title: Myocardial electrotonic response to submaximal exercise in dogs with healed myocardial infarctions: evidence for β-adrenoceptor mediated enhanced coupling during exercise testing.
abstract: BACKGROUND
  Autonomic neural activation during cardiac stress testing is an established risk-stratification tool in post-myocardial infarction (MI) patients. However, autonomic activation can also modulate myocardial electrotonic coupling, a known factor to contribute to the genesis of arrhythmias. The present study tested the hypothesis that exercise-induced autonomic neural activation modulates electrotonic coupling (as measured by myocardial electrical impedance, MEI) in post-MI animals shown to be susceptible or resistant to ventricular fibrillation (VF).
  
  
  METHODS
  Dogs (n = 25) with healed MI instrumented for MEI measurements were trained to run on a treadmill and classified based on their susceptibility to VF (12 susceptible, 9 resistant). MEI and ECGs were recorded during 6-stage exercise tests (18 min/test; peak: 6.4 km/h @ 16%) performed under control conditions, and following complete β-adrenoceptor (β-AR) blockade (propranolol); MEI was also measured at rest during escalating β-AR stimulation (isoproterenol) or overdrive-pacing.
  
  
  RESULTS
  Exercise progressively increased heart rate (HR) and reduced heart rate variability (HRV). In parallel, MEI decreased gradually (enhanced electrotonic coupling) with exercise; at peak exercise, MEI was reduced by 5.3 ± 0.4% (or -23 ± 1.8Ω, P < 0.001). Notably, exercise-mediated electrotonic changes were linearly predicted by the degree of autonomic activation, as indicated by changes in either HR or in HRV (P < 0.001). Indeed, β-AR blockade attenuated the MEI response to exercise while direct β-AR stimulation (at rest) triggered MEI decreases comparable to those observed during exercise; ventricular pacing had no significant effects on MEI. Finally, animals prone to VF had a significantly larger MEI response to exercise.
  
  
  CONCLUSIONS
  These data suggest that β-AR activation during exercise can acutely enhance electrotonic coupling in the myocardium, particularly in dogs susceptible to ischemia-induced VF.
authors.affiliation: Department of Physiology and Cell Biology, The Ohio State University Columbus, OH, USA ; Department of Electrical and Computer Engineering, The Ohio State University Columbus, OH, USA ; Safety Pharmacology, QTest Labs Columbus, OH, USA.
authors.forename: Carlos L
authors.initials: CL
authors.lastname: Del Rio
authors.affiliation: Department of Electrical and Computer Engineering, The Ohio State University Columbus, OH, USA ; Biomedical Engineering, The Ohio State University Columbus, OH, USA.
authors.forename: Bradley D
authors.initials: BD
authors.lastname: Clymer
authors.affiliation: Department of Physiology and Cell Biology, The Ohio State University Columbus, OH, USA ; Davis Heart and Lung Research Institute, The Ohio State University Columbus, OH, USA.
authors.forename: George E
authors.initials: GE
authors.lastname: Billman
keywords: arrhythmic risk; electrotonic coupling; exercise; myocardial infarction; β-adrenoceptor stimulation