1 Protopine (Pro) from Corydalis tubers has been shown to have multiple actions on cardiovascular system, including anti-arrhythmic, anti-hypertensive and negative inotropic effects. Although it was thought that Pro exerts its actions through blocking Ca2+ currents, the electrophysiological profile of Pro is unclear. The aim of this study is to elucidate the ionic mechanisms of Pro effects in the heart. 2 In single isolated Ventricular myocytes from guinea-pig, extracellular application of Pro markedly and reversibly abbreviates action potential duration, and decreases the rate of upstroke (dV/dt)(max), amplitude and overshoot of action potential in a dose-dependent manner. Additionally, it produces a slight, but significant hyperpolarization of the resting membrane potential. 3 Pro at 25, 50 and 100 mu M reduces L-type Ca2+ current (I-Ca,I-L) amplitude to 89.1, 61.9 and 45.8% of control, respectively, and significantly slows the decay kinetics of ICa,L at higher concentration. The steady state inactivation of I-Ca,I-L is Shifted negatively by 5.9-7.0 mV (at 50-100 mu M Pro), whereas the voltage-dependent activation of I-Ca,I-L remains unchanged. In contrast, Pro at 100 mu M has no evident effects on T-type Ca2+ current (I-Ca,I-T). 4 In the presence of Pro, both the inward rectifier (I-Kl) and delayed rectifier (I-K) potassium currents are variably inhibited, depending on Pro concentrations. 5 Sodium current (I-Na), recorded in low [Na+](o) (40 mM) solution, is more potently suppressed by Pro. At 25 mu M, Pro significantly attenuated I-Na at most of the test voltages (-60- + 40 mV, with a 53% reduction at -30 mV. 6 Thus, Pro is not a selective Ca2+ channel antagonist. Rather, it acts as a promiscuous inhibitor of cation channel currents including I-Ca,I-L, I-K, I-Kl as well as I-Na. These findings may provide some mechanistic explanations for the therapeutic actions of Pro in the heart.