Lecture #16: Tubular Reabsorption and Secretion Flashcards
Where is aquaporin-2 found in the tubules of the kidneys?
Present in apical membranes of collecting tubule cells .
Aquaporin-2 is controlled by ADH.
Which aquaporin is controlled by ADH?
aquaporin-2
Where is aquaporin-1 located in the kidneys?
It is widespread, including the renal tubules.
For a substance to be reabsorbed, it must first be transported through what two layers?
Reabsorbed across the tubular epithelial membranes into the renal interstitial fluid.
Reabsorbed through the peritubular capillary membrane back into the blood.
What are the two different routes that water is transported from the tubular cells into the interstitium?
Transcellular Route by Osmosis
Paracellular Routes by Osmosis
Is the paracellular or transcellular pathway the most common osmosis pathway for water?
Transcellular
ATPases establish ionic graadients across nephron cell membranes. What is this ion gradient used for within the kidney?
Gradients drive reabsorption or secretion of many other solutes.
These are then transported by way of “secondary” active transport.
Where is the ENaC channel and what opens and closes it?
ENaC channel is found in apical membrane of nephron cells.
Closed by drug amiloride.
Opened by a number of hormones.
We know that ENaC channels are found in the apical membrane of nephron cells, but what other two channels are also found in some segments of apical membranes of the nephron?
CFTR (Cl-) channels
Potassium (K+) channels
What type of transport occurs through channels or uniporters?
Facilitated Transport (i.e., glucose transport)
What type of transport is directly coupled to an energy source?
Active Transport
What type of transport is coupled indirectly to an energy source (i.e., ion gradient)
Secondary Active Transport
Almost 90 percent of glucose reabsorption occurs in the early proximal tubule. Which of the following mechanisms is responsible for moving glucose from the tubular lumen into the cytoplasmic compartment?
A) Primary active transport via a glucose ATPase pump.
B) Secondary Active Transport via a sodium/glucose co-transporter.
C) Secondary active transport via a sodium/glucose antiporter.
D) Diffusion due to concentration differences between tubular fluid and cytoplasm.
B) Secondary active transport via a sodium/glucose co-transporter.
Name some common ATPases we are studying?
Na/K ATPase
H+ ATPase
H/K ATPase
Ca ATPase
What is the charge in mV in the tubular lumen? Also, what is the charge inside of the tubular epithelial cell?
- 3 mV
- 70 mV
Where are sodium-glucose co-transporters located on tubule cells? Also, what is the name of these co-transporters?
Located on brush border of proximal tubule cells.
SGLT2 and SGLT1
Where is the sodium/glucose co-transporter, SGLT2, located and what percentage of glucose does it transport?
SGLT2 -> reabsorbs 90% of glucose in early proximal tubule.
Where is the sodium/glucose co-transporter, SGLT1, located and what percentage of glucose does it reabsorb?
SGLT1 -> reabsorbs 10% of glucose in late proximal tubule.
What are the two substances that are actively secreted into the renal tubules?
> Creatinine
> Para-aminohippuric acid
What is the transport maximum for glucose?
375 mg/min
What is the filtered load for glucose?
125 mg/min
An increase in which of the following factors will result in an increase in glomerular hydrostatic pressure?
A) arterial pressure B) afferent arteriolar resistance C) efferent arteriolar resistance D) both B and C E) all of the above
B) afferent arteriolar resistance
Of the reasons listed below, which one best describes why some passively reabsorbed substances do not have a transport maximum?
a) The rate of diffusion is determined by electrochemical gradient of the substance.
b) The permeability of the membrane for the substance.
c) Time that the fluid containing the substance remains within the tubule.
d) all of the above
D) all of the above
Where is aquaporin-3 located in the kidney tubules?
Present in basolateral membranes of collecting tubule cells.
What does the rate of transport depend on?
1) The electrochemical gradient
2) Time the substance is in the tubule
- which depends on tubular flow rate
What is “solvent drag”?
Solvent drag is another mechanism that will reabsorb or move some of the solutes.
*Osmotic movement of water can also carry some solutes…..this is solvent drag.
True or False:
Passive water reabsorption by osmosis is coupled mainly to sodium reabsorption.
True
You know that the kidney couples reabsorption of water, chloride, and urea with sodium reabsorption. With that being said, what happens to the lumen potential when sodium is reabsorbed?
There’s an increase in lumen negative potential, thus leading to the passive reabsorption of Chloride.
We know that sodium reabsorption leads to water reabsorption too, but what effect does this have on luminal chloride concentration?
Increases luminal chloride concentration, thus causing passive reabsorption of chloride.
We know that the reabsorption of sodium leads to the reabsorption of water, but what effect does this have on urea concentration in the lumen?
Increases luminal urea concentration, thus leading to the passive reabsorption of urea.
List some of the defining characteristics of the proximal tubule?
> highly metabolic w/large numbers of mitochondria
extensive brush borders on luminal surfaces
extensive intercellular and basal channels on interstitial surfaces
reabsorb
- 65% of filtered sodium, chloride, bicarbonate and potassium
- reabsorb all filtered glucose and amino acids
What percent of filtered sodium, chloride, bicarbonate, and potassium is reabsorbed by the proximal tubule?
65%
What all is reabsorbed from the proximal tubules?
> sodium > chloride > bicarbonate > potassium > water > glucose > amino acids
What is the term “bulk flow” used to describe?
The reabsorption (flow) that is occurring due to hydrostatic and osmotic forces (Starling Forces).
What is secreted in to the proximal tubular lumen?
Protons (H+)
Organic Acids
Bases
What 3 substances are reabsorbed more avidly than water in the proximal tubule?
Bicarbonate
Glucose
Amino Acids
What is the difference in sodium reabsorption between the first half and the second half of the proximal tubule?
