title: Autonomic blockade unmasks maturational differences in rate-dependent atrioventricular nodal conduction and facilitation in the mouse.
abstract: BACKGROUND
  In large animals, rate-dependent AV nodal (AVN) properties of conduction are modulated by autonomic inputs. In this study, we investigated whether the properties of AVN conduction and facilitation are altered by autonomic blockade in the mouse and whether this effect is age dependent.
  
  
  RESULTS
  Young (age 4-6 weeks; n = 11) and adult (age 8-9 months; n = 11) female mice underwent in vivo electrophysiologic testing, before and after autonomic blockade. After autonomic blockade, the adult mice had significantly longer AVN effective refractory period (AVNERP; 67 +/- 14 msec vs 56 +/- 4 msec, P = 0.05) and functional refractory period (AVNFRP; 81 +/- 10 msec vs 72 +/- 4 msec, P = 0.05). With autonomic blockade, the increase from baseline of AVN Wenckebach cycle length (deltaAVW; 1.8 +/- 8.1 msec vs 8.8 +/- 3.3 msec, P = 0.04), as well as of AVNERP (deltaAVNERP; 3.5 +/- 3.5 msec vs 21.4 +/- 12.6 msec, P = 0.002) and AVNFRP (deltaAVNFRP; 2.3 +/- 3.2 msec vs 12.8 +/- 9.0 msec, P = 0.008), was significantly larger in adult than in young mice. Compared with young mice, adult mice were less likely to exhibit AVN facilitation (44% vs 90%, P = 0.03) and had smaller maximal shortening of AVN conduction times after the "test beat" for any coupling of the "facilitating beat" (4 +/- 4 msec vs 7 +/- 3 msec, P = 0.05).
  
  
  CONCLUSIONS
  Complete autonomic blockade significantly increases AVN conduction times and refractory periods in adult but not in young mice. Adult mice also exhibit less AVN facilitation. Our results confirm that, like in larger animals, rate-dependent murine AVN properties of conduction are under autonomic regulation. Adult mice have higher sympathetic AVN inputs at baseline, leading to slower conduction after autonomic blockade.
authors.affiliation: Division of Cardiac Electrophysiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA. sabas@msx.upmc.edu
authors.forename: Samir
authors.initials: S
authors.lastname: Saba
authors.forename: Barry
authors.initials: B
authors.lastname: London
authors.forename: Leonard
authors.initials: L
authors.lastname: Ganz
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000375:Aging / Q000502:physiology; D000818:Animals; D001283:Atrioventricular Node / Q000502:physiology*; D001340:Autonomic Nerve Block / Q000379:methods*; D001341:Autonomic Nervous System / Q000502:physiology; D002304:Cardiac Pacing, Artificial / Q000379:methods*; D004558:Electric Stimulation; D004562:Electrocardiography / Q000379:methods; D006321:Heart / Q000294:innervation / Q000502:physiology; D006339:Heart Rate / Q000502:physiology*; D008297:Male; D051379:Mice; D009435:Synaptic Transmission / Q000502:physiology*