Figure 9. Determinants of use-dependent block by volatile anesthetics. Currents are normalized to the first pulse. Lines are least-square fits of a single exponential to the data, yielding a limiting current and a time constant as parameters. Data are from separate exemplary cells for each protocol. (A) Influence of pulse frequency. Plotted are currents elicited by trains of 20 pulses to 0 mV from a holding potential of -85 mV. At 2 Hz (circles), 1.2 mM halothane reduces the limiting current to 0.50 with a time constant of 0.66 ms. At 5 Hz (squares), 1.2 mM halothane leads to a limiting current of 0.09 and a time constant of 0.34 ms. (B) Influence of pulse duration. Pulses from a holding potential of -85 mV to 0 mV were used, the duration of the depolarizing pulses was varied from 4 ms (circles) to 20 ms (downward triangles), 40 ms (squares), and 100 ms (upward triangles). The recovery interval was kept constant at 500 ms, corresponding to a frequency of approximately 2 Hz. The cell was superfused with 1.2 mM sevoflurane.