> First Half of Proximal Tubule
- reabsorption is via co-transport along with glucose, amino acids, and other solutes.
> Second Half of Proximal Tubule
- reabsorption is mainly with chloride ions
In the proximal tubule, what secondary transport system typically transports most sodium (Na+) out of the tubule?
Most Na+ entry is via antiport with H+
True or False:
Na/K ATPase pumps are not found in the proximal tubule.
False - Na/K ATPase pumps are found in the proximal tubule.
3Na:2K
K can easily diffuse back out of cell.
What is the electrical gradient in the cytoplasm of the proximal tubule cells? Also, what is the electrical gradient of the proximal tubular lumen?
cytoplasm = -70 mV
tubular lumen = -3 mV
In the proximal tubule, what is the luminal sodium concentration?
140 mOsm
What is the cytoplasmic sodium concentration of the proximal tubule?
30 mOsm
In the proximal tubule, why does the proton concentration increase in the lumen?
[H+] increases in lumen due to antiport transport with Na+
How is carbonic acid formed within the lumen of the proximal tubule?
The protons combine with luminal bicarbonate.
What enzyme found in the lumen of the proximal tubule splits carbonic acid into carbon dioxide and water?
Carbonic Anhydrase
What percent of filtered water is reabsorbed in the thin descending segment of the loop of Henle?
Reabsorbs about 20% of filtered water
Is the thin descending segment of the Loop of Henle more permeable to water or solutes, including urea and sodium?
Highly permeable to water and moderately permeable to most solutes, including urea and sodium.
Thus, more permeable to water.
Is the proximal tubule isosmotic, hyposmotic, or hyperosmotic?
Isosmotic
Is the thick ascending Loop of Henle isosmotic, hypo-osmotic, or hyper-osmotic?
Hypo-osmotic
True or False:
The thin ascending segment of the Loop of Henle is impermeable to water.
TRUE
**Dr. Anderson said this WILL be a question on the exam.
In which part of the cell is the Na/K ATPase pump located in the thick ascending segment of the Loop of Henle?
Na/K ATPase pump in basolateral membranes.
**Drives reabsorption of K into cell against concentration gradient.
What is the purpose of the sodium/potassium pump in the basolateral membrane of the thick ascending segment of the loop of Henle?
It drives reabsorption of potassium into the cell against its concentration gradient.
How many ions are moved into the cell by the sodium/potassium/chloride co-transporter in the thick ascending segment of the Loop of Henle?
1-sodium
2-chloride
1-potassium
**remember, this is moving these ions into the cell.
What does the slight back leak of potassium ions into the lumen of the thick ascending segment of the loop of Henle do to the luminal electrical gradient?
Creates a positive charge of +8 mV
What does the luminal electrical gradient of +8 mV, which is caused by the slight back leak of potassium ions into the lumen of the thick ascending segment of the loop of Henle, and the slight back lead of potassium into the lumen do to Mg++ and Ca++ ions?
Forces Mg++ and Ca++ to diffuse through tubular lumen through paracellular space into the interstitial fluid.
True or False:
The thick ascending segment of the Loop of Henle is permeable to water and is not a site of action of powerful “loop” diuretics.
FALSE – the thick ascending segment of the Loop of Henle in NOT permeable to water (impermeable) and IS a site of action of powerful “loop” diuretics.
What does the first portion of the distal tubule form?
Macula Densa
What segment is the distal tubule referred to as?
Diluting Segment because it reabsorbs most of the ions but is impermeable to water and urea.
In the distal tubule, is the Na/Cl co-transporter found in the luminal or basolateral membrane?
Luminal Membrane
In the distal tubule, is the Na/K ATPase pump found in the luminal or basolateral membrane?
Basolateral Membrane
In the early distal tubule, what drug inhibits the Na/Cl co-transporter?
Thiazide Diuretics
Is the early distal tubule impermeable to water and urea?
YES
What ions are reabsorbed in the early distal tubule?
Na
Cl
Ca
Mg
What do principal cells in the late distal tubule and cortical collecting tubule do?
> reabsorb Na and water from tubular lumen
> secrete K into tubular lumen
> uses Na/K ATPase pump
> primary site of K sparing diuretics
True or False:
Principal cells in the late distal tubule and collecting tubule reabsorb sodium and secrete potassium?
TRUE
What is the tubular lumen electrical gradient in the early distal tubule?
-10 mV
What is the tubular lumen electrical gradient of the last distal tubule and cortical collection tubule?
-50 mV
What ions do intercalated cells reabsorb and secrete in the late distal tubule and cortical collecting tubule?
Intercalated cells:
> reabsorb K and Bicarbonate from tubular lumen
> secrete H into tubular lumen
What is the importance3 of intercalated cells secreting H protons into the tubular lumen in the late distal tubule and cortical collecting tubule?
Secretion of H into tubular lumen:
>mediated by H+ATPase transporter
>H is generated through the action of carbonic anhydrase
>for each H secreted, a bicarbonate ion is reabsorbed across the basolateral membrane
**This is important in acid/base control.
What hormone controls the permeability to water in the medullary collecting duct?
ADH
What is the medullary collecting duct capable of doing with H ions?
capable of secreting H+ against a large concentration gradient.
**is important in acid/base control
True or False:
The medullary collecting duct is permeable to urea.
TRUE
Do cuboidal epithelial cells of the medullary collecting duct have few mitochondria and smooth surfaces?
YES
What is reabsorbed in the medullary collecting duct?
Na Cl Water (+ADH) Urea Bicarbonate
What is secreted in the medullary collecting duct?
H (protons)
What is the net reabsorption pressure in the peritubular capillaries?
10 mmHg
