1. Effects of taurine on the reversal potential of Na+ channel (INa) in isolated 17-day-old embryonic chick ventricular cardiomyocytes were examined using whole-cell voltage clamp technique. Experiments were performed at room temperature (22 degrees C). 2. Test pulses were applied between -60 and +50 mV from a holding potential of -90 mV. Addition of taurine (1-20 mM) to the bath solution inhibited the INa at -30 mV in a concentration-dependent manner; by 38.8 +/- 3.7% (n = 14, P < 0.01) at 10 mM and by 49.5 +/- 4.6% (n = 12, P < 0.001) at 20 mM. 3. Simultaneously, the reversal potential was shifted in the hyperpolarizing direction by 10.6 +/- 2.9 mV (n = 10, P < 0.05) at 10 mM and by 12.6 +/- 2.2 mV (n = 9, P < 0.01) at 20 mM. The shift was also produced concentration-dependently. Even when taurine at low concentrations (1 and 5 mM) enhanced INa, the shift occurred. 4. Intracellular taurine level decreased in Langendorff perfused guinea-pig hearts with Ca(2+)- and Mg(2+)-free solution, but not with Ca(2+)-free and 20 mM Mg2+ solution. 5. These results indicate that taurine shifts the reversal potential of the INa due to Na(+)-taurine cotransport, which might play an important role for the cell functions.