I _Kr Impact on Repolarization and Its Variability Assessed by Dynamic Clamp

Abstract

Dunkin-Hartley guinea pigs were euthanized by cervical dislocation under 800 mg/kg chloral hydrate intraperitoneal anesthesia. Ventricular myocytes were isolated by using a retrograde coronary perfusion method previously published, 6 with minor modifications. Rod-shaped, Ca 2+ -tolerant myocytes were used within 12 h from dissociation. This investigation conforms to the Guide to the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH publication no. 85-23, revised 1996) and to the guidelines for Animal Care endorsed by the University of Milano-Bicocca.© 2015 American Heart Association, Inc. Original Article Circ Arrhythm ElectrophysiolBackground-Repolarization and its stability are exquisitely sensitive to I Kr features. Information on the relative importance of specific I Kr abnormalities is missing and would assist in the evaluation of arrhythmogenic risk. Methods and Results-In single guinea-pig myocytes, endogenous I Kr was replaced by modeled I Kr (mI Kr ) by dynamic clamp (DC) at a cycle length of 1 s. mI Kr parameters were systematically modified, and the resulting changes in action potential duration (APD) and its short term variability (SD1) were measured. We observed that (1) I Kr blockade increased SD1 more than expected by its dependency on APD; (2) mI Kr completely reversed APD and SD1 changes caused by I Kr blockade; (3) repolarization was most sensitive to inactivation shifts, which affected APD and SD1 concordantly; (4) activation shifts of the same magnitude had marginal impact on APD, but only when reducing mI Kr , they significantly increased SD1; (5) changes in maximal conductance resulted in a pattern similar to that of activation shifts. Conclusions-The largest effect on repolarization and its stability are expected from changes in I Kr inactivation. APD is less sensitive to changes in other I Kr gating parameters, which are better revealed by SD1 changes. SD1 may be more sensitive than APD in detecting I Kr -dependent repolarization abnormalities. (Circ Arrhythm Electrophysiol.

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“…Furthermore, because QT‐like and RR‐like intervals on MEAs share certain similarities to ECG signals, it could be feasible to apply some of the clinical pro‐arrhythmic evaluation strategies to MEA; QT variability index (Berger, 2003) or beat‐to‐beat variability of the repolarisation duration [BVR (Pueyo et al , 2016; Stams et al , 2016)] can be informative, especially considering the pivotal role of the K v 11.1 channel in shaping the repolarisation stability (Altomare et al , 2015). Data analysis algorithms could allow automatic quantification of both short‐ and long‐term components of the repolarisation variability in a selected series of beats (Sala et al , 2016).…”

Dynamic Clamp

“…Another major insight provided by the Altomare et al study 1 is that repolarization, and its temporal variability was more sensitive to shifts in the voltage dependence of inactivation than any other electrophysiological variable that was tested. Negative shifts in V 0.5 of inactivation to hyperpolarized potentials prolonged APD and increased SD1, whereas positive shifts shortened APD and shortened SD1.…”

“…Altomare et al 1 provide conceptual advances in understanding the relative contributions of changes in gating features and gmax to action potential duration (APD) and its temporal variance. They should be congratulated for their innovative experimental approach and the interesting results.…”

“…1 Poincare plots allowed an assessment of the dynamics of the beat-to-beat variability of APD. Two metrics of beat-to-beat variability were computed: (1) SD1, the mean orthogonal deviation from the line of identity in the Poincare plot comparing the APD of beat n+1 with the APD of the nth beat; (2) SD2 is another metric of mean of the deviations from the identity line.…”

“…There are many fascinating results from the work of Altomare et al 1 First, the command injection of the modeled innate I Kr fully restored the SD1 metric of beat-to-beat variability to control values, but did not restore the SD1/SD2 ratio. Thus, the command modeled I Kr failed to fully substitute for the native I Kr during longer epochs.…”