Reconstitution of Single Potassium Channels from Bovine Gall­ bladder Epithelium*

The study of molecular transport across gall-bladder epithe­ lium may contribute to our understanding of the pathophys­ iology of gall-bladder disease. The aim of this study was to reconstitute and characterize single potassium ion channels in bovine gall-bladder epithelial mucosa - both apical and basolateral aspects. Standard subcellular fractionation tech­ niques were used to form either apical or basolateral closed­ membrane vesicles from the mucosal epithelium offresh gall bladders from healthy young adult cattle. Vesicular ion channels were incorporated into voltage-clamped planar lipid bilayers under known ionic conditions and their con­ ductances, reversal potentials, and voltages were character­ ized. Low-conductancevoltage-insensitive apical membrane vesicle channels of at least four conductance levels were found (mean ± SD): 12 ± 4 pS, n = 10; 40 ± 12 pS, n = 4; 273 ± 31 pS, n =3; and 151 ± 24 pS, n =5. Conductances of potassium ion channels in basolateral membrane vesicles were in the range 9 - 450 pS, and these channels included high-conductance calcium-activated potassium-ion chan­ nels 'K(Ca)' which were voltage- and calcium-dependent.