Angiotensin II inhibits and alters kinetics of voltage-gated K(+) channels of rat arterial smooth muscle

Combined Treatment with KV Channel Inhibitor 4-Aminopyridine and either γ-Cystathionine Lyase Inhibitor β-Cyanoalanine or Epinephrine Restores Blood Pressure, and Improves Survival in the Wistar Rat Model of Anaphylactic Shock

Simple Summary

Allergic diseases are presenting a constant increase all over the world and caused by such different substances as food, drugs, and pollens. Anaphylactic shock is the more severe complication of allergy which can induce death if the treatment is not administered immediately. Some patients do not respond to the recommended treatment, intra venous or intramuscular epinephrine. The pathophysiology of anaphylactic shock is still under investigation. The mediators released after the activation of mast cells and basophiles act on endothelial cells and smooth muscle cells, inducing the vasodilation responsible for hypotension and shock. Nitric oxide and hydrogen sulphide are both intracellular mediators that induce vasodilation. The role of potassium voltage dependent channels is suspected. We aimed to demonstrate the ability of a blocker of potassium voltage dependent channels, 4-aminopyridine, alone or in combination with inhibitors of cystathionine γ-lyase to restore blood pressure and improve survival in an ovalbumin rat anaphylactic shock model. The blockade of potassium voltage dependent channels alone or combined with inhibitors of cystathionine γ-lyase, dl-propargylglycine, or β-cyanoalanine restored blood pressure and improved survival. These findings suggest possible investigative treatment pathways for research concerning epinephrine-refractory anaphylactic shock in patients.

Abstract

The mechanism of anaphylactic shock (AS) remains incompletely understood. The potassium channel blocker 4-aminopyridine (4-AP), the inhibitors of cystathionine γ-lyase (ICSE), dl-propargylglycine (DPG) or β-cyanoalanine (BCA), and the nitric oxide (NO) synthase produce vasoconstriction and could be an alternative for the treatment of AS. The aim of this study was to demonstrate the ability of L-NAME, ICSE alone or in combination with 4-AP to restore blood pressure (BP) and improve survival in ovalbumin (OVA) rats AS. Experimental groups included non-sensitized Wistar rats (n = 6); AS (n = 6); AS (n = 10 per group) treated i.v. with 4-AP (AS+4-AP), epinephrine (AS+EPI), AS+DPG, AS+BCA, or with L-NAME (AS+L-NAME); or AS treated with drug combinations 4-AP+DPG, 4-AP+BCA, 4-AP+L-NAME, or 4-AP+EPI. AS was induced by i.v. OVA (1 mg). Treatments were administered i.v. one minute after AS induction. Mean arterial BP (MAP), heart rate (HR), and survival were monitored for 60 min. Plasma levels of histamine, prostaglandin E2 (PGE2) and F2 (PGF2α), leukotriene B4 and C4, angiotensin II, vasopressin, oxidative stress markers, pH, HCO3, PaO2, PaCO2, and K+ were measured. OVA induced severe hypotension and all AS rats died. Moreover, 4-AP, 4-AP+EPI, or 4-AP+BCA normalized both MAP and HR and increased survival. All sensitized rats treated with 4-AP alone or with 4-AP+BCA survived. The time-integrated MAP “area under the curve” was significantly higher after combined 4-AP treatment with ICSE. Metabolic acidosis was not rescued and NO, ICSE, and Kv inhibitors differentially alter oxidative stress and plasma levels of anaphylactic mediators. The AS-induced reduction of serum angiotensin II levels was prevented by 4-AP treatment alone or in combination with other drugs. Further, 4-AP treatment combined with EPI or with BCA also increased serum PGF2α, whereas only the 4-AP+EPI combination increased serum LTB4. Serum vasopressin and angiotensin II levels were increased by 4-AP treatment alone or in combination with other drugs. Moreover, 4-AP alone and in combination with inhibition of cystathionine γ-lyase or EPI normalizes BP, increases serum vasoconstrictor levels, and improves survival in the Wistar rat model of AS. These findings suggest possible investigative treatment pathways for research into epinephrine-refractory anaphylactic shock in patients.

Ion channel molecular complexes in vascular smooth muscle

Ion channels that influence membrane potential and intracellular calcium concentration control vascular smooth muscle excitability. Voltage-gated calcium channels (VGCC), transient receptor potential (TRP) channels, voltage (K_V), and Ca²⁺-activated K⁺ (BK) channels are key regulators of vascular smooth muscle excitability and contractility. These channels are regulated by various signaling cues, including protein kinases and phosphatases. The effects of these ubiquitous signaling molecules often depend on the formation of macromolecular complexes that provide a platform for targeting and compartmentalizing signaling events to specific substrates. This manuscript summarizes our current understanding of specific molecular complexes involving VGCC, TRP, and K_V and BK channels and their contribution to regulating vascular physiology.

Potassium Channels in the Uterine Vasculature: Role in Healthy and Complicated Pregnancies

A progressive increase in maternal uterine and placental blood flow must occur during pregnancy to sustain the development of the fetus. Changes in maternal vasculature enable an increased uterine blood flow, placental nutrient and oxygen exchange, and subsequent fetal development. K⁺ channels are important modulators of vascular function, promoting vasodilation, inducing cell proliferation, and regulating cell signaling. Different types of K⁺ channels, such as Ca²⁺-activated, ATP-sensitive, and voltage-gated, have been implicated in the adaptation of maternal vasculature during pregnancy. Conversely, K⁺ channel dysfunction has been associated with vascular-related complications of pregnancy, including intrauterine growth restriction and pre-eclampsia. In this article, we provide an updated and comprehensive literature review that highlights the relevance of K⁺ channels as regulators of uterine vascular reactivity and their potential as therapeutic targets.

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.

KV channel trafficking and control of vascular tone

Membrane potential is a principal regulator of arterial contractility. Arterial smooth muscle cells express several different types of ion channel that control membrane potential, including K_V channels. K_V channel activation leads to membrane hyperpolarization, resulting in inhibition of voltage-dependent Ca²⁺ channels, a reduction in [Ca²⁺ ]_i , and vasodilation. In contrast, K_V channel inhibition leads to membrane depolarization and vasoconstriction. The ability of K_V channels to regulate arterial contractility is dependent upon the number of plasma membrane-resident channels and their open probability. Here, we will discuss mechanisms that alter the surface abundance of K_V channel proteins in arterial smooth muscle cells and the functional consequences of such regulation. Cellular processes that will be described include those that modulate K_V channel transcription, retrograde and anterograde trafficking, and protein degradation.

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.

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.

The Action of Smooth Muscle Cell Potassium Channels in the Pathology of Pulmonary Arterial Hypertension

Many different types of potassium channels with various functions exist in pulmonary artery smooth muscle cells, contributing to many physiological actions and pathological conditions. The deep involvement of these channels in the onset and exacerbation of pulmonary arterial hypertension (PAH) also continues to be revealed. In 2013, KCNK3 (TASK1), which encodes a type of two-pore domain potassium channel, was shown to be a predisposing gene for PAH by genetic mutation, and it was added to the PAH classification at the Fifth World Symposium on Pulmonary Hypertension (Nice International Conference). Decreased expression and inhibited activity of voltage-gated potassium channels, particularly KCNA5 (Kv1.5), are also seen in PAH, regardless of the cause, and facilitation of pulmonary arterial contraction and vascular remodeling has been shown. The calcium-activated potassium channels seen in smooth muscle cells also change from BKca (Kca1.1) to IKca (Kca3.1) predominance in PAH due to transformation and have effects including the facilitation of smooth muscle cell migration, enhancement of proliferation, and inhibition of apoptosis. Elucidation of these roles for potassium channels in pulmonary vasoconstriction and remodeling may help bring new therapeutic strategies into view.

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.

Steady-state modulation of voltage-gated K+ channels in rat arterial smooth muscle by cyclic AMP-dependent protein kinase and protein phosphatase 2B

Voltage-gated potassium channels (K_v) are important regulators of membrane potential in vascular smooth muscle cells, which is integral to controlling intracellular Ca2+ concentration and regulating vascular tone. Previous work indicates that K_v channels can be modulated by receptor-driven alterations of cyclic AMP-dependent protein kinase (PKA) activity. Here, we demonstrate that K_v channel activity is maintained by tonic activity of PKA. Whole-cell recording was used to assess the effect of manipulating PKA signalling on K_v and ATP-dependent K+ channels of rat mesenteric artery smooth muscle cells. Application of PKA inhibitors, KT5720 or H89, caused a significant inhibition of K_v currents. Tonic PKA-mediated activation of K_v appears maximal as application of isoprenaline (a β-adrenoceptor agonist) or dibutyryl-cAMP failed to enhance K_v currents. We also show that this modulation of K_v by PKA can be reversed by protein phosphatase 2B/calcineurin (PP2B). PKA-dependent inhibition of K_v by KT5720 can be abrogated by pre-treatment with the PP2B inhibitor cyclosporin A, or inclusion of a PP2B auto-inhibitory peptide in the pipette solution. Finally, we demonstrate that tonic PKA-mediated modulation of K_v requires intact caveolae. Pre-treatment of the cells with methyl-β-cyclodextrin to deplete cellular cholesterol, or adding caveolin-scaffolding domain peptide to the pipette solution to disrupt caveolae-dependent signalling each attenuated PKA-mediated modulation of the K_v current. These findings highlight a novel, caveolae-dependent, tonic modulatory role of PKA on K_v channels providing new insight into mechanisms and the potential for pharmacological manipulation of vascular tone.

Selective down-regulation of KV2.1 function contributes to enhanced arterial tone during diabetes

Enhanced arterial tone is a leading cause of vascular complications during diabetes. Voltage-gated K(+) (KV) channels are key regulators of vascular smooth muscle cells (VSMCs) contractility and arterial tone. Whether impaired KV channel function contributes to enhance arterial tone during diabetes is unclear. Here, we demonstrate a reduction in KV-mediated currents (IKv) in VSMCs from a high fat diet (HFD) mouse model of type 2 diabetes. In particular, IKv sensitive to stromatoxin (ScTx), a potent KV2 blocker, were selectively reduced in diabetic VSMCs. This was associated with decreased KV2-mediated regulation of arterial tone and suppression of the KV2.1 subunit mRNA and protein in VSMCs/arteries isolated from HFD mice. We identified protein kinase A anchoring protein 150 (AKAP150), via targeting of the phosphatase calcineurin (CaN), and the transcription factor nuclear factor of activated T-cells c3 (NFATc3) as required determinants of KV2.1 suppression during diabetes. Interestingly, substantial reduction in transcript levels for KV2.1 preceded down-regulation of large conductance Ca(2+)-activated K(+) (BKCa) channel β1 subunits, which are ultimately suppressed in chronic hyperglycemia to a similar extent. Together, our study supports the concept that transcriptional suppression of KV2.1 by activation of the AKAP150-CaN/NFATc3 signaling axis contributes to enhanced arterial tone during diabetes.

Pharmacogenomic strain differences in cardiovascular sensitivity to propofol

INTRODUCTION

A pharmacogenomic approach was used to further localize the genetic region responsible for previously observed enhanced cardiovascular sensitivity to propofol in Dahl Salt Sensitive (SS) versus control Brown Norway (BN) rats.

METHODS

Propofol infusion levels that decreased blood pressure by 50% were measured in BN.13(SS) rats (substitution of SS chromosome 13 into BN) and in five congenic (partial substitution) strains of SS.13(BN). The effect of superfused 2,6 diisopropylphenol on small mesenteric arterial vascular smooth muscle transmembrane potential was measured in congenic strains before and during superfusion with Rp-adenosine-3',5'-cyclic monophosphorothioate and 2.5 μM (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, inhibitors of protein kinase A and G, respectively. The genetic locus and potential role of the renin gene in mediating vascular smooth muscle sensitivity to propofol were determined in three selected subcongenic SS.BN¹³ strains.

RESULTS

A 30-32% smaller propofol infusion rate reduced blood pressure by 50% in BN.13(SS) compared with BN and the SS.13(BN) congenic containing an 80 BN gene substitution. Compared with the 80 BN gene-containing SS.13(BN) congenic, SS exhibited greater protein kinase A dependent vascular smooth muscle hyperpolarization in response to propofol. Using subcongenics, the increased propofol-induced cardiovascular sensitivity and hyperpolarization was further localized to an eight-gene region (containing the BN renin gene). Blockade of angiotensin receptors with losartan in this subcongenic increased propofol-induced hyperpolarization by threefold to that observed in SS.

CONCLUSIONS

Enhanced cardiovascular sensitivity to propofol in SS (compared with BN) is caused by an altered renin gene. Through modified second messenger function, this differentially regulates vascular smooth muscle contractile state and reduces vascular tone, thereby exacerbating cardiovascular depression by propofol.

Protein kinase C-independent inhibition of arterial smooth muscle K(+) channels by a diacylglycerol analogue

BACKGROUND AND PURPOSE

Analogues of the endogenous diacylglycerols have been used extensively as pharmacological activators of protein kinase C (PKC). Several reports show that some of these compounds have additional effects that are independent of PKC activation, including direct block of K(+) and Ca(2+) channels. We investigated whether dioctanoyl-sn-glycerol (DiC8), a commonly used diacylglycerol analogue, blocks K(+) currents of rat mesenteric arterial smooth muscle in a PKC-independent manner.

EXPERIMENTAL APPROACH

Conventional whole-cell and inside-out patch clamp was used to measure the inhibition of K(+) currents of rat isolated mesenteric smooth muscle cells by DiC8 in the absence and presence of PKC inhibitor peptide.

KEY RESULTS

Mesenteric artery smooth muscle K(v) currents inactivated very slowly with a time constant of about 2 s following pulses from -65 to +40 mV. Application of 1 µM DiC8 produced an approximate 40-fold increase in the apparent rate of inactivation. Pretreatment of the cells with PKC inhibitor peptide had a minimal effect on the action of DiC8, and substantial inactivation still occurred, indicating that this effect was mainly independent of PKC. We also found that DiC8 blocked BK and K(ATP) currents, and again a significant proportion of these blocks occurred independently of PKC activation.

CONCLUSIONS AND IMPLICATIONS

These results show that DiC8 has a direct effect on arterial smooth muscle K(+) channels, and this precludes its use as a PKC activator when investigating PKC-mediated effects on vascular K(+) channels.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Increased sensitivity of serotonin on the voltage-dependent K+ channels in mesenteric arterial smooth muscle cells of OLETF rats

This study examined the mechanisms of hypertension in diabetes. We investigated the effects of serotonin (5-HT) on voltage-dependent K(+) (Kv) channel activity, vasoconstriction, 5-HT receptor expression levels, and the involvement of protein kinase C (PKC) in mesenteric arteries of Otsuka Long-Evans Tokushima fatty (OLETF) rats compared with Long-Evans Tokushima Otsuka (LETO) rats. Blood pressure, body weight, blood glucose level, and mesenteric arterial wall thickness were greater in OLETF rats. The 5-HT-induced vasoconstriction of mesenteric arteries was greater in OLETF rats than in LETO rats and inhibited by the 5-HT(2A) inhibitor inhibitor, ketanserin. The Kv currents in mesenteric arterial smooth muscle cells (MASMCs), determined using a perforated patch clamp technique, was inhibited by 1 mM 4-AP (42.5 +/- 4.1% vs. 63.5 +/- 2.3% in LETO vs. OLETF rats at +40 mV), but was insensitive to 1 mM TEA and 100 nM iberiotoxin. The inhibition of Kv current by 1 microM 5-HT in MASMCs was greater in OLETF rats than in LETO rats (17.1 +/- 2.2% vs. 33.2 +/- 2.7% in LETO vs. OLETF rats at +40 mV), and the inhibition was prevented by treatment with the PKCalpha- and beta- selective inhibitor, Gö6976. The expression level of 5-HT(2A), but not 5-HT(2B), receptor and the expression levels of total PKC, PKCbeta, and PKCepsilon, but not PKCalpha, were higher in the mesenteric arteries of OLETF rats compared with LETO rats. The enhanced expression of 5-HT(2A) receptor together with PKCbeta may promote mesenteric vasoconstriction and increase vascular resistance in OLETF rats.

