How do we measure
In a normal, symmetrical,
or general cell,
we put a little recording
electrode within the cell
and have a secondary electrode in
a bath or in the fluid around it.
That gives us our
Epithelial tissue is
different and that it has
a number of different
The apical membrane has
a membrane potential,
the basolateral membrane
has a membrane potential,
and there’s a membrane potential across
the whole epithelial cell itself.
So we have three membrane
potentials to take into account:
the apical, the basolateral,
and the complete endothelial
cell’s membrane potential
from one side to the other.
So let’s go through some
examples to practice this
because this is a very different
way of looking at voltages.
It’s both across the membrane,
across the second membrane, and
then across the whole cell line.
If we have an apical membrane voltage
of let’s say minus 60 millivolts,
we have a basolateral voltage
of about minus 80 millivolts,
therefore, there is a 20-millivolt
difference between those two sides.
So, it’s possible to involve ions differently
that might want to move across the whole
entire tissue that don’t necessarily want
to move across one membrane or the other.
Let’s keep going through
some examples of this.
Let’s use the transport of sodium
and chloride as an example.
If we have a voltage that is set
up by a sodium-potassium ATPase,
we have sodium traveling in
across the apical membrane
and then maybe potassium traveling
across the basolateral.
It can set up a voltage
difference across the membrane
and that voltage difference
will want to drive
chloride across through
those tight junctions.
Without the transepithelial
the chloride would not want
to travel in this direction.
So it’s traveling towards
a negative charge
on the apical membrane to a
positive charge on the basolateral.
The positive charge is set up by positively
charged ions leaving the basolateral side.
A second example is looking
at sodium chloride secretion.
Here, we are going to use
the sodium-potassium ATPase
as the initial setup on
the basolateral membrane.
We also have another channel that it’s
going to cotransport three things:
sodium, chloride, and potassium.
These are sometimes
known as NCCK channels.
This allows the co-transport of all
three of these ions into the cytosol.
Chloride then is allowed to
travel across the apical membrane
and that sets up a negative charge
across the apical membrane.
If we look at this, that will want
to pull sodium across with it.
Because sodium will want to travel
from a positive to a negative,
and therefore, this transepithelial gradient allows for this negative
pull across the transepithelial
surface or the leaky tight junctions.