Now, how is this secretion of hydrogen
ions controlled and regulated?
About 40% of it is regulated
via the cephalic phase.
So this is simply the
thought of eating
and the thought of food starts those
gastric juices start flowing.
About 40% of it is
in that format.
If you know that it’s done
through this cephalic phase,
you have to think of, “Oh, the
parasympathetic nervous system is
involved, we’re probably
If you went that route, you are right,
and we’ll go back and talk through
that in more specific detail.
The parasympathetic nervous
system involves the vagus nerve,
which is cranial nervy number 10,
and that directly
stimulates acid production.
One is it can directly stimulate acid
production from the parietal cells.
The second way it can do this is it
engages histamine release from ECL cells.
These are enterochromaffin-like cells.
Also, ACh stimulates G
cells to produce gastrin,
and gastrin is another stimulator of
hydrochloric acid or hydrogen ion production.
So it’s interesting to think of
that ACh does something directly,
and then through to
And we’ll talk through why
that extra kind of regulation
is very important for
hydrogen ion production.
The gastric phase accounts for about 50%
of the control and
regulation of hydrogen ions.
And this is primarily done through a
process called vago-vagal reflexes.
So these reflexes are done via
stimulation of the vagus nerve,
which is cranial nerve number 10, and
stimulates more release of acetylcholine.
The other thing that stimulates from the
gastric phase is proteins and peptides,
especially amino acids,
will help stimulate G
cells to release gastrin,
and gastrin is a potent
stimulator of hydrogen ions.
Hydrogen ions though do have
a negative feedback loop,
and that is by
stimulating D cells,
it will produce somatostatin.
And somatostatin helps to decrease
or stop the hydrogen ion production.
It’s important process to
both engage it and stop it.
The last phase that we have
is the intestinal phase,
which only accounts for about
10% of hydrogen ion production.
That is done via protein digestion
through feedback in the intestines.
Okay. So let’s now talk through the precise
way in which acetylcholine directly
stimulates acid release.
The direct way is using the
cranial nerve number X
to stimulate muscarinic
receptors on the parietal cell,
and that is directly done via ACh.
The second way is for ACh
to stimulate ECL cells,
which then cause the release of histamine,
and histamine can then increase the
amount of hydrogen ion production.
And the final way is that acetylcholine
also stimulates G cells to release gastrin,
and gastrin stimulates both the ECL
cells and the parietal cells directly,
and so you get even further increases
in hydrogen ion production.
And I hope you’re thinking right now,
“Okay, there is a lot of ways to
increase hydrogen ion production.”
And this is interesting
because anytime there
are multiple methods
to do the same thing,
it is even more highly regulated than a
process that only uses one kind of way.
So we’re using multiple methods
to stimulate the same thing,
and these redundant mechanisms can
sometimes be targeted pharmacologically
to decrease certain types
of ion production,
and we’ll get into that in a few slides.
Okay. Let’s get into the
nuts and bolts of this.
Acetylcholine stimulates muscarinic
receptors on the parietal cell.
These are G-coupled protein receptors
that increase phospholipase C,
which cause a reaction to
change IP3 and DAG formation.
The nice thing about IP3 is it’s
going to cause calcium release
from the endoplasmic reticulum.
DAG causes protein phosphorylation
of a specially protein kinase C
Both calcium and protein kinase C
will stimulate the hydrogen ion pump.
Now, that’s mechanism number one.
The next mechanism is gastrin.
Gastrin works through
and also stimulates or
activates phospholipase C,
and therefore, releases calcium and
phosphorolyzes protein kinase C
to increase hydrogen
So both acetylcholine and gastrin
work through the same mechanism.
Histamine, which is also a stimulator
of hydrogen ion production,
works through an
It works through histamine
2 receptors that are --
that stimulate another
G-coupled protein receptor,
that stimulates G-alpha S
and adenylate cyclase.
This increases the amount of cyclic AMP and
phosphorylization of protein kinase A,
which then stimulates
the hydrogen ion pump.
You may ask, “Well, here’s
two different pathways
acting potentially at the same
time, why is that important?”
Well, interestingly, they
can potentiate each other.
Meaning that if you stimulate both pathways
at the same time,
you get more acid production than
if you would have stimulated one
or the other by themselves.
So this kind of synergistic
release or combining of pathways
allow for even more
So now that we went through our
three big acid producers,
there has to be something
that shuts this system off.
Well, remember, from D cells,
somatostatin is released,
and that works through
and that shuts off
So there’s less phosphorylation
of protein kinase A,
and therefore, less
hydrogen ion pumping.
The last mechanism that kind of shut
things down are prostaglandins.
Prostaglandins can also work
through adenylate cyclase
to decrease the amount of cyclic
AMP and protein kinase A,
and therefore, decrease the overall
ability to engage the hydrogen ion pump.
Now, why is this important
pharmacologically for disorders
that have too much acid
production in the stomach?
Well, it’s a nice thing because you can
target certain aspects of this system.
Maybe you want to target
just one of the pathways,
such as targeting the
If you block that histamine pathway,
you will still get some acid
production from acetylcholine,
but you won’t get the
potentiation of acid production.
So that’s one way you can have someone
who has too much acid production
to decrease it a little bit
but not shut it off totally.
Another way you could
target this system is
giving a proton pump
inhibitor type of a drug,
and that could decrease the amount of
hydrogen ion pumping into the stomach,
and that’s another way that you can
kind of temper this particular system.