Encyclopedia of Health and Medicine

hormone 135
to those receptors. A receptor is somewhat like an
outlet that has a unique configuration. The hormone
for which the receptor is sensitive matches
that configuration, forming a chemical “lock”
between the hormone molecule and the cell.
Through such binding hormones cause chemical
changes within the cell that may activate enzymes
or alter the cell’s genetic encoding by creating new
proteins (called genetic transcription). Each hormone
has unique receptors. Many cells have
receptors only for certain hormones, eliciting specific
and narrowly focused changes. Only cells in
the TESTES and OVARIES, for example, have receptors
for FOLLICLE-STIMULATING HORMONE (FSH) and
LUTEINIZING HORMONE (LH). Some hormones, such as
GROWTH HORMONE (GH), have receptors in all cells,
in which case the hormone has widespread
actions.
Some hormones stimulate and others inhibit
activity. Most hormonal responses occur in cascades,
with multiple activities resulting from the
hormone’s release. For example, the HYPOTHALAMUS
releases GROWTH HORMONE–RELEASING HORMONE
(GHRH), which stimulates the PITUITARY GLAND to
release GH. Growth hormone initiates metabolic
changes within some cells, such as the BONE and
MUSCLE, and also activates the production of
insulinlike growth factors (IGFs) that induce
metabolic activity in other cells.
Peptide Hormones
Peptide hormones consist of amino acid chains
and are the most abundant form of endogenous
hormone. Scientists further define peptide hormones
as small peptide (fewer than 10 amino
acids), polypeptide (more than 10 and fewer than
100 amino acids), or protein (100 or more amino
acids), depending on the length and configuration
of the amino acid chain. These distinctions influence
the hormone’s mechanisms of action, stability,
and receptor binding. Most of the body’s
hormones are peptide hormones.
Peptide hormones are water soluble and travel
through the bloodstream attached to protein molecules
called protein carriers. These larger structures
keep the hormone intact during transit. Most peptide
hormones cannot penetrate the wall of the cell.
Instead, they bind with protein receptors on the
cell’s surface (also called the plasma membrane).
The binding causes a chemical reaction that activates
proteins within the cell that then carry the
hormone’s message within the cell, indirectly influencing
cell activity. Among the exceptions are the
thyroid hormones, which do cross the cell membrane
to bind with receptors in the cell nucleus and
directly influence the cell’s activity.
PEPTIDE HORMONES
ANTIDIURETIC HORMONE (ADH)
CALCITONIN
cholecystokinin (CCK)
CHORIONIC GONADOTROPIN
CORTICOTROPIN-RELEASING
enterogastrone
HORMONE (CRH)
FOLLICLE-STIMULATING
gastric inhibitive
HORMONE (FSH)
polypeptide (GPI)
gastrin
GLUCAGON
GONADOTROPIN-RELEASING
GROWTH HORMONE (GH)
HORMONE (GNRH)
GROWTH HORMONE–RELEASING
INHIBIN
HORMONE (GHRH)
INSULIN
LUTEINIZING HORMONE (LH)
MELATONIN
motilin
OXYTOCIN
PARATHYROID HORMONE
PROLACTIN
RELAXIN
RENIN
secretin
SOMATOSTATIN
THYROID-STIMULATING
THYROTROPIN-RELEASING
HORMONE (TSH)
HORMONE (TRH)
THYROXINE (T 4 ) TRIIODOTHYRONINE (T 3 )
vasoactive intestinal peptide (VIP)
Steroid Hormones
Steroid hormones are lipid structures that derive
from cholesterol. Scientists further define steroid
hormones as corticosteroids (glucocorticoids and
mineralocorticoids), sex steroids (ANDROGENS,
ESTROGENS, PROGESTERONE), and vitamin D derivatives.
Like peptide hormones, steroid hormones
bind to protein carriers to transport them through
the bloodstream to their target cells. Steroid hormones
penetrate the wall of the cell to bind with
receptors (specialized proteins) within the cytoplasm
or cell nucleus to directly alter the cell’s
activity. Steroid hormones elicit genetic transcription
responses in the cells that contain their receptors.
ENDOGENOUS STEROID HORMONES
ALDOSTERONE CORTISOL ESTROGENS
PROGESTERONE TESTOSTERONE vitamin D