Whole cell recording of H-146 cells revealed that the outward K+ current was completely inhibited by quinidine (IC50 approximately 17 microM). In contrast, maximal concentrations of 4-aminopyridine (4-AP; >/=10 mM) reversibly blocked only approximately 60% (IC50 approximately 1.52 mM). Ten millimolar 4-AP had no effect on the inhibition by hypoxia, which reduced current density from approximately 27 to approximately 13 pA/pF, whereas 1 mM quinidine abolished the hypoxic effect. In current clamp, 10 mM 4-AP depolarized the cell by approximately 18 mV and hypoxia caused further reversible depolarization of approximately 4 mV. One millimolar quinidine collapsed the membrane potential and abrogated any further hypoxic depolarization. RT-PCR revealed expression of the acid-sensitive, twin P domain K+ channel TASK but not of TWIK, TREK, or the known hypoxia-sensitive Kv2.1, which was confirmed by sequencing and further PCR with primers to the coding region of TASK. However, a reduction in extracellular pH had no effect on K+ current. Thus, although the current more closely resembles TWIK than TASK pharmacologically, structurally the reverse appears to be true. This suggests that a novel acid-insensitive channel related to TASK may be responsible for the hypoxia-sensitive K+ current of these cells.