© 2001 American Heart Association, Inc.
Editorial |
Tuning Repolarization in the Heart
A Multitude of Potassium Channels and Regulatory Pathways
From the Department of Cardiology, University of Munich, Munich, Germany.
Correspondence to Michael Näbauer, Department of Cardiology, University of Munich, Marchioninistr. 15, 81377 Munich, Germany. E-mail nabauer{at}med1.med.uni-muenchen.de
Key Words: > • potassium channel • heart failure • arrhythmia • angiotensin • transcription control
 |
Introduction |
|---|
 Top Introduction Molecular Correlates of...Regulatory Pathways of Cardiac...References |
|---|
Approximately one decade after the cloning of the first potassium channel, potassium channel genes of amazing diversity have emerged, many of them expressed in the heart. A magnitude of
subunits forming homotetramers or
heterotetramers, multiple regulatory subunits, and alternative splicing
of genes adds up to a seemingly endless diversity of potassium
channels. Considering the relative uniformity of the major cardiac
inward currents, most of the heterogeneity in action
potential waveforms among different species and anatomical regions
seems to be related to differences in potassium channel expression.
Recent attention has focused on electrical
heterogeneity within atrial and ventricular
myocardium, which also seems predominantly to reflect
differences in potassium channel expression. Most of these studies have
been done on a functional level; now information about molecular
substrates of regional electrical heterogeneity is
emerging. Because of the essential importance for identification of potential targets for therapeutic interventions, much attention has recently focused on 2 aspects: elucidation of the molecular identity of cardiac potassium channels and their correlation to currents, including regional heterogeneity, and identification of regulatory pathways relevant to ion channel expression under normal and pathological conditions.
|
Molecular Correlates of Potassium Currents in the Heart |
|---|
|
TopIntroductionMolecular Correlates of...Regulatory Pathways of Cardiac...References |
|---|
Molecular identification of potassium channels has been guided by macroscopic current characteristics, single-channel analysis, and pharmacological tools, comparing native currents to currents of candidate genes expressed in heterologous expression systems. In addition, the relative abundance of potassium channel transcripts detected in tissue samples of the anatomical region studied was taken into account. Awareness of regional electrical heterogeneity resulted in tissue sampling from areas considered electrically homogenous to allow more meaningful correlation of potassium channel genes with native currents. However, correlation of potassium channel gene expression with currents in individual cells is still based on the assumptions that tissue samples consisted of myocardial cells only and that transcript expression was uniform in the cells of a given sample. These assumptions have been brought into question by the demonstration of major cell-to-cell variability of potassium channel mRNA transcripts, even within confined anatomical regions, leading to the proposal that understanding the full complexity of the molecular basis of K+ current expression in the heart would require coordination of electrophysiological measurements with in situ mRNA detection methods.1
This concept has been addressed by Schultz et al2 in this issue of Circulation Research. In rat ventricular myocytes from midmyocardial layers, measurements of potassium currents were combined with qualitative assessments of the presence or absence of Kv2.1 and Kv4.3 mRNA transcripts by a single-cell reverse transcriptase–polymerase chain reaction (RT-PCR) technique. In contrast to the good correlation between the transient outward current (Ito) and the presence of Kv4.3 mRNA, a large discrepancy was observed between tetraethylammonium (TEA)-sensitive delayed rectifier currents and the presence of Kv2.1 mRNA transcripts, which is thought to contribute a major part of the TEA-sensitive delayed rectifier current in rat ventricle.3 In fact, there was no difference in mean size or decay time of the TEA-sensitive delayed rectifier currents between cells positive and negative for Kv2.1 mRNA transcripts.
How can this finding be interpreted? Of course, there are
technical concerns in that the RT-PCR technique may have missed Kv2.1
transcripts present in a cell. However, recovery of Kv4.3 mRNA was
consistently successful, and the technique has been proven
useful for correlating ion currents to mRNA transcripts in neurons.
