Human Physiology – Regulation of Digestion: Mechanisms for HCl and Pepsinogen Synthesis & Secretion

Human Physiology – Regulation of Digestion: Mechanisms for HCl and Pepsinogen Synthesis & Secretion

February 24, 2020 0 By Ewald Bahringer


>>Dr. Ketchum: All right, so next what we
want to do is turn to the mechanism for hydrochloric acid synthesis and secretion. We’re dealing
with the lumen of the gastric gland. So let’s get yourself oriented here, and then here
are the parietal cells. So we know that the parietal cells of the gastric pits, okay,
or the gastric glands are going to produce hydrochloric acid. But the question is well,
how? How does this happen? So if you recall, we’ve got carbon dioxide in your parietal
cell that interacts with water. And this—this reaction is catalyzed by using carbonic anhydrase,
which you are very familiar with now. And now we form carbonic acid, H2CO3, carbonic
acid. We know that H2CO3 will readily dissociate. When it readily dissociates, you get your
hydrogen ions and your bicarbonate ions. Remember, bicarbonate is HCO3. Let’s focus on the
hydrogen ions first. Remember what the goal is; we want to produce HCl in the lumen. The
hydrogen ions, if you notice here, are at a low concentration inside the cell; they’re
at a high concentration in the lumen. Then we have potassium. So potassium is at a low
concentration in the lumen, but potassium is at a high concentration in the cytoplasm.
So if you look at the purple structure, that’s a carrier protein, and it requires ATP. So
ask yourself, “What type of membrane transport is that?” So if you’d like to, to pause
the video to think about it for a moment, that would be a good idea. All right, so here’s
your hint: The carrier protein is involved, we’re hydrolyzing ATP, we’re pumping hydrogen
ions from a low concentration to high, and we’re pumping potassium from a low concentration
to high. So that makes this a type of primary active transport. So this is yet one other
type of primary active transport that you’ve learned about this semester. So now once we
get those, those potassium ions at a high concentration in the cytoplasm, then they’re going to
move back out through a channel. So this is going to be facilitated diffusion. So they
move back into the lumen through a channel, and that’s facilitated diffusion. Okay,
so now we have the hydrogen ions in the lumen, but we still need chloride ions to get there.
So once we make bicarbonate in the cytoplasm of the cell, we can transport bicarbonate
out of the cell and into the blood. Meanwhile, we can bring chloride ions into the cytoplasm,
and we’re using a carrier protein. There’s no ATP being indicated here, but we are moving
bicarbonate from a high concentration inside the cell toward a lower concentration out
into the blood. And we’re moving chloride ions from a low concentration toward a higher
concentration. So the type of transport occurring here is what? Takes a carrier protein, we’re
moving two solutes—one along a gradient, the other one against—so this must be secondary
active transport. So this specifically will be called chloride-bicarbonate countertransport.
Now, once we have chloride ions at a high concentration in the cytoplasm of the cell,
then we can transport them toward a lower concentration into the cytoplasm. So we’re
moving them through a channel once again, and this is facilitated diffusion. So now
we have hydrochloric acid—we have HCl in the lumen, okay? And that’s we’re we
wanted to get. We wanted to figure out how—what’s the mechanism for hydrochloric acid synthesis
and secretion? So then we should ask ourselves, “Well what stimulates HCl secretion?”
There’s three things. There’s the parasympathetic nervous system, there’s gastrin, which is
a hormone, and there’s histamine, which is a paracrine signal. So histamine acts directly
on parietal cells. Question for you—since histamine is a paracrine signaler, does histamine
travel through the blood? Okay, you should’ve answered no. Paracrine messengers do not go
through the blood; they only go through the interstitial fluid. We’re going to discuss
more on the parasympathetic nervous system in gastrin here in just a moment. Again, remember
histamine direct—acts directly on parietal cells to cause the synthesis and secretion of hydrochloric
acid. Okay, but what about pepsinogen, right? We’ve got to synthesize and secrete pepsinogen
as well. Well we know that the chief cells do that. And that happens right alongside
the synthesis and secretion of the hydrochloric acid. So the parasympathetic nervous system
is involved, gastrin, the hormone, is involved, and then histamine. So histamine also acts
directly on chief cells, because it’s the chief cells that synthesize and secrete pepsinogen.