Reactive oxygen species have been implicated in different types of cardiac arrhythmias including human atrial fibrillation. Kv1.5, the presumed molecular correlate of I(Kur), is an important determinant of human atrial repolarization. The aim of this study was to assess the effects of H(2)O(2), at pathophysiologically relevant concentrations (20-1,000 microM), on Kv1.5 expressed in Chinese hamster ovary cell line. Kv1.5 cDNA in pcDNA3 expression vector and CD8, a surface marker protein, were cotransfected in cells by calcium phosphate precipitation. Kv1.5 activation kinetics were significantly accelerated while the activation curve was negatively shifted by 10 mV (V(1/2) changed from -9.3 to -19.0 mV) in the presence of 100 microM H(2)O(2). The shift in Kv1.5 peak current I-V curve was voltage-dependent, the current amplitude being increased for voltages <+20 mV but decreased for high depolarizing voltages. The rapid activation time constant obtained from a bi-exponential fitting was decreased from 16.1+/-3.4 ms to 8.8+/-1.5 ms for a -20 mV depolarization ( n=9; P=0.01) and from 4.3+/-2.1 ms to 2.3+/-0.4 ms when cells were depolarized to +20 mV ( P<0.05). Kv1.5 steady-state inactivation was not modified by H(2)O(2). Intracellular application of SOD or catalase reduced the H(2)O(2) induced shift of activation I-V curve and SOD significantly decreased Kv1.5 amplitude at +40 mV ( n=9; P<0.05). In conclusion, H(2)O(2) increased Kv1.5 current amplitude at voltages corresponding to the action potential repolarization phase and accelerated Kv1.5 channel opening. These changes can reduce the action potential duration, leading to a shortening of the atrial effective refractory period. H(2)O(2)-induced changes in Kv1.5 properties could thus be involved in initiation or perpetuation of AF.