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 approximately 200 pS in excised patches bathed in symmetrical 150 mM K+, and was blocked by 1 mM Ba2+. Channel open-state 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 microM. 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.
Department of Physiological Sciences, University Medical School, Newcastle upon Tyne, United Kingdom.
M A
MA
Gray
J R
JR
Greenwell
A J
AJ
Garton
B E
BE
Argent
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002384:Catalysis; D005576:Colforsin / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000502:physiology; D010183:Pancreatic Ducts / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D012633:Secretin / Q000494:pharmacology
D015221:Potassium Channels; D012633:Secretin; D005576:Colforsin; D001464:Barium; D003994:Bucladesine; D011494:Protein Kinases; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine