Welcome to the medical zone! In this video 5 hormones will be discussed
that play important roles in the regulation of digestion in the Gastrointestinal tract.
First we will shortly discuss some basic principles. With over thirty different hormones
identified as being produced in the gastrointestinal tract, the gut has
been described as the largest endocrine organ in the body . For the purpose of a correct
definition: Hormones are chemical messengers secreted into blood that modify the physiology
of target cells. So hormones are not secreted into the lumen of the gastrointestinal tract,
but into the circulatory system, which will transport them to their destination. The definition
of a target cell for a particular hormone is a cell that has receptors for that hormone
and can thus respond to it. The endocrine cells within the gastrointestinal
tract are referred to collectively as the ‘enteric endocrine system’. Some of the best-studied
enteric hormones are: Gastrin, Cholecystokinin, Secretin, Motilin and Gastric inhibitory polypeptide,
which is also known as glucose dependent insulinotropic peptide. These are the 5 hormones that will
be discussed in this video. Another thing you must realize is the following:
in contrast to endocrine glands like the anterior pituitary gland, in which essentially all
cells produce hormones, the enteric endocrine system is diffuse. single hormone-secreting
cells are scattered among other types of cells in the mucosa of the stomach and small intestine.
Now you know the basics, let’s have a look at the various roles of the beforementioned
main hormones. Okay, so on this picture you can see a basic
overview of the different parts of the gut that we will be talking about: the esophagus,
the stomach, duodenum, jejunum, liver, gall bladder and the pancreas.
So, the first hormone we will discuss is gastrin. The primary action of gastrin is: stimulation
of gastric acid secretion. Gastrin is secreted by G cells, these are
located in the antrum of the stomach (and actually some in the duodenum as well). It
is secreted in response to stimuli associated with ingestion of a meal, such as distention
of the stomach, the products of proteins, and gastrin releasing peptide, which is released
by nerves of the gastric mucosa during vagal stimulation. So, the primary action of gastrin
is: stimulation of gastric acid secretion. It binds gastric receptors found predominantly
on parietal and enterochromaffin-like or ECL cells. Because gastrin and CCK have structurally
many things in common, the gastrin receptor is also one of the receptors that can bind
cholecystokinin, and is known as the CCK-B receptor. It is a member of the G protein-coupled
receptor family. Stimulation of ECL cells by gastrin leads to histamine release, and
histamine binding to H2 receptors on parietal cells is necessary for full-blown acid secretion.
Thus, fully-stimulated acid secretion by parietal cells occurs when gastrin binds to the gastrin
receptors on parietal cells, along with histamine which is derived from ECL cells.
Let’s get back to gastrin. Apart from the described function, gastrin also hase some
minor additional functions like stimulating parietal cell maturation and fundal growth,
pepsinogen secretion by chief cells, stimulating pancreatic secretions and gallbladder emptying. Now let’s move on to the second hormone, called
cholecystokinin. Cholecystokinin is the principle stimulus
for delivery of bile and pancreatic enzymes into the small intestine, hereby stimulating
the digestion of fat and proteins. As mentioned previously, cholecystokinin and
gastrin are highly similar peptides. Cholecystokinin (or CCK) is secreted by specialized gut endocrine
cells called I cells in the mucosa of the duodenum and jejunum mainly in response to
digestive products of fat, fatty acids, and monoglycerides in the intestinal contents.
CCK mediates digestion in the small intestine various functions. It stimulates the acinar
cells of the pancreas to release digestive enzymes and stimulates the secretion of a
juice rich in pancreatic digestive enzymes. Together these enzymes catalyze the digestion
of fat, protein, and carbohydrates. CCK also causes the increased production of hepatic
bile, and stimulates the contraction of the gall bladder and the relaxation of the Sphincter
of Oddi, resulting in the delivery of bile into the duodenal part of the small intestine,
where the bile in turn plays important roles in emulsifying fatty substances, allowing
them to be digested and absorbed. Cholecystokinin also inhibits stomach contraction moderately.
