Rental Transport Mechanisms

Question 1

How much ultrafiltrate is generated everyday? Why is it necessary?

Answer 1

180L per day

certain metabolic end products (such as urea, creatinine, etc) are eliminated primarily by ultrafiltration.

Question 2

The reabsorption of what ion is the main energy consumer in the KD?

Answer 2

• Because of its high concentration in plasma, Na is the main solute that needs to be reabsorbed by the tubules
• The reabsorption of Na is the main consumer of energy in the kidney.
• Since Na concentration in the urine may vary by orders or magnitude, the kidney needs to generate and maintain steep Na concentration gradients.
• The formation of such steep concentration gradients is restricted to the distal part of the nephron
• Most of the filtered Na is reabsorbed by energetically more favorable mechanisms in the proximal tubule and loop of Henle.

Question 3

Why is the macula densa strategically located at the beginning of the distal tubule?

Answer 3

• Because the distal tubule and collecting ducts have limited Na reabsorptive capacity, the macula densa is strategically located at the beginning of the distal tubule to ensure a constant Na load via the tubuloglomerular feedback mechanism.

Question 4

What reabsorption events take place in the proximal tubule?

Answer 4

• reabsorption of ~2/3rd of filtered water and Na
• ~80% of bicarbonate and phosphate
• and practically all nutrients.
• The secretion of xenobiotics also occurs in the PT.

Question 5

In the early segments of the PT, Na reabsorption is coupled to reabsorption of what?

Answer 5

• HCO3-
• nutrients (glucose, lactate)
• and phosphate.

Question 6

Explain the indirect reabsorption of HCO3.

Answer 6

• The reabsorption of HCO3- is indirect because HCO3- does not actually traverse the apical membrane.
• Instead, the PT secretes H+, which combines with filtered HCO3- to form carbonic acid (H2CO3), which then dissociates into CO2 and H2O, and is reabsorbed as such.
• The formation of CO2 and H2O from H2CO3 in the tubular fluid is catalyzed by a carbonic anhydrase residing at the surface of the apical membrane.
• H+ ions are secreted into the tubular fluid by a Na/H exchanger in the apical membrane.
• This transporter is driven by the low intracellular [Na+] established by a Na/K-ATPase located in the basolateral membrane.
• In the PT cell, CO2 and H2O are converted back into carbonic acid by an intracellular carbonic anhydrase.
• Intracellular H2CO3 then dissociates spontaneously into H+ and HCO3-.
• H+ ions are secreted into the tubular fluid by the Na/H exchanger, while HCO3- is exported to the blood by a Na/HCO3 cotransporter, completing the indirect reabsorption of HCO3-.

Question 7

What effect would carbonic anhydrase inhibitors have of Na excretion?

Answer 7

• The reabsorption of NaHCO3 in the PT is almost completely dependent on the activity of carbonic anhydrase.
• Therefore, carbonic anhydrase inhibitors, like acetazolamide, increase Na excretion, and thus are potent diuretics.

Question 8

What are the some of the co-transporters of Na in the proximal tubule?

Answer 8

• Na-glucose
• Na-amino acids
• Na-lactate
• Na-inorganic phosphate co-transporters.

Question 9

Explain why often more than one co-transporter with similar substrate specificity but with increasing ratio of Na to co-transported substance (e.g. 1Na/1glucose, 2Na/1glucose, etc.) is utilized in series?

Answer 9

• It increases the avidity of nutrient reabsorption along the length of the PT. This arrangement ensures that most nutrients are completely reabsorbed in the PT.

Question 10

Why can high rates of water reabsorption can be achieved with a very small difference in osmolality in the proximal tuble?

Answer 10

• Transport of HCO3-, glucose etc. into the interstitial fluid raises its osmolality, while reducing the osmolality of the tubular fluid.
• This osmotic gradient then drives water into the interstitium.
• Since water permeability of the PT is extremely large (due to the expression of water channels), high rates of water reabsorption can be achieved with a very small difference in osmolality.

Question 11

Why does the osmolality of the tubular fluid along the length of the PT remain practically unchanged?

Answer 11

• Because water immediately follows solutes due to high permeability of water in the PT.

