Sodium, hydrogen antiporter activation by extracellular adenosine triphosphate in biliary epithelial cells
C Elsing1, A Kassner, W Stremmel
1Department of Medicine, University of Heidelberg, Germany. christoph-elsing@krzmail.krz.uni-heidelberg.de
Background & Aims:
Extracellular nucleotides are secretagogues and influence ion permeability. The aim of this study was to investigate whether secretagogues activate transmembrane acid-base carriers, e.g., sodium, hydrogen antiporter in cholangiocytes.
Methods:
Cells were loaded with pH and Ca(2+)-sensitive dyes. Intracellular changes in ion concentrations were monitored by confocal laser scanning microscopy. Adenosine 3', 5'-cyclic monophosphate was determined by standard methods.
Results:
Baseline intracellular pH (pH1) averaged 7.28 +/- 0.17 pH units. Adenosine triphosphate (ATP; 10 mumol/L) increased baseline pH1 by 0.28 +/- 0.06 pH units/10 min (P < 0.01). Ten and 100 mumol/L ATP increased Na+, H+ antiporter-mediated proton flux from 9.4 +/- 4.9 mmol. L-1 min-1 to 17.2 +/- 11.8 and 16.7 +/- 10.3 mmol. L(-1)-min(-1) (P < 0.001), respectively. Under control and ATP-stimulated conditions, 1 mmol/L amiloride blocked pH1 recovery, indicating true activation of Na+, H+ antiporter by extracellular ATP. Inhibition of basolateral Na+, H+ antiporter isoform inhibited stimulation by ATP. Na+, H+ antiporter-mediated proton flux was stimulated by adenosine, uridine triphosphate, adenosine-5'-0-(3-thiotriphosphate), alpha, beta-methylene-ATP, and 2-methylthio-ATP but not prevented by adenosine receptor blocking. Activation by ATP was not influenced by the Ca2+/protein kinase C/calmodulin system but could be mimicked by addition of N6,2'-O-dibutyryladenosine-3',5'-cyclic monophosphate and was inhibited by pertussis toxin.
Conclusions:
Extracellular nucleotides may modulate secretory and absorptive function of cholangiocytes by activating Na+/H+ exchange mechanisms.
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