The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time ...course and magnitude of the whole‐cell intracellular Ca2+ transient by modulating trans‐sarcolemmal Ca2+ influx through L‐type Ca2+ channels (ICa,L) and Na‐Ca exchangers (ICa,NCX). The transient outward potassium current (Ito) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation‐contraction coupling (ECC). The magnitude of Ito varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of Ito in ECC through its influence on ICa,L, and possibly ICa,NCX. We also discuss differential effects of Ito on ECC between different species, between different regions of the heart and in heart disease.
Atrial fibrillation (AF) is the most frequent arrhythmia found in clinical practice. In recent studies, a decrease in the development or recurrence of AF was found in hypertensive patients treated ...with angiotensin-converting enzyme inhibitors or angiotensin receptor-blocking agents. Hypertension is related to an increased wall tension in the atria, resulting in increased stretch of the individual myocyte, which is one of the major stimuli for the remodeling process. In the present study, we used a model of cultured atrial neonatal rat cardiomyocytes under conditions of stretch to provide insight into the mechanisms of the preventive effect of the angiotensin receptor-blocking agent losartan against AF on a molecular level. Stretch significantly increased protein-to-DNA ratio and atrial natriuretic factor mRNA expression, indicating hypertrophy. Expression of genes encoding for the inward rectifier K(+) current (I(K1)), Kir 2.1, and Kir 2.3, as well as the gene encoding for the ultrarapid delayed rectifier K(+) current (I(Kur)), Kv 1.5, was significantly increased. In contrast, mRNA expression of Kv 4.2 was significantly reduced in stretched myocytes. Alterations of gene expression correlated with the corresponding current densities: I(K1) and I(Kur) densities were significantly increased in stretched myocytes, whereas transient outward K(+) current (I(to)) density was reduced. These alterations resulted in a significant abbreviation of the action potential duration. Losartan (1 microM) prevented stretch-induced increases in the protein-to-DNA ratio and atrial natriuretic peptide mRNA expression in stretched myocytes. Concomitantly, losartan attenuated stretch-induced alterations in I(K1), I(Kur), and I(to) density and gene expression. This prevented the stretch-induced abbreviation of action potential duration. Prevention of stretch-induced electrical remodeling might contribute to the clinical effects of losartan against AF.
Fast transient outward potassium currents (Ito,f) are critical determinants of regional heterogeneity of cardiomyocyte repolarization as well as cardiomyocyte contractility. Additionally, Ito,f ...densities are markedly down-regulated in cardiac hypertrophy and heart disease, conditions associated with activation of the serine/threonine phosphatase calcineurin (Cn). In this study, we investigated the regulation of Ito,f expression by Cn in cultured neonatal rat ventricular myocytes (NRVMs) with and without α1-adrenoreceptor stimulation with phenylephrine (PE). Overexpression of constitutively active Cn in NRVMs induced hypertrophy and caused profound increases in Ito,f density as well as Kv4.2 mRNA and protein expression and promoter activity, without affecting Kv4.3 or KChIP2 levels. The effects of Cn on hypertrophy, Ito,f, and Kv4.2 transcription were associated with NFAT activation and were abrogated by NFAT inhibition. Despite activating Cn and inducing hypertrophy in NRVMs, PE resulted in profound down-regulation of Ito,f densities as well as Kv4.2, Kv4.3, and KChIP2 expression. Although hypertrophy and NFAT activation were inhibited by the Cn inhibitory peptide CAIN, Ito,f and Kv4.2 expression were further reduced by CAIN, whereas Cn overexpression eliminated PE-induced reductions in Ito,f and Kv4.2 expression without affecting Kv4.3 or KChIP2 levels. We conclude that Cn increases cardiac Ito,f densities by positively regulating Kv4.2 gene transcription. Consistent with this conclusion, we found that Ito,f was increased in myocytes isolated from young mice overexpressing Cn prior to the development of heart disease. This positive regulation of Kv4.2 transcription by Cn activation is expected to minimize the reductions in Ito,f and Kv4.2 expression observed in hypertrophic cardiomyocytes.
