SODIUM, HYDROGEN ANTIPORTER ACTIVATION BY EXTRACELLULAR ADENOSINE-TRIPHOSPHATE IN BILIARY EPITHELIAL-CELLS

Background & Aims: Extracellular nucleotides are secretagogues and inf luence ion permeability, The aim of this study was to investigate whet her secretagogues activate transmembrane acid-base carriers, e.g., sod ium, hydrogen antiporter in cholangiocytes. Methods: Cells were loaded with pH and Ca2+-sensitive dyes. Intracellular changes in ion concent rations were monitored by confocal laser scanning microscopy. Adenosin e 3',5'-cyclic monophosphate was determined by standard methods. Resul ts: Baseline intracellular pH (pH,) averaged 7.28 +/- 0.17 pH units. A denosine triphosphate (ATP; 10 mu mol/L) increased baseline pH(i) by 0 .28 +/- 0.06 pH units/10 min (P < 0.01). Ten and 100 mu mol/L ATP incr eased 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 acid ATP-stimulated conditio ns, 1 mmol/L amiloride blocked pH(i) recovery, indicating true activat ion of Na+,H+ antiporter by extracellular ATP, Inhibition of basolater al Na+,H+ antiporter isoform inhibited stimulation by ATP. Na+,H+ anti porter-mediated proton flux was stimulated by adenosine, uridine triph osphate, adenosine-5'-O-(3-thiotriphosphate), alpha,beta-methylene-ATP , and 2-methylthio-ATP but not prevented by adenosine receptor blockin g. Activation by ATP was not influenced by the Ca2+/protein kinase C/c almodulin system but could be mimicked by addition of N-6,2'-O-dibutyr yladenosine-3',5'-cyclic monophosphate and was inhibited by pertussis toxin. Conclusions: Extracellular nucleotides may modulate secretory a nd absorptive function of cholangiocytes by activating Na+/H+ exchange mechanisms.