Inactivation of Nav current by AP clamp waveforms. (A) Top trace shows an AP recorded from a rat CC with the perforated patch-method with normal Na⁺ and K⁺ gradients. The AP waveform was then used as a voltage-clamp waveform for all traces in A and B. For a given train of AP clamp waveforms, a cell was held at different holding potentials, −50 through −80 mV, before and between each AP clamp waveform. For the example in A, 100 ms separated the initiation of each individual AP clamp waveform. The traces below the AP waveform show currents activated from a holding potential (h.p.) of −50 mV or −80 mV (bottom), for the first and fifth evoked current in a 10 Hz train. Note that the inward current during the falling phase of the AP command waveform does not change in amplitude between the first and fifth waveform, reflecting Cav current, while the early inward current shows marked diminution, indicative of Nav current inactivation. (B) The full command waveforms for the first four APs in a 10-pulse train are shown along with the currents evoked by the AP command waveforms for holding potentials from −50 mV through −80 mV.