Effects of ouabain and vagal stimulation on sinus nodal function in conscious dogs

We studied the effects of intravenous ouabain administration (15 micrograms/kg) and vagal stimulation on instrumented conscious dogs. In these dogs, a special electrode was implanted on the epicardial surface over the sinus node and plaque electrodes were placed on the superior vena cava and left atrial appendage. Unipolar sinus electrograms were recorded through the terminals of the sinus electrode paired with the superior vena caval electrode. An electrode also was implanted around the desheathed cervical vagosympathetic trunk, and a coiled polyethylene tube with side holes was implanted cranial to it. Ouabain administration resulted in: (1) an increase in the sinus cycle lengths and sinoatrial intervals and increase in beat-to-beat variation of the sinus cycles and sinoatrial intervals, and (2) periods of sinus pacemaker shift, sinus arrest and sinoatrial block that occurred spontaneously or after atrial overdrive stimulation delivered through the electrode on the left atrial appendage. These effects of ouabain were abolished by intravenous administration of atropine (2 mg). Prior to vagal stimulation, intravenous propranolol (0.5 mg/kg) was administered to block sympathetic responses and 0.75% bupivacaine was injected through the polyethylene tube to block afferent transmission. Vagal stimulation in conscious dogs resulted in two types of responses: (1) slowing of sinus pacemaker rate accompanied by a decrease in diastolic slope and followed by sinus pacemaker shifts, and (2) gradual prolongation of the sinoatrial intervals followed by failure of sinoatrial conduction. An intravenous bolus of acetylcholine (0.1 mg) resulted in a response of type 2 in dogs in which vagal stimulation produced a response of type 1. The marked similarity between the effects of ouabain and vagal stimulation on sinus function and the abolition of the effects of ouabain by atropine suggest that the ouabain effects are vagally mediated.