Regulation of colonic apical potassium (BK) channels by cAMP and somatostatin.
High-conductance apical K+ (BK) channels are present in surface colonocytes of mammalian (including human) colon. Their location makes them well fitted to contribute to the excessive intestinal K(+) losses often associated with infective diarrhea. Since many channel proteins are regulated by phosphorylation, we evaluated the roles of protein kinase A (PKA) and phosphatases in the modulation of apical BK channel activity in surface colonocytes from rat distal colon using patch-clamp techniques, having first increased channel abundance by chronic dietary K+ enrichment. We found that PKA activation using 50 micromol/l forskolin and 5 mmol/l 3-isobutyl-1-methylxanthine stimulated BK channels in cell-attached patches and the catalytic subunit of PKA (200 U/ml) had a similar effect in excised inside-out patches. The antidiarrheal peptide somatostatin (SOM; 2 micromol/l) had a G protein-dependent inhibitory effect on BK channels in cell-attached patches, which was unaffected by pretreatment with 10 micromol/l okadaic acid (an inhibitor of protein phosphatase type 1 and type 2A) but completely prevented by pretreatment with 100 micromol/l Na+ orthovanadate and 10 micromol/l BpV (inhibitors of phosphoprotein tyrosine phosphatase). SOM also inhibited apical BK channels in surface colonocytes in human distal colon. We conclude that cAMP-dependent PKA activates apical BK channels and may enhance colonic K+ losses in some cases of secretory diarrhea. SOM inhibits apical BK channels through a phosphoprotein tyrosine phosphatase-dependent mechanism, which could form the basis of new antidiarrheal strategies.
Perry
M D
MD
Institute of Molecular Medicine, St James's University Hospital, Leeds LS9 7TF, United Kingdom.
Sandle
G I
GI
D000818:Animals; D020134:Catalytic Domain; D016764:Cell Polarity; D003106:Colon / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D020536:Enzyme Activators / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism*; D017419:Potassium, Dietary / Q000008:administration & dosage; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013004:Somatostatin / Q000378:metabolism*
D020536:Enzyme Activators; D004791:Enzyme Inhibitors; D051036:Large-Conductance Calcium-Activated Potassium Channels; D017419:Potassium, Dietary; D013004:Somatostatin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D010749:Phosphoprotein Phosphatases; D017027:Protein Tyrosine Phosphatases; D011188:Potassium