Endothelin-I and angiotensin II inhibit arterial voltage-gated K+ channels through different protein kinase C isoenzymes

AIMS

Voltage-gated K+ (Kv) channels of arterial smooth muscle (ASM) modulate arterial tone and are inhibited by vasoconstrictors through protein kinase C (PKC). We aimed to determine whether endothelin-1 (ET-1) and angiotensin II (AngII), which cause similar inhibition of Kv, use the same signalling pathway and PKC isoenzyme to exert their effects on Kv and to compare the involvement of PKC isoenzymes in contractile responses to these agents.

METHODS AND RESULTS

Kv currents recorded using the patch clamp technique with freshly isolated rat mesenteric ASM cells were inhibited by ET-1 or AngII. Inclusion of a PKCepsilon inhibitor peptide in the intracellular solution substantially reduced inhibition by AngII, but did not affect that by ET-1. Kv inhibition by ET-1 was reduced by the conventional PKC inhibitor Gö 6976 but not by the PKCbeta inhibitor LY333531. Selective peptide inhibitors of PKCalpha and PKCepsilon were linked to a Tat carrier peptide to make them membrane permeable and used to show that inhibition of PKCalpha prevented ET-1 inhibition of Kv current, but did not affect that by AngII. In contrast, inhibition of PKCepsilon prevented Kv inhibition by AngII but not by ET-1. The Tat-linked inhibitor peptides were also used to investigate the involvement of PKCalpha and PKCepsilon in the contractile responses of mesenteric arterial rings, showing that ET-1 contractions were substantially reduced by inhibition of PKCalpha, but unaffected by inhibition of PKCepsilon. AngII contractions were unaffected by inhibition of PKCalpha but substantially reduced by inhibition of PKCepsilon.

CONCLUSION

ET-1 inhibits Kv channels of mesenteric ASM through activation of PKCalpha, while AngII does so through PKCepsilon. This implies that ET-1 and AngII target Kv channels of ASM through different pathways of PKC-interacting proteins, so each vasoconstrictor enables its distinct PKC isoenzyme to interact functionally with the Kv channel.

Loss of cerebrovascular Shaker-type K(+) channels: a shared vasodilator defect of genetic and renal hypertensive rats

The cerebral arteries of hypertensive rats are depolarized and highly myogenic, suggesting a loss of K(+) channels in the vascular smooth muscle cells (VSMCs). The present study evaluated whether the dilator function of the prominent Shaker-type voltage-gated K(+) (K(V)1) channels is attenuated in middle cerebral arteries from two rat models of hypertension. Block of K(V)1 channels by correolide (1 micromol/l) or psora-4 (100 nmol/l) reduced the resting diameter of pressurized (80 mmHg) cerebral arteries from normotensive rats by an average of 28 +/- 3% or 26 +/- 3%, respectively. In contrast, arteries from spontaneously hypertensive rats (SHR) and aortic-banded (Ao-B) rats with chronic hypertension showed enhanced Ca(2+)-dependent tone and failed to significantly constrict to correolide or psora-4, implying a loss of K(V)1 channel-mediated vasodilation. Patch-clamp studies in the VSMCs of SHR confirmed that the peak K(+) current density attributed to K(V)1 channels averaged only 5.47 +/- 1.03 pA/pF, compared with 9.58 +/- 0.82 pA/pF in VSMCs of control Wistar-Kyoto rats. Subsequently, Western blots revealed a 49 +/- 7% to 66 +/- 7% loss of the pore-forming alpha(1.2)- and alpha(1.5)-subunits that compose K(V)1 channels in cerebral arteries of SHR and Ao-B rats compared with control animals. In each case, the deficiency of K(V)1 channels was associated with reduced mRNA levels encoding either or both alpha-subunits. Collectively, these findings demonstrate that a deficit of alpha(1.2)- and alpha(1.5)-subunits results in a reduced contribution of K(V)1 channels to the resting diameters of cerebral arteries from two rat models of hypertension that originate from different etiologies.

Identification and functional characterization of protein kinase A-catalyzed phosphorylation of potassium channel Kv1.2 at serine 449

Vascular smooth muscle Kv1 delayed rectifier K+ channels (KDR) containing Kv1.2 control membrane potential and thereby regulate contractility. Vasodilatory agonists acting via protein kinase A (PKA) enhance vascule smooth muscle Kv1 activity, but the molecular basis of this regulation is uncertain. We characterized the role of a C-terminal phosphorylation site, Ser-449, in Kv1.2 expressed in HEK 293 cells by biochemical and electrophysiological methods. We found that 1) in vitro phosphorylation of Kv1.2 occurred exclusively at serine residues, 2) one major phosphopeptide that co-migrated with 449pSASTISK was generated by proteolysis of in vitro phosphorylated Kv1.2, 3) the peptide 445KKSRSASTISK exhibited stoichiometric phosphorylation by PKA in vitro, 4) matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy (MS) and MS/MS confirmed in vitro Ser-449 phosphorylation by PKA, 5) in situ phosphorylation at Ser-449 was detected in HEK 293 cells by MALDI-TOF MS followed by MS/MS. MIDAS (multiple reaction monitoring-initiated detection and sequencing) analysis revealed additional phosphorylated residues, Ser-440 and Ser-441, 6) in vitro 32P incorporation was significantly reduced in Kv1.2-S449A, Kv1.2-S449D, and Kv1.2-S440A/S441A/S449A mutant channels, but Kv1.2-S440A/S441A was identical to wild-type Kv1.2 (Kv1.2-WT), and 7) bath applied 8-Br-cAMP or dialysis with PKA catalytic subunit (cPKA) increased Kv1.2-WT but not Kv1.2-S449A current amplitude. cPKA increased Kv1.2-WT current in inside-out patches. Rp-CPT-cAMPS reduced Kv1.2-WT current, blocked the increase due to 8-Br-cAMP, but had no effect on Kv1.2-S449A. cPKA increased current due to double mutant Kv1.2-S440A/S441A but had no effect on Kv1.2-S449D or Kv1.2-S440A/S441A/S449A. We conclude that Ser-449 in Kv1.2 is a site of PKA phosphorylation and a potential molecular mechanism for Kv1-containing KDR channel modulation by agonists via PKA activation.

Effects of docosahexaenoic acid on large-conductance Ca2+-activated K+ channels and voltage-dependent K+ channels in rat coronary artery smooth muscle cells

AIM

To investigate the effects of docosahexaenoic acid (DHA) on large-conductance Ca(2+)-activated K(+)(BK(Ca)) channels and voltage-dependent K(+) (K(V)) channels in rat coronary artery smooth muscle cells (CASMCs).

METHODS

Rat CASMCs were isolated by an enzyme digestion method. BK(Ca) and K(V) currents in individual CASMCs were recorded by the patch-clamp technique in a whole-cell configuration at room temperature. Effects of DHA on BK(Ca) and K(V) channels were observed when it was applied at 10, 20, 30, 40, 50, 60, 70, and 80 micromol/L.

