The heterogeneous distribution of ion channels in ventricular muscle gives rise to spatial variations in action potential (AP) duration (APD) and contributes to the repolarization sequence in healthy hearts. It has been proposed that enhanced dispersion of repolarization may underlie arrhythmias in diseases with markedly different causes. We engineered dominant negative transgenic mice that have prolonged QT intervals and arrhythmias due to the loss of a slowly inactivating K(+) current. Optical techniques are now applied to map APs and investigate the mechanisms underlying these arrhythmias. Hearts from transgenic and control mice were isolated, perfused, stained with di-4-ANEPPS, and paced at multiple sites to optically map APs, activation, and repolarization sequences at baseline and during arrhythmias. Transgenic hearts exhibited a 2-fold prolongation of APD, less shortening (8% versus 40%) of APDs with decreasing cycle length, altered restitution kinetics, and greater gradients of refractoriness from apex to base compared with control hearts. A premature impulse applied at the apex of transgenic hearts produced sustained reentrant ventricular tachycardia (n=14 of 15 hearts) that did not occur with stimulation at the base (n=8) or at any location in control hearts (n=12). In transgenic hearts, premature impulses initiated reentry by encountering functional lines of conduction block caused by enhanced dispersion of refractoriness. Reentrant VT had stable (>30 minutes) alternating long/short APDs associated with long/short cycle lengths and T wave alternans. Thus, optical mapping of genetically engineered mice may help elucidate some electrophysiological mechanisms that underlie arrhythmias and sudden death in human cardiac disorders.

cium-dependent arrhythmias in transgenic mice with heart failure.

. Atrial contractile dysfunction, fibrosis, and arrhythmias in a mouse model of cardiomyopathy secondary to cardiac-specific overexpression of tumor necrosis factor-␣. Am J Physiol Heart Circ Physiol 289: H1456 -H1467, 2005. First published May 27, 2005; doi:10.1152/ajpheart.00733.2004.-Transgenic mice overexpressing the inflammatory cytokine TNF-␣ in the heart develop a progressive heart failure syndrome characterized by biventricular dilatation, decreased ejection fraction, decreased survival compared with non-transgenic littermates, and earlier pathology in males. TNF-␣ mice (TNF1.6) develop atrial arrhythmias on ambulatory telemetry monitoring that worsen with age and are more severe in males. We performed in vivo electrophysiological testing in transgenic and control mice, ex vivo optical mapping of voltage in the atria of isolated perfused TNF1.6 hearts, and in vitro studies on isolated atrial muscle and cells to study the mechanisms that lead to the spontaneous arrhythmias. Programmed stimulation induces atrial arrhythmias (n ϭ 8/32) in TNF1.6 but not in control mice (n ϭ 0/37), with a higher inducibility in males. In the isolated perfused hearts, programmed stimulation with single extra beats elicits reentrant atrial arrhythmias (n ϭ 6/6) in TNF1.6 but not control hearts due to slow heterogeneous conduction of the premature beats. Lowering extracellular Ca 2ϩ normalizes conduction and prevents the arrhythmias. Atrial muscle and cells from TNF1.6 compared with control mice exhibit increased collagen deposition, decreased contractile function, and abnormal systolic and diastolic Ca 2ϩ handling. Thus abnormalities in action potential propagation and Ca 2ϩ handling contribute to the initiation of atrial arrhythmias in this mouse model of heart failure. heart failure; atrium; atrial fibrillation; cytokines INFLAMMATORY CYTOKINES, including TNF-␣, are increased in the serum and hearts of patients with congestive heart failure (CHF) and may contribute to the pathophysiology of the disease (18,24,25). We recently engineered mice that overexpress TNF-␣ in the heart under the control of the ␣-myosin heavy chain promoter (TNF1.6 mice) and develop a cardiomyopathy characterized by biventricular dilatation, decreased left ventricular ejection fraction, ventricular arrhythmias, and decreased survival compared with nontransgenic littermates (28). Most of the mice exhibit symptoms of CHF before death (tachypnea, cyanosis, and ascites) and evidence of decompensated heart failure at autopsy (pleural effusions, hepatic congestion, and severe atrial and ventricular dilatation).We have previously used optical mapping of voltage and calcium in ventricles from the TNF1.6 mice to show prolongation of action potential duration (APD), prolongation of calcium transient duration, and elevated diastolic and depressed systolic calcium (33). Premature beats had depressed action potential amplitude and slowed conduction velocities that contributed to initiation of reentrant arrhythmias. Lowering extracellular calcium reversed the abnor...

