Applying the whole cell patch clamp technique and established pulse protocols, IK1 was recorded at room temperature (22 ± 1.5°C) from ventricular cardiomyocytes up to 8 hours after preparation using an Axoclamp 200B patch clamp amplifier (Axon Instruments, Union City, CA). Pipettes were formed from aluminosilicate glass (AF150-100-10; Science Products, Hofheim, Germany) with a P-97 horizontal puller (Sutter Instruments, Novato, CA). Their resistances were between 0.8 and 2 MΩ when filled with pipette solution (see below). Data acquisition was performed with pClamp 6.0 software (Axon Instruments) through a 12-bit A-D/D-A interface (Digidata 1200; Axon Instruments). Data were low-pass filtered with 1–10 kHz (−3 dB) and digitized at 10–100 kHz. Data analysis was performed using Clampfit 10.2 (Axon Instruments) and GraphPad Prism 5.01 (San Diego, USA) software. The cells were bathed in 140 mM NaCl, 4 mM KCl, 2 mM CaCl2, 2 mM MgCl2, 5 mM HEPES, 5 mM Glucose, pH = 7.4 (adjusted with NaOH). The pipette solution contained 10 mM NaCl, 140 mM KCl, 2 mM EGTA, 1 mM MgCl2, 0.1 mM Na-GTP, 5 mM Mg-ATP, 10 mM HEPES, pH = 7.2 (adjusted with KOH). Potassium currents were elicited by 500-ms hyper- and depolarizing voltage steps between −140 and −30 mV from a holding potential of −100 mV (see inset in Fig. 1B). Around the resting membrane potential and at more hyperpolarized voltages, the ventricular IK1 conductance is much larger than that of any other potassium current.21 In murine ventricular cardiomyocytes, currents through Kir2.1 channels represent the major component of IK1 consistent with the finding that myocytes isolated from Kir2.1−/− mice completely lacked detectable whole cell IK1 in 4 mM external potassium.19 A potential “contamination” of IK1 by other inward rectifying potassium currents (IK,ATP,34 IK,ACh,35 and IK,Ca35) in our experiments was excluded by the composition of our experimental solutions. Thus, IK,ATP was inhibited by the presence of mM concentrations of ATP in the pipette solution,36,37 and activation of IK,ACh was prevented by the lack of acetylcholine. IK,Ca activation could be excluded because calcium channels do not activate at potentials more negative than −60 mV at which significant IK1 was detectable. Further, calcium was absent from the pipette solution which additionally contained 2 mM of the calcium buffer EGTA (see above). For the determination of IK1 density-voltage relations, the current amplitudes at the end of the test pulses to various potentials were measured. These were then divided by the cell capacitance to obtain current densities. IK1 recordings from both wt and dystrophic cardiomyocytes were always started 5 min after whole cell access was attained to avoid potential artifacts due to time-dependent shifts in current properties.