Enhanced Dispersion of Repolarization and Refractoriness in Transgenic Mouse Hearts Promotes Reentrant Ventricular Tachycardia

Abstract

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.

Imaging cellular signals in the heart in vivo : Cardiac expression of the high-signal Ca 2+ indicator GCaMP2

“…Hearts were perfused and suspended in a customdesigned optical chamber (32). Di-4-ANNEPS and Rhod2 loading, photodiode array mapping, restitution protocols, and analysis were as described (40).…”

“…In a series of experiments, the conduction velocity of the Ca 2ϩ transient was 24.1 Ϯ 3.7 cm͞s and increased 8-20% after perfusion with 1 M iso. The spectral separation between GCaMP2 and commonly used potentiometric dyes (31) also allowed the simultaneous optical recordings of action potential and Ca 2ϩ transients using spatially registered photodiode arrays (32). GCaMP2-expressing hearts were briefly loaded with RH237, illuminated at wavelengths that excited both indicators, and imaged with spatially registered photodiode arrays (Fig.…”

Isolation, Characterization and Differentiation Potential of Cardiac Progenitor Cells in Adult Pigs

“…Despite these differences, significant data have accumulated in which the contractile apparatus, including myosin, has been modified, resulting in hypertrophy or dilation followed by failure. These kinds of important physiological differences can be found in many of the basic parameters underlying cardiac output (Stull et al 2002), cardiac electrophysiology (Baker et al 2000) and calcium flux (Bers et al 2000). Although these concerns do not negate data gathered with the use of the murine models, they underscore the fact that the murine data should invariably be treated with caution as they are applied to human disease (Yutzey et al 2007).…”

Optical Mapping of Action Potentials and Calcium Transients in the Mouse Heart

“…The mouse heart is very sensitive to hypothermia; thus, temperature variations across the heart will cause artificial differences in action potential durations. Consequently, a heating system was implemented in the perfusion chamber in order to maintain a constant temperature of 37°C during the entirety of the experiment 8 . Since a Langendorff model does not retain innervation of the heart, one needs to consider adding neurotransmitters to the perfusate in order to investigate physiological sympathetic and parasympathetic tone 9 .…”