Moreover, the usual worry with PCR, given the extreme sensitivity of
the technique, is false-positives rather than false-negatives. Thus,
the conclusion seems to justify that at the level of Kv2.1 mRNA,
significant cell-to-cell variability exists, with transcripts
present in only 40% to 50% of the myocytes. However, the presence
of Kv2.1 mRNA does not warrant functional protein, in which case the
uniformly observed TEA-sensitive current would be encoded by different
genes. Although this cannot be definitively excluded, the most likely
interpretation of the data is that potassium channel expression within
these cells is heterogenous, with other potassium
channel subunits contributing to the TEA-sensitive current in cells
negative for Kv2.1 mRNA, possibly Kv1.2, Kv1.5, Kv3.1, Kv3.2, or yet
another unidentified potassium channel gene. Earlier findings support
this view, demonstrating significant cell-to-cell variability within
identified anatomical regions by in situ
hybridization.1
This study marks a first step toward high-resolution
correlation of molecular substrates to native ion currents in
theheart; probing for additional potassium channel subunits
and a pharmacological profile of the TEA-sensitive current seems
useful. However, to accomplish a more definitive identification between
K+ channel genes and native currents,
several other approaches should be considered, including
immunohistochemistry with subunit-specific antibodies to confirm the
localization of the subunits in the membrane of the cells, specific
antibodies that alter channel function to confirm the identity of the
current studied, and elimination of responsible genes or transcripts by
a transgenic or antisense approach to eliminate the current considered
related to the
gene.4 5
Regional and genetic diversity of potassium channels makes
correlation of native ion channels with genes a difficult task.
However, it also holds great promise: the multitude of and ß
subunits being expressed may provide many more targets for rationally
designed therapeutic interventions than presently perceived, some
of which may have the potential for regional
specificity.
|
Regulatory Pathways of Cardiac Ion Channel Expression |
|---|
|
TopIntroductionMolecular Correlates of...Regulatory Pathways of Cardiac...References |
|---|
Differential regional expression of ion channels necessitates regulatory pathways to control the specific pattern. A very characteristic distribution has been recognized for the Ca2+-independent transient outward current, Ito, which is strongly influenced by location within the ventricular wall, with 5-fold larger currents in epicardial than endocardial layers.6 Ito seems to be highly regulated, being affected by electrical remodeling in atrial fibrillation and, at the ventricular level, by cardiac hypertrophy and failure, with up to 70% downregulation of the current in these disease states.7 Therefore, regulation of Ito, encoded mostly by Kv4.3 in human and canine ventricle, seems to be an interesting target. Is control of expression of normal development and regional distribution exerted along the same or different pathways as under pathological conditions? Is regulation of Ito attributable to changes in Kv4.3
subunit expression or rather
related to modulatory subunits? Downregulation of
Ito in
human heart failure has been found to correlate with the reduction of
Kv4.3 mRNA levels, suggesting that channel expression is at least in
part regulated by steady-state accumulation of Kv4.3
mRNA.8
Potential mechanisms of Kv4.3 downregulation in cardiac
hypertrophy and failure are addressed by Zhang et
al9 in this issue of
Circulation Research, advancing
a novel concept of dual regulatory control of Kv4.3 mRNA accumulation
by hypertrophic stimuli. In rat neonatal myocytes, both
phenylephrine and angiotensin II caused Kv4.3
mRNA to decrease by 
50%, but with a notably different time course:
decay in response to angiotensin II was much more rapid,
with minimal Kv4.3 mRNA levels reached after only 8 hours, much earlier
than after exposure to phenylephrine. As turnover
measurements revealed that Kv4.3 mRNA was very stable, with a half-life
of >20 hours, destabilization of Kv4.3 mRNA was concluded to be the
mechanism underlying downregulation of Kv4.3 mRNA in response to
angiotensin II. Kv4.3 promoter-reporter constructs
confirmed this concept, because angiotensin II had no
effect on transcriptional activity, whereas phenylephrine
inhibited Kv4.3 promoter activity. Thus, the influence of the
hypertrophic stimuli phenylephrine and
angiotensin II on Kv4.3 mRNA accumulation is exerted by
disparate pathways.
Provided that this finding is applicable to adult tissue,
specifically human myocardium, and that steady-state mRNA
levels of Kv4.3 translate into functional current, this study might
prove to be of utmost importance for understanding expression of ion
channels under normal and pathophysiological
conditions. Angiotensin II has emerged as a central
neurohumoral signal in the pathophysiology of cardiac
hypertrophy and failure. In addition to circulating
angiotensin II, the hormone is produced locally within the
myocardium under normal conditions, with increased activity
during cardiac hypertrophy and
failure.10 Therefore,
involvement of angiotensin II in the downregulation of
Ito in
heart failure, or, more generally, in cardiac ion channel expression,
is an attractive hypothesis. Another perspective offered by this
hypothesis is related to findings of increased levels of
angiotensinogen in human
endocardium.10 Higher levels
of angiotensin II in endocardium might decrease Kv4.3
accumulation and, hence,
Ito
expression, possibly contributing to the transmural gradient of
Ito.
