Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

Abstract

Normal central nervous system development relies on accurate intrinsic cellular programs as well as on extrinsic informative cues provided by extracellular molecules. Migration of neuronal progenitors from defined proliferative zones to their final location is a key event during embryonic and postnatal development. Extracellular matrix components play important roles in these processes, and interactions between neurons and extracellular matrix are fundamental for the normal development of the central nervous system. Guidance cues are provided by extracellular factors that orient neuronal migration. During cerebellar development, the extracellular matrix molecules laminin and fibronectin give support to neuronal precursor migration, while other molecules such as reelin, tenascin, and netrin orient their migration. Reelin and tenascin are extracellular matrix components that attract or repel neuronal precursors and axons during development through interaction with membrane receptors, and netrin associates with laminin and heparan sulfate proteoglycans, and binds to the extracellular matrix receptor integrins present on the neuronal surface. Altogether, the dynamic changes in the composition and distribution of extracellular matrix components provide external cues that direct neurons leaving their birthplaces to reach their correct final location. Understanding the molecular mechanisms that orient neurons to reach precisely their final location during development is fundamental to understand how neuronal misplacement leads to neurological diseases and eventually to find ways to treat them.

Extracellular matrix; Central nervous system; Cerebellum; Neuron; Migration

Braz J Med Biol Res, March 2006, Volume 39(3) 393-403 (Review)

Transient outward potassium current and Ca²⁺ homeostasis in the heart: beyond the action potential

R.A. Bassani

Centro de Engenharia Biomédica, Universidade Estadual de Campinas, Campinas, SP, Brasil

References

^{Correspondence and Footnotes}

Abstract

The present review deals with Ca²⁺-independent, K⁺-carried transient outward current (I_to), an important determinant of the early repolarization phase of the myocardial action potential. The density of total I_to and of its fast and slow components (I_to,f and I_to,s, respectively), as well as the expression of their molecular correlates (pore-forming protein isoforms Kv4.3/4.2 and Kv1.4, respectively), vary during postnatal development and aging across species and regions of the heart. Changes in I_to may also occur in disease conditions, which may affect the profile of cardiac repolarization and vulnerability to arrhythmias, and also influence excitation-contraction coupling. Decreased I_to density, observed in immature and aging myocardium, as well as during several types of cardiomyopathy and heart failure, may be associated with action potential prolongation, which favors Ca²⁺ influx during membrane depolarization and limits voltage-dependent Ca²⁺ efflux via the Na⁺/Ca²⁺ exchanger. Both effects contribute to increasing sarcoplasmic reticulum (SR) Ca²⁺ content (the main source of contraction-activating Ca²⁺ in mammalian myocardium), which, in addition to the increased Ca²⁺ influx, should enhance the amount of Ca²⁺ released by the SR during systole. This change usually takes place under conditions in which SR function is depressed, and may be adaptive since it provides partial compensation for SR deficiency, although possibly at the cost of asynchronous SR Ca²⁺ release and greater propensity to triggered arrhythmias. Thus, I_to modulation appears to be an additional mechanism by which excitation-contraction coupling in myocardial cells is indirectly regulated.

Key words: Action potential, Repolarization, Ca²⁺ current, K⁺ current, Excitation-contraction coupling

Introduction

The action potential (AP) is the triggering signal for contraction in striated muscle. Ion currents through sarcolemmal voltage- and ligand-dependent channels, as well as electrogenic ion transporters, determine the AP waveform. In a typical mammalian cardiac myocyte, 4 phases of the AP (Figure 1) can be identified (1,2):

Figure 1.
Action potential (AP) and ion currents in a rabbit ventricular myocyte. A, AP waveform, in which phases zero (upstroke), 1 (early repolarization), 2 (plateau), and 3 (late repolarization) are indicated. B, Voltage-dependent ion currents that contribute to membrane depolarization during the AP: Na⁺ current (I_Na) and L-type Ca²⁺ current (I_Ca,L). Note: the actual I_Na density is 10-fold greater than that shown in the figure. C, Voltage-dependent K⁺ currents that contribute to membrane repolarization during the AP: transient outward current (I_to), total delayed rectifier K⁺ current (I_K) and inwardly rectifying K⁺ current (I_K1). For clarity, I_K components (I_Kr and I_Ks) were combined, and currents carried by ion transporters (e.g., Na⁺-Ca²⁺ exchanger, Na⁺-K⁺ ATPase), as well as I_Cl(Ca), were omitted. Traces were obtained by simulation with the LabHEART 4.9.5 program (Ref. 3).

