Now that we have how gallbladder
releases bile into the small intestine,
we need to discuss something called the
cystic fibrosis transmembrane regulator.
And this process involves a
very specialized transporter.
This transporter, although in
pathophysiology, can be mutated.
And a mutated CFTR transporter can
cause problems both in the pancreas,
in the gallbladder,
elsewhere in the GI tract.
So how do we know if the CFTR
transporter is working properly or not?
One way is to test in a gland that is
easily accessed than the pancreas.
It’s hard to test this
in the pancreas itself.
So we can test it
in other glands.
So this is a sweat gland.
From a sweat gland, you will
use the CFTR transporter
to move chloride into the
cells of the sweat gland duct
and eventually across the basolateral
membrane into the plasma
or into the interstitial
fluid and then plasma.
So it’s done through a two-stage process
in where you reabsorb chloride.
So the solute that you will
normally be able to reabsorb
will then produce a
How you can use this clinically is if the
CFTR transporter is not working properly,
chloride will not be able to
be reabsorbed in the same way.
It will be left in the lumen of
the sweat gland and sweat it out.
Interestingly, there’s a relationship
between chloride and sodium.
If chloride is left in the luminal
portion of the sweat gland,
it prevents sodium from
being reabsorbed as well.
So sodium is normally reabsorbed
across an ENaC sodium channel,
which is an endothelial
And then across the basolateral membrane
with the sodium-potassium ATPase.
If the CFTR channel is
not working properly and
chloride is left in the
luminal portion of the duct,
then sodium will be also left in
the luminal portion of the duct.
So let’s look at how this
looks in terms of secretions.
This will cause an increase
in the concentration of
both sodium and chloride
in sweat secretions.
How can you use this clinically?
One way is you can test if the CFTR
is working correctly.
This shows and example
of what a CFTR membrane,
cystic fibrosis transmembrane
regulator looks like.
And this is an ABC binding transporter.
So this needs ATP for
it to function properly
to have the chloride go from the
cytosol into the interstitial space.
If we stimulate sweating,
we should get a sweat rate along the X-axis
and a concentration of sodium
and chloride along the Y-axis.
This shows a normal individual who
has a normal functioning CFTR.
If you contrast that with
someone with cystic fibrosis,
who has an abnormal CFTR transporter,
they will have large amounts of sodium
and chloride across any sweat rate.
How can you use this clinically?
Is you can test someone
who has cystic fibrosis,
you can test the sweat glands.
And this is done through a quantitative
pilocarpine iontophoresis test,
which is abbreviated QPIT.
So what does this do?
Well, pilocarpine is a cholinergic agonist,
meaning that it acts like acetylcholine.
You use this particular type
of transmembrane stimulus
by using electrical current to
pass the drug across the skin.
This then causes sweating to occur.
If the person has a low amount of chloride,
then they’re unaffected by cystic fibrosis.
If they have in between 40 to 60
millimolars, it’s hard to tell.
They might have the
disease, they might not.
Diagnostic for cystic fibrosis if you
have more than 60 millimoles per liter.