Two types of transient outward currents in cardiac ventricular cells of mice

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

Postnatal development has a marked effect on ventricular repolarization in mice

To better understand the mechanisms that underlie cardiac repolarization abnormalities in the immature heart, this study characterized and compared K(+) currents in mouse ventricular myocytes from day 1, day 7, day 20, and adult CD1 mice to determine the effects of postnatal development on ventricular repolarization. Current- and patch-clamp techniques were used to examine action potentials and the K(+) currents underlying repolarization in isolated myocytes. RT-PCR was used to quantify mRNA expression for the K(+) channels of interest. This study found that action potential duration (APD) decreased as age increased, with the shortest APDs observed in adult myocytes. This study also showed that K(+) currents and the mRNA relative abundance for the various K(+) channels were significantly greater in adult myocytes compared with day 1 myocytes. Examination of the individual components of total K(+) current revealed that the inward rectifier K(+) current (I(K1)) developed by day 7, both the Ca(2+)-independent transient outward current (I(to)) and the steady-state outward K(+) current (I(ss)) developed by day 20, and the ultrarapid delayed rectifier K(+) current (I(Kur)) did not fully develop until the mouse reached maturity. Interestingly, the increase in I(Kur) was not associated with a decrease in APD. Comparison of atrial and ventricular K(+) currents showed that I(to) and I(Kur) density were significantly greater in day 7, day 20, and adult myocytes compared with age-matched atrial cells. Overall, it appears that, in mouse ventricle, developmental changes in APD are likely attributable to increases in I(to), I(ss), and I(K1), whereas the role of I(Kur) during postnatal development appears to be less critical to APD.

Functional properties of K+ currents in adult mouse ventricular myocytes

Although the K+ currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time- and voltage-dependent outward K+ currents which modulate action potential repolarization. A Ca2+ -independent transient outward K+ current, Ito, can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K+ current, IKur, can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K+ current, Iss, can be obtained by combining these two procedures: (i) inactivating Ito and (ii) eliminating IKur by application of low concentration of 4-AP. Iss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of Ito is more than 20-fold faster than that of IKur. These results demonstrate that the outward K+ currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32 degrees C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K+ current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K+ currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K+ channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of IKur, Ito and Iss during action potential repolarization.

The Fas Receptor-1,4,5-IP3Cascade: A Potential Target for Treating Heart Failure and Arrhythmias

Activation of the Fas receptor in various cell types, including myocytes, triggers apoptotic as well as nonapoptotic effects. Recent studies suggest that Fas activation in the heart participates in the development of major pathologies such as myocarditis and ischemic/reperfusion insults, which are manifested by arrhythmias and mechanical dysfunction. To decipher the contribution of the Fas/FasL pathway to myocardial pathologies, we have investigated the functional consequences of Fas activation in normoxic and hypoxic ventricular myocytes. Our major findings were as follows. (1) Although Fas is constitutively expressed in ventricular myocytes, normoxic myocytes are resistant to Fas-mediated apoptosis. In contrast, hypoxia predisposes myocytes to apoptosis induced by Fas activation. The underlying mechanism is a shift in the balance between proapoptotic proteins (including Fas) and antiapoptotic proteins toward the former. (2) In normoxic myocytes, Fas activation causes a wide range of functional disturbances, which include reduction in resting potential and action potential amplitude, prolonged action potential duration, development of delayed and early after-depolarizations, occasionally culminating into arrhythmias, diastolic [Ca(2+)](i) level increase, decreased I(to) and increased I(Ca,L). (3) The above-mentioned effects in normoxic myocytes (but not Fas-mediated apoptosis in hypoxic myocytes) depend on the phospholipase C --> 1,4,5-IP(3) --> SR [Ca(2+)](i) release cascade. (4) Inhibition of tyrosine kinases with genistein blocks both the apoptotic and nonapoptotic consequences of Fas activation in ventricular myocytes. Based on these studies we propose that tyrosine phosphorylation in ventricular myocytes can serve as a novel potential target for attenuating Fas-mediated dysfunction in normoxic and hypoxic myocardium.

KB130015, a new amiodarone derivative with multiple effects on cardiac ion channels

