SECRETIN CAUSES H-TYPE H+-ATPASE( SECRETION FROM INTRAHEPATIC BILE DUCTULES BY VACUOLAR)

Intrahepatic bile duct epithelial cells contribute to bile formation b y hormone-dependently secreting HCO3- to bile and H+ to periductual fl uid. The present study was undertaken to determine whether the secreti n-induced H+ secretion is due to activation of a H+ --ATPase or Na+-H exchange. H+ secretion was estimate from the rate of intracellular pH (pH(i)) recovery after acid loading (24 mM NH3Cl) of microdissected b ile ductules from pig liver mounted in a flow-through chamber on the s tage of a microscope, pH(i) was measured from an estimated average of 10-15 epithelial cells using the fluorescent pH(i) indicator 2',7'-bis ((carboxyethyl)-5,6-carboxyfluorescein and dual-wavelength excitation of fluorescence. The ducts were superfused with HCO3- free N-2-hydroxy ethylpiperazine-N'-2-ethanesulfonic acid buffers. We found that secret in induced net H+ secretion of 4.53 +/- 0.7 mumol . ml cell volume-1 . min-1. This H+ secretion was blocked by 10(-6) M bafilomycin A1 but w as unaffected by Na+ substitution with choline in the superfusion buff er. 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 th e cellular mechanisms causing secretin-dependent ductular HCO3- secret ion into bile.