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The modifying effect of autonomic blockade on digitoxin-induced changes in monophasic action potential and refractoriness of the right ventricle of the dog heart.

The aim of the present investigation was to study the modifying effect of autonomic blockade with atropine plus propranolol and atropine alone on the electrophysiologic response to digitoxin during an observation period of 8 hours. Twenty-five pentobarbital-anaesthetised Labrador dogs were used. Pacing and programmed electrical stimulation were used to determine heart rate independent changes in repolarisation times and the effective (V-ERP) and the functional (V-FRP) refractory periods. The dogs were divided into three groups and all dogs got digitoxin (2.0 mg) intravenously during 5 min. Fourteen dogs were given digitoxin without blockade, while 5 dogs were pretreated with propranolol 0.5 mg/kg plus atropine 0.05 mg/kg every hour for 8 hours and 6 dogs with atropine in the same dose and with the same interval. Digitoxin alone increased 50 and 90% repolarisation times 2-4 hours after injection of the drug. During autonomic blockade digitoxin induced a shortening of action potential duration, and thus digitoxin interacts with autonomic transmitters resulting in prolongation of action potential duration. The "direct" effects of digitoxin, the action potential shortening effect, the decrease in membrane responsiveness (i.e. V-ERP/50% repolarisation time) reached a maximum 2-3 hours after injection of the drug.