L42. Intestinal Water and Electrolyte Movement Flashcards
(15 cards)
What is the role of water in the intestine?
We need fluid balance in the intestines to provide an environment that supports both digestion and absorption
Digestion
- Dissolving water soluble components enzyme function
Absorption
- Diffusion of nutrients to sites of absorption
- Lubrication for effective transit and elimination
What are the mechanisms of osmotic water flux?
Electrolytes (particles that carry charge Na+, K+, Cl- and HCO3-)
Osmolytes (particles that don’t carry charge - monosaccharides, amino acids, bile salt)
What are the mechanisms of water and solute flux?
The small intestine is where most absorption occurs (bulk absorption)
- Small intestine = leaky = bulk transport
The large intestine is where regulated absorption/transport occurs
- Large intestine = tight= regulated transport
What is absorption of minerals in the small intestines involving?
In general
- Transcellular transport: apical and basolateral transporters and may require intracellular carriers
- Minerals don’t have major influences on fluid movement (small quantities)
Example: Ca2+ absorption
- Actively absorbed in duodenum
- Apical membrane: epithelial Ca2+ channels
- Intracellular: Ca2+ binding proteins
- Basolateral membrane: Ca2+ ATPase
Example: Fe2+ absorption
- Actively absorbed in duodenum
- Apical membrane: co-transported with H+
- Basolateral membrane: exported via ferroportin
What is ion transport like in the large intestine?
Most osmolytes (monosaccharides, amino acids, bile salt) have already been absorbed in the small intestine so the large intestine is mopping up and doesn’t need bulk absorption
Water movement is passive (osmosis) and driven by the transcellular movement electrolytes (Na+, K+, Cl- and HCO3-)
What is the salt stats and ion transport in the large intestine?
- Electrolyte transport in the large intestine (colon) changes based on the amount of salt in the diet
- Regulated fine tuning of salt uptake by the large intestine based on need/homeostasis
- Normal salt uptake = electroneutral
- Salt depleted = electrogenic
What is electroneutral absorption Na+/H+ exchange - high sodium environment?
Electroneutral absorption
- Predominant in salt replete animals
Carbonic Anhydrase (CA):
- Generates HCO3- and H+
Apical NHE-3 exchanger:
- H+ recycled by conversion back to CO2 and H2O
Cl- absorbed transcellularly via:
- Apical Cl-/HCO3- exchange (HCO3- recycled)
- Basolateral Cl- channels
Water movement (osmosis):
- NaCl in interstitial fluid causes a small amount of paracellular H2O flux via osmosis
What is electrogenic Na+ absorption?
Electrogenic Na+ absorption
- Predominant under low salt conditions
Apical Na+ entry via highly selective channel - ENaC
Na+ exits via basolateral Na+/K+-ATPase
- Na+/K+-ATPase and K+ channels creates net charge transfer - electrogenic
Cl- follows via paracellular pathway
- Tight epithelium
- Tight = reduced paracellular permeability
Transcellular water follows salt movement
- Tight therefore most water moves via aquaporins in the cell membrane
How do we turn on electrogenic absorption?
Aldosterone
- Released in response to low Na+, high K+, and low blood pressure
What does role does aldosterone play in regards to Na+ absorption?
Aldosterone released in response to low Na+/low blood pressure
Mechanism
- Binds to gene promotors in colonic epithelial cells to stimulate protein synthesis :
* ENaC subunits
* Na+/K+ pump
* K+ channel
- Serum and glucocorticoid-induced protein kinase (SGK), which cycles ENaC into apical membrane
Effect
- Increases the amount of electrogenic Na+ absorption
- Increased total Na+ absorption
What is involved with the absorption of short chain fatty acids?
Absorption of short chain fatty acids (SCFA)
- Short chain fatty acids are produced by colonic bacteria
- Luminal concentrations can reach 100-150mM
- Na+ absorption via apical SMCT transporter
- No need for a basolateral transporter - these SCFAs are used as energy source for colonocytes
What is involved with intestinal electrolyte and H2O secretion?
Components:
Mucous - goblet cells in crypt and surface epithelium
Electrolytes:
Active secretion of NaCl
HCO3- exchanged for Cl-
High in duodenum
Neutralises acid
Helps hydrate mucus
Active K+ secretion in the colon
Aldosterone sensitive
Water - osmosis driven
Modifies lumen contents
Hydrates mucous
What is intestinal NaCl secretion?
- Occurs in all segments of the intestine (far more occurs in the small intestine)
- Results in isosmotic NaCl solution
- Na+/K+-ATPase generates gradient for Na+
- Cl- uptake via basolateral NKCC1
- Secondary active transport
- Increases intracellular Cl- above electrochemical equilibrium - CFTR allows Cl- to move across the apical membrane
- Paracellular Na+ movment driven by Cl-
- Osmotic gradient drives paracellular water movement
How is intestinal NaCl secretion regulated?
Secondary messengers stimulate intestinal Cl- secretion
- There are 2 main mechanisms (secondary messengers):
- cAMP stimulates CFTR activity
- Sustained secretory response
- Ligands include VIP (ENS) and prostaglandins - Ca2+ stimulates K+ channel activity
- Increased driving force for NKCC and Cl- exit via CFTR
- Transient secretory response
- Ligands include ACh (ENS) and histamine
What is intestinal HCO3- secretion?
- Na+/K+-ATPase generates gradient for Na+
- HCO3- uptake via Na+/HCO3- co-transporter (NBC)
- Secondary active transport using Na+ driving force - HCO3- moves out of apical membrane via CFTR channel or Cl-/HCO3- exchange (modifying)
- HCO3- transport drives paracellular Na+ movement
- Osmotic gradient drives water movement
Mechanism occurs simultaneous to (and requires) Cl- secretion
