Reabsorption/Secretion in the Proximal Tubule

Question 1

T or F. The kidney excretes metabolic waste products such as urea, creatinine and uric acid as fast as they are produced in the body assuming a normally functioning kidney

Answer 1

T, so that their level in body fluid is at a very low, non-toxic level.

Question 2

How many times is plasma filtered through the glomerulus?

Answer 2

The entire plasma is filtered through glomerulus about 60 times every day that is equivalent to filtration of entire body fluid about 5 times a day.

Such high rate of filtration enables the body to get rid of the waste products as fast as they produced in the body.

Question 3

How much of the GF is reabsorbed in the PT?

Answer 3

about 2/3. So, the primary role of PT is to reabsorb most of the water and solutes filtered into glomerulus.

Question 4

What is the main function of the distal tubule?

Answer 4

On the other hand the function of distal tubule is regulation of water and solute absorption/secretion and determine the urine composition to suite the physiological status of body fluid volume and composition.

Question 5

What does the epithelium of the proximal tubule look like apically? Why?

Answer 5

In the luminal side the plasma membrane of epithelial cells is folded into many finger like projections called microvilli.

This increases the surface area of apical membrane by thousands of folds, which is much needed for the massive reabsorption of glomerular filtrate in the proximal tubule.

Question 6

T or F. The reabsorption in the PT is iso-osmotic.

Answer 6

T.

Question 7

Why is the PT also an important regulatory site in controlling ECF?

Answer 7

Due to the volume of fluid absorbed in the PT, any change in the rate of reabsorption here can have significant impact on the volume of ECF.

Question 8

What is the normal rate of plasma filtration through a glomerulus? Reabsorption in the PT?

Answer 8

130ml/min
85ml/min

Therefore, GF is delivered into the LOH at the rate of 45 ml/min

Question 9

T or F. Reabsorption in PT is iso-osmotic, that means there is virtually no change in osmolarity of GF as it flows along the PT.

Answer 9

T.

Question 10

How can we determine the fluid amount rate absorbed in the PT?

Answer 10

Can determine by tracking the concentration of a substance that is not reabsorbed in the PT nor it is secreted by the tubular epithelial cells, namely inulin

So, if you infuse inulin intravenously to maintain a plasma concentration of 1 mg/dl, and measure inulin concentration in the tubular fluid, you can calculate the rate of water absorption in the PT.

You can use a glass micropipette to withdraw TF from the PT.

Eqn on next slide

Question 11

What is the mass flow balance in the PT for inulin?

Answer 11

GFR - reabsorption = the volume of GF delivered into the LOH.

So, GFR times plasma inulin concentration is equal to V-L, that is the rate of flow into LOH, x the inulin concentration in the TF.

Rate of flow into LOH or V-L therefore is GFR x plasma inulin concentration (Pin) divided by inulin concentration in the TF.

According to the numbers we have it is 130 times 1 divided by 3; that is 43 ml/min delivered into LOH.

So, 130-43 is 87 ml/min, what is reabsorbed in the PT, that is 2/3rd of GFR.

Question 12

Describe the relationship between the TF to plasma inulin concentration ratio (x) at different distance from the glomerulus (y) in the PT.

Answer 12

Linearly increasing, indicating that fluid is absorbed through out the length of PT making the inulin more concentrated.

Question 13

T or F. Even though inulin concentration is increased along the length of PT, the osmolarity of TF remains virtually the same from the glomerular end to the loop end.

Answer 13

T. Because the absorption is iso-osmotic in the PT

Question 14

What is the mass flow of solutes into the PT normally? Mass flow of reabsorption in the PT?

Answer 14

The plasma osmolarity is about 290 mOmol/L and the GFR is 130 ml/min, and therefore the flow of solutes in PT is 37.7 mOsmol/min.

Reabsorption is 2/3rd of GFR which is 86.6 ml/min, and at isoosmolar reabsorption condition, the solute reabsoprtion is 25.1 mOsmol/min (0.290*86.6).

