Angiotensin II, angiotensin IIantagonists and spironolactoneand their modulation of cardiacrepolarizationEva Delpon, Ricardo Caballero, Ricardo Gomez, Lucıa Nunez and Juan Tamargo

Department of Pharmacology, School of Medicine, Universidad Complutense, 28040-Madrid, Spain

Angiotensin II and aldosterone produce pro-arrhythmic

effects by several mechanisms, including the modu-

lation of voltage-dependent KC channels involved in

human cardiac repolarization. Drugs that inhibit the

renin–angiotensin–aldosterone system exert anti-

arrhythmic actions that are related to the blockade of

the pro-arrhythmic actions of angiotensin II and aldo-

sterone. These anti-arrhythmic actions include inhi-

bition of electrical and structural cardiac remodeling,

inhibition of neurohumoral activation, reduction of blood

pressure and stabilization of electrolyte disturbances. In

this article, several angiotensin II AT1 receptor antagonists

(candesartan, E3174, eprosartan, irbesartan and losartan)

and aldosterone receptor antagonists (canrenoic acid and

spironolactone) that directly modulate the activity of the

voltage-dependent KC channels are reviewed; the effects

of these antagonists might be useful in the prevention and

treatment of cardiac arrhythmias.

Box 1. Pro-arrhythmic actions of angiotensin II

† Increases the sinus rate and pacemaker activity in the His–Purkinje

system.

† Promotes myocardial ischemia by increasing myocardial oxygen

demands (MVO2) and producing coronary vasoconstriction.

† Increases extracellular Ca2C entry and Ca2C release from intra-

cellular stores.

† Decreases conduction velocity as a result of cell-to-cell uncoupling

and inhibition of the inward NaC current.

† Increases the activity of the sympathetic nervous system and the

release of aldosterone and endothelin-1 from various tissues.

† Produces myocardial remodeling (e.g. dilatation, fibrosis and

The renin–angiotensin–aldosterone system and its

pharmacological modulation

The renin–angiotensin–aldosterone system (RAAS) is anenzymatic cascade in which the enzyme renin acts onangiotensinogen to form angiotensin I (Ang I), which isthen converted by the action of angiotensin-convertingenzyme (ACE) into angiotensin II (Ang II). Ang II, in turn,promotes the release of aldosterone from the zonaglomerulosa of the adrenal cortex and other tissues,including cardiac, endothelial and vascular smoothmuscle cells and the brain. Ang II can also be generatedby pathways other than those catalyzed by ACE [1](Figure 1). Ang II interacts with at least two receptors:angiotensin AT1 and AT2 receptors [2]. The major effects ofAng II, such as vasoconstriction, secretion of aldosterone,vasopressin and endothelin-1, NaC retention, stimulationof the sympathetic nervous system, cardiovascular hyper-trophy and fibrosis (cardiovascular remodeling), increasein cardiac rate and contractility, inflammatory responsesand oxidative stress, are mediated by the AT1 receptor[2–4]. The physiological role of AT2 receptors is only

Corresponding author: Delpon, E. (edelpon@med.ucm.es).

www.sciencedirect.com 0165-6147/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved

partially understood but they appear to counterbalancesome of the effects of AT1 receptor stimulation [5,6]. Aldo-sterone binds to mineralocorticoid receptors and causesNaC retention, cardiovascular fibrosis, KC and Mg2C

depletion, and impairment of baroreceptor function [7].Additionally, Ang II exerts pro-arrhythmic effects, whichare summarized in Box 1. Most of these effects are alsoproduced by aldosterone [7]. Therefore, the RAAS has animportant role in the genesis of hypertension, ischemicheart disease, congestive heart failure (CHF) and cardiacarrhythmias [3,4,7–11].

Several classes of drugs inhibit the RAAS and slow theprogression of cardiovascular diseases, including renininhibitors, ACE inhibitors that decrease the formation ofAng II and inhibit the breakdown of bradykinin, Ang IIreceptor antagonists that bind competitively to AT1

receptors, and aldosterone receptor antagonists. Commontherapeutic indications of ACE inhibitors, Ang II receptorantagonists and aldosterone receptor antagonists includehypertension, CHF and myocardial infarction [3–11]. Inthis article, we review the role of Ang II and aldosteronein the genesis of cardiac arrhythmias and the clinicaland experimental data suggesting that Ang II receptorantagonists and aldosterone receptor antagonists exertanti-arrhythmic effects. Moreover, the direct effects ofAng II receptor antagonists and aldosterone receptorantagonists on human cardiac KC channels that areunrelated to their effects at AT1 or aldosterone receptors

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005

hypertrophy).

