The interrelationship of morphine and the parasympathetic nervous system in digoxin-induced arrhythmias in the guinea-pig.

1. The role of the parasympathetic nervous system and the effect of morphine on cardiac arrhythmias induced by digitalis was assessed in guinea-pigs that had digitalis arrhythmias produced by digoxin 50 micrograms/kg intravenously plus 500 micrograms/kg per h. 2. Morphine produced a dose-dependent rightward shift in the relationship between digoxin dose and arrhythmias. Morphine 5 mg/kg subcutaneously (s.q.) was associated with significantly (P less than 0.05) higher digoxin doses both at the onset of ventricular arrhythmias and at development of fatal arrhythmias compared with the control group that received saline s.q. 3. Vagotomy and atropine prevented the effect of morphine so that the dose-response relationships and the mean digoxin dose at onset of ventricular arrhythmias and the development of fatal arrhythmias were not significantly different from the control group receiving saline. 4. These data indicate that morphine limits the development of digitalis arrhythmias and this effect is, at least in part, mediated through the parasympathetic nervous system.