A, ISO (1 μM) did not stimulate If in EDS cardiomyocytes, 1, control; 2, ISO (1 μM). B, ISO stimulated If in LDS cardiomyocytes (left panel: original recordings; right panel: time course of this experiment, voltage protocol identical to Fig. 2). *Difference in current density between the absence and the presence of ISO was statistically significant (B, histogram, n= 6; P < 0.05, paired t test).