Secretin activates Cl^- channels in bile duct epithelial cells through a cAMP-dependent mechanism

Using patch-clamp recording techniques, we assessed the effects of secretin on membrane ion channel activity in isolated rat bile duct epithelial cells. In the whole cell configuration, secretin activated an inward membrane current at -40 mV in 6 of 13 cells, and increased current density from 17 ± 8 to 98 ± 33 pA/pF. Secretin-stimulated currents reversed near the equilibrium potential for Cl^- and exhibited a linear current- voltage relationship. In the cell-attached configuration, secretin activated low-conductance channels in 73% (11 of 15) of patches. Similar channels were activated by forskolin, suggesting that adenosine 3',5'-cyclic monophosphate (cAMP) is involved as a second messenger. At the resting membrane potential, channels carried inward membrane current and had a slope conductance of 10 ± 1 pS. In excised patches, addition of purified catalytic subunit of cAMP- dependent protein kinase (protein kinase A) to the cytoplasmic surface activated channels in four of six attempts. With equal Cl^- concentrations in bath and pipette, channels had a linear slope conductance of 13 ± 2 pS and currents reversed near 0 mV. Partial substitution of pipette Cl^- with gluconate caused a shift in reversal potential in the direction anticipated for a Cl^--selective channel (gluconate to Cl^- permeability ratio of 0.21 ± 0.05, n = 4). Thus in bile duct epithelial cells, exposure to secretin activates low-conductance, Cl^--selective channels, probably through a cAMP- dependent mechanism. This likely contributes to secretin-dependent choleresis.