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Technical Discussion What exactly are
hormones and how are they different from "non-hormones"?
Hormones are chemical messengers secreted into blood or
extracellular fluid by one cell that affect the functioning of other
cells.
Most hormones circulate in blood, coming into
contact with essentially all cells. However, a given hormone usually
affects only a limited number of cells, which are called target
cells. A target cell responds to a hormone because it bears
receptors for the hormone.
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Lay Interpretation
What are hormones and what makes
them different from chemical compounds that are not hormones?
Hormones are excreted by certain
tissues into the blood stream or space between the cells
(extra-cellular). Hormones only react directly with cells that
have specific receptor sites for a particular hormone. |
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In other words, a particular cell is a target cell
for a hormone if it contains functional receptors for that hormone,
and cells which do not have such a receptor cannot be influenced
directly by that hormone. Reception of a radio broadcast provides a
good analogy.
Everyone within range of a transmitter for
particular station is exposed to that signal (even if they don't
contribute!). However, in order to be a target for a specific
station and thus
influenced directly by their broadcasts, you have to have a receiver
tuned to that frequency.

Hormone receptors are found either exposed on the surface of the
cell or within the cell, depending on the type of hormone. In very
basic terms, binding of hormone to receptor triggers a cascade of
reactions within the cell that affects function.
Additional details
about receptor structure and function are provided in the section on
hormone mechanism of action.
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Without a specific receptor site, cells will not respond directly to
any hormone. The radio station analogy of the previous page or to
the left works well when looking at the spectrum of hormones present
in the blood at every second.
Some cells can have receptors for two or more hormones, allowing
excitation and modulation of actions.
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A traditional part of the definition of hormones
described them as being secreted into blood and affecting cells at
distant sites. However, many of the hormones known to act in that
manner have been shown to also affect neighbouring cells or even have
effects on the same cells that secreted the hormone.
Nonetheless, it
is useful to be able to describe how the signal is distributed for a
particular hormonal pathway, and three actions are defined:
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Endocrine action: the hormone is
distributed in blood and binds to distant target cells.
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Paracrine action: the hormone acts
locally by diffusing from its source to target cells in the
neighbourhood.
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Autocrine action: the hormone acts on the
same cell that produced it.
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the actions of hormones are divided into three categories according
to how they get to their targeted cells. Endocrine action happens
when cells respond to hormones that have been delivered to the via
the blood stream. hGH distribution is a relevant example of
this action.
Paracrine action happens to cells in the immediate vicinity of
the cell producing the hormone. It does not depend on blood to
arrive at its targeted cells.
Autocrine action is where the cell producing the hormone also
responds to that hormone. |
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Two important terms are used to refer to molecules that bind to the
hormone-binding sites of receptors:
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Agonists are molecules that bind the
receptor and induce all the post-receptor events that lead to a
biologic effect. In other words, they act like the "normal"
hormone, although perhaps more or less potently. Natural
hormones are themselves agonists and, in many cases, more than
one distinct hormone binds to the same receptor. For a given
receptor, different agonists can have dramatically different
potencies.
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Antagonists are molecules that bind the
receptor and block binding of the agonist, but fail to trigger
intracellular signalling events. Antagonists are like certain
types of bureaucrats - they don't themselves perform useful
work, but block the activities of those that do have the
capacity to contribute. Hormone antagonists are widely used as
drugs.
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This section is not too important in this side, but describes
agonist and antagonist actions on receptor sites of a cell. If the
hormones are the generals of our army, agonists are the staff who
can initiate similar actions in a limited way; although they are
sometimes more potent than the general!
Antagonists are like a bureaucrat who ties up resources and
blocks action. Endocrine antagonists block receptor sites,
preventing hormonal interaction with the target cell.
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Finally, a comment on the names given hormones and
what some have called the tyranny of terminology. Hormones
are inevitably named shortly after their discovery, when
understanding is necessarily rudimentary. They are often named for
the first physiologic effect observed or for their major site of
synthesis.
As knowledge and understanding of the hormone grow,
the original name often appears inappropriate or too restrictive,
but it has become entrenched in the literature and is rarely
changed. In other situations, a single hormone will be referred to
by more than one name.
The problem is that the names given to hormones
often end up being either confusing or misleading. The solution is
to view names as identifiers rather than strict guidelines to source
or function. |
Don't be fooled into thinking that just because a hormone is called
something that sounds like a function that that action is all it
does. Each hormone has been named shortly after discovery and
definitely before it is fully understood. This means that some
hormones have names that do not reflect accurately the roles they
play in the body. |
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The technical information on these pages are the work of
Professor Bowen, Colorado State University and are reproduced
without endorsement of any kind. The "lay" interpretations are
the work of this site and do not necessarily reflect Professor
Bowen's opinions.