Renal Tubular Transport

Question 1

What are the basic mechanisms for
•Passive or “downhill” transport:

Answer 1

–Simple diffusion: “down” electrochemical gradient via lipid bilayer or aqueous channels

–Facilitated diffusion: “down” electrochemical gradient; specific carriers are required

Question 2

What are the basic mechanisms for active transcellular solute movement?

Answer 2

Energy dependent uphill process.

primary active transport

secondary active transport.

Pinocytosis

Question 3

–Primary active transport:

Answer 3

against electrochemical gradient; ATP hydrolysis provides energy

Question 4

–Secondary active transport:

Answer 4

“downhill” movement of one substance provides energy for “uphill” movement of another substance
•Cotransport, countertransport

Question 5

What percentage of filtrate does the proximal tubule reabsorb?

What is absorbed here?

Answer 5

•Proximal tubule reabsorbs 60-80% of the filtrate

–Most of filtered H2O, Na+, K+, Cl-, bicarbonate, Ca2+, phosphate
–Normally, all the filtered glucose, amino acids

Question 6

What is secreted in the proximal tubule?

Answer 6

•Several organic anions and cations (including drugs, drug metabolites, creatinine, urate) are secreted in proximal tubule

Question 7

What transporter is highlighted for the proximal tubular transport?

Answer 7

Na-K-ATPase

Question 8

Are urea and Cl- secreted by the proximal tubule?

Answer 8

no

Question 9

What facilitates the net unidirectional transport of Na+ in the PCT?

Answer 9

•Polarity of epithelial cell membranes facilitates net unidirectional transport

Question 10

What powers the transport of Na+ in PCT reabsorption?

Answer 10

• Ultimately powered by Na+,K+ ATPase in basolateral membrane
• Na+ reabsorption is usually coupled to transport of or exchange for another solute

Question 11

Na+ Reabsorption is Linked to what kind of transport?

Answer 11

Transcellular Transport

Question 12

Paracellular Reabsorption of Cl- and Urea in Early PCT is not an active process. What does it depent on?

Answer 12

•dependent on Na+ and H2O reabsorption

Question 13

In the early PCT, there are no Cl- transporter. As Na+ and water are reabsorbed, Cl- and urea become more concentrated in luminal fluid. What provides the driving force for paracellular reabsorption?

What else allows transport of Cl-?

Answer 13

–Modest concentration gradient between lumen and peritubular interstitium provides driving force for paracellular reabsorption

–There are specific Cl- channels (typically in the form of anion exchanger) in the later PCT that allows transcellular transport

Question 14

What does the transcellular transport of Cl- lead to?

Answer 14

•This creates a slightly positive charge in the tubular fluid, which helps drive paracellular reabsorption of Ca, Mg, and K

Question 15

Where are organic nutrients (glucose, amino acids) reabsorbed?

Answer 15

The PCT

Question 16

How do the kidneys regulate plasma concentrations of glucose and A.A.’s?

Answer 16

They don’t. Thats the role of the liver and endocrine system

Question 17

What is the Basic Mechanism of Tubular Reabsorption of Glucose & Amino Acids

Answer 17

•Secondary active transport; only transcellular pathways
•Uptake across luminal membrane:
–Against concentration gradient
–Coupled to Na+ entry down its electrochemical gradient
–Ultimately dependent on Na-K-ATPase
•Exit cells through basolateral membrane by facilitated diffusion

Question 18

If filtered amount (load) of glucose (= GFR · P_glucose) exceeds a certain rate:

Answer 18

–Capacity of nephrons to reabsorb all the filtered glucose is exceeded (same with amino acid transport)
–Glucose appears in the urine (glucosuria)
–Osmotic diuresis!

Question 19

•Can you identify a disease in which saturation of the Na-glucose cotransporters occurs?

Answer 19

Diabetes

Question 20

Tubular handling of organic acids and bases is affected by pH of

Answer 20

of luminal fluid, H+ in the tubular lumen favors reabsorption of organic acids, but traps organic bases in the lumen

Question 21

Characterize the descending limb of the loop of henle’s permeability.

Answer 21

•Descending limb is highly permeable to H2O, moderately permeable to solutes

Question 22

Characterize the permeability of the ascending limb of the loop of henle

Answer 22

•Ascending limb is highly permeable to solutes, but impermeable to water

Question 23

What is the first portion of the early distal tubule?

The next part is highly convoluted, what is it permeable to? Impermeable to?

Answer 23

macula densa

•Next part is highly convoluted and permeable to most ions, but is impermeable to urea and water (diluting segment)

Question 24

Describe the permeability of the late distal tubule along with it’s secretion/reabsorption

Answer 24

•Impermeable to urea; reabsorbs Na+ and secretes K+, under hormonal influence; secretes H+ against a large concentration gradient; permeability to water is controlled by ADH

Question 25

What cell types are found in the late distal tubule?

Answer 25

Principle cells

intercalated cells

Question 26

What is the role of principle cells in the late distal tubule?

Answer 26

–Na+ reabsorption and K+ secretion
•K+ diffuses out of cell and into the tubular fluid

–Site of potassium-sparing diuretics
•They inhibit the stimulatory effect of aldosterone at this site
•Can also directly block sodium channels on the luminal membranes, decreasing the effectiveness of the Na-K pump

Question 27

What is the role of the late distal tubules intercalated cells?

Answer 27

–Secrete H+ ions via H-ATPase transporter

Question 28

What is the final site for processing of urine?

Answer 28

Medullary collecting duct

Question 29

The medullary collecting duct is permeable to urea, and can secrete H+ ions against their concentration gradient. How is the permeability of water controlled here?

Answer 29

ADH

Question 30

What are two factors promoting fluid movement into peritubular capillaries?

What is the consequence of this?

Answer 30

• High plasma colloid osmotic pressure
• Low hydrostatic pressure in these capillaries

•Consequence: almost as much fluid is reabsorbed as was initially filtered into Bowman’s capsule

Question 31

Where does aldosterone work? what does it lead to reabsorption of? Secretion of?

Answer 31

Site of action: late distal tubule and collecting duct

Effects: Increase NaCl reabsorption

increase K+ secretion

also H+ secretion

Question 32

Where does angiotensin II work? what does it lead to reabsorption of? Secretion of?

Answer 32

Site of action: proximal tubule, thick ascending loop of henle/distal tubule, collecting tubule

Effects: Increase NaCl reabsorption

increase H+ secretion

Question 33

Where does ADH work? what does it lead to reabsorption of? Secretion of?

Answer 33

Site of action: distal tubule/collecting tubule and duct

Effects: Increase H2O reabsorption

Question 34

Where does ANP work? what does it lead to reabsorption of? Secretion of?

Answer 34

Site of action: distal tubule/collecting tubule and duct

Effects: decrease Na+ reabsorption

Question 35

Where does parathyroid hormone work? what does it lead to reabsorption of? Secretion of?

Answer 35

Site of action: proximal tubule, thick ascending loop of henle/distal tubule

Effects: decrease PO₄^3- reabsorption

Increase Ca²⁺ reabsorption