Phase zero (upstroke): during this brief phase, the rapidly activating Na⁺ current drives the membrane potential (E_m) from its diastolic level (~ -80 mV) to positive values (20-50 mV). The AP peak is limited mainly by Na⁺ channel inactivation and decrease in the transsarcolemmal Na⁺ electrochemical gradient.

Phase 1 (early repolarization): the rapid, transient outward current (I_to) is the predominant contributor to the partial membrane repolarization.

Phase 2 (plateau): this usually long phase may last up to a few hundred milliseconds, and is characterized by slow E_m variation, which is dependent on the delicate balance between depolarizing (mostly L-type Ca²⁺ current, I_Ca,L) and repolarizing (mainly mediated by delayed rectifier K⁺ channels) currents.

Phase 3 (late repolarization): this phase relies on both a decrease in I_Ca,L due to I_Ca,L inactivation/deactivation, and activation of delayed rectifier K⁺ channels. As repolarization proceeds, K⁺ efflux through inwardly rectifying K⁺ channels (I_K1) becomes greater (Figure 1C) due to relief of channel rectification and contributes to late restoration and stabilization of the diastolic E_m.

Recent investigation has provided evidence that the early repolarization phase may considerably influence the AP waveform. I_to, the main current responsible for this phase, has been shown to be carried by distinct ionic components: in addition to the Ca²⁺-independent, voltage- and time-dependent K⁺ current (I_to-1), a Ca²⁺-dependent Cl^- current (I_Cl(Ca) or I_to-2) was also identified. In this review, I will refer to the 4-aminopyridine-sensitive, Ca²⁺-insensitive transient outward K⁺ current (I_to-1) as I_to. More information on I_Cl(Ca) can be found in a recent review (4).

I_to, in turn, is the net result of K⁺ flux through at least two different types of channel associated with different isoforms of the pore-forming protein. The behavior of these channels differs especially in the time course of voltage-dependent inactivation and steady-state recovery from inactivation. Fast I_to (I_to,f) is mediated by channels that recover from inactivation in less than 100 ms, whereas for channels that carry slow I_to (I_to,s) recovery from inactivation takes a few seconds (5,6).

Both experimental data and mathematical models have shown that I_to magnitude and composition (i.e., the relative contributions of I_to,f and I_to,s to total I_to) may markedly affect AP duration and shape (7-13). Expression of I_to,f and I_to,s channel proteins is highly regulated, and may vary with species, developmental stage, and region of the heart (6,11). For instance, I_to,f is more prominent in ventricular epicardial myocytes than in endocardial myocytes. In the former, the AP waveform displays the so-called spike-and-dome configuration, with distinct phases 1 and 2, while in the latter AP duration tends to be greater and repolarization to the plateau level is slower and more gradual. Differences in I_to,f density correlate with differences in the AP profile within and between ventricles (e.g., 5,11,14,15). The I_to,f and AP duration gradients across and along the ventricular wall have been proposed to contribute to the dispersion of ventricular refractoriness, which markedly influences ventricular repolarization path, direction and time course. It has been proposed that region-dependent changes in I_to reported in certain disease states (e.g., post-myocardial infarction, ventricular hypertrophy, heart failure) may underlie increased susceptibility to repolarization abnormalities and reentry arrhythmias (11,16,17). However, in this brief review, I will not deal with the influence of I_to on cardiac electric conduction (see Ref. 16 for further discussion), but its indirect effects, via the AP waveform, on Ca²⁺ homeostasis, which is of paramount importance for the development of adequate ventricular mechanical function and blood pumping.

