Toxin I from the snake Dendroaspis polylepis polylepis: a highly specific blocker of one type of potassium channel in myelinated nerve fiber.

Toxin I from the venom of the black mamba snake Dendroaspis polylepis polylepis specifically blocks one component of the K-current (IKf1) in the frog node of Ranvier, with a high affinity (Kd = 4 X 10(-10) M) without significantly affecting either the others components of the K current (IKf2 and IKs) or the Na-current. Moreover, for toxin concentrations corresponding to 10- or 100-fold the Kd value, the block was almost irreversible.