† Increases the dispersion of cardiac repolarization.

. doi:10.1016/j.tips.2005.01.006

Aldosterone

Angiotensin II

ACE

Angiotensinogen

Angiotensin I

Renin inhibitors

AT1 and AT2receptors

Aldosterone receptorantagonists

Renin

AT1 receptor antagonists

ACE inhibitors

tPA,cathepsin G,

tonin,trypsin,

kallikrein

TRENDS in Pharmacological Sciences

Cathepsin G,trypsin,

kallikrein,chymase,

CAGE

Figure 1. The renin–angiotensin–aldosterone system (RAAS) and sites of action of inhibitors of the bioenzymatic cascade. Angiotensinogen is converted to angiotensin I

(Ang I) by renin. Subsequently, Ang I is converted mainly by angiotensin-converting enzyme (ACE) to angiotensin II (Ang II), which can then act on angiotensin AT1 and AT2

receptors and/or increase the release of aldosterone. Enzymes other than ACE can catalyze the production of Ang II directly from angiotensinogen and from Ang I. The RAAS

has an important role in the genesis of hypertension, ischemic heart disease, congestive heart failure (CHF) and cardiac arrhythmias. Several drugs can inhibit the RAAS. For

example, renin inhibitors inhibit the production of Ang I from angiotensinogen. ACE inhibitors decrease the formation of Ang II by blocking ACE. However, because enzymes

other than ACE can catalyze the formation of Ang II, increased levels of Ang II can exist despite effective ACE inhibition. By contrast, AT1 receptor antagonists block the effects

of Ang II on the AT1 receptor irrespective of how Ang II is synthesized. Aldosterone receptor antagonists block the effects of aldosterone at the level of its receptor.

Aldosterone receptors are found in multiple non-epithelial sites, including the brain, heart and blood vessels. Ang II and aldosterone are synthesized within the

cardiovascular system. Abbreviations: CAGE, chymotrypsin-like angiotensin-generating enzyme; tPA, tissue plasminogen activator.

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005156

and might contribute to their anti-arrhythmic effectswill be described.

Human cardiac repolarization

The human cardiac action potential duration (APD)and the QT interval of the surface electrocardiogram(a measure of ventricular repolarization) are largelydetermined by: (i) several voltage-gated KC channels(Kv), including the 4-aminopyridine-sensitive componentof the transient outward current (Ito1) and the ultrarapid(IKur), rapid (IKr) and slow (IKs) components of the delayedrectifier current; and (ii) the voltage-independent inwardrectifier (IK1) and the ligand-gated channels activated byacetylcholine (IKACh) and ATP (IKATP) [12,13] (Figure 2a).Proteins and genes that encode pore-forming a-subunitsand b-subunits of Kv channels are shown in Figure 2b.The configuration of the action potential and the APD varyconsiderably among different cardiac regions (atria versusventricles) (Figure 2a) and specific areas within theseregions (epicardium versus endocardium). This hetero-geneity mainly reflects differences in the type and/orexpression patterns of the KC channels. It is currentlyassumed that the IKr has a crucial role in the control ofhuman ventricular repolarization, whereas IKur is onlyrecorded in human atria (Figure 2). Lengthening of thecardiac APD together with refractoriness produced by KC

channel blockers is recognized as an anti-arrhythmiceffect and has stimulated the research of old and newcompounds that possess such a property.

Effects of Ang II on cardiac KC currents

Ang II increases the dispersion of cardiac repolarization, apro-arrhythmic effect that can be explained by its contra-dictory effects on cardiac channels [14]. In guinea-pigmyocytes, Ang II increased IKr but inhibited IKs [15] and

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IKATP [16]. However, in mice ventricular myocytes Ang IIdid not modify IKur or IK1, whereas in rat ventricularmyocytes, it produced an inhibition of Ito1 and a pro-longation of the APD; these effects were mediated by AT2

receptor stimulation of the serine/threonine phosphatasetype 2A [17].

