Active K⁺secretion through multiple K_Ca-type channels and regulation by IK_Cachannels in rat proximal colon

Abstract

Colonic K+ secretion stimulated by cholinergic agents requires activation of muscarinic receptors and the release of intracellular Ca2+. However, the precise mechanisms by which this rise in Ca2+ leads to K+ efflux across the apical membrane are poorly understood. In the present study, Northern blot analysis of rat proximal colon revealed the presence of transcripts encoding rSK2 [small conductance (SK)], rSK4 [intermediate conductance (IK)], and rSlo [large conductance (BK)] Ca2+-activated K+ channels. In dietary K+-depleted animals, only rSK4 mRNA was reduced in the colon. On the basis of this observation, a cDNA encoding the K+ channel rSK4 was cloned from a rat colonic cDNA library. Transfection of this cDNA into Chinese hamster ovary (CHO) cells led to the expression of Ca2+-activated K+ channels that were blocked by the IK channel inhibitor clotrimazole (CLT). Confocal immunofluorescence confirmed the presence of IK channels in proximal colonic crypts, and Western blotting demonstrated that IK protein sorted to both the apical and basolateral surfaces of colonic epithelia. In addition, transcellular active K+ secretion was studied on epithelial strips of rat proximal colon using unidirectional 86Rb+ fluxes. The addition of thapsigargin or carbachol to the serosal surface enhanced net 86Rb+ secretion. The mucosal addition of CLT completely inhibited carbachol-induced net 86Rb+ secretion. In contrast, only partial inhibition was observed with the BK and SK channel inhibitors, iberiotoxin and apamin, respectively. Finally, in parallel with the reduction in SK4 message observed in animals deprived of dietary K+, carbachol-induced 86Rb+ secretion was abolished in dietary K+-depleted animals. These results suggest that the rSK4 channel mediates K+ secretion induced by muscarinic agonists in the rat proximal colon and that transcription of the rSK4 channel is downregulated to prevent K+ loss during dietary K+ depletion.

Regulation of cell proliferation by intermediate-conductance Ca2+-activated potassium and volume-sensitive chloride channels in mouse mesenchymal stem cells

“…Similarly, knockdown of KCa3.1 or Clcn3 gene with specific siRNA to decrease IK Ca or I Cl.vol mRNA and KCa3.1 or Clcn3 protein caused a suppression (20 -30%) of mouse MSC prolif- IK Ca has been reported in blood cells and in tissues involved in the transport of fluid and ions, including the lung (40), colon (16), and salivary glands (36). In these tissues, IK Ca underlies K ϩ permeability.…”

Silencing of hSlo potassium channels in human osteosarcoma cells promotes tumorigenesis

“…27 Interestingly, in many cells, BK channels are described to control different steps of the secretary process. 9,[28][29][30] These observations prompted us to investigate whether BK channels could control the tumor growth through a modulation of chemokine expression (Fig. 4).…”

Cyclic AMP-induced K+secretion occurs independently of Cl−secretion in rat distal colon

“…NKCC mediates Cl Ϫ uptake across the basolateral membrane, whereas cystic fibrosis transmembrane regulator (CFTR) mediates active Cl Ϫ secretion across the apical membrane. Intermediate-conductance, Ca 2ϩ -activated K ϩ channels localized to the basolateral membrane have been shown to provide the driving force for agonist (Ca 2ϩ , cAMP)-induced Cl Ϫ secretion in human and mouse colon (22,23 -activated K ϩ channel 3.1, and Kcnn4 (14,17,30). Immunofluorescence studies have localized Kcnn4-like proteins to both the apical and basolateral membranes of human, guinea pig, and rat colon (3,10,11).…”