epithelial transport Flashcards
The apical surface faces the
‘special’ fluid (e.g., food in the gut, urine in the kidney, saliva in the parotid duct) and usually contains the special transporters that endow the epithelium with its specialized transport properties.
The basolateral surface is exposed to the
interstitial fluid and usually has generic transport properties like the plasma membranes of non-epithelial cells (neurons, for example).
Main task of the lungs is to
transport O2 into and CO2 out of the blood.
There are no membrane transporters for.
(active or otherwise) for O2 and CO2; they are lipid soluble molecules that diffuse freely through all membranes.
the endothelial cells in the lung that separate blood from air in the alveoli of the lungs do not possess
special transporters like those in epithelia to transport O2 or CO2
epithelial sheet are glued to their neighbors by
tight junctions.
In most epithelia, however, the tight junctions are
not very tight.
tight junctions examples
sweat glands and the distal parts of kidney tubules,
preventing virtually any substance from passing from one side to the other by passing in between the cells.
Leaky tight junctions are located
of the small and large intestine,
gall bladder, and
proximal part of the kidney tubules are relatively leaky, and
Leaky tight junctions are somewhat selective in their
leakiness; some are relatively more permeable to cations, others to anions.
leak tight junctions provide a
provide a pericellular shunt pathway for the movement of water and solutes.
In general, epithelia engaged in massive transport of substances are ___, while those epithelia doing the finishing work (‘fine tuning’) are _____.
leaky
tight
Leaky epithelia cannot maintain
energy gradients as large as those produced by tight epithelia, because solutes and water leak back across the epithelium through the pericellular shunt.
to get across an epithelium, a substance must follow one (or both) of two possible routes:
it may either cross two membranes by entering the epithelial cell on one side and leaving on the other, or it may cross no membranes at all by passing in between cells through the pericellular shunt pathway.
The driving force for nearly all transport – water, salts, nutrients, non-volatile metabolic wastes – .
is the Na/K pump (always located in the basolateral membrane)
By keeping intracellular sodium ion concentration low, the Na/K pump provides the:
energy to drive a host of secondary transporters
Protons are the main exception to the otherwise universal dependence of
epithelial pumping on the Na/K pump, because primary active transporters have evolved for protons, most notably in the stomach (to secrete acid into the lumen of the stomach) and kidney (to excrete protons, which are a metabolic waste product.)
What value of membrane potential would you expect to record across the basolateral membrane (neuron)?
Answer: because it’s like the plasma membrane of a neuron, Vm would be about -70 mV. The basolateral membrane also contains some chloride channels, and the Na/K pump.
the basolateral membrane is just like many other cells, containing
relatively low sodium permeability and high potassium permeability.
Apical membrane
It is relatively highly permeable to sodium, not potassium.
Na/K pump in the apical membrane.
how are salt and water are transported from apical to basolateral solution:
Sodium ions leak into the cell across the apical membrane, down their electrochemical gradient. They are then pumped out of the other side of the cell by the Na/K pump, across the basolateral membrane.
This results in the net transport across the epithelium of a positive charge, and chloride follows passively, drawn by the electrical force.
The net transport of NaCl produces an
osmotic gradient, which in turn draws water along.
What would happen to the transport if the Na/K pump were blocked?
As the cell filled with sodium (leaking in across the apical membrane), the driving force for further sodium entry across the apical membrane would be reduced, and net transport of sodium, chloride, and water would decrease.
If one were to measure the voltage across this epithelium, what would be the result?
By putting electrodes into apical and basolateral solutions and connecting them to a recording device, one would find that the apical solution is negative with respect to the basolateral solution. Because the movement of sodium out of this solution creates the electrical gradient that draws the chloride along with it.
What would happen to this transepithelial voltage if the chloride ions in the apical solution were replaced with a larger anion, say SO4=, which could not fit through the chloride channel?
In this case, the Na/K pump would continue to operate. Because no anion can follow, the trans- epithelial voltage will increase. Before long, the apical solution would become so negative that net sodium transport would stop.
What is the transepithelial potential difference (transPD)?
TransPD = Vm (Basolateral) – Vm (Apical)
all membrane potentials are written as the
potential of the inside of the cell with respect to the outside (i.e., outside = zero);
the transepithelial potential is written as
the potential of the apical solution with respect to the basolateral (i.e., basolateral = zero)
the cell is
isopotential
The cell is isopotential, so the line across the cell is
horizontal.
The key to understanding epithelial secretion in general and the main defect in those diseases is a
chloride channel in the apical membrane (especially in the GI tract and lungs).
Normally, in a resting cell, Cl- channel is _____.
closed
The secretion is driven by
Cl- leaking out of the cell into the lumen
As the Cl- leaks out of the cell into the lumen, the electrical negativity that it creates draws
Na+ along passively.
isopotential
all voltage drops are at membranes,
so all lines showing electric potential in Fig. 2 are horizontal – no change over distance, except across membranes
when this Cl- channel is activated,
the cell begins to secrete electrolytes and water into the lumen.
The Cl- concentration in the cell is high, because
of a Cl- pump in the basolateral membrane.
The pump is a Na-K-2Cl cotransporter that uses the
downhill leakage of Na+ into the cell to drive the uptake of Cl- from the interstitial fluid
(it is located in the basolateral membrane, not in the apical membrane).
The Na+ flows mostly through the_____ pathway. The resulting osmotic gradient
intercellular shunt
draws water along, too, giving a net secretion of an isotonic solution of NaCl.
So eptithelia have mechanisms both to absorb and to secrete. Which one wins?
at rest, the apical Cl- channels are closed, so absorption wins.
A variety of stimuli can open the Cl- channel and turn on secretion.
In the GI tract, it happens physiologically during digestion (parasympathetic nerve stimulation, hormones in the blood), as chemicals activate receptors in the basolateral membrane (the signal is carried across the inside of the cell from the basolateral receptors to the apical membrane by ‘cell signaling’ mechanisms (Ca++ ions, activated protein kinases, cyclic AMP, etc.)
Pathogens also can activate the
Cl- channels.
