Hormone
Epinephrine (adrenaline), a
catecholamine-type hormone
A hormone (from
Greek ὁρμή - "impetus") is a chemical messenger that carries a
signal from one cell to another. All
multicellular organisms produce hormones;
plant
hormones are also called
phytohormones. Hormones in
animals are
often transported in the blood. Cells respond to a hormone when they
express a specific
receptor for that hormone. The hormone binds to the receptor protein,
resulting in the activation of a
signal transduction mechanism that ultimately leads to cell
type-specific responses.
Endocrine hormone
molecules
are secreted (released) directly into the
bloodstream, while
exocrine hormones (or ectohormones) are secreted directly into a duct,
and from the duct they either flow into the bloodstream or they flow from
cell to cell by
diffusion
in a process known as
paracrine signalling.
Hierarchical nature of hormonal control
Hormonal regulation of some physiological activities involves a hierarchy of
cell types acting on each other either to stimulate or to modulate the release
and action of a particular hormone. The secretion of hormones from successive
levels of
endocrine cells is stimulated by chemical signals originating from cells
higher up the hierarchical system. The master coordinator of hormonal activity
in mammals is
the
hypothalamus, which acts on input that it receives from the
central nervous system.
Other hormone secretion occurs in response to local conditions, such as the
rate of secretion of
parathyroid hormone by the
parathyroid cells in response to fluctuations of ionized
calcium
levels in
extracellular fluid.
Hormone signaling
Hormonal signalling across this hierarchy involves the following:
-
Biosynthesis of a particular hormone in a particular tissue
- Storage and
secretion of the hormone
- Transport of the hormone to the target cell(s)
- Recognition of the hormone by an
associated cell membrane or
intracellular
receptor protein.
- Relay and amplification of the received hormonal signal via a
signal transduction process: This then leads to a cellular response. The
reaction of the target cells may then be recognized by the original
hormone-producing cells, leading to a
down-regulation in hormone production. This is an example of a
homeostatic
negative feedback loop.
- Degradation of the hormone.
As can be inferred from the hierarchical diagram, hormone biosynthetic cells
are typically of a specialized cell type, residing within a particular
endocrine gland (e.g., the
thyroid gland, the
ovaries, or the
testes). Hormones may exit their cell of origin via
exocytosis
or another means of
membrane transport. However, the hierarchical model is an oversimplification
of the hormonal signaling process. Cellular recipients of a particular hormonal
signal may be one of several cell types that reside within a number of different
tissues, as is the case for
insulin,
which triggers a diverse range of systemic physiological effects. Different
tissue types may also respond differently to the same hormonal signal. Because
of this, hormonal signaling is elaborate and hard to dissect.
Interactions with receptors
Most hormones initiate a cellular response by initially combining with either
a specific
intracellular or
cell membrane associated
receptor protein. A cell may have several different receptors that recognize
the same hormone and activate different
signal transduction pathways, or alternatively different hormones and their
receptors may invoke the same biochemical pathway.
For many hormones, including most
protein hormones, the receptor is membrane associated and embedded in the
plasma membrane at the surface of the cell. The interaction of hormone and
receptor typically triggers a cascade of secondary effects within the
cytoplasm
of the cell, often involving
phosphorylation or dephosphorylation of various other cytoplasmic proteins,
changes in
ion
channel permeability, or increased concentrations of intracellular molecules
that may act as
secondary messengers (e.g.
cyclic AMP). Some
protein hormones also interact with
intracellular receptors located in the
cytoplasm
or
nucleus by an
intracrine
mechanism.
For hormones such as
steroid or
thyroid hormones, their receptors are located
intracellularly within the
cytoplasm
of their target cell. In order to bind their receptors these hormones must cross
the cell membrane. The combined hormone-receptor
complex then moves across the nuclear membrane into the nucleus of the cell,
where it binds to specific
DNA sequences, effectively amplifying or suppressing the action of certain
genes, and affecting
protein synthesis.
However, it has been shown that not all steroid receptors are located
intracellularly, some are
plasma membrane associated.
An important consideration, dictating the level at which cellular
signal transduction pathways are activated in response to a hormonal signal
is the effective
concentration of hormone-receptor complexes that are formed.
Hormone-receptor complex concentrations are effectively determined by three
factors:
- The number of hormone molecules available for complex formation
- The number of receptor molecules available for complex formation and
- The
binding affinity between hormone and receptor.
The number of hormone molecules available for complex formation is usually
the key factor in determining the level at which
signal transduction pathways are activated. The number of hormone molecules
available being determined by the concentration of circulating hormone, which is
in turn influenced by the level and rate at which they are secreted by
biosynthetic cells. The number of receptors at the cell surface of the receiving
cell can also be varied as can the
affinity between the hormone and its receptor.
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