Role of the RAAS in cardiac arrhythmias

Ang II slows conduction velocity and decreases cellcoupling within the heart, conditions that enable oneimpulse to excite areas of the heart more than once(reentry) [14] (Box 1). In fact, reentry is the most commoncause of cardiac arrhythmias. Ang II also producescoronary vasoconstriction and increases heart rate andmyocardial oxygen demands, leading to ischemia-inducedcardiac arrhythmias [18]. Moreover, activation of theRAAS leads to cardiac structural remodeling (dilatation,fibrosis and hypertrophy) in a variety of pathological cardio-vascular conditions, which results in impaired impulsepropagation and reentrant arrhythmias [3,9,10,19,20].

Ang II also increases the entry of extracellular Ca2C

through L- and T-type channels [21] and the release ofCa2C from intracellular stores, thus leading to anincrease in the intracellular concentration of Ca2C

{[Ca2C]i} [22]. These effects induce: (i) abnormal auto-maticity (see Glossary) in partially depolarized andischemic ventricular muscle; and (ii) afterdepolarizations{i.e. secondary depolarizations developed before [in Phase2 or 3 of the action potential (Figure 2)] or just aftercomplete action potential repolarization that produce focalactivity and might trigger arrhythmias} [23]. Both after-depolarizations and abnormal automaticity have beeninvolved in the induction of arrhythmias during myocar-dial ischemia–reperfusion [24]. Activation of the sym-pathetic nervous system and the release of endothelin-1

Human atrial K+ repolarizingcurrents

Human ventricular K+ repolarizing currents

Ito1

IKur

IKr

IK1 IKACh

Ito1

IKr

IKs

GeneProtein

α αβ β

Kv1.5 Kvβ2.1 KCNA5 KCNAB2

Kv4.3 KChiP2 KCND3 KCNIP2

Kv11.1 (HERG) MiRP1 KCNH2 KCNE2

Kv7.1 (KvLQT1) minK KCNQ1 KCNE1

IKur

Ito1

IKr

IKs

Current

(a)

(b)

TRENDS in Pharmacological Sciences

IKs, IK1, IKACh, IATP

IKATP

Phase 0

Phase 1

Phase 2

Phase 4

Phase 3

IK1, IKATP

Figure 2. (a) Human atrial (left) and ventricular (right) action potentials. The configuration and duration of action potentials reflect a balance between inward depolarizing and

outward repolarizing currents. The initial upstroke of the action potential in atrial and ventricular cells is due to the activation of the inward NaC current (INa). Repolarization is

mainly due to the outward efflux of KC that counterbalances the depolarizing effects of the inward Ca2C and NaC influx during Phase 2 of the action potentials. The main

voltage-dependent human KC currents are represented together with the inward rectifier currents (IK1, IKACh and IKATP). Note that some currents are exclusive to the atria (IKur)

or more important (IKACh) in the atria than in the ventricles and vice versa (IKr). (b) Genes encoding the a-subunit and b-subunit proteins that co-assemble to form the human

cardiac Kv channels. Kv channels are formed by the co-assembly of four pore-forming a-subunits and b-subunits. b-Subunits are a diverse molecular group, which includes

cytoplasmic proteins (Kvb2.1 and KChiP2) that interact with the intracellular domains of Kv channels and single transmembrane proteins (mink and MiRP1).

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005 157

might have a role in the pro-arrhythmic effects ofthe RAAS [4,22]. Both sympathetic stimulation andendothelin-1 produce a marked vasoconstrictor effectthat reduces coronary blood flow (i.e. myocardial ischemia)and increases myocardial oxygen demands, cardiac auto-maticity and the dispersion of ventricular repolarization,which facilitate the genesis of reentry-type arrhythmias.Furthermore, Ang II, through AT1 receptors, activatesa powerful membrane (NADH/NADPH) oxidase thatenhances the production of oxygen species that inactivatenitric oxide (NO) [25,26]. NO has important effects on

Glossary

Automaticity: The ability of some cardiac cells to generate action potentials

spontaneously.