This means that solute is delivered to LOH at the rate of 12.6 mOsmol/min.

The major solutes that contribute to isoosmotic reabsorption from the PT are sodium, chloride and bicarbonate

Question 15

Where does Na+ absorption primarily occur?

Answer 15

Sodium reabsorption occurs throughout the tubule, although 65% of it is reabsorbed in the PT.

Question 16

How is Na+ reabsorbed?

Answer 16

It is reabsorbed by an active transport mechanism, and the same mechanism is used throughout the tubule.

Due to the involvement of active transport mechanism it is an energy consuming process and accounts for majority of the oxygen consumed in the kidney.

Question 17

Describe the wall of the PT.

Answer 17

The luminal surface of the wall of PT is lined with the epithelium, monolayer of polarized epithelial cells.

At the apical end of epithelial cells the intercellular space is sealed by a selectively permeable junctional complex called a tight junction.

Question 18

What are the tight junctions made from?

Answer 18

made up of complex of proteins providing a barrier function, which means it allows diffusion of selective molecules based on size and charge, while preventing diffusion of most molecules.

It has pore pathways that allows diffusion of specific ions (claudins) and leak pathways that allows diffusion of molecules based on size. But, most ions and solutes have to be absorbed by a transcellular route.

Question 19

T or F. The major portion of Sodium is absorbed by transcellular pathways

Answer 19

T.

Question 20

Describe transcellular reabsorption.

Answer 20

On the luminal membrane there are Na+ channels that allow free diffusion of Na+ from the lumen to the cytoplasm.

The channels for Na absorption in the luminal membrane are several – NHE, Na-Glucose, Na-amino acid and Na-PO4 cotransporters

Question 21

What is the main driving force for NA absorption?

Answer 21

The NKA pump. It is located in the basolateral membrane and transports 3 Na out of the cell and 2 K into the cell at the expense of one molecule of ATP.

Question 22

What are the results of NKA activity?

Answer 22

The decrease in intracellular Na+ concentration and reduction of membrane potential to almost -70 mV. Both reduction of Na conc. in the cytoplasm and negative membrane potential are poised to drive Na reabsorption from the TF into the cytoplasm.

The potential energy of downhill movement of sodium at the luminal membrane is used in the transport of solutes such as glucose and amino acids.

Question 23

Sodium reabsorption in the proximal tubule is accompanied by what?

Answer 23

The absorption of equivalent amounts of anions to maintain electro neutrality.

Question 24

What happens to the TF as sodium is reabsorbed?

Answer 24

As the sodium is reabsorbed by an active transport process, the TF becomes more and more negative.

It can be 5 mV more negative than the interstitial fluid. This lumen negative electrical potential drives the transport of chloride and bicarbonate from the TF into the interstitial fluid.

Question 25

How is Cl transported in the PT?

Answer 25

paracellular routes.

Cldn 4 in the tight junction forms chloride selective pores.

The majority of chloride is absorbed via paracellular route. But, there could be small portion transported by active transport mechanism via anion exchangers.

Question 26

How does HCO3- reabsorption occur?

Answer 26

Bicarbonate transport is coupled to NHE, this is an active transport process.

Therefore, more HCO3 is reabsorbed in the proximal part of proximal tubule compared to chloride reabsorption.

Question 27

T or F. Bicarbonate reabsorption is prefered over Cl in the PT.

Answer 27

T. Unlike Cl transport, which is a passive diffusion, bicarbonate transport is active and driven by proton secretion.

Question 28

Where all does HCO3- reabsorption occur?

Answer 28

Bicarbonate absorption occurs in the PT, DT and CD of nephron, and it is coupled to proton secretion into the lumen.

Question 29

What is the difference between the HCO3- reabsorption in the PT, DT and CD?

Answer 29

the mechanism involved in proton secretion.

NOTE: In all segments the intracellular enzyme carbonic anhydrase catalyzes formation of carbonic acid from carbon dioxide and water. Carbonic acid under physiologic pH dissociates into proton and bicarbonate. Proton is secreted into lumen, while bicarbonate is transported into interstitial fluid.

