2+) (CaV1.2) channels, the voltage-gated K(+) (KV) channels, and the large-conductance Ca(2+)-activated K(+) (BK) channels. First, we provide a brief overview on the physiological role of vascular CaV1.2, KV and BK channels in regulating arterial tone. Second, we discuss the current understanding of the expression changes and regulation of CaV1.2, KV and BK channels in the vasculature during hypertension. Third, based on available proof-of-concept studies, we describe the potential therapeutic approaches targeting these vascular ion channels in order to restore blood pressure to normotensive levels.

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Different potassium channels are involved in relaxation of rat renal artery induced by P1075

The ATP-sensitive K(+) channels opener (K(ATP)CO), P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N″-3-pyridylguanidine], has been shown to cause relaxation of various isolated animal and human blood vessels by opening of vascular smooth muscle ATP-sensitive K(+) (K(ATP)) channels. In addition to the well-known effect on the opening of K(ATP) channels, it has been reported that vasorelaxation induced by some of the K(ATP)COs includes some other K(+) channel subtypes. Given that there is still no information on other types of K(+) channels possibly involved in the mechanism of relaxation induced by P1075, this study was designed to examine the effects of P1075 on the rat renal artery with endothelium and with denuded endothelium and to define the contribution of different K(+) channel subtypes in the P1075 action on this blood vessel. Our results show that P1075 induced a concentration-dependent relaxation of rat renal artery rings pre-contracted by phenylephrine. Glibenclamide, a selective K(ATP) channels inhibitor, partly antagonized the relaxation of rat renal artery induced by P1075. Tetraethylammonium (TEA), a non-selective inhibitor of Ca(2+)-activated K(+) channels, as well as iberiotoxin, a most selective blocker of large-conductance Ca(2+) -activated K(+) (BK(Ca)) channels, did not abolish the effect of P1075 on rat renal artery. In contrast, a non-selective blocker of voltage-gated K(+) (K(V)) channels, 4-aminopyridine (4-AP), as well as margatoxin, a potent inhibitor of K(V)1.3 channels, caused partial inhibition of the P1075-induced relaxation of rat renal artery. In addition, in this study, P1075 relaxed contractions induced by 20 mM K(+) , but had no effect on contractions induced by 80 mM K(+). Our results showed that P1075 induced strong endothelium-independent relaxation of rat renal artery. It seems that K(ATP), 4-AP- and margatoxin-sensitive K(+) channels located in vascular smooth muscle mediated the relaxation of rat renal artery induced by P1075.

Role of Kv1 potassium channels in regulating dopamine release and presynaptic D2 receptor function

Dopamine (DA) release in the CNS is critical for motor control and motivated behaviors. Dysfunction of its regulation is thought to be implicated in drug abuse and in diseases such as schizophrenia and Parkinson's. Although various potassium channels located in the somatodendritic compartment of DA neurons such as G-protein-gated inward rectifying potassium channels (GIRK) have been shown to regulate cell firing and DA release, little is presently known about the role of potassium channels localized in the axon terminals of these neurons. Here we used fast-scan cyclic voltammetry to study electrically-evoked DA release in rat dorsal striatal brain slices. We find that although G-protein-gated inward rectifying (GIRK) and ATP-gated (K_ATP) potassium channels play only a minor role, voltage-gated potassium channels of the Kv1 family play a major role in regulating DA release. The use of Kv subtype-selective blockers confirmed a role for Kv1.2, 1.3 and 1.6, but not Kv1.1, 3.1, 3.2, 3.4 and 4.2. Interestingly, Kv1 blockers also reduced the ability of quinpirole, a D2 receptor agonist, to inhibit evoked DA overflow, thus suggesting that Kv1 channels also regulate presynaptic D2 receptor function. Our work identifies Kv1 potassium channels as key regulators of DA release in the striatum.