Cardiac inward rectifier K + currents ( I K1 ) play an important role in maintaining resting membrane potential and contribute to late phase repolarization. Members of
the K ir2.x channel family ...appear to encode for I K1 . The purpose of this study was to determine the molecular composition of cardiac I K1 in rabbit ventricle. Western blots revealed that K ir2.1 and K ir2.2 , but not K ir2.3 , are expressed in rabbit ventricle. Culturing rabbit myocytes resulted in a â¼50% reduction of I K1 density after 48 or 72 h in culture which was associated with an 80% reduction in K ir2.1 , but no change in K ir2.2 , protein expression. Dominant-negative (DN) constructs of K ir2.1 , K ir2.2 and K ir2.3 were generated and tested in tsA201 cells. Adenovirus-mediated over-expression of K ir2.1dn , K ir2.2dn or K ir2.1dn plus K ir2.2dn in cultured rabbit ventricular myocytes reduced I K1 density equally by 70% 72 h post-infection, while AdK ir2.3dn had no effect, compared to green fluorescent protein (GFP)-infected myocytes. Previous studies indicate that the Ba 2+ required for half-maximum block (IC 50 ) differs significantly between K ir2.1 , K ir2.2 and K ir2.3 channels. The dependence of I K1 on Ba 2+ revealed a single binding isotherm which did not change with time in culture. The IC 50 for block of I K1 was also unaffected by expression of the different DN genes after 72 h in culture. Taken together, these results demonstrate
functional expression of K ir2.1 and K ir2.2 in rabbit ventricular myocytes and suggest that macroscopic I K1 is predominantly composed of K ir2.1 and K ir2.2 heterotetramers.
G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (Ito) in Normal and Diseased Myocardium. Journal ...of Molecular and Cellular Cardiology (2001) 33, 851–872. The Ca2+-independent transient outward potassium current (Ito) plays an important role in early repolarization of the cardiac action potential. Itohas been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac Itohave been identified: fast Ito, called Ito,f, and slow Ito, called Ito,s. Recent findings suggest that Ito,fis formed by assembly of Kv4.2and/or Kv4.3alpha pore-forming voltage-gated subunits while Ito,sis comprised of Kv1.4and possibly Kv1.7subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac Itohave been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of Itoin rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in Itodensity with a loss of heterogeneity of Itoexpression and associated action potential prolongation. Alterations of Itoin rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca2+homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac Itoand the functional consequences of changes in Itothat occur in cardiovascular disease.
Volume regulatory Cl- channels are key regulators of ischemic preconditioning (IPC). Because Cl- efflux must be balanced by an efflux of cations to maintain cell membrane electroneutrality during ...volume regulation, we hypothesize that I(K1) channels may play a role in IPC. We subjected cultured cardiomyocytes to 60-minute simulated ischemia (SI) followed by 60-minute of simulated reperfusion (SR) and assessed percent cell death using trypan blue staining. Ischemic preconditioning (10-minute SI/10-minute SR) significantly (P<0.0001) reduced the percent cell death in nontransfected cardiomyocytes IPC(CM) 18.0+/-2.1% versus control (C(CM)) 48.3+/-1.0%. IPC protection was not altered by overexpression of the reporter gene (enhanced green fluorescent protein, EGFP). However, overexpression of dominant-negative Kir2.1 or Kir2.2 genes using adenoviruses (AdEGFPKir2.1DN or AdEGFPKir2.2DN) encoding the reporter gene EGFP prevented IPC protection both IPC(CM)+AdEGFPKir2.1DN 45.8+/-2.3% (mean+/-SEM) and IPC(CM)+AdEGFPKir2.2DN 47.9+/-1.4% versus IPC(CM); P<0.0001 in cultured cardiomyocytes (n=8 hearts). Transfection of cardiomyocytes with AdEGFPKir2.1DN or AdEGFPKir2.2DN did not affect cell death in control (nonpreconditioned) cardiomyocytes (both C(CM)+ AdEGFPKir2.1DN 45.8+/-0.7% and C(CM)+AdEGFPKir2.2DN 46.2+/-1.3% versus C(CM); not statistically significant). Similar effects were observed in both cultured (n=5 hearts) and freshly isolated (n=4 hearts) ventricular cardiomyocytes after I(K1) blockade with 20 micromol/L BaCl2 plus 1 micromol/L nifedipine (to prevent Ba2+ uptake). Nifedipine alone neither protected against ischemic injury nor blocked IPC protection. Our findings establish that I(K1) channels play an important role in IPC protection.
Mortality in patients with cardiogenic shock after out-of-hospital cardiac arrest remains high despite advances in resuscitation and early revascularization strategies. Recent studies suggest a ...reduced mortality in survivors of cardiac arrest subjected to mild therapeutic hypothermia, but the underlying mechanisms are not yet clear. Because positive hemodynamic effects of mild therapeutic hypothermia have been suggested, we aimed at testing the hypothesis that patients in cardiogenic shock might benefit from mild therapeutic hypothermia.