RESULTS

When DHA concentrations were greater than 10 micromol/L, BK(Ca) currents increased in a dose-dependent manner. At a testing potential of +80 mV, 6.1%+/-0.3%, 76.5%+/-3.8%, 120.6%+/-5.5%, 248.0%+/-12.3%, 348.7%+/-17.3%, 374.2%+/-18.7%, 432.2%+/-21.6%, and 443.1%+/-22.1% of BK(Ca) currents were increased at the above concentrations, respectively. The half-effective concentration (EC(50)) of DHA on BK(Ca) currents was 37.53+/-1.65 micromol/L. When DHA concentrations were greater than 20 micromol/L, K(V) currents were gradually blocked by increasing concentrations of DHA. At a testing potential of +50 mV, 0.40%+/-0.02%, 1.37%+/-0.06%, 11.80%+/-0.59%, 26.50%+/-1.75%, 56.50%+/-2.89%, 73.30%+/-3.66%, 79.70%+/-3.94%, and 78.1%+/-3.91% of K(V) currents were blocked at the different concentrations listed above, respectively. The EC(50) of DHA on K(V) currents was 44.20+/-0.63 micromol/L.

CONCLUSION

DHA can activate BK(Ca) channels and block K(V) channels in rat CASMCs, and the EC(50) of DHA for BK(Ca) channels is lower than that for K(V) channels; these findings indicate that the vasorelaxation effects of DHA on vascular smooth muscle cells are mainly due to its activation of BK(Ca) channels.

Mechanistic basis of differential conduction in skeletal muscle arteries

The goal of this investigation was to probe intercellular conduction in skeletal muscle feed arteries and to address why smooth muscle-initiated responses fail to robustly spread like their endothelial counterpart. Using computational and experimental approaches, two interrelated rationales were developed to explain this apparent discrepancy in cell-to-cell communication. The first rationale stressed that smooth muscle electrical responses, if initiated, will be actively dissipated as they spread from cell-to-cell along the arterial wall. Charge dissipation is promoted within arteries by the structural and connectivity properties of vascular cells. The second rationale centred on the idea that when agents other than KCl stimulate a limited number of smooth muscle cells, they fail to generate the currents required to elicit a localized membrane potential (V(M)) response. This insufficiency results in part from charge loss, via gap junctions, to neighbouring unstimulated cells. Experiments confirmed the latter rationale by showing that focal phenylephrine application: (1) elicited a localized constriction insensitive to L-type Ca(2+) channel blockade; and (2) failed to substantially depolarize vascular smooth muscle cells. Further investigation revealed that while focal phenylephrine-induced constriction was V(M) independent, it was reliant on internal Ca(2+) mobilization and the activation of inositol 1,4,5-trisphosphate (IP(3)) receptors. The preceding findings illustrate that by using computational modelling and experimentation in a complementary manner, one can isolate key cellular properties and rationally examine their role in limiting the conduction of smooth muscle-initiated responses. Functionally, these observations enable investigators to assign the concept of 'local and global' blood flow control to the electrical and/or non-electrical behaviour of specific cell types.

Electrical remodeling in a transgenic mouse model of alpha1B-adrenergic receptor overexpression

Cardiac-specific overexpression of wild-type alpha(1B)-adrenergic receptors (alpha(1B)-AR) in mice predisposes to dilated cardiomyopathy and sudden death. Although alpha-adrenergic stimulation is thought to contribute to induction of arrhythmias in heart failure, the electrophysiological consequences of chronic alpha(1)-adrenergic activation have not been clearly defined. Thus we characterized ventricular repolarization and monitored incidence of spontaneous arrhythmias in end-stage heart failure alpha(1B)-AR mice (9-12 mo) and younger alpha(1B)-AR mice (2-3 mo) that do not present signs of heart failure. Compared with aged-matched controls, the corrected QT interval was 34% longer in the 9- to 12-mo alpha(1B)-AR mice, and the action potential durations were also significantly prolonged in these mice. These changes were associated with a decrease in the density of the outward K(+) currents, Ca(2+)-independent transient, ultrarapid delayed rectifier, and steady state (at +30 mV, reduction of 68, 64, and 41%, respectively), and underlying K(+) channel expression. Electrocardiogram (ECG) recordings revealed that older alpha(1B)-AR mice exhibited spontaneous ventricular arrhythmias. The alterations in repolarization can contribute to these rhythm abnormalities and are likely caused by chronic alpha(1B)-AR activity. Additional data obtained in 2- to 3-mo alpha(1B)-AR mice clearly showed that electrical remodeling was already observed in younger transgenic animals. However, it appeared to be slightly less pronounced than in older mice. These results suggest that there are two waves of remodeling: one due to chronic alpha(1B)-AR activity, and a second due to heart failure. Taken together, these data provide strong evidence for a pathological role of chronic alpha(1B)-AR activity in the development of repolarization defects and ventricular arrhythmias.

Cellular localization of mitochondria contributes to Kv channel-mediated regulation of cellular excitability in pulmonary but not mesenteric circulation

Mitochondria are proposed to be a major oxygen sensor in hypoxic pulmonary vasoconstriction (HPV), a unique response of the pulmonary circulation to low oxygen tension. Mitochondrial factors including reactive oxygen species, cytochrome c, ATP, and magnesium are potent modulators of voltage-gated K(+) (K(v)) channels in the plasmalemmal membrane of pulmonary arterial (PA) smooth muscle cells (PASMCs). Mitochondria have also been found close to the plasmalemmal membrane in rabbit main PA smooth muscle sections. Therefore, we hypothesized that differences in mitochondria localization in rat PASMCs and systemic mesenteric arterial smooth muscle cells (MASMCs) may contribute to the divergent oxygen sensitivity in the two different circulations. Cellular localization of mitochondria was compared with immunofluorescent labeling, and differences in functional coupling between mitochondria and K(v) channels was evaluated with the patch-clamp technique and specific mitochondrial inhibitors antimycin A (acting at complex III of the mitochondrial electron transport chain) and oligomycin A (which inhibits the ATP synthase). It was found that mitochondria were located significantly closer to the plasmalemmal membrane in PASMCs compared with MASMCs. Consistent with these findings, the effects of the mitochondrial inhibitors on K(v) current (I(Kv)) were significantly more potent in PASMCs than in MASMCs. The cytoskeletal disruptor cytochalasin B (10 microM) also altered mitochondrial distribution in PASMCs and significantly attenuated the effect of antimycin A on the voltage-dependent parameters of I(Kv). These findings suggest a greater structural and functional coupling between mitochondria and K(v) channels specifically in PASMCs, which could contribute to the regulation of PA excitability in HPV.

Visualizing the temporal effects of vasoconstrictors on PKC translocation and Ca2+ signaling in single resistance arterial smooth muscle cells

Arterial smooth muscle (ASM) contraction plays a critical role in regulating blood distribution and blood pressure. Vasoconstrictors activate cell surface receptors to initiate signaling cascades involving increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and recruitment of protein kinase C (PKC), leading to ASM contraction, though the PKC isoenzymes involved vary between different vasoconstrictors and their actions. Here, we have used confocal microscopy of enhanced green fluorescence protein (eGFP)-labeled PKC isoenzymes to visualize PKC translocation in primary rat mesenteric ASM cells in response to physiological vasoconstrictors, with simultaneous imaging of Ca(2+) signaling. Endothelin-1, angiotensin II, and uridine triphosphate all caused translocation of each of the PKC isoenzymes alpha, delta, and epsilon; however, the kinetics of translocation varied between agonists and PKC isoenzymes. Translocation of eGFP-PKCalpha mirrored the rise in [Ca(2+)](i), while that of eGFP-PKCdelta or -epsilon occurred more slowly. Endothelin-induced translocation of eGFP-PKCepsilon was often sustained for several minutes, while responses to angiotensin II were always transient. In addition, preventing [Ca(2+)](i) increases using 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl) ester prevented eGFP-PKCalpha translocation, while eGFP-PKCdelta translocated more rapidly. Our results suggest that PKC isoenzyme specificity of vasoconstrictor actions occurs downstream of PKC recruitment and demonstrate the varied kinetics and complex interplay between Ca(2+) and PKC responses to different vasoconstrictors in ASM.

