Objective To investigate the effects of BmkTXKβ, a newly purified ‘long chain peptide inhibitor of K+ channels from the Chinese scorpion Buthus martensi Karsch (BmK), on the electrophysiological properties of isolated rabbit atrial myocytes. Methods The standard whole-cell patch-clamp technique was used to study the effects of multiple concentrations of BmkTXKβ on potassium currents and action potentials. Results BmkTXKβ produced concentration-dependent prolongation of action potential duration at 20%, 50%, and 90% repolarization (APD20,50,90) without any use-dependence. Meanwhile, it had no significant effect on RMP, APA, or Vmax (n=9). At a dose of 1 μmol/L, BmkTXKβ decreased Ito by 41.4% (n=10, P