Hemodynamic effects of mild therapeutic hypothermia in 20 consecutive patients admitted in cardiogenic shock after successful resuscitation from out-of-hospital cardiac arrest were investigated. A historic normothermic control group was matched (one-to-one) by means of a propensity score. Patients were cooled to 33°C for 24 hrs using an endovascular cooling device and hemodynamic variables were continuously recorded by means of pulse contour analysis. Cardiac performance was determined by echocardiography.
Mild therapeutic hypothermia induced a significant decrease in heart rate from 74 to 64 beats per minute. Despite the reduction in heart rate, cardiac index remained unchanged under mild therapeutic hypothermia likely due to an increase in ejection fraction from 43 ± 4% to 55 ± 4%. Mean arterial pressure increased rapidly from 75 ± 2 mm Hg to 84 ± 3 mm Hg (p = .001) upon induction of hypothermia paralleled by an initial increase in systemic vascular resistance. Accordingly, patients with mild therapeutic hypothermia required lower cumulative doses of vasopressors and inotropes.
We conclude that in cardiogenic shock mild therapeutic hypothermia provides circulatory support and an increase in systemic vascular resistance that leads to reduced vasopressor use and may result in lower oxygen consumption. These findings suggest that mild therapeutic hypothermia could be a therapeutic option in hemodynamically unstable patients independent of cardiac arrest and further randomized clinical studies are needed.
Background
Asymptomatic diabetic patients have a high incidence of clinically unrecognized left ventricular dysfunction with an abnormal cardiac response to exercise. We, therefore, examined ...subclinical defects in the contraction–relaxation cycle and intracellular Ca
2+
regulation in myocardium of asymptomatic type 2 diabetic patients.
Methods
Alterations in the dynamics of the intracellular Ca
2+
transient and contractility were recorded in right atrial myocardium of type 2 diabetic patients and non-diabetic control tissue loaded with fura-2. In order to gain an insight into mechanisms underlying the altered Ca
2+
handling in diabetic myocardium levels of mRNA, protein expression and phosphorylation of key proteins in sarcoplasmic Ca
2+
handling were determined.
Results
In isolated atrial trabeculae of diabetic myocardium the rise of systolic Ca
2+
was significantly prolonged, but relaxation of the Ca
2+
transient was unaltered compared to control tissue. Accordingly, the levels of expression of mRNA and protein of the Ca
2+
release channel (RyR2) of the sarcoplasmic reticulum were reduced by 68 and 22%, respectively. Endogenous phosphorylation of RyR2 by protein kinases C, however, was increased by 31% in diabetic myocardium, as assessed by the back-phosphorylation technique. Levels of expression of SERCA2 and phospholamban were unaltered between both groups.
Conclusions
Intracellular Ca
2+
release is prolonged in non-failing myocardium of type 2 diabetic patients and this may be primarily due to a decreased expression of RyR2. This defective Ca
2+
release may represent an early stage of ventricular dysfunction in type 2 diabetes and would favor the abnormal response to exercise frequently observed in asymptomatic diabetic patients.
Prolonged action potentials (APs) and decreased transient outward K+ currents (I(to)) are consistent findings in hypertrophic myocardium. However, the connection of these changes with cardiac ...hypertrophy is unknown. The present study investigated the effects of changes in I(to) and the associated alterations in AP on myocyte hypertrophy induced by phenylephrine.
Chronic incubation of cultured neonatal ventricular rat myocytes (NVRMs) with phenylephrine (PE) reduced I(to) density and prolonged AP duration, leading to a 2-fold increase in the net Ca2+ influx per beat and a 1.4-fold increase in Ca2+-transient amplitude. PE treatment of chronically paced (2-Hz) NVRM also induced increases in cell size, protein/DNA ratio, atrial natriuretic factor mRNA expression, as well as beta/alpha myosin mRNA ratio. These hypertrophic changes were associated with a 2.4-fold increase in activation of nuclear factor of activated T-cells (NFAT), indicating increased activity of the Ca2+-dependent phosphatase calcineurin. Overexpression of Kv4.2 channels using adenovirus prevented the AP duration prolongation as well as the increases in Ca2+ influx and Ca2+-transient amplitude induced by PE. Kv4.2 overexpression also prohibited the PE-induced increases in cell size, protein/DNA ratio, atrial natriuretic factor expression, beta/alpha myosin mRNA ratio, and NFAT activation.
Our results demonstrate that PE-mediated hypertrophy in NRVMs seems to require I(to) reductions and AP prolongation associated with increased Ca2+ influx and Ca2+ transients as well as calcineurin activation. The clinical implications of these studies and the possible involvement of other signaling pathways are discussed.
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