Question 12

What is the concept of “solvent drag” and paracellular movement?

Answer 12

• In addition to the high water permeability of the PT cell membrane, the tight junctions in this segment are also highly permeable to water.
• Therefore significant osmotic movement of water also occurs paracellularly.
• Because these tight junctions are also “leaky” to solutes, some of the electrolytes and other solutes are also swept along.
• This process is called solvent drag, and is analogous to the movement of solutes through the capillary membrane in response to the Starling forces.

Question 13

Why does tubular concentration of Cl- increase through the proximal tubule? What is the consequence of this on Na+ and other cations in paracellular transport?

Answer 13

• Due to the preferential reabsorption of Na+ with HCO3-, phosphate and other anions, the concentration of Cl- in the tubular fluid gradually increases toward the later segments of the PT.
• This concentration difference allows Cl- to diffuse through the paracellular pathway into the interstitium.
• The diffusion of Cl- out of the tubule creates a lumen positive voltage, which further aids reabsorption of Na+ and other cations in the later segments of the PT through the paracellular pathway.

Question 14

The second half of the proximal tubule also reabsorbs NaCl via a transcellular mechanism. What are the transport mechanisms?

Answer 14

• This transport is mediated by the combined operation of the apical Na/H exchanger and a Cl/organic anion exchanger.
• The apical Na/H exchanger is driven by the Na gradient established by the basolateral Na/K-ATPase.
• The uphill movement of Cl- into the cell is driven by the higher concentrations of organic anions (mainly formate) inside the cell than in the tubular fluid.

Question 15

How is this concentration gradient is also maintained by the Na/H exchanger?

Answer 15

• secretion of H+ neutralizes the secreted organic anions in the lumen, while the resulting cellular alkalinization aids the dissociation of acid into an anion and H+ inside the cell.
• The key event is that the neutralized organic acids become lipophilic, and diffuse through the lipid bilayer into the cell.
• The end result of this mechanism is uptake of NaCl into the cell.

Question 16

How is Na and Cl transported on from the cell to the blood on the basolateral side of the proximal tubule?

Answer 16

• Na is then moved toward the blood by the basolateral Na/K-ATPase, while Cl- exits via a basolateral K/Cl cotransporter.

Question 17

What is nonionic diffusion? Is it active or passive?

Answer 17

• The movement of lipid soluble weak acids and bases in their undissociated forms through the cell membrane is known as nonionic diffusion.
• This type of transport is passive

Question 18

For permeable weak acids and bases in their undissociated forms, how can net secretion or reabsorption be modulated?

Answer 18

• If the membrane is impermeable to the dissociated form, and there is a pH gradient across the membrane, such transport can mediate net secretion or reabsorption.
• This mechanism is important in the renal handling of NH3/NH4+ and also results in the pH dependent excretion of several drugs (e.g aspirin and barbituates).

Question 19

What are the advantages and disadvantages of PT transport mechanisms?

Answer 19

• The advantage of the above transport events is that large amounts of Na and water are reabsorbed with relatively little expenditure of energy.
• The downside of this efficiency is that transport has to take place through a very permeable epithelium, and thus is prone to back-leak.

Question 20

What is the rate limiting step of the two step process in the PT vs other sections of the tubule?

Answer 20

• In other parts of the nephron the rate limiting step is the first one.
• However, the PT is so leaky and its rate of transport is so high that the rate of uptake into the peritubular capillary can become limiting.
o Consequently, changes in interstitial hydrostatic pressure and the hydrostatic and oncotic pressure in the capillary have a significant impact on the rate of net fluid reabsorption in the PT.

Question 21

Define Glomerulotubular balance.

Answer 21

• An important aspect of the regulation of salt and water reabsorption in the PT is the phenomenon referred to as glomerulotubular balance. (Not to be confused with the tubuloglomerular feedback mechanism.)
• The essence of this mechanism is that even though fluid load arriving to the PT can vary due to spontaneous changes in GFR (that were not corrected by autoregulation), the PT always reabsorbs a constant fraction of the filtered load.
• The purpose of this response is to stabilize the rate at which Na and water are delivered to the loop of Henle, and ultimately to the distal nephron, which has limited transport capacity.

