Endocrinology

Endocrinology is a branch of biology and medicine dealing with the

endocrine system, its diseases, and its specific secretions known as
hormones. It is also concerned with the integration of developmental
events proliferation, growth, and differentiation, and the psychological
or behavioral activities of metabolism, growth and development, tissue
function, sleep, digestion, respiration, excretion, mood, stress,
lactation, movement, reproduction, and sensory perception caused by
hormones.

There are three basic types of hormones: lipid-derived, amino acid-

derived, and peptide hormone. Lipid-derived hormones are structurally
similar to cholesterol and include steroid hormones such as estradiol
and testosterone. Amino acid-derived hormones are relatively small
molecules and include the adrenal hormones epinephrine and
norepinephrine. Peptide hormones are polypeptide chains or proteins
and include the pituitary hormones, antidiuretic hormone
(vasopressin), and oxytocin.

Chemical Classification of Hormones:

Hormones secreted by different endocrine glands vary widely in

chemical structure. All hormones, however, can be divided into a few
chemical classes.

1. Amines: These are hormones derived from the amino acids

tyrosine and tryptophan. They include the hormones secreted by
the adrenal medulla, thyroid, and pineal glands.
2. Polypeptides and proteins: Polypeptide hormones generally
contain less than 100 amino acids; an example is anti-diuretic
 hormone. Protein hormones are polypeptides with more than 100
amino acids; growth hormone is an example. The distinction
between polypeptide and protein hormones is blurred in the case
of insulin, which is composed of two polypeptide chains that are
both derived from the same protein precursor.
3. Glycoproteins: These molecules consist of a long polypeptide
(containing more than 100 amino acids) bound to one or more
carbohydrate groups. Examples are follicle-stimulating hormone
(FSH) and luteinizing hormone (LH).
4. Steroids: These are lipids derived from cholesterol. They include
the hormones testosterone, estradiol, progesterone, and cortisol.

In terms of their actions in target cells, hormone molecules can be

divided into those that are polar, and therefore water-soluble, and
those that are nonpolar, and thus insoluble in water. Since the
nonpolar hormones are soluble in lipids, they are often referred to as
lipophilic hormones. Unlike the polar hormones, which cannot pass
through plasma membranes, lipophilic hormones can gain entry into
their target cells. These lipophilic hormones include the steroid
hormones and thyroid hormones. Steroid hormones are secreted by
only two endocrine glands: the adrenal cortex and the gonads. The
gonads secrete sex steroids; the adrenal cortex secretes corticosteroids
(including cortisol and aldosterone) and small amounts of sex steroid.

The major glands that make up the endocrine system are the:

 hypothalamus

 pituitary

 thyroid
  parathyroids

 adrenals

 pineal body

 the ovaries

 the testes

Below are some examples of hormones that are produced by the

endocrine system.
Hormone Secreting gland(s) Function
adrenaline adrenal increases blood pressure, heart rate,
and metabolism in reaction to stress
aldosterone adrenal controls the body’s salt and water
balance
cortisol adrenal cortex plays a role in stress response

dehydroepiandrosterone sulfate (DHEA) adrenal aids in production of body odor and

growth of body hair during puberty
estrogen ovary works to regulate menstrual cycle,
maintain pregnancy, and develop
female sex characteristics; aids in
sperm production
follicle stimulating hormone (FSH) Anterior pituitary controls the production of eggs and
sperm
glucagon pancreas helps to increase levels of blood
glucose
insulin pancreas helps to reduce your blood glucose
levels
luteinizing hormone (LH) Anterior pituitary controls estrogen and testosterone
production as well as ovulation
melatonin pineal controls sleep and wake cycles

oxytocin Posterior pituitary helps with lactation, childbirth, and

mother-child bonding
parathyroid hormone parathyroid controls calcium levels in bones and
blood
progesterone ovary helps to prepare the body for
pregnancy when an egg is fertilized
prolactin Anterior pituitary promotes breast-milk production

testosterone ovary, testes, adrenal contributes to sex drive and body

density in males and females as well
as development of male sex
characteristics
thyroid hormone thyroid help to control several body
functions, including the rate of
metabolism and energy levels
Thyrotropin-releasing hormone (TRH) Hypothalamus Stimulates secretion of TSH and
prolactin

Growth hormone–releasing hormone Hypothalamus Causes release of growth hormone

(GHRH)

Dopamine or prolactin-inhibiting factor Hypothalamus Inhibits release of prolactin

(PIF)
Growth hormone Anterior pituitary Stimulates protein synthesis and
overall growth of most cells and
tissues
Adrenocorticotropic hormone (ACTH) Anterior pituitary Stimulates synthesis and secretion
of adrenocortical hormones
(cortisol, androgens, and
aldosterone)
Antidiuretic hormone Posterior pituitary Increases water reabsorption by the
kidneys and vasopressin causes
vasoconstriction and increased
blood pressure
Calcitonin Thyroid Promotes deposition of calcium in
the bones and decreases
extracellular fluid calcium ion
concentration
Leptin Adipocytes Inhibits appetite, stimulates
thermogenesis

Cholecystokinin (CCK) Small intestine Stimulates gallbladder contraction

and release of pancreatic enzymes

Renin Kidney Catalyzes conversion of

angiotensinogen to angiotensin I
(acts as an enzyme)
 Mechanism of hormone action
Generally hormone works in 2 way.

