The development of H2O2-induced changes in membrane potentials, membrane currents and corresponding contractile activity (shortening) were studied in rat and guinea-pig ventricular myocytes using the suction-pipette whole-cell clamp method. The cells exhibited a different sensitivity to 30 microM H2O2 in terms of time development of the changes, which were fully irreversible. The observed changes are described in three phases: (1) prolongation of action potential duration (APD) accompanied by increased contractility. With a prolonged exposure, the increased APD was accompanied by early afterdepolarizations (EADs), delayed afterdepolarizations (DADs) and aftercontractions. The changes in APD and the EADs were fully and permanently abolished by tetrodotoxin (TTX) but not by nifedipine, while the DADs and aftercontractions were abolished by ryanodine. These changes preceded phase (2), which was characterized by APD shortening, a decrease in contractility, membrane depolarization, single or multiple extrasystoles, or steady spontaneous activity; this phase could not be prevented by any of the above pharmacological interventions and resulted in a final phase (3) characterized by full depolarization and inexcitability. All the above changes were prevented by intracellular application of iron chelator-deferoxamine, indicating that .OH generated intracellularly in the presence of Fe3+ induces the observed changes. The examination of membrane currents indicated that the increased APD may be due to an increase in the TTX-sensitive Na+ current as well as to the decreased delayed current, while L-type Ca2+ channels appear to be unaffected. The shortening of APD during the second phase was associated with a large increase in the delayed K+ current. The increased contractility in the first stage appears to be due to increased sarcolemmal Ca2+ influx via Na(+)-Ca2+ exchange (among other possible mechanisms), leading to a loading of sarcoplasmic reticulum that eventually results in Ca2+ overload and functional failure.