Question 30

How is proton secretion mediated in the PT?

Answer 30

In the PT, proton secretion is mediated by the luminal membrane channel called NHE, which is an antiporter.

One hydrogen ion is secreted out when one Na is transpotted into the cell.

Question 31

What does the secreted H ion do once in the TF?

Answer 31

Secreted H ion combines with the bicarbonate in TF to form carbonic acid.

Question 32

What happens to the carbonic acid?

Answer 32

A membrane bound carbonic anhydrase enzyme in the luminal membrane breaks carbonic acid into carbon dioxide and water. Carbon dioxide freely diffuse into cell and across the cell into interstitial fluid. The bicarbonate formed during the production of proton by intracellular CA is actively transported across the BL membrane into interstitial fluid by a Na/bicarboate co-transporter.

Question 33

Due to the active transport process, most of bicarbonate is reabsorbed from the PT.

Answer 33

The filtered HCO3 conc is 25 mml/L. HCO3 conc in TF is 3.8 mmol/L. So the mass flow of HCO filtered into PT is 3.25 mmol/min [(130mL/min)(0.025 mmol/ml)] and mass flow of HCO from PT to LOH is 0.17 mmol/min [(44 mL/min)(0.0038 mol/min)].

Therefore, 95% of HCO is reabsorbed in the PT.

Question 34

It is not just a neutralization of HCO3. The luminal HCO3 is neutralized by the proton secreted by the cells. However, a HCO3 is formed inside the cell during proton production by CA.

Answer 34

This HCO is transported into IF. So, there is a net absorption of one molecule of HCO for each HCO neutralized in the lumen. Net result is HCO absorption.
This is an active transport mechanism coupled to active transport of Na.

Question 35

How is the water absorbed from the PT?

Answer 35

Water absorption is driven by osmotic gradient. The massive solute transport of Na+ reabsorption followed by passive diffusion of anions results in a drop in the osmolarity of TF and an increase in omsolarity in the interstitial fluid.

Question 36

What ways does water cross the epithelium out of the lumen?

Answer 36

Water transport occurs by both paracellular route in so called leaky epithelium and

by transcellular pathway via the water channels called aquaporins on the luminal membrane and basolateral membrane.

Question 37

What are the forms of aquaporins?

Answer 37

AQ-1 is at the luminal membrane, AQ-4/5 are in the basolateral membrane.

AQ-2 is in DT, regulated by ADH or vasopressin.

Question 38

Once the fluid with salts is absorbed across the epithelium from the lumen of PT, how is it absorbed into the peritubular capillary?

Answer 38

Three different starling forces are involved in this process:

1) Positive interstitial fluid pressure.
2) Low hydrostatic pressure in the peritubular capillary
3) High oncotic pressure in peritubular capillary

Question 39

How does positive interstitial fluid pressure promote capillary uptake of fluid from the interstitial field?

Answer 39

You have higher hydrostatic pressure in the interstitial space due to fluid accumulation. That favors fluid uptake into the capillary.

Question 40

How does Low hydrostatic pressure in the peritubular capillary promote capillary uptake of fluid from the interstitial field?

Answer 40

As a portion of plasma is filtered through glomerular capillary bed the volume of blood delivered to peritubular capillary is about 20% lower than that in the afferent arteriole. This favors fluid uptake.

Question 41

How does high oncotic pressure in peritubular capillary promote capillary uptake of fluid from the interstitial field?

Answer 41

Due to ultra filtration of plasma in the glomerular capillary the protein conc. in the peritubular capillary is increased resulting in increased oncotic pressure. This also favors fluid uptake by the capillary.

Question 42

T or F. Fluid reabsorption from the PT is iso-osmotic. However, the solute absorption in the PT is selective, not all solutes are absorbed at the same rate. Some are absorbed faster than others and some are not absorbed at all.

