Effect of a novel class I antiarrhythmic agent, TYB-3823, on the calcium current in single guinea pig ventricular myocytes

The effect of TYB-3823 on Ca2+ current (ICa) of guinea pig ventricular myocytes was investigated by means of whole-cell patch-clamp technique. TYB-3823 (100–1,000 μM) caused a concentration-dependent decrease in ICa. Furthermore, a reduction of ICa induced by TYB-3823 (1,000 μM) was progressively accentuated by repetitive membrane depolarization, indicating a rate-dependent block of ICa. However, the inhibitory potency on ICa was approximately 1 1000 of a Ca2+ antagonistic agent, verapamil hydrochloride. Considering evidence that 3–30 μM TYB-3823 decreased the maximum upstroke velocity of the action potential of guinea pig ventricular muscles, it is indicated that the drug does not show its Ca2+ antagonistic property in the usual concentration range as a class I antiarrhythmic agent.