Renal Transport Mechanisms

Question 1

What gets reabsorbed in the Proximal Convoluted Tubule? Proximal straight tubule?

Answer 1

All glucose and amino acids should be reabsorbed here as well as most other filtered substances:

• sodium (70%)
• urea (50%)
• potassium (70%)
• phosphate (70%)
• Ca2+ (70%)
• Mg (30%)
• H2O (70%)

PST: ~15% of phosphate

Question 2

In order for reabsorption to occur, what do things have to journey through?

Answer 2

1. Stuff has to cross the apical border
2. Navigate through the cell interior
3. Cross the basolateral border
4. Then enter the peritubular capillary

• Reabsorption is powered by Na-K-ATPase
• reabsorption usually driven by Na+ coupled with something else

Question 3

How does stuff cross the apical border?

Answer 3

-by symporter or antiporter

Question 4

How do things cross the basolateral border?

Answer 4

• Na+ gets out via Na-K-ATPase

- Special transporters handle the rest

Question 5

What transporters exist in the PCT?

Answer 5

apical: SGLT, amino acid-Na+ symp, phosphate-lactate-citrate-Na symp, NHE (Na-H exchanger)
basolateral: Na-K-ATPase, GLUT, Amino Acid, bicarb, phosphate, lactate, citrate

Water moves transcellularly via AQP and bulk flow into blood

Question 6

What gets reabsorbed form the TAL?

Answer 6

NO water !!

• Important for Na, K, Ca, and Mg (most happens here ~60%)
• NKCC2 (N, 2Cl, K) and ROMK are important here, back leak of K+ via ROMK is critical to passive paracellular diffusion of Mg and Ca from lumen

Question 7

What is unique about the Distal Convoluted tubule?

Answer 7

-least amount of solutes reabsorbed here
-highly regulated, ADH and aldosterone, variable
secretion/reabsorption of solutes depending on
conditions
-Principal cells, alpha and beta intercalated cells

Question 8

What do principal cells reabsorb and secrete?

Answer 8

reabsorb: Na+, and H2O
Secrete: K+

Question 9

What do alpha intercalated cells reabsorb and secrete?

Answer 9

reabsorb: K+, HCO3-
Secrete: H+

Question 10

What do beta intercalated cells reabsorb and secrete?

Answer 10

reabsorb: H+, Cl-
Secrete: K+, HCO3-

Question 11

What gets reabsorbed in the DCT?

Answer 11

-relatively impermeable to water
-continued NaCl reabsorption via thiazide sensitive NCC
further dilutes tubular fluid
-Ca2++ crosses apically via TRPV5 and basolaterally via
NCE (Na-Ca exchanger)
-ENaC in late DT and collecting duct is acted upon by
aldosterone to reabsorb Na by upregulating its
expression

Question 12

What channels allow principal cells and beta intercalated cells to secrete K+ into the tubular lumen?

Answer 12

BK and ROMK

Question 13

How do alpha intercalated cells reabsorb K+?

Answer 13

1. H-K ATPase moves K into the cells from the tubular
   lumen.
2. K diffuses across the basolateral membrane through K
   channels.

Question 14

What are the most important factors that stimulate sodium reabsorption?

Answer 14

1. Na+ deficiency
2. low Na+ diet
3. hyponatremia
4. Na+ loss through severe diarrhea

Question 15

What are the most important factors that stimulate sodium secretion?

Answer 15

1. hypernatremia
2. ANP
3. Renal prostaglandins

Question 16

What stimulates K+ secretion by principal cells and beta intercalated cells?

Answer 16

increased serum concentration and aldosterone

Question 17

What stimulates K+ reabsorption by alpha intercalated cells?

Answer 17

(1) K+ deficiency;
(2) low K+diet;
(3) hypokalemia;
(4) K+ loss through severe diarrhea

Question 18

What does ADH respond to? How?

Answer 18

1. Hyperosmolality (very sensitive)
2. Volume depletion

How:
1. Increases the water permeability of the principal cells of
the late distal tubule and collecting ducts.
•Aquaporins inserted into apical membrane of
principal cells (not present w/o ADH)
•Best known and most important mechanism!!!!
2. Increases urea permeability in the inner medullary
collecting ducts (but not in the cortical or outer
medullary collecting ducts)
3. Increases the activity of the Na+-K+-2Cl−cotransporter
(NKCC2) of the thick ascending limb

Question 19

How is urine concentrated?

Answer 19

Countercurrent mechanisms (multiplier and exchanger)

Question 20

Describe countercurrent multiplication.

Answer 20

1. The single effect
   •NaCl leaves ascending limb, interstitium becomes
   hyperosmotic (salty medulla!)
   •Water leaves descending
   limb to equalize the interstitium osmolality

2. Fluid flow
   •Fluid always flowing through tubule
   •New fluid enters descending limb from above
   •Pushes tubular fluid downward
   •Gradient develops
   •Effect multiplies

Question 21

What is countercurrent exchange?

Answer 21

• Passive movement of water from descending limb into
interstitium, and reabsorbed into vasa recta
• Increased osmotic gradient results in more water
reabsorption (ADH)
• Blood flow rate impacts equilibration

Question 22

What creates medullary osmolality?

Answer 22

the presence of NaCl and urea

Question 23

What is the equation fro osmolar clearance?

Answer 23

Cosm = (Uosm*V)/(Posm)

Question 24

What is the equation for free water clearance?

Answer 24

Ch2o = V - Cosm or V - (Uosm*V)/(Posm)

when (-): excess solutes are removed, water conserved

when (+): water is being excreted, dilute urine

Question 25

what is obligatory urine volume?

Answer 25

The maximal concentrating ability of the kidney dictates how much urine volume must be excreted each day to rid the body of metabolic waste products and ions that are ingested.

OUV = minimum solute excretion/max U conc ability

Question 26

Define diuresis.

Answer 26

Continuously reabsorb solutes and fail to reabsorb water
•High volume, dilute urine
•Low medullary solute concentrations

Question 27

Define Natriuresis

Answer 27

The excretion of an excessively large amount of sodium in the urine especially disproportional to the excretion of water.

*Causes
•Drug (natriuretic)
•Hormone (atrial natriuretic peptide)
•Significantly elevated renal perfusion pressure (pressure natriuresis).