KB130015 (KB015), a new drug structurally related to amiodarone, has been proposed to have antiarrhythmic properties. In contrast to amiodarone, KB015 markedly slows the kinetics of inactivation of Na(+) channels by enhancing concentration-dependently (K(0.5) asymptotically equal to 2 microM) a slow-inactivating I(Na) component (tau(slow) asymptotically equal to 50 ms) at the expense of the normal, fast-inactivating component (tau(fast) asymptotically equal to 2 to 3 ms). However, like amiodarone, KB015 slows the recovery from inactivation and causes a shift (K(0.5) asymptotically equal to 6.9 microM) of the steady-state voltage-dependent inactivation to more negative potentials. Despite prolonging the opening of Na(+) channels KB015 does not lengthen but often shortens the action potential duration (APD) in pig myocytes or in multicellular preparations. Only short APDs in mouse are markedly prolonged by KB015, which frequently induces early afterdepolarizations. KB015 has also an effect on other ion channels. It decreases the amplitude of the L-type Ca(2+) current (I(Ca-L)) without changing its time course, and it inhibits G-protein gated and ATP-gated K(+) channels. Both the receptor-activated I(K(ACh)) (induced in atrial myocytes by either ACh, adenosine or sphingosylphosphorylcholine) and the receptor-independent (GTPgammaS-induced or background) I(K(ACh)) are concentration-dependently (K(0.5) asymptotically equal to 0.6 - 0.9 microM) inhibited by KB015. I(K(ATP)), induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP), is equally suppressed. However, KB015 has no effect on I(K1) or on I(to). Consistent with the effects in K(+) currents, KB015 does not depolarize the resting potential but antagonizes the APD shortening by muscarinic receptor activation or by DNP. Intracellular cell dialysis with KB015 has marginal or no effect on Na(+) or K(+) channels and does not prevent the effect of extracellularly applied drug, suggesting that KB015 interacts directly with channels at sites more easily accessible from the extracellular than the intracellular side of the membrane. At high concentrations KB015 exerts a positive inotropic action. It also interacts with thyroid hormone nuclear receptors. Its toxic effects remain largely unexplored, but it is well tolerated during chronic administration.

Cytotoxic lymphocytes and cardiac electrophysiology

It is widely recognized that immune effector mechanisms contribute to cardiac dysfunction in major cardiac pathologies, such as myocarditis and the consequent dilated cardiomyopathy, Chagas' disease and heart transplant rejection. Of the wealth of immune mechanisms known to affect cardiac function, this review will deal with the adverse effects caused by cytotoxic T lymphocytes (CTL, CD4(+) and CD8(+) T lymphocytes), which participate in a broad range of heart pathologies. The interaction between cytotoxic lymphocytes and their target cells can set off two different effector mechanisms: (1) The perforin/granzymes, and (2) The Fas/FasL. In this review, I will discuss these mechanisms, and present experimental evidence showing that both can adversely affect cardiac myocytes in vitro, in a way that can contribute to a decline in the overall cardiac function.

Modulation of outward potassium currents in aligned cultures of neonatal rat ventricular myocytes during phorbol ester-induced hypertrophy

Protein kinase C-stimulating phorbol esters induce a strong hypertrophic response when applied in vitro to cardiac ventricular myocytes. The aim of this study was to determine if this in vitro model of hypertrophy is associated with changes in the expression of voltage-gated K(+)channels. Myocytes were isolated from 3--4-day-old neonatal rats and cultured on aligned collagen thin gels. Membrane currents were measured with the use of the whole-cell arrangement of the patch clamp technique and the expression levels of the Kv1.4, Kv4.2 and Kv2.1 alpha subunits quantified using Western blot analysis. Voltage steps positive to -30 mV resulted in the activation of both a transient (I(to)) and a sustained (I(sus)) component of outward K(+)current in the aligned myocytes. Overnight exposure to phorbol 12-myristate 13-acetate (PMA) caused a 55% increase in myocyte size and a three-fold reduction in the peak amplitude of I(to). No differences in the half-maximal voltages required for activation and steady-state inactivation were observed between I(to)measured in control and PMA-treated myocytes. In contrast, PMA treatment resulted in a 62% increase in a tetraethylammonium-sensitive component of I(sus)(TEA-I(sus)) and was associated with the appearance of a slow component of current decay. Expression levels of the Kv1.4 and Kv4.2 alpha subunits were strongly depressed in the hypertrophic myocytes, while the density of the Kv2.1 alpha subunit was enhanced. PMA-induced changes in the Kv alpha subunits were partially prevented through inhibition of the mitogen-activated protein kinase (MAPK) pathway. Thus, PMA-induced hypertrophy of cultured ventricular myocytes is associated with an altered expression of voltage-gated K(+)channels.

Four kinetically distinct depolarization-activated K+ currents in adult mouse ventricular myocytes

