Abstract
The ROMK (Kir1.1; Kcnj1) gene is believed to encode the apical small conductance K(+) channels (SK) of the thick ascending limb (TAL) and cortical collecting duct (CCD). Loss-of-function mutations in the human ROMK gene cause Bartter's syndrome with renal Na(+) wasting, consistent with the role of this channel in apical K(+) recycling in the TAL that is crucial for NaCl reabsorption. However, the mechanism of renal K(+) wasting and hypokalemia that develop in individuals with ROMK Bartter's syndrome is not apparent given the proposed loss of the collecting duct SK channel. Thus, we generated a colony of ROMK null mice with approximately 25% survival to adulthood that provides a good model for ROMK Bartter's syndrome. The remaining 75% of null mice die in less than 14 days after birth. The surviving ROMK null mice have normal gross renal morphology with no evidence of significant hydronephrosis, whereas non-surviving null mice exhibit marked hydronephrosis. ROMK protein expression was absent in TAL and CCD from null mice but exhibited normal abundance and localization in wild-type littermates. ROMK null mice were polyuric and natriuretic with an elevated hematocrit consistent with mild extracellular volume depletion. SK channel activity in TAL and CCD was assessed by patch clamp analysis in ROMK wild-type ROMK(+/+), heterozygous ROMK(+/-), and null ROMK(-/-) mice. In 313 patches with successful seals from the three ROMK genotypes, SK channel activity in ROMK (+/+ and +/-) exhibited normal single channel kinetics. The expression frequencies are as follows: 67 (TAL) and 58% (CCD) in ROMK(+/+); about half that of the wild-type in ROMK(+/-), being 38 (TAL) and 25% (CCD); absent in both TAL or CCD in ROMK(-/-) between 2 and 5 weeks in 15 mice (61 and 66 patches, respectively). The absence of SK channel activity in ROMK null mice demonstrates that ROMK is essential for functional expression of SK channels in both TAL and CCD. Despite loss of ROMK expression, the normokalemic null mice exhibited significantly increased kaliuresis, indicating alternative mechanisms for K(+) absorption/secretion in the nephron.
Highlights
The ROMK (Kir1.1; Kcnj1) gene is believed to encode the apical small conductance K؉ channels (SK) of the thick ascending limb (TAL) and cortical collecting duct (CCD)
In non-surviving ROMK(Ϫ/Ϫ) mice, the size of the kidney was about one-third of normal (Fig. 2, E and F); the renal cortex was considerably thinner than in wild-type (Fig. 2, A and B) or heterozygous (Fig. 2, C and D) mice, and the renal pelvis surrounding the renal papilla and the pelvic fornices at the level of the outer medulla were extensively dilated. These changes indicated that significant hydronephrosis was present in the non-surviving null mice, but this was still somewhat milder than that seen in the original ROMK-deficient mice [69]
We cannot exclude the possibly that mild hydronephrosis may be present in a small number of our ROMK(Ϫ/Ϫ) mice given the limited number of animals examined in this study
Summary
37881–37887, 2002 Printed in U.S.A. Absence of Small Conductance K؉ Channel (SK) Activity in Apical Membranes of Thick Ascending Limb and Cortical Collecting Duct in ROMK (Bartter’s) Knockout Mice*. The ROMK (Kir1.1; Kcnj1) gene is believed to encode the apical small conductance K؉ channels (SK) of the thick ascending limb (TAL) and cortical collecting duct (CCD). Loss-of-function mutations in the human ROMK gene cause Bartter’s syndrome with renal Na؉ wasting, consistent with the role of this channel in apical K؉ recycling in the TAL that is crucial for NaCl reabsorption. The kidney small conductance Kϩ channel (SK) expressed in the apical membranes of thick ascending limb (TAL) and cortical collecting duct (CCD) cells mediates Kϩ secretion in the distal nephron, thereby playing an essential role in Kϩ balance [1].
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