Chapter 3 Properties of Epithelial Potassium Channels

Basolateral membrane potassium conductance of A6 cells

To study the properties of the basolateral membrane conductance of an amphibian epithelial cell line, we have adapted the technique of apical membrane selective permeabilization (Wills, N.K., Lewis, S.A., Eaton, D.C. 1979b, J. Membrane Biol. 45:81-108). Monolayers of A6 cells cultured on permeable supports were exposed to amphotericin B. The apical membrane was effectively permeabilized, while the high electrical resistance of the tight junctions and the ionic selectivity of the basolateral membrane were preserved. Thus the transepithelial current-voltage relation reflected mostly the properties of the basolateral membrane. Under "basal" conditions, the basolateral membrane conductance was inward rectifying, highly sensitive to barium but not to quinidine. After the induction of cell swelling either by adding chloride to the apical solution or by lowering the osmolarity of the basolateral solution, a large outward-rectifying K+ conductance was observed, and addition of barium or quinidine to the basolateral side inhibited, respectively, 82.4 +/- 1.9% and 90.9 +/- 1.0% of the transepithelial current at 0 mV. Barium block was voltage dependent; the half-inhibition constant (Ki) varied from 1499 +/- 97 microM at 0 mV to 5.7 +/- 0.5 microM at -120 mV. Cell swelling induces a large quinidine-sensitive K+ conductance, changing the inward-rectifying basolateral membrane conductance observed under "basal" conditions into a conductance with outward-rectifying properties.

Reconstitution of a calcium-activated potassium channel in basolateral membranes of rabbit colonocytes into planar lipid bilayers

A highly enriched preparation of basolateral membrane vesicles was isolated from rabbit distal colon surface epithelial cells employing the method described by Wiener, Turnheim and van Os (Weiner, H., Turnheim, K., van Os, C.H. (1989) J. Membrane Biol. 110:147-162) and incorporated into planar lipid bilayers. With very few exceptions, the channel activity observed was that of a high conductance. Ca2+-activated K+ channel. This channel is highly selective for K+ over Na+ and Cl-, displays voltage-gating similar to "maxi" K(Ca) channels found in other cell membranes, and kinetic analyses are consistent with the notion that K+ diffusion through the channel involves either the binding of a single K+ ion to a site within the channel or "single-filing" ("multi-ion occupancy"). Channel activity is inhibited by the venom from the scorpion Leiurus quinquestriatus, Ba2+, quinine, and trifluoperazine. The possible role of this channel in the function of these cells is discussed.

Osmotically induced basolateral K+ conductance in turtle colon: lidocaine-induced K+ channel noise

The basolateral membrane of amphotericin-treated turtle colon can exhibit two distinct types of K+ conductance, one of which is associated with cell swelling and is blocked by quinidine or lidocaine. Fluctuations in basolateral K+ currents were analyzed under swelling (mucosal KCl) and nonswelling (mucosal K gluconate) conditions. Under nonswelling conditions, it was not possible to detect a spontaneous Lorentzian component in the power density spectrum (PDS) and the addition of lidocaine neither inhibited the macroscopic current nor induced a Lorentzian component in the PDS. Under swelling conditions, however, lidocaine induced a Lorentzian component in the PDS and the corner frequency increased linearly with blocker concentration as expected for reversible blockade of the channel. The gating and conductance properties of osmotically induced channels estimated from a two-state model were similar to those determined recently in single-channel recordings from isolated colonic cells.