In the experiments here, the time- and voltage-dependent properties of the Ca2+-independent, depolarization-activated K+ currents in adult mouse ventricular myocytes were characterized in detail. In the majority (65 of 72, approximately 90%) of cells dispersed from the ventricles, analysis of the decay phases of the outward currents revealed three distinct K+ current components: a rapidly inactivating, transient outward K+ current, Ito,f (mean +/- SEM taudecay = 85 +/- 2 ms); a slowly (mean +/- SEM taudecay = 1,162 +/- 29 ms) inactivating K+ current, IK,slow; and a non inactivating, steady state current, Iss. In a small subset (7 of 72, approximately 10%) of cells, Ito,f was absent and a slowly inactivating (mean +/- SEM taudecay = 196 +/- 7 ms) transient outward current, referred to as Ito,s, was identified; the densities and properties of IK,slow and Iss in Ito,s-expressing cells are indistinguishable from the corresponding currents in cells with Ito,f. Microdissection techniques were used to remove tissue pieces from the left ventricular apex and from the ventricular septum to allow the hypothesis that there are regional differences in Ito,f and Ito,s expression to be tested directly. Electrophysiological recordings revealed that all cells isolated from the apex express Ito,f (n = 35); Ito,s is not detected in these cells (n = 35). In the septum, by contrast, all of the cells express Ito,s (n = 28) and in the majority (22 of 28, 80%) of cells, Ito,f is also present. The density of Ito,f (mean +/- SEM at +40 mV = 6.8 +/- 0.5 pA/pF, n = 22) in septum cells, however, is significantly (P < 0.001) lower than Ito,f density in cells from the apex (mean +/- SEM at +40 mV = 34.6 +/- 2.6 pA/pF, n = 35). In addition to differences in inactivation kinetics, Ito,f, Ito,s, and IK,slow display distinct rates of recovery (from inactivation), as well as differential sensitivities to 4-aminopyridine (4-AP), tetraethylammonium (TEA), and Heteropoda toxin-3. IK,slow, for example, is blocked selectively by low (10-50 microM) concentrations of 4-AP and by (>/=25 mM) TEA. Although both Ito,f and Ito,s are blocked by high (>100 microM) 4-AP concentrations and are relatively insensitive to TEA, Ito,f is selectively blocked by nanomolar concentrations of Heteropoda toxin-3, and Ito,s (as well as IK,slow and Iss) is unaffected. Iss is partially blocked by high concentrations of 4-AP or TEA. The functional implications of the distinct properties and expression patterns of Ito,f and Ito,s, as well as the likely molecular correlates of these (and the IK,slow and Iss) currents, are discussed.

Cardiac dysfunction in murine autoimmune myocarditis

We have investigated the pathophysiological basis of cardiac dysfunction in autoimmune myocarditis and in the resulting dilated cardiomyopathy. To this end we utilized the myosin-induced autoimmune myocarditis model in BALB/c mice. Myocarditis has been found to be associated with massive ventricular lymphocyte infiltration and a 50% reduction in tail artery blood flow, reflecting the depressed cardiac function in myocarditis. Action potential characteristics of control and diseased isolated ventricular myocytes were (mean+/-SEM): resting potential: -68.1+/-1. 1,-68.3+/-0.7 mV; action potential amplitude: 96.5+/-10.4, 92.3+/-4. 4 mV; action potential duration at 80% repolarization (APD80) 38+/-5, 116+/-24* ms; * P<0.05. We utilized the whole cell voltage clamp technique to explore ion currents involved in APD prolongation and arrhythmogenic activity, and found that in diseased myocytes the transient outward current (Ito) was markedly attenuated. At a membrane potential of +40 mV, in control and in diseased myocytes, I(to) current density was 14.7+/-1.5 and 6.5+/-1.4 pA/pF, respectively, P<0.005. In contrast, the L-type Ca2+current (ICa,L) remained unchanged. To further explore the basis for cardiac impairment, we simultaneously measured [Ca2+]i transient and contraction in isolated normal and diseased myocytes. The major findings indicated that both the relaxation kinetics of [Ca2+]i transients and myocyte contraction were significantly faster in the diseased myocytes. In conclusion, substantial, potentially reversible, electrophysiological and mechanical perturbations in ventricular myocytes from mice with myosin-induced autoimmune myocarditis appear to contribute to disease-related cardiac dysfunction.

Regulation of voltage-gated K+ channel expression in the developing mammalian myocardium

As in neurons, depolarization-activated, Ca2+-independent outward K+ currents play prominent roles in shaping the waveforms of action potentials in myocardial cells. Several different types of voltage-gated K+ currents that contribute to the distinct phases of action potential repolarization have been characterized in myocardial cells isolated from different species, as well as in cells isolated from different regions of the heart in the same species. Important among these are the transient outward current, I(to), similar to the neuronal K+ current IA, and several components of delayed rectification, including I(Kr)[IK(rapid)], I(Ks)(IK(slow)], and I(Kur)[IK(ultrarapid)]. The properties of these currents in different species and cell types are remarkably similar, suggesting that the molecular correlates of functional voltage-gated K+ channel types are also the same. A number of voltage-gated K+ channel (Kv) pore-forming (alpha) and accessory (beta) subunits have now been cloned from heart cDNA libraries, and a variety of experimental approaches are being exploited to determine the molecular relationships between these subunits and functional voltage-gated myocardial K+ channels. Considerable progress has been made recently in defining these relationships, and the results obtained to date indeed suggest that distinct molecular entities underlie the different types of voltage-gated K+ channels characterized electrophysiologically in myocardial cells. Marked changes in the densities and/or the properties of voltage-gated K+ currents occur during normal cardiac development, as well as in conjunction with myocardial damage or disease, and there is considerable interest in understanding the molecular mechanisms underlying these changes. Although there is evidence for transcriptional, translational, and posttranslational regulation of functional voltage-gated K+ channel expression, we are only beginning to understand the underlying mechanisms; further studies focussed on delineating the molecular mechanisms controlling functional K+ channel expression are clearly warranted.

