Physiology of hormones
Most cells are capable of producing one or more molecules, which act as
signalling molecules to other cells, altering their growth, function, or
metabolism. The classical hormones produced by cells in the
endocrine glands mentioned so far in this article are cellular products,
specialized to serve as regulators at the overall organism level. However they
may also exert their effects solely within the tissue in which they are produced
and originally released.
The rate of hormone biosynthesis and secretion is often regulated by a
homeostatic
negative feedback control mechanism. Such a mechanism depends on factors
which influence the
metabolism
and excretion
of hormones. Thus, higher hormome concentration alone can not trigger the
negative feedback mechanism. Negative feedback must be triggered by
overproduction of an "effect" of the hormone.
Hormone secretion can be stimulated and inhibited by:
- Other hormones (stimulating- or releasing-hormones)
- Plasma concentrations of ions or nutrients, as well as binding
globulins
- Neurons
and mental activity
- Environmental changes, e.g., of light or temperature
One special group of hormones is the
tropic hormones that stimulate the hormone production of other
endocrine glands. For example,
thyroid-stimulating hormone (TSH) causes growth and increased activity of
another endocrine gland, the
thyroid,
which increases output of
thyroid hormones.
A recently-identified class of hormones is that of the "hunger hormones" -
ghrelin,
orexin and
PYY 3-36 - and "satiety hormones" - e.g.,
leptin,
obestatin,
nesfatin-1.
In order to release active hormones quickly into the
circulation, hormone biosynthetic cells may produce and store biologically
inactive hormones in the form of
pre- or
prohormones.
These can then be quickly converted into their active hormone form in response
to a particular stimulus.
Hormone effects
Hormone effects vary widely, but can include:
- stimulation or inhibition of growth,
- In puberty hormones can affect
mood and
mind
- induction or suppression of
apoptosis
(programmed cell death)
- activation or inhibition of the
immune system
- regulating
metabolism
- preparation for a new activity (e.g.,
fighting,
fleeing,
mating)
- preparation for a new phase of life (e.g.,
puberty,
caring for offspring,
menopause)
- controlling the
reproductive cycle
In many cases, one hormone may regulate the production and release of other
hormones
Many of the responses to hormone signals can be described as serving to
regulate
metabolic activity of an organ or tissue.
Chemical classes of hormones
Vertebrate hormones fall into three chemical classes:
- Amine-derived
hormones are derivatives of the
amino
acids
tyrosine and
tryptophan. Examples are
catecholamines and
thyroxine.
-
Peptide hormones consist of chains of amino acids. Examples of small
peptide hormones are
TRH and
vasopressin. Peptides composed of scores or hundreds of amino acids are
referred to as
proteins.
Examples of protein hormones include
insulin
and growth hormone. More complex protein hormones bear
carbohydrate side chains and are called
glycoprotein hormones.
Luteinizing hormone,
follicle-stimulating hormone and
thyroid-stimulating hormone are glycoprotein hormones.
- Lipid and
phospholipid-derived hormones derive from lipids such as
linoleic acid and
arachidonic acid and phospholipids. The main classes are the
steroid hormones that derive from
cholesterol and the
eicosanoids. Examples of
steroid hormones are
testosterone and
cortisol.
Sterol
hormones such as
calcitriol are a
homologous system. The
adrenal cortex and the
gonads are
primary sources of steroid hormones. Examples of
eicosanoids are the widely studied
prostaglandins.
Pharmacology
Many hormones and their
analogues are used as
medication.
The most commonly-prescribed hormones are
estrogens
and
progestagens (as methods of
hormonal contraception and as
HRT),
thyroxine (as
levothyroxine, for
hypothyroidism) and
steroids (for
autoimmune diseases and several
respiratory disorders).
Insulin is
used by many
diabetics. Local preparations for use in
otolaryngology often contain
pharmacologic equivalents of
adrenaline, while
steroid and
vitamin D
creams are used extensively in
dermatological practice.
A "pharmacologic dose" of a hormone is a medical usage referring to an amount
of a hormone far greater than naturally occurs in a healthy body. The effects of
pharmacologic doses of hormones may be different from responses to
naturally-occurring amounts and may be therapeutically useful. An example is the
ability of pharmacologic doses of
glucocorticoid to suppress
inflammation.
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