DISTAL TUBULAR DISTAL TUBULAR FUNCTIONFUNCTION

Prof Harbindar Jeet SinghFaculty of Medicine

Universiti Teknologi MARA

Objectives of this lecture

Distal tubular handling of NaCl

Calcium and magnesium

Potassium

Phosphate

The distal tubule is composed of three morphologically distinct segments

1. Thick ascending limb of the loop of Henle (pars recta)

2. Macula densa

3. Distal convoluted tubule (pars convoluta)

The thick ascending limb can be divided into

i) a medullary segment

i) a cortical segment

The thick ascending limb is involved in the transport of NaClfrom the lumen to the surrounding interstitium

Its epithelium is impermeable to water

The reabsorption of NaCl is mediated by an Na+-K+-2Cl-

cotransport mechanism

It reabsorbs about 30 % of the NaCl filtered at the glomerulus

Transport of NaCl in the TAL

Na+ exits the cell at the basolateral membrane via the Na+-K+-ATPase, Cl- exits via the K+/Cl- co-transporter

The energy for this reabsorptive process is provided by theNa+-K+-ATPase that is located in the basolateral plasma

Vasopressin (ADH) strongly upregulates this co-transporter in the thick ascending limb

This can be inhibited by loop diuretics like furosemide and bumetanide

Calcium handling by the TALH

About 15-20% of the filtered load of Ca 2+ is reabsorbed in the TAL

About half of calcium reabsorption in TALH is passive and the other halfis active.

The passive transport is via the paracellular route, driven by the positive lumen PD created by the active reabsorption of NaCl and the luminal membrane recycling of K+.

The active component is transcellular.Calcium enters via ECaC passively andthen extruded actively either through Ca pump or Na+/Ca 2+ exchange andactive sodium extrusion

PTH increases Ca 2+reabsorption in the TALH.

It increases the paracellular permeabilityto calcium via an effect on claudin-16,and perhaps also the some activecomponent of calcium transport.

The TAL is the major site for magnesium reabsorption, accounting for 60-70% of magnesium reabsorption

Like calcium, the mechanism of reabsorption of Mg 2+in the TALH is paracellular and passive, involving a specific tight junction protein called paracellin-1/Claudin-16. The driving force for paracellular magnesium reabsorption is the positive lumen PD

Loop diuretics, like furosemide, which inhibit the Na+-K+-2Cl-co-transporter, and prevent the establishment of the lumen positive PD inhibit Ca 2+ and Mg 2+ reabsorption

Magnesium handling by the TALH

Lumen PTH enhances the reabsorption ofMg 2+ by increasing NaCl reabsorption generating a trans-epithelial PD, and increasingthe passive permeability of TALHto paracellular transport

Potassium handling in the TAL

The medullary TAL, which has a high basolateral-to-apical K+ permeability ratio exhibits net K+ reabsorption

The corticol TAL, which has a low basolateral-to-apical K+ permeability ratio exhibits net K+ secretion

There is net K+ reabsorption in the TAL, where about 25% of the filtered load is reabsorbed.

Reabsorption is both passive via paracellular route because of lumen-positive voltage, and transcellular route involving secondary

active Na+-K+-Cl cotransport (NKCC2).

Inhibited by loop diuretics like furosemide

Tubular function (Distal Convoluted tubule - DCT)

The DCT begins beyond the macula densa and extends to the collecting duct

Reabsorption of NaCl is present in the DCT mainly via the transcellular routeinvolving the thiazide sensitive Na+/Cl cotransporter and the basolateral Na+-K+-ATPase.

The Na+/Cl- co-transporter is up-regulated by aldosterone.

In the CCT Na+ reabsorption is mainly transcellular and mediated by the principal cell.

About 5% of the filtered load is handled at the DCT and CCT.

Sodium crosses the apical membrane of the principal cell via the ENaC, which is blocked by amiloride.

The medullary CT only reabsorbs about 3 % of the filtered load.

The late distal tubule (CNT) together with the initial collecting tubule (ICT)has the greatest capacity for K+ secretion

K+ secretion is sodium chloride dependent, and is inhibited by thiazide diuretics

The secretion of potassium is a two-stage process

a) Uptake by the cell from the interstitium by Na+-K+-ATPase at the basolateralsurface

b) Passive diffusion of potassium from the cell to the tubular lumen

The luminal or apical membrane of the principal cells is highly permeable to K+

In cases of potassium depletion, there is net reabsorption of K+ in the late distal tubule. It occurs through the α intercalated cells.

