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Flashcards in Eight Deck (13):

What are the six parts of the distal nephron? What is the function of it?

The distal nephron begins at the macula densa and includes
several segments with unique functional properties (Fig. 8.1):
• Early distal convoluted tubule (DCT1)
• Late distal convoluted tubule (DCT2)
• Connecting segment (CS)
• Cortical collecting duct (CCD)
• Outer medullary collecting duct (OMCD)
• Inner medullary collecting duct (IMCD)
Ten percent of the filtered solute and water (~20 L) reaches
the distal nephron, and <1 mEq/L, when needed.


Describe NaCl transport in the DCT. What transporters are involved? What things modify these transporters?

The distal convoluted tubule (DCT) is responsible for the reabsorption
of approximately 5% of the filtered NaCl. Reabsorption
of sodium occurs via a sodium chloride cotransporter
(NCC). NCC is predominately expressed in the early to mid
DCT or DCT1 (Fig. 8.2). NCC belongs to the SLC12 family
of electroneutral cation chloride cotransporters. NCC responds
to changes in the delivered load of sodium and is specifically inhibited by the thiazide type diuretics. The thiazide diuretics
bind to the chloride ion site on NCC and interfere with
transport function. The basolateral Na-K-ATPase provides a
favorable chemical gradient for sodium reabsorption. Chloride
efflux occurs through CLC-KB, which is expressed on
the basolateral membrane. Recent studies have shown that
NCC is regulated by the WNK (with no lysine) family
of serine-threonine kinases. Four mammalian WNK kinases
have been identified. They are expressed broadly in tissues;
however, WNK1 and WNK4 localize to the DCT and cortical
collecting duct. Loss-of-function mutations of WNK1 and
gain-of-function mutations of WNK4 inhibit NCC activity.


Describe the pathophys and clinical manifestations of Gitelmans syndrome.

In 1966, Gitelman described a syndrome characterized
by hypokalemic metabolic alkalosis and salt wasting.
These patients also presented with hypocalciuria and
hypermagnesuria. The clinical manifestations of the
syndrome were analogous to those described in patients
abusing thiazide type diuretics, suggesting impaired
function of NCC. Recent molecular studies indicate that
the majority of patients with Gitelman’s syndrome have inactivating mutations of NCC. A small percentage of
patients have inactivating mutations of CLC-KB, perhaps
explaining the phenotypic variations seen in some affected


Describe Mg transport in the DCT.

Urinary dilution also occurs in the DCT since this segment
is relatively impermeable to water. Urine can achieve an
osmolality as low as 50 mOsm/L at this site.
More than 85% of the filtered magnesium is reabsorbed
via passive mechanisms in the proximal tubule and loop of
Henle. The DCT (specifically DCT1) plays a critical role in
regulating transcellular magnesium transport and governs the
final plasma and urine concentration of magnesium. TRPM6
(Transient Receptor Potential channel Melastatin type 6) is a
magnesium channel expressed predominately in the early and
mid DCT (Fig. 8.3). It is the sixth member of the melastatin
subfamily of transient receptor potential (TRP) channels The TRP channels comprise a family of cation channels that
are subdivided into seven subfamilies with unique ion conductance
properties. The efflux of magnesium from the DCT
is poorly understood. Many regulatory factors have been
implicated in the control of magnesium transport; however,
nothing has emerged as the primary regulatory factor. Lossof-
function mutations in TRPM6 are associated with severe


Describe the transport of Ca in the DCT. Describe the regulation of plasma Ca levels. Describe the regulation of calcium in the DCT.

The majority of calcium (~80%-90%) is reabsorbed passively
along the proximal tubule and loop of Henle. The concentration
of calcium in plasma is regulated via a complex
system that involves intestinal absorption of calcium, mobilization
from bone and kidney transport. Transcellular calcium
transport in the DCT plays a pivotal role in regulating the total
body calcium content (Fig. 8.4). TRPV5 (Transient Receptor
Potential Vanilloid type 5) is a calcium channel expressed
almost exclusively in DCT2. Reabsorption of calcium involves
influx of calcium through TRPV5, binding to the cytoplasmic
carrier protein, calbindin-D28k, which facilitates the diffusion
of calcium to the basolateral membrane, and cellular efflux via
a sodium/calcium antiporter (NCX1) and a calcium-ATPase
transport protein (PMCA1b). Parathyroid hormone (which is
sensitive to changes in the plasma concentration of calcium)
facilitates calcium reabsorption in this segment by increasing
the expression of TRPV5, calbindin-D28k, and NCX1.
The active form of vitamin D (1,25 dihydroxyvitamin D) also
increases the transcription of these calcium transport pathway


Describe transport in the connecting segment.

