Secretin causes H+ secretion from intrahepatic bile ductules by vacuolar-type H(+)-ATPase

Intrahepatic bile duct epithelial cells contribute to bile formation by hormone-dependently secreting HCO3- to bile and H+ to periductular fluid. The present study was undertaken to determine whether the secretin-induced H+ secretion is due to activation of a H(+)-ATPase or Na(+)-H+ exchange. H+ secretion was estimated from the rate of intracellular pH (pHi) recovery after acid loading (24 mM NH4Cl) of microdissected bile ductules from pig liver mounted in a flow-through chamber on the stage of a microscope. pHi was measured from an estimated average of 10-15 epithelial cells using the fluorescent pHi indicator 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein and dual-wavelength excitation of fluorescence. The ducts were superfused with HCO3(-)-free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffers. We found that secretin induced net H+ secretion of 4.53 +/- 0.7 mumol.ml cell volume-1 x min-1. This H+ secretion was blocked by 10(-6) M bafilomycin A1 but was unaffected by Na+ substitution with choline in the superfusion buffer. The experiments also showed that bafilomycin A1 did not block Na(+)-H+ exchange. The secretin-induced H+ secretion is probably caused by a vacuolar-type H(+)-ATPase and may constitute an important element of the cellular mechanisms causing secretin-dependent ductular HCO3- secretion into bile.