Primary structure and functional expression of a cortical collecting duct Kir channel

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.