TASK, A HUMAN BACKGROUND K+ CHANNEL TO SENSE EXTERNAL PH VARIATIONS NEAR PHYSIOLOGICAL PH

TASK is a new member of the recently recognized TWIK K+ channel family , This 395 amino acid polypeptide has four transmembrane segments and two P domains, In adult human, TASK transcripts are found in pancreas< placenta<brain<lung, prostate<heart, kidney<uterus, small intestine an d colon, Electrophysiological properties of TASK were determined after expression in Xenopus oocytes and COS cells, TASK currents are K+-sel ective, instantaneous and non-inactivating. They show an outward recti fication when external [K+] is low ([(K+)](out) = 2 mM) which is not o bserved for high [K+](out) (98 mM). The rectification can be approxima ted by the Goldman-Hodgkin-Katz current equation that predicts a curva ture of the current-voltage plot in asymmetric K+ conditions, This str ongly suggests that TASK lacks intrinsic voltage sensitivity, The abse nce of activation and inactivation kinetics as well as voltage indepen dence are characteristic of conductances referred to as leak or backgr ound conductances, For this reason, TASK is designated as a background K+ channel, TASK is very sensitive to variations of extracellular pH in a narrow physiological range; as much as 90% of the maximum current is recorded at pH 7.7 and only 10% at pH 6.7, This property is probab ly essential for its physiological function, and suggests that small p H variations may serve a communication role in the nervous system.