Chloride transport in microperfused interlobular ducts isolated from guinea‐pig pancreas

Abstract

Isolated interlobular ducts from the guinea‐pig pancreas secrete a HCO₃⁻‐rich fluid in response to secretin. To determine the role of Cl⁻ transporters in this process, intracellular Cl⁻ concentration ([Cl⁻]_i) was measured in ducts loaded with the Cl⁻‐sensitive fluoroprobe, 6‐methoxy‐N‐ethylquinolinium chloride (MEQ). [Cl⁻]_i decreased when the luminal Cl⁻ concentration was reduced. This effect was stimulated by forskolin, was not dependent on HCO₃⁻ and was not inhibited by application of the anion channel/transporter inhibitor H₂DIDS to the luminal membrane. It is therefore attributed to a cAMP‐stimulated Cl⁻ conductance, probably the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. [Cl⁻]_i also decreased when the basolateral Cl⁻ concentration was reduced. This effect was not stimulated by forskolin, was largely dependent on HCO₃⁻ and was inhibited by basolateral H₂DIDS. It is therefore mediated mainly by Cl⁻/HCO₃⁻ exchange. With high Cl⁻ and low HCO₃⁻ concentrations in the lumen, steady‐state [Cl⁻]_i was 25‐35 mm in unstimulated cells. Stimulation with forskolin caused [Cl⁻]_i to increase by approximately 4 mm due to activation of the luminal anion exchanger. With low Cl⁻ and high HCO₃⁻ concentrations in the lumen to simulate physiological conditions, steady‐state [Cl⁻]_i was 10–15 mm in unstimulated cells. Upon stimulation with forskolin, [Cl⁻]_i fell to approximately 7 mm due to increased Cl⁻ efflux via the luminal conductance. We conclude that, during stimulation under physiological conditions, [Cl⁻]_i decreases to very low levels in guinea‐pig pancreatic duct cells, largely as a result of the limited capacity of the basolateral transporters for Cl⁻ uptake. The resulting lack of competition from intracellular Cl⁻ may therefore favour HCO₃⁻ secretion via anion conductances in the luminal membrane, possibly CFTR.