Epithelia (Absorptive Epithelia (Electrical Driving Force:
- AVP = Vasopressin
- Water permeability of CCT, Inner and Outer Medullary Collecting Duct is facilitated by vasopressin
- Movement of ions makes basolateral side hypotonic, leads to water secretion trans and paracellularly.
- Severely disturbed if Cl channel is mutated or defect (Cystic fibrosis)
- 'The Dude' generates low sodium concentration in cell, used by transporters on apical AND basolateral side.
- Na+ gradient used by Na+/K+/2Cl- transporter.
- Facilitates secretion of Na+, K+ and 2Cl-
- Chloride then secreted apically via Cl channel.
- Secretion results in negative lumen and positive intersitium, drives paracellular Na+ secretion.
- Acinar cells of the salivary gland represent a leaky secretory epithelium.
- Produces an isotonic primary fluid (ions + water)
Electrical Driving Force:
Electrical gradient + Chemical gradient = Electrochemical gradient.
- In conjunction with the chemical driving force
- Na+/K+ ATPase = 2+ in, 3+ out.
- Generation of a negative charge inside the cell.
- Cell membrane is K+ permeable, becomes even more negative.
- Negative internal charge attracts more positive ions into the cell.
- Simple diffusion: can move through
- Facilitated diffusion: requires a specific membrane protein (AQPs, GLUTs)
- Active transport: specific membrane protein + energy (Na+/K+ ATPase)
- Diffusion occurs at constant rate, cannot be saturate
- Mediated transport can be saturated as depends on the capacity of the transporter.
Function: Transport from mucosal to serosal solution, driven by Na+ transport
- Once glucose has entered the cell coupled to Na+ has established high gradient, can be released via facilitated diffusion GLUT1 or 2.
- Can be absorbed against a gradient as Na+ acts as a driving force for the absorption.
- AQP3 and AQP4 expressed on basolateral membrane.
- Membrane = impermeable
- Na+ has chemical driving force but need SGLT1 or 2 to cross membrane (secondary active), enables glucose absorption.
- In a leaky epithelia, consequence of Na+ absorption is negative lumen and positive interstitium: drives paracellular absorption of Cl-
- Also establishes osmotic gradient and drives the paracellular absorption of water.
- Basal membrane + lateral intercellular membrane
- Properties are fairly similar amongst (high water and K+ permeability)
- No microvilli, does have membrane infoldings.
- Expresses Na+/K+ ATPase
- Contains microvilli - highly reabsorptive or secretory.
- Varying properties e.g. PCT = leaky epithelium, high water permeability: AQP1 + paracellular. CCT = tight epithelium, low water permeability, only AQP2.
- NO Na+/K+ATPase
- Apical Side: Mucosal Solution
- Basolateral Side: Serosal Solution
- Cells are connected, have polarity, have different protein expression.
- Different epithelia = different behaviour
- Different epithelia = different regulation
- Airways: Ciliated
- Blood Brain Barrier: Protects the brain, secretion of metabolites from the brain.
- Tubule in Kidney: Absorption and secretion of ions/metabolites e.g. glucose, urea, amino acids.