Tubular Reabsorption and Secretion

Question 1

What is the path of reabsorption?

Answer 1

Filtered solutes and water from tubular fluid, epithelial cell tubule, peritubular capillaries (circulation)

Question 2

What is the path of secretion?

Answer 2

Movement of solutes from circulation through peritubular capillaries, epithelial cell tubule, tubular fluid

Question 3

How is most solute transport from lumen to peritubular space energy-wise? What is another way of transporting solute?

Answer 3

Active, driven by energy released upon ATP hydrolysis or by coupling to energy stored in transmembrane ion concentration gradients; passive transport

Question 4

How is water transport driven from lumen to peritubular space? Where osmolarity is high, what does that mean for water?

Answer 4

passive (occurs by osmosis);

Water is low; will move down its concentration gradient

Question 5

What are two ways to move solute and water across the renal tubular epi?

Answer 5

Transcellular (uptake into and efflux from the cell); paracellular as solute and water moves through junctions

Question 6

Where does one find the Na-K ATPase? What does this do for the inside of the cell? What does this ATPase allow for with regard to Na and another solute?

Answer 6

Basolateral membrane;
Maintains an inside negative membrane potential difference;
allows for Na gradient to help drive secondary transport of another solute against its gradient!!

Question 7

What are some examples of ATPases? What name is synonymous with ATPase? What do ATPases help drive?

Answer 7

Na-K, H, H-K;
pumps;
Solute transport across a membrane in a direction against a solute electrochemical potential gradient

Question 8

What is the driving solute most often in the cause of secondary active co-transporters? In what direction do counter-transporters couple transport? What ultimately determines the direction of counter-transporters?

Answer 8

Na (EC to IC electrochemical potential gradient);
opposite directions;
depends on which solute has the largest electrochemical potential gradient

Question 9

What does passive transport not require relative to active transport? What is the movement of solute?

Answer 9

Coupling to a source of energy;

down a solute electrochemical potential gradient

Question 10

What type of transport is paracellular transport (A or P)? What determines if epithelia can maintain a transepithelial voltage difference or osmotic gradient?

Answer 10

Passive;

cell-to-cell junctional resistance or “leakiness” of the epi to solute and water transport across the junction

Question 11

What is the phenomenon that would enable water to move through the intercellular junctions followed by solute?

Answer 11

Solvent drag, which would entrain movement of solute (usually paracellular)

Question 12

How are active and passive transporters coordinated in cells? How can you have tubular reabsorption or secretion of solute?

Answer 12

In series;
P at basolateral membrane, A at luminal, and vice-versa for reabsorption; A at basolateral membrane, P at luminal and vice-versa for secretion

Question 13

What limits transcellular reabsorption and secretion? What could inhibit these two processes?

Answer 13

Saturable and has a maximum rate (TMax);

inhibitable by drugs (diuretics) and circulating metabolites

Question 14

With renal handling of glucose, what function does the kidney not perform (filtration, excretion, reabsorption, secretion)? What is the only location of glucose reabsorption?

Answer 14

Secretion;

proximal tubule

Question 15

At normal levels of plasma glucose (5-10 mM), is there any glucose excreted? When would we see some glucose excreted? What is the reason for why you’re not able to reabsorb the glucose?

Answer 15

No; at levels around or greater than 15 mM;

because of saturation kinetics of transporters at the proximal tubule, leading to more glucose being excreted!!

Question 16

What is a threshold plasma concentration at which glucose enters the urine? For what pathology would the kidney not be able to return filtered glucose to the circulation? What should the clearance of glucose relative to clearance of inulin normally be?

Answer 16

11 mM;
Poorly controlled diabetes mellitus;
At 0!!

Question 17

For reabsorption of glucose at the lumenal membrane, what two transporters are present at the early and late proximal tubule? Are either electrogenic? How does glucose move across the basolateral membrane?

Answer 17

Na-Glu cotransporters with Na moving down its solute electrochemical gradient;
both are (1:1 and 2:1 for Na:Glu);
passively through facilitated diffusion

Question 18

Which of the following functions isn’t designated for phosphorous (filtration, secretion, reabsorption, excretion)? What kind of kinetics is displayed for phosphate transporters?

Answer 18

Secretion (like glucose);

saturation, with a tubular maximum

Question 19

How much of the phosphate filtered out ends up in the urine? What happens to the rest? Where is most of the phosphate reabsorbed?

Answer 19

A small amount lingers in the tubular fluid; the rest is reabsorbed;
Proximal tubule (approximately 90%)

Question 20

What are the transporters for phosphate like at the lumenal and basolateral membranes? Are they electrogenic?

Answer 20

Active at lumenal, facilitated diffusion at level of basolateral membrane;
yes (3:1 and 2:1 for Na/PO4)

Question 21

For aa’s, like PO4 and glucose, what function is lacking? What are the transporters like at the luminal vs. the basolateral membrane?

Answer 21

Secretion;

Na-symport at the luminal membrane, and facilitated diffusion at the basolateral membrane

Question 22

Where are aa’s reabsorbed? What types of kinetics are displayed? What is the normal clearance of aa’s? What can result from high AA levels or aa reabsorption tubular defects?

Answer 22

proximal tubule;
saturation with a tubular maximum;
close to zero;
hyperaminoaciduria

Question 23

What can be secreted? What is the direction/path of secretion?

Answer 23

1. foreign to body (drugs or toxins)
2. metabolized by kidney
3. metabolized by liver (e.g. glucoronidation, sulfation)
4. organic/inorganic solutes regulated in blood;
   Peritubular space to tubular lumen

Question 24

What function is lacking with p-aminohippuric acid (of the four)?

Answer 24

Reabsorption

Question 25

At low PAH concentrations, what is its clearance and what’s left in the renal vein? What happens with too much PAH relative to transporter kinetics and location? What is PAH’s clearance relative to inulin? At low PAH what does its clearance equal?

Answer 25

Complete clearance and should not be found in renal vein;
Saturation hit and Tm achieved, and now found in the renal vein;
Considerably higher;
RPF

Question 26

At the basolateral membrane of the proximal tubule, what types of transporters are there? What type of transport is this (1, 2, 3?) What types of transporters are present at the luminal membrane?

Answer 26

Na-driven (secondary) to move in divalent anion, with divalent ion to help move PAH into the cell;
tertiary;
Facilitated and anion gradient-driven antiport (anion in, PAH out)

Question 27

At low PAH plasma concentrations, where should all the PAH end up?

Answer 27

Urine, via filtration and secretion

Question 28

How is salicylate filtered? Where is salicylate secreted and what are the transport mech’s across the basolateral and luminal membranes? What can salicylate use regarding transporters?

Answer 28

In HA and A- forms;

active at basolateral, passive at luminal of the A- form; same as PAH

Question 29

Where does reabsorption of salicylate occur mostly? How so? When would reabsorption decrease? Increase?

Answer 29

Distail nephron, passively through nonionic diffusion;
increased tubular fluid pH and flow rates;
decreased tubular fluid pH and flow rates

Question 30

At what pH would clearance of salicylate increase? Decrease? What would Csal/Cinulin greater than one indicate netwise?

Answer 30

High urine pH; low urine pH;

Secretion