Myocardial Ca²⁺ cycling

Ca²⁺-induced Ca²⁺ release (CICR) is the most accepted mechanism underlying excitation-contraction coupling (ECC) in the mammalian myocardium (18,19). Ca²⁺ influx through voltage-dependent, sarcolemmal I_Ca,L during the AP is the main trigger for the release of a greater amount of Ca²⁺ by the sarcoplasmic reticulum (SR), which is the source of most Ca²⁺ that activates contraction. SR Ca²⁺ release causes an increase in the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i), which results in greater interaction of the ion with myofilament proteins and development of force and cell shortening. Thus, the AP is the signal for contraction development, and Ca²⁺ acts as the second messenger in the electro-mechanical coupling process. Contraction is limited by Ca²⁺ removal from the cytosol by several transporters, which promote [Ca²⁺]_i decline, Ca²⁺ dissociation from myofilaments and cell relaxation. The dominant transporter that ultimately determines relaxation is the SR Ca²⁺-ATPase (SERCA), which is responsible for 70-90% of cytosolic Ca²⁺ clearance and repletion of the SR Ca²⁺ store. Ca²⁺ efflux is mainly mediated by the Na⁺-Ca²⁺ exchanger (NCX), a sarcolemmal counter-transporter that is driven by the transsarcolemmal Na⁺ and Ca²⁺ electrochemical gradients. NCX accounts for 7-30% of total cytosolic Ca²⁺ removal during relaxation, which is approximately equivalent to the amount of Ca²⁺ entering the cell via I_Ca,L during excitation (1,20,21).

The Ca²⁺ transient amplitude is an important determinant of contraction amplitude and is greatly dependent on the amount of Ca²⁺ released by the SR during ECC. During each AP, the SR releases a fraction of its content, which increases with increasing trigger (i.e., I_Ca,L) amplitude and/or SR Ca²⁺ content (12,22,23). Moreover, ECC efficiency may be modulated by additional regulation of the SR Ca²⁺ channel activity by ions (e.g., Mg²⁺) and proteins (e.g., Ca²⁺-

calmodulin-dependent protein kinase II, FK506-binding protein, sorcin) (1).

It is important to note that Ca²⁺ transport by the main influx (I_Ca,L) and efflux (NCX) pathways during the cardiac cycle is strongly influenced by E_m: the former because of the voltage dependence of I_Ca,L activation and inactivation (and also because of the driving force for Ca²⁺ flux), and the latter because the direction and driving force for Ca²⁺ transport are determined by the difference between E_m and the exchanger reversal potential (E_NCX = 3E_Na - 2E_Ca, where E_Na and E_Ca are the Nernst equilibrium potentials for Na⁺ and Ca²⁺, respectively (24)). The consequences of the voltage dependence of I_Ca,L and NCX operation for ECC, [Ca²⁺]_i and contraction amplitude are many. The amplitude and time course of the trigger Ca²⁺ signal were shown to markedly affect its ability to induce SR Ca²⁺ release (19) and the so-called fractional SR Ca²⁺ release, i.e., the fraction of the SR Ca²⁺ content that is released at a twitch (22). On the other hand, Ca²⁺ efflux by the NCX is thermodynamically favored by membrane repolarization. Changes in NCX function may influence SR Ca²⁺ content and vulnerability to triggered arrhythmias (1). Thus, one can conclude that the AP does not represent simply an impulse to trigger ECC, but a complex input waveform that can modulate directly or indirectly ECC efficiency. On the other hand, Ca²⁺ cycling may conversely modulate the AP waveform, because of the effects of SR-released Ca²⁺ on membrane currents, for instance, inactivation of I_Ca,L, activation of Ca²⁺-dependent Cl^- channels and NCX-mediated current (25).

Transient outward K⁺ current (I_to)

I_to is characteristic of neurons and cardiac muscle, and its voltage-dependent activation and inactivation kinetics is much faster than that of other cardiac K⁺ currents. The channel is a macromolecular protein complex composed of pore-forming subunits a (which belong to the Kv gene subfamily), accessory ß subunits (several types of ß subunit have been identified) and other regulatory proteins, such as minimal K⁺ channel subunit homologues, K⁺ channel-associated proteins (possibly chaperone proteins) and K⁺ channel-interacting proteins (KChIP, which belong to a family of neuronal Ca²⁺ binding proteins). KChIP2 is present in the heart, and its co-expression with Kv4.2, but not with Kv1.4, increases current amplitude, changes the biophysical channel properties and allows the channel to be regulated by protein kinase A. The a subunit presents 6transmembrane domains, a K⁺-selective pore region and a highly charged S4 domain, which is considered to be the region where the voltage sensor is located. The functional channel consists of the assembly of 4 a subunits. In rodent heart, it seems that the channel that mediates I_to,f consists of Kv4.2 and/or 4.3 subunits co-assembled with KChIP2. A detailed description of the molecular aspects of the channels that mediate I_to can be found elsewhere (2,6,11,17).

