Cell Communication and Endocrine overview
I.
General Info
i. What is cell communication?
each other using chemicals.
Cells talk to
If the cells
are part of a multicellular organism, cells
talk to each other to alter and coordinate
each other’s activity and keep the organism
alive.
This is what we will focus on!
B. In a multicellular organism, cells communicate
with chemicals in 3 general ways:
i. Synapses- a neuron spits chemicals
(neurotransmitters) onto a cell, either
another neuron or another type of cell.
For
example, your muscle cells contract because
a neuron told them to.
ii. Paracrine communication- cells in a local
area can talk to each other.
Chemicals used
locally are called a variety of terms, we
will use “paracrine factors.”
iii. Endocrine communication- cells in one area
of the body can talk to cells far away.
Chemicals that are used for this type of
long-distance communication are called
hormones.
Hormones are released into the
blood, and any cell of the body that has a
receptor for a given hormone will respond.
These are the chemicals we will draw our
examples from.
-from here on out I will refer to chemical signals, whether
they are neurotransmitters, paracrine factors, or hormones,
as “signals” unless I am using a specific example.1
C. A cell can only “hear” a chemical signal from
another cell if it has a receptor for that
signal.
A receptor is a protein that can
specifically bind to the chemical signal.
Receptors may be embedded into the membrane and
stick into the exterior space or may be in the
cytosol.
Generally, water-soluble signals bind
to receptors on the EXTERIOR surface of the cell.
These signals do NOT enter the cell.
Lipid-
soluble signals generally diffuse into the cell
and bind a receptor in the cytosol.
II.
The action of chemical signals: how they send a
message and how receiving cells respond to that
message
A. What signals cause cells to do:
i. Turn genes on: remember that genes generally
code for proteins, so "turning on" a gene
will cause a protein (such as an enzyme or
channel) to be made.
ii. Activate
or inactivate an existing enzyme
or other protein in the cell
iii. Cause a gated channel to open
- Overall, the effects of these actions can:
1. Alter membrane permeability or solute
absorption by opening or closing
channels (#ii, iii), or causing them to
be made (#i) (for example, calcitriol
causes the production of Ca2+ channels
in cells lining the small intestine, so
that dietary Ca2+ may be absorbed;
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insulin causes the activation of
glucose channels so that cells may take
in glucose from the blood)
2. Induce secretory activity; for example,
by the production or activation of
enzymes involved with making products
for secretion
3. Stimulate cell division; for example,
by the production or activation of
enzymes involved with replication of
organelles or DNA
4. Alter metabolic activity; for example,
by the production or activation of
enzymes involved with ATP production
5. Alter the rate of transcription or
translation, ie, make particular
proteins/enzymes faster or slower.
B. How Water-Soluble Signals work
i. There are 3 general types of receptors that
can bind to water-soluble signals:
1. Ion channel receptors: these are
actually gated channels.
binds to them, they open.
When a signal
For example,
when a neuron tells a muscle cell to
contract, it spits the neurotransmitter
acetylcholine onto the muscle cell.
Acetylcholine binds to a receptor on
the muscle cell.
This receptor is
actually a Na+ channel that has been
closed.
It opens when Acetylcholine
binds, and Na+ rushes in.
That event
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will eventually lead to a muscle
contraction.
2. Second-messenger linked receptors: when
a signal binds to a second-messenger
linked receptor, the cell will
experience a set of changes that will
cause it to change in some way (ie, 1-5
above).
Following is an example of how
second-messenger linked receptors can
work:
ii. How a cell responds when water-soluble
signals bind to second-messenger linked
receptors: signal transduction
1. When a signal binds one of these
receptors, the receptor will change
shape or move on the INTERIOR part of
the cell.
This event will set forth a
series of chemical reactions that will
eventually lead to the activation or
inactivation of specific proteins.
2. Binding of signal to the receptors
causes a cascade of chemical events
within the cell. In many pathways, the
ultimate goal is to activate protein
kinases, enzymes that phosphorylate
other chemicals (for example, kinases
could phosphorylate enzymes that drive
the production of thyroid hormones in
thyroid cells). Why does this matter?
Many proteins are activated or
inactivated by the addition or removal
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of phosphate; so by adding phosphate to
proteins, kinases can turn them "on" or
"off."
3. The substance that activates the
kinases is called a second messenger.
Two common second messengers that are
used in signal transduction pathways
are cyclicAMP and Ca2+.
