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Departments of Medicine, 1 University of Colorado Health Sciences Center, Denver, Colorado 80262; 2 Duke University Medical Center, Durham, North Carolina 27710; and 3 Mayo Clinic, Rochester, Minnesota 55905
To evaluate whether ATP in bile serves as a
signaling factor regulating ductular secretion, voltage-clamp studies
were performed using a novel normal rat cholangiocyte (NRC) model. In
the presence of amiloride (100 µM) to block
Na+ channels, exposure of the
apical membrane to ATP significantly increased the short-circuit
current (Isc)
from 18.2 ± 5.9 to 52.8 ± 12.7 µA
(n = 18). The response to
ATP is mediated by basolateral-to-apical Cl
transport because it is
inhibited by 1) the
Cl
channel blockers
4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (1 mM),
diphenylanthranilic acid (1.5 mM), or
5-nitro-2-(3-phenylpropylamino)benzoic acid (50 or 100 µM) in the
apical chamber, 2) the
K+ channel blocker
Ba2+ (5 mM), or
3) the
Na+-K+-2Cl
cotransport inhibitor bumetanide (200 µM) in the basolateral chamber.
Other nucleotides stimulated an increase in
Isc with a rank
order potency of UTP = ATP = adenosine 5'-O-(3)-thiotriphosphate, consistent with P2u purinergic
receptors. ADP, AMP, 2-methylthioadenosine 5'-triphosphate, and
adenosine had no effect. A cDNA encoding a rat
P2u receptor
(rP2uR) was isolated from a liver
cDNA library, and functional expression of the corresponding mRNA in
Xenopus laevis oocytes resulted in the
appearance of ATP-stimulated currents with a similar pharmacological
profile. Northern analysis identified hybridizing mRNA transcripts in
NRC as well as other cell types in rat liver. These findings indicate
that exposure of polarized cholangiocytes to ATP results in luminal
Cl
secretion through
activation of P2u receptors in the
apical membrane. Release of ATP into bile may serve as an autocrine or
paracrine signal regulating cholangiocyte secretory function.
liver; chloride channel; ion transport; bile
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