Abstract: AIM:To test the hypothesis that antiarrhythmic properties of U50,488H may be mediated by preserving Cx43 protein via regulating intracellular calcium. METHODS: Using an in vivo arrhythmia study, arrhythmia was induced by temporary occlusion of the left anterior descending (LAD) coronary artery with a silk suture for 30 min in Sprague Dawley rats weighing 250 to 350 g. U50,488H, a κ-opioid receptor (κ-OR) agonist, Nor-BNI, a κ-opioid receptor antagonist, Bay K8644, a calcium channel agonist, nifedipine, a calcium channel inhibitor and heptanol, a Cx43 inhibitor were chosen to be intravenously (i.v.) injected into a femoral vein prior to ischemia. A sham group underwent the same surgical procedures except that the suture underneath the LAD was left untied. In an in vitro arrhythmia study, each heart was quickly removed and underwent an initial 10 min of normal baseline perfusion and was subjected to perfusion at 37℃ for 30 min (for analysis of arrhythmia) or 90 min (for Western blotting). The hearts were then randomly divided into three groups: Con (normal calcium perfusion, 1.5 mmol/L), high calcium (high calcium perfusion, 3.3 mmol/L) and low calcium (low calcium perfusion, 0.5 mmol/L). Before and during the ischemia period, electrocardiogram (ECG) was used to measure the incidence of arrhythmias. RESULTS: By performing ECG monitoring and immunoblotting in isolated Langendorff-perfused rat hearts, high concentrations of calcium-perfused rat hearts exhibited increased cardiac arrhythmias. Diminished expression of Cx43 protein was observed. U50,488H dose-dependently inhibited L-type calcium current in single ventricular myocytes of rats using whole-cell patch clamp techniques. These effects were blocked by nor-BNI, a selective κ-OR antagonist. Administration of U50,488H before myocardial ischemia attenuated total arrhythmia scores. This effect was blocked by nor-BNI, antagonized by Bay K8644, an L-type calcium channel agonist, and by the Cx43 uncoupler heptanol. Finally, immunoblotting data demonstrated that the preservation of Cx43 protein conferred by U50,488H was reversed in the presence of Bay K8644. CONCLUSION: κ-OR activation with U50,488H may confer antiarrhythmic effects via modulation of the calcium-Cx43 pathway.