Answer 42

T. If you can measure the concentration of each solute in the TF at different lengths of PT you can have some idea about their transport properties

Question 43

Why is the absorption of Na and Cl isoosmotic?

Answer 43

Their concentrations do not change along the length of PT because water absorption is the result of osmotic gradient created by NaCl reabsorption.

NOTE: There is a slight increase in Cl concentration at the end of the PT; this is due to a more dominant absorption of HCO3 in the convoluted PT (HCO3- is slightly dilute).

Question 44

Why is HCO3 absorbed faster than Cl-?

Answer 44

This is because Cl absorption occurs by passive diffusion, whereas bicarbonate absorption involves active transport mechanism

Question 45

How does the concentration of glucose, AA, and organic acids change along the length of the PT?

Answer 45

Glucose, aa and organic acids need to be conserved and are completely reabsorbed by the nephron. They are absorbed by active transport mechanisms and therefore, their conc in the TF are reduced gradually along the length of PT.

Question 46

How does the concentration of inulin or PAH change along the length of the PT?

Answer 46

Inulin is not an endogenous solute. But, if you infuse inulin intravenously and measure its concentration in TF along the length of PT you will find that the concentration increases along PT

this is because inulin is not reabsorbed.

Unlike inulin PAH is actively secreted by PT, and therefore its concetration in the TF is much higher than inulin.

Question 47

How is glucose reabsorption in the PT controlled?

Answer 47

the Na-Glu co-transporter in the luminal membrane of the epithelial cells, which allows transport of one molecule of glucose along with one equivalent of Na from the TF into the cytoplasm.

Two isoforms of glucose transporter are SGLT1 and SGLT2. So, glucose transport is coupled to Na electrochemical gradient, and therefore it is an active transport mechanism. This active transport is essential as all of glucose in GF needs to be reabsorbed in the PT.

Question 48

Can any amount of glucose be reabsorbed?

Answer 48

No, glucose is only completely reabsorbed until it reaches a threshold level of plasma glucose which is 200-220 mg/dl (aka threshold).

Question 49

Describe the relationship of glomerular filtration of glucose with plasma glucose level (x)

Answer 49

glomerular filtration of glucose is increased linearly with the increase in plasma glucose level. 100% of filtered glucose is reabsorbed until a certain concentration of plasma glucose. Until this conc., rate of filtration is equal to rate of reabsorption,

Question 50

Why at a certain conc does the reabsorption of glucose decrease from 100%?

Answer 50

because some of the glucose transporter molecules gets saturated.

so unabsorbed glucose is excreted in urine.

Question 51

As the filtered glucose level is increased above the threshold more and more glucose transporters are saturated and finally reach a state when all transporters are occupied, and the rate reabsorption remains constant. Any further increase in glucose in the filtrate would be excreted out in the urine. Not all nephrons have same capacity. So, at threshold some nephrons are saturated and begin to excrete glucose in urine. With further increase in glucose all nephrons are saturated.

This concentration is called what?

Answer 51

transport maximum, which is either 375 mg/min or 350 mg/dl

Tm is reduced in chronic renal failure due to reduced number of functional nephrons.

Question 52

What are physiological causes of glucosuria? Pathologic?

Answer 52

pregnancy, and

pathophysiologic causes are diabetes mellitus and mutation in SGLT1 and SGLT2, a condition known as familial renal glucosuria.

Question 53

How are AAs reabsorbed from the PT?

Answer 53

coupled to Na electrochemical gradient via Na-amino acid co-transporters

Question 54

Are all AAs transported by the same transporters?

Answer 54

No, There are several isoforms of Na-amino acid cotransporters, some are specific for neutral amino acids, whereas others specific for either acidic or basic amino acids.

Similar to glucose, almost all amino acids are reabsorbed from the PT.

Question 55

How are metabolic intermediates reabsorbed from the PT?

Answer 55

Also coupled to Na electrochemical gradient.

Question 56

Is the plasma conc of metabolic intermediates normally low or high?

