Cloning and functional expression of a novel cardiac two-pore background K+ channel (cTBAK-1)

We have cloned from a mouse heart cDNA library a novel K+ channel subunit that has two pore-forming domains and four transmembrane regions. Its amino acid sequence shares 25% identity with mouse TWIK-1, 22% with mouse TREK-1, and 33% with a putative K+ channel of Caenorhabditis elegans (C40C9). Strikingly abundant mRNA for this clone was expressed in the heart. The mRNA was also detected in kidney, brain, skin, testis, lung, skeletal muscle, small intestine, and stomach but not in liver, thymus, or spleen. Reverse transcription-polymerase chain reaction analyses of single cells showed that the mRNA of the clone was expressed in both atrial and ventricular myocytes per se. Xenopus oocytes injected with the cRNA of the clone expressed a Ba2+-sensitive K+-selective current with an almost linear steady-state current-voltage relationship. In cell-attached patches, the expressed channel exhibited short-lasting openings with a mean open time of approximately 2 milliseconds and a unitary conductance of approximately 16 pS (150 mmol/L [K+]o). The K+ current was insensitive to intracellular Na+ (50 mmol/L), Ca2+ (0.1 mmol/L), H+ (pH 6.4), and arachidonic acid (10 micromol/L) in inside-out patches. Thus, the current flowing through the channel may contribute to the cardiac cellular electrical activity as a linear background K+ conductance. Therefore, we designated the clone cTBAK (cardiac two-pore background K+ channel).