Selenium (Se) has long been recognized as both an essential mammalian nutrient and a hazardous element. Sodium selenite is commonly used as a dietary supplement for the treatment of Se deficiency. On the other hand, chronic Se toxicity has been demonstrated to affect the major organs, including the heart, in experimental animals. This study examines the effects of high concentrations of extracellular selenite (in the range of 0.001-1 mM) application into the recording bathon the electrical properties of rat papillary muscles. Conventional glass semifloating microelectrodes were used to record intracellular action potentials (APs) in isolated rat papillary muscles. The amplitude of APs and the resting membrane potential of papillary muscles were not changed following a 20-min selenite (1 mM) application compared to the first minute of its application. Freshly isolated ventricular myocytes by an enzymatic method were used to determine the selenite-induced alterations in Na+ currents. Na+ currents, measured at 22 degrees C, by the whole-cell patch-clamp technique, decreased by 38 +/- 8% in the presence of 1 mM selenite for 5 min. These selenite-induced effects were not reversed, but are restored by dithiothreitol (1 mM). These results demonstrated that toxic concentrations of selenite induced a significant shortening in AP duration as a result of an increase in the rate of repolarization. Our findings also suggest that a decrease in Na+ currents, in addition to Ca2+ currents, may play a role in the shortening of AP duration in rat papillary muscles.