Optical imaging of arrhythmias in the cardiomyocyte monolayer.

In recent years, cultured cardiac cell monolayers have become a contemporary experimental preparation for the study of fundamental mechanisms that underlie normal and pathologic electrophysiology at the tissue level. Ion channels and gap junctions in the cardiomyocyte monolayer may be modulated using drugs that suppress or enhance certain channels/junctions, or by genetic silencing or overexpression. The cardiomyocyte monolayer is particularly well suited for studies of functional electrophysiologic properties of mixtures of cardiac and noncardiac cells (eg, myofibroblasts), which otherwise would be difficult to investigate. Optical mapping of monolayers has provided insight into mechanisms that can set the stage for arrhythmias, such as unidirectional conduction block, gap junction uncoupling, ischemia, alternans, and anisotropy, and continues to enhance our understanding of basic electrophysiologic mechanisms

Early voltage/calcium uncoupling predestinates the duration of ventricular tachyarrhythmias during ischemia/reperfusion.

BACKGROUND: Abnormal intracellular calcium (Ca(i)) kinetics during ischemia/reperfusion (I/R) can alter membrane voltage (V(m)) and destabilize wavefront propagation. OBJECTIVE: We used optical mapping to investigate the hypothesis that early V(m)/Ca(i) uncoupling during a ventricular tachyarrhythmia (VT) can play a primary role in perpetuation of VT episodes. METHODS: Seventeen Langendorff-perfused guinea pig hearts were subjected to 15 min I/15 min R. Simultaneous optical recordings of V(m) and Ca(i) signals were obtained using a dual-photodiode array. Spatiotemporal entropy (E) was used to quantify differences in V(m)/Ca(i) kinetics during VT and compare wavefront topology during the first 500 ms of a VT episode. RESULTS: A total of 39 episodes of VT were analyzed; VT was classified as self-terminating (ST, n = 28) and non-self-terminating (NST, n = 11). The ST/VTs were further classified into short ST/VT (1 to 5 s in duration; n = 16) and long ST/VT (>5 s, n = 12). E values for NST/VTs were significantly higher than E values for both short and long ST/VTs separately as well as E values for ST/VTs as a group. Further, E values for long ST/VTs were significantly higher than E values for short ST/VTs. Wave breaks were consistently identified during periods of high E. CONCLUSION: High E during the first 500 ms of the onset of VT (the first 2 to 3 beats) is significantly correlated with long ST or NST episodes. This may be related to destabilization of wave propagation that helps to perpetuate VT. Early V(m)/Ca(i) uncoupling can predestinate the development of a malignant NST/VT

Electrotonic suppression of early afterdepolarizations in the neonatal rat ventricular myocyte monolayer

Pathologies that result in early afterdepolarizations (EADs) are a known trigger for tachyarrhythmias, but the conditions that cause surrounding tissue to conduct or suppress EADs are poorly understood. Here we introduce a cell culture model of EAD propagation consisting of monolayers of cultured neonatal rat ventricular myocytes treated with anthopleurin-A (AP-A). AP-A-treated monolayers display a cycle length dependent prolongation of action potential duration (245 ms untreated, vs. 610 ms at 1 Hz and 1200 ms at 0.5 Hz for AP-A-treated monolayers). In contrast, isolated single cells treated with AP-A develop prominent irregular oscillations with a frequency of 2.5 Hz, and a variable prolongation of the action potential duration of up to several seconds. To investigate whether electrotonic interactions between coupled cells modulates EAD formation, cell connectivity was reduced by RNA silencing gap junction Cx43. In contrast to well-connected monolayers, gap junction silenced monolayers display bradycardia-dependent plateau oscillations consistent with EADs. Further, simulations of a cell displaying EADs electrically connected to a cell with normal action potentials show a coupling strength-dependent suppression of EADs consistent with the experimental results. These results suggest that electrotonic effects may play a critical role in EAD-mediated arrhythmogenesis. © 2013 The Physiological Society

First measurement of the ratio B(t -> Wb)/B(t -> Wq) and associated limit on the Cabibbo-Kobayashi-Maskawa element vertical bar V-tb vertical bar RID C-1693-2008 RID A-5169-2010

We present the first measurement of the ratio of branching fractions R equivalent to B(t --> Wb)/B(t --> Wq) from pa collisions at roots = 1.8 TeV. The data set corresponds to 109 pb(-1) of data recorded by the Collider Detector at Fermilab during the 1992-95 Tevatron run. We measure R = 0.94(-0.24)(+0.31)(stat + syst) or R > 0.61 (0.56) at 90% (95)% C.L., in agreement with the standard model predictions. This measurement yields a limit on the Cabibbo-Kobayashi-Maskawa quark mixing matrix element \V-tb\ under the assumption of three generations and unitarity