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Am J Physiol Gastrointest Liver Physiol 297: G159-G167, 2009. First published April 30, 2009; doi:10.1152/ajpgi.00132.2009
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HORMONES AND SIGNALING

Regulation of colonic apical potassium (BK) channels by cAMP and somatostatin

Institute of Molecular Medicine, St James's University Hospital, Leeds, United Kingdom

Submitted 4 April 2009 ; accepted in final form 23 April 2009

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 µmol/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 µmol/l) had a G protein-dependent inhibitory effect on BK channels in cell-attached patches, which was unaffected by pretreatment with 10 µmol/l okadaic acid (an inhibitor of protein phosphatase type 1 and type 2A) but completely prevented by pretreatment with 100 µmol/l Na⁺ orthovanadate and 10 µmol/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.

BK channels; colon; patch clamp; potassium secretion