Catechol: a potent and specific inhibitor of the fast potassium channel in frog primary afferent neurones.

The effects of catechol on various ionic channels of isolated primary afferent neurones of the bull‐frog were examined by a single‐suction‐electrode clamp system, which combined internal perfusion and current or voltage clamp using an electronic switching circuit. Catechol was found to inhibit rather specifically the fast K+ current as does 4‐aminopyridine (4‐AP). Ca2+, Na+ and slow K+ currents were not affected. Although both 4‐AP and catechol were inhibitors of the fast K+ channels, their sites of action were quite different. Catechol was effective when applied on the external surface of the cell membrane whereas 4‐AP acted preferably internally. We assumed that a single fast K+ channel has two distinct sites for blockers: the catechol site is exposed to the external medium or situated at the outer orifice of the pore, and the 4‐AP site is located within the same channel but is more easily accessible from inside the nerve cell than outside. The 4‐AP and catechol sites were not, however, completely separate and independent of each other since a synergistic interaction was observed between catechol and 4‐AP.