Two types of a subunits may form channels with different kinetic properties. Kv4.2/4.3 expression correlates with I_to,f, which shows fast inactivation and recovery from steady-state inactivation (milliseconds). Kv4.2 appears to be the pore-forming subunit in rodent atria, whereas Kv4.3 mediates I_to,f in canine and human ventricle. Kv1.4 is thought to form the channels that mediate I_to,s, which displays a longer time course, especially of recovery from steady-state inactivation (seconds) (5,6,11,17).

The expression of these rapidly activating K⁺ channels is influenced by several factors.

Species

Kv4.2/4.3 and I_to,f are strongly expressed in the ventricular myocardium of adult rodents, ferrets, dogs, and humans, with a lesser contribution of Kv1.4 and I_to,s (6,12). I_to,f has been considered to be responsible for the typically brief rodent ventricular AP (6,26). In other species (e.g., rabbit), I_to,s and Kv1.4 are the dominant I_to component and channel isoform, respectively (6,12). In the guinea pig ventricle, I_to is absent, a fact that may contribute to the prolonged AP in this species (6).

Developmental stage

In several species, even those in which the heart presents large I_to expression during adulthood, I_to density is considerably small or absent during the fetal and neonatal period. In neonatal rodent ventricle, I_to,f and I_to,s (whose density is paralleled by Kv4.2/4.3 and Kv1.4 expression, respectively) show similar contributions to total I_to, while in adults the former contributes over 90% (2,11,27,28). In the neonatal myocardium, the AP is longer than in the adult, and AP shortening with maturation coincides with an increase in myocardial I_to density and channel isoform switch (28). Rabbit ventricle also shows a developmental increase in I_to, but in this species the dominant component shifts from I_to,f to I_to,s (29). On the other hand, aging is associated with a decrease in I_to density and AP prolongation (30).

Region of the ventricle

I_to density (particularly I_to,f) is greater in epicardium vs endocardium, in the apex vs base, and in right vs left ventricle. This variation is considered to underlie the regional differences in the AP profile, as well as in the dispersion of repolarization (5,11,14,15). While in rodents this regional variation may rely on a gradient of Kv4.3 expression only, it has not yet been established whether the origin of this variation in canine and human ventricle depends on the expression of Kv4.3, KChIP2, or both (17,31,32).

Disease

I_to,f down-regulation associated with AP prolongation has been reported following myocardial infarction, in hypertension, diabetic cardiomyopathy, ventricular hypertrophy, and heart failure (7-9,32-35), although in some cases AP lengthening is not accompanied by changes in I_to density (36). The mechanisms responsible for these changes have not been ascertained, but it is possible that they involve increased activity of the sympathetic and renin-angiotensin-aldosterone systems, since norepinephrine (via a-adrenoceptors), angiotensin II and aldosterone may negatively regulate I_to,f-mediating channels (11,17,35,37,38). Although in most cases a decrease in I_to,f is accompanied by Kv4.2/4.3 down-regulation, sometimes with an increase in I_to,s and Kv1.4 up-regulation (9,32,33,38), the functional change may be also associated with direct modification of the biophysical properties of the current (e.g., by angiotensin II) (37). Because of the decrease in I_to,f in the epicardium (where current density is greater) during disease, the transmural heterogeneity of AP duration is largely suppressed or even reversed, an event that may lead to repolarization abnormalities and possibly increase the predisposition to reentry arrhythmias (11,15).

How I_to can affect Ca²⁺ homeostasis

In the physiological context, I_to regional variability within the heart is thought to partially underlie regional differences in Ca²⁺ transient amplitude (15,39). The role of I_to magnitude and composition in cell Ca²⁺ homeostasis and contraction is mediated by I_to influence on AP waveform, especially the rate at which a plateau is attained, as well as its amplitude and duration. These AP features affect voltage-dependent Ca²⁺ transport pathways involved in ECC, in relaxation and in general regulation of cell Ca²⁺ load, such as I_Ca,L and NCX (Figure 2).