Long QT and ventricular arrhythmias in transgenic mice expressing the N terminus and first transmembrane segment of a voltage-gated potassium channel

Voltage-gated potassium channels control cardiac repolarization, and mutations of K+ channel genes recently have been shown to cause arrhythmias and sudden death in families with the congenital long QT syndrome. The precise mechanism by which the mutations lead to QT prolongation and arrhythmias is uncertain, however. We have shown previously that an N-terminal fragment including the first transmembrane segment of the rat delayed rectifier K+ channel Kv1.1 (Kv1.1N206Tag) coassembles with other K+ channels of the Kv1 subfamily in vitro, inhibits the currents encoded by Kv1.5 in a dominant-negative manner when coexpressed in Xenopus oocytes, and traps Kv1.5 polypeptide in the endoplasmic reticulum of GH3 cells. Here we report that transgenic mice overexpressing Kv1.1N206Tag in the heart have a prolonged QT interval and ventricular tachycardia. Cardiac myocytes from these mice have action potential prolongation caused by a significant reduction in the density of a rapidly activating, slowly inactivating, 4-aminopyridine sensitive outward K+ current. These changes correlate with a marked decrease in the level of Kv1.5 polypeptide. Thus, overexpression of a truncated K+ channel in the heart alters native K+ channel expression and has profound effects on cardiac excitability.

Electrophysiology of ion channels of the heart

The contribution of Na+, Ca2+, and various K+ currents to the shape of the cardiac action potential is outlined based on the relation between electrophysiological properties and structure of channel molecules. These currents have also been found in human ventricular myocytes, where the most prominent K+ current is a transient outward current that is not influenced by methylsulfonanilide antiarrhythmic drugs. Combined knowledge of electrophysiological and molecular properties of ion channels is likely to form the basis for rational design of future drugs.

Characterization of a voltage-gated K+ channel beta subunit expressed in human heart

Voltage-gated K+ channels are important modulators of the cardiac action potential. However, the correlation of endogenous myocyte currents with K+ channels cloned from human heart is complicated by the possibility that heterotetrameric alpha-subunit combinations and function-altering beta subunits exist in native tissue. Therefore, a variety of subunit interactions may generate cardiac K+ channel diversity. We report here the cloning of a voltage-gated K+ channel beta subunit, hKv beta 3, from adult human left ventricle that shows 84% and 74% amino acid sequence identity with the previously cloned rat Kv beta 1 and Kv beta 2 subunits, respectively. Together these three Kv beta subunits share > 82% identity in the carboxyl-terminal 329 aa and show low identity in the amino-terminal 79 aa. RNA analysis indicated that hKv beta 3 message is 2-fold more abundant in human ventricle than in atrium and is expressed in both healthy and diseased human hearts. Coinjection of hKv beta 3 with a human cardiac delayed rectifier, hKv1.5, in Xenopus oocytes increased inactivation, induced an 18-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 8.0 msec vs. 35.4 msec at -50 mV). hKv beta 3 was localized to human chromosome 3 by using a human/rodent cell hybrid mapping panel. These data confirm the presence of functionally important K+ channel beta subunits in human heart and indicate that beta-subunit composition must be accounted for when comparing cloned channels with endogenous cardiac currents.

Molecular and functional diversity of cloned cardiac potassium channels

Action potential duration is an important determinant of refractoriness in cardiac tissue and thus of the ability to propagate electrical impulses. Action potential duration is controlled in part by activation of K+ currents. Block of K+ channels and the resultant prolongation of action potential duration has become an increasingly attractive mode of anti-arrhythmic intervention. Detailed investigation of individual cardiac K+ channels has been hampered by the presence of multiple types of K+ channels in cardiac cells and the difficulty of isolating individual currents. We have approached this problem by employing a combination molecular cloning technology, heterologous channel expression systems, and biophysical analysis of expressed channels. We have focused on six different channels cloned from the rat and human cardiovascular systems. Each channel has unique functional and pharmacological characteristics, and as a group they comprise a series of mammalian K+ channel isoforms that can account for some of the diversity of channels in the mammalian heart. Each channel appears to be encoded by a different gene with little or no evidence for alternate splicing of RNA transcripts to account for the differences in primary amino acid sequence. In addition to the unique kinetic properties of these channel isoforms when expressed as homotetrameric assemblies, the formation of heterotetrameric K+ channels is also observed. The formation of heterotetrameric channels from the different gene products to create new channels with unique kinetic and pharmacological properties might further account for cardiac K+ channel diversity.

Heteromultimeric assembly of human potassium channels. Molecular basis of a transient outward current?

