Fig. 3. Reversal potentials of K⁺ current and hyperpolarisation-induced current. (A) Sample traces of tail currents recorded subsequently in one fibre. Left: deactivating K⁺ currents, mainly consisting of A currents. For K⁺ current activation the fibre was first depolarised from –70 to +10 mV for 80 ms, then the voltage was stepped to different levels as seen in B. Right: deactivation of hyperpolarisation-dependent currents. For activation the voltage was stepped from –50 mV to –120 mV for 4 s, followed by deactivating jumps to –50 mV or more positive levels as seen in B. (B) Mean current–voltage relationships of peak tail-currents obtained, as in A, from paired recordings of four fibres from two animals. Note that the ordinate gives relative sizes of the currents, i.e. normalised to I_K at V=–30 mV and to hyperpolarisation-dependent current at –50 mV, respectively. [K⁺]_o and [Cl^–]_o were standard, i.e. 10 mmol l^–1 and 95 mmol l^–1, respectively. The mean reversal potential was –55.2±4.1 mV for the K⁺ current and –20.5±4.0 mV for the hyperpolarisation-activated current (l=2, f=4). The current electrode contained 3 mol l^–1 KCl; recordings of the hyperpolarisation-dependent currents were performed 5 to 10 min after impaling the fibre.