Proximal Tubular reabsorption and secretion Flashcards
Where is the PCT mainly found?
Cortex
What increases surface area for reabsorption of the proximal tubule?
Microvilli on luminal surface
What does the proximal tubule reabsorb?
Na+
Cl-
Glucose
Protein/AA
HCO3-
Water
What is paracellular reabsorption?
Leakiness of tight junctions
What is transcellular reabsorption?
Dependent on presence of transporters
How do solutes move from the lumen to the interstitial fluid?
Concentration gradients
Electrochemical gradients
Na+K+ ATPase pump
How do solutes move from the interstitial fluid to vasculature?
Passive diffusion
Oncotic pressure
How is Na+ cleared?
Freely filtered, actively reabsorbed, not secreted
How is Na+ reabsorbed in the first half of the PCT?
With HCO3- and glucose
How is Na+ reabsorbed in the second half of the PCT?
With Cl- and glucose
Where is Na+K+ATPase pump restricted to?
Basolateral side of the cell
How does Na+ enter the cell from the lumen in the first half of PCT?
Via the Na+-H+ antiporter
How does Na+ enter the ISF from the cell in the first half of PCT?
Na+/K+ATPase pump
Na+-HCO3- symporter
How is the lumen negative potential difference across cells in early PCT created?
By Na+ cotransporters - bring net +ve charge into cell leaving -ve charge behind
How is glucose reabsorbed in PCT?
Na+ dependent - no transepithelial or electrochemical gradient present for glucose
How does glucose enter the cell from the lumen?
SLGT2 symporters
How does glucose enter the ISF from the cell?
Glucose uniporters (GLUT2)
Why does tubular Cl- fluid rise along length of PCT?
More water being reabsorbed than Cl-
Preferential reabsorption of Na+ with HCO3-
How does Cl- enter the cell from the lumen in the second half of PCT?
Cl- anion antiporter
How does Cl- enter the ISF from the cell in the second half of the PCT?
K+-Cl_ symporter
What are the glucose transporters in the second half of the PCT?
SGLT1
GLUT1
How is water reabsorbed in the PCT?
Aquaporin water channels on apical and basolateral membrane
What is solvent drag in the PCT?
Osmotic water flow results in reabsorption of some solutes, eg. K+, Ca2+ and Mg2+, by solvent drag along paracellular routes
What does the PT secrete?
Organic anions and cations (metabolism products)
Exogeneous organic compounds (drugs and toxic chemicals)
Where do secreted substances originate from?
Crossing epithelial layer from renal interstitium
Synthesis within epithelial cells
Does paracellular secretion occur?
No
What is tubular secretion coupled to?
Reabsorption of Na+
Where does tubular secretion move substances from?
Peritubular capillaries into tubular lumen
How are anions moved into the cell from the ISF?
Basolateral OAT antiporters move anions in exchange for endogenous dicarboxylic acid eg. alpha ketoglutarate
How do alpha ketoglutarate move into the cell?
Via sodium dicarboxylate symporters (NaDC3)
How are anions transported across apical membrane?
By OAT and MRP proteins along conc. gradients
What can cause gout?
Elevated plasma conc. of urate
What is apical cation transport fuelled by?
Na+/K+ATPase pump
What drives H+ out of the cell into the lumen?
Sodium gradient by NHE3
How are small cations moved out of the cell into the lumen?
Apical OCTN proteins in exchange for H+
How are larger cations transported out of the cell?
Via MDR1 ATPase transporter on apical membrane
How are cations transported across basolateral membrane?
Passive diffusion
Different OCT proteins along conc. gradients
What is the transport maximum of a system?
Amount of solute delivered to the tubule per minute that just saturates its transport process
What is the Tm for glucose?
380mg/min
