After the Langendorff cell isolation procedure the ventricular cardiomyocytes were plated on cover slips, and, 90 min later, fixed in 3.5% paraformaldehyde for 10 min. Cell culture medium was removed, the cells were washed 3 times with PBS, permeabilized in 0.1% Triton X-100 for 5 min at room temperature, and washed again 3 times with PBS. This was followed by a 2 hour block with 10% horse serum and 0.01% azide in PBS. Thereafter, the cells were incubated with Anti-Kir2.1 antibody (APC-026, Alomone Labs; 1:500 in PBS) at 4°C overnight. On the next day, the cells were washed 3 times with PBS, and incubated for 60 min with the corresponding secondary antibody (Alexa Fluor 594, #A21207, Invitrogen; 1:500 in PBS) at room temperature. Subsequently, after 3 more PBS washing steps, the cells were mounted, dried and stored at 4°C. The slides were finally analyzed using a LSM 510 confocal microscope (Zeiss, Jena, Germany). For the immunostaining experiments 3 wt and 3 mdx mice (15–25 week-old) were used for cell isolation.