Proximal Tubule Transport

Question 1

Normal kidney flitration in L/day:

Answer 1

180

Question 2

Normal renal plasma flow (mL/min) through Afferent arteriole?

Answer 2

600 mL/min

Question 3

Normal GFR in mL/min:

Answer 3

125 mL/min

Question 4

Normal plasma flow in Efferent arteriole?

Answer 4

475 mL/min

Question 5

Proportion of filtered solutes and water reabsorbed in the proximal tubule?

Answer 5

2/3

Question 6

What is secondary active transport?

Answer 6

When one solute moves down its electrochemical gradient and drags another solute with it.

Question 7

Proximal tubule lumen concentration of Na?

Answer 7

140 mM

Question 8

intracellular concentration of Na?

Answer 8

4-10 mM

Question 9

Peritubular capilary conc of Na?

Answer 9

140 mM

Question 10

Explain the basolateral membrane events of Na and K:

Answer 10

Na-K-ATPase pumps 3 Na out into the peritubular capillary, and 2 K into the cell. The K recycles by leaving the cell passively through a K channel down its concentration gradient. (See page 4 lecture notes)

Question 11

Four types of Na transporters on the apical (luminary) side of the cell to bring Na in:

Answer 11

1. Na/H Antiporter
2. Na/Gluc Symporter (SGLT-1/2)
   - 90% glu reabsorbed in prox tub via SGLT-2
3. Na/AA Symporter
4. Na/PO4 Symporter

***Glu, AA, PO4 all being brought in AGAINST their conc gradient

Question 12

What is Tm with regards to Glucose? When is it reached?

Answer 12

Maximum transport of glucose SGLTs. Saturates at 15 mM.

*beyond this, gluc will remain in urine

Question 13

Why is glucosuria NOT a marker for diabetes?

Answer 13

Can have normal serum glucose AND glucose present in urine if proximal tubule is damaged.

Question 14

How does Cl- get back into the cell in the proximal tubule?

Answer 14

Via formate (HCOO-) antiporter

**formate forms formic acid with H+ in the lumen that have been kicked out via Na/H antiporter.

**formic acid comes back into the cell and dissociates thus recycling the formate

Question 15

How does Cl- get back into the capillary?

Answer 15

K/Cl symporter (from cell)

Cl channel (down electrochem gradient)

Paracellularly (once Cl is concentrated in later portion of tubule, conc gradient is created)
-can bring Na with

Question 16

% of bicarb that gets “reclaimed” in the proximal tubule:

Answer 16

80%

Question 17

How is bicarb reclaimed in the cell?

Answer 17

Carbonic anhydrase:

CO2 (ubiquitous) + H20 (reabsorbed) –> CA –> HCO3- + H+

Question 18

How does bicarb get from the cell back into the blood?

Answer 18

Na/HCO3 symporter

**requires 3 bicarbs for every Na

Question 19

Three ways water get reabsorbed?

Answer 19

1. diffusion
2. pericellularly – straight to capillary
3. aquaporins

Question 20

What causes the shift from favoring filtration is the glomerulus to favoring reabsorption in the peritubular capillaries?

Answer 20

A small increase in the intracapillary oncotic pressure
-29 mmHg –> 33mmHg

A larger decrease in the hydrostatic pressure
-60 mmHg –> 20 mmHg

**both favor reabsorption leading to net reabsorption force of 10 mmHg

(see page 9 lecture notes)

Question 21

What things don’t have channels or transporters and cannot be reabsorbed because they can’t diffuse across lipid membranes?

Answer 21

Polar molecules

Ex: waste products, toxins, drugs, ….. all the stuff we WANT to get rid of
-cyt P450 in liver makes things polar so they get excreted

**this is why we filter 180 L per day!!!

Question 22

What things don’t have channels or transporters but can get back in across lipid membranes?

Answer 22

steroid hormones

cholesterol

O2

**lipophilic stuff

Question 23

What are OATs and OCTs?

Answer 23

Organic Anion/Cation Transporters

-Actively transport weak organic acids and bases out of blood and into cell for facilitated transport to tubule lumen

Question 24

Which WOAs and WOBs do we want to keep?

Answer 24

MCAs (monocarboxylic acids)

-pyruvate, ketones, lactate

Question 25

How do we get MCAs back into the cell from tubular lumen?

Answer 25

Na/MCA Symporter

**driven by Na strong conc gradient