title: Altered re-excitation thresholds and conduction of extrasystolic action potentials contribute to arrhythmogenicity in murine models of long QT syndrome.
abstract: OBJECTIVE
  QT interval prolongation reflecting delayed action potential (AP) repolarization is associated with polymorphic ventricular tachycardia and early after depolarizations potentially initiating extrasystolic APs if of sufficient amplitude. The current experiments explored contributions of altered re-excitation thresholds for, and conduction of, such extrasystolic APs to arrhythmogenesis in Langendorff-perfused, normokalaemic, control wild-type hearts and two experimental groups modelling long QT (LQT). The two LQT groups consisted of genetically modified, Scn5a(+/ΔKPQ) and hypokalaemic wild-type murine hearts.
  
  
  METHODS
  Hearts were paced from their right ventricles and monophasic AP electrode recordings obtained from their left ventricular epicardia, with recording and pacing electrodes separated by 1 cm. An adaptive programmed electrical stimulation protocol applied pacing (S1) stimulus trains followed by premature (S2) extrastimuli whose amplitudes were progressively increased with progressive decrements in S1S2 interval to maintain stimulus capture. Such protocols culminated in either arrhythmic or refractory endpoints.
  
  
  RESULTS
  Arrhythmic outcomes were associated with (1) lower conduction velocities in their initiating extrasystolic APs than refractory outcomes and (2) higher conduction velocities in the LQT groups than in controls. Furthermore, (3) the endpoints were reached at longer S1S2 coupling intervals and with smaller stimulus amplitudes in the LQT groups compared with controls. This was despite (4) similar relationships between conduction velocity and S1S2 coupling interval and between re-excitation thresholds and S1S2 coupling interval in all three experimental groups.
  
  
  CONCLUSIONS
  Arrhythmias induced by extrasystolic APs in the LQT groups thus occur under conditions of higher conduction velocity and greater sensitivity to extrastimuli than in controls.
authors.affiliation: Physiological Laboratory, University of Cambridge, Cambridge, UK. rmd42@cam.ac.uk
authors.forename: R M
authors.initials: RM
authors.lastname: Duehmke
authors.forename: S
authors.initials: S
authors.lastname: Pearcey
authors.forename: J D
authors.initials: JD
authors.lastname: Stefaniak
authors.forename: L
authors.initials: L
authors.lastname: Guzadhur
authors.forename: K
authors.initials: K
authors.lastname: Jeevaratnam
authors.forename: C
authors.initials: C
authors.lastname: Costopoulos
authors.forename: T H
authors.initials: TH
authors.lastname: Pedersen
authors.forename: A A
authors.initials: AA
authors.lastname: Grace
authors.forename: C L-H
authors.initials: CL
authors.lastname: Huang
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000235:genetics / Q000503:physiopathology*; D004558:Electric Stimulation; D023381:Endpoint Determination; D005260:Female; D006329:Heart Conduction System / Q000503:physiopathology; D007008:Hypokalemia / Q000235:genetics / Q000503:physiopathology; D008133:Long QT Syndrome / Q000235:genetics / Q000503:physiopathology*; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D008817:Mice, Mutant Strains; D023421:Models, Animal; D062554:NAV1.5 Voltage-Gated Sodium Channel / Q000235:genetics / Q000502:physiology; D009431:Neural Conduction / Q000235:genetics / Q000502:physiology*; D013997:Time Factors; D018879:Ventricular Premature Complexes / Q000235:genetics / Q000503:physiopathology*
chemical_list: D062554:NAV1.5 Voltage-Gated Sodium Channel; C568321:Scn5a protein, mouse