Artifactual expression of maxi-K+ channels in basolateral membrane of gallbladder epithelial cells

To patch clamp the basolateral cell membrane, sheets of Necturus gallbladder epithelium were stripped of the subepithelial tissue layers and affixed apical side down on cover slips coated with Cell-Tak [F. Wehner, L. Garretson, K. Dawson, Y. Segal, and L. Reuss. Am. J. Physiol. 258 (Cell Physiol. 27): C1159-C1164, 1990]. In 90% of the patches we observed K+ channels identical to the maxi-K+ channels previously demonstrated in the apical membrane (Y. Segal and L. Reuss. J. Gen. Physiol. 95: 791-818, 1990). To ascertain whether these channels were present in the native tissue, we carried out intracellular-microelectrode studies. We tested for activation of basolateral membrane K+ conductance by depolarization or by elevation of intracellular Ca2+ and for tetraethylammonium sensitivity of the basolateral membrane voltage and fractional resistance. The results were negative, indicating that maxi-K+ channels are not expressed in the basolateral membrane of the "intact" epithelium. Using the same intracellular-microelectrode protocol on the apical membrane, we demonstrated the presence of an apical K+ conductance attributable to maxi-K+ channels. Additional experiments revealed a Ba(2+)-sensitive basolateral K+ conductance in the native epithelium. We conclude that in the stripped preparation there is artifactual expression of maxi-K+ channels. In addition, the native basolateral membrane K+ channels either are not expressed in this preparation or have a low conductance and cannot be discerned from the background noise.