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 mu M) 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 mu M). 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 (1 mu M) 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. 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 a Cl--rich fluid while unstimulated. This became HCO3--rich when secretin was applied. 7. Addition of H2DIDS and MIA (10 mu M) 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.