Figure 3. Effects of two different extracellular [K⁺] on peak I–V relationship and activation properties.

A, experimental I–V relationship for I_Kv4.3, 2 mm (○, n = 5) and 98 mm (•, n = 4). Data were obtained using 1000 ms voltage steps from a holding potential of −90 mV to variable voltages ranging between −120 and +50 mV in 10 mV steps (see inset in A). B, experimental steady-state activation functions were obtained for I_Kv4.3 using a two-pulse protocol. Ten-millisecond voltage pulses, P1, applied from a holding potential −90 mV ranged between −120 and +50 mV (10 mV increment), with P2 set at −40 mV and 500 ms duration (inset in B). Data recorded at [K⁺]_o 2 (n = 3) and 98 mm (n = 5), are represented by ○ and •, respectively. C, the time constants of activation (τ_act) were determined by fitting the rising phase of I_Kv4.3 obtained during both 10 and 1000 ms depolarizing pulses (inset in C). Current traces were then fitted with the function f_a(t) = C(1 − exp(−t/τ_act))⁴. The data obtained with a 1000 ms pulse are shown by ○ (2 mm[K⁺]_o, n = 5) and • (98 mm[K⁺]_o, n = 4). Experimental data obtained at 2 mm[K⁺]_o using a 10 ms pulse (n = 4) are shown as □. Dashed and continuous lines show fits of τ_act(V) with single- and double-exponential functions, respectively. All data shown in this figure were obtained using the cut-open oocyte voltage clamp technique.