Tubule transport Flashcards
Functions/ structure of PCT
Leaky epithelia
Bulk absorber
Later segments do fine tubing
Absorptive role of lots of filtered solutes
Brush border microvilli, S1/2/3 segments
S1 and 2 convoluted, 3 straight
High permeability for ions and water
Absorption by PCT
All filtered organic solutes including glucose and a.a
2/3 filtered NaCl and H20
Isotonic
Active Na+ transport underlying all
First half = Na+ with organic solutes, phosphate and bircarbonate
Second half = Na+ with Cl-
Glucose reabsorption
Glucose enters through SGLT (SGLT2 followed by SGLT 1 to increase gradient and make sure all reabsorbed)
D-glucose specific
Exit through GLUT
Transporters have finite limit = Tm for glucose capacity, excess glucose = overspill into urine
Amino acids reabsorption
Similar mechanism to glucose
Na+ coupled transport
Cationic, anionic, neutral and glycine transporters
L amino acids
Bicarbonate reabsorption
H+ with NHE on apical go into lumen
React with bicarbonate in lumen and produce carbonic acid, dissociates with CA on apical membrane
H20 and C02 enter the cell, re-hydrated with CA and then dissociate
Chloride reabsorption
Paracellular, from electrochemical gradient from other solutes moving
Transcellular: with sodium
NHE works in tandem with anion exchanger
H+ combines with anion in lumen, enters cell with Cl- coupled transport
Calcium absorption
Transcellular through ECaC
Active transporters on apical, calcium ATPase and sodium calcium exchanger
doesn’t need Na+
Secretion of organic anions
Movement into lumen (basolateral -> apical)
Basolateral secondary active transporter, accumulation of ions such as alpha -KG from blood
Alpha-KG can then diffuse out of cell down gradient in exchange for PAH (para amino hippurate), so PAH into cell
PAH diffuse across apical with anion coupled exchange (eg. with OH-)
Potential difference along tubule
S1: lumen is negative from movement of sodium
S2/3: lumen is positive for movement of Cl-
- positive lumen pd. drives Na+ reabsorption via paracellular
Water reabsorption
Moves passively down gradient
No discernible osmotic gradient, so overall is iso-osmotic
PCT very permeable to water
different SGLT transporters
SGLT 2 first, has 1:1 ratio and absorbs majority
SGLT 1 later on, has 2:1 ration so greater driving force for absorbing all the glucose
what happens after bicarbonate has been re-formed in the cell
H+ goes back out on apical membrane through NHE, keeps exchanger going
bicarbonate exits on transporter on basolateral, geenrally Na+ coupled symporter, so Na+ also pumped out
bicarbonate efflux
3 HCO3-: 1 Na+
- sodium being pumped out, energy from bicarbonate gradient created
- 3:1 gradient increases energetic power
chloride transcellular specifics
indirect Na-H antiporter in tandem with AE
- H+ enters lumen, react with anions that become neutral and enter cell
- dissociate in cell back into anion and H+
- H+ recycles out again
- Anion diffuses out with a Cl- coupled transporter, so Cl- enters cell
- anions sucks as formate
where does chloride absorption begin
about halfway along
