Proximal renal tubule physiology

1. PROXIMAL TUBULAR
PHYSIOLOGY
Ahad Lodhi
Mentor: Dr Karniski

2. Mechanism of Transport
1. Primary Active Transport
2. Secondary Active Transport
3. Pinocytosis
4. Passive Transport

4. Two pathways of the absorption:
Lumen
Plasma
Cells
Transcellular
Pathway
Paracellular
transport

5. • The proximal tubule is often divided
into:
Ultrastructural
Pars convoluta (PCT)(convoluted part)
Proximal straight tubule (PST) (straight
part)
Histological
S1-segment
S2-segment
S3-segment

6. Reabsorb about 65 percent of the filtered sodium, chloride, bicarbonate, and
potassium and essentially al the filtered glucose and amino acids.
Secrete organic acids, bases, and hydrogen ions into the tubular lumen.

7. • The main function of the proximal tubule is the
isosmotic reabsorption of about 60-65% of the
glomerular filtrate.
• Quantitatively, however, marked differences exist along
the tubule:
– reabsorption of sodium, water, glucose and bicarbonate in
the early proximal tubule (S1) is about three-fold greater
than that in the mid-portion of the convoluted proximal
tubule (S2), and nearly ten times that of the straight
segment of the tubule (S3).
– All segments of the proximal tubule are capable of
reabsorbing the same solutes .
– Proximal tubular reabsorption therefore plays a crucial role
in the maintenance of fluid and electrolyte balance of the
body

8. Objectives of the lecture
Proximal tubular function
Proximal tubular handling of
 Glucose
 Na with Cl,
 Potassium
 Phosphate
 Calcium
 Magnesium
 Amino acid
 Bicarbonate & H+
 Urea
 Water

9. Filtration, reabsorption, and excretion rates of substances by the
kidneys
Filtered Reabsorbed Excreted Reabsorbed
(meq/24h) (meq/24h) (meq/24h) (%)
Glucose (g/day) 180 180 0 100
Bicarbonate (meq/day) 4,320 4,318 2 > 99.9
Sodium (meq/day) 25,560 25,410 150 99.4
Chloride (meq/day) 19,440 19,260 180 99.1
Water (l/day) 169 167.5 1.5 99.1
Urea (g/day) 48 24 24 50
Creatinine (g/day) 1.8 0 1.8 0

10. Proximal convoluted tubule PCT
Reabsorption of
• 65% of Na+ ( 1ry active)
• 65 % of K+ (2ry active), water, urea & Cl- (secondary active and
passive)
• 100% of glucose & amino acids ( 2ry active)
• 90% Ca
• Po4 , Mg+, nitrate, sulfate
• Bicarbonate
formed inside the cell from carbonic acids by the help
of carbonic anhydrase to give HCO3&H2
HCO3 is reabsorbed &H2 is secreted

11. Transport of solutes out of the proximal tubule can
be described to occur in two phases.
In the first phase, essential nutrients such as
glucose, sodium bicarbonate, and amino acids are
predominantly reabsorbed.
The second phase predominantly involves NaCl
reabsorption

12. Na-K ATPase Pump

14. Glucose reabsorption

15. Glucose reabsorption
• Glucose reabsorption in the proximal tubule
occurs in two steps
– Carrier mediated, Na/glucose co-transport across the
apical membrane
• Followed by facilitated glucose transport and active sodium
extrusion
• Two specific Na coupled carriers have been
identified in the apical membrane
-SGLT-1 and SGLT-2

16. Figure 1 Glucose reabsorption in the proximal tubule
Mather, A. & Pollock, C. (2010) Renal glucose transporters: novel targets for hyperglycemia management
Nat. Rev. Nephrol. doi:10.1038/nrneph.2010.38

18. • These depend on the sodium gradient and
glucose transport is therefore a secondary
active step as sodium gradient has to be
actively maintained.
• Transport of glucose across the basolateral
membrane involves the GLUT i.e. GLUT 2 in
the early PT and GLUT 1 in the late PT

19. SGLT & GLUT
• Glucose reabsorption is maximum in
the S1 segment and slows as the
tubular fluid progresses from S1 to S3.
• However the affinity for glucose rises
from S1 to S3 as indicated by the Km
– (Km is defined as the concentration of
substrate at which a half-maximal rate
of transport is attained)
– The Km for S1 is about 2 mM and for S3
it is 0.4 mM
• The different affinities for glucose in
the different proximal segments is due
to the presence of the two SGLT
carriers, i.e. 1 and 2.

