Download Endocrine System

Hormones and Endocrinology
The endocrine glands, by producing and releasing
hormones, play a critical role in homeostasis.
By definition, endocrine glands are organs or tissues
that produce chemical signals released into the
bloodstream and effect target cells that may be
located anywhere in the body.
The definition distinguishes endocrine function from
the chemical signaling that occurs between neurons;
that chemical is not released into the bloodstream,
but only into the cleft between nerve cells.
Some endocrine glands release their hormone(s) into
the circulatory system at the gland itself.
Others (labeled neurosecretory cells in fig. 26.1B)
release their hormone(s) at some distance from the
cell body of the hormone producing cell. However,
they are still released into the circulatory system.
There are a variety of chemicals that act as
hormones:
1. short chains of amino acids called peptides, or
longer ones that are proteins
2. amines (e.g. thyroxin, derived from the amino
acid tyrosine)
3. steroids (derived from cholesterol)
There are only two ways hormones communicate their
signals to target cells:
A. Peptide, protein, and amine hormones communicate
using a “second messenger” system. The process:
1. Circulating hormone binds to cell
surface receptors on the target cell
membrane.
2. Binding causes a conformation
change that affects “second
messenger” molecules (protein
kinases or phosphatases).
3. Through a chain of reactions the
cellular response is activated.
B. Steroid hormones are lipid soluble, and can pass
through the cell membrane. They bind to an
intracellular receptor and act inside the
nucleus of target cells.
The receptor-hormone complex
binds directly to DNA at very
specific sites, activating the
transcription of specific genes,
and leading to the production of
specific protein(s).
There are a very large number of hormones produced
by glands that occur throughout the head and torso of
your body. We can’t cover all of them, but here’s
where they are produced:
The hypothalamus and the pituitary
The so-called “master gland” is the hypothalamus.
It produces two peptide hormones that are released
from neurosecretory
cells that extend into
the posterior portion
of the pituitary. It also
releases ‘releasing
factors’ that stimulate
the anterior pituitary
to release its hormones.
The posterior pituitary hormones are:
Oxytocin –
this hormone causes contraction of target
muscle cells. At the end of pregnancy, it causes
contraction of muscle cells in the uterine wall
that are the key to birth of the fetus
(parturition).
Later, it causes muscle cells surrounding milk
glands in the breasts to contract, leading to milk
ejection.
ADH (or antidiuretic hormone) –
promotes the retention (reabsorption) of water
from the filtrate in the distal tubules and
collecting ducts of nephrons in the kidney. It is
part of the control system to maintain the
osmolarity of the blood.
The releasing factors (individually) stimulate the
release of hormones from the anterior pituitary.
The hormones produced by the pituitary are all
proteins. Some of the hormones are basically
involved in metabolic responses:
TSH – thyroid stimulating hormone – causes the
thyroid to accelerate production of T3 and T4
(triiodothyronine and tetraiodothyronine).
Note that this, like the other anterior pituitary
hormones, has as its target another endocrine
organ.
Also, that this hormone is released by a
hypothalamic stimulus – TRF – thryroid
stimulating hormone releasing factor.
When the feedback system that adjusts the releasing
factor in response to circulating T3 and T4 levels,
problems arise:
If feedback does not slow or shut off TRF, too much
thyroid hormone ends up in circulation. The result,
called Graves’ disease, more frequently caused by an
autoimmune problem, has apparent symptoms:
weight loss, excessive sweating,
bulging eyes (exopthalmos).
Insufficient thyroxin causes the release of more TRF,
and in turn more TSH. In adults, the thyroid grows
(hypertrophy) and the apparent symptom is a goiter
Adult hypothyroidism can
readily be treated with oral
thyroxine.
Childhood hypothyroidism is
more serious. Developmental
effects include stunted growth
and mental retardation (the
medical term is cretinism). It,
too, if detected early, can be
treated with oral thyroxine.
GH or growth hormone – its release is stimulated by
another releasing factor from the hypothalamus –
GRF (growth hormone releasing factor).
Growth hormone has a very broad spectrum of
effects and targets: 1) amino acid uptake and
protein synthesis are stimulated; 2) long bone
extension (height growth) is stimulated indirectly,
by stimulating the release of growth factors from
the liver.
Overproduction leads to giantism – height, but also
pronounced brow ridges and other effects.
Underproduction leads to pituitary dwarfism – all
parts in proportion but small.