Abnormal automaticity: Automatic behavior generated in cells that ordinarily

lack spontaneous activity (e.g. in ventricular cells).

Cardioversion: The process of restoring the normal rhythm of the heart from an

abnormal rhythm. It can be chemical (by taking an anti-arrhythmic drug) or

electrical (applying a synchronized electrical current through the chest wall to

the heart through special electrodes or paddles that are applied to the skin of

the chest and back).

His–Purkinje system: A specialized conduction system that enables a rapid

propagation of the impulse from the atrioventricular node to the ventricles. It

comprises the His bundle and the Purkinje fibers.

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sympathetic discharge, heart rate variability and baro-receptor function [27,28]. A decrease in the production ofNO can be restored by ACE inhibitors, AT1 receptors andaldosterone receptor antagonists [29–31].

In a myocardial infarction rat model, mineralocorticoidreceptor stimulation triggered electrical remodelingcharacterized by an increase in Ca2C current (ICa) and adecrease in Ito1 that preceded cardiac hypertrophy andwas prevented by mineralocorticoid receptor antagonism[32]. Furthermore, ACE inhibitors, Ang II receptorantagonists and aldosterone receptor antagonists exhi-bited anti-arrhythmic effects in models of ischemia–reperfusion or post-myocardial infarction in rats and inAT1 receptor knockout mice [24,33] and attenuated thearrhythmogenic remodeling (fibrosis) inpatients withhyper-tension, atrial fibrillation (AF) and CHF [3,4,7–10,18–20].Moreover, these drugs decreased sudden cardiac deathin patients with CHF, an effect attributed to a decreasein the incidence of complex arrhythmias [8,10,34–39] andQT dispersion in patients with hypertension, left ventri-cular hypertrophy and CHF [40–43]. AF is associated withthe activation of the RAAS, and ACE inhibitors and Ang IIreceptor antagonists reduced the risk of developing AF

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005158

and the recurrences of AF associated with hypertensionand CHF [10,44–50]. Thus, ACE inhibitors, Ang II recep-tor antagonists and aldosterone receptor antagonistsexert ‘anti-arrhythmic’ actions that, empirically, havebeen attributed to blockade of the pro-arrhythmic effectsof the RAAS.

Effects of spironolactone on human cardiac repolarizing

channels

Spironolactone is an aldosterone receptor antagonistthat exhibits a KC-sparing diuretic action and is used inthe treatment of hyperaldosteronism, hypertension, CHFand cirrhotic ascites [9]. The spironolactone ‘renaissance’started with the finding that it reversed aldosterone-induced vascular and cardiac fibrosis [9]. Two old studiesdemonstrated that spironolactone lengthened the APD inrabbit and rat multicellular preparations, effects thatwere attributed empirically to a decrease in KC conduc-tance [51,52]. Very recently, it was demonstrated thatspironolactone and its main metabolite, canrenoic acid(CA), inhibited Kv11.1 [human ether-a-go-go (HERG)]channel currents recorded in stably transfected Chinesehamster ovary (CHO) cells and IKr recorded in guinea-pigventricular myocytes [53]. Spironolactone and CA alsoblocked human Kv1.5 (hKv1.5), Kv4.3 and Kv7.1CminKchannels [54]. In humans, spironolactone is metabolized toactive metabolites (canrenone and/or CA) that have longerhalf-lives than spironolactone (1.5 h compared with16.5 h). Thus, the clinical relevance of the effects ofspironolactone was precluded by the fact that it ismetabolized too rapidly to be detected in plasma [55].A surprising finding was the morphology of the CAconcentration–response curves in all four channels. At awide range of concentrations (0.01–1000 nM in Kv11.1channels), CA-induced block was almost concentrationindependent and at these low concentrations block of thefour voltage-dependent KC channels reached almost 25%.Because the peak free plasma concentrations of CA afteradministration of therapeutic doses of spironolactonevaries between 3 nM and 16 nM [55], these results demon-strated that at therapeutic concentrations CA blocksKv11.1, hKv1.5, Kv4.3 and Kv7.1CminK channels [53,54].

