TASK-2 is a member of the Tandem-pore domain Acid-Sensing K⁺ channel (TASK) family. In common with the other TASK channel family members, it is sensitive to extracellular pH, and inhibited by acidity. TASK-2 is also sensitive to changes in the osmotic potential of the external medium and participates in cell volume regulation (Niemeyer et al. 2001). It is located in various epithelial tissues including the pancreas, placenta, lung, small intestine, colon and especially the kidney, yet is absent from excitable tissues such as the nervous system, skeletal muscle and the heart (Lesage & Lazdunski, 2000). TASK-2 deficient animals have a metabolic acidosis and hypotension due to the urinary loss of NaHCO₃^– (Warth et al. 2004). TASK-2 is thought to be located in the basolateral membranes of proximal tubule cells, where it participates in the reclamation of filtered HCO₃^– by maintenance of the basolateral membrane potential. Efflux of HCO₃^– across the basolateral membrane on the NBC cotransporter is thought to alkalinise the extracellular fluid in the restricted microenvironment surrounding the basolateral surface of the cells, raising the local pH and activating TASK-2. We have investigated the regulation of TASK-2 using a mutational approach. Channel function was studied by transient expression of wild-type and mutant channels in CHO cells and subsequent whole-cell and single channel patch clamp. The open probability of K2P channels is generally independent of voltage, yielding linear current/voltage (I/V) curves. Despite these properties, we found that these channels showed distinct inward rectification immediately upon the establishment of whole-cell clamp, which became progressively less pronounced with time. This rectification was unaffected by polyamines or Mg²⁺ (agents that cause rectification in Kir channels) but was mimicked by inclusion of Na⁺, in the pipette solution (Morton et al. 2004). In excised inside-out patches, Na⁺ reduced the amplitude of single channel currents, indicative of rapid block and unblock of the pore. Mutations in the selectivity filter abolished Na⁺-induced rectification, suggesting that Na⁺ binds within the selectivity filter in wild-type channels. This sensitivity to intracellular Na⁺ may be an additional potential regulatory mechanism of TASK-2 channels. The pH-sensing mechanism was different from that of TASK-1 and -3, where a single pore-neighbouring residue is largely responsible, involving the combined action of several charged residues in the large extracellular M1-P1 loop (Morton et al. 2005). Neutralisation of no single amino acid in isolation gave substantial loss of pH-sensitivity. However, the combined removal of five charged amino acids (E28, K32, K35, K42 & K47) resulted in a marked reduction in pH-sensitivity. Wild-type channels contain two M1-P1 loops, but a concatemeric construct, comprised of one wild-type subunit and one containing the five mutations, was fully pH sensitive, indicating that only one M1-P1 loop is required to yield a fully pH sensitive channel. Unless prevented, channel activity in the whole cell mode decreases with time, such that roughly half of the channel activity is lost within 5 min. This run-down depends upon phosphorylation state and PKC activity via a mechanism involving the C-terminus, and may involve channel trafficking. Thus the regulation of TASK-2 channels is multifactorial, as in most channels, and the prime determinant of channel activity may yet turn out to be something other than the extracellular pH.