Small-conductance Ca2+-activated K+ (SK) channels are important participants in inhibitory neurotransmission in gastrointestinal smooth muscles. Three isoforms of an SK channel family were cloned from murine proximal colon smooth muscle. The transcripts encoding these subunits (SK1, SK2, and SK3) were detected in murine proximal colon and other peripheral tissues. The mRNA of each subunit was expressed at different levels in murine and canine colonic smooth muscles. The mRNA quantitative ratio of SK transcriptional expression in murine proximal colon is SK2 > SK3 > SK1; transcriptional expression of SK isoforms in canine proximal colon is minimal. SK3 immunohistochemical localization in murine small intestine (jejunum) and proximal colon showed immunoreactivity in circular and longitudinal muscularis. In transversely sectioned muscularis, staining was localized at the cell membrane in smooth muscle cells. Immunoreactivity was more intense in myenteric ganglia between longitudinal and circular muscularis and neuronal processes in circular 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+) gradients. Our results suggest that SK channel expression varies in colonic myocytes from different species and may contribute differentially to inhibitory junction potentials.