Question 22

What are the three mechanisms are involved in glomerulotubular balance?

Answer 22

• The portion of proximal tubular Na reabsorption that is mediated by co-transporters is limited by the availability of the co-transported substances (such as bicarbonate and nutrients). An increase in filtration provides more substrate to fuel additional Na-coupled reabsorption.
• When GFR changes due to a change in efferent arteriolar tone, significant changes occur in Starling forces.
o For instance, with efferent constriction, hydrostatic pressure in the glomerulus increases while in the peritubular capillary it declines.
o The increased glomerular capillary hydrostatic pressure results in enhanced filtration from a diminished blood flow.
o Consequently, the oncotic pressure entering the peritubular capillaries increases significantly (as more protein-free ultrafiltrate is removed from the blood). The increased oncotic pressure combined with the decreased hydrostatic pressure in the peritubular capillary then facilitates fluid removal from the interstitium and thereby reduces back-leak.
• Luminal flow is sensed by the PT through bending of microvilli. An increase in flow stimulates Na transport.

Question 23

What is the main hormone that stimulates Na reabsorption?

Answer 23

• The main hormone that stimulates Na reabsorption in the PT is Angiotensin II (AII), which stimulates the activity of the Na/H exchanger

Question 24

What part of the renal tubule do sympathetic nerves innvervate?

Answer 24

• Sympathetic nerves directly innervate PT cells.

Question 25

When does sympathetic tone have effect?

Answer 25

• Under normal conditions, sympathetic tone to the kidney is minimal, however, severe volume depletion results in increased renal sympathetic activity, which in turn stimulates salt and water reabsorption.

Question 26

Explain the difference between renal sympathetic blood volume receptors and classical baroreceptors.

Answer 26

• The increased sympathetic activity during volume depletion arises primarily from low-pressure receptors residing in the atria and in large veins, while the classical baroreceptor reflexes originating from the carotid sinus and aortic arch play only a minor role.

Question 27

What is the main mechanism for the stimulation of Na reabsorption?

Answer 27

• The main mechanism for the stimulation of Na reabsorption by renal sympathetic nerves is increased activity of apical Na/H exchanger, and this effect is mediated by alpha-adrenergic receptors

Question 28

The two main inhibitors of Na reabsorption in the PT are? The production of both hormones is stimulated by?

Answer 28

• Dopamine: an autocrine inhibitor (it is produced locally by the PT from L-dopa).
• Atrial Natriuretic Peptide (ANP): a circulating hormone, while dopamine acts

Question 29

The production of both hormones is stimulated by?

Answer 29

• ECFV expansion.
• The stimulus for ANP release is the stretching of the atria, while the signal that enhances dopamine production during volume expansion remains unknown.

Question 30

Explain how Xenobiotics often bypass the glomerulus and are secreted into the tubule. Is this an efficient or inefficient process?

Answer 30

• Another important function of the PT is the elimination of foreign substances.
• Often the first line of defense against toxic substances is their binding to plasma albumin.
• Although this limits the cells’ exposure to these toxicants, it also reduces their filterability by the glomerulus.
• Such substances are often taken up actively through the basolateral membrane via high-affinity transporters and are then secreted into the tubular fluid.
• This is a very efficient process and therefore such substances can often be cleared at a rate comparable to renal blood flow. This is the basis for using PAH clearance for the measurement of RPF.

Question 31

What are the three main categories of xenobiotic transporters?

Answer 31

• organic anion transporters
• organic cation transporters
• multidrug resistance (MDR) proteins
• (Many (~ 50) genes encode transporters that are involved in the elimination of xenobiotics.)

Question 32

Which of the three main categories of xenobiotic transporters are based in secondary active transport and which is based on ATP hydrolysis?

Answer 32

• Anion and cation transporters mediate secondary active transport of xenobiotics (i.e. transport is driven by an existing electrochemical gradient), while MDRs are actual efflux pumps driven by ATP hydrolysis.

Question 33

Explain the reabsorption of albumin and the filtration and degradation of smaller proteins and peptides.

