title: Interactions among heart rate, autonomic activity, and arterial pressure upon the multiple repetitive extrasystole threshold in the dog.
abstract: The effects of heart rate, autonomic stimulation, and arterial blood pressure upon the multiple repetitive extrasystole threshold (MRET) were studied after coronary occlusion in anesthetized dogs. The MRET was unaffected by changes in heart rate. Sympathetic stimulation alone reduced the MRET from 52.4 +/- 3.7 (mean +/- SEM) to 41.3 +/- 2.7 mA. Vagal stimulation alone was without effect. During simultaneous sympathetic and vagal stimulation the threshold was 48.7 +/- 3.0 mA. This supports the contention that vagal activity serves mainly to oppose the tendency for sympathetic activity to lower the MRET. Reduction of blood pressure significantly decreased the average MRET from 38.6 +/- 2.5 to 34.4 +/- 1.8 mA. The presence of a post junctional site for the observed sympathetic-vagal interaction was investigated using isoproterenol infusion and vagal stimulation. Infusion of isoproterenol lowered the MRET. Vagal stimulation in this situation raised the MRET; the effect varied directly with the rate of isoproterenol infusion. This suggests that part of the vagal effect is exerted postjunctionally; that is, directly upon the myocardial cell and not merely by inhibition of adrenergic transmitter release at sympathetic nerve endings.
mesh_terms: D000320:Adrenergic Fibers / Q000502:physiology; D000818:Animals; D001341:Autonomic Nervous System / Q000502:physiology*; D001794:Blood Pressure; D005117:Cardiac Complexes, Premature / Q000503:physiopathology*; D000242:Cyclic AMP / Q000096:biosynthesis; D004285:Dogs; D004594:Electrophysiology; D006339:Heart Rate; D007545:Isoproterenol / Q000008:administration & dosage; D014630:Vagus Nerve / Q000502:physiology
chemical_list: D000242:Cyclic AMP; D007545:Isoproterenol