Calcium sparklets in arterial smooth muscle

Voltage-dependent, L-type Ca2+ channels (LTCC) play an essential role in arterial smooth muscle contraction and, consequently, the regulation of arterial diameter, tissue perfusion and blood pressure. However, the spatial organization of functional LTCC in arterial myocytes is incompletely understood. Total internal reflection fluorescence and swept-field confocal microscopy revealed that the opening of a single or a cluster of LTCC produces local elevations in [Ca2+]i called Ca2+ sparklets. In arterial myocytes, Ca2+ sparklets are produced by the opening of Cav1.2 channels. The Ca2+ sparklet activity is bimodal. In low activity mode, rare stochastic openings of solitary LTCC produce limited Ca2+ influx ('low activity Ca2+ sparklets'). In contrast, discrete clusters of LTCC associated with protein kinase Ca (PKCa) operate in a sustained, high-activity mode resulting in substantial Ca2+ influx ('persistent Ca2+ sparklets'). The Ca2+ sparklet activity varies regionally within a myocyte depending on the relative activities of nearby PKCa and opposing protein phosphates 2A and 2B. Low- and high-activity persistent Ca2+ sparklets modulate local and global [Ca2+]i in arterial smooth muscle, suggesting that this Ca2+ signal may play an important role in the regulation of vascular function.

GPCR signalling in hypertension: role of GRKs

Hypertension is a prevalent condition in the developed world and disease severity is directly correlated with additional cardiovascular complications. It is estimated that 30% of the adult population in the United States has hypertension, which is classified as a systolic blood pressure > or =140 mmHg and/or a diastolic blood pressure > or =90 mmHg. A prolonged increase in afterload ultimately leads to congestive heart failure in the majority of cases. Currently, medication designed to treat hypertension is inadequate, thus new therapies need to be explored. Blood pressure is tightly regulated by blood vessel radius, which is established by hormones and/or peptides binding to GPCRs (G-protein-coupled receptors). Catecholamines and peptide hormones, such as AngII (angiotensin II), are elevated in hypertension and, therefore, signalling by these GPCRs is increased. Their signalling is tightly controlled by a class of proteins, the GRKs (GPCR kinases). Elevated levels of either GRK2 or GRK5 in both the lymphocytes and VSM (vascular smooth muscle) are associated with human hypertension and animal models of the disease. The focus of the present review is on the role GRKs, and their regulation of GPCRs, play in high blood pressure.

Mechanisms underlying angiotensin II-induced calcium oscillations

To gain insight into the mechanisms that underlie angiotensin II (ANG II)-induced cytoplasmic Ca2+ concentration ([Ca]cyt) oscillations in medullary pericytes, we expanded a prior model of ion fluxes. ANG II stimulation was simulated by doubling maximal inositol trisphosphate (IP3) production and imposing a 90% blockade of K+ channels. We investigated two configurations, one in which ryanodine receptors (RyR) and IP3 receptors (IP3R) occupy a common store and a second in which they reside on separate stores. Our results suggest that Ca2+ release from stores and import from the extracellular space are key determinants of oscillations because both raise [Ca] in subplasmalemmal spaces near RyR. When the Ca2+-induced Ca2+ release (CICR) threshold of RyR is exceeded, the ensuing Ca2+ release is limited by Ca2+ reuptake into stores and export across the plasmalemma. If sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps do not remain saturated and sarcoplasmic reticulum Ca2+ stores are replenished, that phase is followed by a resumption of leak from internal stores that leads either to [Ca]cyt elevation below the CICR threshold (no oscillations) or to elevation above it (oscillations). Our model predicts that oscillations are more prone to occur when IP3R and RyR stores are separate because, in that case, Ca2+ released by RyR during CICR can enhance filling of adjacent IP3 stores to favor a high subsequent leak that generates further CICR events. Moreover, the existence or absence of oscillations depends on the set points of several parameters, so that biological variation might well explain the presence or absence of oscillations in individual pericytes.

AKAP150 is required for stuttering persistent Ca2+ sparklets and angiotensin II-induced hypertension

Hypertension is a perplexing multiorgan disease involving renal primary pathology and enhanced angiotensin II vascular reactivity. Here, we report that a novel form of a local Ca2+ signaling in arterial smooth muscle is linked to the development of angiotensin II-induced hypertension. Long openings and reopenings of L-type Ca2+ channels in arterial myocytes produce stuttering persistent Ca2+ sparklets that increase Ca2+ influx and vascular tone. These stuttering persistent Ca2+ sparklets arise from the molecular interactions between the L-type Ca2+ channel and protein kinase Calpha at only a few subsarcolemmal regions in resistance arteries. We have identified AKAP150 as the key protein, which targets protein kinase Calpha to the L-type Ca2+ channels and thereby enables its regulatory function. Accordingly, AKAP150 knockout mice (AKAP150-/-) were found to lack persistent Ca2+ sparklets and have lower arterial wall intracellular calcium ([Ca2+]i) and decreased myogenic tone. Furthermore, AKAP150-/- mice were hypotensive and did not develop angiotensin II-induced hypertension. We conclude that local control of L-type Ca2+ channel function is regulated by AKAP150-targeted protein kinase C signaling, which controls stuttering persistent Ca2+ influx, vascular tone, and blood pressure under physiological conditions and underlies angiotensin II-dependent hypertension.

Decreased function of voltage-gated potassium channels contributes to augmented myogenic tone of uterine arteries in late pregnancy

Increased pressure-induced (myogenic) tone in small uteroplacental arteries from late pregnant (LP) rats has been previously observed. In this study, we hypothesized that this response may result from a diminished activity of vascular smooth muscle cell (SMC) voltage-gated delayed-rectifier K+(Kv) channels, leading to membrane depolarization, augmented Ca2+influx, and vasoconstriction (tone). Elevation of intraluminal pressure from 10 to 60 and 100 mmHg resulted in a marked, diltiazem-sensitive rise in SMC cytosolic Ca2+concentration ([Ca2+]i) associated with a vasoconstriction of uteroplacental arteries of LP rats. In contrast, these changes were significantly diminished in uterine arteries from nonpregnant (NP) rats. Gestational augmentation of pressure-induced Ca2+influx through L-type Ca2+channels was associated with an enhanced SMC depolarization, the appearance of electrical and [Ca2+]ioscillatory activities, and vasomotion. Exposure of vessels from NP animals to 4-aminopyridine, which inhibits the activity of Kvchannels, mimicked the effects of pregnancy by increasing pressure-induced depolarization, elevation of [Ca2+]i, and development of myogenic tone. Furthermore, currents through Kvchannels were significantly reduced in myocytes dissociated from arteries of LP rats compared with those of NP controls. Based on these results, we conclude that decreased Kvchannel activity contributes importantly to enhanced pressure-induced depolarization, Ca2+entry, and increase in myogenic tone present in uteroplacental arteries from LP rats.