Answer 56

Normally, the concentration of metabolic intermediates in the plasma is low.

But, it can be high under pathophysiologic conditions, such as diabetes mellitus. In such conditions, the plasma concentration of metabolites may exceed the capacity of PT to reabsorb, and therefore a part of it may be excreted in the urine.

Diabetic ketosuria is example of that.

Question 57

T or F. Normally, protein in the GF is low, but peptides are filtered through the glomerulus.

Answer 57

PT epithelial cells have peptide transporters that can reabsorb the filtered peptides.

Question 58

When could protein appear in the GF?

Answer 58

Protein may appear in GF in conditions such as multiple sclerosis, hemoglobinemia or myoglobinemia.

Since these proteins cannot be reabsorbed, they will be excrete in the urine

Question 59

How are phosphates reabsorbed from the PT?

Answer 59

Also coupled to Na electrochemical gradient and therefore uses the potential energy associated with the downhill transport of Na. Therefore, it is an active transport mechanism.

Question 60

T or F. Phosphate reabsorption has low threshold

Answer 60

T, and hence phosphate is partially excreted in the urine.

Threshold is normally poised to match the physiological concentration of phosphate in the plasma.

Therefore, any excess of phosphate in daily intake is excreted oiut in the urine

Question 61

What regulates transport maximum for phosphate?

Answer 61

hormones. For example PTH, the parathyroid hormone, decreases Tm for phosphate and therefore facilitates phosphate excretion in the urine

Question 62

How is Cl- reabsorbed from the PT?

Answer 62

Cl is reabsorbed by passive diffusion across the renal tubular epithelium.

Question 63

What are driving forces of Cl-reabsorption?

Answer 63

• concentration gradient created by water reabsorption,
• electrochemical gradient created by Na reabsorption.

While 66% of fluid is reabsorbed in the PT, only 60% of Cl is reabsorbed due to competition by HCO, which is reabsorbed by an active transport mechanism.

Question 64

How is K+ reabsorbed from the PT?

Answer 64

Reabsorption of potassium also occurs by passive transport mechanism. Predominantly, it is transported via tight junctions. Some of the claudins in the tight junctions form cation selective pores.

Question 65

How is urea reabsorbed from the PT? Is it effective?

Answer 65

The passive transport of urea is relatively slow and only 50% of filtered urea is reabsorbed, therefore 50% of filtered urea is excreted in the urine.

Urea clearance in urine can be increased by increasing the urinary flow.

Question 66

Substances that are freely filtered through glomerulus, but not reabsorbed can cause what?

Answer 66

Increase the TF osmolarity and cause diuresis, that is excessive water excretion in the urine. Such substances have clinical significance to stimulate diuresis and reduce ECFV.

Examples of such conditions are intracranial and intra ocular pressure and edema. It is also indicated to promote excretion of toxins form the body.

Question 67

What is Mannitol?

Answer 67

an inert monosaccharide, not produced in the body nor degraded in the body.

It is water soluble, freely filtered into GF, not reabsorbed nor secreted in the PT and is typically used in the clinic to induce diuresis

Question 68

If you continuously infuse mannitol at concentration of 30 mml/L the plasma osmolarity will be increased by 30 mOsm/l, so the total plasma osmoloarity will be 320 mOsm/L instead of 290 mOml/L.

Answer 68

Mannitol filtered into GF would be 3.9 mO,/min, when GFR is 130 ml/min. Under such conditions, the delivery of manntiol to LOH was 67.8 mMol/L. Theoretically, if the water rabsorption was unaffected the mannitol should be delivered to LOH at a concentration of 90 mMol/L.

This suggests that there was a reduction in fluid reabsorption in the PT equivalent to the amount of mannitol in the TF.

Question 69

What are some examples of substances that are secreted into the PT?

Answer 69

• organic acids and bases,
• creatinine,
• PAH or paraamino hippuric acid and others.

Mostly this process occurs by active transport mechanisms. Na co-transporters and other co-transporters are involved in tubular secretion of substances.