Therefore, at the same time that this hormone causes emptying of the gallbladder and pancreatic
secretion, it also slows the emptying of food from the stomach to give adequate time for
digestion of the fats in the upper intestinal tract. Cholecystokinin is also produced by
neurons in the enteric nervous system, and is widely and abundantly distributed in the
brain. A nice fact, worth sharing is that the name cholecystokinin has a nice origin.
It means to move the bile sac, so the gallbladder. Also CCK secretion is regulated by a feedback
mechanism: digestion and absorption of the very molecules that stimulate cholecystokinin
secretion, thus this will in turn decrease CCK secretion. An interesting fact to know
is that when given high enough doses, gastrin and CCK can produce all the effects of the
other. The third hormone is Secretin.
Secretin functions as a type of fireman: in response to acid, it stimulates the pancreas
and bile ducts to release a flood of bicarbonate base, which neutralizes the acid, and the
secretion of secretin is turned off. Secretin was the first gastrointestal hormone
discovered and is secreted by specialized enteroendocrine cells called S cells in the
mucosa of the duodenum in response to acidic gastric juice emptying into the duodenum from
the pylorus of the stomach. Secretin has a mild effect on motility of the gastrointestinal
tract and acts to promote pancreatic secretion of bicarbonate which in turn helps to neutralize
the acid in the small intestine. This will enhance the action of intestinal digestive
enzymes. It has a direct effect on the pancreatic acinar cells as well as the duct cells. There
is also a vagal-mediated secretory response. Secretin results in a bicarbonate-rich pancreatic
secretion. Apart from this main stimulation of bicarbonate
release by the pancreas, a similar, but quantitatively less important response to secretin is elicited
by bile duct cells, resulting in additional bicarbonate being dumped into the small gut.
Also it stimulates pepsinogen secretion from the stomach (by chief cells). As acid is neutralized
by bicarbonate, the intestinal p H rises toward neutrality,
secretin is sometimes also refered to as nature s antacid. Number four is called motilin.
Motilin is released cyclially and stimulates waves of gastrointestinal motility called
migrating motor complexes, which move through the stomach and small intestine every 90 minutes
in a fasted person. Motilin is secreted by endocrine M cells in
the duodenum and jejunum during fasting, and the only known function of this hormone is
to increase gastrointestinal motility. These M cells are by the way not the same as M cells
in Peyer s patches, of which you might have heard before. So, motilin is released cyclially
and stimulates waves of gastrointestinal motility called migrating motor complexes that move
through the stomach and small intestine every 90 minutes in a fasted person. Motilin secretion
is inhibited after ingestion of food by mechanisms that are not fully understood; at low p H
it inhibits gastric motor activity, whereas at high p H it has a stimulatory effect. These
bursts of motilin secretion are like “housekeeping contractions”, which sweep the stomach and
small intestine clear of undigested material. Last but not least, we will have a look at
the hormone “gastric Inhibitory Peptide”. It stimulates insulin secretion in a glucose
dependent manner and inhibits gastric acid secretion and motor activity of the stomach.
It is a hormone that is produced by enteroendocrine K -cells which are mainly located at the duodenum
and proximal jejunum. It is released into the circulation in response to fatty acids
and amino acids but to a lesser extent in response to carbohydrate. A nice thing to
remember: it s the only hormone released by all three major types of food.
The only action that is known to be physiologically significant is the stimulation of insulin
release in the presence of glucose. This hormone is responsible for the finding that an oral
glucose load is cleared from the blood more rapidly than an intravenous glucose load of
the same magnitude. For this action, it has also been referred to as glucose-dependent
insulinotropic peptide, it stimulates insulin secretion in a glucose dependent manner. And
as its name implies, “gastric inhibitory peptide” inhibits gastric acid secretion. Also It has
a mild effect in decreasing motor activity of the stomach and therefore slows emptying
of gastric contents into the duodenum when the upper small intestine is already overloaded
with food products. This is the end of this video.
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