EFFECTS OF PROPAFENONE ON CALCIUM CURRENTS IN SINGLE VENTRICULAR MYOCYTES OF GUINEA-PIG

1 The effects of propafenone on the inward calcium current (I(Ca)) were investigated in isolated single ventricular myocytes of the guinea-pig by the whole-cell clamp method. Propafenone inhibited I(Ca) in a dose-dependent manner at concentrations of propafenone ranging from 1 x 10(-8) to 1 x 10(-3) m and half maximal block Of I(Ca) occurred at a propafenone concentration of 1.5 x 10(-6) m. Propafenone did not change the current-voltage relation Of I(Ca) other than a reduction in amplitude and showed no clear use- or frequency-dependent effects upon I(Ca) (StiMUlation frequencies from 0.03 to 2 Hz). Propafenone did not alter the steady-state inactivation process: the half maximal activation potentials were 18.5 +/- 2.2 mV in the control state and 20.9 +/- 5.0 mV in the presence of 1 x 10(-6) M propafenone (n = 12, NS). Propafenone (I X 10(-6) M) increased the half-time of reactivation by 73.9% (n = 6, 212.3 +/- 1.2 ms vs 369.2 +/- 1.5 ms, P< 0.05). 2 We conclude that propafenone blocks I(Ca) in a concentration-dependent and a channel state-, use- or frequency-independent manner. The I(Ca) blockade elicited by propafenone at clinically therapeutic plasma concentration is significant and may be involved in its anti-arrhythmic effects.