concentration ([Ca 2ϩ ]i) handling and vulnerability to arrhythmias in a species-dependent manner. The effects of uncouplers were investigated in perfused mouse hearts labeled with rhod-2/AM or 4-[␤-[2-(di-n-butylamino)-6-naphthyl]vinyl]pyridinium (di-4-ANEPPS) to map [Ca 2ϩ ]i transients (emission wavelength ϭ 585 Ϯ 20 nm) and action potentials (APs) (emission wavelength Ͼ 610 nm; excitation wavelength ϭ 530 Ϯ 20 nm). Confocal images showed that rhod-2 is primarily in the cytosol. DAM (15 mM) and cyto-D (5 M) increased AP durations (APD 75 ϭ 20.0 Ϯ 3 to 46.6 Ϯ 5 ms and 39.9 Ϯ 8 ms, respectively, n ϭ 4) and refractory periods (45.14 Ϯ 12.1 to 82.5 Ϯ 3.5 ms and 78 Ϯ 4.24 ms, respectively). Cyto-D reduced conduction velocity by 20% within 5 min and DAM by 10% gradually in 1 h (n ϭ 5 each). Uncouplers did not alter the direction and gradient of repolarization, which progressed from apex to base in 15 Ϯ 3 ms. Peak systolic [Ca 2ϩ ]i increased with cyto-D from 743 Ϯ 47 (n ϭ 8) to 944 Ϯ 17 nM (n ϭ 3, P ϭ 0.01) but decreased with DAM to 398 Ϯ 44 nM (n ϭ 3, P Ͻ 0.01). Diastolic [Ca 2ϩ ]i was higher with cyto-D (544 Ϯ 80 nM, n ϭ 3) and lower with DAM (224 Ϯ 31, n ϭ 3) compared with controls (257 Ϯ 30 nM, n ϭ 3). DAM prolonged [Ca 2ϩ ]i transients at 75% recovery (54.3 Ϯ 5 to 83.6 Ϯ 1.9 ms), whereas cyto-D had no effect (58.6 Ϯ 1.2 ms; n ϭ 3). Burst pacing routinely elicited long-lasting ventricular tachycardia but not fibrillation. Uncouplers flattened the slope of AP restitution kinetic curves and blocked ventricular tachycardia induced by burst pacing. optical action potentials; action potentials; intracellular calcium; restitution kinetics MOLECULARLY ENGINEERED MICE have been extensively used to genetically alter a specific component of a complex signaling process and to develop models of human diseases. Transgenic mice are used as models for various cardiac diseases and offer an effective strategy to elucidate the mechanisms underlying long QT-related arrhythmias, metabolic diseases, and the pathology of heart failure (29). A limitation of mouse models is the small size of the heart, making it difficult to study changes in contractility, electrophysiology, and vulnerability to arrhythmias in intact hearts. The challenge of measuring changes in cardiac phenotype has been partly overcome by applying optical technique to map electrical activity, but a major technical difficulty in the application of optical techniques to measure action potentials (APs) and intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) transients has been the distortion of the signals by movement due to muscle contractions.Several approaches have been used to reduce movement artifacts: 1) perfusion in Ca 2ϩ -free Tyrode solution to abolish contractions, an approach applicable to amphibian hearts (38); 2) design perfusion chambers to mechanically stabilize the heart (18, 37); 3) perfusion with an inhibitor of L-type voltage- Chemical uncouplers can potentially provide a practical approach to block movement artifacts and have been used to...

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