Maxi K⁺ channels and their relationship to the apical membrane conductance in Necturus gallbladder epithelium

Using the patch-clamp technique, we have identified large-conductance (maxi) K⁺ channels in the apical membrane of Necturus gallbladder epithelium, and in dissociated gallbladder epithelial cells. These channels are more than tenfold selective for K⁺ over Na⁺, and exhibit unitary conductance of ∼200 pS in symmetric 100 mM KCl. They are activated by elevation of internal Ca²⁺ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca²⁺ sensitivities of the macroscopic apical membrane conductance (G_a). G_a was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180 μS/cm² at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 μS/cm² at -25 mV. We have related maxi K⁺ channel properties and G_a quantitatively, relying on the premise that at any apical membrane voltage G_a comprises a leakage conductance and a conductance due to maxi K⁺ channels. Comparison between G_a and maxi K⁺ channels reveals that the latter are present at a surface density of 0.09/μm², are open ∼15% of the time under control conditions, and account for 17% of control G_a. Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca₂₊ and voltage dependencies of single maxi K⁺ channels, results from intracellular microelectrode experiments indicate that maxi K⁺ channel activity in situ is higher than predicted from the measured apical membrane voltage and estimated bulk cytosolic Ca²⁺ activity. Mechanisms that could account for this finding are proposed.