Figure 6. Tonic block of Na sup + currents by washin of tetracaine and N-alkyl tetracaine derivatives. Outward Na sup + currents were elicited by a test pulse of +30 mV for 5 ms applied after a prepulse of -130 mV for 100 ms. The protocol was repeated every 0.5 min while the cell was kept at a holding potential of -100 mV under reversal Na sup + gradient conditions. (A) Superimposed consecutive traces recorded before (time 0) and after the external application of tetracaine (left panel; 100 micro Meter for a period of 0.5'-6') and N-hexyl-tetracaine (right panel; 100 micro Meter for a period of 0.5'-6'). (B) Normalized peak Na sup + currents were plotted as a function of time after external washin of 100 micro Meter N-ethyl tetracaine (C2, [open triangle]; n = 4), N-butyl tetracaine (C4, [open inverted triangle]; n = 5), and N-hexyl tetracaine (C6, [square, open]; n = 5). Normalized peak Na sup + currents after external 100 micro Meter tetracaine washin (C0, [circle, open]; n = 4) were also plotted for comparison. Each data set was force-fitted by a single exponential function (solid line) to extract the relative wash-in rate (tau) of 9.9 +/- 3.3 min, 4.9 +/- 0.9 min, 3 +/- 0.1, and 0.16 +/- 0.03 for C2, C4, C6, and C sub 0, respectively. Although some data points were not fitted well by a single exponential function, for comparison these fittings provide the relative wash-in rates of N-alkyl tetracaine derivatives.