Bicarbonate secretion in interlobular ducts from guinea-pig pancreas

1. The transport of HCO3- across the luminal membrane of pancreatic duct cells was studied by monitoring the luminal pH of isolated guinea-pig interlobular ducts after microinjection of an extracellular fluoroprobe, the dextran conjugate of 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF-dextran). Luminal Cl- concentration was also measured by microfluorometry following microinjection of the dextran conjugates of 6-methoxy-N-(4-aminoalkyl)quinolinium bromide (ABQ-dextran) and Cl-NERF (Cl-NERF-dextran). 2. When HCO3-/CO2 was admitted to the bath, a transient acidification of the duct lumen was observed, followed by a marked alkalinization. The latter was abolished when the luminal Cl- concentration was reduced to 25-35 mM by replacement with glucuronate and may. therefore, be attributed to Cl--HCO3- exchange at the luminal membrane. 3. Secretin, forskolin and acetylcholine stimulated HCO3- secretion into the lumen even when the luminal Cl- concentration was reduced to approximately 7 mM. Furthermore, agonist-evoked HCO3- secretion was not inhibited by luminal glibenclamide, dihydro-4,4'diisothiocyanostilbene-2,2'-disulphonic acid (H2DIDS) or 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB). These observations are not easily reconciled with HCO3- transport across the luminal membrane being mediated by Cl--HCO3- exchange in parallel with a Cl- conductance. 4. Agonist-stimulated HCO3- secretion was blocked by omitting Na+ from the bath but not by addition of N-methyl-N-isobutylamiloride (MIA) or bafilomycin A(1). This supports our previous conclusion that HCO3- entry into duct cells from the extracellular fluid requires Na+ but is not dependent on Na+-H+ exchange or vacuolar-type H+-ATPase activity. 5. The three actions of secretin on guinea-pig pancreatic duct cells described in this and the accompanying paper - stimulation of a relatively Cl--insensitive luminal HCO3- efflux pathway, stimulation of basolateral Na+-HCO3- cotransport, and lack of effect on intracellular pH - require the current model of pancreatic HCO3- secretion to be modified.