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.