Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) enhances triggered afterdepolarizations in rat ventricular myocytes

The effects of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) on action potential and afterdepolarizations were studied in rat ventricular myocytes using nystatin-perforated whole-cell patch-clamp technique. TCDD treatment, in the concentration range of 1 to 100 nM, significantly prolonged action potential duration measured at 90% of repolarization (APD(90)). The triggered delayed-afterdepolarizations (DADs) were observed in 6 out of 8 cells after exposure of TCDD (10 nM). In the presence of isoproterenol (ISO, 10 nM) or Bay K 8644 (1 mu M), TCDD (10 nM) markedly augmented the amplitude and frequency of the arrhythmogenic DADs and triggered sustained spontaneous firings in ventricular myocytes. Voltage-clamp data indicated that TCDD (10 nM) exposure significantly enhanced the transient inward current (I-ti). The triggered early-afterdepolarizations (EADs) were evoked only in cells simultaneously exposed to TCDD (10 nM) and ISO (or Bay K 8644). Further study indicated that TCDD treatment increased L-type Ca2+ current. These results indicate that activation of TCDD signaling pathway can prolong action potential duration and cause abnormal triggered afterdepolarizations. These effects may lead to clinically relevant ventricular arrhythmia especially when susceptible individuals are under elevated sympathetic stress or suffering from other myocardiopathies coincided with Ca2+-overload.