PROXIMAL TUBULAR FUNCTION

Professor Harbindar Jeet SinghFaculty of Medicine

Universiti Teknologi Malaysia

Objectives of the lecture

2. Proximal tubular function

Proximal tubular handling of

NaCl, Water,Glucose,Calcium and magnesium,Potassium,Phosphate

1. Characteristics of the proximal tubule

Reabsorption and secretion in the kidney takes place in the

proximal tubule,

loop,

distal tubule and

the collecting duct

Ultrafiltration takes place in the glomerulus

The proximal tubule is often divided into:

Proximal tubule - Pars convoluta (PCT)(convoluted part)

- Proximal straighttubule (PST)(straight part)

S1-segment

S2-segment

S3-segment

Histological Ultrastructural

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

In addition to the variable reabsorptive capacity between the early and late proximal segments, in the earliest parts of the proximal tubule (S1) there is also a preferential reabsorption of organic solutes (glucose, and amino acids), sodium bicarbonate, lactate, acetate, phosphate and citrate.

The early segments have a greater membrane surface area, and more mitochondria.

The rate of reabsorption along the proximal tubule is therefore not constant or the same throughout the tubule

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

S1 S2 S3

Glucose reabsorption in the proximal tubule occurs in two steps

(i) Carrier mediated, Na+/glucose cotransport across the apical membrane

(ii) 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

Glucose reabsorption

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 latePT.

It is a passive process

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.

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-1 is found located in the outer medulla, where S3 is located.

Exit of glucose from the proximal tubular cells is via GLUT,in particular GLUT2 and GLUT1, 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).

GLUT2 mutation is present in humans who present with Fanconi syndrome, which is glycosuria with generalisedproximal tubular dysfunction.

1. SGLT-2 binds to one Na+ per glucose

Properties of SGLT transporters

2. SGLT-1 carries two Na+ per glucose

3. SGLT-2 has high glucose carrying capacity

4. SGLT-1 has a low glucose carrying capacity

5. SGLT-2 has a low affinity for glucose (Km 2 mM)

6. SGLT-1 has a high affinity for glucose (Km 0.4 mM)

7. SGLT-1 has a Km for sodium of 50 mM

8. SGLT-2 has a Km for sodium of 228 mM

9. SGLT-1 has an affinity for D-galactose that is 10-fold higher than SGLT-2

Patients with rare congenital disorder of glucose-galactose malabsorption have a partial defect in renal reabsorption of glucose, whereas patients with renal glycosuria have normal intestinal glucose transport.

Renal tubular glucose handling

Since glucose transport is carrier dependent it exhibits saturation kinetics.

Renal threshold for glucose

Transport maximum TM

for glucose

In an adult human, thetransport maximum for glucose is 375 mg/min

Reabsorption of sodium chloride (NaCl)

Sodium chloride reabsorption occurs along the entire nephron

Na+ is avidly reabsorbed with glucose, amino acids and bicarbonate in the early part of the proximal tubule. This can be considered the first phase.

The second phase of Na+ reabsorption is together with chloride

Both passive and active processes contribute to NaCl reabsorption in the second phase

The percentage of passive NaCl absorption varies from one third to two thirds of the total NaCl absorption in the second phase of proximal absorption

Diffusion is probably the main driving force for passive sodium transport in the second phase, in addition to the lumen PD and Na+ gradient.

Both these forces are generated by the preferential active reabsorption of sugar, amino acids and HCO3

- in the first phase

The high Cl- concentration results in paracellular movement of Cl- from the lumen to the peritubular plasma creating a lumen positive PD, which then drives the passive paracellular sodium transport

Active NaCl reabsorption in the second phase involves electrogenic Na+ reabsorption, involving the basolateral Na+, K+-ATPase

The lumen-negative PD created by active sodium reabsorptionthen also provides the driving force for paracellular Cl- reabsorption

The entry of sodium into the tubular cell is favoured by the low Na+ concentration inside the cell

Renal handling of sodium chloride

Site of aldosterone action (2-3% of filtered sodium is under humoral control.

Aldosterone acts on principal cells of CD.

Aldosterone increases apical sodium channels, apical K+ channels, basolateral sodium pump activity, and mitochondrialmetabolism.

Proximal tubular calcium reabsorption

The proximal tubule reabsorbs about 50-60% of the filtered calcium

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

About 55% of the total serum calcium is filterable at the glomerulus

98 – 99% of the filtered calcium is reabsorbed

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+/Ca 2+ exchanger driven by Na+-K+ ATPase or Ca 2+ - ATPase.

