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OVERVIEW ENDOCRINE SYSTEM
ENDO BLOCK 412
Dr. Shaikh Mujeeb Ahmed
Assistant Professor
AlMaarefa College
Objectives
• Understanding the common aspects of neural
and endocrinal regulations.
• Describing the chemical nature of hormones
• Recalling the overall hormonal functions
• Understanding the different mechanisms of
hormonal action & concept of second
messenger system
Objectives
• Understanding the common aspects of neural
and endocrinal regulations.
• Describing the chemical nature of hormones
• Recalling the overall hormonal functions
• Understanding the different mechanisms of
hormonal action & concept of second
messenger system
Introduction
• Body systems always works to maintain
homeostasis
• Two major regulatory systems of body are
– Nervous system
– Endocrine system
Nervous and Endocrine Systems
• Act together to coordinate functions of all body systems
• Nervous system
– Nerve impulses/ Neurotransmitters
– Faster responses, briefer effects, acts on specific target
• Endocrine system
– Hormone – mediator molecule released in 1 part of the body
but regulates activity of cells in other parts
– Slower responses, effects last longer, broader influence
Fig. 13.2
Comparison between Nervous and
Endocrine system
Nervous system
•The nervous system exerts
point-to-point control through
nerves, similar to sending
messages by intercom. Nervous
control is electrical in nature and
fast.
Hormones travel via the bloodstream
to target cells
•The endocrine system broadcasts its
hormonal messages to essentially all
cells by secretion into blood and
extracellular fluid. Like a radio
broadcast, it requires a receiver to get
the message - in the case of endocrine
messages, cells must bear a receptor
for the hormone being broadcast in
order to respond.
General Principles of Endocrinology
• The endocrine system consists of the ductless
endocrine glands that are scattered throughout the
body.
• The endocrine glands are not connected anatomically
• They constitute a system in a functional sense.
• Secreting hormones into the blood
• Once secreted, a hormone travels in the blood to its
distant target cells, where it regulates or directs a
particular function.
• Endocrinology is the study of the homeostatic
chemical adjustments and other activities that
hormones accomplish.
A cell is a target because is has a
specific receptor for the hormone
Most hormones circulate in blood, coming into contact
with essentially all cells. However, a given hormone usually
affects only a limited number of cells, which are called
target cells.
A target cell responds to a hormone because it bears
receptors for the hormone.
Hormone receptors
Types of Hormone
• According to the distance the hormone travel
– Endocrine
– Paracrine
– Autocrine
– Intracrine
• According to solubility
– Hydrophilic eg. Peptide, hormone
– Lipophilic eg. Steroid & thyroid hormone
Types of Hormone
Hormone types
– Circulating – circulate in
blood throughout body
– Local hormones – act
locally
• Paracrine – act on
neighboring cells
• Autocrine – act on the
same cell that secreted
them
Characteristics of Peptide hormone
• Produced and processed by the endoplasmic reticulum and Golgi
complex of the endocrine cell
• Stored in secretory vesicles until signaled for release by exocytosis.
• Circulate in the blood largely dissolved in the plasma
• Bind with surface membrane receptors of their target cells,
• Act primarily through second-messenger pathways to alter the
activity of preexisting proteins, such as enzymes, to produce their
physiologic response.
• The peptide molecule prepared first is usually a larger precursor
called as Preprohormone; It is cleaved to form Prohormone; further
cleavage of Prohormone, forms the final mature Hormone.
Example:
Preproinsulin → Proinsulin → Insulin
This hormone is stored in vesicles of endocrine cells and released on
demand
Blood capillary
1 Binding of hormone (first messenger)
to its receptor activates G protein,
which activates adenylate cyclase
Water-soluble
hormone
Adenylate cyclase
Receptor
Second messenger
G protein
ATP
cAMP
2 Activated adenylate
cyclase converts
ATP to cAMP
6 Phosphodiesterase
Protein kinases
inactivates cAMP
3 cAMP serves as a
Activated
protein
kinases
second messenger
to activate protein
kinases
4 Activated protein
Protein
kinases
phosphorylate
cellular proteins
ATP
ADP
Protein—
P
5 Millions of phosphorylated
proteins cause reactions that
produce physiological responses
Target cell
Characteristics of lipophilic Steroid
hormone
• By stepwise modifications of a basic cholesterol
precursor molecule.
• Steroidogenic (“steroid-producing”) organs specialize in
the type of hormones they produce because each of
these organs has the enzymes necessary to produce
only one or several, not all, of the steroid hormones.
• Steroid hormones act primarily by activating genes on
binding with receptors inside the cell, thus bringing
about formation of new proteins in the target cell that
carry out the desired response.
• lipophilic hormones are largely bound to plasma
proteins.
