H ow ste ro id h o rm o n es w ork • Figure 9.2
Steroid hormones go directly into the target
cell and stimulate the making of new proteins,
which alter the cell's activity. The entire pro-
cess of steroid hormone action takes a signifi-
cant amount of time (minutes to hours).
Free hormone
Blood capillary
Transport
protein
The activated hormone-receptor
complex binds to specific genes
of the target cell’s DNA and
causes these genes to
be expressed.
Messenger RNA (mRNA)
from the hormone-activated
genes leaves the nucleus
and starts making new
proteins.
Lipid-soluble steroid hormone enters the
Q -------target cell directly through the cell membrane
and binds to a specific receptor (activation).
Cytosol
Target cell
Some organs—such as the hypothalamus, pancreas,
thymus, ovaries, testes, heart, liver, and kidneys—
contain endocrine cells.
Some tissues, such as adipose tissue, secrete hormones.
The endocrine system also controls long-term changes,
such as growth and the onset of puberty.
The components of the endocrine system function in
this general manner. In direct response to a change in some
physiological parameter or by stimulation from the nervous
system, an endocrine cell secretes a chemical signal called
a hormone into the bloodstream. Although the hormone
circulates throughout the body, it acts only on specific cells,
called target cells. The target cells contain the appropriate
receptor for a secreted hormone, called a hormone receptor;
imagine the hormone receptor as a lock that can be opened
only by a specific key (the hormone). The hormone-recep-
tor complex elicits some action within the target cells that
restores the changed parameter to normal levels. Once nor-
mal levels are restored, the secretion of the endocrine cells
is usually stopped via negative feedback (see Chapter 1). In
this way, the endocrine system helps maintain homeostasis.
Hormones come in two major types, steroid and non-
steroid. Steroid hormones dissolve in fats or lipids, while
nonsteroid hormones dissolve in water. Steroid hormones
are made from cholesterol, while nonsteroid hormones are
usually made from small amino acids, peptides, or large
proteins. Steroid hormones can go directly through the tar-
get cell membrane, which is made of lipids, while nonster-
oid hormones usually must bind to a hormone receptor on
the surface of the cell membrane to enter the cell. These
two modes of entry lead to entirely different mechanisms of
action. Let’s look at steroid hormones first.
Steroid Hormones Go Directly
into the Target Cell
Steroid hormones pass directly through the cell membrane
and stimulate specific genes to make new proteins, which
alter the activity of the target cell (Figure 9.2). Because
new proteins must be made, steroid hormones take longer
to act than their nonsteroid counterparts. However, the
effects of steroid hormones are generally longer lasting.
Different endocrine cells produce various steroid hor-
mones, which act on a variety of targets. For example, the
ovaries, testes, and adrenal glands produce androgens and
estrogens, which act on many cells to produce male and
female characteristics. The adrenal cortex secretes min-
eralocorticoids, which act on the kidneys to increase sodi-
um and water reabsorption and potassium excretion. The
kidneys produce calcitriol, the active form of vitamin D,
which acts on the intestinal lining to promote absorption
of calcium and phosphate.
Now, let’s look at how nonsteroid hormones work.
Hormones Act on Target Cells 255
PROCESS DIAGRAM
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