Tubular transport mechanisms

[Ref: AV6:Chp4]

 

Transporters vs channels

Transports

• Highly specific
• Low rate of transport
  * Can be saturated
• Protein undergoes conformational changes during the transport

Channels

• Specificity relatively low
• Faster movements

 

Reabsorption via transcellular route

In proximal tubule

Step 1

• Na-K ATPase pumps sodium from tubular cells into interstitial fluid
• Na+ moves from lumen into cells due to concentration gradient
  * Mainly via Na-H antiporter

Step 2

• Anions move with Na+ to balance the charge

Step 3

• Water moves into interstitial fluid due to osmotic gradient

Step 4

• Water and electrolyte moves from interstitial fluid into peritubular capillaries driven by Starling forces.

Glucose

• Enters epithelial cells across the apical membrane via a symporter with Na+
  * i.e. SGLT2
• Exits epithelial cells via an uniporter
  * i.e. GLUT2

 

Reabsorption via paracelluar route

As water and sodium (and associated anions) get reabsorbed

--> Concentration of solutes not specifically transported via the transcellular route will become concentrated
* e.g. Urea, K, Cl-, Ca2+, Mg2+

--> Diffuse down the concentration gradient via the paracellular route

Glucose

• Tight junctions are not permeable to saccharides
• Gluose cannot be transported via this route no matter how large the concentration gradient is
• Transcelluar route only

 

Electric charge of lumen

• Early in the proximal tubule, lumen slightly negative
• Later in the proximal tubule, lumen slightly positive

 

Tm vs gradient-limited system

Transport mechanism can be either

1. Tubular maximum-limited (Tm), or
2. Gradient-limited

Tm limited system

e.g. Glucose in proximal tubule

• Transporters moving the substance become saturated
• Further increase in solute concentration does not increase the movement speed

Gradient-limited system

• Transporters moving the substance are NOT saturated
• However, passive leakage (usually via the tight junctions) caused to a very large gradient may be high enough to offset the transports

e.g. Sodium in proximal tubule

• When a very large sodium gradient exists between the interstitum and the lumen, sodium may leak back into the lumen just as fast as Na-K ATPase pumps may be absorbing
  --> Movement of sodium across membrane is high, but there is no net movement