induced by direct vagus nerve stimulation and 4 acetylcholine antagonizes the intracellular production of cyclic amp by catecholamines therefore the effects we observe with infusion of acetylcholine and norepinephrine likely do not reproduce quantitatively the effects of autonomic nerve activity nevertheless these qualitative effects demonstrate an important modulation of atrial flutter excitable gap which can be clinically significant atrial flutter protocol all experiments described were in accordance with institutional guidelines for animal experimentation fourteen mongrel dogs of either sex weight 29 45 kg were studied in the post absorptive state general anaesthesia was induced with sodium thiopental 25 mg kg iv and maintained with chloralose 80 mg kg iv bolus supplemented by 20 mg kg hr maximum as needed the dogs were intubated and ventilated harvard pump with room air 10 breaths min tidal volume to achieve a maximum inspiratory pressure of 20 cm water to maintain arterial ph 7 35 7 45 and pao 2 gt 80 mm hg arterial and venous cannulae were inserted in the left femoral artery and vein by direct cut down for blood pressure monitoring and drug administration respectively an additional venous cannulae was inserted in the right femoral vein or in the right internal jugular vein for blood sampling muscular relaxation was then induced with gallamine triethiodide flaxedil 100 3 mg kg intravenously a right thoracotomy was performed via the fourth or fifth intercostal space and the pericardium was incised to provide access to the vena cava and the right atrium according to the procedure described by frame et al 1986 the tissue on a line extending from the superior to.