SECRETIN CAUSES H+ HCO3- SECRETION FROM PIG PANCREATIC DUCTULES BY VACUOLAR-TYPE H+-ADENOSINE TRIPHOSPHATASE/

Background/Aims: Secretin stimulates pancreatic ductules to secrete HC O3- into pancreatic juice and H+ into interstitial fluid. The aim of t he present study was first to examine whether ductular H+ secretion is inhibited by micromolar concentrations of bafilomycin A(1), which blo cks vacuolar H+-adenosine triphosphatase by specific action, and secon dly to test for evidence of ductular Na+/HCO3- cotransport. Methods: D uctular H+ secretion was estimated from the rate of intracellular pH r ecovery after acid-loading (24 mmol/L NH4Cl) microdissected pancreatic ductules from pig, mounted in a flow-through perfusion chamber on the stage of a fluorescent microscope. Intracellular pH was measured usin g the fluorescent pH indicator 2'7'-bis(carboxyethyl)-5,6-carboxyfluor escein and dual-wavelength excitation of fluorescence. The ducts were superfused with either HCO3--free HEPES-containing buffers or HCO3--co ntaining buffers. Results: Secretin (10(-8) mol/L) induced a net H+ se cretion of 1.87 +/- 0.23 mu mol . mL cell vol(-1). min(-1) that was bl ocked by 10(-6) mol/L bafilomycin A(1) and was unaffected by Na+ subst itution with choline using HEPES superfusion buffers. Secretin-stimula ted ductules superfused with bicarbonate-containing, Cl--free buffers showed Na+-dependent and 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid-inhibitable alkalinization of intracellular pH. Conclusions: Secr etin causes H+/HCO3- secretion from pancreatic ductules by a mechanism involving vacuolar-type H+-adenosine phosphatase. Pancreatic ductules also show Na+/HCO3- cotransport, which may account for a small fracti on of secreted bicarbonate.