Abstract
The human ether-a-go-go-related gene (HERG) product forms the pore-forming subunit of the delayed rectifier K(+) channel in the heart. Unlike the cardiac isoform, the erg K(+) channels in native smooth muscle demonstrate gating properties consistent with a role in maintaining resting potential. We have cloned the smooth muscle isoform of HERG, denoted as erg1-sm, from human and rabbit colon. erg1-sm is truncated by 101 amino acids in the C terminus due to a single nucleotide deletion in the 14th exon. Sequence alignment against HERG showed a substitution of alanine for valine in the S4 domain. When expressed in Xenopus oocytes, erg1-sm currents had much faster activation and deactivation kinetics compared with HERG. Step depolarization positive to -20 mV consistently produced a transient outward component. The threshold for activation of erg1-sm was -60 mV and steady-state conductance was approximately 10-fold greater than HERG near the resting potential of smooth muscle. Site-directed mutagenesis of alanine to valine in the S4 region of erg1-sm converted many of the properties to that of the cardiac HERG, including shifts in the voltage dependence of activation and slowing of deactivation. These studies define the functional role of a novel isoform of the ether-a-go-go-related gene K(+) channel in smooth muscle.
Highlights
The human ether-a-go-go related gene (HERG)1 encodes for a Kϩ channel that is essential for normal repolarization of the cardiac action potential
1) the threshold for activation of the heterologously expressed cardiac isoform of the human ether-a-go-go-related gene (HERG) current and its native form, IKr, is positive to Ϫ 50 mV [5, 19], whereas the resting potential of gastrointestinal smooth muscle generally lies around Ϫ60 mV, 2) unlike the native cardiac myocytes, where large tail currents occur upon repolarization [20, 21], smooth muscle cells do not demonstrate large tail currents upon repolarization, and 3) it is not clear how recovery from inactivation of the cardiac HERG can generate sufficient outward current during repolarization of shorter duration of action potential trains that occur from a plateau potential of Ϫ30 mV in gastrointestinal smooth muscle
The findings of the present study demonstrate the functional properties of the smooth muscle isoform of the ether-a-go-gorelated gene Kϩ channel and show the differences compared with the cardiac isoform
Summary
The human ether-a-go-go related gene (HERG) encodes for a Kϩ channel that is essential for normal repolarization of the cardiac action potential. Recent studies suggest that an inwardly rectifying Kϩ conductance that is active near the resting membrane potential of gastrointestinal smooth muscle cells [12, 13], pituitary cells (14 –16), carotid body [15,16,17], and microglia [18] has properties similar to that of HERG channels. In these cells HERG conductance has been directly demonstrated in single cells, and in esophageal and stomach smooth muscle the presence of a “window” current within the range of the resting potential and depolarization and contraction by HERG channel blockers of whole tissue segments strongly suggest a role for ether-a-gogo-related gene Kϩ conductance in maintaining resting membrane potential [12]. We show that there is a unique substitution of a conserved amino acid in the S4 voltage sensor region between the cardiac and smooth muscle isoforms that confers significant hyperpolarizing shift in the voltage dependence of activation for erg1-sm and results in steady-state conductance within the potential range for maintaining the resting membrane potential and demonstrates ki-
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