However, caution is warranted: it has not yet been determined whether
the gradient in
Ito is
related to a similar transmural expression gradient of Kv4.3
subunit; other factors, such as distribution of modulatory subunits,
could be responsible as well. Canine endocardial and epicardial
myocytes, at least, did not reveal any changes in Kv4.3 mRNA abundance
after incubation with angiotensin II, suggesting that
mechanisms other than altered Kv4.3 mRNA accumulation may be
operative.11
The electrical homogeneity of myocardium in situ does not seem to reflect electrophysiological homogeneity at a cellular and molecular level, where significant regional and cell-to-cell heterogeneity can be detected, especially related to the diversity of potassium channel expression. Identification of the pathways of ion channel regulation holds great promise for therapeutic interventions if we succeed in unraveling the complexities of ion channel diversity and the relation of that diversity to the electrical stability of the heart.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
|
TopIntroductionMolecular Correlates of...Regulatory Pathways of Cardiac...References |
|---|
1. Brahmajothi MV, Morales MJ, Liu S, Rasmusson RL, Campbell DL, Strauss HC. In situ hybridization reveals extensive diversity of K+ channel mRNA in isolated ferret cardiac myocytes. Circ Res. 1996;78:1083–1089.
2.
Schultz J-H, Volk
T, Ehmke H. Heterogeneity of Kv2.1 mRNA expression and
delayed rectifier current in single isolated myocytes from rat left
ventricle. Circ Res. 2001;88:483–490.
3.
Himmel HM, Wettwer
E, Li Q, Ravens U. Four different components contribute to outward
current in rat ventricular myocytes.
Am J Physiol. 1999;277:H107–H118.
4. Tamkun MM, Bennett PB, Snyders DJ. Cloning and expression of human cardiac potassium channels. In: Zipes D, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. 2nd ed. Philadelphia, Pa: Saunders; 1995:21–31.
5.
Xu H, Barry DM, Li
H, Brunet S, Guo W, Nerbonne JM. Attenuation of the slow component of
delayed rectification, action potential prolongation, and triggered
activity in mice expressing a dominant-negative Kv2 subunit.
Circ Res. 1999;85:623–633.
6.
Liu DW, Gintant GA,
Antzelevitch C. Ionic bases for
electrophysiological distinctions among
epicardial, midmyocardial, and endocardial myocytes from the free wall
of the canine left ventricle. Circ
Res. 1993;72:671–687.
7. Näbauer M, Kääb S. Potassium channel down-regulation in heart failure. Cardiovasc Res. 1998;37:324–334.[Medline] [Order article via Infotrieve]
8.
Kääb S, Dixon
J, Duc J, Ashen D, Näbauer M, Beuckelmann DJ, Steinbeck G, McKinnon
D, Tomaselli GF. Molecular basis of transient outward potassium current
downregulation in human heart failure: a decrease in Kv4.3 mRNA
correlates with a reduction in current density.
Circulation. 1998;98:1383–1393.
9.
Zhang T-T, Takimoto
K, Stewart AFR, Zhu C, Zhu C, Levitan ES. Independent regulation of
cardiac Kv4.3 potassium channel expression by angiotensin
II and phenylephrine. Circ
Res. 2001;88:476–482.
10. Kawaguchi H, Kitabatake A. Alterations of signal transduction system in heart failure. Jpn Heart J. 1997;38:317–332.[Medline] [Order article via Infotrieve]
11.
Yu H, Gao J, Wang
H, Wymore R, Steinberg S, McKinnon D, Rosen MR, Cohen IS. Effects of
the renin-angiotensin system on the current
Ito in
epicardial and endocardial ventricular myocytes from the
canine heart. Circ Res. 2000;86:1062–1068.
This article has been cited by other articles:
 |
|
 |
|
  M. Akyol, S. Jalilzadeh, M. F. Sinner, S. Perz, B. M. Beckmann, C. Gieger, T. Illig, H.-E. Wichmann, T. Meitinger, S. Kaab, et al. The common non-synonymous variant G38S of the KCNE1-(minK)-gene is not associated to QT interval in Central European Caucasians: results from the KORA study Eur. Heart J., February 1, 2007; 28(3): 305 - 309. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Pfeufer, S. Jalilzadeh, S. Perz, J. C. Mueller, M. Hinterseer, T. Illig, M. Akyol, C. Huth, A. Schopfer-Wendels, B. Kuch, et al. Common Variants in Myocardial Ion Channel Genes Modify the QT Interval in the General Population: Results From the KORA Study Circ. Res., April 1, 2005; 96(6): 693 - 701. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||