Heparin-binding EGF-like growth factor mediates oxyhemoglobin-induced suppression of voltage-dependent potassium channels in rabbit cerebral artery myocytes

Oxyhemoglobin (OxyHb) can suppress voltage-dependent K(+) channel (K(V)) currents through protein tyrosine kinase activation, which may contribute to cerebral vasospasm following subarachnoid hemorrhage. Here we have tested the hypothesis that shedding of heparin-binding EGF-like growth factor (HB-EGF) and the resulting activation of the tyrosine kinase EGF receptor (EGFR) underlie OxyHb-induced K(V) channel suppression in the cerebral vasculature. With the use of the conventional whole cell patch-clamp technique, two EGFR ligands, EGF and HB-EGF, were found to mimic OxyHb-induced K(V) suppression in rabbit cerebral artery myocytes. K(V) current suppression by OxyHb or EGF ligands was eliminated by a specific EGFR inhibitor, AG-1478, but was unaffected by PKC inhibition. Compounds (heparin and CRM-197) that specifically interfere with HB-EGF signaling eliminated OxyHb-induced K(V) suppression, suggesting that HB-EGF is the EGFR ligand involved in this pathway. HB-EGF exists as a precursor protein that, when cleaved by matrix metalloproteases (MMPs), causes EGFR activation. MMP activation was detected in OxyHb-treated arteries by gelatin zymography. Furthermore, the MMP inhibitor (GM-6001) abolished OxyHb-induced K(V) current suppression. We also observed K(V) current suppression due to EGFR activation in human cerebral artery myocytes. In conclusion, these data demonstrate that OxyHb induces MMP activation, causing HB-EGF shedding and enhanced EGFR activity, ultimately leading to K(V) channel suppression. We propose that EGFR-mediated K(V) suppression contributes to vascular pathologies, such as cerebral vasospasm, and may play a more widespread role in the regulation of regional blood flow and peripheral resistance.

Increased inhibition of inward rectifier K+ channels by angiotensin II in small-diameter coronary artery of isoproterenol-induced hypertrophied model

OBJECTIVE

We investigated the effects of angiotensin II (Ang II) on inward rectifier K+ (Kir) channels in small-diameter coronary arterial smooth muscle cells (SCASMCs) of control and isoproterenol (Iso)-induced hypertrophied rabbits.

METHODS AND RESULTS

Kir current amplitude and Kir channel protein expression were definitely lower in the Iso-induced hypertrophied model than in the control. In a pressurized arterial experiment, 15 mmol/L K+-induced vasodilation was greater in the control arteries than in the arteries of Iso-induced hypertrophied model. Ang II reduced the Kir current in a concentration-dependent manner, and this inhibition was greater in SCASMCs from Iso-induced hypertrophied model than from control. Although, there was no difference in the expression of Ang II type 2 (AT2) receptor between SCASMCs of control and Iso-induced hypertrophied model, the expression of Ang II type 1 (AT1) receptor and phosphorylated PKC alpha were greater in SCASMCs of Iso-induced hypertrophied model than of control.

CONCLUSION

Ang II inhibits Kir channels more prominently in SCASMCs of Iso-induced hypertrophied model owing to increases in the expression of AT1 receptor and the activation of PKC alpha. Our findings about the differential expression of Kir channels and different modulation of Kir channels by a vasoconstrictor (Ang II) in a hypertrophy model are important for better understanding the responsiveness of small-diameter arteries during hypertrophy.

Down-regulation of D2 dopamine receptor and increased protein kinase Cmu phosphorylation in aldosterone-producing adenoma play roles in aldosterone overproduction

CONTEXT

The mechanism associated with the overproduction of aldosterone by aldosterone-producing adenomas (APA) is unknown.

OBJECTIVE

The objective of the study was to explore the role of the D2 dopamine receptor (D2R) on aldosterone synthesis and secretion and clarify the clinical importance of this role on aldosterone overproduction in APA.

RESULTS

D2R expression in APA was examined in 24 patients and was much less than that in the nontumorous adrenal cortex. D2R mRNA levels in APA were inversely correlated with CYP11B2 mRNA levels and the patient's plasma aldosterone concentration. Angiotensin II (AII)-stimulated aldosterone secretion and CYP11B2 mRNA expression in human adenocarcinoma cells (H295R) was attenuated by the D2 agonist, bromocriptine (BMC). BMC selectively attenuated AII-induced protein kinase C (PKC)-mu phosphorylation and its translocation to the cell membrane. PKCmu-specific short-hairpin RNA significantly decreased AII-induced CYP11B2 mRNA expression and aldosterone secretion. BMC also attenuated the AII-induced increase in cytoplasmic calcium, partially through an inhibition of cytoplasmic inositol 1,4,5 triphosphate production. Despite similar total PKCmu levels in APA and the nontumorous adrenal cortex, expression of phosphorylated PKCmu in APA was much higher.

CONCLUSION

This is the first study to demonstrate that the D2R modulated aldosterone secretion and synthesis through a specific attenuation of PKCmu activity, as well as the intracellular calcium level. Down-regulation of the D2R in APA, in turn, increased PKCmu activity and led to overproduction of aldosterone in affected patients. The D2R may thus serve as a potential treatment target for primary aldosteronism.

Activators of the PKA and PKG pathways attenuate RhoA-mediated suppression of the KDR current in cerebral arteries

This study tested whether activation of protein kinase A (PKA) and G (PKG) pathways would attenuate the ability of RhoA to suppress the delayed rectifier K(+) (K(DR)) current and limit agonist-induced depolarization and constriction. Smooth muscle cells from rat cerebral arteries were enzymatically isolated, and whole cell K(DR) currents were monitored with conventional patch-clamp electrophysiology. The K(DR) current averaged 21.2 +/- 2.3 pA/pF (mean +/- SE) at +40 mV and was potently inhibited by UTP. Current suppression was eliminated in the presence of C3 exoenzyme, confirming that this modulation is dependent on RhoA. Activation of PKA (dibutyryl-cAMP, forskolin) or PKG (dibutyryl-cGMP, sodium nitroprusside, nitric oxide) similarly abolished the ability of UTP to suppress K(DR) and did so without effect on baseline current. Using pressure myography techniques, we stripped cerebral arteries of endothelium and preconstricted them with UTP; these were subsequently shown to hyperpolarize and dilate to both forskolin and sodium nitroprusside. An increase in K(V) channel activity was found to partly underlie these associated changes, as constriction to 4-aminopyridine (K(DR) channel blocker) was greater after PKA or PKG activation. We conclude from our electrophysiological and functional observations that the PKA and PKG pathways attenuate the ability of UTP to depolarize and constrict cerebral arteries in part by minimizing the RhoA-mediated suppression of the K(DR) current.

Activation of NFATc3 down-regulates the beta1 subunit of large conductance, calcium-activated K+ channels in arterial smooth muscle and contributes to hypertension

Large conductance, Ca2+-activated K+ (BK) channels modulate the excitability and contractile state of arterial smooth muscle. Recently, we demonstrated that during hypertension, expression of the accessory beta1 subunit was decreased relative to the pore-forming alpha subunit of the BK channel. Reduced beta1 subunit expression resulted in BK channels with impaired function due to lowered sensitivity to Ca2+. Here, we tested the hypothesis that activation of the calcineurin/NFATc3 signaling pathway down-regulates beta1 expression during angiotensin II-induced hypertension. Consistent with this hypothesis, we found that in vivo administration of angiotensin II-activated calcineurin/NFATc3 signaling in arterial smooth muscle. During angiotensin II infusion, arterial smooth muscle BK channel function was decreased in wild type (WT) but not in NFATc3 null (NFATc3-/-) mice. Accordingly, beta1 expression was decreased in WT but not in NFATc3-/- arteries. Angiotensin II-induced down-regulation of the beta1 subunit required Ca2+ influx via L-type Ca2+ channels. However, in the absence of angiotensin II, moderate elevation of [Ca2+]i alone was not sufficient to activate NFAT transcriptional activity and, thus, decrease beta1 subunit expression. Importantly, angiotensin II infusion increased systemic blood pressure to a lower extent in NFATc3-/- than in WT mice, indicating that this transcription factor is required for the development of severe hypertension during chronic angiotensin II signaling activation. We conclude that activation of calcineurin and NFATc3 during sustained angiotensin II signaling down-regulates the expression of the beta1 subunit of the BK channel, which in turn contributes to arterial dysfunction and the development of hypertension.

