PHOSPHORYLATION-REGULATED LOW-CONDUCTANCE CL-CHANNELS IN A HUMAN PANCREATIC DUCT CELL-LINE

A low-conductance Cl- channel has been identified in the apical membrane of the human pancreatic duct cell Capan-1 using patch-clamp techniques. Cell-attached channels were activated by the vasoactive intestinal polypeptide (VIP, 0.1 mu mol/l), dibutyryl-adenosine 3',5'-cyclic monophosphate (db-cAMP, 1 mmol/l), 8-bromo adenosine 3',5'-cyclic monophosphate (8-Br-cAMP, 1 mmol/l), 3-isobutyl-1-methyl-xanthine (IBMX, 100 mu mol/l) and forskolin (10 mu mol/l). No channel activity was observed in non-stimulated control cells. In both cell-attached and excised inside-out patches, the channel had a linear current/voltage relationship and a unitary conductance of 9 pS at 23 degrees C and 12 pS at 37 degrees C. Its opening probability was not voltage dependent although pronounced flickering was induced at negative potentials. Anionic substitution led to the selectivity sequence CI- > I- much greater than > HCO3- > gluconate. In inside-out excised patches, the channel activity declined spontaneously within a few minutes. Reactivation of silent excised channels was achieved by adding protein kinase A (PKA, in the presence of ATP, cAMP and Mg2+). Conversely, active channels were silenced in the presence of alkaline phosphatase. The PKA-activated Cl- channel was 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 100 mu mol/l) and 4-acetamido-4'isothi ocyanatostilbene-2,2'-disulphonic acid (SITS, 100 mu mol/l) insensitive, but was blocked by diphenylamine-2-carboxylic acid (DPC, 100 mu mol/l). These results demonstrate that the apical low-conductance Cl- channel in Capan-1 is regulated on-cell by VIP receptors via cAMP and off-cell by PKA and phosphatases. They provide evidence that this channel is closely related to the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel.