Morphine and the endogenous opioid dynorphin in the brain attenuate digoxin-induced arrhythmias in guinea pigs.

The effects of the opioid receptor agonists morphine and dynorphin on digoxin-induced arrhythmias were examined in guinea pigs that had received intravenous digoxin (50 mu/kg bolus plus 500 mu/kg/hr intravenously). Animals received either morphine (50 or 100 micrograms/kg) or dynorphin A(1-13) (50 or 100 micrograms/kg) or saline (the diluent) into the lateral cerebral ventricle (intracerebroventricularly) prior to digoxin. Morphine and dynorphin produced significant (P < 0.05) dose-dependent increases in the threshold of digoxin-induced arrhythmias. The mean digoxin dose at the development of fatal arrhythmias was 775 +/- 42 micrograms/kg in the control group but was significantly higher namely 958 +/- 45 micrograms/kg after 100 micrograms/kg of morphine ICV, and 984 +/- 47 micrograms/kg after 100 micrograms/kg of dynorphin A (1-13) intracerebroventricularly. In the absence of digoxin, the highest doses of each of these opioids did not produce arrhythmias. Changes in blood pressure and heart rate were unlikely explanations for the observed actions of these opioids as morphine accentuated the increase in blood pressure that accompanied digoxin while dynorphin was associated with a lower blood pressure with digoxin, despite similar effects on arrhythmias. In the control group, fatal digoxin-induced arrhythmias were ventricular tachyarrhythmias in two-thirds of cases and complete heart block in the rest. Morphine and dynorphin reduced the development of ventricular tachyarrhythmias. The role of the cholinergic system was explored, with morphine, utilizing atropine sulfate which crosses the blood brain barrier and atropine methylnitrate which does not enter the CNS. Atropine sulfate but not atropine methylnitrate reversed the effects of morphine on digoxin-induced arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)