Fluid secretion in interlobular ducts isolated from guinea-pig pancreas

1. Pancreatic HCO₃^- and fluid secretion were studied by monitoring luminal pH (pH(L)) and luminal volume simultaneously in interlobular duct segments isolated from guinea-pig pancreas. The secretory rate and HCO₃^- flux were estimated from fluorescence images obtained following microinjection of BCECF-dextran (70 kDa, 20 μM) into the duct lumen. 2. Ducts filled initially with a Cl^--rich solution swelled steadily (2.0 nl min^-1 mm^-2) when HCO₃^-/CO₂ was introduced, and the luminal pH increased to 8.08. When Cl^- was replaced by glucuronate, spontaneous fluid secretion was reduced by 75%, and pH(L) did not rise above 7.3. 3. Cl^--dependent spontaneous secretion was largely blocked by luminal H₂DIDS (500 μM). We conclude that, in unstimulated ducts, HCO₃^- transport across the luminal membrane is probably mediated by Cl^--HCO₃^- exchange. 4. Secretin (10 nM) and forskolin (1μM ) both stimulated HCO₃^- and fluid secretion. The final value of pH(L) (8.4) and the increase in secretory rate (1.5 nl min_-1 mm^-2) after secretin stimulation were unaffected by substitution of Cl^-. 5. The Cl^--independent component of secretin-evoked secretion was not affected by luminal H₂DIDS. This suggests that a Cl^--independent mechanism provides the main pathway for luminal HCO₃^- transport in secretin-stimulated ducts. 6. Ducts filled initially with a HCO₃^--rich fluid (125 mM HCO₃^-, 23 mM Cl^-) secreted Cl^--rich fluid while unstimulated. This became HCO₃^--rich when secretin was applied. 7. Addition of H₂DIDS and MIA (10 μM) to the bath reduced the secretory rate by 56 and 18%, respectively. Applied together they completely blocked fluid secretion. We conclude that basolateral HCO₃^- transport is mediated mainly by Na⁺-HCO₃^- cotransport rather than by Na⁺-H⁺ exchange.