Cyclic AMP regulates bicarbonate secretion in cholangiocytes through release of ATP into bile.
OBJECTIVE
Bicarbonate secretion is a primary function of cholangiocytes. Either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) can mediate bicarbonate secretion, but these are thought to act through separate pathways. We examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R) in mediating bicarbonate secretion because this is the only intracellular Ca(2+) release channel in cholangiocytes.


METHODS
Intrahepatic bile duct units (IBDUs) were microdissected from rat liver then luminal pH was examined by confocal microscopy during IBDU microperfusion. Cyclic AMP was increased using forskolin or secretin, and Ca(2+) was increased using acetylcholine (ACh) or adenosine triphosphate (ATP). Apyrase was used to hydrolyze extracellular ATP, and suramin was used to block apical P2Y ATP receptors. In selected experiments, IBDUs were pretreated with short interfering RNA (siRNA) to silence expression of specific InsP3R isoforms.


RESULTS
Both cAMP and Ca(2+) agonists increased luminal pH. The effect of ACh on luminal pH was reduced by siRNA for basolateral (types I and II) but not apical (type III) InsP3R isoforms. The effect of forskolin on luminal pH was reduced by a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor and by siRNA for the type III InsP3R. Luminal apyrase or suramin blocked the effects of forskolin but not ACh on luminal pH.


CONCLUSIONS
Cyclic AMP-induced ductular bicarbonate secretion depends on an autocrine signaling pathway that involves CFTR, apical release of ATP, stimulation of apical nucleotide receptors, and then activation of apical, type III InsP3Rs. The primary role of CFTR in bile duct secretion may be to regulate secretion of ATP rather than to secrete chloride and/or bicarbonate.
Minagawa
Noritaka
N
Department of Medicine, Yale University School of Medicine, New Haven, Connecticut, USA.
Nagata
Jun
J
Shibao
Kazunori
K
Masyuk
Anatoliy I
AI
Gomes
Dawidson A
DA
Rodrigues
Michele A
MA
Lesage
Gene
G
Akiba
Yasutada
Y
Kaunitz
Jonathan D
JD
Ehrlich
Barbara E
BE
Larusso
Nicholas F
NF
Nathanson
Michael H
MH
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001081:Apyrase / Q000494:pharmacology; D001639:Bicarbonates / Q000378:metabolism*; D001646:Bile / Q000378:metabolism*; D001653:Bile Ducts, Intrahepatic / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000037:antagonists & inhibitors / Q000502:physiology; D005765:Gastrointestinal Agents / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000187:drug effects / Q000502:physiology; D008297:Male; D020033:Protein Isoforms / Q000502:physiology; D034741:RNA, Small Interfering / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012633:Secretin / Q000494:pharmacology; D013498:Suramin / Q000494:pharmacology
D001639:Bicarbonates; D005765:Gastrointestinal Agents; D053496:Inositol 1,4,5-Trisphosphate Receptors; D020033:Protein Isoforms; D034741:RNA, Small Interfering; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D012633:Secretin; D005576:Colforsin; D013498:Suramin; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D001081:Apyrase; D002118:Calcium