Cloning and expression of the large-conductance Ca(2+)-activated K+ channel from colonic smooth muscle.

We have cloned cDNAs encoding the alpha- and beta-subunits of a large-conductance Ca(2+)-activated K+ channel (BK channel) from canine colonic smooth muscle (cslo-alpha and cslo-beta). Nucleotide sequence homology of cslo-alpha with mslo and dslo suggests that it is the canine homologue of these genes. The carboxy-terminal end of the protein is the most diverse between species, and we have also found alternative exons in cslo-alpha in this region. We have identified a unique splice site in the carboxy-terminal region of cslo-alpha, which we term site 5. Northern analysis demonstrates expression of both alpha- and beta-subunits in all canine vascular and visceral smooth muscles tested. Expression of alpha-1 alone and alpha + beta-subunit cRNA in Xenopus oocytes results in a Ca(2+)- and voltage-dependent conductance. The activity of alpha/beta-channels, measured as either changes in the voltage of half-maximal activation (V0.5) in open probability (NP0) or in the normalized conductance (G/Cmax), was more sensitive to [Ca2+]free than channels composed of the alpha-subunit alone. Neither alpha- nor alpha/beta-channels expressed in membrane patches of Xenopus oocytes were found to be regulated by protein kinase G.