To gain insight into the molecular basis of cardiac repolarization, we have expressed K+ channels cloned from ventricular myocardium in Xenopus oocytes. A recently identified human cardiac K+ channel isoform (human Kv1.4) has properties similar to the 4-aminopyridine-sensitive calcium-insensitive component of the cardiac transient outward current. However, these channels recovered from inactivation much slower than native channels. Hybrid channels consisting of subunits from different K+ channel clones (delayed rectifier channels [Kv1.1, Kv1.2, and Kv1.5] and Kv1.4) were created by coinjection of cRNAs in oocytes. Multimeric channels consisting of Kv1.4:Kv1.1, Kv1.4:Kv1.2, and Kv1.4:Kv1.5 were expressed and compared. The hybrid channels displayed characteristics of heterotetrameric channels with kinetics that more closely resembled a native cardiac transient outward current. The inactivation and recovery from inactivation of the heteromeric channels indicated that the presence of a single inactivating subunit (Kv1.4) was probably sufficient to cause channel inactivation. The results demonstrate that expression of different K+ channel genes can produce channel protein subunits that assemble as heteromultimers with unique properties. It is shown that certain combinations of voltage-gated K+ channels probably do not contribute to native transient outward current. However, one combination of subunits could not be excluded. Therefore, this mechanism of channel assembly may underlie some of the functional diversity of potassium channels found in the cardiovascular system.

Multiple levels of native cardiac Na+ channels at elevated temperature measured with high-bandwidth/low-noise patch clamp

Currents through single Na+ channels were studied in cell-attached patches of enzymatically dispersed heart cells of the mouse with a low-noise patch-clamp technique that allows evaluation of current levels at temperatures of up to 35 degrees C with bandwidths of up to 13 kHz. Noise arising from the pipette and the holder was reduced by the use of short (total length 8 mm) patch pipettes, which were sealed at their end with oil and inserted for only 1.5 mm into an appropriately tipped holder. At 9 degrees C (filter 5 kHz), channel openings were regularly dominated by one open level, and amplitude histograms could be fitted with high accuracy with a sum of Gaussian curves. Above 24 degrees C (filter 10 or 13 kHz), however, channel-open levels were heterogeneous with maximum levels of up to 4.5 pA at -50 mV. Amplitude histograms with improved resolution, based on variance calculation with window widths of 75 microseconds or 195 microseconds, confirmed the observed heterogeneity of open levels. Regular level patterns were not found. The frequency of the largest levels strongly varied from patch to patch and intermediate levels were always the most frequent. A corresponding dissociation of amplitudes was also observed at 35 degrees C. Averaged currents, formed from trace ensembles including only levels below arbitrarily set borders, obeyed equal kinetics. It is concluded that at low temperature the conductance of single Na+ channel currents is much more homogeneous than at 24 degrees C and above, where the same channels have multiple open states with different conductance.

Molecular biology of the voltage-gated potassium channels of the cardiovascular system

K+ channels represent the most diverse class of voltage-gated ion channels in terms of function and structure. Voltage-gated K+ channels in the heart establish the resting membrane K+ permeability, modulate the frequency and duration of action potentials, and are targets of several antiarrhythmic drugs. Consequently, an understanding of K+ channel structure-function relationships and pharmacology is of great practical interest. However, the presence of multiple overlapping currents in native cardiac myocytes complicates the study of basic K+ channel function and drug-channel interactions in these cells. The application of molecular cloning technology to cardiovascular K+ channels has identified the primary structure of these proteins, and heterologous expression systems have allowed a detailed analysis of channel function and pharmacology without contaminating currents. To date six different K+ channels have been cloned from rat and human heart, and all have been functionally characterized in either Xenopus oocytes or mammalian tissue culture systems. This initial research is an important step toward understanding the molecular basis of the action potential in the heart. An important challenge for the future is to determine the cell-specific expression and relative contribution of these cloned channels to cardiac excitability.

Age-related appearance of outward currents may contribute to developmental differences in ventricular repolarization

Ventricular repolarization significantly influences contractility, refractoriness, and ion channel state. Factors affecting repolarization will thus affect these secondary phenomena. To understand the influence of age on ventricular repolarization, we studied neonatal, young, and adult dogs using electrocardiogram, action potential, and whole-cell voltage-clamp recordings from single epicardial myocytes. Hearts of neonatal and 57-58-day-old dogs require a significantly longer time for repolarization than those of adult dogs, as determined by analysis of rate-corrected QT and JT (QT minus QRS) intervals. Epicardial action potentials of neonates are significantly longer than those of adults, as determined by measurements of duration at 50% and 90% repolarization. The adult action potential is characterized by a large phase 1 notch that is absent from neonatal recordings. This notch develops between 58 and 64 days of age, and by 64-68 days of age, it is equal to that in adults. In addition, action potentials recorded from adult and young epicardial muscle are more greatly affected by rapid pacing and superfusion of 2 mM 4-aminopyridine than are potentials recorded from neonatal tissue. Whole-cell voltage-clamp recordings reveal a 4-aminopyridine-sensitive transient outward current in adult myocytes that is absent from neonatal myocytes. The correlation between developmental changes in the 4-aminopyridine-sensitive current, the action potential, and the QT interval suggests that the transient outward current may be an important determinant in the relation between age and repolarization.