Figure 2.
How lengthening of the action potential (AP) plateau by transient outward current (I_to) suppression may affect excitation-contraction coupling in mammalian cardiac myocytes. Prolonged depolarization increases Ca²⁺ influx mainly by voltage-dependent L-type Ca²⁺ channels, and decreases the driving force of Ca²⁺ extrusion via the sarcolemmal Na⁺-Ca²⁺ exchanger (NCX). Both changes enhance Ca²⁺ availability for uptake by the sarcoplasmic reticulum (SR) Ca²⁺-ATPase, leading to an increase in SR Ca²⁺ content. Both increased Ca²⁺ influx and SR Ca²⁺ load synergistically increase the fractional SR Ca²⁺ release during systole, which causes greater myofilament activation and contraction. Increased Ca²⁺ influx can also permit a greater direct contribution to myofilament activation by Ca²⁺ originating from the extracellular medium.

Most of I_Ca,L develops during the AP plateau, in the voltage range at which the current peak is nearly maximal (1). A decrease in I_to density results in AP prolongation, especially of the plateau phase. It is expected that a longer AP is associated with greater total Ca²⁺ influx, and this has been confirmed by experimental evidence (12, 21,40). Paucity of I_to and prolonged AP are observed in some conditions in which the SR function is depressed, such as immaturity and senescence, ventricular hypertrophy and heart failure. In these cases, increased Ca²⁺ influx may contribute not only to the contraction-activating cytosolic Ca²⁺ pool, but also to facilitating ECC by enhancement of the fractional SR Ca²⁺ release (22,23). It is tempting to speculate whether there is a negative relationship between I_to functional expression and the SR relative contribution to ECC. I_to density has been found to be higher in cardiomyocytes in which Ca²⁺ cycling between the SR and cytosol is more prominent, such as rodents and ferrets vs rabbits, and adults vs neonates (6,11,12,41). In adult rodents and ferrets, a short AP would allow large and short-lived I_Ca,L, better suited for triggering synchronized SR Ca²⁺ release than for providing Ca²⁺ for contraction and SR loading (see below).

During early postnatal development, SR contribution to Ca²⁺ cycling, although important, is smaller than in adult myocardium (1,41), probably due to structural SR underdevelopment and paucity of sarcolemma-SR specialized junctions, as well as to diminished sensitivity of the CICR mechanism (18). Because of the small volume and reduced Ca²⁺ buffering capacity in immature myocytes (42), a greater Ca²⁺ influx should have a considerable impact directly on cytosolic [Ca²⁺] and/or indirectly via the induction of SR Ca²⁺ release, and thus on contractile activity. It has been recently shown that immature human myocytes (in which AP is long) stimulated with a waveform similar to the adult AP show depressed Ca²⁺ transients, which indicates that prolonged AP in developing myocytes is of paramount importance for proper Ca²⁺ cycling (43). Similar results were observed in myocytes from senescent animals (44), which present, as neonatal cells, reduced I_to density and prolonged AP (30).

In adults, enhanced cardiac workload due to increased hemodynamic load and/or decreased myocardial contractile function, such as in chronic ventricular hypertrophy induced by pressure overload, myocardial infarction and heart failure, are commonly associated with diminished total I_to or selectively I_to,f, and AP prolongation (7-9,35). These changes are accompanied by greater total Ca²⁺ influx during the long AP waveform, which may result in maintenance of Ca²⁺ transient amplitude, just as observed in immature and senescent ventricle. Interestingly, in these conditions the SR function is depressed, usually in association with SERCA down-regulation and diminished CICR sensitivity (1). One could thus interpret these findings from the viewpoint that I_to reduction may enable a presumably adaptive increase in Ca²⁺ influx during the long AP, so as to maintain Ca²⁺ cycling and contractile function compatible with survival.

