Tubular transport Flashcards
Are transport proteins in the epithelia asymmetrically distributed and why?
Yes permits vectorial transport
What are the types of transport proteins?
- Pumps
- Carriers
- Channels
What are the transport processes?
- Active
- Secondary active
- Passive
How do Na/K pumps work?
Through the hydrolysis of ATP it moves 3 Na out of the cell and 2 K into the cell. This is against their concentration gradients
How do SGLT cotransporters work?
They use secondary active transport to move Na down its concentration gradient and it brings a glucose with it
How do GLUT uniporters work?
Passive transport through the uniporter
How is glucose absorbed?
The Na/K pump creates low Na in the cell so that SGLT can move in sodium and glucose on the apical side. On the basolateral side, GLUT allows for the passive movement of glucose into the blood
Should there be glucose in the urine?
No, it should all be absorbed in the proximal tubule
What is a condition in which you could find glucose in the urine?
Diabetes mellitus, the amount of glucose exceeds the Tm and glucose is excreted
What is an important physiological buffer?
HCO3-
Reabsorption of filtered HCO3 relies on what?
Na gradient maintained by the Na/K ATPase
What inhibits reabsorption of HCO3?
CA inhibitors (acetazolamide)
What does the PT efficiently recover?
- Filtered amino acids
- Oligopeptides
- Low molecular weight proteins
How is sodium reabsorbed in the proximal tubule?
- Na coupled mechanisms when reabsorbing molecules (transcellular processes)
- Chloride mediated Na transport (paracellular processes)
What is chloride-mediated Na transport?
Cl is driven across leaky tight junctions and Na follows. This is because the lumen is negatively charged and the Cl- is repelled
Is the PT highly water permeable?
Yes
What features facilitate PT solute absorption?
- Subcellular specializations
- Secondary structural features
- Tertiary structural features
What are some subcellular specializations of the PT?
- Mitochondrial rich
- Specialized transporters
What is a secondary structural feature of PT?
Brush border
What are tertiary structural features of PT?
- Convolutions of early PT
- Proximity of all nephronal segments to the vasa recta
How is HCO3 reabsorbed?
It is combined with H+ by CA to make CO2 and H2O the Co2 can now diffuse into the cell. The CO2 is then combined with H2O and makes HCO3 again. This is then shuttled out of the cell through Na HCO3 cotransporter
How is transport of solutes iso-osmotic?
PT solute and water transport are proportionate
How are LMPs absorbed?
They enter through endocytosis and fuses with a lysosome. The lysosome is degraded and fuses with a multivesicular body and then transported to the interstitial space
What is Bartter syndrome?
Nacl absorption is impaired so there is no dilution of luminal fluid. There is now low medullary interstitial osmolarity
What is different about apical NaCl transporters?
Don’t depend on the presence of K and has different drug sensitivity
How is calcium absorbed in the DT?
Ca enters through ion channel and then binds to calbindin. This then releases the Ca at the basolateral membrane and it exits across the membrane via Na Ca exchanger or pumps
What is the effect of parathyroid hormone on the DT?
- Increases Ca absorption
- Stimulate Vitamin D production
What hormones regulate the collecting duct?
Aldosterone and ADH
What do Principle cells in the collecting ducts do?
Absorb Na on apically located Na channel (ENaC)
What hormone regulates ENaC?
Aldosterone
What blocks ENaC?
Amiloride
What is the effect of ADH on principal cells?
Causes an increase the amount of aquaporins (AQP2) are on the apical side and water can flow into the cell
What is nephrogenic diabetes insipidus?
Loss of function mutation where the collecting duct can no longer respond to ADH
What inhibits the NaK2Cl cotransporter?
Diuretics (lasix, Bumex)
How is NaCl transported in the TAL?
There is a sodium gradient made by NaK pump. The NaK2Cl then moves Na down its concentration gradient. On the basolateral side, Cl and Na are pumped out and K is pumped out on the apical side
Why do you get dilution of luminal fluid in the TAL?
NaCl is filtered from the lumen but water is impermeable so you are taken solutes out with the solvent staying the same resulting in dilute fluid
What are the different cells of the collecting duct?
- Principle cells
- alpha intercalated cells
- beta intercalated
Which AQP in the CD is regulated?
AQP2 (apical sde)
Which AQP in the CD is constitutive?
AQP3,AQP4 (basolateral side)
What do alpha intercalated cells secrete?
Acid (HCO3 secreted back into the blood)
What do Beta intercalated cells secrete?
Base (bicarbonate secreted back into the lumen)
