title: Effect of the autonomic blockade on the automaticity of the A-V junctional pacemaker in awake dogs.
abstract: The effect of the autonomic blockade on the automaticity of the A-V junctional pacemaker was evaluated in 15 awake dogs with experimentally induced A-V junctional rhythm. The duration of asystole after overdrive (D.A.O.) in these dogs was prolonged significantly in accordance with increase in the drive rate, and the mean +/- SD of the D.A.O. reached 4.7 +/- 1.1 seconds (N = 15) after overdrive at 2.5 times the spontaneous heart rate. After administration of atropine (0.4 mg/kg; i.v.) to eight dogs, the mean +/- SD of the D.A.O. at the same rate decreased from 4.5 +/- 0.9 to 3.4 +/- 1.2 seconds. After administration of practolol (0.5 mg/kg; i.v.) to the seven other dogs, the mean +/- SD of the D.A.O. at the same rate increased remarkably from 4.9 +/- 1.3 to 9.4 +/- 3.0 seconds. Intravenous injection of practolol (0.5 mg/kg) had no effect upon the D.A.O. in the five dogs with sinus rhythm. Thus, it is suggested that (1) the sympathetic nerve might play a more important role in regulating the automaticity of the A-V junctional pacemaker than the vagus and (2) it physiologically might take over 5.0 seconds for the A-V junctional pacemaker to initiate an escape beat during longstanding sinus arrest, if a marked dysfunction of the A-V junctional pacemaker occurs due to a decrease in tension of the sympathetic nerve.
mesh_terms: D000818:Animals; D001283:Atrioventricular Node / Q000187:drug effects / Q000503:physiopathology*; D001285:Atropine / Q000494:pharmacology; D002304:Cardiac Pacing, Artificial; D004285:Dogs; D004562:Electrocardiography; D006327:Heart Block / Q000503:physiopathology*; D006329:Heart Conduction System / Q000503:physiopathology*; D006339:Heart Rate / Q000187:drug effects; D011217:Practolol / Q000494:pharmacology; D013564:Sympathetic Nervous System / Q000502:physiology
chemical_list: D001285:Atropine; D011217:Practolol