Influence of acute myocardial ischaemia on ventricular cyclic AMP concentrations in naive and morphine-treated rats.

1. Ventricular cyclic AMP (cAMP) concentrations of naïve and morphine-treated rats subjected to acute myocardial ischaemia were examined. 2. In naïve rats, ventricular cAMP levels were increased at 3 min but decreased 5 and 10 min after left coronary artery ligation. Statistically significant changes were observed in the right ventricle after 3 min and in the left ventricle after 10 min. 3. Acute morphine treatment did not significantly alter ventricular cAMP content in rats subjected to either sham operation or acute left coronary artery ligation. 4. Ventricular cAMP concentrations were significantly lower in sham-operated rats after 5 weeks of chronic morphine treatment while after 3 weeks of chronic morphine treatment, this phenomenon was seen only after acute coronary ligation. The reductions were reversed by opiate withdrawal. 5. These observations support the theory that elevated cAMP levels may contribute to the production of early ventricular arrhythmias during acute myocardial ischaemia. It is suggested that the reduction in ventricular cAMP concentrations may partly account for the previously reported decreased occurrence of early ventricular arrhythmias in chronic morphine-treated rats subjected to acute left coronary artery ligation.