GTP-binding proteins regulate high conductance anion channels in rat bile duct epithelial cells

Epithelial cells from the intrahepatic bile duct contribute to bile formation, but little is known of the cellular mechanisms responsible. In these studies, we have characterized the endogenous GTP-binding proteins (G proteins) present in these cells and evaluated their role in regulation of high conductance anion channels. G proteins were identified in purified plasma membranes of isolated bile duct epithelial cells using specific antisera on Western blots, and ion channel activity was measured in excised inside-out membrane patches using patch-clamp recording techniques. In patches without spontaneous channel activity, addition of cholera toxin to the cytoplasmic surface had no effect (n = 10). Addition of pertussis toxin caused an activation of channels in 13/34 (38%) attempts, as detected by an increase in channel open probability. Activated channels were anion selective (gluconate/Cl- permeability ratio of 0.17 +/- 0.04) and had a unitary conductance of approximately 380 pS. Channel open probability was also increased by the nonhydrolyzable GDP analogue guanosine 5'-0-(2-thiodiphosphate) in 8/14 (57%) attempts. In contrast, channel open probability was rapidly and reversibly decreased by the nonhydrolyzable analogue of GTP 5' guanylylimidodiphosphate in 7/9 (78%) attempts. Western blotting with specific antisera revealed that both Gi alpha-2 and Gi alpha-3 were present in significant amounts, whereas Gi alpha-1 and Go alpha were not detected. These studies indicate that in bile duct epithelial cells, high conductance anion channels are inhibited, in a membrane-delimited manner, by PTX-sensitive G proteins.