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The endocrine system is a very important and very involved system within our
bodies.
It allows our body to communicate over long distances
and is a major player in regulating a stable environment
or in other words preserving homeostasis. This system employs the use of
hormones
which are chemicals secreted from glands and enter the bloodstream
where they circulate until exerting an effect on a downstream target cell.
We're going to be specifically talking about the two types of mechanisms
in which hormones exert their effect on target cells. But to start off
here's a brief overview what hormones are and how they work.
There are three basic group of hormones.
1. Amino acid derivatives. 2.
Peptide hormones and 3.
Lipid derivatives or steroid hormones. For amino acid derivatives,
these are small molecules and they come from amino acids like tyrosine and
tryptophan.
One hormone derived from tryptophan
is melatonin. A hormone involved in our sleep-wake cycles.
You can get tryptophan from foods like turkey, chocolate
or milk. Peptide hormones are made up of
a bunch of amino acids. Some examples would be
thyroid-stimulating hormone, oxytocin or prolactin.
Lipid derivatives or steroid hormones are considered lipophilic
meaning they have an affinity for lipid structures which we'll take a look at
shortly.
They circulate in the blood bound to carrier proteins
so they usually last in the circulation longer than other types of hormones.
There are two main mechanisms
for which a hormone can affect a target cell. The first
is by and non steroid action. This mechanism
is employed by an amino acid hormone or a peptide hormone.
These hormones cannot freely cross a membrane on a target cell
so it first acts by binding a cell surface receptor.
There is then an intracellular signaling cascade that occurs in order
for the desired effect to take place.
There are proteins on the intracellular side of the target cell
that are associated with the receptor. Most often
this involves what's called a G-protein. This G protein is found next to an
enzyme that converts
ATP to a molecule called cyclic AMP
or cAMP. This is something that makes this mechanism
unique. Cyclic AMP is called a second messenger
because it signals a cascade of events that eventually
change the enzymatic activity in a cell to cause the target cell response.
Examples of hormones that use the cyclic AMP
second messenger system include ACTH,
calcitonin, epinephrine,
glucagon, parathyroid hormone
and ADH. The second mechanism is the steroid action.
Because lipid derivative or steroid hormones are lipophilic
they can freely move through the membrane and into the cell.
From there they can either bind to a receptor within the cell
or they can move freely into the nucleus where the hormone
or hormone complex causes a change in gene activity.
This then increases transcription
and mRNA production which then leads to an increase in protein production
within the cell. Examples of hormones that exert effects via steroid hormone
mechanisms
are testosterone, estrogen,
progesterone, aldosterone,
and calcitriol.