Mechanisms underlying rate-dependent remodeling of transient outward potassium current in canine ventricular myocytes.

Transient outward K+ current (I to) downregulation following sustained tachycardia in vivo is usually attributed to tachycardiomyopathy. This study assessed potential direct rate regulation of cardiac I(to) and underlying mechanisms. Cultured adult canine left ventricular cardiomyocytes (37 degrees C) were paced continuously at 1 or 3 Hz for 24 hours.

Calcium-handling abnormalities underlying atrial arrhythmogenesis and contractile dysfunction in dogs with congestive heart failure.

Background: Congestive heart failure (CHF) is a common cause of atrial fibrillation. Focal sources of unknown mechanism have been described in CHF-related atrial fibrillation. The authors hypothesized that abnormal calcium (Ca(2+)) handling contributes to the CHF-related atrial arrhythmogenic substrate.

Cardiac-directed parvalbumin transgene expression in mice shows marked heart rate dependence of delayed Ca2+ buffering action.

Relaxation abnormalities are prevalent in heart failure and contribute to clinical outcomes. Disruption of Ca2+ homeostasis in heart failure delays relaxation by prolonging the intracellular Ca2+ transient. We sought to speed cardiac relaxation in vivo by cardiac-directed transgene expression of parvalbumin (Parv), a cytosolic Ca2+ buffer normally expressed in fast-twitch skeletal muscle.

Cardiac transgenic and gene transfer strategies converge to support an important role for troponin I in regulating relaxation in cardiac myocytes.

Elucidating the relative roles of cardiac troponin I (cTnI) and phospholamban (PLN) in beta-adrenergic-mediated hastening of cardiac relaxation has been challenging and controversial. To test the hypothesis that beta-adrenergic phosphorylation of cTnI has a prominent role in accelerating cardiac myocyte relaxation performance we used transgenic (Tg) mice bearing near complete replacement of native cTnI with a beta-adrenergic phospho-mimetic of cTnI whereby tandem serine codons 23/24 were converted to aspartic acids (cTnI S23/24D). Adult cardiac myocytes were isolated and contractility determined at physiological temperature under unloaded and loaded conditions using micro-carbon fibers.

Targeted ablation of ILK from the murine heart results in dilated cardiomyopathy and spontaneous heart failure.

A requirement for integrin-mediated adhesion in cardiac physiology is revealed through targeted deletion of integrin-associated genes in the murine heart. Here we show that targeted ablation of the integrin-linked kinase (ILK) expression results in spontaneous cardiomyopathy and heart failure by 6 wk of age. Deletion of ILK results in disaggregation of cardiomyocytes, associated with disruption of adhesion signaling through the beta1-integrin/FAK (focal adhesion kinase) complex.

Properties of a time-dependent potassium current in pig atrium: evidence for a role of kv1.5 in repolarization.

Cardiac electrical activity is modulated by potassium currents. Pigs have been used for antiarrhythmic drug testing, but only sparse data exist regarding porcine atrial ionic electrophysiology. Here, we used electrophysiological, molecular, and pharmacological tools to characterize a prominent porcine outward K(+) current (I(K,PO)) in atrial cardiomyocytes isolated from adult pigs.

Comparison of Ca2+-handling properties of canine pulmonary vein and left atrial cardiomyocytes.

Cardiac tissue in the pulmonary vein sleeves plays an important role in clinical atrial fibrillation. Mechanisms leading to pulmonary vein activity in atrial fibrillation remain unclear. Indirect experimental evidence points to pulmonary vein Ca(2+) handling as a potential culprit, but there are no direct studies of pulmonary vein cardiomyocyte Ca(2+) handling in the literature.

Atrial fibrillation-associated minK38G/S polymorphism modulates delayed rectifier current and membrane localization.

Background: Atrial fibrillation (AF) is a common acquired arrhythmia with multi-factorial pathogenesis. Recently, a single nucleotide polymorphism (SNP, A/G) at position 112 in the KCNE1 gene, resulting in a glycine/serine amino acid substitution at position 38 of the minK peptide, was associated with AF occurrence (AF more frequent with minK38G); however, the functional effect of this SNP is unknown.

Methods And Results: We used patch clamp recording, confocal microscopy and protein biochemistry to study the effect of this SNP on delayed-rectifier current expression and mathematical simulation to identify potential functional consequences.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. I. Experimental studies.

Two genetic experimental approaches, de novo expression of parvalbumin (Parv) and overexpression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a), have been shown to increase relaxation rates in myocardial tissue. However, the relative effect of Parv and SERCA2a on systolic function and on beta-adrenergic responsiveness at varied pacing rates is unknown. We used gene transfer in isolated rat adult cardiac myocytes to gain a fuller understanding of Parv/SERCA2a function.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. II. Mathematical modeling studies.

We developed a mathematical model specific to rat ventricular myocytes that includes electrophysiological representation, ionic homeostasis, force production, and sarcomere movement. We used this model to interpret, analyze, and compare two genetic manipulations that have been shown to increase myocyte relaxation rates, parvalbumin (Parv) de novo expression, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) overexpression. The model was used to seek mechanistic insights into 1) the relative contribution of two mechanisms by which SERCA2a overexpression modifies Ca2+ sequestration, i.

Parvalbumin corrects slowed relaxation in adult cardiac myocytes expressing hypertrophic cardiomyopathy-linked alpha-tropomyosin mutations.

Hypertrophic cardiomyopathy mutations A63V and E180G in alpha-tropomyosin (alpha-Tm) have been shown to cause slow cardiac muscle relaxation. In this study, we used two complementary genetic strategies, gene transfer in isolated rat myocytes and transgenesis in mice, to ascertain whether parvalbumin (Parv), a myoplasmic calcium buffer, could correct the diastolic dysfunction caused by these mutations. Sarcomere shortening measurements in rat cardiac myocytes expressing the alpha-Tm A63V mutant revealed a slower time to 50% relengthening (T50R: 44.

Targeting diastolic dysfunction by genetic engineering of calcium handling proteins.

Diastolic heart failure (HF) is associated with significant morbidity and mortality, and is a growing medical problem in this country. Diastolic dysfunction is defined as an abnormality in myocardial relaxation that impairs filling during diastole and contributes to the clinical syndrome of HF. Effective clinical strategies to treat diastolic dysfunction are limited.

Divergent abnormal muscle relaxation by hypertrophic cardiomyopathy and nemaline myopathy mutant tropomyosins.

Mutations in tropomyosin (Tm) have been linked to distinct inherited diseases of cardiac and skeletal muscle, hypertrophic cardiomyopathy (HCM), and nemaline myopathy (NM). How HCM and NM mutations in nearly identical Tm proteins produce the vastly divergent clinical phenotypes of heightened, prolonged cardiac muscle contraction in HCM and skeletal muscle weakness in NM is currently unknown. We report here a direct comparison of the effects of HCM (A63V) and NM (M9R) mutant Tm on membrane-intact myocyte contractile function as assessed by adenoviral gene transfer to fully differentiated cardiac muscle cells.

Optimal range for parvalbumin as relaxing agent in adult cardiac myocytes: gene transfer and mathematical modeling.

Parvalbumin (PV) has recently been shown to increase the relaxation rate when expressed in intact isolated cardiac myocytes via adenovirus gene transfer. We report here a combined experimental and mathematical modeling approach to determine the dose-response and the sarcomere length (SL) shortening-frequency relationship of PV in adult rat cardiac myocytes in primary culture. The dose-response was obtained experimentally by observing the PV-transduced myocytes at different time points after gene transfer.