The effects of CA on Kv11.1 channels were both voltageand frequency independent: the block appeared before thechannels were activated, and channel activation only ledto a small increase in block. CA did not modify the voltagedependence of inactivation or the time-course of reactiva-tion of Kv11.1 channels, indicating that CA did not bind tothe inactivated state of the channel. These results sug-gested that CA binds to a closed state (or active but notopen) from which opening proceeds, in addition to the openstate of the Kv11.1 channels. CA-induced block of hKv1.5and Kv7.1Cmink channels increased in the voltage rangeof channel activation, was not observed at the beginning ofthe depolarizing pulses and mono-exponentially increasedas the channels were opened, suggesting that CA blocksthe open state of these channels. In Kv4.3 channels, CAaccelerated the decline in current, which is suggestiveof an open-channel block mechanism, and modified thevoltage dependence of inactivation, the block increasing

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with channel inactivation, which indicates that CA blockedthe inactivated state of the channel [54].

The CA-induced block of the four Kv channels mightaccount for the described lengthening of the APD [51,52].In vivo, however, the effects of CA on Kv11.1 channels andcardiac repolarization might be blunted because spirono-lactone produces hyperkalemia (high levels of KC) [31]and, unlike most other KC currents, the current ampli-tude of Kv11.1 channels increases following elevation ofthe extracellular concentration of KC {[KC]o} [56]. How-ever, hyperkalemia is not expected to reduce the blockingeffects on hKv1.5, Kv4.3 and Kv7.1CminK channelcurrents; instead, a further decrease in channel currentshould be produced.

Acute intravenous administration of aldosterone hasbeen shown to rapidly (within 4–6 min) prolong the mono-phasic atrial action potentials recorded in patients withsupraventricular arrhythmias, which led to the proposalof a non-genomic mechanism for this effect [57]. If thelengthening effects of aldosterone are not mediated by itsinteraction with aldosterone receptors, it is not expectedthat the administration of spironolactone and CA willcounteract the prolonging effects of aldosterone. In fact,aldosterone reduced Ito1 [58], IKr and IKs, and addition ofCA in the presence of aldosterone did not antagonize theeffects of aldosterone; instead, a further decrease in cur-rent was observed that was similar to that produced by CAalone (i.e. block was additive) [53].

A possible lengthening of human cardiac repolariz-ation, together with the anti-fibrotic effects of spirono-lactone, might be highly desirable for the treatment ofboth ventricular and supraventricular arrhythmias [19,31].Therefore, direct effects of CA on cardiac repolarizationmight contribute to the spironolactone-induced reductionof QT dispersion, ventricular arrhythmias [59] and suddencardiac death in patients with CHF [37].

Effects of Ang II receptor antagonists on cardiac

repolarizing channels

The effects of five Ang II receptor antagonists [candesartan,eprosartan, irbesartan, losartan and its active metaboliteE3174, with different chemical structures, receptor affin-ity, type of AT1 receptor antagonism and pharmacokineticproperties, have been analyzed on cloned cardiac KC

channels. Micromolar concentrations (0.1–1 mM) of losar-tan, E3174, candesartan and eprosartan blocked hKv1.5channels that were stably expressed in mouse fibroblasts(LtkK cells) [60,61]. The block was frequency dependent,reaching 75% block for the most potent drug (E3174).Maximum plasma concentrations reached after the admini-stration of therapeutic doses of candesartan, losartan andeprosartan ranged between 0.2 mM and 4.5 mM. However,caution should be taken in extrapolating the in vitroresults of these drugs because they markedly bound toplasma proteins (90–95%). Interestingly, these Ang IIreceptor antagonists produced a profound modification ofthe voltage-dependence of hKv1.5 channel gating, theblock increasing concomitantly with channel opening, andat potentials at which channels are fully open a shallowvoltage-dependent unblock appeared. Furthermore, theactivation curve of hKv1.5 channels became biphasic,