Functional expression of an inactivating potassium channel cloned from human heart

Recently a putative K+ channel with homology to the Shaker family of potassium channels has been cloned from human ventricular myocardium. However, proof that the cDNA encodes a K+ channel requires appropriate translation and expression of a functional ion-selective channel. Therefore, expression of this putative human K+ channel DNA was attempted by cytoplasmic injections of in vitro transcribed cRNA into Xenopus laevis oocytes and screening by two-electrode voltage-clamp methods. This resulted in expression of voltage-gated channels that rapidly inactivated (time constant of inactivation, 47.6 +/- 3.6 msec; 0 mV; n = 10) and were at least 50 times more selective for K+ than Na+ (Na+/K+ permeability ratio of 0.02). The channels showed voltage-dependent activation (half-maximal voltage, -34 +/- 0.7 mV; n = 5), and 50% of the channels were inactivated within 2 seconds when the membrane potential was clamped near -60 mV (half-maximal voltage, -62 +/- 7 mV; n = 10). The expressed protein resulted in a K+ current that had many properties similar to the 4-aminopyridine-sensitive calcium-insensitive component of the cardiac transient outward current that is observed in native cardiac myocytes and thus may serve as one molecular substrate for this current.

L-type calcium channels, potassium channels, and novel nonspecific cation channels in a clonal muscle cell line derived from embryonic rat ventricle

We have characterized the membrane currents in the H9c2 clonal muscle cell line derived from embryonic rat ventricle. These cells, established by selective serial passage and clonal proliferation, have been found by Hescheler and coworkers to express dihydropyridine-sensitive calcium channels that respond to beta-adrenergic stimulation. We have investigated the macroscopic and elementary currents in these cells by using standard patch-clamp methods. In cells that are kept confluent for 3-4 weeks, we have confirmed the expression of L-type calcium channels and additionally establish that the unitary conductance of many, but not all, of these channels (25 pS in 70 mM barium) is equal to that of cardiac rather than skeletal muscle. When the cells are proliferating rapidly, calcium channels are sparse or absent, but at least two distinct potassium channels and a nonspecific cation channel are observed. The nonspecific channel exhibits a conductance of 30 pS in physiological saline and conducts sodium, potassium, and calcium with nearly equal efficacy. Several unusual properties distinguish this nonspecific channel from others described previously. Gating is voltage dependent, with slow activation and marked increases in open probability at positive potentials. Unlike voltage, activation and marked increases in open probability at positive potentials. Unlike voltage, changes in [Ca2+] or in membrane stretch do not noticeably influence activity. In conclusion, our work and that of Hescheler et al indicate that H9c2 cells are potentially valuable surrogates for the investigation of ion channel regulation and muscular gene expression.

Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle

1. Regional variations in the shape of early repolarization of the action potential have been correlated to differences in transient outward K+ current, I(t), in myocytes isolated from the epicardial surface, the endocardial trabeculae and the papillary muscles of rabbit left ventricles. Temperature was 35 degrees C during whole-cell, and 22-23 degrees C during cell-attached experiments. 2. Membrane resting potentials were very similar regionally. At 0.1 Hz stimulation the action potential plateau amplitude in papillary muscle cells was significantly higher (104.7 mV) than in epicardial cells (96.47 mV). Exposure to 4-aminopyridine or increases in the rate of stimulation from 0.1 Hz to 3.3 Hz increased plateau height and diminished the initial notch on repolarization. These effects were correlated to the magnitude of I(t) in these cells. At low rates of stimulation I(t) caused a 'spike and dome' morphology of the action potential. 3. Voltage clamp experiments confirmed a higher current density of I(t) in epicardial cells (7.66 pA/pF at +20 mV) than in endocardial (6.45 pA/pF) or papillary muscle cells (3.69 pA/pF). I(t) at 35 degrees C was faster and larger than previously reported and individual currents inactivated almost completely during 100 ms pulses to plateau potentials. No differences in the kinetics or voltage dependence of whole-cell currents were found. Thus, the half-inactivation potential was -37.8 mV in cells from all three regions. 4. Cell-attached recordings from endocardial and epicardial cells showed very similar single-channel amplitudes, burst open probabilities and ensemble averages. The peak channel open probability soon after the start of depolarizing voltage clamp pulses did not change between cell types (P approximately 0.8). The slope conductance of I(t) channels was 13.0 pS with an intercept near the resting potential of the cell. 5. We conclude that regional variations in the shape of initial repolarization in cells from rabbit left ventricle are caused by variations in the magnitude of the transient outward K+ current, I(t). Epicardial cells have the largest, and papillary muscle cells the smallest I(t). The differences are not explained by alterations in the whole-cell kinetics or single-channel kinetics and conductance. The most likely explanation for variations in whole-cell current density is therefore a decrease in channel density in endocardium and papillary muscle compared with epicardial tissue. We estimate the density of I(t) channels per cell to be 1495 (one per 3-4 micron2) in epicardium, 1175 (one per 4-5 micron2) in endocardium, and 875 (one per 6 micron2) in papillary muscle cells.