Answer 33

Although the permselectivity of the glomerular filtration barrier is fairly impressive, the albumin concentration of the ultrafiltrate is still ~0.05% of that of plasma. Thus, the total amount of albumin entering the tubular fluid is ~8 g/d, which, if excreted, would constitute significant proteinuria. However, the proximal tubules extract practically all filtered albumin from the tubular fluid, and thus normal urine contains only trace amounts of protein. In contrast to albumin, smaller proteins and oligopeptides (such as hormones) are relatively freely filtered by the glomerulus. These peptides are subsequently degraded in the PT. Consequently, renal elimination is the main determinant of the metabolic clearance of many peptide hormones. Therefore it is not surprising that the half-life of many hormones (e.g. glucagon, PTH, ANP) is prolonged several fold in end-stage renal disease.

Question 34

What are the two mechanisms through which filtered proteins are eliminated from the tubular fluid?

Answer 34

• oligopeptides are degraded by ectopeptidases in the brush border into amino acids and reclaimed as such
• Larger proteins are taken up into the cell by receptor-mediated endocytosis and degraded intracellularly. Amino acids are then returned to the blood through the basolateral membrane.

Question 35

What percentage of Na and H2O does the loop of henle reabsorb?

Answer 35

• It reabsorbs ~25% of filtered Na but only ~15% of filtered water.

Question 36

Explain the differences between the descending and ascending limbs and the cortex and the medulla?

Answer 36

• Water reabsorption is restricted to the descending limb, while the majority of Na reabsorption occurs in the thick ascending limb (TAL).
• The ascending limb is the first segment of the nephron, which is practically water impermeable.
• Consequently, water cannot follow the reabsorbed salt, and thus the tubular fluid becomes hypoosmotic while the medullary interstitium becomes hyperosmotic due to the accumulation of reabsorbed salt.
• It is this hyperosmotic medullary environment that drives the reabsorption of water from the thin descending limb and also allows water to be reabsorbed from the collecting ducts if antidiuretic hormone (ADH) is present (see below).

Question 37

Explain the transcellular reabsorption of Na in the TAL.

Answer 37

• A small fraction of transcellular Na transport occurs via Na/H exchange, similarly as in the PT.
• However, the main route of Na entry into the cell is through a luminal Na/K/2Cl cotransporter.
• Na is then pumped out from the cell by a basolateral Na/K-ATPase, while Cl- exits through a basolateral Cl channel.
• Since the concentration of K in the tubular fluid is much lower than that of Na, continued operation of the cotransporter can only be maintained by recycling K through an apical K channel.

Question 38

Explain paracellular reabsorption of Na, Ca, and Mg in the TAL.

Answer 38

• Although the cotransporter itself is electroneutral (1 Na+, 1 K+, 2 Cl-), the return of K+ into the lumen (secretion) generates a lumen-positive voltage.
• Since the tight junctions of the TAL are highly permeable to cations, the lumen-positive voltage generated by the secretion of K+ allows Na+ to be also reabsorbed paracellularly at a rate comparable to the transcellular pathway.
• The lumen-positive voltage also drives other cations, particularly divalent ones like Ca++ and Mg++ through the paracellular pathway, and thus a sizable fraction of these ions are reabsorbed in this segment.

Question 39

Explain paracellular reabsorption of Na, Ca, and Mg in the TAL.

Answer 39

• Although the cotransporter itself is electroneutral (1 Na+, 1 K+, 2 Cl-), the return of K+ into the lumen (secretion) generates a lumen-positive voltage.
• Since the tight junctions of the TAL are highly permeable to cations, the lumen-positive voltage generated by the secretion of K+ allows Na+ to be also reabsorbed paracellularly at a rate comparable to the transcellular pathway.
• The lumen-positive voltage also drives other cations, particularly divalent ones like Ca++ and Mg++ through the paracellular pathway, and thus a sizable fraction of these ions are reabsorbed in this segment.

Question 40

Explain the target of Loop Diuretics and when they are used.

Answer 40

The luminal Na/K/2Cl cotransporter is the target of a group of potent diuretics that include furosemide (Lasix) and bumetinide (Bumex). These drugs are also known as loop diuretics, and are used mainly to treat the edema associated with heart failure and hepatic cirrhosis. Mutations in this cotransporter are the cause of Type 1 Bartter’s syndrome, which is a severe neonatal salt wasting condition. Since the operation of the cotransporter is linked to recycling of K via an apical K channel and exit of Cl through a basolateral Cl channel, mutations in these channels (Type 2 and 3 Bartter’s syndrome) result in a similar phenotype.

