CO₂ permeability and bicarbonate transport in microperfused interlobular ducts isolated from guinea‐pig pancreas

Abstract

Permeabilities of the luminal and basolateral membranes of pancreatic duct cells to CO2 and HCO3− were examined in interlobular duct segments isolated from guinea‐pig pancreas. Intracellular pH (pHi) was measured by microfluorometry in unstimulated, microperfused ducts loaded with the pH‐sensitive fluoroprobe 2′7′‐bis(2‐carboxyethyl)‐5(6)‐carboxyfluorescein (BCECF). When HCO3−/CO2 was admitted to the bath, pHi decreased transiently as a result of CO2 diffusion and then increased to a higher value as a result of HCO3− uptake across the basolateral membrane by Na+‐HCO3− cotransport. When HCO3−/CO2 was admitted to the lumen, pHi again decreased but no subsequent increase was observed, indicating that the luminal membrane was permeable to CO2 but did not allow HCO3− entry to the cells from the lumen. Only when the luminal HCO3− concentration was raised above 125 mm was HCO3− entry detected. The same was true of duct cells stimulated with forskolin. Recovery of pHi from an acid load, induced by exposure to an NH4+ pulse, was dependent on basolateral but not luminal Na+ and could be blocked by basolateral application of methylisobutylamiloride and H2DIDS. This indicates that the Na+‐H+ exchangers and Na+‐HCO3− cotransporters are located exclusively at the basolateral membrane. In the presence of HCO3−/CO2, substitution of basolateral Cl− with glucuronate caused larger increases in pHi than substitution of luminal Cl−. This suggests that the anion exchanger activity in the basolateral membrane is greater than that in the luminal membrane. We conclude that the luminal and basolateral membranes are both freely permeable to CO2, but while the basolateral membrane has both uptake and efflux pathways for HCO3−, the luminal membrane presents a significant barrier to the re‐entry of secreted HCO3−, largely through the inhibition of the luminal anion exchanger by high luminal HCO3− concentrations.

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

“…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.…”

Substance P inhibits bicarbonate secretion from guinea pig pancreatic ducts by modulating an anion exchanger

“…The effects of DIDS and amiloride are likely to be confined to the basolateral transporters because 1) DIDS is unlikely to gain rapid access to the lumen of the sealed ducts because of its charged sulfonic acid groups, and 2) there are no amiloride-sensitive transport steps at the apical membrane of the small interlobular ducts used in this study. In addition, basolateral DIDS will block the basolateral anion exchanger (8,31), which will normally act to transport HCO 3 Ϫ out of the cell. Thus the rate at which pH i acidifies after exposure to DIDS and amiloride will reflect the intracellular buffering capacity and the rate at which HCO 3 Ϫ effluxes (i.e., is secreted) across the apical membrane on Cl Ϫ /HCO 3 Ϫ exchangers and CFTR Cl Ϫ channels (2, 27).…”

“…HCO 3 Ϫ is accumulated across the basolateral membrane of the duct cell by Na-(HCO-3 )n cotransporters, Na ϩ /H ϩ exchangers, and proton pumps (2,9). Also on the basolateral membrane are Cl Ϫ /HCO 3 Ϫ exchangers (8,31). The function of these basolateral anion exchangers is unclear because, given the prevailing transmembrane Cl Ϫ concentration gradient, they would be predicted to transport HCO 3 Ϫ from the cell into the blood, i.e., to oppose HCO 3 Ϫ secretion.…”

The effect of cystic fibrosis transmembrane conductance regulator on the function of acid/base transporters of human pancreatic duct cells

“…This suggests that HCO 3 -influx through transport proteins increases at the apical membrane when CFTR is present, as found by Ishiguro et al (2000) using isolated guinea pig ducts.…”