Chloride conductance pathways in heart

Nonelectrogenic movement of Cl- is believed to be responsible for the active accumulation of intracellular Cl- in cardiac muscle. The electro-neutral pathways underlying this nonpassive distribution of Cl- are believed to include Cl(-)-HCO3- exchange, Na(+)-dependent cotransport (operating as Na(+)-Cl- and Na(+)-K(+)-2Cl- cotransport), and K(+)-Cl- cotransport. The electrogenic movement of Cl- in cardiac muscle is particularly interesting from a historical perspective. Until recently, there was some doubt as to whether Cl- carried any current in the heart. Early microelectrode experiments indicated that a Cl- conductance probably played an important role in regulating action potential duration and resting membrane potential. Subsequent voltage-clamp experiments identified a repolarizing, transient outward current that was believed to be conducted by Cl-, yet further investigation suggested that this transient outward current was more likely a K+ current, not a Cl- current. This left some doubt as to whether Cl- played any role in regulating membrane potential in cardiac muscle. More recent studies, however, have identified a highly selective Cl- conductance that is regulated by intracellular adenosine 3',5'-cyclic monophosphate, and it appears that this Cl- current may play an important role in the regulation of action potential duration and resting membrane potential.

Properties of transient K+ currents and underlying single K+ channels in rat olfactory receptor neurons

The transient potassium current, IK(t), of enzymatically dissociated rat olfactory receptor neurons was studied using patch-clamp techniques. Upon depolarization from negative holding potentials, IK(t) activated rapidly and then inactivated with a time course described by the sum of two exponential components with time constants of 22.4 and 143 ms. Single-channel analysis revealed a further small component with a time constant of several seconds. Steady-state inactivation was complete at -20 mV and completely removed at -80 mV (midpoint -45 mV). Activation was significant at -40 mV and appeared to reach a maximum conductance at +40 mV (midpoint -13 mV). Deactivation was described by the sum of two voltage-dependent exponential components. Recovery from inactivation was extraordinarily slow (50 s at -100 mV) and the underlying processes appeared complex. IK(t) was reduced by 4-aminopyridine and tetraethylammonium applied externally. Increasing the external K+ concentration ([K+]o) from 5 to 25 mM partially removed IK(t) inactivation, usually without affecting activation kinetics. The elevated [K+]o also hyperpolarized the steady-state inactivation curve by 9 mV and significantly depolarized the voltage dependence of activation. Single transient K+ channels, with conductances of 17 and 26 pS, were observed in excised patches and often appeared to be localized into large clusters. These channels were similar to IK(t) in their kinetic, pharmacological, and voltage-dependent properties and their inactivation was also subject to modulation by [K+]o. The properties of IK(t) imply a role in action potential repolarization and suggest it may also be important in modulating spike parameters during neuronal burst firing. A simple method is also presented to correct for errors in the measurement of whole-cell resistance (Ro) that can result when patch-clamping very small cells. The analysis revealed a mean corrected Ro of 26 G omega for these cells.

Nonlinear charge movement in mammalian cardiac ventricular cells. Components from Na and Ca channel gating

Intramembrane charge movement was recorded in rat and rabbit ventricular cells using the whole-cell voltage clamp technique. Na and K currents were eliminated by using tetraethylammonium as the main cation internally and externally, and Ca channel current was blocked by Cd and La. With steps in the range of -110 to -150 used to define linear capacitance, extra charge moves during steps positive to approximately -70 mV. With holding potentials near -100 mV, the extra charge moving outward on depolarization (ON charge) is roughly equal to the extra charge moving inward on repolarization (OFF charge) after 50-100 ms. Both ON and OFF charge saturate above approximately +20 mV; saturating charge movement is approximately 1,100 fC (approximately 11 nC/muF of linear capacitance). When the holding potential is depolarized to -50 mV, ON charge is reduced by approximately 40%, with little change in OFF charge. The reduction of ON charge by holding potential in this range matches inactivation of Na current measured in the same cells, suggesting that this component might arise from Na channel gating. The ON charge remaining at a holding potential of -50 mV has properties expected of Ca channel gating current: it is greatly reduced by application of 10 muM D600 when accompanied by long depolarizations and it is reduced at more positive holding potentials with a voltage dependence similar to that of Ca channel inactivation. However, the D600-sensitive charge movement is much larger than the Ca channel gating current that would be expected if the movement of channel gating charge were always accompanied by complete opening of the channel.

Two components of transient outward current in canine ventricular myocytes

Repolarization during phase 1 of cardiac action potential is important in that it may influence both impulse conduction in partially depolarized tissue and action potential duration. Thus, it is important to know the properties and regulation of the underlying currents. In about 50% of canine ventricular myocytes, the actin potential displays a phase 1 of fast repolarization and a prominent notch between phase 1 and the plateau. A transient outward current is responsible for both. This current is composed of two components: one (Ito1) blocked by 4-aminopyridine and the other (Ito2) blocked by manganese. In the present study, we characterized each of the components in isolation from the other. Both had an activation threshold between -30 and -20 mV. At the same voltage, Ito1 was larger than Ito2 and had a shorter time to peak. The peak current-voltage relationship for Ito1 was almost linear, but that for Ito2 was bell-shaped. Ito1 decayed during sustained depolarization with a single exponential time course: tau less than 30 msec at all voltages. It recovered from inactivation with a voltage-dependent time course: tau = 70 msec at -90 mV and 720 msec at -40 mV. Ito2 was augmented by elevating [Ca2+]o or by isoproterenol. It was inhibited by caffeine, ryanodine, or a preceding transient inward current, suggesting that it was activated by intracellular calcium released from sarcoplasmic reticulum. We conclude that Ito1 and Ito2 in canine ventricle are similar to those described for many other cardiac tissues, but the kinetics of Ito1 are significantly faster than in other tissues.

