Transport Mechanism in the Proximal Tubule

Question 1

What is the proximal tubule?

Answer 1

• Makes up much of the cortical mass in the
  kidney
• Responsible for the initial processing of
  glomerular filtrate
• Contains the most metabolically active cells in
  the kidney

Question 2

Compare the osmolality of the tubular fluid and the plasma?

Answer 2

The osmolality of the tubular fluid falls only very slightly below that of the plasma along the length of the tubule
- The filtrate in the PT is iso-osmotic ≈ 300
mOsmols/L

Question 3

What is the driving force for sodium reabsorption?

Answer 3

The Na+/K+ -ATPase pump located on the basal and lateral sides of the cells of the PT

• lowers the sodium concentration inside the cell to around 5-10 mmol/L
• Na+ reabsorption is almost always by active transport via a Na+/K+ ATPase pump

Question 4

What important role does Na+ reabsorption fulfil?

Answer 4

Na+ reabsorption provides the energy and the means for reabsorbing most other solutes

Question 5

How does Na+ reabsorption aid in the reabsorption of Cl-?

Answer 5

Cl- ions follow Na+ passively as a result of electrical gradient created into the interstitial fluid

Question 6

Describe how Na+ reabsorption aids in the reabsorption of water?

Answer 6

Water follows by osmosis due to osmotic gradient created by NaCl, organic nutrients and selected cations by secondary (coupled) active transport

Question 7

What percentage of filtered substances are reabsorbed and what are these substances?

Answer 7

65% of filtered water, Na+, Cl-, K+ and other solutes

Question 8

Sodium reabsorption is associated with?

Answer 8

• Complete reabsorption of filtered glucose and amino acids

- Almost complete reabsorption of bicarbonate and phosphate ions

Question 9

Glucose reabsorption is known as what kind of process?

Answer 9

transcellular - moves from lumen to the proximal tubular cell via the Na+/glucose co-transport

Question 10

What is secondary active transport?

Answer 10

when solutes move against their electrochemical gradient

Question 11

Describe the 2 types of carriers?

Answer 11

1. co-transport carrier
   
   • Na-glucose co-transporterr,
2. counter transport
   
   • This sodium hydrogen exchanger (NHE-3)

Question 12

Describe how a co-transporter works?

Question 13

Describe how a counter transporter works?

Question 14

Describe how solutes are reabsorbed in the late proximal tubule?

Answer 14

Na+ is reabsorbed with Cl- across both the transcellular and paracellular

Question 15

What is paracellular reabsorption?

Answer 15

solutes flow in between cells

Question 16

Paracellular reabsorption of NaCl occurs because?

Answer 16

the rise in [Cl-] in the tubular fluid in the early proximal tubule creates a concentration gradient of Cl- (140mmol/l in the lumen vs. 105 mmol/l in the interstitium)

Question 17

Glucose is efficiently reabsorbed where?

Answer 17

first third of the PT

Question 18

How much glucose is reabsorbed by the PT?

Answer 18

99.9% of tubular glucose load is reabsorbed, 96.5% proximally. The urine ultimately excreted is glucose free

Question 19

What is the transport maximum?

Answer 19

when the plasma levels start to increase the filtered load will increase and it can surpass the ability of the nephron to totally reabsorb it
- anything filtered beyond that maximal amount reabsorbed will begin appearing in urine

Question 20

What is renal plasma threshold?

Answer 20

The minimal plasma concentration of a substance that results in the excretion of that substance in urine

Question 21

What is the renal plasma threshold of glucose?

Answer 21

300mg/100ml

Question 22

What is the renal plasma threshold?

Answer 22

150mg /100ml

Question 23

What is the renal plasma threshold?

Answer 23

150mg /100ml

Question 24

What is glycosuria?

Answer 24

occurs when the glucose plasma concentration exceeds renal plasma threshold

Question 25

Describe the mechanism of protein reabsorption?

Answer 25

• Protein reabsorption begins when the proteins are partially degraded by the enzymes on the surface of the proximal tubule cells
• The partially degraded proteins are taken into the cell by endocytosis
• Once inside the cell, enzymes digest the proteins and peptides into their constituent amino acids
• Amino acids exit the cell across the basolateral membrane and return to the blood

Question 26

How much protein is reabsorbed?

Answer 26

all of it

Question 27

The driving force for water reabsorption is?

Answer 27

the transtubular osmotic gradient established by solute reabsorption (Na+, Cl+, glucose etc)

Question 28

The high water permeability is due to?

Answer 28

is due to presence of water channels that are formed by aquapolin-1 on both luminal and basolateral membranes
- Because the proximal tubule is highly permeable to water, water will flow by osmosis by paracellular and transcellular routes

Question 29

What is solvent drag?

Answer 29

K+ and Ca2+, are carried along in the reabsorbed fluid

Question 30

Name the organic anions and cations secreted by the proximal tubules?

Answer 30

1. p-aminohippuric acid (PAH)
2. drugs (e.g. Penicillin)
3. nonesteroidal anti-inflamatory agents (e.g. ibuprofen, indomethacin, and naproxen)
4. antiviral drug (e.g. Adefovir)