A model of the interaction between N-type and C-type inactivation in Kv1.4 channels

Kv1.4 channels are Shaker-related voltage-gated potassium channels with two distinct inactivation mechanisms. Fast N-type inactivation operates by a ball-and-chain mechanism. Slower C-type inactivation is not so well defined, but involves intracellular and extracellular conformational changes of the channel. We studied the interaction between inactivation mechanisms using two-electrode voltage-clamp of Kv1.4 and Kv1.4ΔN (amino acids 2-146 deleted to remove N-type inactivation) heterologously expressed in Xenopus oocytes. We manipulated C-type inactivation by introducing a lysine-tyrosine point mutation (K532Y, equivalent to Shaker T449Y) that diminishes C-type inactivation. We used experimental data to develop a comprehensive computer model of Kv1.4 channels to determine the interaction between activation and N- and C-type inactivation mechanisms needed to replicate the experimental data. C-type inactivation began at lower voltage preactivated states, whereas N-type inactivation was coupled directly to the open state. A model with distinct N- and C-type inactivated states was not able to reproduce experimental data, and direct transitions between N- and C-type inactivated states were required, i.e., there is coupling between N- and C-type inactivated states. C-type inactivation is the rate-limiting step determining recovery from inactivation, so understanding C-type inactivation, and how it is coupled to N-type inactivation, is critical in understanding how channels act to repetitive stimulation.

Quantitative rt-PCR methods for investigation of low copy potassium channel gene expression in native murine arteries

Voltage-gated K+ channels (K(V) channels) are encoded by the KCNx gene family and have such a wide range of properties that it is necessary to identify the precise expression profile that is instrumental in governing the electrical phenotype of a cell and its response to extrinsic factors. Real-time quantitative RT-PCR methodology has been developed and validated for specific RNA species in vascular smooth muscle cells. We have shown that most of the KCNA gene family, encoding the major K(V)alpha1 subunits, was markedly up-regulated in the resistance artery compared to the thoracic aorta, in line with reported patch-clamp recordings. Thus quantitative real-time RT-PCR data can be translated into physiological response.

Voltage gated K+ channel expression in arteries of Wistar-Kyoto and spontaneously hypertensive rats

BACKGROUND

We have previously demonstrated differences in the gene expression of voltage-gated K v1.X channel alpha-subunits in arteries from Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs). The purpose of this study was to test the hypothesis that these differences are also present at the protein level.

METHODS

Proteins were isolated from the aorta, mesenteric (MAs) and tail arteries (TAs) of 12- to 15-week-old male WKY and SHR, and analyzed by immunoblotting. K(v) currents were recorded from MA myocytes by patch clamp methods.

RESULTS

Expression of Kv1.2, Kv1.5, and Kv2.1 was higher in MAs but was not different in aortas of SHRs as compared to WKYs. In the TA, expression of Kv1.2 and Kv1.5 was higher while that of Kv2.1 was lower in SHR compared to WKY. In the MA, the larger expression of an 80 kDa species of Kv1.2 in SHRs was associated with a lower expression of a 60 kDa species. Kv2.1 gene expression was larger in MAs from SHRs but not different in TAs. K(v) currents associated with Kv1.X and Kv2.1 channels were both larger in MA myocytes from SHRs but less than expected based upon differences in K(v) alpha-subunit protein expression.

CONCLUSIONS

For the MA, K(v) protein expression and current components between WKYs and SHRs were qualitatively consistent, but differences in gene and protein expression were not closely correlated. The higher expression of K(v) subunits in small mesenteric arteries (SMAs) of SHR would tend to maintain normal myogenic activity and vasoconstrictor reserve, and could be viewed as a form of homeostatic remodeling.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Kv1.5 is a major component underlying the A-type potassium current in retinal arteriolar smooth muscle

Little is known about the molecular characteristics of the voltage-activated K⁺ (K_v) channels that underlie the A-type K⁺ current in vascular smooth muscle cells of the systemic circulation. We investigated the molecular identity of the A-type K⁺ current in retinal arteriolar myocytes using patch-clamp techniques, RT-PCR, immunohistochemistry, and neutralizing antibody studies. The A-type K⁺ current was resistant to the actions of specific inhibitors for K_v3 and K_v4 channels but was blocked by the K_v1 antagonist correolide. No effects were observed with pharmacological agents against K_v1.1/2/3/6 and 7 channels, but the current was partially blocked by riluzole, a K_v1.4 and K_v1.5 inhibitor. The current was not altered by the removal of extracellular K⁺ but was abolished by flecainide, indicative of K_v1.5 rather than K_v1.4 channels. Transcripts encoding K_v1.5 and not K_v1.4 were identified in freshly isolated retinal arterioles. Immunofluorescence labeling confirmed a lack of K_v1.4 expression and revealed K_v1.5 to be localized to the plasma membrane of the arteriolar smooth muscle cells. Anti-K_v1.5 antibody applied intracellularly inhibited the A-type K⁺ current, whereas anti-K_v1.4 antibody had no effect. Co-expression of K_v1.5 with K_vβ1 or K_vβ3 accessory subunits is known to transform K_v1.5 currents from delayed rectifers into A-type currents. K_vβ1 mRNA expression was detected in retinal arterioles, but K_vβ3 was not observed. K_vβ1 immunofluorescence was detected on the plasma membrane of retinal arteriolar myocytes. The findings of this study suggest that K_v1.5, most likely co-assembled with K_vβ1 subunits, comprises a major component underlying the A-type K⁺ current in retinal arteriolar smooth muscle cells.