Kassiri et al. (45) reported that I_to inhibition in cultured cardiac myocytes was effective in inducing myocyte hypertrophy by a Ca²⁺ influx-dependent mechanism. Expression and function of Kv4.2/4.3 channels are inhibited by signaling pathways normally activated during cardiac overload, possibly via phosphorylation by protein kinase C resulting from stimulation of angiotensin, a₁-adrenergic and endothelin-1 receptors (11, 37,46). Additionally, mineralocorticoid receptor activation has been implicated in early channel down-regulation following myocardial infarction (35). Thus, it has been speculated whether hypertrophy induction by activation of these pathways would partially involve enhanced Ca²⁺ influx resulting from I_to depression (45). Greater cell Ca²⁺ cycling, in addition to contributing to the preservation of cardiac mechanical function, might also play a role in excitation-transcription coupling, for instance via Ca²⁺-calmodulin-dependent kinases and phosphatases (calcineurin), in the development of ventricular hypertrophy and remodeling, which may eventually deteriorate to heart failure (1,36,45). However, although there is strong evidence of the implication of Ca²⁺-dependent biochemical pathways in hypertrophy development (see, e.g., 1,36,38), I_to,f reduction may not be the only way by which cell Ca²⁺ cycling can be increased. A few days after aortic banding (before hypertrophy development), Ca²⁺ cycling is enhanced by augmented SR function, without signs of increased Ca²⁺ influx (47), whereas a few weeks later the observed AP prolongation may be associated with an increase in I_Ca,L density, rather than a decrease in I_to (36). Also, Bodi et al. (38) observed that I_to suppression, reversal of Kv4.2/4.3 vs Kv1.4 dominance and AP prolongation in transgenic mice overexpressing I_Ca,L occurred only several months after hypertrophy development. Thus, I_to suppression may also be a consequence, rather than a cause, of greater Ca²⁺ cycling. For instance, enhancement of Ca²⁺ transient amplitude by SERCA overexpression has been shown to cause Kv4.2/4.3 and KChIP2 down-regulation, a decrease in I_to density and AP lengthening, even in the absence of hypertrophy or heart failure (48). In summary, there is no compelling evidence supporting the hypothesis that I_to suppression is necessarily involved in hypertrophy signaling. Part of the discrepancies among studies might be due to the multiplicity of experimental hypertrophy models and of the signaling pathways involved.

Although greater Ca²⁺ influx during a long AP may help prevent a dramatic depression of Ca²⁺ cycling in disease conditions, this compensation may be only partial in heart failure, because: a) a decrease in SR Ca²⁺ content in this condition (49) probably limits the amount of released Ca²⁺, even though influx is increased, and b) the slow AP phase 1 to phase 2 transition may slow down the I_Ca,L time course. To investigate the latter aspect, Sah et al. (50) employed stimulating AP waveforms of similar total duration, but with different rates of phase 1 repolarization. They observed that when early repolarization is slowed, I_Ca,L shows a decreased peak and slower kinetics, in spite of similar or greater total Ca²⁺ influx compared to that with fast early repolarization. Slow-developing I_Ca,L results in asynchronous SR Ca²⁺ release, which is less efficient to rapidly increase [Ca²⁺]_i at systole (50,51). This alteration may become more accentuated, with progression to heart failure, when intrinsic reduction of I_Ca,L amplitude may occur (51). One of the expected consequences would be slower and weaker contraction activation. Thus, an increase in Ca²⁺ influx due to I_to suppression and AP prolongation may provide a partial adaptation that is eventually offset by decreased ECC efficiency with maintenance and worsening of the disease condition.

Thus, the kinetic aspect involves an additional, subtler aspect of I_Ca,L modulation by I_to, i.e., the timing and rate of early repolarization. Linz and Meyer (52) showed that, during the respective AP waveforms, I_Ca,L is briefer and attains greater amplitude in rat (strong I_to) than in guinea pig myocyctes (where I_to and AP phase 1 are absent), even though the plateau is much shorter in the former. This difference was attributed to the ability of the large rodent I_to to rapidly drive E_m to a range at which the driving force for I_Ca,L is greater, before the channels rapidly inactivate and deactivate, causing I_Ca,L to assume a quasi impulse-like waveform. Although total Ca²⁺ influx is lower, it develops much faster in the rat than in the guinea pig, which is consistent with an optimal triggering signal for SR Ca²⁺ release. On the other hand, comparing epicardial vs endocardial canine ventricular myocytes (both of which present much longer APs than observed in rodents), the greater I_to density in the former causes the AP to assume the spike-and-dome configuration, which apparently favors Ca²⁺ channel reopening during the plateau (see secondary I_Ca,L peak in Figure 1B), resulting in greater Ca²⁺ influx (53). Thus, it appears that the fine-tuning of Ca²⁺ influx by the modulation of the AP waveform by I_to strongly depends on the type, density and behavior of the ion current profile present in a specific cell type and/or animal species.

