Lecture One - Introduction Flashcards
Transcellular
transcport via the cell, i.e. basolateral to apical or vice versa
Paracellular
transport in between cells
Four examples of leaky epithelia
proximal tubule, gallbladder, small intestine and chloroid plexus
Three examples of tight epithelia
Distal tubule, stomach and frog skin
what was frog skin used for?
model epithelium for understanding net transport pathways
Net transport occurs due to…
presence of different transport proteins on basolateral and apical membranes
Rte
Transepithelial resistance
Define transepithelial resistance
resistance across the epithelium to movement
High Rte
not a lot of transport across the epithelium in either direction
High Rte is usually….
tight epithelia
High Rte is typically….
larger than 2000 ohms cm2
Low Rte
lots of net transport
Low Rte is usually…
leaky epithelia
Low Rte is typically….
under 200 ohms cm2
Net transport for transcellular is…
similar for all epithelia
What decides if a cell is tight or leaky?
typically paracellular (transport between cells)
Vte in leaky epithelia
0mV
Vte in tight epithelia
50mV
Flux in leaky epithelia
typically large as it is isosomotic
Flux in tight epithelia
small
Leaky epithelia has … H2O permeability
high
Tight epithelia has … H2O permeability
low
Vte
transepithelial potential
Define transepithelial potential
net transport of ions and charge generates a potential across the epithelium
Define isosmotic
movement by ions and solutes is followed by water
negative Vte means
more anions or less cations
positive Vte means
less anions or more cations
How is Vt generated?
sum of basolateral and apical membrane potentials, determined by net movement of charge
Principal cell negative Vte example
ATPase and K+ basolateral channels set the driving force for sodium uptake, so sodium enters the cell and then pumped out the basolateral membrane. Net movement of positive charge from the apical to basolateral membranes therefore loss of positive charge results in a negative charge
Thick ascening limb positive Vte example
ATPase and K+ channel drives NKCC2, which is electro-neutral. Sodium and chloride is reabsorbed but K+ is recycled through ROMK. less anions therefore positively charged
Intracellular microelectrode measures
intracellular potential
Patch clamp measures
single or whole cell current
Two electrode voltage clamp measures
cell current
Ussing chamber measures
transepithelial potential and resistance which can then be used ot measure short circuit current
describe the Ussing chamber set up
two blocks of perspex with the epithelium of interest clamped in between. Epithelium is perfused with Krebs/ringer solution. use buffer to maintain pH. Reference and recording electrode, second set of electrodes connected to a current electrode box
Two electrodes in Ussing chamber function
measure Vte directly by comparing reference and recording
Second set of electrodes connected to a current electrode box in Ussing chamber function
allows injection of a known current across the entire cell membrane to cause a Rte shift but the amount of change is dependent on the reisstance
Ohms law
V = I*R
Ohms law to measure Rte
Rte = change in V/ I injected
Ohms law to measure Isc
Vte/Rte
what is the absolute Vte
the Vte with no current injected
What is Jnet?
Isc
Tracer studies
take radioactive sodium24 in apical solution and measure how much appears in basolateral membrane to measure influx and vice versa for outflux
what proteins are present on basolateral membrane of the upper airway model?
Na+/K+ATPase, basolateral K+ channel and NKCC1
What proteins are present on the apical membrane of the upper airway model?
CFTR and ENaC
Describe the upper airway model
Sodium influx driving force on basolateral membrane set by NKCC1
Chloride accumulates in cell to above the electrochemical equilibrium so CFTR has net chloride secretion.
Driving force for sodium reabsorption through ENaC
Balance between Chloride secretion and sodium reabsorption in upper airway model
sets the height of the pericilliary layer which is the first line of defence against infection
