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

Abstract

Using the patch-clamp technique, we have identified large-conductance (maxi) K § channels in the apical membrane of Neaurus 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 KC1. They are activated by elevation of internal Ca 2+ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca ~+ sensitivities of the macroscopic apical membrane conductance (Ga). Ga was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180/~S/cm ~ at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 #S/cm 2 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~ and maxi K § channels reveals that the latter are present at a surface density of 0.09//zm 2, are open ~15% of the time under control conditions, and account for 17% of control G v Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca 2 § 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 Ca ~ § activity. Mechanisms that could account for this finding are proposed.

Stretch-activated single K + channels account for whole-cell currents elicited by swelling

“…2 A (Insert). At more positive voltages, the relative increase in I K is smaller because of the supralinear contribution of apical-membrane maxi-K ϩ channels activated by membrane depolarization (19). Fig.…”

“…The electrodiffusive Na ϩ permeability of both apical and basolateral membranes is negligible (20,21). From intracellularmicroelectrode studies in the assembled epithelium, the K ϩ current is likely to correspond to both apical-membrane maxi-K ϩ channels (19) and to basolateral-membrane channels with different properties (17,22), whereas the increase in K ϩ conductance by cell swelling is entirely basolateral (18). In the absence of protein kinase A or protein kinase C activation, the Cl Ϫ current is exclusively basolateral (23,24).…”

Effects of changes in mucosal solution Cl- or K+ concentration on cell water volume of Necturus gallbladder epithelium.

“…3 and 4). This difference is attributable to activation of apical membrane maxi K + channels by the depolarization elicited by elevating external [K + ] (Segal and Reuss, 1990a).…”

Cell-Volume Changes and Ion Conductances in Amphibian Gallbladder Epithelium

“…The function of the voltage-sensitive Clc hannels could be to prevent cell shrinkage when [Ca 2+ ] i is elevated , e.g., in response to cholinergic agonists [14]. Under these conditions, the hyperpolarization by activation of maxi-K + channels [7] would enhance Cl -loss. Thus, this particular cell type, and may be others, abhor shrinkage more than swelling.…”

“…I K was measured at E Cl (-3 mV and 4 mV in isosmotic and hyposmotic solutions, respectively) and I Cl was measured at E K (-88 mV). From previous studies, the K + current is likely to include apical-membrane maxi-K + channels [7] and basolateral-membrane channels with different properties [8], whereas the increase in K + conductance by cell swelling is entirely basolateral [5]. In the absence of PKA or PKC activation, the Cl -current is exclusively basolateral [9].…”