Chloride Secretion and Cystic Fibrosis Notes

Chloride Secretion

  • Chloride secretion involves using combinations of channels and transporters, including secondary active transporters, primary active transporters, and ion channels.
  • Key players in chloride secretion:
    • Sodium pump (Na+/K+ ATPase)
    • Chloride channel
    • Sodium-potassium-chloride cotransporter

Cellular Mechanisms of Chloride Secretion

  • Epithelial cells have tight junctions at the luminal margin, creating a fence and barrier that restricts movement via the paracellular pathway.
  • The tight junctions create two membrane domains: apical and basolateral.
  • The basolateral membrane contains the sodium-potassium ATPase, a primary active transporter that exchanges three sodium ions leaving the cell for two potassium ions entering, using ATP hydrolysis.
  • The sodium-potassium ATPase sets up ion gradients: low sodium inside the cell and high potassium.
  • The basolateral membrane also contains the sodium-potassium-chloride cotransporter, a carrier-mediated symport or cotransport system that uses the sodium gradient to move potassium and chloride against their electrochemical gradients.
  • The cotransporter moves one sodium, one potassium, and two chloride ions.
  • This transport is electroneutral.
  • Chloride is accumulated above its electrochemical equilibrium inside the cell.
  • The apical membrane contains a chloride channel that allows chloride to diffuse down its electrochemical gradient into the lumen.
  • The movement of chloride from the blood to the lumen creates a negative charge, attracting sodium and water via the paracellular pathway.

Pump Leak Hypothesis

  • To maintain the chloride gradient, sodium is recycled by the sodium-potassium ATPase.
  • Potassium diffuses down its electrochemical gradient, maintaining the negative membrane potential.
  • Sodium and water are removed via the paracellular pathway, matching electron neutrality and osmosis.
  • The process results in isotonic fluid secretion, where the osmolarity is the same as the body fluid.
  • This involves moving a volume of water from inside the body to the lumen of the gut without affecting cell volume.

Rate-Limiting Step: Chloride Channel Regulation

  • The rate-limiting step in chloride secretion is the removal of chloride across the apical membrane via the chloride channel.
  • Chloride cannot passively diffuse across the lipid bilayer and requires an ion channel or carrier.
  • Ion channels have an open probability and undergo gating.
  • Without an open chloride channel, no chloride secretion occurs.
  • The opening of the chloride channel is strictly regulated.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

  • The chloride channel has been identified at the molecular level as the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR).
  • Cystic fibrosis is a defect in chloride secretion involving the CFTR channel.
  • CFTR was named transmembrane conductance regulator because its structure was initially unclear and did not resemble known chloride channels.
  • CFTR is a chloride channel; overstimulation can cause secretory diarrhea, while mutations can cause cystic fibrosis.

Secretory Diarrhea

  • Excessive stimulation of secretory cells in the crypts of the small intestine or colon.
  • Can be due to high concentrations of endogenous secretagogues (hormones and neurotransmitters) produced by tumors or inflammation.
  • Often caused by bacterial infections, such as Vireo cholerae, which activates the chloride channel.
  • Enterotoxins activate cell signaling pathways, leading to the activation of adenylate cyclase, producing cyclic AMP, which stimulates the CFTR chloride channel.
  • This continual stimulation of secretion overwhelms the capacity to reabsorb the solution, leading to secretory diarrhea.

Cell Signaling Pathway in Chloride Secretion

  • Secretagogues bind to G-coupled proteins, activating adenylate cyclase and creating cyclic AMP.
  • Cyclic AMP phosphorylates CFTR, opening the channel in the apical membrane.
  • In cholera, enterotoxins produced by the bacteria bind to adenylate cyclase and irreversibly activate it, causing sustained secretion.

Gut Structure and Function

  • The gut has a highly invaginated system with villi and crypts.
  • Secretion occurs in epithelial cells in the crypt cells, while glucose absorption occurs in the cells in the villi.
  • The cells are the same; stem cells in the crypt reproduce and become secretory cells, then migrate to the villus to become epithelial cells for glucose absorption.
  • Secretion and absorption are balanced, with about nine liters of fluid secreted a day and 8.5 liters reabsorbed.
  • Oral rehydration therapy can treat secretory diarrhea by stimulating glucose-stimulated water reabsorption.

Cystic Fibrosis

  • Complex inherited disorder affecting children and young adults.
  • Inherited in an autosomal recessive pattern.
  • Disease frequency varies among ethnic groups; prevalent in northern Europeans.
  • Symptoms include:
    • Airways: clogging and blocking of breathing passages, mucus and infection in the lungs, respiratory insufficiency.
    • Liver: blocking of small ducts of the bile tubes.
    • Pancreas: occlusion of the ducts, pancreatitis.
    • Small intestine: blockage of the intestine.
    • Reproductive duct: infertility in males.
    • Skin: very salty sweat.
  • The common theme is that epithelial tissues are involved in airway production, enzyme production, movement and secretion of fluid.
  • Management includes chest percussion, antibiotics for lung infections, pancreatic enzyme replacement, and nutritional attention.

Historical Perspective of Cystic Fibrosis

  • 1938: First comprehensive description of cystic fibrosis.
  • 1950s: Demonstration that excessive salt loss was associated with cystic fibrosis.
  • 1980s: Demonstration that chloride secretion in epithelial cells was the primary defect.
  • Localization of the disease to chromosome seven.
  • Patch clamp studies showed that the chloride channel was defective.
  • 1989: Cloning of the gene.
  • Twenty first century therapies will be specific for CFTR mutation.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Structure and Function

  • CFTR is a chloride channel that everyone has, but it might be mutated.
  • It is a transmembrane protein with properties of a channel.
  • It has nucleotide binding domains that bind ATP and a regulatory domain that can close the channel.
  • CFTR is highly regulated and linked to cell signaling pathways.
  • Cyclic AMP activates protein kinase A which phosphorylates the R domain, ATP binds to nucleotide binding domains, conformational change which opens the channel.

CFTR and Lung Cells

  • There is a balance between secretion and absorption which keeps the lung surface moist, you need fluid surface to promote gas exchange.
  • In cystic fibrosis, there is a defective chloride channel in the apical membrane, so there is no fluid secretion and there is more absorption.
  • The lung surface becomes dry, the mucus sticks, colonisation of bacteria, leads to immune responses, lung tissue is killed.

Sweat Formation and Cystic Fibrosis

  • People with cystic fibrosis have very salty sweat.
  • Formation of sweat occurs in two processes:
    • Primary isotonic fluid secretion.
    • Secondary reabsorption of sodium and chloride, that isn't water, produces a hypotonic solution.
  • Failure of the epithelial cells in the duct cells of the sweat glands to reabsorb the sodium chloride that produces the salty sweat in cystic fibrosis patients.
  • Two different ion channels, one that's activated by noradrenaline, which is CFTR, and one that's activated by acetylcholine, which is the calcium regulated chloride channel.
  • Different changes in electrochemical gradient, and you're changing the water permeability.
  • CFTR is a chloride channel, because chloride can go in either direction through that channel, depending on electrochemical gradient.
  • This reinforced fact of actually CFTR is a chloride channel.
  • Normally there is an electrical gradient, and with cystic fibrosis you get a non functional chloride channel, and you don't allow the sodium ion to move without the chloride, so then the sodium chloride is retained on the skin causing the salty sweat.