REGULATION OF MAXI-K+ CHANNELS ON PANCREATIC DUCT CELLS BY CYCLIC AMP-DEPENDENT PHOSPHORYLATION

Using the patch-clamp technique we have identified a Ca2+-sensitive, voltage-dependent, maxi-K+ channel on the basolateral surface of rat pancreatic duct cells. The channel had a conductance of ∼200 pS in excised patches bathed in symmetrical 150 mm K+, and was blocked by 1 mm Ba2+. Channel openstate probability (Po) on unstimulated cells was very low, but was markedly increased by exposing the cells to secretin, dibutyryl cyclic AMP, forskolin or isobutylmethylxanthine. Stimulation also shifted the Po/voltage relationship towards hyperpolarizing potentials, but channel conductance was unchanged. If patches were excised from stimulated cells into the inside-out configuration, Po remained high, and was not markedly reduced by lowering bath (cytoplasmic) Ca2+ concentration from 2 mm to 0.1 μm. However, activated channels were still blocked by 1 mm Ba2+. Channel Po was also increased by exposing the cytoplasmic face of excised patches to the purified catalytic subunit of cyclic AMP-dependent protein kinase., We conclude that cyclic AMP-dependent phosphorylation can activate maxi-K+ channels on pancreatic duct cells via a stable modification of the channel protein itself, or a closely associated regulatory subunit, and that phosphorylation alters the responsiveness of the channels to Ca2+. Physiologically, these K+ channels may contribute to the basolateral K+ conductance of the duct cell and, by providing a pathway for current flow across the basolateral membrane, play an important role in pancreatic bicarbonate secretion. © 1990 Springer-Verlag New York Inc.