Tubular Reabsorption - RS

Question 1

Which side of the membrane is the Na/K ATPase located? What is it’s function (think broad)?

Answer 1

It is restricted to the basolateral side (peritubular side) of all renal epithelial cells in all segments of the nephron. It maintains a steady state negative charge in the cell. Ultimately it functions to drive many of the secondary transporters.

Question 2

In cotransporters in which directions do the two gradients point?

Answer 2

The gradients are in opposite directions

Question 3

In countertransporters in which directions do the two gradients point?

Answer 3

The gradients are in the same direction. (Draw this out and you will see it. The high concentration is on the same side and the low is on the same side, but the molecules move in opposite directions. 1 drives the other.)

Question 4

Is paracellular transport active or passive? Do solutes move across it?

Answer 4

It is passive. Solutes move in what is known as solute drag, but that depends on the leakiness of the connections between cells.

Question 5

Do drugs and metabolies affect transcellular or paracellular transport?

Answer 5

Transcellular.

Question 6

In which direction is reabsorption defined? Secretion?

Answer 6

Reabsorption is tubular lumen to peritubular space/ capillary. Secretion is peritubular space/ capillary to tubular lumen.

Question 7

Where is glucose reabsorbed? What is the formula for solving for reabsorbed glucose.

Answer 7

The proximal tubule ONLY.
Reabsorbed glucose = Pglu x GFR - Uglu x V
Reabsorption = filtration - excretion

Question 8

In the early PT which glucose transporter is acting? What does it exchange? How good is it?

Answer 8

SGLT2 exchanges 1 glucose for 1 Na, it can concentrate as high as 70 (this is the lumenal side basolateral is passive)

Question 9

In the late PT which glucose transporter is acting? What does it exchange? How good is it?

Answer 9

SGLT1 exchanges 1 glucose for 2 Na, it can concentrate as high as 4900 (this is the lumenal side basolateral is passive)

Question 10

Describe the renal handling of phosphate (PO4):

Answer 10

Note, like glucose, the renal handling of phosphate at normal physiological levels results in virtually complete reabsorption of the filtered load of phosphate and a small, but important, amount of phosphate remains in the tubular fluid and is excreted in the urine.

Question 11

What is the stoichiometry of phospate transport?

Answer 11

It is 2 or 3 Na to 1 phosphate making it electrogenic and sensitive to voltage potential.

Question 12

Describe the renal handling of amino acids:

Answer 12

AA’s are freely filtered at the glomerulus and are completely (> 98%) reabsorbed across the proximal tubule by AA-specific, transcellular transport mechanisms mediating active uptake at the luminal membrane (Na-symport) and passive efflux at the basolateral membrane (facilitated diffusion). The PT is the only segment that absorbs AA’s.

Question 13

How are drugs secreted by the kidney?

Answer 13

They enter the tubule by transcellular secretion from the peritubular space to the tubular lumen across various segments of the nephron.

Question 14

How does the kidney hand p-aminohippuric acid?

Answer 14

The renal handling of PAH is an example of a solute which is secreted into the tubular fluid, but not reabsorbed from the tubular fluid. Accordingly, the amount of PAH excreted in the urine is a function of the amount filtered (filtered load) as well as the amount secreted by the tubule.

Question 15

What is PAH and why is PAH used?

Answer 15

It is an artificial compound that is not metabolized. It is used to measure renal plasma flow. The clearance of PAH is much greater than the clearance of inulin. At low plasma PAH the clearance equals renal plasma flow.

Question 16

What does the clearance of PAH measure?

Answer 16

Renal plasma flow

Question 17

Describe the renal handling of salicylate:

Answer 17

Secretion and reabsorption both occur. Like PAH, transcellular transport of salicylate across the proximal tubule occurs by active uptake across the basolateral membrane and passive efflux across the lumenal membrane (same transporters too).

Question 18

Salicylate diffuses in a nonionic fashion what affects its transport?

Answer 18

pH is the driving force. pH dependence of salicylate absorption from the distal tubular fluid back into the general circulation resulting from nonionic diffusion of salicylate.

Question 19

At high pH what is true of the clearance of salicylate?

Answer 19

The clearance is high. Where tubular fluid pH is high, nonionic diffusion of salicylate out of the tubular fluid is decreased and the clearance of salicylate from the circulation is increased, due to decreased return of filtered salicylate to the circulation and increased renal excretion of salicylate

Question 20

At low pH what is true of the clearance of salicylate?

Answer 20

The clearance is low.