Effects of taurine on the spontaneous action potentials in rabbit sino-atrial nodal cells were examined at different extracellular Ca2+ concentrations ([Ca]o). Experiments were performed at 36 degrees C. The firing rate of spontaneous activity was 132.5 +/- 12.1 beats/min (n = 18) in normal Tyrode's solution ([Ca]o = 1.8 mM). Increasing [Ca]o level from 0.9 to 10.8 mM significantly changed the maximum rate of depolarization. Other parameters of the action potentials were unaffected. When [Ca]o was 0.9 mM, application of taurine (1 to 20 mM) tended to cause a positive chronotropic effect and hyperpolarized the maximum diastolic potential. In the normal solution (at 1.8 mM [Ca]o), taurine significantly enhanced only the maximum rate of depolarization. In contrast, under high [Ca]o (5.4 and 10.8 mM), taurine at 1 and 5 mM had a negative chronotropic effect, but 20 mM taurine had a positive chronotropic effect. Also, taurine shortened the action potential duration and hyperpolarized the maximum diastolic potential. The maximum rate of depolarization was inhibited. In 10.8 mM [Ca]o solution, irregular spontaneous activity (dysrhythmias) occurred in 4 of 6 preparations, and addition of taurine (1 to 20 mM) abolished it. These results indicate that taurine modulates the action potential configuration in the sino-atrial nodal cells dependent on [Ca]o.