Vasopressin stimulates action potential firing by protein kinase C-dependent inhibition of KCNQ5 in A7r5 rat aortic smooth muscle cells

[Arg(8)]-vasopressin (AVP), at low concentrations (10-500 pM), stimulates oscillations in intracellular Ca(2+) concentration (Ca(2+) spikes) in A7r5 rat aortic smooth muscle cells. Our previous studies provided biochemical evidence that protein kinase C (PKC) activation and phosphorylation of voltage-sensitive K(+) (K(v)) channels are crucial steps in this process. In the present study, K(v) currents (I(Kv)) and membrane potential were measured using patch clamp techniques. Treatment of A7r5 cells with 100 pM AVP resulted in significant inhibition of I(Kv). This effect was associated with gradual membrane depolarization, increased membrane resistance, and action potential (AP) generation in the same cells. The AVP-sensitive I(Kv) was resistant to 4-aminopyridine, iberiotoxin, and glibenclamide but was fully inhibited by the selective KCNQ channel blockers linopirdine (10 microM) and XE-991 (10 microM) and enhanced by the KCNQ channel activator flupirtine (10 microM). BaCl(2) (100 microM) or linopirdine (5 microM) mimicked the effects of AVP on K(+) currents, AP generation, and Ca(2+) spiking. Expression of KCNQ5 was detected by RT-PCR in A7r5 cells and freshly isolated rat aortic smooth muscle. RNA interference directed toward KCNQ5 reduced KCNQ5 protein expression and resulted in a significant decrease in I(Kv) in A7r5 cells. I(Kv) was also inhibited in response to the PKC activator 4beta-phorbol 12-myristate 13-acetate (10 nM), and the inhibition of I(Kv) by AVP was prevented by the PKC inhibitor calphostin C (250 nM). These results suggest that the stimulation of Ca(2+) spiking by physiological concentrations of AVP involves PKC-dependent inhibition of KCNQ5 channels and increased AP firing in A7r5 cells.

Oxyhemoglobin-induced suppression of voltage-dependent K+ channels in cerebral arteries by enhanced tyrosine kinase activity

Cerebral vasospasm following aneurysmal subarachnoid hemorrhage (SAH) has devastating consequences. Oxyhemoglobin (oxyhb) has been implicated in SAH-induced cerebral vasospasm as it causes cerebral artery constriction and increases tyrosine kinase activity. Voltage-dependent, Ca(2+)-selective and K(+)-selective ion channels play an important role in the regulation of cerebral artery diameter and represent potential targets of oxyhb. Here we provide novel evidence that oxyhb selectively decreases 4-aminopyridine sensitive, voltage-dependent K(+) channel (K(v)) currents by approximately 30% in myocytes isolated from rabbit cerebral arteries but did not directly alter the activity of voltage-dependent Ca(2+) channels or large conductance Ca(2+)-activated (BK) channels. A combination of tyrosine kinase inhibitors (tyrphostin AG1478, tyrphostin A23, tyrphostin A25, genistein) abolished both oxyhb-induced suppression of K(v) channel currents and oxyhb-induced constriction of isolated cerebral arteries. The K(v) channel blocker 4-aminopyridine also inhibited oxyhb-induced cerebral artery constriction. The observed oxyhb-induced decrease in K(v) channel activity could represent either channel block, or a decrease in K(v) channel density on the plasma membrane. To explore whether oxyhb altered trafficking of K(v) channels to the plasma membrane, we used an antibody generated against an extracellular epitope of K(v)1.5 channels. In the presence of oxyhb, staining of K(v)1.5 on the plasma membrane surface was markedly reduced. Furthermore, oxyhb caused a loss of spatial distinction between staining with K(v)1.5 and the general anti-phosphotyrosine antibody PY-102. We propose that oxyhb-induced suppression of K(v) currents occurs via a mechanism involving enhanced tyrosine kinase activity and channel endocytosis. This novel mechanism may contribute to oxyhb-induced cerebral artery constriction following SAH.

Metamizol acts as an ATP sensitive potassium channel opener to inhibit the contracting response induced by angiotensin II but not to norepinephrine in rat thoracic aorta smooth muscle

Clinically metamizol (MZ) has been related to alteration on haemodynamic parameters and modifications on blood pressure in humans when administered intravenously. These effects have been observed at MZ therapeutic doses. Experimentally, MZ is able to induce relaxation on several types of vascular smooth muscles and modulates the contraction induced by phenylephrine. However, the mechanism underlying the MZ effects on vascular reactivity is not clear. Potassium channels (K) present on vascular smooth muscle cells closely regulate the vascular reactivity and membrane potential. There are four described types of K in vascular tissue: K voltage sensitive (K(V)), K calcium sensitive (K(Ca)2+), K ATP sensitive (K(ATP) and K inward rectification (K(IR), voltage sensitive). The aim of this work was to investigate MZ effects on angiotensin II (AT II) and noradrenaline (NA) induced contraction and to evaluate the K participation on MZ modulating effect on vascular smooth muscle contraction, using isometric and patch clamp techniques. MZ induces relaxation in a concentration dependent manner. Furthermore, MZ strongly inhibits in a concentration dependent fashion the contraction induced by AT II. However, MZ inhibition on NA induced contraction was moderated compared with that observed on AT II. MZ effects on AT II induced contraction was blocked by glybenclamide (a specific K(ATP) blocker, 3 microM, *p < 0.01). In patch clamp experiments, MZ (3 mM) induces an increase on potassium current (K+) mediated by K(ATP) in similar way as diazoxide (a specific K(ATP) opener, 3 microM). Our results suggest that MZ induces relaxation and inhibits contraction induced by AT II acting as a K(ATP) opener.

Glucose reduces endothelin inhibition of voltage-gated potassium channels in rat arterial smooth muscle cells

Prolonged hyperglycaemia impairs vascular reactivity and inhibits voltage-activated K(+) (Kv) channels. We examined acute effects of altering glucose concentration on the activity and inhibition by endothelin-1 (ET-1) of Kv currents of freshly isolated rat arterial myocytes. Peak Kv currents recorded in glucose-free solution were reversibly reduced within 200 s by increasing extracellular glucose to 4 mm. This inhibitory effect of glucose was abolished by protein kinase C inhibitor peptide (PKC-IP), and Kv currents were further reduced in 10 mm glucose. In current-clamped cells, membrane potentials were more negative in 4 than in 10 mm glucose. In 4 mm d-glucose, 10 nm ET-1 decreased peak Kv current amplitude at +60 mV from 23.5 +/- 3.3 to 12.1 +/- 3.1 pA pF(-1) (n = 6, P < 0.001) and increased the rate of inactivation, decreasing the time constant around fourfold. Inhibition by ET-1 was prevented by PKC-IP. When d-glucose was increased to 10 mm, ET-1 no longer inhibited Kv current (n = 6). Glucose metabolism was required for prevention of ET-1 inhibition of Kv currents, since fructose mimicked the effects of d-glucose, while l-glucose, sucrose or mannitol were without effect. Endothelin receptors were still functional in 10 mm d-glucose, since pinacidil-activated ATP-dependent K(+) (K(ATP)) currents were reduced by 10 nm ET-1. This inhibition was nearly abolished by PKC-IP, indicating that endothelin receptors could still activate PKC in 10 mm d-glucose. These results indicate that changes in extracellular glucose concentration within the physiological range can reduce Kv current amplitude and can have major effects on Kv channel modulation by vasoconstrictors.

