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brief review
Roles of Na-K-2Cl and Na-Cl cotransporters and ROMK
potassium channels in urinary concentrating mechanism1
STEVEN C. HEBERT
Division of Nephrology, Vanderbilt University, Nashville Tennessee 37232
distal convoluted tubule; Bartter’s syndrome; thick ascending limb of
Henle
THE SEPARATION of NaCl from water in the thick ascending limb of Henle (TAL) and the distal convoluted
tubule (DCT) is crucial to urinary concentration and
dilution. This is accomplished by the active reabsorption of NaCl in these relatively watertight epithelia.
Major advances have been made over the past few
years in our understanding of NaCl transport mechanisms in these nephron segments. This has been fueled
by the molecular identification of the Na-K-2Cl cotransporter (BSC1; see Refs. 2, 13) and K1 secretory channels (ROMK; see Refs. 8, 20) expressed in the TAL and
the Na-Cl transporter (TSC1; see Refs. 2, 3) expressed
in the DCT (see Fig. 1A).
The TAL and DCT reabsorb about 10–15% and
5–7%, respectively, of the filtered NaCl. The NaCl
reabsorption mechanisms shown in Fig. 1 for TAL and
DCT are secondary active transport processes that
depend on low intracellular Na1 activities maintained
by active extrusion of Na1 from the cell by the basolateral Na1-K1-ATPase (Na1 pump). K1 that enters the
TAL cell by Na-K-2Cl cotransport recycles back to the
tubular urine through potassium channels (4–6). This
latter process has two major consequences: 1) it replenishes the urinary K1 that would otherwise be lost
through absorption by the Na-K-2Cl cotransporter,
ensuring a virtually inexhaustible supply of tubular K1
necessary for Na1 reabsorption; and 2) it also results in
a lumen-positive transepithelial voltage that provides
the driving force for paracellular transport of one-half
of the total Na1 reabsorption (7).
1 This report is the second in a series of minireviews, which are
based on a symposium on the urinary concentrating mechanism, held
at Experimental Biology ’97 in New Orleans, LA.
The Na-K-2Cl and Na-Cl cotransporters, together
with the recently cloned K-Cl cotransporters, comprise
a newly identified family of proteins, i.e., the electroneutral cation-chloride cotransporters. Polyclonal antibodies that are directed against each of these three proteins demonstrate transporter-specific fluorescence on
apical membranes of these nephron segments (9, 14).
Expression of these transporters in Xenopus laevis
oocytes gives rise to the expected ion transport function
[Na-K-2Cl cotransport (2) or Na-Cl cotransport (2) or K
channel (8)]. Moreover the biophysical, electrophysiological and regulatory characteristics of the cloned
channel correspond closely to that of the native low
conductance, inwardly rectifying (and ATP- and pHregulated) K1 channels found in the apical membranes
of TAL (8, 20).
The cation-chloride transporters are glycoproteins
with molecular weights of ,150 and 165 kDa for the
Na-Cl and Na-K-2Cl cotransporters, respectively (Fig.
1B). These cotransporter proteins have similar overall
topologies with a large hydrophobic central region of
possibly 12 membrane-crossing helixes flanked by large
hydrophilic regions that appear to face the interior of
the cell. N-linked sugar residues are present on the
large extracellular loop between the 7th and 8th membrane-spanning segment: two for the Na-Cl cotransporter and up to three for the Na-K-2Cl cotransporter.
The specific ion and diuretic binding regions on these
cotransporters have not been identified.
The ROMK inwardly rectifying K1 channel has a
novel topology that consists of only two potential
membrane-spanning helixes flanked by cytosolic amino
and carboxy terminals (Fig. 1C). It has been suggested
that the functional ROMK channel may be composed of
four identical subunits that form a central pore, but
0363-6127/98 $5.00 Copyright r 1998 the American Physiological Society
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Hebert, Steven C. Roles of Na-K-Cl and Na-Cl contransporters and
ROMK potassium channels in urinary concentrating mechanism. Am. J.
