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Figure 3 Effect of isoproterenol in the LQT3 model. ATX-II (20 nmol/L) produced a marked prolongation of APD of the M cell, more than that of the epicardial cell and a dramatic increase in TDR. Isoproterenol (Iso, 100 nmol/L) in the continued presence of ATX-II always abbreviated the APD of both cells, resulting in a persistent decrease in TDR. APD = action potential duration; LQT3 = long QT syndrome 3; TDR = transmural dispersion of repolarization.