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1 Department of Physiology and Biophysics, Chicago Medical School, Finch University of Health Sciences, North Chicago, Illinois 60064; and 2 Department of Physiology, Chung-Ang University, Seoul 156-756, Korea
A mammalian
K+ channel subunit (TBAK-1/TASK-1)
containing two pore domains and four transmembrane segments and whose
mRNA is highly expressed in the heart has been cloned recently. TBAK-1 and TASK-1 are identical except for the additional nine amino acids in
the NH2 terminus of TBAK-1. We
examined their kinetic properties, pH sensitivity, and regional cardiac
mRNA expression and determined whether a native cardiac
K+ channel with similar kinetic
properties was present. When TBAK-1 or TASK-1 was transiently expressed
in COS-7 cells, time- and voltage-independent whole cell currents were
observed. Single-channel conductances of TBAK-1 and TASK-1 were 14.6 ± 1.0 and 13.8 ± 2.8 pS, respectively, at 
80 mV in 140 mM extracellular K+, and the mean
open times were 0.8 ± 0.1 and 0.6 ± 0.1 ms, respectively. Both
TBAK-1 and TASK-1 were highly sensitive to extracellular pH such that a
decrease from 7.2 to 6.4 reduced their open probability (Po) by 81 ± 14% and 80 ± 16%, whereas a decrease in intracellular pH
from 7.2 to 6.4 reduced the
Po by 42 ± 10% and 47 ± 12%, respectively. TBAK-1/TASK-1 mRNA was expressed
in all regions of the rat heart, with the highest level of expression
in the right atrium. A 14-pS K+
channel with kinetic properties similar to those of TBAK-1/TASK-1 was
identified in rat atrial and ventricular cells. These results indicate
that TBAK-1/TASK-1 represents a functional native
K+ channel in the rat heart.
two-pore potassium channel; atrial cell; pH
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