Jm. Mcgill et al., GTP-BINDING PROTEINS REGULATE HIGH-CONDUCTANCE ANION CHANNELS IN RAT BILE-DUCT EPITHELIAL-CELLS, The Journal of membrane biology, 133(3), 1993, pp. 253-261
Epithelial cells from the intrahepatic bile duct contribute to bile fo
rmation, but little is known of the cellular mechanisms responsible. I
n these studies, we have characterized the endogenous GTP-binding prot
eins (G proteins) present in these cells and evaluated their role in r
egulation of high conductance anion channels. G proteins were identifi
ed in purified plasma membranes of isolated bile duct epithelial cells
using specific antisera on Western blots, and ion channel activity wa
s measured in excised inside-out membrane patches using patch-clamp re
cording techniques. In patches without spontaneous channel activity, a
ddition 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 proba
bility. Activated channels were anion selective (gluconate/Cl- permeab
ility ratio of 0.17 +/- 0.04) and had a unitary conductance of approxi
mately 380 pS. Channel open probability was also increased by the nonh
ydrolyzable GDP analogue guanosine 5'-0-(2-thiodiphosphate) in 8/14 (5
7%) attempts. In contrast, channel open probability was rapidly and re
versibly decreased by the nonhydrolyzable analogue of GTP 5' guanylyli
midodiphosphate in 7/9 (78%) attempts. Western blotting with specific
antisera revealed that both G(i)alpha-2 and G(i)alpha-3 were present i
n significant amounts, whereas G(i)alpha-1 and G(o)alpha were not dete
cted. These studies indicate that in bile duct epithelial cells, high
conductance anion channels are inhibited, in a membrane-delimited mann
er, by PTX-sensitive G proteins.