An important side effect of I_to down-regulation and AP prolongation present in disease states may be the greater vulnerability to arrhythmias. In addition to reentry-predisposing changes in refractoriness dispersion (16), triggered activity is often associated with increased AP duration (1). Prolonged depolarization may result in diminished Ca²⁺ efflux via NCX (due to a decreased driving force), which, in combination with greater Ca²⁺ influx, leads to a greater cell and SR Ca²⁺ load. Although this might cause a further increase in Ca²⁺ transient and contraction amplitude because SR Ca²⁺ content greatly influences the fractional SR Ca²⁺ release (22,23), it may also facilitate arrhythmogenesis. SR Ca²⁺ overload is often accompanied by enhanced diastolic SR Ca²⁺ release (1,54) that results in the generation of a depolarizing, inward membrane current by electrogenic efflux of the leaked Ca²⁺ via NCX (stoichiometry of 1 Ca²⁺:3 Na⁺) (24). If of sufficient magnitude, this current can drive E_m to the excitation threshold and give rise to triggered arrhythmias. In the particular case of heart failure, arrhythmogenesis by this mechanism would be additionally favored by NCX up-regulation and E_m instability due to decreased I_K1 density (1).

Finally, it is also worthwhile to consider the relationship between AP duration and cycle length. This relation seems to stem from multifactorial mechanisms: I_to may be one of the underlying mechanisms in some species, in addition to modulation of I_Ca,L and other Ca²⁺-dependent currents by SR-released Ca²⁺, and a rate-dependent increase in density of delayed-rectifier K⁺ currents (e.g., 25,55). In most large mammals, including man, which present marked Kv4.2/4.3-dependent I_to, AP duration is decreased or little affected by increasing rate, especially in epicardial cells (16,56,57). This response might be important to allow adequate relaxation and ventricular filling during the shortened diastole. However, in rabbit and hamster myocardium, in which total I_to density is lower and I_to,s is the dominant component, the AP is lengthened with increasing rate (12,56,58). This might be due, at least in part, to the slow I_to,s recovery from inactivation, which would decrease channel availability at short intervals (2,55,56). A positive relationship between AP duration and rate may limit ventricular function and predispose the heart to triggered arrhythmias at high rates. As pointed out earlier, I_to,f down-regulation has been described in canine and human hypertrophied and failing ventricle. However, this event per se does not seem to affect the AP duration-rate relation, since the AP is prolonged at long, but not short cycle lengths (16,25). Recent results from computer simulations predicted that the positive rate-AP duration relationship relies on the presence of I_to,s, rather than on a decrease in I_to,f, since a negative relation is still present if I_to is totally suppressed (12). Interestingly, our prediction is in agreement with experimental data that show a negative AP duration-rate curve in the guinea pig ventricle (in which I_to is absent (55,59)) which is not changed by chronic ventricular hypertrophy induced by pressure overload (59). The finding that diabetic cardiomyopathy is associated with a frankly positive AP duration-rate relationship in the rat is intriguing, in contrast with the weak rate dependence observed in controls (60). However, information on I_to,s or Kv1.4 expression in this condition is lacking, and thus a possible link between I_to,s dominance and positive AP duration-rate still requires experimental confirmation.

Changes in I_to density and profile caused by pathological cardiovascular conditions may be overall adaptive, as they contribute to increase Ca²⁺ influx and may confer some protection against reentry due to prolonged refractoriness, although excessive Ca²⁺ loading, in conjunction with a decrease in membrane electrical stability, may favor the appearance of triggered arrhythmias. However, the fact that these changes are superimposed on the naturally occurring regional heterogeneity of this current (and its components) in the adult ventricle makes it difficult to predict their net effect on vulnerability to arrhythmia. This also complicates the development of pharmacological and gene therapy strategies targeted to I_to channels. Hopefully, this might be overcome with novel information to be gathered in the coming years.

Acknowledgments

I am grateful to Dr. José W.M. Bassani for helpful criticism and comments about this manuscript, and to Mr. Rafael A. Ricardo for assistance with the illustrations, both from Departamento de Engenharia Biomédica, FEEC, UNICAMP, Campinas, SP, Brazil.

Address for correspondence: R.A. Bassani, Centro de Engenharia Biomédica, UNICAMP, Caixa Postal 6040, 13084-971 Campinas, SP, Brasil. Fax: [phone]. E-mail: [e-mail]

Publication supported by FAPESP. Received May 18, 2005. Accepted November 24, 2005.

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Correspondence and Footnotes

Publication Dates

• Publication in this collection
  06 Mar 2006
• Date of issue
  Mar 2006

History

• Accepted
  24 Nov 2005
• Received
  18 May 2005