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
- Tubular maximum-limited (Tm), or
- 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