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(Note: Transcellular pathway is the major pathway in the TALH and DCT).

Proximal tubular reabsorption of calcium is decreased by volume expansion and increased by volume depletion.

Calcium transport in the proximal tubule

Calcium handling by the nephron

Effects of PTH

There are 2 types of PTH receptors.PTH1R and PTH2R. PTH1R binds to PTH and PTHrP, PTH2R binds only to PTH.

Action.

At the glomerulus decreases kf reducing gfr and filtered load of calcium,

In the proximal tubule PTH inhibit NaHCO3

-reabsorption calcium reabsorption.

The increased distal delivery of HCO3-

Increases calcium reabsorption by Increasing the apical calcium channelsand paracellular permeability

Magnesium reabsorption

70-80% of the serum magnesium is freely filtered at the glomerulus

Only about 3% of the filtered load appears in the urine

The proximal tubule reabsorbs between 5-15% of the filtered magnesium load

The mechanism of magnesium reabsorption in the proximal tubule is not clearly understood but may be paracellular and a passive process

Magnesium handling by the nephron

Potassium reabsorption

Potassium is freely filtered at the glomerulus

There is net potassium reabsorption in the early part of the proximal tubule.

Under normal situations it is rigidly coupled to that of Na+ and water

However, there occurs a net entry of K+ into the lumen at the proximal straight tubule (S3) and the thin descending limb.

Approximately 65% of the filtered K+ is reabsorbed in the proximal convoluted tubule mainly by the paracellular route

K+ reabsorption in the proximal tubule is primarily passiveand mainly paracellular

Weinstein proposed a model for this paracellular movement

i) A decrease in K+ conc in the lateral interspaces due to active K+ uptake by the N+-K+-ATPase

iii) Diffusion of K+ from lumen to the lateral spaces

ii) Low or absent diffusion from plasma to the interspace

There occurs

Potassium reabsorption in the proximal tubule

Potassium handling by the kidney

Phosphate reabsorption

Plasma phosphate concentration varies between 0.8 and 1.5 mM.

The filterable phosphate varies between 0.7 – 1.3 mM (ionised and complexed)(i.e. 86 % of the total phosphate concentration).

mg/dl mM Ionised H2PO4

- and HPO42- 2.1 0.7

Diffusible phosphate 1.5 0.5 Non-diffusible phosphate 0.6 0.2 Total phosphate 4.2 1.4

At a blood pH of 7.4, 80% of the ionised phosphate is HPO42- and the

rest as H2PO4-

At a GFR of 180 litres/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.

Proximal tubule

Phosphate-anionexchanger

Water reabsorption

In the proximal tubule water movement from the tubular lumento the peritubular capillary is the passive consequence of activesolute reabsorption

It is determined by the transepithelial driving forces for water and the passive transepithelial water permeability (Pf)

The Pf of the proximal tubule is very high, which allows smallosmotic pressure differences to drive water transport.

Both paracellular and transcellular pathways are believed to be involved in the movement of water. The transcellular pathwayis more dominant.

Water moves across the apical membrane through aquaporins(more details in the lecture on urine concentration mechanisms)

Amino acid transport.

The proximal tubule apical membrane transports all amino acids

i) Na+ dependentii) Na+ independent

Amino acid transporters can be divided into two major groups

About 90% of the filtered load of amino acids is reabsorbedin the first part of the proximal tubule.

The remaining 9-10% is reabsorbed in the late proximal tubule

There are a smaller number of amino acid transport proteins than there are amino acids, implying that some transporters accept multiple amino acids

i) Neutral amino acid transportersii) Acid or anionic amino acid transportersiii) Basic or cationic amino acid transporters

The AA transport systems have also been divided according the AA transported

Transport of acidic amino acids (anionic transporters)

This transporter is responsible for the transport of aspartate and glutamate

The acidic or cationic amino acid transporters carry a negative charge, and the amino acid transport is coupled to the transport of at least two Na+ ions or in some instances by acountertransport of K+ and H+.

Both however carry sodium and the glycine transporter alsocarries chloride

The other transports glycine only.

One of these transports all the three neutral amino acids namely; glycine, proline, and hydroxyproline

Two distinct amino acid transporters have been identified for the transport of neutral amino acids

Transport of neutral amino acids

Transport of basic amino acids (cationic transporters)

Amino acids lysine and arginine are transported by the same amino acid transporter that transports the neutral amino acid cystine.

L

Handling of amino acids along the nephron

THANK YOU

Amino acid transport systems found in the kidney