SECRETIN ACTIVATES CL- CHANNELS IN BILE-DUCT EPITHELIAL-CELLS THROUGHA CAMP-DEPENDENT MECHANISM

Using patch-clamp recording techniques, we assessed the effects of sec retin on membrane ion channel activity in isolated rat bile duct epith elial cells. In the whole cell configuration, secretin activated an in ward membrane current at -40 mV in 6 of 13 cells, and increased curren t density from 17 +/- 8 to 98 +/- 33 pA/pF. Secretin-stimulated curren ts reversed near the equilibrium potential for Cl- and exhibited a lin ear current-voltage relationship. In the cell-attached configuration, secretin activated low-conductance channels in 73% (11 of 15) of patch es. Similar channels were activated by forskolin, suggesting that aden osine 3',5'-cyclic monophosphate (cAMP) is involved as a second messen ger. At the resting membrane potential, channels carried inward membra ne current and had a slope conductance of 10 +/- 1 pS. In excised patc hes, addition of purified catalytic subunit of cAMP-dependent protein kinase (protein kinase A) to the cytoplasmic surface activated channel s in four of six attempts. With equal Cl- concentrations in bath and p ipette, channels had a linear slope conductance of 13 +/- 2 pS and cur rents reversed near O mV. Partial substitution of pipette Cl- with glu conate caused a shift in reversal potential in the direction anticipat ed for a Cl--selective channel (gluconate to C1(-) permeability ratio of 0.21 +/- 0.05, n. = 4). Thus in bile duct epithelial cells, exposur e to secretin activates low-conductance, Cl--selective channels, proba bly through a cAMP-dependent mechanism. This likely contributes to sec retin-dependent choleresis.