Cellular Mechanism of Hormone Action - Medical Video Lecture - MBBS

After coursing throughout the
blood vessels of our bodies,
a hormone eventually
meets a receptor
that was created specifically
for it by the target cells
that it was sent to stimulate.
And the way that hormone
interacts with the receptor
happens in one of two
very characteristic ways.
And I want to show you
today how that happens.
And so the first major
mechanism of hormone action
at the cell that I'm
going to start with
is by secondary messengers.
And I'm going to start
with secondary messengers
because, historically, they're
a little bit more confusing.
But essentially what's
happening is a hormone
is binding to a
receptor on a cell.
So let me draw a cell
and its receptor.
And I'll draw a
hormone binding to it.
But the process of that binding,
instead of just stimulating
the effect, it really sets
off this chain reaction
that leads to
secondary messengers
inside the cell being released.
So let me draw those.
And you've got these secondary
messengers being released,
and these are actually
what's stimulating
whatever effect is
desired, whether that
may be insulin being released
or glucose being taken up
inside the cell, or any of
the other countless things
taking place in our bodies that
are controlled by hormones.
And so in order to give you a
visual for how this might take
place in the cell, I've
pre-drawn some pictures
in the key.
So let me pull those in here.
And I want you to be sure
that these drawings are not
to scale in any way.
But really, this is the
best I can at least do
in explaining the process,
because all of this
is happening at
the atomic level,
and atoms are really tiny.
But anyway, what we have in
this picture is a receptor.
And I've drawn that
in pink, and it's
located inside
the cell membrane.
So this is the phospholipid
bilayer of the cell membrane,
and we've got the inside of
the cell, and the outside.
And then also in
the cell membrane,
we've got a G protein, and
I've drawn that in green.
It's called a G
protein because it
binds to molecules that
include the nucleotide guanine.
And that's the same
G from the DNA bases
that you might be a little
bit more familiar with.
But it's currently
bound to a molecule
called guanine diphosphate.
And then we'll see
how that changes.
But we also have this
adenylate cyclase enzyme
that's in the cell membrane.
And remember that an
enzyme speeds up reaction.
So we're going to see how
adenlyate cyclase speeds up
a reaction.
But what starts this
process off is the hormone
is going to bind
to the receptor.
So it's going to look like that.
And you're going to have the
hormone bound to the receptor.
And once that hormone
binds to the receptor,
it's going to change shape.
And that's going to allow it
to interact with the G protein
here.
And so that looks like this.
And what you saw happen was
that as the G protein interacts
with the receptor in
a hormone complex,
it's going to exchange that
GDP that it started with,
that guanine
diphosphate for GTP.
So essentially, it's
exchanging a guanine
bound to two phosphates
for a guanine bound
to three phosphates.
And what happens is that
enables the G protein
to move through
the cell membrane
and interact with
adenylate cyclase.
And so that activates
the adenylate cyclase.
And as an enzyme, that
activated adenylate cyclase
facilitates the
conversion of ATP,
which is the energy currency
of the cell, into cAMP.
And cAMP stands for cyclic
adenosine monophosphate.
So we've taken this ATP,
or adenosine triphosphate,
and created a cyclic
adenosine monophosphate.
And we also have the
additional two phosphates.
But it is this
cAMP molecule that
activates the protein inside
the cell whose effect is really
being targeted by the
hormone in the first place.
And so eventually,
this system resets,
but not before several
adenlyate cyclase enzymes
were activated, resulting in
a lot of cAMP being produced.
And so we call this
signal amplification.
And what I mean by
signal amplification
is that, in theory, one
hormone can bind to a receptor.
And that process can
set off a chain reaction
that leads to a lot of
cAMP being produced.
And so it can mean that
less hormone is required
to ultimately activate the
protein or the effect that's
being desired.
And so secondary
messengers are a means
in which hormones
act on the cell.
But really, if I'm being
honest, this effect
happens differently
for a lot of cells.
And all of the mechanisms of
secondary messenger hormone
action aren't known right now.
And honestly, there are a
lot of secondary messengers
other than cAMP.
But really, the takeaway
is that for the majority
of the hormones in your
body, binding to the cell
surface activates a
series of reactions
that initiate a response
inside the cell.
And so it's very similar to
the use of a phone service
provider.
And because we as
people-- let me draw us--
because we're unable
to communicate directly
with people sometimes, because
of separation or convenience
or lack of efficiency,
we use a phone service
to project our voice to them
through a phone conversation.
So through a phone
conversation, we'll
direct our voice to people that
we want to communicate with.
Or maybe we use the
delivery of a text message.
A text message will transmit
some intended message
that we have for people that
we can't communicate directly
with, for whatever reason.
And so that's very similar to
how secondary messengers assist
hormones that can't communicate
with a receptor directly
inside of the cell.
And so peptide hormones and
catecholamines, both of which
can't cross the cell membrane,
use secondary messengers
to communicate.
And then the other major method
of hormone action on a cell
is as a primary messenger.
Certain hormones, like
steroids and thyroid hormones,
can actually cross
the cell membrane.
And it eliminates this
entire middleman system
that we set up before.
So let me pull in
another cell membrane.
And so the hormone
crosses the cell membrane
and it binds to
a receptor that's
located either in the
cytosol or in the nucleus.
And so we could
imagine a nucleus,
and there might be DNA inside.
But when the hormone binds
to the receptor that's
either in the cytoplasm
or inside the nucleus,
that binding process is going
to directly affect transcription
in the nucleus, or
translation in the cytoplasm,
of the protein that's being
activated by the hormone.
And this process has quite
a few less moving parts
than the secondary
messenger system did.
But again, it stems
back to the fact
that these are steroid
and thyroid hormones that
are typically lipid-based
and are capable of crossing
through the cell
membrane on their own.
And so they don't need all of
that extra machinery set up
for them.
But anyway, these are
both primary messengers
and secondary messengers.
And those are the
two main processes
by which hormones
act on the cell
that they're created to target.