Lecture 8 - Absorptive and Secretory Epithelia Flashcards
How is electroneutrality achieved in the absorption of Na+ and water?
Movement of Cl-
What type of process is Na+ coupled solute cotransport of SGLT1 and GLUT2?
Electrogenic process
What type of transport is the movement of glucose?
Active transport for glucose to get inside cell and then passive to get from inside cell to basolateral
What SGLT do we have in the early PT?
In the early prox. tubule SGLT2 re-absorbs 90% of the filtered glucose
What SGLT do we have in the late PT?
In the late prox. tubule SGLT1 is the main glucose
transporter
What are the common principles of Na+ coupled solute cotransport in the kidney? (e.g. glucose)
- Secondary active (Na+ coupled) uptake through apical membrane (uphill)
- Facilitated diffusion through basolateral membrane (downhill)
What type of osmotic absorption is Na+ coupled solute cotransport in leaky absorptive epithelium?
Isosmotic absorption
What type of osmotic absorption is Na+ transport in tight absorptive epithelium?
Hyperosmotic absorption
Glucose is
A. absorbed at the basolateral membrane in a sodium dependent manner.
B. absorbed entirely via facilitated diffusion.
C. secreted.
D. transported against its gradient at the apical membrane.
D. transported against its gradient at the apical membrane.
What are the electrical properties of secretory epithelia?
– leaky for water secretion
– tight (e.g. K+ secretion in cortical duct)
What is the ion transport (water secretion) in secretory epithelia?
– actively secrete Cl- ions
– Na+ and water follows passively
– secretion of isosmotic NaCl solution
What are the apical membrane properties (water secretion) of secretory epithelia?
High permeability to Cl- due to the presence of Cl channels
Cl- channels need to be activated to open
What is the driving force for Cl- exit through apical membranes?
Driving force is electrical gradient not concentration gradient
What are the two important epithelial Cl- channels?
cAMP-activated Cl- channel
Ca2+-activated Cl- channel (TMEM16A)
What is Ca2+-activated Cl- channel (TMEM16A) activated by?
Activated by an increase in intracellular Ca2+
What is TMEM16A?
Ca2+-activated Cl- channel
What is CFTR?
Cystic Fibrosis Transmembrane conductance
Regulator (CFTR)
What type of channel is CFTR?
cAMP-activated Cl- channel
What channel is defective in cystic fibrosis?
Cystic Fibrosis Transmembrane conductance
Regulator (CFTR) - cAMP-activated Cl- channel
What is the hydraulic conductivity of secretory epithelia?
– high
– secretion of isosmotic NaCl solution
What is the hormonal/nervous control of secretory epithelia?
- under precise control
- low basal rates of secretion increased by nervous or hormonal control as required
What are some examples of secretory epithelia?
Exocrine glands
– Sweat glands
– Salivary glands
– Pancreas
– Liver
Small & Large intestine
Respiratory epithelium
Reproductive tract
– Male - epididymis
– Female - vagina, uterus and fallopian tubes
What is the physiological function of secretory epithelia in the GIT?
Gastrointestinal tract, both exocrine glands and intestinal epithelium
Secretion of enzymes for the digestion of food
What is the physiological function of secretory epithelia in the respiratory epithelium?
Keep surface moistened to maintain mucociliary clearance
What is the physiological function of secretory epithelia in the reproductive tract?
– Male - maturation of sperm
– Female - survival and fertilisation of gamete
What is the physiological function of secretory epithelia in sweat glands?
Thermoregulation
How do the two important types of epithelial Cl- channels differ?
How they are regulated differs, Changes in cAMP levels in cell vs changes in intracellular calcium
What is NKCC?
Na+, K+, 2Cl- secondary active cotransporter
Cl– uptake against its electrochemical gradient
What are the common functional concepts in absorptive and secretory epithelia?
- Electrochemical gradients determine transport of charged solutes (e.g. ions)
- ‘Two membranes’ (different transporters)
- Combination of active (uphill) and passive (downhill) transport for each ion/solute to cross the epithelium.
What are the active and passive transporters for Na+?
Active: Na/K-ATPase
Passive: ENaC
What are the active and passive transporters for Cl-?
Active: NKCC
Passive: CFTR
What are the active and passive transporters for Glucose?
Active: SGLT
Passive: GLUT
Water secretion
A. takes only place in the kidney.
B. is mainly facilitated by chloride ions.
C. is mainly facilitated by sodium ions.
D. requires glucose absorption.
B. is mainly facilitated by chloride ions.
What type of ion movement is impaired in cystic fibrosis?
Cl– secretion is impaired in cystic fibrosis
Why do people with cystic fibrosis have viscous mucus in various organs?
No CFTR (mutation)
Cl- secretion decrease
water secretion decrease
What is VIP?
VIP – vasoactive intestinal peptide (hormone) that activates Cl– secretion
