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

1. Permeabilities of the luminal and basolateral membranes of pancreatic du ct cells to CO2 and HCO3- were examined in interlobular duct segments isola ted from guinea-pig pancreas. Intracellular pH (pH(1)) was measured by micr ofluorometry in unstimulated, microperfused ducts loaded with the pH-sensit ive fluoroprobe 2'7'-bis(2-carboxyethyl)-5(6) -carboxy-fluorescein (BCECF). 2. When HCO3-/CO2 was admitted to the bath, pH(i) 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. 3. When HCO3-/CO2 was admitted to the lumen, pH, again decreased but no sub sequent increase was observed, indicating that the luminal membrane was per meable to CO2 but did not allow HCO3- entry to the cells from tile lumen. O nly when the luminal HCO2 concentration was raised above 125 mM was HCO3- e ntry detected. The same was true of duct cells stimulated with forskolin. 4. Recovery of pH(i) from an acid load, induced by exposure to an NH4+ puls e, was dependent on basolateral but not luminal Na+ and could be blocked by basolateral application of methylisobutylamiloride and H2DIDS. This indica tes that Na+-H+ exchangers and Na+-HCO3- cotransporters are located exclusi vely at the basolateral membrane. 5. In the presence of HCO3-/CO2, substitution of basolateral Cl- with glucu ronate caused larger increases in pH(i) than substitution of luminal Cl- Th is suggests that the anion exchanger activity in tile basolateral membrane is greater than that in the luminal membrane. 6. We conclude that the luminal and basolateral membranes are both freely p ermeable to CO2, but while the basolateral membrane has both uptake and eff lux pathways for HCO3- the luminal membrane presents a significant barrier to tile re-entry of secreted HCO3- largely through the inhibition of the lu minal anion exchanger by high luminal KCO3- concentrations.