Ankyrin B is an intracellular polypeptide involved in targeting of ion transporters, i.e. the Na/K-ATPase (NKA). Ankyrin B is found in various cell types. In cardiomyocytes, ankyrin B is suggested to play a special role in anchoring a macromolecular complex consisting of the NKA, the Na/Ca-exchanger and the IP3-receptor (Mohler, P.J et al.2005). Ankyrin B dysfunction is linked to cardiac arrhythmias and sudden cardiac death, shown both in mice and man (Mohler, P.J et al.2003). We investigated the role of ankyrin B for regulation of NKA current in mouse cardiomyocytes. To study the interaction between NKA and ankyrin B, we used cell permeable interaction inhibiting peptides directed against the binding motifs for ankyrin B on NKA. After the cardiomyocytes were incubated with peptide for 1 hour, we measured NKA current using the voltage clamp technique. The results showed a reduction in NKA current compared to a control group incubated with a biologically inactive peptide. To evaluate whether the NKA current reduction is associated with an increased intracellular Na concentration, we have performed preliminary studies in cardiomyocytes using the flourescent sodium indicator SBFI. In conclusion, these data suggest that ankyrin B is important for normal NKA function.