1) synthesis of new protein molecule

2) changing cell permiability

Hormones circulate in the blood but their concentration can increase

or decrease based on the requirement of the body. This is controlled by
feedback mechanisms.  
These mechanisms control the secretion of endocrine glands by
stimulating the hypothalamus, pituitary or both, which inturn governs
the secretion of a particular hormone. In positive feedback, the
secretion of the hormone increases where as in negative feedback
further secretion of hormone slows down. Feedback mechanisms are
the key factors for maintaining homeostasis in our body.
 
Hormones are classified into three major groups as peptide hormones,
steroid hormones and amino acid derived hormones based on their
chemical structure.
·  Peptide hormones cannot cross the phospolipid cell membrane and
bind to the receptors on the exterior cell surface. They are are
transported to the golgi, which is the site of modification. It acts as
a first messenger in the cell. Hormones on binding to their receptors do
not enter the target cell but generate the production of second
messengers such as cyclic AMP (c AMP), which in turn regulates
cellular metabolism. This is catalyzed by the enzyme adenylate cyclase.
The interaction between the hormone at the surface and the effect
brought out by cAMP within the cell is known as signaling cascade. At
each step there is a possibility of amplification. (Figure 11.17).
1. One hormone molecule may bind to multiple receptor molecules
before it is degraded.
2. Each receptor may activate several adenylate cyclases each of which
make much c AMP.
3. Thus there is more signal after each step.
The actions of cAMP are terminated by phosphodiesterases. The effect
of peptide hormones like insulin, glucagon, somatotropin are usually
short lived because they work through second messenger system.

· Steroid hormones can easily cross the cell membrane, and bind to
their receptors, which are intracellular or intranuclear. Upon binding to
the receptors, they pair up with another receptor – hormone complex
(dimerize).This dimer can then bind to DNA and alter its transcription.
(Figure 11.18).
·The effect of steroid hormones such as aldosterone, estrogen, FSH are
long lived, as they alter the amount of mRNA and protein in a cell.

·Amino acid derived hormones are derived from one or two aminoacid
with a few additional modifications. Thyroid hormone is synthesised
from tyrosine and includes the addition of several iodine atoms.
Epinephrine an amino acid derivative may function through second
messenger system like peptide hormones or they may actually enter
the cell and function like steroid hormones.
The regulatory molecules epinephrine, norepinephrine, dopamine, and
serotonin are in the chemical family known as monoamines. Serotonin
is derived from the amino acid tryptophan. Epinephrine,
norepinephrine, and dopamine are derived from the amino acid
tyrosine and form a subfamily of monoamines called the
catecholamines. Epinephrine (also called adrenaline) is a hormone
secreted by the adrenal gland, not a neurotransmitter, while the closely
related norepinephrine functions both as a hormone and a
neurotransmitter. Like ACh, monoamine neurotransmitters are released
by exocytosis from presynaptic vesicles, diffuse across the synaptic
cleft, and interact with specific receptor proteins in the membrane of
the postsynaptic cell. The stimulatory effects of these monoamines, like
those of ACh, must be quickly inhibited so as to maintain proper neural
control. The inhibition of monoamine action is due to (1) reuptake of
monoamines into the presynaptic neuron endings, (2) enzymatic
degradation of monoamines in the presynaptic neuron endings by
monoamine oxidase (MAO), and (3) the enzymatic degradation of
catecholamines in the postsynaptic neuron by catechol-O-
methyltransferase (COMT). The monoamine neurotransmitters do not
directly cause opening of ion channels in the postsynaptic membrane.
Instead, these neurotransmitters act by means of an intermediate
regulator, known as a second messenger. In the case of some synapses
that use catecholamines for synaptic transmission, this second
messenger is a compound known as cyclic adenosine monophosphate
(cAMP).Although other synapses can use other second messengers.
Binding of norepinephrine, for example, with its receptor in the
postsynaptic membrane stimulates the dissociation of the Gprotein
alpha subunit from the others in its complex. This subunit diffuses in
the membrane until it binds to an enzyme known as adenylate cyclase
(also called adenylylcyclase). This enzyme converts ATP to cyclic AMP
(cAMP) and pyrophosphate (two inorganic phosphates) within the
postsynaptic cell cytoplasm. Cyclic AMP in turn activates another
enzyme, protein kinase, which phosphorylates (adds a phosphate group
to) other proteins. Through this action, ion channels are opened in the
postsynaptic membrane.
 
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