Gallamine triethiodide selectively blocks voltage-gated potassium channels in Ranvier nodes

The effects of gallamine on ionic currents in single intact Ranvier nodes of the toad Xenopus were investigated. The following fully reversible effects were observed: 1. With a test concentration of 1 mmol/1 the current-voltage relation of steady-state potassium currents, I_Kss exhibited a complete block of I_Kss up to about V = 110 mV; with stronger depolarisations the block was incomplete. The peak sodium currents, in contrast, were not affected. 2. At the same test concentration the potassium permeability constant P̄_K was reduced by 92 % from its normal value, while the sodium permeability constant P̄_Na decreased by only 8%. 3. Concentration-response relations of the block of P̄_K yielded an apparent dissociation constant of 30 μmol/1 and a steepness parameter of unity. Patch-clamp experiments on cloned Kv1.1, Kv1.2, Kv1.3 and Kv3.1 channels yielded apparent dissociation constants of 86, 19, ≫100 and 121 μmol/1, respectively. Our findings show that gallamine is particularly well suited for separating potassium and sodium currents in axonal current ensembles. They also strongly suggest that potassium currents in Ranvier nodes of Xenopus are mainly carried by an ensemble of Kv1.1 and 1.2 channels.