AJP - Renal Physiology, Vol 273, Issue 5 825-F836, Copyright © 1997 by American Physiological Society
Primary structure and functional expression of a cortical collecting duct Kir channel
P. A. Welling
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201, USA.
Maintenance of a negative membrane potential in the cortical collecting
duct (CCD) principal cell depends on a small-conductance, inward-rectifying
basolateral membrane K+ (Kir) channel. In the present study, a candidate
cDNA encoding this K+ channel, CCD-IRK3, was isolated from a mouse
collecting duct cell line, M1. CCD-IRK3 shares a high degree of homology
with a human brain inward-rectifier K+ channel (Kir 2.3). By Northern
analysis, CCD-IRK3 transcript (2.9 kb) was readily detected in M1 CCD cells
but not in Madin-Darby canine kidney, LLC-PK1, Chinese hamster ovary, or
monkey kidney fibroblast cell lines. CCD-IRK3-specific reverse
transcription-polymerase chain reaction confirmed bonafide expression in
the kidney. Functional expression studies in Xenopus oocytes revealed that
CCD-IRK3 operates as strongly inward-rectifying K+ channel. The cation
selectivity profile of CCD-IRK3 [ionic permeability values (PK/Pi), Tl >
or = Rb > or = K+ >> NH4 > Na; inward-slope conductance
(GK/Gi), Tl > or = K+ >> NH4 > Na > Rb] is similar to the
macroscopic CCD basolateral membrane K+ conductance (GK/Gi, K+ >> NH4
> Rb; PK/Pi, Rb approximately equal to K+ >> NH4). CCD-IRK3 also
exhibits the pharmacological features of the native channel. Patch-clamp
analysis reveals that CCD-IRK3 functions as a high open probability,
voltage-independent, small-conductance channel (14.5 pS), consistent with
the native channel. Based on these independent lines of evidence, CCD-IRK3
is a possible candidate for the small-conductance basolateral Kir channel
in the CCD.
