Fluid secretion in interlobular ducts isolated from guinea‐pig pancreas

Abstract

1. Pancreatic HCOצ and fluid secretion were studied by monitoring luminal pH (pHL) and luminal volume simultaneously in interlobular duct segments isolated from guinea-pig pancreas. The secretory rate and HCOצ flux were estimated from fluorescence images obtained following microinjection of BCECF-dextran (70 kDa, 20 ìÒ) into the duct lumen. 2. Ducts filled initially with a Cl¦-rich solution swelled steadily (2·0 nl min¢ mm¦Â) when HCOצÏCOµ was introduced, and the luminal pH increased to 8·08. When Cl¦ was replaced by glucuronate, spontaneous fluid secretion was reduced by 75%, and pHL did not rise above 7·3. 3. Cl¦-dependent spontaneous secretion was largely blocked by luminal HµDIDS (500 ìÒ). We conclude that, in unstimulated ducts, HCOצ transport across the luminal membrane is probably mediated by Cl¦-HCOצ exchange. 4. Secretin (10 nÒ) and forskolin (1 ìÒ) both stimulated HCOצ and fluid secretion. The final value of pHL (8·4) and the increase in secretory rate (1·5 nl min¢ mm¦Â) after secretin stimulation were unaffected by substitution of Cl¦. 5. The Cl¦-independent component of secretin-evoked secretion was not affected by luminalHµDIDS. This suggests that a Cl¦-independent mechanism provides the main pathway for luminal HCOצ transport in secretin-stimulated ducts. 6. Ducts filled initially with a HCOצ-rich fluid (125 mÒ HCOצ, 23 mÒ Cl¦) secreted a Cl¦-rich fluid while unstimulated. This became HCOצ-rich when secretin was applied. 7. Addition of HµDIDS and MIA (10 ìÒ) to the bath reduced the secretory rate by 56 and 18%, respectively. Applied together they completely blocked fluid secretion. We conclude that basolateral HCOצ transport is mediated mainly by Na¤-HCOצ cotransport rather than by Na¤-H¤ exchange.7803

Functional coupling of apical Cl−/HCO3− exchange with CFTR in stimulated HCO3− secretion by guinea pig interlobular pancreatic duct

“…HCO 3 Ϫ concentration in the lumen ([HCO 3 Ϫ ]L) was estimated from pHL with assumed values for CO2 solubility of 0.03 mM/mmHg and pK of the HCO 3 Ϫ /CO2-buffer system of 6.1 (17). The rate of HCO 3 Ϫ secretion into resealed duct lumens was calculated from the fluid secretory rate and changes in [HCO 3 Ϫ ]L. Measurement of Vm.…”

“…The rate of fluid secretion into the lumen of resealed ducts was measured as previously described (17). Luminal fluorescence images were acquired at 1-min intervals via a charge-coupled device camera and transformed to binary images by using ARGUS 50 software (Hamamatsu Photonics, Hamamatsu, Japan).…”

“…The pH of the duct lumen (pH L) was estimated by microfluorimetry as described previously (17,21). The lumen of sealed ducts was punctured with a double-barreled (theta-glass) micropipette.…”

“…Although HCO 3 Ϫ and Cl Ϫ transport by the microperfused pancreatic duct of guinea pig has been studied (15)(16)(17), direct study of regulated luminal Cl Ϫ /HCO 3 Ϫ exchange by pH i measurement has until recently not been reported. We have detected apical membrane Cl Ϫ /base exchange activity in forskolin-stimulated interlobular pancreatic duct.…”

“…Secretin-stimulated HCO 3 Ϫ secretion in guinea pig pancreatic duct is not dependent on elevated luminal [Cl Ϫ ] (21) (brackets denote concentration) and occurs even in the presence of luminal fluid containing 125 mM HCO 3 Ϫ and 23 mM Cl Ϫ (17). As luminal [Cl Ϫ ] falls to or below this level (with corresponding elevation of [HCO 3 Ϫ ]), CFTR Cl Ϫ permeability may fall while that for HCO 3 Ϫ may rise (34).…”