Gallamine triethiodide selectively blocks voltage-gated potassium channelsin Ranvier nodes

The effects of gallamine on ionic currents in single intact Ranvier nodes o f the toad Xenopus were investigated. The following fully reversible effect s were observed: 1. With a test concentration of 1 mmol/l the current-voltage relation of st eady-state potassium currents, IK,, exhibited a complete block of IK,, UP t o 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 PK wa s reduced by 92 % from its normal value, while the sodium permeability cons tant (P) over bar Na decreased by only 8%. 3. Concentration-response relations of the block of PK yielded an apparent dissociation constant of 30 mu mol/l and a steepness parameter of unity. Patch-clamp experiments on cloned Kv1.1, Kv1.2, Kv1.3 and Kv3.1 channels yi elded apparent dissociation constants of 86, 19, much greater than 100 and 121 mu mol/l, respectively. Our findings show that gallamine is particularl y well suited for separating potassium and sodium currents in axonal curren t 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 channel s.