Proximal Tubule Flashcards
Describe the constitutive reabsorption properties of the proximal tubule with respect to NaCl and water?
The PT reabsorbs approximately 67% of both filtered water as well as filtered NaCl iso-osmotically
Describe organic solute reabsorption (Constitutive/regulatory and saturable/nonsaturable) and secretion in the proximal tubule?
Almost all organic solute reabsorption occurs in the the PT and is constitutive and saturable. The PT is also the location for both cation and anion organic solute secretion in the nephron.
Describe the change in transepithelial voltage as you go down the PT, what causes this?
The voltage changes from -3mV to +3mV. This results from exit of more positive cationic charged solutes in the first 25% of the PT and exit of more anionic negative charged solutes further along the PT
Describe the transcellular transport of Na+ across the apical and basolateral membrane in the PT?
Na+ transport results from passive uptake across the apical membrane and active efflux across the basal and lateral membrane. The Na+ gradient across the apical membrane is utilized for secondary active transport of other organic solutes (i.e. glucose)
Describe the paracellular transport of Na+ in the PT?
In the early PT back leak of Na+ occurs due to a negative lumen voltage. This reverses in the late PT and forces Na+ out of the lumen.
Describe the Paracellular transport of Cl- in the PT? Compare early vs late PT
In the early PT paracellular Cl- transport is driven by a lumen negative voltage. In the late PT lumenal Cl- is elevated above plasma Cl- and paracellular transport is driven by a concentration gradient (this efflux also contributes in creating the positive lumen voltage in the late PT)
Describe the Transcellular transport of Cl- in the PT?
Occurs mostly in the late PT. Active uptake across apical membrane via anion coutertransport. Passive efflux across basolateral membrane via Cl- channel and K+-Cl- symporter
Describe the Reabsorption of HCO3- in the Proximal Tubule?
About 85% of filtered HCO3- is reabsorbed in the PT and is relatively unregulated. Lumen HCO3- is converted to CO2 and OH-. OH- combines with H+ (Supplied by Na+/H+ antiporter) to form H2O. Both CO2 and H2O enter the cell. In the cell H2O dissociates to H+ (recycled via Na+/H+ antiporter) and OH-. CAII converts the CO2 and OH- back into HCO3- which is transported out of the cell.
Does trancellular reabsorption of HCO3- involve a net secretion of H+?
No the H+ is recycled
Describe the two ways by which the PT secretes H+?
The PT secretes H+ as a titratable acid by combining H+ with HPO4 2- to form H2PO4-. It also secretes H+ as NH4+ via diffusion trapping.
What is effect does H+ secretion have on HCO3- in the plasma?
Every time a H+ is secreted a new HCO3- is created and transported across the basolateral membrane. This helps replenish the HCO3- that is used to titrate systemic acid creation from a variety of processes. Thus HCO3- levels can be controlled by regulating H+ secretion
What is the source of intracellular NH3 that is used for H+ secretion in the PT?
Glutamine Metabolism
