The ACh-evoked Ca2+-activated K+ current in mouse mandibular secretory cells. Single channel studies

Although acetylcholine (ACh) is able to activate voltage- and Ca2+-sensitive K+ (BK) channels in mouse mandibular secretory cells, our recent whole cell studies have suggested that these channels, like those in sheep parotid secretory cells, do not contribute appreciably to the conductance that carries the ACh-evoked whole cell K+ current. In the present study, we have used cell-attached patch clamp methods to identify and characterize the K+ channel type responsible for carrying the bulk of this current. When the cells were bathed in a NaCl-rich solution the predominant channel type activated by ACh (1 mu mol/l or 50 nmol/l) had a conductance only of 40 pS; it was not blocked by TEA but it was sensitive to quinine and it conducted Rb+ to an appreciable extent. BK channels, which could be seen in some but not all patches from resting cells, also showed increased activity when ACh was added to the bath, but they were much less conspicuous during ACh stimulation than the 40-pS channels. When the cells were bathed in a KCl-rich rather than a NaCl-rich solution, a small-conductance K+ channel, sensitive to quinine but not to TEA, was still the most conspicuous channel to be activated by ACh although its conductance was reduced to 25 pS. Our studies confirm that the ACh-evoked whole-cell K+ current is not carried substantially by BK channels and show that it is carried by a small-conductance K+ channel with quite different properties.