Muster: Proximal Tubule Flashcards
Via what transporter is 90 % of glucose reabsorbed in the proximal tubule?
SGLT 2 (sodium-glucose linked transporter)
Transport Maximum
The limit of saturation of the sodium/glucose transporters - at which no additional glucose will be absorbed and will remain in the urine
Normally, what percentage of filtered glucose is reabsorbed in the proximal tubule?
100 %
What level of glucose saturates the sodium-glucose transporters?
15 mM glucose
If you see normal blood glucose but glucose is present in the urine, where do you know you have a problem?
Proximal tubule!
What three things are pumped UP their electrochemical gradient via secondary active transport with sodium?
Glucose
Amino Acids
Phosphorous
What 4 things drive the Na+/H+ pump on the luminal side of the proximal tubule cells?
Increase in CO2 Increase in Angiotensin II Increase in sympathetic NS Decrease in pH -->example is an acidosis
List 5 general steps that occur in the proximal tubule that begin with sodium extrusion and end with bulk water reabsorption?
- Active extrusion of Na+ from epithelial cell to interstitium via Na+/K+ ATPase
- Passive entrance of Na+ from tubular lumen across apical membrane (mostly via Na+/H+ secondary active transporter) into cell to replace sodium removed in step 1
- Parallel movement of anions that must accompany the sodium to preserve electroneutrality (chloride and bicarbonate)
- Osmotic flow of water and solute from interstitium into peritubular capillary
- Bulk flow of water and solute from interstitium into peritubular capillary
What accumulates as it moves through the proximal tubule and is exchanged for formate from the inside of the endothelial cell?
Chloride
What is the point of the formate in the chloride/formate relationship?
Purely recycling!
What is the fate of formate once it leaves the cell?
It combines with H+ (from the Na+/H+ secondary transporter) to form HF (formic acid) which then enters the cell!
Where and how is the H+ ion made that is an antiporter/secondary transporter for Na+?
Inside the cell via carbonic anhydrase!
Via carbonic anhydrase, what else is made besides the H+ ion?
Bicarbonate
What is the fate of bicarbonate?
Leaves the cell and is put back into the blood stream by way of a Na+/HCO3- symporter (3 HCO3- : 1 Na+ ratio).
What is the process called when bicarbonate is put back into the blood stream?
Reclamation
What percent of bicarbonate that you filter is reclaimed in the proximal tubule?
80 %
By what 3 mechanisms does water get reabsorbed?
- Diffusion
- Aquaporins
- Paracellularly
What is meant by the term “iso-osmotic” in the proximal tubule?
Summed total of solutes reabsorbed is proportional to water reabsorbed
What forces govern the facilitation of net uptake of the reabsorbed solutes?
Starling Forces
Was the net in the glomerulus filtration or absorption?
Filtration!
Is the net outcome filtration of absorption in the peritubular capillary?
Reabsorption!
What happens to polar substances in the proximal tubule?
They are trapped in the lumen and you pee them out!
What is “Liver Transformation?”
The liver can make drugs polar via CYP450 enzymes to keep them in the proximal tubule lumen so they are excreted in the urine!
Why are non-polar substances (ex. steroids, cholesterol, oxygen) not excreted?
They can diffuse across the lipid bilayer!
What is one thing that all weak organic acids and bases that we need to save have in common?
They are Monocarboxylic acids
What nondisciminatory channels allow us to reabsorb WOA and WOBs?
Organic Anion channels and organic Cation Channels
If the term “filtrate” is used, what is the location?
Lumen of the tubule
How much do the kidneys filter?
125 mL/min
What is secondary transport?
One solute moves down its electrochemical gradient and that drives the movement of the other.
How does sodium get into the cell to begin with?
Na+/H+ transporter (Na+ in, H+ out).
What is the concentration of sodium in the peritubular capillary?
140 mM
What is glucosuria?
Abnormal finding = Glucose in urine
Tm has been reached.
Where are most of the formate anti-porters?
Late sections of the proximal tubule
Sum up the reabsorption of the proximal tubule (Na, CL, water, glucose, bicarbonate).
66 % of Na+, Cl-, H20 reabsorbed
100 % of glucose reabsorbed
80 % of bicarbonate reabsorbed
What is the location for secretion/reabsorption for weak organic aids and bases?
Proximal Tubule