K channel kinetics during the spontaneous heart beat in embryonic chick ventricle cells

By averaging the current that passes through cell-attached patches on beating heart cells, while measuring action potentials with a whole-cell electrode, we were able to study K channels during beating. In 7-d chick ventricle in 1.3 mM K physiological solutions at room temperature, delayed-rectifier channels have three linear conductance states: 60, 30, and 15 pS. The 60 and 15 pS conductances can exist alone, but all three states may appear in the same patch as interconverting conductance levels. The delayed-rectifier conductance states have low densities (less than 10 channels per 10-microns diam cell), and all have a reversal potential near -75 mV and the same average kinetics. Outward K current through delayed-rectifier channels follows the upstroke without appreciable delay and lasts throughout the action potential. No inward current flows through delayed-rectifier channels during beating. The early outward channel has a nonlinear conductance of 18-9 pS depending on the potential. It also turns on immediately after the upstroke of the action potential and lasts on average only 50 ms. The early outward channel has an extrapolated reversal potential near -30 mV; no inward current flows during beating. The inward-rectifier has an extrapolated conductance and reversal potential of 2-3 pS and -80 mV in 1.3 mM K. Channel kinetics are independent of external K between 10 and 120 mM, and the channel conducts current only during the late repolarization and diastolic phases of the action potential. No outward current flows through inward-rectifier channels during beating. This work parallels a previous study of Na channels using similar techniques (Mazzanti, M., and L. J. DeFelice. 1987, Biophys. J. 52:95-100).

A novel cardiac potassium channel that is active and conductive at depolarized potentials

We report the existence of a novel potassium channel revealed in single-channel recordings from guinea-pig ventricular heart cells. The channel, observed in approximately 10% of patches, demonstrates a 14 pS conductance at physiological potassium concentrations, does not rectify over the voltage range of the action potential, and is quite selective for K ions. The channel activates with depolarization, but does not require intracellular Ca2+ ions to open. Open channel probability increases rapidly (less than 10 ms) to a plateau in response to depolarizing voltage steps, and demonstrates no detectable inactivation (greater than 600 ms). These features clearly distinguish this channel from other known K channels in cardiac muscle. Because of its high activity at plateau potentials, we propose the name iKp.

Properties of the transient outward current in rabbit atrial cells

1. Whole-cell and patch clamp techniques have been used to study the steady-state voltage dependence and the kinetics of a transient outward current, It, in single cells from rabbit atrium. 2. The steady-state voltage dependence of both activation and inactivation of It are well described by Boltzmann functions. Inactivation is fully removed at potentials negative to -70 mV and it is complete near 0 mV. The threshold for activation of It is near -30 mV and it is fully activated at +30 mV. The region of overlap between the activation and inactivation curves indicates that a steady non-inactivating current will be recorded over a membrane potential range from approximately -30 to 0 mV. 3. In general, the time course of inactivation at potentials in the range 0 to +50 mV is best described as a sum of two exponential functions. The kinetic parameters controlling these processes exhibit only very weak voltage dependence. 4. Comparison of the time course of the development of inactivation in response to long depolarizing voltage clamp steps with the development of inactivation in response to trains of brief depolarizing pulses indicates that inactivation develops very quickly and decays relatively slowly at potentials near the resting potential (e.g. -70 mV). Thus, in response to (i) a train of voltage-clamp pulses or (ii) a series of action potentials, the magnitude of It decreases due to a progressive increase in the amount of inactivation. 5. A simple model of channel gating is presented: it can account for the major aspects of the voltage dependence and kinetics of It (cf. Aldrich, 1981). 6. Cell-attached patch clamp recordings have been used to identify the single-channel or unitary events underlying the current, It. In general, only one active channel is present per patch. The single-channel conductance in normal Tyrode solution is approximately 14 pS and the current-voltage relationship is approximately linear between +50 and +150 mV with respect to rest. This information, in combination with the fully activated current-voltage characteristics from the whole-cell data, can be used to estimate the number and density of It channels per cell: these are 1600 and one per 3-4 micron 2, respectively. 7. Ensemble averages obtained from patch recordings are very similar in time course to the macroscopic or whole-cell current itself: the ensemble current rises to a peak within approximately 5 ms and decays with a biexponential time course in response to depolarizations to approximately +50 mV.(ABSTRACT TRUNCATED AT 400 WORDS)