Physiol. 275 (Renal Physiol. 44): F325–F327, 1998.—The cloning of the
apical transport proteins involved in NaCl absorption in the thick
ascending limb of Henle and the distal convoluted tubule has ushered in a
new era that promises to provide molecular details of the urinary
concentrating and diluting processes. We now have at hand tools to
examine the regulation of these proteins that can affect the concentrating
and diluting capacity.
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BRIEF REVIEW
this has not been verified. Several alternatively spliced
isoforms of ROMK have been identified in the rat (1, 8,
20) and human (16) kidney. In the rat, three ROMK
isoforms with distinct amino terminals have been
identified that are differentially expressed along the
nephron from medullary TAL to outer medullary collecting duct (1, 11).
Lessons from mutations in the human Na-K-2Cl
cotransporter (Slc12a1), NaCl cotransporter (Slc12a3)
and apical K1 channel (KCNJ1 or ROMK) genes. Lifton
and co-workers (17, 18) at Yale University and the
International Collaborative Study Group for Bartterlike Syndromes (10) have identified mutations in both
the Slc12a1 (Na-K-2Cl cotransporter; chromosome 16)
and the KCNJ1 (ROMK apical K1 channel; KIR1.1;
chromosome 11) genes in a number of families with
Bartter’s syndrome, including the antenatal hypercalcemic variant. Moreover, these same groups have identi-
fied mutations in the Slc12a3 (Na-Cl cotransporter)
gene on chromosome 16q13 in the Gitelman’s variant of
Bartter’s syndrome (12, 15, 19). Bartter’s syndrome is
associated with salt wasting and hypokalemic alkalosis, and individuals exhibit a major defect in urinary
concentrating and diluting capacity. Moreover, the classic and antenatal Bartter’s variants are accompanied
by hypercalciuria, whereas in Gitelman’s syndrome
hypocalciuria is found. Some of these mutations are
missense, resulting in alterations in single amino acids,
whereas others result in deletions of portions of the
transporter proteins. These mutations apparently result in loss of transporter (Na-K-2Cl or Na-Cl cotransporter, or K1 channel) function, although actual functional studies have not yet been reported.
These exciting discoveries demonstrate that Bartter’s syndrome is genetically heterogeneous and provide clear evidence that the Slc12a1 and Slc12a3 genes
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Fig. 1. Salt transporters contributing
to the separation of NaCl from water in
the loop of Henle (TAL) and the distal
convoluted tubule (DCT). A: cell models
of ion transport in the DCT and the
TAL of Henle. The Na-K-2Cl cotransporter (BSC1; gene Slc12a1; chromosome 16) and the apical K1 recycling
channel (ROMK; KCNJ1; chromosome
11) in the TAL and the Na-Cl cotransporter (TSC1; Slc12a3; chromosome
16q13) in the DCT have been cloned.
B: proposed topology of the bumetanidesensitive Na-K-2Cl and thiazide-sensitive Na-Cl cotransporters. C: topology
model of the ROMK inward-rectifying
K1 channel.
BRIEF REVIEW
8.
9.
10.
11.
12.
S. C. Hebert is supported by National Institute of Diabetes and
Digestive and Kidney Diseases Grants DK-37605, DK-36803, and
DK-45792.
13.
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encode the apical Na-K-2Cl cotransporter in TAL and
the Na-Cl cotransporter in DCT, respectively. The
finding that mutations in KCNJ1 (ROMK) also lead to
an identical clinical syndrome provides strong evidence
that ROMK encodes the apical K1 channel in the TAL
and that normal function of this K1 channel is vital to
maintenance of NaCl reabsorption by the TAL. The
deficits in concentrating and dilution capacity that
these individuals exhibit is readily explained by their
loss of function of Na-Cl, Na-K-2Cl, or K1 channels,
which are vital to the separation of NaCl and water in
the nephron.
Summary. The cloning of the apical transport proteins involved in NaCl absorption in the TAL and DCT
has ushered in a new era that promises to provide
molecular details of the urinary concentrating and
diluting processes. We now have at hand tools to
examine the regulation of these proteins and genes
under a variety of circumstances that can affect the
concentrating and diluting capacity.
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