Heteromeric TASK-1/TASK-3 is the major oxygen-sensitive background K+ channel in rat carotid body glomus cells

摘要

Carotid body (CB) glomus cells from rat express a TASK-like background K⁺ channel that is believed to play a critical role in the regulation of excitability and hypoxia-induced increase in respiration. Here we studied the kinetic behaviour of single channel openings from rat CB cells to determine the molecular identity of the ‘TASK-like’ K⁺ channels. In outside-out patches, the TASK-like background K⁺ channel in CB cells was inhibited >90% by a reduction of pHo from 7.3 to 5.8. In cell-attached patches with 140 mm KCl and 1 mm Mg²⁺ in the bath and pipette solutions, two main open levels with conductance levels of ∼14 pS and ∼32 pS were recorded at a membrane potential of −60 mV. The K⁺ channels showed kinetic properties similar to TASK-1 (∼14 pS), TASK-3 (∼32 pS) and TASK-1/3 heteromer (∼32 pS). The presence of three TASK isoforms was tested by reducing [Mg²⁺]o to ∼0 mm, which had no effect on the conductance of TASK-1, but increased those of TASK-1/3 and TASK-3 to 42 pS and 74 pS, respectively. In CB cells, the reduction of [Mg²⁺]o to ∼0 mm also caused the appearance of ∼42 pS (TASK-1/3-like) and ∼74 pS (TASK-3-like) channels, in addition to the ∼14 pS (TASK-1-like) channel. The 42 pS channel was the most abundant, contributing ∼75% of the current produced by TASK-like channels. Ruthenium red (5 μm) had no effect on TASK-1 and TASK-1/3, but inhibited TASK-3 by 87%. In CB cells, ruthenium red caused ∼12% inhibition of TASK-like activity. Methanandamide reduced the activity of all three TASKs by 80–90%, and that of TASK-like channels in CB cell also by ∼80%. In CB cells, hypoxia caused inhibition of TASK-like channels, including TASK-1/3-like channels. These results show that TASK-1, TASK-1/3 and TASK-3 are all functionally expressed in isolated CB cells, and that the TASK-1/3 heteromer provides the major part of the oxygen-sensitive TASK-like background K⁺ conductance.