20. SGLT & GLUT
• SGLT-2 has a high capacity but low affinity and is found
in the early proximal tubule, whereas SGLT-1 has high
affinity but low capacity and is found in the late
proximal tubule.
• As the early part of the proximal tubule is in the outer
cortex,SGLT-2 is found predominantly located there,
whereas SGLT-1is found located in the outer medulla,
where S3 is located
• Exit of glucose from the proximal tubular cells is via
GLUT, in particular GLUT2 , which is a high-capacity,
low affinity baso-lateral transporter found in tissues
with large glucose fluxes, such as intestine, liver,
pancreas and proximal tubule (S1 and S2) .
• Fanconi Syndrome: GLUT 2 Mutation

21. Sodium and chloride (Na and Cl)
Reabsorption

23. • The majority (70%) of sodium is reabsorbed in the proximal
tubule. It is reabsorbed into the cytosol of the epithelial cells
either alone by diffusion through ion channels followed by
water or together with another product such as chloride,
glucose or AA using a co-transporter by secondary active co-transport.
• First Phase: Along with glucose, amino acids Phosphate and
bicarbonate in the early part of the proximal tubule(co-transporter)
secondary active
• Second Phase (along with chloride):
Active & Passive involving electric (ion channels) and
concentration gradient resulting in ATP utilization and
diffusion.

24. • Sodium chloride reabsorption occurs along the entire nephron
• The percentage of NaCl absorption varies from one third to
two thirds of the total NaCl absorption in the second phase of
proximal absorption

25. Nacl
• Historically, sodium reabsorptive pathways have been
emphasized over chloride, but in truth, both ions must
be reabsorbed to defend or expand extracellular
volume and, in fact, some authors have argued that
chloride balance actually takes precedence over
sodium in determining extracellular volume and blood
pressure
• In the proximal tubule, chloride reabsorption is
indirectly linked to sodium reabsorption through
potential and concentration gradients resulting from
active sodium transport

26. • Passive Paracellular Chloride Transport in the
Proximal Tubule
• Paracellular chloride flux can consist of two
components:
– diffusion driven by the electrochemical chloride
gradient, and
– solvent drag driven by convective forces resulting
from bulk solvent flow.
Solvent drag has been invoked since early days of renal
physiology although the bulk of evidence indicates that
solvent drag does not play a significant role in sodium
chloride reabsorption

27. Active Transcellular Chloride
Transport in the Proximal Tubule
• Apical chloride entry pathways
Warnock and Yee
demonstrated the presence
of bicarbonate-independent
chloride-base (presumably
hydroxide) exchange in
brush border vesicles that
could support such a
transport model

28. chloride/formate exchange
• Karniski and Aronson found that rabbit
brush border vesicles contain Cl-formate
exchange activity
• In the case of formate recycling, the
process appears to be indirectly
coupled to sodium transport through
activity of the apical sodium-proton
exchanger NHE3 and the consequent
acidification of the luminal fluid
• NHE3 supports electroneutral
exchange of sodium ions for protons.
The inwardly directed sodium gradient
generated by the sodium-potassium
ATPase thus drives extrusion of
protons and acidification of the
luminal fluid.
• The low pH in the lumen would result
in a fraction of the extruded formate
becoming protonated to formic acid,
which as a small uncharged molecule,
has some significant membrane
permeability and could diffuse into the
cell across the apical membrane.

29. chloride-oxalate exchanger
• oxalate-driven transport is not inhibited by
blockers of NHE3 or in the NHE3 knockout mice
• acidification-independent recycling pathway
• oxalate-dependent absorption was found to
require not only active sodium transport but
also lumenal sulfate
• Proximal tubule brush border vesicles were
found to exhibit anion exchange activity that
could exchange oxalate for sulfate
• Brush border sulfate uptake is carried out by a
sodium-sulfate cotransporter (due to Na
gradient) that carries two sodium ions and one
sulfate ion into the cell
• The high intracellular concentration of sulfate
drives oxalate into the cell via a sulfate-oxalate
exchanger, and the resulting elevated
intracellular concentration of oxalate drives
chloride into the cell via the chloride-oxalate
exchanger

