Figure 2.

Transient outward currents are carried mainly by K⁺. (A) Whole-cell current recordings under conditions with 100 mM Cs⁺ in the pipette solution. The voltage protocol was similar to that given in Fig. 1B (note large K⁺_in currents at potentials negative of −100 mV). Arrow shows zero current level. (B) Tail current recordings of the transient outward currents. From a holding potential of −180 mV, transient outward currents were activated by a voltage step to +100 mV. Tail currents of the transient outward currents were recorded at the indicated membrane potentials (I_AP Tail). (C) Tail currents of K⁺_in currents recorded in the same cell as in B. After a prepulse potential of −180 mV, where K⁺_in currents were activated, tail currents were recorded at the indicated membrane potentials ranging from −140 mV to +80 mV with +20 mV increments (K⁺_in Tail). At increasingly depolarized tail potentials, the decay times of K⁺_in current deactivation became shorter and therefore could be clearly distinguished from the transient outward currents. (D) Tail currents from recordings in B and C were plotted as a function of applied membrane potentials. ru, relative units. I_AP tail currents and K⁺_in tail currents were normalized by the currents at +40 mV and −60 mV, respectively. Pipette solutions with zero [Ca²⁺]_i and the standard bath solution were used.