Molecular properties of small-conductance Ca2+-activated K+ channels expressed in murine colonic smooth muscle

Small-conductance Ca2+-activated K+ (SK) channels are important participant s in inhibitory neurotransmission in gastrointestinal smooth muscles. Three isoforms of an SK channel family were cloned from murine proximal colon sm ooth muscle. The transcripts encoding these subunits (SK1, SK2, and SK3) we re detected in murine proximal colon and other peripheral tissues. The mRNA of each subunit was expressed at different levels in murine and canine col onic smooth muscles. The mRNA quantitative ratio of SK transcriptional expr ession in murine proximal colon is SK2. SK3. SK1; transcriptional expressio n of SK isoforms in canine proximal colon is minimal. SK3 immunohistochemic al localization in murine small intestine (jejunum) and proximal colon show ed immunoreactivity in circular and longitudinal muscularis. In transversel y sectioned muscularis, staining was localized at the cell membrane in smoo th muscle cells. Immunoreactivity was more intense in myenteric ganglia bet ween longitudinal and circular muscularis and neuronal processes in circula r and longitudinal muscularis. Transient expression of mSK1, mSK2, and mSK3 in COS cells resulted in Ca2+-activated voltage-independent channels. mSK1 is less sensitive to apamin compared with SK2 and showed intracellular Ca2 + sensitivity (10(-8) to 10(-6) M) in asymmetrical K+ (5/140 mM K+) gradien ts. Our results suggest that SK channel expression varies in colonic myocyt es from different species and may contribute differentially to inhibitory j unction potentials.