Heterogeneous Kv1 function and expression in coronary myocytes from right and left ventricles in rats

Coronary blood flow control is not uniform along the vascular tree and particularly between the right coronary artery and the left anterior descending artery. Resting membrane potential that contributes largely to the vascular tone is mainly regulated by K(+) channels in coronary myocytes. In the present study, we hypothesized that right coronary artery (RCA) and left coronary artery (LCA) exhibited a cell-specific function of K(+) channels. The net outward current was markedly greater in RCA compared with LCA cells, and this difference was due to a larger 4-aminopyridine (4-AP)-sensitive voltage-gated potssium (Kv) current in RCA cells, whereas the iberiotoxin (IbTx)-sensitive, large conductance Ca(2+)-dependent potassium (BK(Ca)) current was smaller in RCA cells. To go further in the molecular identity of this Kv current, we used 50 nM correolide, which specifically blocked Kv1 family alpha-subunits. Outward currents generated by ramp depolarization protocols were highly sensitive to correolide in both RCA and LCA cells, suggesting that Kv1 contributed for a large part to the net outward current. 4-AP-induced contractions in isolated RCA, and LCA were greater than IbTx-induced contraction. Furthermore, the 4-AP-induced contraction in RCA was significantly greater than that in LCA, which is in agreement with the electrophysiological data. Finally, the Kv1.2 alpha-subunit but not the Kv1.5 was detected in both RCA and LCA using primary specific antibody in Western blotting and immunofluorescence assay, and expression of Kv1.2 alpha-subunit was markedly higher in RCA compared with LCA. In summary, we reported for the first time a heterogeneous function and expression of Kv1 alpha-subunits in rat coronary myocytes isolated from RCA or LCA.

Contribution of Kv channels to phenotypic remodeling of human uterine artery smooth muscle cells

Vascular smooth muscle cells (VSMCs) perform diverse functions that can be classified into contractile and synthetic (or proliferating). All of these functions can be fulfilled by the same cell because of its capacity of phenotypic modulation in response to environmental changes. The resting membrane potential is a key determinant for both contractile and proliferating functions. Here, we have explored the expression of voltage-dependent K+ (Kv) channels in contractile (freshly dissociated) and proliferating (cultured) VSMCs obtained from human uterine arteries to establish their contribution to the functional properties of the cells and their possible participation in the phenotypic switch. We have studied the expression pattern (both at the mRNA and at the protein level) of Kvalpha subunits in both preparations as well as their functional contribution to the K+ currents of VSMCs. Our results indicate that phenotypic remodeling associates with a change in the expression and distribution of Kv channels. Whereas Kv currents in contractile VSMCs are mainly performed by Kv1 channels, Kv3.4 is the principal contributor to K+ currents in cultured VSMCs. Furthermore, selective blockade of Kv3.4 channels resulted in a reduced proliferation rate, suggesting a link between Kv channels expression and phenotypic remodeling.

Paracrine role for periadventitial adipose tissue in the regulation of arterial tone

Recent studies propose a paracrine role for periadventitial adipose tissue in the regulation of vascular tone. This regulation depends on the anatomical integrity of the periadventitial adipose tissue and involves adipocyte-derived relaxing factor (ADRF). Although the nature of ADRF is largely unknown, it is released by periadventitial adipocytes and induces vasorelaxation by opening K+ channels in the plasma membrane of smooth muscle cells. Alterations in the paracrine role of periadventitial adipose tissue might have a role in vascular dysfunction in hypertension and metabolic disease. Therefore, understanding alterations in ADRF release and the K+ channels involved will help further our understanding of the increased cardiovascular risk and development of chronic vascular disease in obesity. Furthermore, ADRF and perhaps its putative targets might represent exciting new targets for the development of drugs to treat cardiovascular disorders.