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005 159

exhibiting a steeper component responsible for 75% ofthe activation process and a shallower component thatappeared at the more depolarized potentials (Figure 3a).Because hKv1.5 channels exhibited multiple openstates (C4C4.4C4O14O24I14I2 [62]), the second(shallower) component could be the consequence of a fastselective block of the first open state of the channel,followed by a very fast unblock from the second open state,which appeared at more depolarized potentials. A secondpossibility is that the anionic form of these drugs (whichpredominates at physiological pH) binds to a receptorlocated at the intracellular mouth of the channel pore. Inthis case, the voltage-dependent unblock can be explainedconsidering that the anionic form of the drugs reaches thereceptor site from the inside by crossing z20% of themembrane electrical field. Another possibility is thatthe binding site for Ang II receptor antagonists is locatedin the external mouth of the pore, in such a way thatoutward KC efflux hinders the binding of the drug to itsexternal receptor site with a resultant relief of block. Infact, when the [KC]i was reduced to 25%, the voltage-dependent unblock induced by candesartan and epro-sartan was abolished [61].

At therapeutic free plasma concentrations (0.7–1.1 mM)irbesartan blocks hKv1.5 channels [63]. Irbesartan exhi-bited a high affinity for hKv1.5 channels but a low effi-cacy of block because the maximum block reached was

–8+6

50 ms

Control

–80 mV+50 mV

1 s

–80 mV+60 mV

−80 –60 –40 –20 0 20 40 600

50

100

150

Membrane potential (mV)

hKv1.5

Kv4.3 Kv7.1+minK

(a) (

(c) (d)

ControlLosartan

hKv1

.5 (

pA)

500

pA

1 nA

Irbesartan(1 µM)

1 nA

(1µM)

Figure 3. Effects of angiotensin II (Ang II) AT1 receptor antagonists on cardiac Kv chan

activation. The continuous lines represent the fit of the data to a single Boltzmann equati

activation, Vm is the membrane potential and k represents the slope factor of the curve), w

components. This result demonstrates that losartan (purple) markedly modifies the volta

stably transfected LtkK cells in the absence, in the presence and after washout of 0.1

irbesartan partially inhibit hKv1.5 channel currents. (c) Reduction of the peak and accelera

effects produced by 0.1 mM candesartan on maximum outward and tail Kv7.1CminK cha

and tail Kv11.1 channel currents. Currents in panels c–e were recorded in transiently tra

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w30% (Figure 3b). The reasons for this behavior areunknown but it is possible that when the concentration ofbulky molecules of irbesartan near the binding siteincreases, the steric hindrance interactions betweenthem might decrease the efficacy of block. In fact, mol-ecular modeling used to define the energy-minimizeddocking for irbesartan in hKv1.5 channels revealed acomplex interaction between irbesartan and the channelprotein [63]. At therapeutic free plasma concentrationsirbesartan also blocked Kv4.3 channels (Figure 3c),whereas the blocking effects of candesartan and epro-sartan appeared at total maximum plasma concentrationsdescribed after the administration of therapeutic doses.The voltage- and time-dependence of drug-inducedblock suggested an interaction with the open state ofKv4.3 channels. Irbesartan, which modified the voltage-dependence of Kv4.3 channel inactivation, also binds tothe inactivate state.

Candesartan was the most potent Ang II receptorantagonist to block Kv7.1CminK channels. At 0.1 mMcandesartan blocked the current by 60% by binding to theopen state of the channel (Figure 3d) [61], whereas theconcentration of irbesartan needed to reach this level ofblock (500 mM) was distant from the range of therapeuticconcentrations [63].

The effects of Ang II receptor antagonists on Kv11.1channels were highly variable [60,61,63]. Losartan and

200 ms

Control

0 mV0 mV

Control

–40 mV

hKv1.5

2 s

–80 mV+10 mV –60 mV

Control

b)

(e)

Irbesartan (0.1 µM)Washout

E3174(1 µM)

200

pA

Candesartan(0.1 µM)

Kv11.1

nels. (a) Effects of losartan (1 mM) on the voltage dependence of hKv1.5 channel

on, yZA/{1Cexp[(VhKVm)/k]} (where A is the amplitude term, Vh is the midpoint of

hereas the dotted line (purple) shows the fit obtained with a sum of two Boltzmann

ge-dependence of hKv1.5 channel opening (b) hKv1.5 channel currents recorded in

mM irbesartan. As can be observed, therapeutically free plasma concentrations of

tion of the Kv4.3 channel currents produced by 1 mM irbesartan. (d) Marked blocking

nnel currents. (e) Increasing effects produced by 1 mM E3174 on maximum outward

nsfected CHO cells. Reproduced, with permission, from [60,61,63].

