[Inhibitory effect of tocainide on calcium current and potassium current in guinea pig ventricular myocytes]

In whole cell voltage clamp experiment with isolated guinea pig ventricular myocytes, tocainide was shown to exhibit a concentration-dependent inhibition of calcium current (ICa) and delayed rectifier potassium current (IK). Tocainide with a therapeutic concentration of 50 mumol.L-1 significantly inhibited the ICa and Itail (deactivating tail current of IK) by 16% and 3% respectively. Inhibition of ICa by tocainide was also shown to be dependent upon stimulating frequencies, with a greater blockade occurring at 2.0 Hz (57%) than at 0.2 Hz (17%). Tocainide was found to inhibit the IK,ATP by 74% at 200 mumol.L-1 but not at 50 mumol.L-1. These inhibitory effects on ICa and IK can probably explain the therapeutic effect of tocainide on supraventricular tachycardia, and the shortening of plateau phase of action potential.