Functional up-regulation of KCNA gene family expression in murine mesenteric resistance artery smooth muscle

This study focused on the hypothesis that KCNA genes (which encode K(V)alpha1 voltage-gated K(+) channels) have enhanced functional expression in smooth muscle cells of a primary determinant of peripheral resistance - the small mesenteric artery. Real-time PCR methodology was developed to measure cell type-specific in situ gene expression. Profiles were determined for arterial myocyte expression of RNA species encoding K(V)alpha1 subunits as well as K(V)beta1, K(V)alpha2.1, K(V)gamma9.3, BK(Ca)alpha1 and BK(Ca)beta1. The seven major KCNA genes were expressed and more readily detected in endothelium-denuded mesenteric resistance artery compared with thoracic aorta; quantification revealed dramatic differential expression of one to two orders of magnitude. There was also four times more RNA encoding K(V)alpha2.1 but less or similar amounts encoding K(V)beta1, K(V)gamma9.3, BK(Ca)alpha1 and BK(Cabeta)1. Patch-clamp recordings from freshly isolated smooth muscle cells revealed dominant K(V)alpha1 K(+) current and current density twice as large in mesenteric cells. Therefore, we suggest the increased RNA production of the resistance artery impacts on physiological function, although there is quantitatively less K(+) current than might be expected. The mechanism conferring up-regulated expression of KCNA genes may be common to all the gene family and play a functional role in the physiological control of blood pressure.

Estradiol binding to maxi-K channels induces their down-regulation via proteasomal degradation

Estrogens exert their biological action via both genomic and non-genomic mechanisms. Proteins different from classical estradiol receptors are believed to mediate the latter effects. Here we demonstrate that the maxi-K channel functions as an estrogen-binding protein in transfected HEK293 cells. Whole-cell maxi-K channel currents and protein expression were attenuated by exposure to either 17alpha- or 17beta-estradiol. This effect was dose-dependent for 17beta-estradiol at concentrations ranging from 10 nm to 1 microm, while 17alpha-estradiol inhibited channel expression only at 1 microm. These effects were mediated by direct low affinity binding of estradiol to the maxi-K channel but not to its accessory beta1-subunit, as revealed by cell membrane estradiol binding assays. However, specific binding of estradiol to the channel was facilitated by the presence of the beta1 subunit. Addition of MG-132, a blocker of proteasomal degradation, stabilized channel expression. These data suggest that channel down-regulation is mediated by estrogen-induced proteasomal degradation, similar to the pathway used for estrogen receptor degradation. Membrane expression of endogenous maxi-K channels in cultured vascular smooth muscle cells was also attenuated by prolonged exposure to 17alpha- and 17beta-estradiol. Thus our studies demonstrate that estrogen binds to maxi-K channels and may directly regulate channel expression and function. These results will have important implications in understanding estradiol-induced effects in multiple tissues including vascular smooth muscle.

Voltage-gated K+ channels in rat small cerebral arteries: molecular identity of the functional channels

Voltage-gated potassium (KV) channels represent an important dilator influence in the cerebral circulation, but the composition of these tetrameric ion channels remains unclear. The goals of the present study were to evaluate the contribution of KV1 family channels to the resting membrane potential and diameter of small rat cerebral arteries, and to identify the alpha-subunit composition of these channels using patch-clamp, molecular and immunological techniques. Initial studies indicated that 1 micromol l(-1) correolide (COR), a specific antagonist of KV1 channels, depolarized vascular smooth muscle cells (VSMCs) in pressurized (60 mmHg) cerebral arteries from -55 +/- 1 mV to -34 +/- 1 mV, and reduced the resting diameter from 152 +/- 15 microm to 103 +/- 20 microm. In patch clamped VSMCs from these arteries, COR-sensitive KV1 current accounted for 65 % of total outward KV current and was observed at physiological membrane potentials. RT-PCR identified mRNA encoding each of the six classical KV1 alpha-subunits, KV1.1-1.6, in rat cerebral arteries. However, only the KV1.2 and 1.5 proteins were detected by Western blot. The expression of these proteins in VSMCs was confirmed by immunocytochemistry and co-immunoprecipitation of KV1.2 and 1.5 from VSMC membranes suggested KV1.2/1.5 channel assembly. Subsequently, the pharmacological and voltage-sensitive properties of KV1 current in VSMCs were found to be consistent with a predominant expression of KV1.2/1.5 heterotetrameric channels. The findings of this study suggest that KV1.2/1.5 heterotetramers are preferentially expressed in rat cerebral VSMCs, and that these channels contribute to the resting membrane potential and diameter of rat small cerebral arteries.