Maxi K+ channels from the apical membranes of rabbit oviduct epithelial cells

Large conductance (approximately 210 pS), K(+)-selective channels were identified in excised, inside-out patches obtained from the apical membranes of both ciliated and nonciliated epithelial cells grown as monolayers from the primary culture of rabbit oviduct. The open probability of channels showing stable gating was increased at positive membrane potentials and was sensitive to the concentration of free calcium ions at the cytosolic surface of the patch ([Ca2+]i). In these respects, the channel resembled "maxi K+ channels" found in a number of other cell types. The distributions of dwelltimes in the open state were most consistently described by two exponential components. Four exponential components were fitted to the distributions of dwelltimes in the closed state. Depolarizations and [Ca2+]i increases had similar effects on the distribution of open dwelltimes, causing increases in the two open time constants (tau o1 and tau o2) and the fraction of events accounted for by the longer component of the distribution. In contrast, calcium ions and voltage had distinct effects on the distribution of closed dwelltimes. While the three shorter closed time constants (tau c1, tau c2 and tau c3) were reduced by depolarizing membrane potentials, increases in [Ca2+]i caused decreases in the longer time constants (tau c3 and tau c4). It is concluded that oviduct large conductance Ca(2+)-activated K+ channels can enter at least two major open states and four closed states.