Lecture 18: Traffic Across Epithelial Cells: Chloride Secretion

Understand the cellular mechanisms involved in chloride secretion
Understand how a defect in chloride secretion can cause the diverse clinic symptoms associated with secretory diarrhea and cystic fibrosis
In addition gain insight into the excitement associated with physiological research

Location: Lumen and Blood.
Key Players:
- Tight junctions: Divide cells into apical and basolateral membrane domains.
- Na+/K+ ATPase pump: Establishes ion gradients (2K+, 3Na+).
- Na-K-2Cl symporter: Actively accumulates chloride above its electrochemical gradient, utilizing the energy from the Na+ gradient.
- Chloride channel: Allows Cl- to leave the cell via passive diffusion.
- Na+ channels and Na+/K+ ATPase: Facilitate Na+ exit; K+ exits via channels.
Process:
1. Tight junctions divide cells into apical and basolateral membrane domains
2. Na+/K+ ATPase pump creates ion gradients.
3. Na-K-2Cl symporter accumulates Cl-.
4. Cl- exits through the Cl- channel.
5. Na+ exits via basolateral Na-pump, K+ via channels.
6. Cl- transport induces paracellular Na+ and water fluxes.

Cl- accumulation occurs above electrochemical equilibrium. Cl- cannot leave the cell unless the Cl- channel is open. Opening of the Cl- channel is tightly regulated (gated). Therefore, the opening of the Cl- channel is the rate-limiting step in Cl- secretion.
CFTR: The Cl- channel is identified as the Cystic Fibrosis Transmembrane conductance Regulator (CFTR).
Implications: CFTR overstimulation is linked to secretory diarrhea, while its dysfunction causes cystic fibrosis.

Is caused by excessive stimulation of the secretory cells in the crypts of the small intestine and colon
Triggers:
- Abnormally high concentrations of endogenous secretagogues produced by tumors or inflammation.
- More commonly it’s due to the secretion of Enterotoxins from bacteria like Vibrio cholerae.
Enterotoxins irreversibly activate adenylate cyclase, causing maximal stimulation of CFTR.
Result: Secretion overwhelms the absorptive capacity of the colon.

Cholera toxin irreversibly activates adenylate cyclase, leading to CFTR activation. This process involves a G-protein coupled receptor and results in increased cAMP levels. The increased cAMP leads to phosphorylation (P) and activation of CFTR and thus sustained secretion.

Oral Rehydration Therapy: Used to treat secretory diarrhea caused by cholera. Utilizes glucose-stimulated water flux to aid reabsorption. A possible issue is Overstimulation of secretion.

A complex inherited disorder affecting children and young adults. It is inherited in an autosomal recessive fashion
- Heterozygotes have No symptoms but are carriers.
- Offspring of two carriers have a 1 in 4 chance of having CF.
Disease frequency varies among ethnic groups

Patients with CF have a variety of symptoms. A common theme is no matter the organ epithelial tissue is involved. Most cases of mortality are due to respiratory failure.

CFTR is a Cl- channel and is highly Regulated by protein kinase A-dependent phosphorylation of the R domain and causes binding of ATP to the NBD (nucleotide-binding domain). Then ATP causes a conformational change which opens the channel, then chloride can diffuse down it’s concentration gradient.

Normal Lung Epithelial Cells: Balance between secretion and absorption maintains lung surface moisture without excessive fluid buildup.
Lung Epithelial Cells in CF: Defective Cl- channel prevents isotonic fluid secretion and enhances Na+ absorption, resulting in a dry lung surface.

Normally, wet, thin mucus traps inhaled particles, and cilia push mucus to the throat for removal.
In CF, mucus becomes thick and difficult to remove. Bacteria proliferate and attract immune cells, causing damage to healthy tissue. DNA released from bacteria and lung cells adds to the stickiness.
Airways become plugged and deteriorate.

CFTR gene defect leads to defective ion transport. This results in airway surface liquid depletion and defective mucocillary clearance. This causes Mucus obstruction, infection, and inflammation follow.

People with CF have very salty sweat.
Two-stage process of sweat formation:
- Primary isotonic secretion of fluid by acinar cells.
- Secondary reabsorption of NaCl (but not water) in the ducts producing a hypotonic solution.
In CF, epithelial cells in the ducts of sweat glands fail to reabsorb NaCl, leading to salty sweat.

Secretion: Primary secretion occurs in the secretory coil driven by Parasympathetic stimulation (ACh)
Reabsorption: Secondary reabsorption occurs in the reabsorptive duct driven by Sympathetic stimulation (NA).
Key Ions and Channels: $Na^+$ , $Cl^-$ , $K^+$ , $Ca^{2+}$ , CLCA, CFTR, ENaC, NKCC1, ATP-ase.
Cell Signaling: Cyclic AMP, PKA, InsP3.

In duct cells, the membrane potential is depolarized, and Cl- wants to enter the cell down its electrochemical gradient.
In CF patients, defective CFTR causes Cl- to accumulate in the duct lumen, resulting in salty sweat.