Figure 2.

Na⁺ current transients recorded by the three vaseline gap voltage-clamp method from single fibres of frog semitendinosus muscle in the absence and presence of eutomers of tocainide (Toc) and of its proline-like derivatives without methyl groups (To5), with a methyl on the aminic (To6) or amidic (To7) nitrogen atom and with methylene moieties on both nitrogen atoms (To8). In particular, the effects produced by 500 μM of R-Toc were compared to those obtained in the presence of 100 μM of R-To5, S-To6, S-To7, and S-To8. In each group of traces, the control current (in the absence of drug) is the greatest one obtained with single depolarizing pulses from −100 to −20 mV for 10 ms. A similar depolarizing stimulus elicited around 10 min after the application of each compound allowed to estimate the tonic block exerted by the drug. Afterwards, the depolarizing stimulus was repetitively applied at a 10-Hz frequency for 20 – 30 s, producing a cumulative block over the tonic one due to a use-dependent behaviour, until a new equilibrium was reached. The smallest traces correspond to the residual current at the end of the 10 Hz stimulation protocol.