Voltage-clamp experiments were performed to determine the effects of veratrine (1 microgram/ml) on Na and K currents in isolated rabbit ventricular cardiomyocytes. 2. Veratrine did not affect the inward rectifier K current, increased the inactivation time constant of the transient outward current (I(to)) and induced a slowly decaying inward current component (Iv), which was sensitive to tetrodotoxin. 3. Inactivation of fast Na channels by application of short depolarizing prepulses to potentials between -90 and -50 mV prevented the development of Iv.Iv decayed biexponentially with time constants equal to 139 +/- 9.0 ms and 776 +/- 47 ms. The net amplitude of Iv and the time constants for its rapidly and slowly inactivating components were little affected by trains of conditioning prepulses to 0 mV. The contributions, however, of the fast and slow components to the net current were significantly altered by repetitive depolarizations. 4. These components of Iv are likely due to modification of open cardiac Na channels by veratrum alkaloids.