Here’s how the G-protein linked receptor/cAMP system works
(you are NOT responsible for the following information):
First, be aware that there are 3 types of membrane
proteins you need to know about: 1) the hormone
receptor, which spans the membrane from the outside
surface to the interior 2) the G-protein, which is
bound to the receptor on the interior portion of the
membrane. The G-protein has a molecule of GDP
attached to it. 3) Adenylate Cyclase, an enzyme that
spans the membrane. Take a minute to draw a portion of
the membrane with these 3 proteins. As you read each
of the following steps, redraw the pictures, showing
what's going on at each step.
Let's use a liver cell responding to epinephrine. One
of the things that liver cells do in response to E is
break down glycogen to release glucose to the blood
(what other hormone has this effect?). When
epinephrine binds to its receptor, the receptor
changes shape and causes the G-protein to eject its
GDP. When GDP is ejected, the molecule GTP takes its
place. The G-protein is now activated and released
from the receptor.
The G-protein moves along the interior part of the
cell membrane once it's released. It will bump into
and bind an Adenylate Cyclase.
Adenylate Cyclase, upon being bumped and bound, will
drive the reaction:
ATP --> cAMP. That is, it will convert ATP to cAMP.
Again, cAMP is the 2nd messenger, whose job it is to
activate kinases. So, cAMP cruises around the cytosol,
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activating kinases. Incidentally, there are enzymes
ready to degrade cAMP almost as soon as it's made.
Now, we have a bunch of kinases running around really
getting the job done: activating or inactivating
proteins that will have the effect desired by the
hormone. For example, in liver cells, one of the
enzymes that gets activated by these kinases is
responsible for chopping glucose units off of glycogen
chains.
-you are now responsible for the following information
againC. How lipid-soluble signals work: these are a
little more straightforward. Lipid-soluble
signals, such as steroid hormones, diffuse into
the cell freely. They will bind with a receptor
in the cytosol (or sometimes in the nucleus).
Together, the signal chemical and the receptor
will go into the nucleus and bind DNA adjacent to
specific genes. That binding will cause genes to
be activated, and the cell will build more of the
target protein. For example, in muscle cells,
testosterone activates genes that code for
contractile proteins (actin and myosin).
III. The endocrine system: here, the “signal” chemicals
are called hormones
A. Overview of endocrine glands
B. Why hormones are released and what they do:
i. Hormones are released in response to
changing conditions of the body. They cause
cells to alter their activity, and maintain
homeostasis. Some examples:
-when blood Ca2+ levels drop, parathyroid
hormone is released. This causes bone cells
to dissolve Ca2+ and release it to the
blood. Blood Ca2+ levels are restored.
-after fasting for several hours, blood
glucose levels drop. This causes the
release of glucagon. Glucagon “tells” liver
cells to release their stored glucose.
Soon, blood glucose levels are restored.
ii. Some hormones are released by endocrine
glands that monitor a specific body
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condition, and release their hormone when
that condition changes.
-Both examples from part i above illustrate
this. Ca2+ is monitored by the parathyroid
glands, and they release PTH. Glucose is
monitored by the pancreas, and it releases
glucagon.
iii. Some hormones are released by endocrine
glands that must receive a message from the
hypothalamus/pituitary gland. In this case,
it is the hypothalamus that is monitoring
the body condition.
-an example of a hormone whose release is
controlled by the hypothalamus/pituitary is
thyroid hormone. The hypothalamus monitors
several aspects of the blood to determine
how much thyroid hormone should be released.
One aspect it monitors is blood temperature.
When blood temperature drops, the
hypothalamus sends a chemical message to the
pituitary gland. In response, the pituitary
gland releases another chemical into the
blood. When that chemical reaches the
thyroid gland, the thyroid gland will
release thyroid hormone… then FINALLY the
body can respond: cells will increase their
metabolic activity, and the body will warm
up.
-Hormones released by the hypothalamus to
the pituitary gland are called releasing
hormones. Those released by the pituitary
gland are called stimulating hormones. So,
for our above example, the hypothalamus will
send TRH to the pituitary. In response, the
pituitary releases TSH to the blood. In
response, the thyroid gland releases thyroid
hormone to the blood, and most cells of the
body respond.
C. Chemical structure of hormones- Hormones fall
under 3 broad umbrella categories based on
structure:
i. Amino acid derivatives- most are watersoluble; what does that mean about where
they bind receptors? Ex, thyroid hormone
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(this is actually the one exception: it’s
lipid soluble!)
ii. Peptides- water-soluble, ex. PTH
iii. Sterols- lipid-soluble; what does that mean
about where they bind receptors? Ex,
estrogen
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