The ICT and the CCT have approximately 70% principal cells (which secrete K+) and30% intercalated cells (some of which reabsorb K+)

The capacity for K+ secretion diminishes in the MCD. If anything there might be some K+ reabsorption in the MCD

Mechanism of potassium secretion in the distal tubule

a) Uptake by the cell from the interstitium by Na+-K+-ATPase

b) Passive diffusion ofpotassium from the cell to the tubular lumen

Renal tubular handling of potassium

A number of factors influence distal tubule K+ secretion

ii) NaCl delivery to the distal tubule

Increase NaCl delivery increases K+ secretion and reduced delivery decreases K+ secretion by the distal tubule

iii) Effect of distal tubular flow rate increases K+ secretion

For a given increment in flow, distal secretion is greater when K+ is high then when it is moderate

When K+ intake is low, flow has little effect on secretion

i) Increased ECF K+ conc

a) by stimulation of Na+-K+-ATPaseb) increased aldosterone secretion

As might occur in increased ECFV or diuretic use

Potassium excretion as a function of plasma concentration

Intake

K+ secretion as function of distal tubular flow

Diet

iv) Aldosterone

Aldosterone stimulates active reabsorption of Na+ by the principal cells, which is mediated through Na+-K+-ATPaseat the basolateral surface

Aldosterone increases the permeability of the luminal membrane for potassium

v) Acute acidosis decreases potassium secretion, whereas alkalosis increase potassium secretion

Chronic acidosis leads to an increase in K+ secretion

Acidification leads to complete cessation of channel activity. conductance of both K+ and Na+ decreases

vi) Role of anions in potassium secretion

Infusion of sodium bicarbonate stimulates K+ excretion

K+ secretion rises sharply in the presence of poorly permeableanions such as sulphate

vii) Effect of diuretics

Diuretics affect K+ excretion in several ways

a) Directly acting on the transport mechanism

e.g. carbonic anhydrase inhibitors stimulate secretion in the distal tubule

b) Kaliopenic diuretics such as , spironolactone, amiloride, triamterene inhibit K+ secretion

vii) Circadian fluctuations

K+ excretion by the kidney is lowest in the early morning (5-6 am) and rises to peak in the late afternoon (4-6 pm)

This diurnal rhythm is independent of food intake or activity.

c) Furosemide and loop diuretics are powerful inhibitors of Na+/K+/2Cl- co-transport mechanism in the TAL.

This collapses the positive luminal PD difference therebyincreasing potassium leakage.

The increased flow to the distal region also further contributes to the increased secretion

viii) Other hormones

a) Vasopressin

Vasopressin stimulates Na+ reabsorption and K+ secretion in the distal tubule

The rise in Na+ reabsorption is triggered by activation of basolateral V2 receptors, which increase apical membrane Na+ conductance that is dependent on cAMP and PKA.

Concomittant depolarisation of the apical membrane stimulates K+ secretion

Vasopressin also increases the activity of the low-conductance K+ channels in the apical membrane

b) Insulin

Insulin reduces K+ excretion by the kidney independently of changes in plasma K+ conc and aldosterone

It is possible that this action is associated with a reduction in Na+ reabsorption secondary to the inhibition of apical sodium channels by insulin

c) Glucocorticoids

Glucocorticoids stimulate K+ excretion by increased flow and distal sodium delivery

Calcium and magnesium handling by the distal tubule

Approximately 10-15% of filtered calcium is reabsorbed in the distal tubule

As the PD is lumen negative and the calcium concentration is below that of plasma, Ca 2+ absorption is therefore active in this segment

The DCT is the primary site for PTH action, stimulating Ca 2+ reabsorption via c’AMP

Calcium enters apically through ECaC, diffuses across the cytosol bound to calbindin-D28K and exits basolaterally through a calcium pump, or a sodium-calcium exchanger

The distal convoluted reabsorbs about 5-10% of the filtered load of magnesium.

Phosphate reabsorption

Approximately 10% of the filtered load of phosphate might be reabsorbed in the DCT and the CCT.

This might only be evident during instances of phosphate deficiency.

THANK YOU

Potassium transport along the nephron