to those receptors. A receptor is somewhat like an
outlet that has a unique configuration. The hor-
mone 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 hor-
mone has unique receptors. Many cells have
receptors only for certain hormones, eliciting spe-
cific and narrowly focused changes. Only cells in
the TESTESand OVARIES, for example, have recep-
tors 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 cas-
cades, with multiple activities resulting from the
hormone’s release. For example, the HYPOTHALAMUS
releases GROWTH HORMONE–RELEASING HORMONE
(GHRH), which stimulates the PITUITARY GLANDto
release GH. Growth hormone initiates metabolic
changes within some cells, such as the BONEand
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 hor-
mones 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 influ-
ence the hormone’s mechanisms of action, stabil-
ity, 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 mole-
cules called protein carriers. These larger structures
keep the hormone intact during transit. Most pep-
tide 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 acti-
vates proteins within the cell that then carry the
hormone’s message within the cell, indirectly influ-
encing cell activity. Among the exceptions are the
thyroid hormones, which do cross the cell mem-
brane 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 deriva-
tives. Like peptide hormones, steroid hormones
bind to protein carriers to transport them through
the bloodstream to their target cells. Steroid hor-
mones penetrate the wall of the cell to bind with
receptors (specialized proteins) within the cyto-
plasm or cell nucleus to directly alter the cell’s
activity. Steroid hormones elicit genetic transcrip-
tion responses in the cells that contain their recep-
tors.ENDOGENOUS STEROID HORMONES
ALDOSTERONE CORTISOL ESTROGENS
PROGESTERONE TESTOSTERONE vitamin Dhormone 135