Activation of Electrogenic K Secretion Requires BK Channels in Guinea Pig Distal Colon

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A variety of hormones and neurotransmitters activate electrogenic K+ secretion in guinea pig distal colon, generally together with Cl secretion. Blockers of BK channels (KCa1.1, Kcnma1), iberiotoxin (IbTx) and paxilline, inhibited the short-circuit current (Isc) associated with K+ secretion. Consistent with this K+secretion occurring via apical membrane BK, mucosal addition of IbTx inhibited epinephrine (epi) activation with an IC50 for Isc (epiIsc) and for transepithelial conductance (epiGt) of ~200 nM. However, maximal inhibition was only ~50%. Mucosally added paxilline [10 μM] also inhibited epiIsc and epiGt by ~50%. IbTx and paxilline each inhibited Isc activated by mucosal ATP, supporting apical BK as an absolute requirement for this K+ secretion. Sensitivity to IbTx and paxilline demonstrated K+ secretion during activation of Cl secretion by prostaglandin-E2and a cholinergic agonist. Distal colonic epithelial cells expressed BKα mRNA with the ZERO splice variant and 3 splice variants for the C-terminus. These cells also expressed the regulatory β-subunits BKβ1 and BKβ4. Immuno-localization demonstrated BKα in apical and basolateral membranes of surface and crypt cells. Together these results support a cellular mechanism for electrogenic K+ secretion involving activation of apical membrane BK, but epi activated K+ secretion also required opening of other K+ channel types.

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