Human axons contain at least five types of voltage‐dependent potassium channel

1 We investigated voltage‐gated potassium channels in human peripheral myelinated axons; apart from the I, S and F channels already described in amphibian and rat axons, we identified at least two other channel types. 2 The I channel activated between ‐70 and ‐40 mV, and inactivated very slowly (time constant 13.1 s at ‐40 mV). It had two gating modes: the dominant (‘noisy’) mode had a conductance of 30 pS (inward current, symmetrical 155 mM K + ) and a deactivation time constant (τ) of 25 ms (‐80 mV); it accounted for most (≈50‐75 %) of the macroscopic K + current in large patches. The secondary (‘flickery’) gating mode had a conductance of 22 pS, and showed bi‐exponential deactivation (τ= 16 and 102 ms; ‐80 mV); it contributed part of the slow macroscopic K + current. 3 The I channel current was blocked by 1 μM α‐dendrotoxin (DTX); we also observed two other DTX‐sensitive K + channel types (40 pS and 25 pS). The S and F channels were not blocked by 1 μM DTX. 4 The conductance of the S channel was 7‐10 pS, and it activated at slightly more negative potentials than the I channel; its deactivation was slow (τ= 41.7 ms at ‐100 mV). It contributed a second component of the slow macroscopic K + current. 5 The F channel had a conductance of 50 pS; it activated at potentials between ‐40 and +40 mV, deactivated very rapidly (τ= 1.4 ms at ‐100 mV), and inactivated rapidly (τ= 62 ms at +80 mV). It accounted for the fast‐deactivating macroscopic K + current and partly for fast K + current inactivation. 6 We conclude that human and rat axonal K + channels are closely similar, but that the correspondence between K + channel types and the macroscopic currents usually attributed to them is only partial. At least five channel types exist, and their characteristics overlap to a considerable extent.