REGULATION BY TAURINE OF THE SPONTANEOUS ACTIVITY IN YOUNG EMBRYONIC CHICK CARDIOMYOCYTES

Effects of taurine on the spontaneous action potential (AP) and the ionic currents in isolated 3-day-old embryonic chick ventricular cardiomyocytes were examined at different intracellular Ca2+ concentrations ([Ca](i)) by whole-cell voltage-clamp and current-clamp methods. Experiments were performed at room temperature (22 degrees C). The firing rate of spontaneous activity was 61.2 +/- 2.1 beats/min (n = 6) at pCa 10, and 68.2 +/- 1.8 beats/min (n = 6) at pCa 7. When [Ca](i) was pCa 10, taurine 10 and 20 mM caused a positive chronotropic effect and enhanced the maximum rate of depolarization. In contrast, at pCa 7, taurine 10 and 20 mM had a negative chronotropic effect. In voltage-clamp experiments, addition of taurine to the bath solution at pCa 7 inhibited the L-type Ca2+ current (I-Ca(L)) and the delayed rectifying K+ current (I-K). At pCa 10, both I-Ca(L) and I-K currents were enhanced by taurine 10 and 20 mM. The voltage of half-maximum inactivation for I-Ca(L) was not modified, nor was voltage of half-maximum activation for I-K. These results indicate that taurine modulates the I-Ca(L) and I-K currents dependent on [Ca](i), resulting in regulation of spontaneous activity.