Free hormone
Blood capillary
1 Lipid-soluble
Transport
protein
hormone
diffuses into cell
2 Activated
Nucleus
Receptor
receptor-hormone
complex alters
gene expression
DNA
Cytosol
mRNA
3 Newly formed
mRNA directs
synthesis of
specific proteins
on ribosomes
Ribosome
New
protein
4 New proteins alter
cell's activity
Target cell
Characteristics of lipophilic thyroid
hormone
• synthesized by a unique pathway within the
thyroid gland but functions at its target cells
by means similar to those used by lipophilic
steroids.
OVERALL FUNCTIONS OF THE
ENDOCRINE SYSTEM
1. Regulating organic metabolism and H2O and
electrolyte balance
2. Inducing adaptive changes to help the body
cope with stressful situations
3. Promoting smooth, sequential growth and
development
4. Controlling reproduction
5. Regulating red blood cell production
6. Along with the autonomic nervous system,
controlling and integrating activities of both
the circulatory and digestive systems
TROPIC HORMONES
• A hormone that has as its primary function
the regulation of hormone secretion by
another endocrine gland is classified
functionally as a tropic hormone (tropic
means “nourishing”).
• Example: Thyroid stimulating hormone (TSH),
Adrenocorticotropic hormone (ACTH) etc.
COMPLEXITY OF ENDOCRINE
FUNCTION
1. A single endocrine gland may produce multiple
hormones n(Anterior Pituitary).
2. A single hormone may be secreted by more than one
endocrine gland. For example, both the hypothalamus
and pancreas secrete the hormone somatostatin.
3. Frequently, a single hormone has more than one type
of target cells (Vasopressin)
4. The rate of secretion of some hormones varies
considerably over the course of time in a cyclic
pattern.
5. A single target cell may be influenced by more than
one hormone.
Cont..
6. The same chemical messenger may be either
a hormone or a neurotransmitter
7. Some organs are exclusively endocrine in
function (they specialize in hormone
secretion alone, the anterior pituitary being
an example), whereas other organs of the
endocrine system perform non endocrine
functions in addition to secreting hormones
(the testes).
Mechanisms of Hormone Action
• Response depends on both hormone and
target cell
• Lipid-soluble hormones bind to receptors
inside target cells
• Water-soluble hormones bind to receptors on
the plasma membrane
– Activates second messenger system
– Amplification of original small signal
Cell mechanism & Second messengers
• G Protein–Linked Hormone Receptors.
– Many hormones activate receptors that indirectly
regulate the activity of target proteins (e.g.,
enzymes or ion channels) by coupling with groups
of cell membrane proteins called heterotrimeric
GTP-binding proteins (G proteins)
Second messengers for cell-surface
receptors
 Second messenger systems include:
 Adenylate cyclase which catalyzes the conversion of ATP to
cyclic AMP;
 Guanylate cyclase which catalyzes the conversion of GMP
to cyclic GMP (cyclic AMP and cyclic GMP are known
collectively as cyclic nucleotides);
 Calcium and calmodulin; phospholipase C which catalyzes
phosphoinositide turnover producing inositol phosphates
and diacyl glycerol.
Adenyl Cyclase–cAMP Second
Messenger System
• Hormone binds to the
receptor
• GDP is released from G
protein
• Activate adenyl cyclase
which convert ATP to cAMP
• cAMP activates tyrosine
kinase A
• Tyrosine kinase A
phosphorylate specific
protein for physiological
action
IP3 Mechanism
• Hormone binds to the
receptor
• Phospholipase C liberates
DAG & IP3
• IP3 mobilizes Ca++ from ER
• Ca++ and DAG activates
tyrosine kinase C
• Tyrosine kinase C
phosphorylate specific
protein for physiological
action
Calcium-Calmodulin Second
Messenger System
• Hormone binds to the receptor
• Open Ca++ channels & release Ca++ from ER
• Ca++ binds to calmodulin to produce
physiological action
Table 18-3, p. 674
Sequence of Steroid hormone action
Summary of Chemical Structure, Synthesis,
and Transport of Hormones
Properties
Peptides
Synthesis and
storage
Polyribosomes -ER
as
preprohormones/
Vesicles
Cytosol/ ATP
Ca++ /
Chromagranin
Vesicles
Colloid of follicular cells Mitochondria/
pendrin
cytosol
(iodide/chloridetransp
orter
Release from
parent cell
Exocytosis
Exocytosis
thyroglobulin to
megalin endocytosis/
lysosomal degradation
diffusion
Transport in
blood
Most are free
Loosely bound
to albumin
TBG, transthyretin,
albumin
Albumin
sex hormone or
sex hormone or
corticosteroid
BG
Minutes
1-2 Minutes
T4- 7days, T3- 18 days
hours
plasma membrane
plasma
membrane
intracellular
intracellular
protein
phosphorylation
genomic
protein
phosphorylation
genomic
genomic
genomic
Half life
Location of
receptor
Response to
receptor-ligand
binding
Catechol-amines
Thyroid
Hormone
Steroids
References
 Human physiology, Lauralee Sherwood, seventh
edition.
 Text book physiology by Guyton &Hall,11th
edition.
 Physiology by Berne and Levy, sixth edition.
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