30. Basolateral chloride exit pathways
Possible pathways for chloride exit across the
basolateral membrane of proximal tubule cells,
including a
• chloride channel (top),
• a potassium chloride cotransporter
(middle),
• a sodium dependent chloride bicarbonate
exchanger (bottom)

31. Potassium reabsorption

33. Phosphate reabsorption

34. • At a blood pH of 7.4, 80% of the ionized
phosphate is HPO4-2 and rest H2PO4-
• At a GFR of 180 l/day approximately 7000 mg of
phosphate is filtered per day. Nearly 90% of
the filtered phosphate is reabsorbed.
• The proximal tubule reabsorbs 80% of the filtered
load
• Phosphate reabsorption is sodium dependent
and enters the apical membrane by secondary
active transport and leaves the basolateral
membrane passively

36. Calcium reabsorption

37. • The proximal tubule reabsorbs about 50-60%
of the filtered calcium
• The transcellular reabsorption probably
accounts for about 10-15%of the total calcium
reabsorption in the proximal tubule and
present in the S1 segment of the PCT

38. • Calcium reabsorption in the S2
segment of the proximal tubule
is mainly passive and paracellular
• It parallels the reabsorption of
sodium and water
• Claudin-2 is proposed to be the
paracellular calcium channel
• There is also a possibility
that there is active transport of
Ca 2+,which is transcellular
involving passive movement of
calcium into the cell through
epithelial calcium channels and
then a basolateral extrusion by
Na+/Ca2+
exchanger driven by Na+-
K +ATPase or Ca2+- ATPase.

39. Magnesium reabsorption

41. The Secretion of H+ and the
Reabsorption of HCO3
-

43. The Secretion of H+
• The secretion of H+ in this section of the nephron is mainly
a result of the Na+/H+ exchanger
– This is an antiporter in the apical membrane
– Energy for this process is provided by the Na/K ATPase in the
basolateral membrane
– Therefore it is secondary active transport
– The ATPase pumps sodium out of the cell into the interstitium
– This maintains a low intracellular Na which creates a gradient for
the absorption of sodium by the Na+/H+ antiporter
– This allows it to drive H against its concentration gradient
– Maintains a negative intracellular potential
– It is essential that HCO3
- is removed from the cells by the co-transporter
with sodium to ensure efficient H+ secretion.

44. Reabsorption of HCO3
-
• Very efficient reabsorption mechanism
• 90% in first 1-2mm of tubule
• Lots of luminal carbonic anhydrase
• Stops the accumulation of H2CO3 in the lumen
• Keeps H+ concentration low - helps antiporter
• Roles of carbonic anhydrase
- from h20 and CO2
– In the cell forms HCO3
– In the tubule it works in reverse forming CO2 and H2O from
the intermediate H2CO3 which forms from HCO3
- and H+
– Allows continuous H+ secretion and HCO3
- reabsorption

45. Urea
• 50% of filtered urea is reabsorbed in the
proximal tubule. However the concentration
of urea actually increases thanks to the
reabsorption of 70% of the filtered water in
the same portion of the nephron. Urea is not
able to be reabsorbed from this point until it
reaches the lower portion of the collecting
duct therefore its concentration further
increases with the reabsorption of water.

46. Secretory Function

49. passive reabsorption or secretion
depending upon the urine pH
• Salicylic acid, for example, exists both as the intact acid
and the organic anion
• Salicylic acid <—> H+ + salicylate-
• The intact acid, but not the organic anion, can freely
diffuse across cell membranes because it is nonpolar
• This difference makes salicylate excretion pH-dependent
• Raising the urine pH (which lowers the free H+
concentration) will shift the above reaction to the right.
The ensuing fall in the urinary salicylic acid
concentration will minimize the back diffusion of
secreted salicylic acid out of the tubular lumen,
thereby increasing total drug excretion

50. Water reabsorption

51. The transepithelial water permeability (Pf) of
the proximal tubule is very high, which allows
small osmotic pressure differences to drive
water transport.
• Both paracellular and transcellular pathways
are believed to be involved in the
movement of water. The transcellular pathway
is more dominant and involves aquaporins

53. Thank you