RACTK1 is a pH-sensitive K+ channel cloned from rabbit renal collecting tubule cells. To characterize the function of this K+ channel in more detail, RACTK1 was transfected to an established cell line and the patch-clamp study was performed. cDNA of RACTK1 was inserted in the pMAM vector and transfected to Chinese hamster ovary cells. In one of 36 cell lines, the channel protein as expressed by the dexamethasone-induced mRNA and was detected by the specific antibody. The RACTK1 K+ channel with 80 pS was consistently observed. In inside-out patch, Ca2+ at concentrations higher than 500 nM activated the channel. Open probability was decreased by protein kinase A (from 45 to 4.2%, n+6) but not by protein kinase C. Whole cell currents of the transformed cells represented K+ conductance that was not blocked by an addition of charybdotoxin but by apamin. RACTK1 K+ channel has similar, though not identical, characteristics to the Ca2+ -activated K+ channel. RACTK1 might therefore encode a subunit of the intermediate conductance Ca2+ -activated K+ channel observed in the apical membrane of the rabbit renal collecting duct.