L4: Tubular Transport

Question 1

What is filtered load? What is excretion rate?

Answer 1

FL: Amount of material in glomerular filtrate
ER: Amount of material lost in urine

Question 2

Equation for transport rate

Answer 2

Tx = FL - ER

if these are =, then Tx is 0

Question 3

What does a positive Tx indicate? A negative Tx?

Answer 3

\+ = some material was removed from filtrate by REABSORPTION
- = some material was added to filtrate by SECRETION

Question 4

What types of transport is used in tubular reabsorption?

Answer 4

Simple diffusion
Facilitated diffusion
Primary active transport
Secondary active transport
Endocytosis

Question 5

Which membrane is the Na/K ATPase on? What kind of transport is it? Describe what it does.

Answer 5

Basolateral membrane (capillary side)
Primary active transport
Takes Na from inside the cell and pushes it out, takes K outside of cell and brings it in (both against conc. gradient)

Question 6

Describe how glucose is transported in the proximal tubule to be reabsorbed?

Answer 6

Apical (luminal/nephron side) membrane: Na being brought in via facilitated diffusion (thanks to Na/K ATPase); G being brought in from lumen into cell via secondary active transport (Na and G sharing a protein)

Basolateral membrane: G being pushed out of cell into capillary via facilitated diffusion

Question 7

How much water is reabsorbed from proximal tubule?

Answer 7

2/3 of water brought in from nephron to capillary at the proximal tubule

Question 8

What is the primary force driving fluid movement from the BL into PTC?

Answer 8

PC (proteins very concentrated in capillaries)

Question 9

Describe the 5 step process of fluid movement from the nephron lumen to peritubular capillaries.

Answer 9

1. Active transport of Na across BL membrane
2. Sligh osmotic gradient pulls water from nephron lumen into BL compartments
3. Anions (Cl-, etc.) follow Na
4. # PC is the primary force driving fluid movement from BL compartments to PTC!!!!!!
5. Reabsorption of H2O and electrolytes is considered to be ISOSMOTIC in the proximal tubule

Question 10

How do organic molecules enter the cell with Na? How does sodium leave the cell?

Answer 10

-Na enters PT cells via cotransport w/ organics and antiport with H (NHE)
(Basically, from nephron side, Na is symported with everything except with H protons)

-Na leaves the cell to capillary via Na/K ATPase or cotransport with HCO3- (all the organic molecules get to blood via uniports)

Question 11

How does Cl enter the PTC?

Answer 11

Paracelluar routes

Question 12

How does ATII affect Na reabsorption in the proximal tubule?

Answer 12

Stimulates Na/H exchange across apical membrane (NHE!!!)

Increases Na reabsorption and H secretion

Question 13

How does the SNS affect Na reabsorption in the proximal tubule?

Answer 13

Stimulates Na reabsorption

Question 14

How does PTH affect Na reabsorption in the proximal tubule?

Answer 14

-PTH is released when there is low serum Ca
-PTH INHIBITS Na-phosphate cotransport
-Increases urinary excretion of phosphate
(Causes Ca to be released from bone > we don’t want phosphate and Ca in blood together due to calcification risk)

Question 15

Describe what TF/P is

Answer 15

Ratio of conc. of material in tubule fluid/conc. in plasma

Question 16

How does reabsorption affect the TF/P ratio?

Answer 16

Produces very small TF/P ratio (inulin has a ratio of 1 because it is completely excreted)

Question 17

What happens to the osmolarity in the proximal tubule?

Answer 17

It does not change! Fluid is isosmotic when it enters and leaves

Question 18

If reabsorption has been less than water OR there has been net secretion of the substance, then the TF/P ratio

Answer 18

TF/P > 1.0

Question 19

If reabsorption has been exactly proportional the the reabsorption of water, then the TF/P ratio

Answer 19

TF/P = 1.0

Question 20

If reabsorption of substance has occurred to a greater extent than water, then TF/P ratio

Answer 20

TF/P < 1.0

Question 21

What is tubular maxima?

Answer 21

Maximum rate of movement/transport across membrane (has to do with saturation of membrane transport proteins)

Question 22

If the solute is transported below the Tm, then

Answer 22

all of the filtered load is reabsorbed

Question 23

If the solute is transported above the Tm, then

Answer 23

the portion of the load above Tm is excreted

Question 24

Where are Tm’s typically found?

Answer 24

Proximal tubule

Question 25

What is the threshold for Tm (in regards to glucose) and what is it dependent on?

Answer 25

Concentration of plasma where glucose first appears in urine

Depends on GFR

Question 26

What happens to the threshold if you decrease GFR?

Answer 26

Threshold increases

Question 27

Name 6 other solutes with Tm’s

Answer 27

• Sugars (fructose, galactose)
• Amino acids
• Metabolic intermediates (lactate, ketone bodies, etc.)
• Phosphate ions
• Water-soluble vitamins
• Proteins and peptides

Question 28

What is the equation for Tm?

Answer 28

Tx = FL - ER (same as transport rate)

Question 29

Describe where mannitol works and how it works as a diuretic.

Answer 29

• Works in proximal tubule
• Excess unreabsorbed mannitol inhibits osmotic water flow from lumen to basolateral space
• This causes Na back diffusion into lumen of tubule with increased Na and water loss

Question 30

Are the transport mechanisms for secretions of organic cations and anions specific or nonspecific?

Answer 30

Very nonspecific

Question 31

Through what type of transport are organic ANIONS (PAH) secreted? Describe the process.

Answer 31

Tertiary active transport

• PAH is taken up into the cell from the blood in exchange for alpha-KG (uphill step)
• PAH leaves the cell on the apical side via a PAH-anion antiporter (type of OAT) (downhill step)

*Most organic anion secretion involves OAT-1 and OAT-3 transporters on basolateral membrane of proximal tubule

Question 32

Can OATs and OCTs be saturated?

Answer 32

Yes - both exhibit saturation kinetics (Tm)

Question 33

Where does most secretion occur?

Answer 33

Late proximal tubule (same for both organic anions and cations)