760
U N I T 9
Endocrine System
Hormones bring about their effects on cell activity by binding to specific cell
receptors.There are two general types of receptors: (1) cell surface receptors
that exert their actions through cytoplasmic second messenger systems, and (2)
intracellular nuclear receptors that modulate gene expression by binding to DNA or
promoters of target genes.
U N D E R S T A N D I N G
Hormone Receptors
Cell Surface Receptors.
Water-
soluble peptide hormones, such as
parathyroid hormone and glucagon,
which cannot penetrate the lipid
layer of the cell plasma membrane,
exert their effects through intracel-
lular second messengers. They bind
to a portion of a membrane receptor
that protrudes through the surface
of the cell. This produces a struc-
tural change in the receptor mol-
ecule itself, causing activation of a
hormone-regulated signal system
located on the inner aspect of the
cell membrane. This system allows
the cell to sense extracellular events
and pass this information to the
intracellular environment. There are
several types of cell surface recep-
tors, including G-protein–coupled
receptors that mediate the actions
of catecholamines, prostaglandins,
thyroid-stimulating hormone, and
others. Binding of the hormone to
the receptor activates a G protein,
which in turn acts on an effector
(such as adenyl cyclase) to generate
a second messenger (such as cyclic
adenosine monophosphate, cAMP).
The second messenger, in turn, acti-
vates other enzymes that participate
in cellular secretion, gene activation,
or other target cell responses.
1
G-protein
Effector
Adenosine
triphosphate
(ATP)
Cyclic AMP
(cAMP)
Receptor
Hormone
(First messenger)
(Transducer)
Adenyl cyclase
Second messenger
Target cell response
Extracellular fluid
Intracellular fluid