KATP channel conductance of descending vasa recta pericytes

Using nystatin-perforated patch-clamp and whole cell recording, we tested the hypothesis that K(ATP) channels contribute to resting conductance of rat descending vasa recta (DVR) pericytes and are modulated by vasoconstrictors. The K(ATP) blocker glybenclamide (Glb; 10 microM) depolarized pericytes and inhibited outward currents of cells held at -40 mV. K(ATP) openers pinacidil (Pnc; 10 microM) and P-1075 (1 microM) hyperpolarized pericytes and transiently augmented outward currents. All effects of Pnc and P-1075 were fully reversed by Glb. Inward currents of pericytes held at -60 mV in symmetrical 140 mM K(+) were markedly augmented by Pnc and fully reversed by Glb. Ramp depolarizations in symmetrical K(+), performed in Pnc and Pnc + Glb, yielded a Pnc-induced, Glb-sensitive K(ATP) difference current that lacked rectification and reversed at 0 mV. Immunostaining identified both K(IR)6.1, K(IR)6.2 inward rectifier subunits and sulfonurea receptor subtype 2B. ANG II (1 and 10 nM) and endothelin-1 (10 nM) but not vasopressin (100 nM) significantly lowered holding current at -40 mV and abolished Pnc-stimulated outward currents. We conclude that DVR pericytes express K(ATP) channels that make a significant contribution to basal K(+) conductance and are inhibited by ANG II and endothelin-1.

Heteromultimeric Kv1 channels contribute to myogenic control of arterial diameter

Inhibition of vascular smooth muscle (VSM) delayed rectifier K+ channels (K(DR)) by 4-aminopyridine (4-AP; 200 micromol/L) or correolide (1 micromol/L), a selective inhibitor of Kv1 channels, enhanced myogenic contraction of rat mesenteric arteries (RMAs) in response to increases in intraluminal pressure. The molecular identity of K(DR) of RMA myocytes was characterized using RT-PCR, real-time PCR, and immunocytochemistry. Transcripts encoding the pore-forming Kvalpha subunits, Kv1.2, Kv1.4, Kv1.5, and Kv1.6, were identified and confirmed at the protein level with subunit-specific antibodies. Kvbeta transcript (beta1.1, beta1.2, beta1.3, and beta2.1) expression was also identified. Kv1.5 message was approximately 2-fold more abundant than that for Kv1.2 and Kv1.6. Transcripts encoding these three Kv1alpha subunits were approximately 2-fold more abundant in 1st/2nd order conduit compared with 4th order resistance RMAs, and Kvbeta1 was 8-fold higher than Kvbeta2 message. RMA K(DR) activated positive to -50 mV, exhibited incomplete inactivation, and were inhibited by 4-AP and correolide. However, neither alpha-dendrotoxin or kappa-dendrotoxin affected RMA K(DR), implicating the presence of Kv1.5 in all channels and the absence of Kv1.1, respectively. Currents mediated by channels because of coexpression of Kv1.2, Kv1.5, Kv1.6, and Kvbeta1.2 in human embryonic kidney 293 cells had biophysical and pharmacological properties similar to those of RMA K(DR). It is concluded that K(DR) channels composed of heteromultimers of Kv1 subunits play a critical role in myogenic control of arterial diameter.

Inhibition of K+ conductance in descending vasa recta pericytes by ANG II

We tested whether K(+) channel inhibition accompanies ANG II-induced depolarization of descending vasa recta (DVR) pericytes. An increase in extracellular K(+) concentration ([K(+)](o)) from 5 to 100 mM depolarized resting pericytes but had no effect after prolonged (10 nM, 20 min) ANG II exposure. In contrast, reduction of extracellular Cl(-) concentration ([Cl(-)](o)) from 154 to 34 mM had a minor effect on resting membrane potential but strongly depolarized pericytes treated with ANG II. The K(+) channel blockers BaCl(2) (0.1, 1 mM) and tetraethylammonium (TEA; 30 mM) depolarized resting pericytes but did not affect membrane potential of ANG II-treated pericytes. Pericyte whole cell currents were reduced by ANG II and nearly eliminated by combined ANG II exposure and the Cl(-) channel blocker niflumic acid (100 muM). Augmentation of inward current induced by raising [K(+)](O) from 5 to 50 mM was eliminated by preexposure to ANG II. TEA- and BaCl(2)-sensitive outward currents, generated by depolarizing pericytes from -80 to -40 mV, were eliminated by ANG II. We conclude that ANG II depolarizes DVR pericytes by a combination of Cl(-) channel activation and K(+) channel inhibition.

Gender differences in ANG II levels and action on multiple K+ current modulation pathways in diabetic rats

Gender differences were studied in ventricular myocytes from insulin-deficient (Type 1) diabetic rats. Cells were obtained by enzymatic dispersion of hearts from control male and female rats and from rats made diabetic with streptozotocin (100 mg/kg) 7-14 days before experiments. ANG II content, measured by ELISA, was augmented in diabetic males but unaltered in diabetic females. In diabetic ovariectomized females, ANG II levels were augmented as in males. ANG II affects multiple cellular pathways including activation of protein kinase C (PKC) and several tyrosine kinases as well as inhibition of protein kinase A (PKA). The involvement of these pathways in modulating outward K(+) currents was studied. Transient and sustained outward K(+) currents were measured using the whole cell voltage-clamp method. In males, these currents are attenuated under diabetic conditions but are augmented by the ANG II-converting enzyme inhibitor quinapril. Activation of PKA by 8-bromo-cAMP enhanced both K(+) currents in cells from diabetic males. The augmentation of these currents by quinapril was blocked when PKA inhibition was maintained with the Rp isomer of 3',5'-cyclic monophosphorothioate. Inhibition of tyrosine kinases by genistein also augmented K(+) currents in cells from diabetic males. Action potentials were abbreviated by 8-bromo-cAMP and genistein. However, both genistein and 8-bromo-cAMP had no effect on K(+) currents in cells from diabetic females. In cells from ovariectomized diabetic females, 8-bromo-cAMP and genistein enhanced these K(+) currents as in males. Inhibition of PKC augmented the transient and sustained K(+) currents in cells from diabetic males and females. A contribution of non-ANG II-dependent activation of PKC is suggested. These results describe some of the mechanisms that may underlie gender-specific differences in the development of cardiac disease and arrhythmias.

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.

Pyrimidine nucleotides suppress KDR currents and depolarize rat cerebral arteries by activating Rho kinase

This study examined whether, and by what signaling and ionic mechanisms, pyrimidine nucleotides constrict rat cerebral arteries. Cannulated cerebral arteries stripped of endothelium and pressurized to 15 mmHg constricted in a dose-dependent manner to UTP. This constriction was partly dependent on the depolarization of smooth muscle cells and the activation of voltage-operated Ca(2+) channels. The depolarization and constriction induced by UTP were unaffected by bisindolylmaleimide I, a PKC inhibitor that abolished phorbol ester (PMA)-induced constriction in cerebral arteries. In contrast, the Rhokinase inhibitor Y-27632 attenuated the ability of UTP to both constrict and depolarize cerebral arteries. With patch-clamp electrophysiology, a voltage-dependent delayed rectifying K(+) (K(DR)) current was isolated and shown to consist of a slowly inactivating 4-aminopyridine (4-AP)-sensitive and an -insensitive component. The 4-AP-sensitive K(DR) current was potently suppressed by UTP through a mechanism that was not dependent on PKC. This reflects observations that demonstrated that 1) a PKC activator (PMA) had no effect on K(DR) and 2) PKC inhibitors (calphostin C or bisindolylmaleimide I) could not prevent the suppression of K(DR) by UTP. The Rho kinase inhibitor Y-27632 abolished the ability of UTP to inhibit the K(DR) current, as did inhibition of RhoA with C3 exoenzyme. Cumulatively, these observations indicate that Rho kinase signaling plays an important role in eliciting the cerebral constriction induced by pyrimidine nucleotides. Moreover, they demonstrate for the first time that Rhokinase partly mediates this constriction by altering ion channels that control membrane potential and Ca(2+) influx through voltage-operated Ca(2+) channels.