Fig. 3. Use-dependent block of amitriptyline and bupivacaine. (A ) Pulse protocol and representative tracings. Holding potential was −140 mV, followed by a test pulse to +30 mV. Duration of the test pulse was 24 ms. This cycle was obtained for a total of 60 pulses at a frequency of 5 Hz. Tracings are shown at pulse number 1, 3, 5, 7, 10, 15, 20, 30...60 for control as well as amitriptyline and bupivacaine. (B ) After steady state was obtained at a concentration of 3 μm for amitriptyline and bupivacaine, the consecutive high-frequency stimulation revealed an additional block of 50.3 ± 0.3% for amitriptyline versus 14 ± 0.3% for bupivacaine. Even at high use-dependent stimulation, no block was achieved when no drug was applied. The data were well fitted by a single exponential function with a time constant of 5.9 ± 0.05 pulse for amitriptyline and 6.0 ± 0.2 pulse for bupivacaine.