This nephron segment bridges the DCT and collecting duct.
The epithelium in this relatively short segment expresses a
small amount of TRPV5. However, the connecting segment
primarily contains a rich population of principal cells and
intercalated cells. These cells, which express ROMK, ENaC
(amiloride sensitive sodium channel), and H-ATPase (V-type
or vacuolar hydrogen-ATPase), are involved in acid-base
homeostasis (H-ATPase), sodium reabsorption (ENaC), and
potassium excretion (ROMK). The percent contribution of the
connecting segment (CS) to overall acid-base and electrolyte
homeostasis, relative to the collecting duct, is controversial.


Generally, describe the cellularity and the transport that occurs in the 3 parts of the collecting duct.

The CCD, OMCD, and initial one-third of the IMCD are
comprised almost entirely of principal cells and intercalated
cells. The terminal collecting duct consists of a cell type that
is distinct, and referred to as the IMCD cell. The relative
abundance of these cell types (and their functional roles) in
each segment is difficult to define, since the collecting duct
is relatively inaccessible to detailed morphologic and physiologic
studies. Generally, the CCD is involved in acid-base
homeostasis, sodium reabsorption, and potassium secretion. The OMCD is primarily involved in regulating acid-base, but
may also participate in potassium reabsorption. The IMCD is
involved in acid-base homeostasis as well as sodium handling
(via atrial natriuretic peptide). All segments of the collecting
duct express aquaporins and are sensitive to the action of
antidiuretic hormone (ADH).


What is the main type of cell in the CCD? What is its function there? What channels does it contain? How can these channels be regulated?

The principal cell in the CCD (and perhaps CS) is the primary
cell type responsible for potassium secretion in the
kidney (Fig. 8.5). Several potassium channels are expressed
in the apical membrane of the principal cell. Since ROMK
is abundantly expressed and has a high open probability, it
appears to be the primary potassium channel involved in apical
In addition, principal cells contribute to sodium conservation
(see Fig. 8.5). Sodium reabsorption occurs through an apical channel, which is blocked by the weak diuretic,
amiloride. This channel is known as ENaC (Epithelial sodium
Channel). ENaC consists of a complex of three subunits (α,β,
and γ) in a ratio of 2:1:1, respectively (controversy regarding
the exact subunit stoichiometry exists). Activation of the α
subunit promotes sodium reabsorption. Nedd4-2, an E3 ubiquitin
ligase, tags ENaC with ubiquitin, resulting in internalization,
thus, terminating its activity.


Describe the pathophys and clinical manifestations of Liddle syndrome.

Liddle syndrome was first described in 1963 as a familial
disorder characterized by severe hypertension and
suppression of plasma aldosterone levels. The syndrome
was exquisitely sensitive to ENaC inhibitors, triamterene
and amiloride, but unresponsive to aldosterone receptor
antagonists. Mutations involving the γ- or β-subunits of
ENaC are responsible for the majority of Liddle syndrome.
The mutated subunits are resistant to the action of Nedd4-2 and, therefore, ENaC remains constitutively active
(expressed) resulting in uncontrolled sodium retention and


What are the results of sodium reabsorption in the CCD? What is the role of WNK1 and WNK4 in the CCD?

Sodium reabsorption in the CCD promotes a negative
potential in the lumen with respect to the basolateral aspect
of the cell (i.e., transepithelial potential). The transepithelial
potential of the CCD is quite large and may exceed −60 mV; a
potential far greater than in any other segment of the nephron.
The transepithelial potential of the CCD facilitates secretion
of several cations, in particular, potassium and hydrogen ions.
The negative transepithelial potential also promotes the reabsorption
of chloride. Claudin 4, expressed in the tight junction
of the CCD, is highly permeable to chloride ions. Emerging
evidence indicates that WNK1 and WNK4 play a pivotal role
in regulating chloride ion conductance, ROMK expression,
and ENaC expression in the CCD. Generally, WNK4 inhibits
ion transport in NCC, NKCC2, ROMK, and Claudin 4, whereas, WNK1 inhibits WNK4. These emerging interactions
will undoubtedly contribute to our understanding of electrolyte
homeostasis. They have already provided unique insights into
several genetic disorders of electrolyte balance, but these rare
disorders are beyond the scope of this discussion. The Na-KATPase
also contributes to sodium and potassium transport in
the CCD by raising and lowering the intracellular potassium
and sodium concentration, respectively.


What is the major regulating factor of Na and K transport in the CCD? Where is it synthesized? What is its molecular mechanism?