Question 41

How does an increase in salt delivery from the glomerulus into the tubule affect Na reabsorption in the TAL? What is the mechanism of this regulation?

Answer 41

• Similarly as the PT, the TAL also increases its rate of Na reabsorption with an increase in salt delivery.
• The purpose of this response is the same as that of glomerulotubular balance in the PT, i.e. to buffer changes in salt load arriving to the distal tubule.
• The mechanism is thought to be mainly hydrodynamic, triggered by the bending of primary cilia.

Question 42

The TAL reabsorbs significant amounts of Ca++ and Mg++ without water into the medullary interstitium. Ca and Mg have limited solubility and are thus prone to precipitate at high concentrations. How does the kidney protect itself against interstitial calcification?

Answer 42

• The kidney protects itself against interstitial calcification (nephrocalcinosis) by expressing a receptor that senses extracellular [Ca++] on the basolateral side of TAL cells.
• Activation of this receptor inhibits apical K channels in the TAL, which generates the voltage that drives the paracellular reabsorption of Ca++.
• Although this feedback protects against nephrocalcinosis, it also results in inhibition of Na reabsorption by the TAL.

Question 43

Na reabsorption in the LH is also regulated by several hormones, including What? How important is this regulation?

Answer 43

• prostaglandins and nitric oxide (NO).
• Surprisingly, despite the quantitative importance of the TAL in overall Na reabsorption (25% of the filtered load), hormonal regulation of transport in the TAL is not of major importance.

Question 44

What is the type of Na transporter in the macula densa? Why is the macula densa important?

Answer 44

• Just like in the TAL, Na transport in the macula densa (MD) is mediated by the Na/K/2Cl transporter.
• Although the amount of salt reabsorbed by the MD is negligible in comparison to the filtered salt, the signals emanating from this small segment are very important for the overall regulation of Na excretion.
• The MD senses the NaCl load arriving to the distal tubule, and initiates the tubuloglomerular feedback, which assures that the downstream segments are not overwhelmed by a Na load.
• The MD also contributes to the regulation of renin secretion.

Question 45

What percentage of NaCl is reabsorbed in the DT? Is this through transcellular and/or paracellular pathways?

Answer 45

• The distal tubule (DT) reabsorbs ~6% of the filtered NaCl load with a typical Na intake.
• Na transport in the DT proceeds almost entirely through a transcellular pathway.

Question 46

Explain the Na and Cl transporters in the DT. Is water reabsorbed in the DT? How do DT diuretics work?

Answer 46

• NaCl enters the cell through a luminal Na/Cl cotransporter and departs via a basolateral Na/K-ATPase, while Cl exits via a basolateral Cl channel.
• Like the TAL, the DT is also impermeable to water, and therefore the tubular fluid is further diluted in this segment.
• The DT is the site of action of thiazide diuretics, which block Na reabsorption by inhibiting the luminal Na/Cl cotransporter. A defect in this cotransporter is the cause of the inherited kidney disease called Gitelman syndrome, a salt-wasting condition. Gitelman patients present with symptoms practically identical to those with chronic thiazide use.

Question 47

What percentage of Na and water does the collecting duct reabsorb? What is the name of the cells that transport the Na and water?

Answer 47

• On a typical Na and water intake, the collecting duct reabsorbs ~4% of the filtered NaCl load and 15% of filtered water.
• Na and water transport takes place in the majority cell type of the CD, called principal cells.
• (The neighboring intercalated cells are involved in the regulation of acid/base balance).

Question 48

Is the Na transport in the CD transcellular and/or paracellular?

Answer 48

• As in the DT, Na transport in the CD is strictly transcellular.

Question 49

What is the apical Na channel of the CD called? What drives the Na transport into the cell?

