00:01
How do we measure
membrane potential?
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.
00:17
That gives us our
membrane potential.
00:20
Epithelial tissue is
different and that it has
a number of different
membrane potentials.
00:26
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.
00:40
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.
00:54
So let’s go through some
examples to practice this
because this is a very different
way of looking at voltages.
01:02
It’s both across the membrane,
across the second membrane, and
then across the whole cell line.
01:09
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.
01:27
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.
01:40
Let’s keep going through
some examples of this.
01:45
Let’s use the transport of sodium
and chloride as an example.
01:50
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.
02:05
It can set up a voltage
difference across the membrane
and that voltage difference
will want to drive
chloride across through
those tight junctions.
02:18
Without the transepithelial
voltage difference,
the chloride would not want
to travel in this direction.
02:24
So it’s traveling towards
a negative charge
on the apical membrane to a
positive charge on the basolateral.
02:33
The positive charge is set up by positively
charged ions leaving the basolateral side.
02:39
A second example is looking
at sodium chloride secretion.
02:45
Here, we are going to use
the sodium-potassium ATPase
as the initial setup on
the basolateral membrane.
02:53
We also have another channel that it’s
going to cotransport three things:
sodium, chloride, and potassium.
03:01
These are sometimes
known as NCCK channels.
03:06
This allows the co-transport of all
three of these ions into the cytosol.
03:11
Chloride then is allowed to
travel across the apical membrane
and that sets up a negative charge
across the apical membrane.
03:21
If we look at this, that will want
to pull sodium across with it.
03:29
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.