Maxi-K+ channel in single isolated cochlear efferent nerve terminals.

Patch clamp recordings were obtained from isolated cochlear efferent nerve terminals. Channel activity was found in 85% of membrane patches, was present in on-terminal and excised patches and was characterized to originate from a maxi-K+ channel. An average of 2.0 +/- 0.1 (N = 33) maxi-K+ channels were found per active patch. In symmetrical solutions, the current-voltage relationship was linear and the single-channel conductance was 221 +/- 5 pS (N = 22). The open probability of the maxi-K+ channel increased with depolarization of the membrane potential and with an increasing free Ca2+ concentration on the cytosolic side. The open probability was insensitive to changes in the free Ca2+ concentration on the extracellular side. TEA (20 mM) and charybdotoxin (10(-7) M) decreased the open probability to nearly zero from the extracellular side but had no effect from the cytosolic side. The high incidence with which this channel was found suggests that the maxi-K+ channel is physiologically relevant which might include protection against overstimulation of the efferent synapse.