The Endocrine System
same tissue) but aﬀect cell types other than those releasing the
paracrine chemicals. For example, somatostatin released by one
population of pancreatic cells inhibits the release of insulin by a
diﬀerent population of pancreatic cells.
Check Your Understanding
For each of the following statements, indicate whether it
applies more to the endocrine system or the nervous system:
rapid; discrete responses; controls growth and development;
Which two endocrine glands are found in the neck?
What is the difference between a hormone and a paracrine?
For answers, see Appendix H.
Describe how hormones are classiﬁed chemically.
Describe the two major mechanisms by which hormones
bring about their effects on their target tissues.
Explain how hormone release is regulated.
List three kinds of interaction of different hormones acting
on the same target cell.
The Chemistry of Hormones
Although a large variety of hormones are produced, nearly all
of them can be classiﬁed chemically as either amino acid based
Amino acid based:
Most hormones are amino acid based.
Molecular size varies widely in this group—from simple
amino acid derivatives (which include thyroxine constructed
from the amino acid tyrosine and amines), to peptides (short
chains of amino acids), to proteins (long polymers of amino
Steroid hormones are synthesized from cholesterol.
Of the hormones produced by the major endocrine organs,
only gonadal and adrenocortical hormones are steroids.
Some researchers add a third class,
noyds), which include
all cell membranes release these biologically active lipids (made
from arachidonic acid). Leukotrienes are signaling chemicals
that mediate inﬂammation and some allergic reactions. Pros-
taglandins have multiple targets and eﬀects, ranging from
raising blood pressure and increasing the expulsive uterine
contractions of birth to enhancing blood clotting, pain, and
Because the eﬀects of eicosanoids are typically highly lo-
calized, aﬀecting only nearby cells, they generally act as para-
crines and autocrines and do not ﬁt the deﬁnition of true
hormones, which inﬂuence distant targets. For this reason,
we will not consider this class of hormonelike chemicals here.
Instead, we note their important eﬀects in later chapters as
Mechanisms of Hormone Action
All major hormones circulate to virtually all tissues, but a hor-
mone inﬂuences the activity of only those tissue cells that have
receptors for it. Tese cells are its
. Hormones bring
about their characteristic eﬀects by
target cell activity.
In other words, they increase or decrease the rates of normal
Te precise response depends on the target cell type. For ex-
ample, when the hormone epinephrine binds to certain smooth
muscle cells in blood vessel walls, it stimulates them to con-
tract. Epinephrine binding to cells other than muscle cells may
have a totally diﬀerent eﬀect, but it does not cause those cells to
A hormone typically produces one or more of the following
Alters plasma membrane permeability or membrane poten-
tial, or both, by opening or closing ion channels
Stimulates synthesis of enzymes and other proteins within
Activates or deactivates enzymes
Induces secretory activity
How does a hormone communicate with its target cell? In
other words, how is hormone receptor binding harnessed to
the intracellular machinery needed for hormone action? Te
answer depends on the chemical nature of the hormone and
the cellular location of the receptor. In general, hormones act at
receptors in one of two ways.
(all amino acid–based hormones
except thyroid hormone) act on
receptors in the plasma
. Tese receptors are usually coupled via regu-
latory molecules called G proteins to one or more intra-
cellular second messengers which mediate the target cell’s
(steroid and thyroid hormones) act
receptors inside the cell
, which directly activate genes.
Tis will be easy for you to remember if you think about
why the hormones must bind where they do. Receptors for
water-soluble hormones must be in the plasma membrane
since these hormones
enter the cell, and receptors for
lipid-soluble steroid and thyroid hormones are inside the cell
because these hormones
enter the cell. Of course, things
are not quite that clear-cut—steroid hormones can exert some
of their more immediate eﬀects via plasma membrane recep-
tors, and the second messengers of some water-soluble hor-
mones can turn genes on.
Plasma Membrane Receptors
and Second-Messenger Systems
With the exception of thyroid hormone, amino acid–based
hormones exert their signaling eﬀects through intracellular
generated when a hormone binds to a re-
ceptor in the plasma membrane. You are already familiar with