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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.
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