Review TRENDS in Pharmacological Sciences Vol.26 No.3 March 2005160

eprosartan inhibited Kv11.1 channel repolarizing tailcurrents at 1 mM, whereas candesartan and, particularly,E3174 (Figure 3e) increased the maximum current andtail currents. Moreover, the effects of E3174 on IKr wereconfirmed in isolated guinea-pig ventricular myocytes. Bycontrast, much higher concentrations of irbesartan(100 mM) were needed to block Kv11.1 channels. Usuallyboth cardiovascular and non-cardiovascular drugs exhi-bited a higher affinity for Kv11.1 channels than for othercardiac KC channels [64,65]. This has been attributed tothe larger volume of the inner vestibule of the channel,which enables the presence of large molecules, and tothe presence of two aromatic residues (Y652 and F656),which established specific interactions with the aromaticmoieties of the drugs [66]. Excessive block of Kv11.1channels leads to an excessive prolongation of the ven-tricular APD and the development of polymorphic ven-tricular tachycardia called ‘torsades de pointes’ [64]. Thisfinding increases the interest in understanding the mol-ecular determinants of drug binding to Kv11.1 channels todevelop safer and more effective drugs. From this point ofview, the analysis of the chemical structure of the Ang IIreceptor antagonists studied, particularly that of irbe-sartan, and its comparison with other cardiovascular andnon-cardiovascular Kv11.1 channel blocking drugs couldprovide clues for the design of safer drugs with low affinityfor Kv11.1 channels.

From these results two main conclusions can beobtained. First, there is not a common ‘class effect’ ofAng II receptor antagonists on cardiac repolarizing Kvchannels. The concentration-, voltage- and time-dependenceof the block produced were different for each single drugon each channel and unrelated to their affinity for AT1

receptors. It should be stressed that the effects of theAng II receptor antagonists appeared in the absence ofAng II; thus, they were not attributable to the antagonismof the effects of Ang II at the AT1 receptor but were ‘directeffects’ produced by their interaction with the channel-forming proteins. Second, at therapeutic free plasmaconcentrations irbesartan selectively blocked hKv1.5 andKv4.3 channels without affecting Kv11.1 and Kv7.1CminKchannels. Inhibition of currents exclusively (IKur) or pre-dominantly (Ito1) [67] present in the atria, together withthe antagonism of the effects of Ang II at the AT1 receptor,might be highly desirable for the treatment of supra-ventricular arrhythmias [68] and might explain whyirbesartan decreases the recurrence of AF in patientstreated with amiodarone (a class III anti-arrhythmicagent) after cardioversion [45]. In fact, in the presenceof irbesartan three different effects would tend to prolongthe human atrial APD: a direct KC channel-blockingeffect, the antagonism of the AT1 receptor-mediated pro-arrhythmic effects of Ang II and the selective stimulationof AT2 receptors.

Concluding remarks

Ang II and aldosterone produce pro-arrhythmic effects byseveral mechanisms (Box 1), including the modulation ofcardiac Kv channels. Ang II receptor antagonists andaldosterone receptor antagonists can exert anti-arrhythmiceffects related to the inhibition of the pro-arrhythmic effects

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of the RAAS in addition to a direct modulation of Kvchannels. The mechanisms involved in the anti-arrhythmiceffects of RAAS inhibitors include favorable hemodynamicchanges, inhibition of electrical and structural (fibrosis,dilatation and hypertrophy) remodeling and neurohumoralactivation, reduction of blood pressure and stabilizationof electrolyte disturbances. All these effects produced byRAAS inhibitors together with the favorable safety profileof these drugs compared with currently used anti-arrhythmic agents suggests that they could be used inthe prevention and treatment of cardiac arrhythmias inpatients with hypertension, myocardial infarction, CHFand AF [48,50]. However, further prospective, controlledand randomized trials are needed before RAAS inhibitorscan be recommended routinely for patients with cardiacarrhythmias.

AcknowledgementsSupported by Comision Interministerial de Ciencia y Tecnologıa(SAF2002-02304), Red Tematica de Investigacion Cardiovascular andPfizer Foundation Grants.

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