ACTH or adrenocorticotrophic hormone – again
controlled by a hypothalamic releasing factor,
stimulates hormone-producing cells in the adrenal
cortex to release steroid hormones called
collectively corticosteroids.
One group are called mineralocorticoids. They
affect salt and water balance, some by affects on
kidney reabsorption.
The other group are called glucocorticoids. They
affect the conversion of glycogen (stored starch) to
glucose, as well as (when necessary) breaking down
muscle proteins to make glucose. They are also
anti-inflammatory.
Other hormones are not directly metabolic in their
end effects:
Prolactin – under the control of hypothalamic
hormones. This hormone has different affects on
different animal groups.
In us, it stimulates milk production by milk glands
in the mammary.
In fish, it is involved in blood osmolarity and its
regulation.
In amphibians, it regulates development during the
tadpole stage.
In birds, it is important in fat metabolism and
regulation of reproductive physiology.
Hormones affecting the testes or the ovaries:
FSH and LH – both hormones are produced by the
anterior pituitary in both sexes. Effects clearly
differ.
In males: FSH stimulates production and
maturation of sperm cells; LH stimulates secretion
of sex hormones (mainly testosterone) from
interstitial cells in the testes.
In females: FSH stimulates the growth of a follicle
and the maturation of an egg; LH promotes the
secretion of estrogen, then the maintenance of a
corpus luteum and its production of progesterone.
Endorphins, sometimes called ‘natural morphine’,
are also released by the anterior pituitary (and by
other parts of the brain). They seem to mainly
affect pain receptors, but their function(s) are still
somewhat hazy.
Remember, thus far we’ve considered only the
anterior pituitary, though also mentioning some of the
hormones released by trophic hormones (those
stimulating other endocrine cells to secrete
hormones). Now for those other hormones…
Many hormones are regulated more directly, by
sensing blood chemistry directly. That is the case for
the hormones regulating blood calcium ion (Ca++).
Two opponent hormones increase blood calcium
when it drops below a set point (parathyroid
hormone from the parathyroid glands) and
calcitonin from the thyroid gland.
There are four parathyroid glands, two on the surface
of each of the paired thyroid glands. Their hormone
causes increased calcium re-absorption in the
kidneys, increased calcium uptake in the gut, and
release of calcium from the bones. Calcitonin has
opposite effects.
Control of the amount of glucose in circulation is
managed by two opponent hormones made by two
kinds of islet cells in the pancreas.
When blood sugar increases above the homeostatic
level (90mg/100ml blood), insulin is released by β
cells of the islets of Langerhans. Insulin causes cells
to take up more glucose from blood, and causes the
liver to take up sugar and store it as glycogen.
When blood sugar decreases below the level,
different islet cells,  cells, release glucagon, which
has opposite effects – the liver breaks stored
glycogen down and releases glucose.
The disease resulting from inadequate regulation of
blood sugar is well known – Diabetes mellitus.
There are two forms of the disease:
1) Failure of islet cells to produce any or sufficient
insulin. This is insulin-dependent or type I
diabetes. Insulin is protein that cannot be taken
orally; it would be digested. Control is by insulin
injection.
2) Failure to respond sufficiently to circulating
insulin (to take up enough sugar). This form is
typically adult onset (age > 40). It is treated by
some combination of diet, exercise, and drugs
(glucose spargers, uptake/pancreatic stimulators)
The opposite problem can also occur – hypoglycemia
or low blood sugar.
It can occur due to over-responsive β cells that
produce too much insulin after a meal, or it can occur
as a result of treatment for diabetes (insulin injection
or stimulation of production).
The symptoms are sweating, nervousness (even the
shakes) and weakness. If blood sugar remains too
low, convulsions and diabetic coma can occur, and
can lead to death. In one famous case (movie:
Reversal of Fortune), Claus von Bulow injected his
wife Sonny with sufficient insulin to drive her into an
irreversible diabetic coma.
Stress responses: the adrenal medulla
The adrenal medulla releases stress hormones when
stimulated by nerve signals (not hormones) from the
hypothalamus. The hormones (amines chemically)
are epinephrine and norepinephrine.
These hormones are part of the “flight-or-fight”
response. They cause:
1) glycogen breakdown and increased blood
glucose
2) increased blood pressure
3) increased ventilation rate
4) increased metabolic rate
5) altered circulation (< to digestive system)