The ACh-evoked, Ca2+-activated whole-cell K+ current in mouse mandibular secretory cells. Whole-cell and fluorescence studies

In our previous studies on sheep parotid secretory cells, we showed that the K+ current evoked by acetylcholine (ACh) was not carried by the high-conductance voltage- and Ca2+-activated K+ (BK) channel which is so conspicuous in unstimulated cells, notwithstanding that the BK channel is activated by ACh. Since several studies from other laboratories had suggested that the BK channel did carry the ACh-evoked K+ current in the secretory cells of the mouse mandibular gland, and that the current could be blocked with tetraethylammonium (TEA), a known blocker of BK channels, we decided to investigate the ACh-evoked K+ current in mouse cells more closely. We studied whether the ACh-evoked K+ current in the mouse is inhibited by TEA and quinine. Using the whole-cell patch-clamp technique and microspectrofluorimetric measurement of intracellular Ca2+, we found that TEA and quinine do inhibit the ACh-evoked K+ current but that the effect is due to inhibition of the increase in intracellular Ca2+ evoked by ACh, not to blockade of a K+ conductance. Furthermore, we found that the K+ conductance activated when ionomycin is used to increase intracellular free Ca2+ was inhibited only by quinine and not by TEA. We conclude that the ACh-evoked K+ current in mouse mandibular cells does not have the blocker sensitivity pattern that would be expected if it were being carried by the high-conductance, voltage- and Ca2+-activated K+ (BK) channel. The properties of this current are, however, consistent with those of a 40 pS K+ channel that we have reported to be activated by ACh in these cells [16].