lec 10 - epithelial transport

functional classifications of epithelia

specialised for absorption, secretion, or to act as a barrier

protective epithelia - cornea, skin 

transporting epithelia - secretory epithelia, absorptive epithelia 

exchange epithelia - endothelium of blood vessels 

highly specialised epithelia - podocytes 

ohms law equation

R=V/I

transepithelial resistance (RTE)

a meaasure of how much the epithelium opposes ion flow

in ohms

R=V/I

transepithelial conductance (GTE) 

the inverse of resistance

indicates how easily ions move across the epithelium

relationshipb between R and G

if the resistance is small, the conductance will be high

if the resistance is large the conductance will be low 

epithelia as electrical circuits

can be modelled as a group of resistors

two pathways in parallel:

transcellular - (Ra+Rb in series), paracellulae = tight junctions

ion channels and transporters alter the transcellular conductance

nephron low resistance, bladder epithelium super high

transepithelial voltage

the difference in electrical potential across the epithelium (lumen vs interstitial)

the interplay of electrogenic transport and junctional resistance influences the ability of an epithelium to maintain a significant transepithelial volrage

epithelial voltage in tight vs leaky epithelia

tight = large VTE - limited paracellular ion flow

leaky = small VTE - low resistance TJ enable paracellular ion flow

electrical properties of leaky epithelia

high current flow through both the paracellular and transcellular pathways

low transepithelial resistance - 10-10 ohms/cm

transepithelial voltage is -5 to 0mV

hormonal control is low to absent - prioritise speed and bulk recovery over precise regulation 

electrical properties of tight epithelia

tight junctions restrict paracelliular ion movement, and low ion transport through the transcellular pathway

transepithelial resistance 300-10000 ohm/cm

transepithelial voltage -20 to -100mV

hormonal control - highly regulated

• activity and expression of ion channels and transporters are under close control

• transport can be precisely adjusted to maintain homeostasis

water permeability in leaky vs tight epithelia

leaky - hgih hydraulic conductivity - hgih expressio fo AQPs, TJs allow paracellular movement, transport large aounts of isosmotic fluid 

tight - low hydraulic conductivity, tight junctions restrict or prevent paracellular water transport, low transport via AQPs, required for regulation of fluid osmolarity (e.g. urine concentration in medulla) 

ADH and water permeability

ADH increases water permeability in tight distal nephron segments

leaky epithelia (proximal tubule) have intrinsically high water permeability

right epithelium has low baseline water permeability

can be upregulated by the hormone ADH to increase expression of AQP in the apical membrane

tight vs leaky epithelial location

leaky - proximal regions of organs (PCT, duodenum and jejunum of the small intestine) 

tight - distal regions of organs (DCT, distal colon of large intestine) 

epithelial summary table