Figure 1A shows typical original traces of whole cell potassium currents recorded from a wild type (wt) cardiomyocyte, and from dystrophic (mdx and mdx-utr) cardiomyocytes. The currents were elicited by depolarizing and hyperpolarizing voltage steps outgoing from a holding potential of −100 mV (pulse protocol, displayed in the inset of Fig. 1B). A summary of the current density-voltage relationships, derived from a series of such experiments, is presented in Fig. 1B. It can be seen that the current densities of dystrophic cardiomyocytes were substantially decreased when compared with those in wt cells. A comparison of the current densities in wt and dystrophic cardiomyocytes at −100 mV revealed a highly significant difference (Fig. 1C). Figs. 1B and 1C do also show that both types of dystrophic cardiomyocytes displayed similar potassium inward currents. Thus, the current densities of mdx and mdx-utr cardiomyocytes were almost identical over the whole voltage range studied (Fig. 1B). Together these data suggests that dystrophin-deficiency (mdx and mdx-utr) in cardiomyocytes generated the observed decrease in IK1 density when compared to the wt, with no additional effect of utrophin-deficiency (only mdx-utr). Figure 1.IK1 in wt and dystrophic ventricular cardiomyocytes. (A) Typical original potassium current traces of a wt, mdx, and mdx-utr cardiomyocyte elicited from a holding potential of −100 mV by 500-ms steps to various voltages (the pulse protocol is shown in the inset of Fig. 1B). The dashed line indicates the zero current level. (B) Current density-voltage relationships derived from a series of experiments as shown in Fig. 1A (n = 38 for wt, 36 for mdx, and 13 for mdx-utr, respectively). The current levels at the end of the test pulse were evaluated and plotted against the applied voltages. The lines through the data points represent fits with the function: y = Y0/(1 + exp((x-V05)/k)). (C) Current density values at −100 mV (means ± SEM) are compared between wt and dystrophic (mdx and mdx-utr) cardiomyocytes. *** indicates that ANOVA revealed a highly significant difference between the tested groups (p < 0.001). (D) Current density values at −100 mV of wt and dystrophic cardiomyocytes from all experiments on female (♀) mice (n = 14 for wt, 15 for mdx, and 13 for mdx-utr). ** p < 0.01, ANOVA. E: Current density values at −100 mV of wt and mdx cardiomyocytes from all experiments on male (♂) mice (n = 24 for wt and 21 for mdx). * p < 0.05 (p = 0.011), Student's t-test.