Aldosterone (a hormone produced by the adrenal gland)
is the major factor involved in regulating transport of sodium
and potassium in the CS and CCD. Aldosterone stimulates the
activity and synthesis of the Na-K-ATPase, thus, affecting the
intracellular concentration of sodium and potassium. Aldosterone
also induces a conformational change (likely because
of subunit phosphorylation via SGK1, also known as serumglucocorticoid-
kinase 1) in ENaC and ROMK that promotes
ion conductance and stabilizes expression in the apical membrane.
A high potassium diet or increased plasma potassium
concentration also promote potassium secretion in the CS and CCD via an increase in circulating aldosterone. This constitutes
a negative feedback loop, which maintains the serum
concentration of potassium within a very narrow range (usually
4-5 mEq/L).


What types of cells are located in the OMCD? What is their function there? What transporters are located in these cells ? What happens if these transporters don't work ?

The OMCD is primarily involved in regulating acid-base
homeostasis; however, potassium reabsorption also occurs at
this site. This site consists almost entirely of cells that express
H-ATPase and chloride-bicarbonate exchanger (also known as
anion exchanger). Although principal cells have been described
in the OMCD, potassium secretion does not occur at this site.
It is likely that principal cells are primarily involved in water
reabsorption via aquaporins 2 and 4. The intercalated cell is
thought to be the primary cell type responsible for renal acidification
in the OMCD. At least two subtypes of intercalated
cells have been described on the basis of protein expression.
Type-A intercalated cells express abundant H-ATPase on the
apical membrane and anion exchanger 1 (AE1) on the basolateral
membrane (Fig. 8.6). H-ATPase is a large multi-subunit enzyme (14 proteins) consisting of two major subunit complexes,
a cytosolic domain (V1) and a membrane spanning
domain (V0), which mediates proton transport.

44 C L I N I C A L I M P L I C AT I O N
Mutations in several subunit proteins have been
characterized. These loss-of-function mutations impair
renal acidification and produce a type of metabolic acidosis
known as distal renal tubular acidosis. Since these proteins
are expressed at other sites (inner ear), they are sometimes
accompanied by other defects (eg, deafness).

Non-type-A intercalated cells express the anion exchanger,
pendrin, on the apical membrane and H-ATPase on the basolateral
membrane (Fig. 8.7). Pendrin is an anion exchanger
that was originally described in patients with the Pendred syndrome
(hypothyroidism, goiter, and sensorineural deafness).
Classically, type-A cells are considered the primary cell
type involved in acid excretion, whereas, non-type-A cells
are involved in bicarbonate excretion. The functional significance
of this is described in detail in Chap 13. Type-A cells
also express H-K-ATPase on the luminal membrane, and are therefore involved in potassium homeostasis, specifically
potassium conservation. For example, H-K-ATPase is likely
important in potassium reabsorption in settings characterized
by a low potassium intake or when the plasma concentration
of potassium is low. The contribution of H-K-ATPase
to acid-base homeostasis appears to be minimal. All intercalated
cells express carbonic anhydrase II. Recent studies
suggest that type-A intercalated cells are expressed from the
late DCT to the initial one-third of the IMCD. Non-type-A
cells appear to be concentrated in DCT2, although, bicarbonate
secretion has been clearly demonstrated throughout the
collecting duct.


What cells are located in the initial part of the IMCD and what is their funciton there? What cells are located in the terminal portion of the IMCD and what is their function there? What channels are located in them and how are they regulated?

The initial one-third of the IMCD consists of intercalated cells
and some principal cells. The principal cells are not involved in
potassium or sodium homeostasis, but rather serve to increase
water permeability in response to ADH.
The terminal segment of the IMCD is comprised of a
unique cell type simply referred to as the IMCD cell. These
cells share some ultrastructural features with principal cells but are functionally distinct. Characteristics of these cells
• Amiloride sensitive sodium channels that are distinct
from ENaC
• Receptors for atrial natriuretic peptide (ANP)
• Expression of H-K-ATPase
• Expression of aquaporins 2 and 4, and receptors for ADH
Several studies suggest that sodium enters the IMCD cell
via an amiloride-sensitive sodium channel, which is regulated by
ANP and aldosterone (Fig. 8.8). ANP inhibits luminal sodium
entry and contributes to the natriuretic effect of this hormone
in vivo. cGMP mediates the transport effects of ANP. Aldosterone
promotes sodium reabsorption in the IMCD by increasing
ion conductance of the amiloride sensitive sodium channel
and activity of Na-K-ATPase. It is worth emphasizing that the
IMCD is the final arbiter of urinary sodium concentration.
Since the sodium concentration in this segment of the
nephron is low (