Proximal Tubule Transport Flashcards
Normal kidney flitration in L/day:
180
Normal renal plasma flow (mL/min) through Afferent arteriole?
600 mL/min
Normal GFR in mL/min:
125 mL/min
Normal plasma flow in Efferent arteriole?
475 mL/min
Proportion of filtered solutes and water reabsorbed in the proximal tubule?
2/3
What is secondary active transport?
When one solute moves down its electrochemical gradient and drags another solute with it.
Proximal tubule lumen concentration of Na?
140 mM
intracellular concentration of Na?
4-10 mM
Peritubular capilary conc of Na?
140 mM
Explain the basolateral membrane events of Na and K:
Na-K-ATPase pumps 3 Na out into the peritubular capillary, and 2 K into the cell. The K recycles by leaving the cell passively through a K channel down its concentration gradient. (See page 4 lecture notes)
Four types of Na transporters on the apical (luminary) side of the cell to bring Na in:
- Na/H Antiporter
- Na/Gluc Symporter (SGLT-1/2)
- 90% glu reabsorbed in prox tub via SGLT-2 - Na/AA Symporter
- Na/PO4 Symporter
***Glu, AA, PO4 all being brought in AGAINST their conc gradient
What is Tm with regards to Glucose? When is it reached?
Maximum transport of glucose SGLTs. Saturates at 15 mM.
*beyond this, gluc will remain in urine
Why is glucosuria NOT a marker for diabetes?
Can have normal serum glucose AND glucose present in urine if proximal tubule is damaged.
How does Cl- get back into the cell in the proximal tubule?
Via formate (HCOO-) antiporter
**formate forms formic acid with H+ in the lumen that have been kicked out via Na/H antiporter.
**formic acid comes back into the cell and dissociates thus recycling the formate
How does Cl- get back into the capillary?
K/Cl symporter (from cell)
Cl channel (down electrochem gradient)
Paracellularly (once Cl is concentrated in later portion of tubule, conc gradient is created)
-can bring Na with
% of bicarb that gets “reclaimed” in the proximal tubule:
80%
How is bicarb reclaimed in the cell?
Carbonic anhydrase:
CO2 (ubiquitous) + H20 (reabsorbed) –> CA –> HCO3- + H+
How does bicarb get from the cell back into the blood?
Na/HCO3 symporter
**requires 3 bicarbs for every Na
Three ways water get reabsorbed?
- diffusion
- pericellularly – straight to capillary
- aquaporins
What causes the shift from favoring filtration is the glomerulus to favoring reabsorption in the peritubular capillaries?
A small increase in the intracapillary oncotic pressure
-29 mmHg –> 33mmHg
A larger decrease in the hydrostatic pressure
-60 mmHg –> 20 mmHg
**both favor reabsorption leading to net reabsorption force of 10 mmHg
(see page 9 lecture notes)
What things don’t have channels or transporters and cannot be reabsorbed because they can’t diffuse across lipid membranes?
Polar molecules
Ex: waste products, toxins, drugs, ….. all the stuff we WANT to get rid of
-cyt P450 in liver makes things polar so they get excreted
**this is why we filter 180 L per day!!!
What things don’t have channels or transporters but can get back in across lipid membranes?
steroid hormones
cholesterol
O2
**lipophilic stuff
What are OATs and OCTs?
Organic Anion/Cation Transporters
-Actively transport weak organic acids and bases out of blood and into cell for facilitated transport to tubule lumen
Which WOAs and WOBs do we want to keep?
MCAs (monocarboxylic acids)
-pyruvate, ketones, lactate
How do we get MCAs back into the cell from tubular lumen?
Na/MCA Symporter
**driven by Na strong conc gradient
