Figure 8. Tonic block of Na sup + currents by internal perfusion of 50 micro Meter N-alkyl tetracaine derivatives. (A) Superimposed current traces were recorded before and after internal perfusion of 50 micro Meter tetracaine (left panel) and 50 micro Meter N-hexyl tetracaine (right panel). The block reached steady-state values at 30 min and 24 min, respectively. (B) Tonic block of Na sup + currents by internal 50 micro Meter N-alkyl tetracaine derivatives is compared with means +/- SEM. With internal application of N-ethyl tetracaine, the peak Na sup + current amplitudes were on average 5.8 +/- 0.8%(C2, n = 6) of the untreated control; with N-butyl tetracaine, the average was 6.2 +/- 1.9%(C4, n = 6); and with N-hexyl tetracaine, the average was 4.6 +/- 1.2%(C6, n = 6). In contrast, with 50 micro Meter tetracaine applied internally, the peak Na sup + current amplitudes were on average 49 +/- 4%(C0, n = 9). The difference between N-alkyl tetracaine derivatives and tetracaine was significant (P < 0.05). Among N-alkyl tetracaine derivatives, the differences in tonic block were not significant. The intracellular perfusion was achieved by the gravity force and the internal solution was perfused at a constant rate of 15 micro liter/min. Time required to observe macroscopic current change was estimated to be 7–10 min after the perfusion began. New steady-state block could be reached within 20–40 min.