Abstract

TASK-2 is a K-2P K+ channel considered as a candidate to mediate CO2 sensing in central chemosensory neurons in mouse. Neuroepithelial cells in zebrafish gills sense CO2 levels through an unidentified K-2P K+ channel. We have now obtained zfTASK-2 from zebrafish gill tissue that is 49 % identical to mTASK-2. Like its mouse equivalent, it is gated both by extra- and intracellular pH being activated by alkalinization and inhibited by acidification. The pH(i) dependence of zfTASK-2 is similar to that of mTASK-2, with pK (1/2) values of 7.9 and 8.0, respectively, but pH(o) dependence occurs with a pK (1/2) of 8.8 (8.0 for mTASK-2) in line with the relatively alkaline plasma pH found in fish. Increasing CO2 led to a rapid, concentration-dependent (IC50 similar to 1.5 % CO2) inhibition of mouse and zfTASK-2 that could be resolved into an inhibition by intracellular acidification and a CO2 effect independent of pH(i) change. Indeed a CO2 effect persisted despite using strongly buffered intracellular solutions abolishing any change in pH(i), was present in TASK-2-K245A mutant insensitive to pH(i), and also under carbonic anhydrase inhibition. The mechanism by which TASK-2 senses CO2 is unknown but requires the presence of the 245-273 stretch of amino acids in the C terminus that comprises numerous basic amino acids and is important in TASK-2 G protein subunit binding and regulation of the channel. The described CO2 effect might be of importance in the eventual roles played by TASK-2 in chemoreception in mouse and zebrafish.