Answer 49

• Na enters principal cells via an apical channel that is highly selective for Na+ (called epithelial Na channel; ENaC).
• Entry of Na+ into the cell is driven by both the low intracellular Na concentration (i.e. chemical gradient) and by the negative potential inside the cell.
• Given this large electrochemical gradient, the CD can render the tubular fluid, and thus the urine, practically Na-free.

Question 50

Explain the coupling of Na reabsorption and K secretion in the CD.

Answer 50

• As in other nephron segment, Na exits the cell via the basolateral Na/K-ATPase, in exchange for K, which is secreted into the tubular fluid through an apical K+ channel.
• Na reabsorption and K secretion are thus coupled.

Question 51

Why are CD diuretics called “K-sparing” diuretics?

Answer 51

• Na transport in the CD is the target for a class of diuretics, known as K-sparing diuretics.
• These drugs either block ENaC (amiloride, triamterene) or prevent the binding of aldosterone (see below) to its receptor (spironolactone, eplerenone).
• The term “K-sparing” reflects the fact that in contrast to other diuretics that cause K-wasting, these diuretics reduce K excretion due to the coupling of Na reabsorption to K secretion in the CD.

Question 52

Explain how there could be a gradual loss of the efficacy of loop-diuretics.

Answer 52

• Na reabsorption in the DT still exhibits some load-dependence but this response is mainly manifested in a chronic setting.
• For instance, chronic use of loop-diuretics increases the Na load arriving to the DT, which results in an upregulation of the NaCl cotransporter in the DT.
• This upregulation may lead to a gradual loss of the efficacy of loop-diuretics.

Question 53

Where are the two sites of aldosterone action? Which is the main site?

Answer 53

• Although the main site of aldosterone action is the CD, it also increases Na reabsorption in the DT by upregulating the expression of the NaCl cotransporter.
• This effect also occurs slowly and becomes more relevant in a chronic setting.

Question 54

What is the most important hormone regulator of Na transport? How does it work?

Answer 54

• Na transport is under strict hormonal regulation in the CD.
• Aldosterone is by far the most important regulator of Na transport
• It stimulates Na reabsorption by the principal cells.

Question 55

What effect does ANP and Urodilatin have on Na reabsorption in the CD?

Answer 55

• ANP and Urodilatin, a similar peptide produced in the kidney, inhibit Na reabsorption by the CD, but these peptides play only a minor role in the overall regulation of Na transport in the CD.

Question 56

Explain the upregulation of the AQP2 channel.

Answer 56

• The apical membrane of principal cells is inherently water impermeable but can be rendered highly water permeable via the insertion of Aquaporin 2 (AQP2) water channels.
• In the absence of stimulation AQP2 resides in intracellular vesicles.
• The signal for their insertion is initiated by antidiuretic hormone (ADH) binding to receptors on the basolateral membrane.
• The insertion of AQP2 water channels allows water to be reabsorbed into the neighboring interstitium, which has been rendered hypertonic by the deposition of salt without water in the ascending limb of the LH.

Question 57

How do ANP, urodilatin, prostaglandins and divalent ions (Ca++, Mg++) act as regulators of water transport?

Answer 57

• ANP, urodilatin, prostaglandins and divalent ions (Ca++, Mg++) act as negative regulators of water transport by antagonizing the effect of ADH.

Question 58

Explain the reabsorption and secretion of urea.

Answer 58

• Urea is a major constituent of urine and its excretion is linked to the excretion of water.
• Urea is freely filtered, and it undergoes both passive reabsorption as well as passive secretion in different parts of the nephron.
• ~50% of filtered urea is reabsorbed in the PT by paracellular diffusion and solvent drag.
• The thin limb of the LH has high urea permeability.
• Thus, in this segment urea diffuses passively into the tubular fluid from the medullary interstitium, which under normal hydration has high urea concentration.
• In the subsequent nephron segments not much urea is reabsorbed since these segments have low constitutive urea permeability.
• This allows up to 70% of the filtered load of urea to be excreted when water excretion is high.
• However, during water deprivation ADH increases urea permeability of inner medullary CD, which allows urea to be reabsorbed, and thus only ~15% of the filtered urea is excreted.
• The purpose of this regulation is to maximize water conservation during dehydration: if the CD would be impermeable to urea but permeable to water, urea excretion would require the excretion of additional water.