Fig. 4. Time constants of inactivation and voltage-dependence of recovery from inactivation of Na⁺ currents of inner muscle. (A) The decaying phase of Na⁺ currents illustrated in Fig. 3B were fitted to single exponentials and the time constants plotted against the amplitude of the depolarizing pulse. The rate of inactivation can be seen to be strongly voltage-dependent. (B) The rate of recovery of inactivation was measured using a paired two-pulse protocol. A 5 ms depolarization to 20 mV was applied to completely inactivate the Na⁺ currents. This was then followed by a 0.5–9 ms recovery step to one of a range of potentials (from –70 to –150 mV in 10 mV steps). The amplitude of currents evoked by a second test pulse (of identical duration and amplitude to the first) was plotted against the duration of the recovery period. The data thus derived were fitted to a single exponential function, the estimated time constants of which were plotted against the membrane potential of the recovery period (C), revealing a strong voltage dependence of the rate of recovery of inactivation. (D) The inward currents observed in Na⁺-isolating salines are blocked by the application of micromolar concentrations of TTX, confirming their identity as Na⁺ currents.